May 6, 2011

Running from an EF-5: Part One

Filed under: Severe — Erin @ 4:24 pm

Note: This is something a little bit different from the usual fare for this blog. Nothing particularly notable is occurring in the South except for the river flooding, and at this point that is a matter of concern for the engineers and hydrologists more than meteorologists. (I could rant about the Corps of Engineers, but that would be better suited for my other blog.) Since this is the case, and since I have felt that I needed to write about my experience on the tornado outbreak of April 27, I’m going to do that. This will be a three-part piece about my experience involving the EF-5 tornado that went through Neshoba, Winston, Kemper, and Noxubee Counties that day. The first part does not even occur on the 27th; it is from a week earlier. I’ve come to the conclusion, though, that it is part of the same story and an important factor in why I chose what I did that dark day.

Wednesday, April 20, 2011. I sit parked on the side of Highway 45, somewhat north of Crawford, in a line of other vehicles whose drivers have made the same decision.  We are all being pelted by hail that I estimate is up to an inch in diameter, mixed in with some rain.  No one dares to drive any farther.  This is, I believe, the third time that hail in this storm has forced me to pull off the road in my drive south to Macon from Starkville that afternoon.  I’ve almost lost count, and that’s not even counting the other storm that produced some hail before I even left Starkville.

I may be in the minority these days in having a cell phone that does not have Internet capability.  Ordinarily that is not a problem for me, but right at this particular moment, I find myself really wishing that I had a radar picture in front of me.  I call my sister and ask her to pull up a radar image on the Internet.  I have some urgent questions for her.

I obviously don’t want the car to be damaged by the hail, but what is currently happening to me is not my primary fear.  See, I have been at Mississippi State University as a graduate student in meteorology, I am well aware that the storm I am under is a supercell thunderstorm, and most likely, I am in what is called the “core” of the storm—the area of the heaviest precipitation.  Supercell thunderstorms are the ones that are most likely to produce tornadoes, and if there is a tornado, the core of the storm is located to the northeast of it.  And the storm is moving northeast.

In this particular situation, I could easily be in far more danger after the hail ends, and that is what I want my sister to tell me about.  I give her my approximate location.  “I think you’re in the hook,” she says uneasily.  (A hook echo on radar is an indication of strong rotation and possibly a tornado.)  My nerves tense at this, but if she can see a hook, it means that there is a slot of low to no precipitation, which I am most definitely not in.  “No, I’m not in the hook,” I say.  “I’m in the core.  I’m getting hammered.  But you are saying there is a hook with this storm?  Is there a tornado warning?”  It turns out that there is, and it is radar- rather than sighting-based.  That’s no surprise, and it certainly doesn’t mean that there isn’t a tornado.  The perils of high-precipitation supercells strike again.  I ask some more questions.  It seems that I am on the north edge of the core, and that based on the motion of the storm, I would be best off staying put until the whole thing is east of me.  The hook should pass south of me if I do that.  I just want to get out of this.  I’m not opposed at all to storm chasing, but it’s really not something I’m inclined to do without another person in the car or live radar available.

Once I am on the road again, I get a glimpse of the wall cloud off to my east with the aid of the nearly constant cloud-top positive lightning flashes that the storm would put out as it intensified.  I recall from my thesis reading that intensifying supercells often do produce a great deal of positive lightning.  This particular supercell would go on to produce larger hail and threaten Columbus.  I take several hours to come down from the adrenaline rush.  One week later, I would be very glad I had this experience.

Part Two of this will be about what happened to me personally on April 27. Part Three will be the aftermath of that event.

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.

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