May 1, 2018: Moderate Risk in the Central Plains

May 1, 2018: Moderate Risk in the Central Plains

Update: 9:15 PM Tuesday

Taking a look at today’s storm reports from SPC, it looks like we have had quite the severe weather day. As of this writing, there are 20 tornado reports (note that some of these reports are likely the same tornado, so the actual number of tornadoes is almost certainly less than this number), 15 damaging wind reports (including a wind gust to 80 MPH near Wymore, Neb.), and an astounding 116 large hail reports (the largest is a report of four-inch diameter hail in Susank, Kan.). As for this morning’s forecast, with the hypothetical target near Minneapolis, Kan., it appears we have two separate tornadoes, both within about 25 miles of Minneapolis. One tornado was just south of the city of Concordia in Cloud County (aptly named?), and a particularly large tornado near Tescott in Ottawa County.

Storm reports received by the National Weather Service as of 9:17 PM CDT.

Right now, the biggest game in town appears to be the supercell over Clay County, Kan. where Doppler radar recently measured peak rotational velocity of 62.6 knots (72.0 MPH). The dual-polarization products do not indicate a tornado debris signature, but storm chasers have confirmed a visual on a tornado with this storm.

0.5 degree tilt reflectivity (left) and velocity (right) of a confirmed tornadic thunderstorm in Clay County, Kansas at 9:08 PM CDT. This view of the Topeka, Kansas radar was generated using GR2Analyst.

Update: 3:30 PM Tuesday

We are now underway in near-west Kansas, as expected. The National Weather Service already has Severe Thunderstorm Warnings out on pretty much every cell, and we already have a report of 1.50-inch diameter hail in Ellis County, Kansas just west of Hays. The image below is from the new GOES-16 Global Lightning Mapper showing the lightning flashes in the rapidly developing thunderstorms. I’ll try to do some updates through the afternoon as I see interesting things happening.

GLM and visible satellite courtesy College of DuPage.

Original Post:

The first big severe weather day of the year in the Great Plains appears to be upon us. The Storm Prediction Center is forecasting a moderate risk for severe thunderstorms in central Kansas, with the primary hazard being large hail. Damaging winds and tornadoes are also possible.

Storm Prediction Center Day 1 Convective Outlook issued at 1644 UTC (11:44 AM CDT) on May 1, 2018.

Starting in the upper levels, the 500 mb analysis shows deep cyclonic flow across the western United States, with a trough axis roughly located from western Montana through the Great Basin and into Southern California. This trough has amplified significantly over the past few days, and is the impetus for surface cyclogenesis across the lee of the Rocky Mountains and into the High Plains.

500 mb objective analysis valid at 1200 UTC (7:00 AM CDT) May 1, 2018.

One of my favorite ways to get in tune with what is going on meteorologically, especially in a mesoscale weather event such as a severe weather episode, remains hand analysis. Sure there are charts out there for a quick look, but I am a proponent of hand analysis since it really lets you get deep into the data. My surface analysis I did for around 1400 UTC (9 AM CDT) revealed that upper 50s dewpoints extend as far north as the Iowa-Minnesota border, with 60s dewpoints well into central Kansas. Central Kansas lies in the warm sector ahead of a surface low over southwest Kansas. A warm front extends from this low northeastwards into eastern Nebraska, with a cold front extending back to the west into northern New Mexico.

Surface hand analysis performed around 1400 UTC (9 AM CDT) May 1, 2018.

As the day progresses, and the surface low deepens (i.e. the pressure continues to drop), temperatures and dewpoints should only climb. Combined with very steep lapse rates of around 8 C/km, instability should be quite high by late afternoon – on the order of 2,500 J/kg (around 1,000 J/kg is generally considered the minimum threshold for severe weather, with values over 4,000 J/kg occurring on some of the more prolific severe weather outbreaks).

My forecast process generally goes something like this: first, I do my chart analysis (both upper-level and surface). The surface analysis above shows pressure (black lines), temperature (red dashed lines), and dewpoint (green solid lines) as well as relevant surface features such as lows, high, fronts, and outflow boundaries (there is actually an outflow boundary near the Iowa-Missouri state line in this particular analysis). After the analysis phase, I head over the satellite and radar. Visible satellite imagery this morning showed some cloud cover over central Kansas, but this cloud cover has since dissipated. As a result, temperatures have already risen into the mid 70s ahead of the front. In fact, there are already some hints at cumulus development ahead of the front – a sign that the atmosphere is becoming increasingly unstable.

Visible satellite image over Central Kansas from around 12:50 PM CDT May 1, 2018. Image source: College of DuPage.

The last step of my forecast process is to look at the convection-allowing models (CAMs) and see if they fit my conceptual model. A look at the High-Resolution Rapid Refresh (HRRR) and the National Severe Storms Laboratory’s Weather Research and Forecasting Model (NSSL-WRF) both depicted similar scenarios, with convection initiating along the front over near-west Kansas (in the vicinity of Russell down towards Dodge City), then track it east into central Kansas. Looking at a forecast sounding ahead of this convection to see what kind of environment these storms are in, we can see that the environment should be quite favorable for both tornadoes and large hail.

HRRR forecast sounding near Minneapolis, KS valid 2300 UTC (6:00 PM CDT) May 1, 2018.

There were several things on this forecast sounding that jumped out at me, but in particular are the Effective Shear (EBWD) of 50 knots, effective storm relative helicity (SRH) of 216 m2/s2, and 700-500 mb lapse rates of 8.7 C/km. EBWD (an abbreviation for “effective bulk wind difference”) values above 40 knots are generally required for supercell thunderstorms, and given the modest capping and the good, but not overly strong upper-level forcing, we should be able to maintain discrete convection for some time. The bottom line is that supercell thunderstorms are likely, and we all know that supercells are associated with the most significant severe weather. Finally, the effective SRH of 216 is quite good. Values over 100 are generally supportive of tornadoes. In fact, a parameter known as significant tornado parameter (STP) can be used for an even quicker look at the tornado environment, with values over one generally associated with tornado events, and values over five generally associated with violent tornadoes (i.e. EF4 or EF5 tornadoes). The STP on this particular forecast sounding is 3.2, so we are well into “tornado territory”.

I am unable to storm chase, but will be watching from home in Fort Worth today. My hypothetical chase target if I could chase would probably be somewhere in the vicinity of Minneapolis, Kansas. Today is probably one of the better severe weather set ups I have seen in the Great Plains for some time, and there are probably going to be some tornadoes, and given the instability and really steep mid-level lapse rates, probably some very large hail too. And we are just getting started: SPC has a moderate risk tomorrow as well in northeast Kansas and northwest Missouri, with an enhanced risk across much of the Central Plains. And on Day 3 (Thursday), there is a slight risk across a large area from Central Texas all the way to the Mid-Mississippi Valley. May is here!

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