Tag Archives: thunderstorms

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, 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.

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!

Forecasting, Mid-latitude cyclones, Severe, Tropical

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.