Epic Storm to Bring Severe Weather to the South

Any “weather weenie” is well aware of the fact that a major storm is coming for the eastern United States (yes, it really is that big). Accuweather.com and the Weather Channel are beginning the hype, Twitter is abuzz with forecasts and speculation (at least in the weather-weenie corner of it), and even the stolid National Weather Service has gotten in on this, with the Storm Prediction Center calling for “Particularly Dangerous Situation” winter storm warnings. (A PDS ordinarily is issued for severe weather—typically, in fact, for the type of severe weather event that has a historic tendency to generate violent tornadoes.) Indeed, blizzard conditions will be widespread across the Midwest, probably affecting an area as large as the “Storm of the Century” of March 1993. The culprits are a low pressure center and a mass of arctic air coming from Canada. The unseasonably warm air that we have enjoyed in the Southeast is going to fuel the system, so to speak.

I am not going to touch on the snow and ice aspects of this. The area that will undergo winter storm or even blizzard conditions is simply too large for me to give anything other than a large-scale overview, for one. And two, this is primarily a Southeastern blog as far as actual forecasts are concerned, and we in the South are going to have to deal with the system in a different way: severe thunderstorms. That warm Gulf air that will feed the system is going to be generating what’s known as a “warm sector.”

Mid-latitude cyclones such as this one have three distinct sectors: one to the east of the cold front, one to the west, and one that remains to the north. The north sector is what will be producing the snow and ice; north sides of cyclones typically have a constant flow of precipitation, because the cold front associated with the low does not actually pass over these regions. The west side is the “dry” side, in general (though there are exceptions); this is also typically a cold part of the system. That’s what’s going to bring a return to winter temperatures for the South; it will follow the passage of the cold front. And the warm sector is the southeastern side—the side that will have southerly flow from the Gulf of Mexico. These 60s and 70s that we’ve been experiencing, in other words.

There will be several factors that will create severe weather for us. I am going to illustrate these with images from the North American Mesoscale (NAM) model, downloaded from TwisterData.com. First is the location and intensity of the jet stream:

That image shows that the jet at 300 mb (~9000 m) will have 140-knot winds over ArkLaTex on Tuesday evening. The jet typically does have strong winds on the order of 80 knots, which is why it is a death sentence (via shear) for hurricanes in the summer, but these same winds that rip warm-core tropical cyclones to shreds create conditions that enhance cold-core cyclones in the cool season, when the jet is much farther south. 140 is quite strong even for winter. Furthermore, the location of the jet indicates that air will be drawn to the northwest along the eastern end of the strongest points—the exit region, in meteorological parlance. The image doesn’t appear to depict this, but the southeastern flow I am referring to is taking place vertically, and this image only shows winds at the 300 mb level. This will create areas of divergence in Mississippi, Louisiana, and eastern Arkansas. Divergence leads to a “vacuum” of sorts—an area of lower pressure—and therefore to rising air.

The second factor is the extreme temperature gradient.

Again, take a good look at Arkansas. This is an image from the same time as the one depicting the jet stream, but much lower in the atmosphere (850 mb, or 1500 m). The 850-mb low is located around Fayetteville, and you can see the wind flow around it. But look at the colors, which represent temperature. That is a very strong cold front, and the reason for it is the extremely unseasonable warm weather across the Gulf states. You can see the southerly flow of warm, moist air, creating the warm sector. The wind flow will be moving warmer air into areas of cooler air (the wind barbs are crossing isotherms), a process called warm-air advection. Look at the strength of the winds at this level, too. There are 65-knot winds forecast at 850 mb! This is hurricane-force.

Finally, the pressure gradient itself:

I will direct your attention to the isobars over the Plains. The low hasn’t even fully intensified yet, but already there is a 40 mb gradient because the high is so darn strong. It is this pressure gradient that will be generating the intense winds with this system, producing blizzard conditions north of the freezing line, and severe weather south of it.

Now, the $64,000 question: What type of severe weather are we talking about? None of the above sounds like particularly good news, but fortunately (for north MS, at any rate), I do have some comparatively good news now. I do not think our tornado risk is going to be all that high, nor our large hail risk. Instead—you guessed it—I think we are in line for some very strong winds as the cold front approaches us late Tuesday night/early Wednesday morning, combined with a lot of rain and the chance of small hail. There will be some instability, but CAPE levels are not going to be much higher than about 750 J/kg across Mississippi. The helicity index, a parameter reasonably predictive of supercell formation, is not expected to be very high. I think there is a chance of supercells in south MS (let’s say south of I-20), but at the present time I am not seeing this type of severe weather event on a large scale.

However, as people in north MS can certainly attest after the repeat punches of severe weather over the past year, high winds can be damaging enough on their own, and this system has the potential to generate some very strong winds for us. All in all, though, I think we are dodging not one, but several bullets with it.

Winter Storm Expectations

I have been very hesitant to make any kind of forecast regarding this system, because it has been a wretchedly difficult storm to predict. Winter storms in the Deep South usually are. And it really doesn’t help when the computers, supposed objective arbiters of the atmospheric data, can’t make up their “minds” either. (Models are supposed to be guidance for the forecasters, a way of looking at possibilities in a straightforward and easy-to-grasp way. They are NOT supposed to replace thinking about one’s own forecast and understanding meteorology.) Just one model, the GFS, has shown nothing, a rain event, a mega snowstorm (in Deep South terms), a crippling ice storm, and wintry slop. And that’s just over the past week. Fortunately, two of those options seem to have been ruled out. We’re going to get frozen precipitation. That’s a given.

This system is one of those where we humans have had to sort through things ourselves and make a judgment call. That’s good for learning, but I’m quite glad I am not in a position where my forecast (or input for a forecast) will have any discernible effect upon the local economy and opening/closing decisions. The question really is whether we will get a foully unpleasant, persistent, cold mix of slop for a day and a half, which makes things pretty miserable but doesn’t require anything to be shut down, or whether it will be a crippling winter storm. Oh, and if it is a crippling winter storm, what type that will be. Mega snowstorm or destructive ice storm?

Wouldn’t want that judgment call either, would you?

Unfortunately, I began leaning toward a worst-case scenario a couple of days ago, though I was not confident enough in it to post such a forecast on the Internet. That’s good; I was prepared to call for an ice storm for everywhere from I-20 to Highway 78. Things began pointing in a different, snowier direction yesterday.

OK, I’m going to put this up first and then go into a bit of meteorological explanation for it. This is not an official forecast from any weather agency, public or private-sector. This is MY forecast.

The system is going to be formed from the mixing of a cold arctic air mass coming down from the north and a moist tropical-type low in the Gulf of Mexico. This type of system is very good at bringing significant winter weather conditions to the Southeast, because it gets all the ingredients in the right place and all that becomes important is the amount of each type of ingredient, whether warm air, cold air, or moisture. Usually, systems from the north that are cold enough to bear snow are too dry; warm air intrusion often kills the chances of frozen precipitation for those that are not too dry. It’s a balancing act.

The precipitation is going to begin as snow across much of the state. I think an exception will be areas south of the I-20 corridor, who will probably get sleet first. This is because the cold air will have arrived and the system from the Gulf is just getting started. In areas where the precipitation type will not remain snow, I doubt there will be much snow accumulation from this part of the storm, because high temperatures have been in the 40s and 50s ever since the New Year’s Day tornado event.

As the Gulf low cranks up, warm air advection from the south will increase. The surface temperatures will still be around or below freezing, but air aloft will be warmed by this intensification of the Gulf low. This will result in a changeover from all snow to a mix of sleet and snow from Starkville-Columbus down to perhaps De Kalb. Areas south of that will probably see a changeover to freezing rain. I have been very concerned about the possibility of a major ice storm for the I-20 corridor for several days, as I watched round after round of forecast Skew-Ts come in with a freezing rain signature. This has all the hallmarks of being just that. The National Weather Service is saying up to half an inch of freezing rain (this is a solid coating of ice); I would put the maximum at 3/4 of an inch. Meridian and Jackson are both going to be clobbered by this part of the storm.

The precipitation should continue in these forms through Sunday night. I think that in areas where there is a snow/sleet mix, what percentage of each type you will see will depend on your latitude. I am inclined to put the belt of maximum snowfall a bit north of Highway 82.

After balking at it, I have come around to agreeing that somebody will get a major snow accumulation on the order of 8 inches. There may be isolated areas where a foot of snow falls. As disruptive as that can be, and as risky as it indeed is for power, I think the greater threat exists in the ice storm corridor. Please, please, please, if you are in this area, make sure you will be able to stay warm!

Everyone from that area remember the ice storm of February 1996? I don’t blame you if you’ve put it aside, but this may refresh your memories.

Freezing rain occurred over all the above counties causing widespread damage to trees and power lines. Accumulations of one-half to one inch of ice were common over this area. Over one hundred thousand customers were without power during the event. Most roads and bridges were impassable, and some of the roads had to be closed.

Though I do not think ice storm conditions will envelop the exact same area as before, a repeat of this event is very possible on Sunday for the I-20 corridor!

Stay warm, stay safe, and if you are in line for snowfall, enjoy it!

A Big Storm, Cold, Thunder, and Perhaps Winter Precip?

Think it was pretty cold this past week? Well, wait till Sunday and Monday. But before we get to that point, we’ll have had quite a system to pass through the area, featuring thunderstorms, strong winds (and the accompanying frigid wind chills), and—though the National Weather Service isn’t officially forecasting it—I think an outside chance of frozen precipitation. Areas outside the Southeast are in line for much more frozen stuff.

Yup, it’s meteorological winter (December through February), all right.

The low that will quickly become a powerful cyclone is analyzed right now as 1002 mb. In a short period of time, it should be located in the Plains states and closing on the Midwest. By the time it reaches Iowa (approximately early tomorrow morning), it will be generating rain and snow to its north as the warm sector (that’s where we are) forms.

Look at this image from the North American Model (NAM). Classic cyclonic shape.

This is a forecast for late Saturday night or the wee hours of Sunday morning, which may be a bit slow. Our storms—generated by uplift along the cold front—may arrive earlier than this. The timing is going to be critical, especially in consideration of the outside chance of wintry precipitation. If the moist sector of the cyclone pushes through faster (forced along by cold, dry air on the other side of the cold front), then we’re not going to get anything of that kind. Jackson NWS doesn’t want to forecast any such precipitation, either; I freely acknowledge that the odds aren’t great and it makes sense that they would not want to go out on a relatively unsupported limb.

However, there is some model support for my thinking on this. Here’s the NAM forecast for 6 hours later.

Observe that precipitation is occurring north of the 0-degree line. Observe where the 0-degree line is.

Looking at imagery for the lower levels of the atmosphere, we can see that it will be a very, very close call for northeast Mississippi, and the type of frozen precipitation (if any) cannot be guessed at with any accuracy. This is because when a cold front passes, there is not a straight vertical line dividing the area of retreating warm air from the advancing cold air. Instead, it is an angle. Since cold air sinks, we are far more likely to have cold air at the surface and some warm air still lingering aloft in the first couple of hours after the “edge of the front” (a very fuzzy demarcation, but you get my point) moves through.

If we do get any winter weather as a parting gift from this cold front, it’s likely to be mixed with rain, and the ground will already be wet from the rain we already would have had (and warm from having been in the warm sector prior to the frontal passage). Accumulation is not even on the table.

What about those thunderstorms? Are we in line for another severe weather outbreak like the one from the end of last month? Probably not. That event had CAPE values that would raise eyebrows in the spring, let alone the fall. These potential energies just will not be present for this front. However, thunderstorms are expected to occur Saturday evening, likely bringing a lot of rain and lightning, and an outside chance of small hail.

After the front has passed, we are going to get a glancing blow from the arctic air mass that is behind the system. Highs are not likely to reach 40 in too many areas north of I-20 on Sunday and Monday. The wind chill on Monday morning is going to be dangerously cold, approaching 10 degrees and getting close to zero around highway 82 and points north. Take note of this if you have to go to work or school.

In the wake of the previous cold spell and the one that is coming up soon, I’m hearing some grumbling about the forecast that NOAA made for a “warmer and drier winter than average” for the Southeast. That forecast is still on tap. However, “warmer and drier than average” typically does not mean that it will be so much warmer and drier that we will be able to notice it every day! We are talking about a couple or three degrees on average for the entire winter season. That allows for plenty of below-average events, such as this upcoming one. And indeed, if you look at the long-term GFS, it shows a rather significant warmup to temperatures approaching 70 degrees until the next system pushes through just before Christmas and drives temperatures down to the 40s again. Such long-term forecasts cannot be trusted in the winter season, because the weather is so volatile, but it’s definitely food for thought!

Snow in Dixie Watch: Tuesday-Wednesday, Dec. 7-8?

I’ve been keeping an eye on this for several days, but the models have been cagey about it, and I have been reluctant to make a blog post about it. For the past two years, predicting snow for central Mississippi in December has been, let’s just say, a fiasco, even for the National Weather Service office, and I’m not overly inclined to get burned a third time running. As a general rule, snow in the South tends to occur in late winter. That said, this is looking promising enough to warrant a blog post.

It’s not a big system. In fact, it’s not easy to identify just what is going on with a surface chart, because the snow (if any) will be driven primarily by upper-atmospheric troughs that do not extend to the surface. What we have got here is a low pressure system off Atlantic Canada and a very strong jet streak that has dived as far south as the Gulf of Mexico.

Here is one level of the troposphere, 500 mb. The upper-level trough is evident enough here, and would you look at that—its base is right over us:

However, there is no obvious trough over the South on a surface map:

What you see in the surface map that appears to match up with the trough at 500 mb is actually a temperature gradient. The thin black lines are the pressure lines, and the thicker lines that correspond with the thick lines at 500 mb are actually temperature lines. There is a reason why this temperature gradient shows up as a trough at the upper levels. When the air is cold, it contracts. When it is warm, it expands. When you consider that the temperature change occurs over time, it can be seen as cold or warm air moving in at a given layer. This process is called thermal (or thickness) advection. Here are simple diagrams indicating how thermal advection happens at the mid levels:

  

These two diagrams are by no means representative of every advection scenario that can occur. Thermal advection can happen at any level of the atmosphere. Whether the advection results in forced ascent from the surface or forced descent to the surface depends on exactly at what level of the atmosphere the strongest contraction/expansion is occurring and what is happening below and above it.

We are going to be in a forced ascent situation, as shown in the second diagram. This results in pressure levels that are expected at a given altitude in the atmosphere (in this example, 18,000 feet) actually occurring at a lower altitude, thus the “trough.” But look: Relative to the lower levels of the atmosphere, there is a high. This is shown on the surface map. What’s important is that there will be forced ascent, not that there will be relatively high pressure at the surface.

This feature is what is going to generate precipitation. By creating a relative low aloft, it causes air at the surface to rise (nature abhors a vacuum), generating cloudiness. Precipitation is expected to fall as snow because there is no warm air getting between the surface and the layer of clouds, owing to that extremely southerly and extremely strong jet.

Will there be snowfall? Probably. The big question seems to be whether any of it will stick. It has been cold enough lately to freeze the ground, but a big question mark is a layer of dry air at the middle to lower levels of the atmosphere. The forced uplift may take a while to fill this layer of air with moisture. We’ll have to see. In any case, keep an eye to the sky tomorrow night!

From La Nina to What?

We are currently in a La Nina episode of moderate strength. La Nina is the stage of the El Nino-Southern Oscillation in which the equatorial Pacific is cooler than average:

La Nina

This episode has likely played a large part in the 2010 Atlantic hurricane season’s intensity and length. It probably also affected large-scale weather patterns during the intense drought over the American South (which still continues, despite rain, because have a significant rain deficit to make up.) La Nina is expected to continue through the winter. La Nina winter conditions for the Southeast are, in my opinion, wretched: Warmer and drier than normal. If we get a snow event, it’ll be in spite of the ENSO pattern. However, by no means should snow be ruled out; winter precipitation is determined by short-term synoptic-scale weather systems more than anything else. Indeed, our first freeze this year happened earlier than last year, which was an El Nino winter (El Nino winters for the Southeast are cooler and wetter than usual).

This La Nina event is following a pattern and timeline of strengthening most similar to the 1998/1999 La Nina event, as the official list of ENSO statuses indicates. That year was not known as a “wintry” winter for the South. There was a devastating ice storm in some areas, but the storm’s description makes it quite clear that this was an aberration that effectively defied the La Nina conditions that had been controlling the winter previously:

Much of the fall and early winter season of late 1998 had been very mild for southerners. But on Tuesday December 22nd, a strong surge of shallow Arctic air drove southward and spelled a quick end to the unseasonable warmth and helped set the stage for one of winter’s nasty tricks. A moist southwest flow above the cold air brought several upper level disturbances. This combination of cold air and moisture held together for several days and brought periods of freezing rain and sleet to the three state region, beginning Tuesday night and not ending until Christmas morning. Up to 2 inches of ice accumulated on power lines and much of the area experienced long power outages, nearly seven days in some cases. Hardest hit was an area from northeast Louisiana to north central Mississippi where tree and power line damage was moderate to severe. The prior warmth had left most trees with a little more foliage than usual and greater potential for ice accumulation.

(Please note that I am NOT forecasting an ice storm for the Southeast this winter! It is not possible to forecast an event like this far in advance.)

I have a growing suspicion that the most profound impact, or influence perhaps, of this year’s La Nina will actually be felt next year. There is a link between intense tornado seasons and a La Nina the previous winter. Some notorious examples are 2008 (the Super Tuesday tornado outbreak, among others), 1999 (the record F5 tornado in Oklahoma), 1974 (the unprecedented and historically unique Super Outbreak, which had either 6 or 7 F5 tornadoes), and 1971 (the most recent F5 tornado in Mississippi). The mechanics of this phenomenon are not well understood yet, and the statistical link is not as highly correlated as it might be. In fact, there may even be a lesser link between El Nino winters and an active tornado season. But there is a growing consensus in meteorology that La Nina winters tend to precede active springtime tornado seasons.

The long-range ENSO computer models are suggesting that this La Nina episode may go away by next year, leaving us in a slightly negative neutral state:

ENSO models

If this plays out, we should consider another long-term implication, the effect of the assumed demise of La Nina on next year’s hurricane season. From this far ahead in time, we have no skill at forecasting any other factor except the Atlantic multi-decadal oscillation that entered a warm phase in 1995, so the status of ENSO would play a large part in long-range hurricane season forecasts. From the current warm phase of the AMO, hurricane seasons in which a La Nina episode from the previous winter met its demise include 2008, 2001, 2000, and 1996. From the previous warm AMO phase, such years include 1968, 1963, 1957, and 1951. However, I do not think that these years from the 50s and 60s are that applicable to this year, because this year’s La Nina is already stronger than any of those were. A couple of them were quite shallow, in fact. And three of the years saw the development of an El Nino following the La Nina, an occurrence that right now seems unlikely.

Given the strength of our current La Nina already, I actually would not be that surprised if the computer models shifted to predict the continuation of La Nina through next winter. Some of them do, in fact. In any case, whether we shift to neutral by next summer or remain in a (weakened but reintensifying) La Nina, it seems that next year’s hurricane season is likely to be active just as this year’s was. 2011 has the potential of being quite a headline-generating weather year!

Major Storm Unfolding in the Midwest

The megastorm that weather people have been talking about for several days has materialized, and it is already bringing very high wind and long lines of severe squalls and tornadic supercells to the Midwest and upper Tennessee Valley. Rain and thunderstorms associated with the cyclone extend as far south as the Gulf states. Blizzard conditions are expected for areas on the cold side of the storm, which is expected to bottom out its pressure in the 960 mb range. The low, centered over Minnesota as of this writing, is already at 966 mb. The factors causing this powerful system are many: Very unseasonable heat in the Southeast has led to a powerful warm sector for the cyclone, and a surge of cool air to contrast with this has caused development of a strong jet stream (>100 knots) in the upper levels of the atmosphere. Had Hurricane Richard’s remnant low been drawn more to the east, as the models were suggesting a couple of days ago, the system would have become even more powerful. A mid-latitude cyclone of this magnitude is not common. It is a credit to modern technology that our computer models were able to accurately predict this rare of a system several days in advance.

The Storm Prediction Center issued a high risk for today, a rare occurrence, but with dozens of tornado warnings already called for the Midwest where the storm’s warm sector meets its area of highest vorticity advection, I expect that this will end up verifying.

Well, This Is Certainly Interesting

Since I posted last night, the range of possibilities for our coming weather in the Deep South has expanded quite a bit, and the “interesting” scenario that I hinted at towards the end is suddenly looking a lot more likely to actually unfold. I am talking about the possibility that Tropical Depression 19, now upgraded to Richard, gets into the central/east Gulf of Mexico and interacts with one of the coming troughs rather than dissipating in Mexico or extreme south Texas.

The HWRF model, which was alone last night among the well-known models in showing Richard going to the northeast, has been joined now by the GFDL, ECMWF, and the majority of other models, as this Google Earth screencap shows.

The National Hurricane Center has made note of this trend as well and is expecting to move its forecast path to the east if the trends hold up. They tend to err on the side of consistency, avoiding what has been dubbed “the windshield wiper effect” when models make dramatic shifts in their forecasts. But it seems likely right now that they are already leaning in favor of an eastern track for this tropical system. The two storm-centric models, the HWRF and GFDL, turn Richard into a major hurricane and slam it into Florida, but the Google graphic indicates that there is actually quite a wide range of possible landfall locations. If the shear in the Gulf of Mexico drops off as forecast, there is really no reason why a major storm couldn’t happen (though I think there are some limits on just what is possible). Ida last year almost became a major in the Gulf in November during an El Nino autumn, after all.


(No, I do not believe Richard will actually approach Category 5.)

What about our cold-core cyclones, then? And that possible early freeze?

The GFS has (and this should surprise exactly no one) backed off its screwball idea of winter precipitation for Mississippi in the first week of November. As of 06Z’s run, it was not on board with the eastern path for Richard, which throws a major wrench into matters, but let’s look at the evolution of the trough before Richard might enter the picture. If he does get into the east-central Gulf, it’s going to be about a week from now before we can consider a landfall.

The first trough, the one expected to bring rain and thunderstorms to Mississippi on Sunday and Monday, is still on schedule. This model has increased the amount of rainfall that we are apparently going to get out of this, showing up to 2 inches in a small area and widespread totals over 1 inch. This is the trough that would pull Richard up, up, and away, sending it somewhere into the Gulf Coast and adding even more rain. (Remember, the GFS rainfall totals as of this run assume that Richard does not get into this part of the Gulf and is not picked up by this trough.) I’m having a hard time accepting a hurricane of major-plus intensity (let’s say Category 4) in the Gulf in a strong trough situation in late October, though stranger things have happened. It seems likely to me that if this trough gets it, Richard will begin to transition into post-tropical and lose some of its intensity. Cold-core cyclones do not get as strong as tropical cyclones, either in minimum pressure or in wind speeds. (There have been some non-tropical lows that went down to the 920 mb range, but these were at extremely northern latitudes. It’s much less unusual to see sub-980 mb cold lows close to the poles.)

When the energy of a tropical cyclone is entrained into a trough, the trough benefits from it. These types of systems have spawned infamous nor’easters, such as the storm created by the transitioning Hurricane Noel in 2007. The GFS already turns the trough into a 990 mb low and possible nor’easter, and as I said, that run doesn’t even take into account the possibility of tropical-origin vorticity being advected into the dominant low. The South could very easily be in for a major flooding rain event (it is unbelievable that I could realistically use the word “flood” when we are technically in a drought), but if we have the situation of a former major hurricane being pulled into a strong autumn trough, the Northeast could get a monster storm. AccuWeather.com is well-known for hyping weather events, but I think they may be onto something with their current news story about this possible storm. Let’s just say that, while nothing is definite, the potential definitely exists.

The GFS still predicts the first freeze to occur just before Halloween. This freeze would follow the second trough, which would dump even more rain on us. The freeze would be a dry freeze (in other words, a believable one) and the day following it, Halloween, would possibly not lift out of the 50s for highs. This has happened before; I distinctly recall a Halloween in the mid-90s (I am thinking 1993 or 1994) in which it did freeze overnight. After that, the cold air lifts out. As I said before, this model run has dropped the “early November Southern winter storm” foolishness of the third trough it shows.

I am going to be very mindful of the evolution of Richard and the strength of the trough that the NHC and models are increasingly convinced will get it. Things are getting interesting here, folks.

Is a Major Pattern Change On the Way?

After weeks upon weeks of dry weather, we have had two rain events in the past couple of weeks as fronts moved through. The rain has not made a noticeable difference in the drought situation, though it should be noted that we are not under fire weather watches every day anymore.

However, the models are starting to suggest a major pattern shift in the long range. It’s important not to focus on specific details this far out, because quite frankly, some of the details are almost certainly utter rubbish. The idea is that we are shifting into a precipitation-bearing trough pattern at last. The GFS 4-5 days out is showing a fairly robust system of about 1000 mb that is expected to bring rain. Jackson’s NWS office is already talking about this system and has given it a 40% chance of producing thunderstorms on Monday. This is potentially an interesting situation as far as severe weather is concerned, and it seems pretty likely that this particular rain event will play out in some form over the Deep South.

In runs from the past several weeks, the GFS has been indicating, for the most part, a return to dry conditions for as far as the model shows. More recently, it’s stopped doing this. After the Monday event, the GFS shows a potentially more extreme event developing over the 28th—and a freeze.


This image shows the storm event. The freeze would follow.

This is where you start to raise an eyebrow at the output and take it with a grain of salt. This would be early in the year for a first frost, but it has happened before. We’ll need to keep an eye on this and see if it stays in the runs and if other models start to pick up on it.

After the proto-event #2, the GFS then kind of goes off, showing a winter storm situation unfolding. It’s exactly what I mean when I say that the details are not what counts here, but the suggestions of cooler air and more precipitation. This is at the end of the run, which is notoriously poor in accuracy. The blue “0” line represents the freezing point.

As I said, the freeze aspect of this is almost certainly complete foolishness. A third rain event, however, is not out of the question at some point in the two-week range, and such a cyclone could easily pose a winter storm problem for areas along the East Coast, as a nor’easter has already done this year.

We are reaching a time of year when the gates start to close for tropical activity. In part, this is because the jet stream is reforming at a more southerly location. The high wind shear generated by the jet stream is a death knell for tropical activity, but it is encouraging of frontal and cold-core cyclonic development.

Tropical Depression 19
However, tropical storms will continue to form as long as they have the necessary conditions, and as I type this, I see that a new depression has been classified. TD 19 is located in the Caribbean and is forecast to intensify a bit and move over the Yucatan Peninsula. The circulation center of this depression is sheared and somewhat displaced from the main convection, but it seems that the convection and low-level center are moving closer together as the source of the shear moves out. The system is expected to get into the western Gulf. It probably won’t affect the central Gulf states, but it should be noted that the HWRF model explodes this into a major hurricane and sends it into the west coast of Florida. This is currently an outlier, but that is a good model, so it should be watched closely. The Gulf is cooling, but there is definitely enough heat in it to sustain a serious storm. Because of high shear, the Gulf currently cannot host a tropical storm, but the shear is supposed to largely disappear by the time TD 19 emerges from the Yucatan.

It will be interesting to see what type of weather system we get if TD 19 does move farther east and north than expected and the Gulf states get significantly cooler weather. The output of the GFS that shows those rain events does not take into account the possibility of tropical energy entrainment, because it favors a southern path for the tropical depression.

Keep an eye out; things could get interesting.

Return of the Heat, Paula, and a Decent Chance for Rain

The blog has been inactive recently because the weather in the Deep South has been profoundly, well, boring. After a brief cool spell brought on by a pair of cold fronts, we saw a return to the high 80s and further intensification of the drought. For Jackson, it was in fact the driest September on record and the maximum temperatures were among the highest (though the cool overnight lows resulted in a moderation of the overall daily averages). Incidentally, with the warmup came an increase in the number of red paper wasps that—apparently re-energized by the heat—wanted to use my house to overwinter, and one of them decided to light on an antique doorknob of virtually the same color as itself. If you have never been stung by this type of wasp before, believe me, you do not want to be.

Rain/Thunderstorm Event
However, at last there is a respectable possibility for some rain, and better yet, it is coupled with another cool-off. A low is moving in that is expected to bring rain on Tuesday. Jackson is even concerned about the threat of severe weather, and the SPC has put us under a Slight risk for Tuesday:

If cumulonimbi materialize, unplug your computers, modems, and printers. Trust me on this one.

Jackson’s forecast discussion speaks of air mass modification by the low pressure system. This is exactly what it sounds like; an air mass with very different characteristics moves in and alters the presiding air mass. A look at the water vapor map makes it very clear what is going on with this:

The curved form over Kansas is the upper level low. The leading edge of it is already approaching the area, and you can tell the difference between even the current humidity of the air and what is coming our way. Drier air has already been pushed out, as you may have noticed from the widespread cumulus cloud formation today.

Here is what Jackson has to say about the incoming system and the potential for severe weather:

THE AIR MASS WILL DESTABILIZE QUICKLY IN THE PATH OF THE DEEP UPPER
LOW AND MOISTURE WILL CONTINUE TO INCREASE AHEAD OF THE SYSTEM.
PRECIPITABLE WATER VALUES AHEAD OF THE SYSTEM ARE EXPECTED TO BE
AROUND 1.5 INCHES AND VERTICAL TOTALS ARE FORECAST IN THE UPPER 20S
FROM NE LA INTO WCNTRL AND NE MS BY 11/00Z. RADAR TRENDS ARE THAT
CONVECTION ASSOCIATED WITH THE OUTFLOW MOVING INTO EXTREME WRN ZONES
IS WEAKENING BUT FORCING REMAINS BETTER FARTHER UPSTREAM CLOSER TO
THE FRONT AND UPPER LOW. CONSIDERING THE EXTREME AIR MASS
MODIFICATION THAT OCCURRED JUST UPSTREAM EARLIER TODAY AND THE FACT
THAT IT WONT TAKE MUCH FOR STRONG CONVECTION TO DEVELOP IN AN AIR
MASS WITH THESE LAPSE RATES…WILL MAINTAIN MENTION OF STRONG
POSSIBLY SEVERE STORMS PER PREVIOUS FORECAST. LOW LEVEL MOISTURE
RETURN WILL BE HIGHEST OVER SWRN AREAS THIS EVENING AND…ALTHOUGH
LAPSE RATES WILL BE HIGHER OVER AREAS TO THEIR NORTH…SWRN ZONES
WILL SEE SUFFICIENTLY UNSTABLE CONDITIONS TO SUPPORT THE POTENTIAL
FOR STRONG…POSSIBLY SEVERE…CONVECTION ALSO. THE UPPER JET
REMAINS PARKED OVER CENTRAL/SRN ZONES THROUGH TUESDAY AND PERIODS OF
HEALTHY DIVERGENCE ALOFT WILL HELP SUPPORT CONVECTION THROUGH
TOMORROW.

VERTICAL TOTALS/LAPSE RATES WILL REMAIN HIGH THROUGH TUESDAY WHEN
THE UPPER LOW PIVOTS INTO THE AREA…AS WILL SYNOPTIC LIFT ACROSS
MUCH OF THE FORECAST AREA. ALTHOUGH LAPSE RATES WILL REMAIN HIGH IN
RESPONSE TO THE COOL UPPER LOW…AVAILABLE MOISTURE WILL BEGIN TO
DIMINISH FROM THE WEST EARLY TUESDAY. MODEL SOLUTIONS DISAGREE ON
THE EVOLUTION OF THE LOW LEVEL MOISTURE FIELD TOMORROW. THEY AGREE
THAT THE SURFACE BOUNDARY WILL BECOME MORE DIFFUSE AS IT MOVES
ACROSS THE FORECAST AREA BUT THE GFS REDUCES SURFACE DEWPOINTS
INTO THE MID 50S BY 13/00Z WHILE THE NAM/EURO MAINTAIN DEWPOINTS IN
THE LOW TO MID 60S. THE SREF LOWERS DEWPOINTS RELATIVELY QUICKLY BUT
HANGS ONTO PRECIP WELL INTO THE AFTERNOON IN THE WEST. FOLLOWED THE
HIGHER POPS FROM THE PREVIOUS FORECAST FOR TOMORROW OVER THE LATEST
MAV MOS IN CONSIDERATION OF THE SLOWER DRYING INDICATED BY SEVERAL
OF THE MODEL SOLUTIONS.

What all this means is that we have an interesting setup for mid-October unfolding and that if all the parameters come together, we could see some severe weather along with our rain. Probably not tornadic activity, but some significant thunderstorms could develop.

Tropical Storm Paula
Meanwhile, in the tropics, we’ve already seen Hurricane Otto come and go and are now on to Tropical Storm Paula. Otto was never a threat to anyone, but it was interesting to observe a classic subtropical-to-tropical transition take place. It also became a hurricane, which is uncommon with systems that begin as subtropical cyclones.

Paula has been a persistent disturbance for quite some time before finally getting its name (later than it should have, in my opinion) as a 60 mph system. It is not moving fast and is not expected to pick up much speed, but it will be moving over some warm water and should intensify into the season’s ninth hurricane. It is probably not going to be a problem for the central Gulf Coast, and is more likely to be yet another Florida event, but right now its forecast is uncertain because steering mechanisms are expected to essentially disappear in 3 days, leaving it drifting around in the Yucatan Channel as (probably) a hurricane. Most of the guidance then has it picked up and hauled northeast, and that is indeed the most climatologically likely track for systems of this kind at this time of year. However, storms in this general area in October are infamously hard to forecast; a particularly unfortunate example was Hurricane Mitch. Paula is likely to be an interesting system to watch and a troublesome one to forecast, and in that respect it is actually a welcome change of pace. Heretofore my synopsis of the 2010 hurricane season would be “a whole lot of normal.” Not normal numbers, of course; the season has obviously been above average in activity, but none of the systems except Hurricane Alex were particularly difficult to forecast or did anything unexpected. I like late season storms because they require a bit more of the human factor in forecasting.

Drought Conditions Develop

There isn’t much to say about the weather in the Deep South, unfortunately. A cold front is set to move through the region this weekend, dropping temperatures down about 10 degrees again and making the air outside seem truly fall-like. However, it is going to be a dry cold front. Nothing has changed as far as rain is concerned; there is none predicted as far as the forecasts are made. The Weather Service in Jackson has begun issuing fire weather products for the state, and the office out of Birmingham has issued red flag warnings. Burn bans are also in effect for large areas, though it’s highly inadvisable to burn even if you aren’t officially under a ban. It’s hard to believe that within a half a day’s drive, the Carolinas are getting drenched with the rainfall of a trough and the remains of Tropical Storm Nicole entrained into the cyclone, while we in the Deep South are now officially under drought conditions.

With the state as parched as this, it’s reaching a point where we will look anywhere for something that might send us some moisture—or at least, that has not yet been eliminated as a contender for doing that. Out in the tropics, there is a new system in the Central Atlantic, 97L, probably one of the last to develop in places like that this year. However, while the track and development of this system are still very much up in the air, the 12Z GFS doesn’t seem particularly interested in the disturbance. It does, in the two-week range, show a big, wet cold-core cyclone coming through the Southeast and finally ending the drought. I am going to keep an eye on this model and see if it retains this feature; the models are beginning to show some skill in long-range forecasts. And besides, hope springs eternal.