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.

April 13, 2011

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

Filed under: Forecasting,Mid-latitude cyclones,Severe — Erin @ 4:46 pm

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.

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