Dec 302010
 

Friday, 31 December 2010: Update

Numerous tornadoes did indeed occur from central to east central Missouri into west central Illinois…
 

Storm reports from the SPC for December 31, 2010

Storm reports from the SPC for December 31, 2010


 

The forecast has verified pretty well, with a high concentration of tornado reports across central to east central Missouri and west central Illinois. Unfortunately, three more people have died in Missouri (central Missouri, near Rolla) as a result of one of these tornadoes. Several other tornadoes caused damage in the St. Louis area and over parts of west central Illinois.

Three fatalities in Missouri:
http://www.usatoday.com/weather/storms/tornadoes/2010-12-31-tornado-arkansas_N.htm

http://news.yahoo.com/s/ap/20101231/ap_on_re_us/us_tornadoes

As outlined yesterday, this setup was not ideal. Low level moisture was on the marginal side and so was instability (CAPE). There was also a relatively small window of opportunity for these storms to produce tornadoes before the system turned into a linear squall line. Yet, there are 20 tornado reports in Missouri and Illinois combined, (with 8 reports in southern Mississippi http://www.wlbt.com/Global/story.asp?S=13766768 ).

I can’t even imagine what kind of a huge outbreak this would have been if conditions had been just a tad better in terms of moisture and instability. Let’s be glad they were not. These storms were moving northeast between 55 and 60 mph, and warning lead times were pretty small.

- Jim

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Friday, 31 December 2010: 1:00 p.m. EST Update
 

Confirmed tornado kills 3 in northwestern Arkansas this morning.

http://news.yahoo.com/s/ap/20101231/ap_on_re_us/us_severe_weather_ark

At least one tornado damages numerous homes in the St. Louis, MO area between 11:00 a.m. and 12:00 p.m. EST (via SPC reports page)

There isn’t much to say, really. It’s just terrible when lives are lost from tornadoes. Hopefully residents in the St. Louis area found shelter before the tornado(s) over there moved through.

- Jim

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Friday, 31 December 2010: 8:45 a.m. EST Update

 

Tornado watch valid 7:30 a.m. CST December 31, 2010 from the SPC

Tornado watch valid 7:30 a.m. CST December 31, 2010 from the SPC


 

A tornado has already been reported in far northwestern Arkansas. A couple supercells are ongoing over northwestern Arkansas extending into southwestern Missouri. But more storms/supercells are expected to develop further north and east into central/eastern Missouri over the next few hours. Some of he storms along the developing squall line over western Missouri extending into central Missouri over the next couple hours may even become supercellular at times at times, posing a tornado threat, as the wind shear increases.

The ideas outlined yesterday look pretty good. But, the northern extent of the target area looks cut off now by a faster moving cold front and convective debris. Nevertheless, I still like the area near I-70 in central Missouri. Though I would extend that taret area further south and still stretch it into extreme west central Illinois over the next few hours into early this afternoon.

- Jim

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Thursday, 30 December 2010

 

With the Northeast blizzard still fresh on my mind (and many others I’m sure), it’s nice to see a severe weather setup with low topped supercells possible tomorrow afternoon (New Years Eve). The region I’m interested in stretches from northern Missouri and west central Illinois south into central Arkansas. But the two areas of focus IMO are 1)north central to northeastern Missouri and 2)central to eastern Arkansas.

Below you can see the GFS forecast surface map for midday Friday (Dec 31). Low pressure is located over northwestern Missouri with a dryline/pre-frontal trough mixture stretching southward from west central Missouri into central Arkansas. This will be the primary focus mechanism for thunderstorms beginning midday Friday throughout the afternoon, as the boundary rapidly propagates east into the evening hours.

 

12z Dec 30 GFS surface foreast valid 18z Friday, December 31

12z Dec 30 GFS surface foreast valid 18z Friday, December 31


 

The surface low pressure will be associated with a very fast and intense upper trough moving very quickly through the mid Mississippi Valley. Below you can see the 500 mb wind/height forecast from the GFS for midday tomorrow. Notice how 500 mb winds are generally out of the southwest/south-southwest from around 70 knots in Arkansas to 90 knots in Missouri! Immediately the 90 knots in Missouri catches my attention as they provide incredible 0-6 km speed shear, which will be helpful in generating severe thunderstorms, possibly supercells, if other factors are present.

 

12z Dec 30 GFS 500mb forecast valid 18z Friday, Dec 31

12z Dec 30 GFS 500mb forecast valid 18z Friday, Dec 31


 

The next image (below) is the 850 mb heights/winds from the GFS for midday Friday. Notice winds are a bit stronger (50 knots) and more backed across much of central to northern Missouri. Meanwhile winds are a bit more veered and not quite as strong (35-45 knots) across central Arkansas. The disparity is because the surface low is located further north across northwestern Missouri, and it is rapidly moving northeast with the upper trough. So now we know there is more 0-6 km speed shear across Missouri and it looks like the 850mb flow is more conducive for supercell development in Missouri — especially north central Missouri — vs. central to eastern Arkansas.

 

12z Dec 30 GFS 850mb forecast valid 18z Dec 31

12z Dec 30 GFS 850mb forecast valid 18z Dec 31


 

Finally, lets take a look at how much instability is forecast for tomorrow across both regions of interest. According to the GFS, mid 50s surface dewpoints should be in place by midday tomorrow as far north as northern Missouri, while lower 60s dewpoints are expected across much of central to eastern Arkansas. Immediately Arkansas does look more favorable from a moisture standpoint, and that does (at first glance) look like one of the main inhibiting factors for the north central Missouri region tomorrow. However, one good thing going for Missouri is 500 mb temperatures will be lower given the closer proximity to the upper trough, helping to boost instability despite the somewhat limited low level moisture.

Given those circumstances, take a look at the GFS CAPE forecast for midday tomorrow (below). Sure enough, there is a 750 j/kg CAPE maximum over north central/northeastern Missouri, likely due to a combination of cold air aloft and adequate surface dewpoints. In Arkansas, you have to head into the southern part of the state before you reach the 750 j/kg of CAPE. Now let me clarify. In typical spring setup you need a good 1500 j/kg to 2000 j/kg or more CAPE to support supercells. But this setup is unique in that more dynamics are in place and the wind shear is so extreme it is compensating for quite low CAPE values.

 

12z Dec 30 GFS CAPE valid 18z Dec 31

12z Dec 30 GFS CAPE valid 18z Dec 31


 

This is not a great setup, and factors will only be present for severe storms during a short window midday tomorrow into the early afternoon hours across north central Missouri extending quickly into extreme west central Illinois. I think if things come together just right, severe storms will develop around 1 pm near I-70 in north central Missouri along the dryline/pre-frontal trough. If enough instability is present (this will depend on how well early morning convective debris is able to erode and move east of the area) and the GFS is correct in forecasting 850 mb winds out of the south-southwest near 50 knots at 18z, then I think low topped supercells will develop and move rapidly north-northeast.

Given the high degree of 0-6 km speed shear and the combination of speed and directional shear in the lowest levels, some tornadoes will be possible in this region. Furthermore, surface temperatures may only reach the upper 50s to lower 60s. This means cloud bases will be pretty low, further enhancing the tornado potential. But this will all occur within a 3-4 hour window, maybe even less, and storms will be moving north-northeast at greater than 50 mph. In other words, good luck chasing them!

Further south across Arkansas the conditions are not that much different than in Missouri, but enough to make a difference. Given the further distance form the surface low, low level wind fields due appear to be more veered in general, which will hurt the tornado potential. The instability will also be a bit lower given the warmer temperatures aloft. Severe storms are still likely across central to eastern Arkansas, some of which will also be low-topped supercells. But If I had to choose a chase target, I’d pick north central Missouri into extreme western Illinois.

Again, this whole setup is mediocre. But for new years eve, it’s not too bad. Happy new year!

- Jim

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