April 26, 2011

Dangerous Severe Weather Situation for Tennessee Valley

Filed under: Forecasting,Mid-latitude cyclones,Severe — Erin @ 9:57 pm

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.

Another Multi-Day Tornado Outbreak Expected

Filed under: Forecasting,Mid-latitude cyclones,Severe — Erin @ 12:56 am

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.

January 30, 2011

Epic Storm to Bring Severe Weather to the South

Filed under: Mid-latitude cyclones,Severe — Erin @ 5:45 pm

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.

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