Category Archives: Mid-latitude cyclones

Mid-latitude cyclones, winter storms, baroclinic cyclones, extratropical cyclones, non-tropical cyclones

Forecasting, Mid-latitude cyclones, Severe

Dangerous Severe Weather Situation for Tennessee Valley

So far the severe weather outbreak of late April 2011 has brought devastation to many different areas in the southern Plains and Southeast. A tornado that was probably of a violent intensity hit a small town in Arkansas last night, reportedly removing pavement from the ground, and tornadoes have touched down from Texas through Arkansas and Louisiana into Mississippi, Tennessee, and Kentucky today. The severe weather is going to continue through Wednesday as the second of two low pressure centers intensifies over the mid-South.

A powerful mid-level jet has formed, with winds screaming at 70 knots or more:


Winds and pressure at 700 mb

This will provide powerful upward forcing. Additionally, strong thermal advection feeding into the developing low pressure center (which has become quite strong today, checking in at 989 mb as of this writing) will promote instability as well as additional vertical uplift:


Temperature and wind at 850 mb

For specific tornado threat regions, the NAM model has been fixing on an area centered over the Tennessee Valley. Yesterday the bull’s eye appeared to be right over the state of Tennessee, but today, it has shifted somewhat south.

The 18 UTC run of the NAM had CAPE over 4000 J/kg in central Mississippi at peak (mid-afternoon Wednesday), with a local maximum roughly over the Golden Triangle region and Tupelo, MS:

This run also had 3 km energy helicity values literally off the charts over the same area:

The 00 UTC run has CAPE peaking around the same value as before.

It has, however, moved, the helicity indices to extreme northwest Alabama:

Bottom line? I think that at this stage, such deviations are just model noise. Observations will be needed to determine exactly what area will have the highest risk of severe weather tomorrow. However, it is important to note that the model has been very consistent with an elevated tornado threat Wednesday afternoon in the Tennessee Valley area. With that in mind, I offer up my own subjective severe weather threat map for this event. I am not assigning probabilities to any of the color regions of this map; they are present only to indicate the areas that I think are in increasingly higher danger of seeing powerful severe weather, including long-tracked tornadoes.

Forecasting, Mid-latitude cyclones, Severe

Another Multi-Day Tornado Outbreak Expected

The year is off to a record start in tornadic activity, and more is on the way. Another three-day tornado outbreak is currently underway, with day 1 having brought close to 40 tornado reports and over 250 hail and wind reports.

The system is expected to bring more severe weather to the eastern part of the U.S. through Tuesday and Wednesday. At the time of this writing, a squall line has developed in the Mississippi River Delta that is expected to push east overnight, bringing strong winds, rain, lightning, and the risk of embedded tornadoes.

This scenario is more complicated than the setup for the last big outbreak, which had a single powerful system to generate the intense weather. A closed low located over Arkansas is responsible for the day 1 activity. This low formed today from a shortwave kink in an upper trough. This low is expected to be blocked by a strong high pressure system off the Atlantic coast, causing it to move north and eventually northeast to die out over the Great Lakes area. However, a second shortwave kink is expected to enter the mid-South and undergo cyclogenesis on Tuesday afternoon or evening.


Surface low at 36 hours

The combination of the current cyclone’s development, the upper-atmospheric jet that is causing all this shortwave activity (see below), and the next cyclone’s appearance on the scene will result in there being significant sources of uplift.


250 mb jet stream


700 mb upward vertical velocities

The highest values of instability in the event are currently prognosticated by the models to occur around midday tomorrow. The NAM and GFS generally agree on the areas of high CAPE, with each model forecasting at least 3000 J/kg (and it should be noted that models do not do well with CAPE and have a tendency to underforecast. Keep an eye on observations such as soundings).


Surface-based CAPE at 18 hours, NAM


Surface-based CAPE at 18 hours, GFS

For Wednesday’s event, the highest CAPE values are expected to be over Mississippi. The NAM and GFS agree on the maximum values but have the location and orientation of the high CAPE axis different.


Surface-based CAPE at 45 hours, NAM


Surface-based CAPE at 45 hours, GFS

The Energy Helicity Index (EHI) values for 18 hours and 45 hours (midday Tuesday and afternoon Wednesday) indicate the areas that the models forecast are most conducive for tornadic supercell development. Here is what the NAM indicates for the two times:


EHI at 18 hours, NAM


EHI at 45 hours, NAM

I am especially concerned about the middle part of Tennessee on Wednesday if that is accurate. Those EHI values are almost off the scale, and they coincide with an area of at least 3000 J/kg CAPE. That area has a history of tornadoes, and between the strong instability, powerful uplift, and helical pattern to the winds, I think it is quite likely that the Tennessee valley may be in the worst part of this outbreak.

The Storm Prediction Center has already put up a Moderate risk for Tuesday and Wednesday. It is thought highly likely that at least one, if not both, of these days will see that risk upgraded to High.

Forecasting, Mid-latitude cyclones, Severe

Tornado and Severe Weather Outbreak in OK/AR/MO Thursday

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I am expecting a tornado and severe weather outbreak to occur tomorrow in what I would call the heart of Tornado Alley.

However, that’s a worthless statement from any perspective—forecasting, meteorological, geographical—without a qualification of the term “Tornado Alley.” I have long thought that the true “heart” of Tornado Alley is not over Oklahoma City at all, but is instead farther east. In truth, where that heart is depends on what one chooses to look at. (For long-tracked violent tornadoes, the bull’s eye is, beyond a shadow of a doubt, over Mississippi.) Some meteorologists developed the idea of three distinct tornado alleys: Plains Alley, the traditional storm-chasing zone in the Plains states that is often referred to with the general appellative of “Tornado Alley”; Dixie Alley, a broad region encompassing much of the Southeast but not including parts of Texas that are climatologically and geologically more similar to Plains Alley; and Hoosier Alley, which is essentially the Midwest. However, if you really take a good look at it, I think you’ll find that the entire eastern region of the country between the two mountain ranges is Tornado Alley. There is no distinct border where you can say, “There are not nearly as many tornadoes in this small region as there are in either ‘Alley’ on each side of it,” and in the absence of such a thing, the only reasonable thing to do is to say it’s all one Alley. (Besides, “Hoosier Alley”? Really?) With the whole of the U.S. between the mountain ranges as Tornado Alley, it seems reasonable to expect the core somewhere east of Oklahoma City, and the meteorological data bear this fact out.

Now that that’s out of the way, let’s take a look at the science behind this and see why there will be severe weather, as well as where it should occur.

The weather-maker will be a low pressure center forming east of the Rocky Mountains. A low that is currently analyzed at 1009 mb (which is to say, not much of a low) is located over Kansas right now. This cyclone is forecast by the NAM to open up as it weakens and become more of a broad trough. This should be occurring right now, in fact. After that low dies out, a strong upper-level low (700 mb) is set to close off its circulation over the TX/OK panhandles on Thursday. Upper-level cyclogenesis should have occurred by 9:00 to 10:00 tomorrow morning (CDT). The usual vertical tilt of these kinds of cyclones will result in the surface low being over central Oklahoma at this time, and it should be at about 1000 mb by midday tomorrow.

Thermal advections will be strong, with the greatest warm-air advection (WAA) occurring in an area just west of Tulsa, OK to north TX to the AR/LA border. Temperature advection drops off in northeast Arkansas.

Surface dewpoint temperatures will be quite high, approaching 65-70 from Tulsa south along the OK/AR border in a narrow swath. A larger area of 60-65 F dewpoints will cover the area from Oklahoma City through a diagonal line bisecting Arkansas NW-SE.

However, despite all this moisture around the surface, a dry socket should be present at the critical 700 mb level in the atmosphere. This shows up on a synoptic model forecast map as a medium brownish-gray blotch of low relative humidity.

All this will lead to mixed-layer CAPE values approaching 2500 J/kg.

The instability will definitely be present, and between the thermal advection and the diffluence-based forcing from the low, lift should be present over the target area. Decent upward vertical velocities are forecast by the models to be present over the area of interest.

A cap of CIN will be in place early in the day, but as the surface heats up, the surface temperature should remove the temperature inversion, making it easier for moist parcels of air to rise.

The disappearance of the capping inversion also shows up in forecast soundings for the area bounded by Tulsa, Fort Smith, and Fayetteville. They indicate that a substantial warm surface layer will have developed by late afternoon, and with CAPE values as high as they are expected to be, this will set off convective growth.

Helicity values are also supposed to be quite high, with 0-3 km at 300-500 over the target area, at a time coinciding with the high CAPE values and the eroded cap, which will support supercellular development instead of linear. This increases the risk of tornadoes and large hail.

The Storm Prediction Center has already put up a Moderate risk for this general area, which I agree with 100%. I wouldn’t be overly surprised to see them go with a High by tomorrow morning, though this is a small enough area that they may not do it on the basis of its size. In any case, I am growing quite confident of a risk of supercells tomorrow for eastern Oklahoma, western Arkansas, and southwest Missouri. If I were to pick one city that I think has the highest chance of a significant impact, I’d go with Tulsa. However, a slight shift in any direction of the low’s movement would result in a shifting of the severe weather risk, and these events are never points, but areas. It is looking like a potentially dangerous day tomorrow for this region of the country, so people living there should make sure they keep aware of what is going on in the afternoon and be ready to act.

Mid-latitude cyclones

Another Winter Storm Coming Very Soon

Here in Mississippi, we are on the verge of getting yet another round of wintry weather. We missed the worst parts of the Groundhog Day Blizzard and even the severe weather was, for the most part, a whimper rather than a roar. However, our luck is about to run out. A system from the Gulf of Mexico is entering the area, and after vacillating for days, the forecasters (both human and computer) have reached a certain amount of consensus about what type of precipitation it will produce and for where.

The writing was actually on the wall last night, in my opinion, when both the 00Z NAM and the 00Z GFS came in with very similar QPF forecasts, both in amount, in type, and in geographic location. What they predicted was not good: up to 3/4 inch of freezing rain, based on the soundings, plus snow, sleet, and rain mixed in (what they call a “wintry mix”). I am not completely sold on 3/4 of an inch of freezing rain now, because it is a bit colder today than expected. This observation ties in with my general philosophy for using the computer models. I don’t pay that much attention to what the big flagship models say for short-term forecasts less than a day out, because they do not accommodate actual observations in their forecasts. Models exist that do, and if you are going to use any models for “nowcasting,” these should be the ones rather than models like the GFS and NAM. So whatever those two are saying today, I’m not paying attention to it. Instead, I am looking at what is happening, and what is happening is that there is more precipitation showing up on radar than the models “believed,” and the temperatures are colder. Not noticeably colder, mind, but just cold enough that it could, I hope, mitigate the amount of freezing rain by changing over to sleet and snow instead.

I think that areas north of I-20 are all going to get something that is frozen. Unlike the last snow event that we had in the South, the one of January 9-11, I do not expect that north Mississippi—the areas that are in the Memphis National Weather Service office’s region—will hit the jackpot. Instead, all indications are that QPF and therefore accumulated precipitation will drop off sharply north of Highway 82 (perhaps Highway 78). The belt of maximum precipitation with this system should be a bit farther south than the January storm.

There will be freezing rain for pretty much every area that gets anything frozen. Fortunately, I don’t think it’s going to be as bad as it looked last night, and it should be mixed in with sleet and snow. The atmosphere is hovering around freezing at several levels, and this will continue until it warms up Friday morning and the precipitation shifts to plain rain. The rain may help to melt whatever falls tonight and early Friday morning. Most of what happens on Friday will be a tapering off, especially since the observed QPF with this system is coming in stronger and a bit earlier than the computers predicted.

There is actually the possibility of yet another round of something in the middle of next week, after a brief “warm” spell this weekend (don’t hold your breath waiting for springlike conditions like what we had last weekend, however; the warmest high will be about 52 on Sunday). It’s too early to say just what this will be, especially since the safe thing to do with respect to such forecasts in the South is to be conservative and forecast cold rain at first, and then modify the forecast as the event approaches—if conditions warrant. Right now, a high of 38F on Wednesday does not warrant modifying the long-term forecast, as utterly miserable as rain would be in that. We’ll have to wait a bit longer to see how next week’s system shakes out, so let’s take this one winter storm at a time for now.

Mid-latitude cyclones, Severe

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.

Mid-latitude cyclones, Severe

Winter Storm Expectations

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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!

Forecasting, Mid-latitude cyclones

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!

Mid-latitude cyclones, Severe

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.

Forecasting, Mid-latitude cyclones, Tropical

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.

Forecasting, Mid-latitude cyclones, Tropical

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.