Severe Weather Threats for Central Plains Monday-Wednesday

After a start to a 2018 tornado season which has featured numerous tornadoes across the Deep South and even scattered tornadoes out West, but not a single tornado in Nebraska, Kansas or Oklahoma, it appears near certain the tornado drought for the Great Plains will come to an end early next week. Something which as been missing thus far…pattern favorable to severe for widespread severe thunderstorms across the Central and Southern Plains…will ramp up beginning Monday across the High Plains, shift eastward Tuesday with a peak higher-end risk for more widespread severe storms Wednesday. The jet stream, the river of air separating the cold Arctic from the warmer mid-latitudes will send a major trough of low pressure over the Western US, temporarily cooling that region down, warming up the Plains, bringing in greater moisture from the Gulf of Mexico and setting up the ingredients for multiple days of severe storms.

 

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European Model forecast depiction of trough of low pressure in the upper-atmosphere over the Western US (forecast for 7 am CDT Tuesday May 1st). This system will contribute to severe weather for the Great Plains Monday-Wednesday.

A brief review since it’s been forever since the Plains have had severe weather and there might finally be something in my neck of the woods. Severe weather is defined by the phenomena. In the US, the criteria, which weather warnings revolve around are 1) large hail of 1 inch or larger, 2) damaging wind gusts of 50 knots/58 mph or higher or 3) a tornado. Severe convection (thunderstorms) needs three major ingredients to maximize their potential. 1) Instability, 2) Moisture, 3) Wind Shear. Instability is positive buoyancy (tendency to rise). This is aided not only by heat, but also by moisture as moist air is less dense than dry air at the same temperature. Wind shear is the change in speed and direction of the wind with height. Winds which turn and increase in speed rapidly with height can promote storm rotation, allow them to form isolated cellular structures called supercells. These can be long-lived, self-maintained and produce the most intense severe weather.

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Schematic of a classic supercell thunderstorm.

Of the three days I’m most concerned about for severe weather this week, Wednesday appears to be the most serious for the Central/Southern Plains for significant severe weather. But let’s take a quick look at all three days.

Monday, April 30th-

day3otlk_0730

The National Weather Service Storm Prediction Center has a Slight Risk of severe weather (2/5 on the scale) for much of the high plains from Texas through Kansas and then, extending farther eastward into Central/NE Nebraska into SE South Dakota. This covers a 15% chance of severe thunderstorms within 25 miles of a point. A more “Marginal Risk” exists surrounding it. This would be for the afternoon and evening hours as a weak disturbance moves out of the Rockies, increasing wind shear and temperature-based instability (upper-atmosphere cooling relative to warming near the surface…warm air rises into colder air) modestly for isolated severe weather. Large hail and damaging winds are the primary hazards, but moisture will be limited, keeping the event from being widespread.

Tuesday, May 1st-

day4prob (1)

Beyond Day 3, there are now categorical outlooks, only probabilities. A 15% chance of severe thunderstorms within 25 miles of a point exists over Eastern Nebraska, Western Iowa, much of northern and Central Kansas into Western Oklahoma. This will likely be a bit more vigorous event from Monday, with the Tuesday disturbance being stronger with better shear profiles, more low level moisture available, and the combination of abnormally warm temperatures and higher moisture will mean higher atmospheric instability for tall, intense thunderstorms with strong updrafts. The storms will likely begin as supercells across Nebraska and Kansas before merging in the evening into an organized structure known as a “mesoscale convective system”. Basically a larger scale complex which can bring locally heavy rain and extensive damaging wind gusts. The initial storms will form along a cold front and threaten damaging winds, large hail and an isolated tornado.

Wednesday, May 2nd-

day5prob (1)

Wednesday is currently the most serious day for severe weather, but some uncertainty still exists. A 30% chance of severe thunderstorms within 25 miles of a point exists from extreme SE Nebraska, across much of Kansas, into western and central Oklahoma. A greater 15% area extends beyond  that, including my area of Lincoln, NE. Wednesday, the main upper-level trough of low pressure over the West (seen in the above map) begins to shift eastward and a surface mid-latitude cyclone sets up over the central and southern  Plains. A dryline (boundary separating warm, moist air from the Gulf of Mexico to the east from dry desert air from the Southwest US) will be located north-south somewhere over central KS/OK with a warm front either over Southeast Nebraska or Northeast KS (this is in question). The ingredients overall suggest robust thunderstorms forming along the dryline and near the area of low pressure (at the the intersection of the dryline and warm front) either in the afternoon or early evening Wednesday which vigorous supercells capable of producing large hail, some significant, damaging winds and multiple tornadoes. A possibility exists for a few of the tornadoes to be strong (EF2+; see more about the Enhanced Fujita Scale) and because of the persistent upper-level dynamics and buoyancy, storms could last after dark, posing nocturnal hazards. Later, storms will eventually merge producing greater high wind and heavy rain threats. Isolated flash flooding could be an issue Wednesday night from any heavy rain events.

For me personally, the the greatest threat for severe weather Wednesday seems to be to my south, but given the lead time, I’m watching to see how the position of the warm front ends up. If it migrates northward in the forecast and my areas is more solidly in the “warm sector”, then we will be just under as much of a hazard as the current 30% area is now. However, I note from forecast experience that warm fronts in severe storm events are notoriously challenging to forecast for as even the day of the event as they can have difficulty moving as far northward as expected because of the cold air they must erode out ahead of them. Much can also depend on the storms the previous day and how they effect the overall regional environment (temperature profiles, areas of instability, position of fronts, etc). But Monday-Wednesday all have potential to be hazardous days with Wednesday being a more potentially significant tornado day after months of silence.

9-km ECMWF USA Surface undefined undefined 108
European Model forecast depiction of Precipitable Water at 7 pm CDT Wednesday (how much atmospheric moisture is available for precipitation in an instantaneous moment). This does not say how much will fall, but just how moisture laden the atmospheric column is. The plume of abnormally high PW will mean the potential for storms with locally heavy rainfall.
9-km ECMWF USA Surface Nebraska 2-m Dew Point 114
European Model forecast depiction of dew point temperature at 1 am CDT Thursday May 3rd. Dew point is a measure of atmospheric moisture. The air behind the warm front and ahead of the dry line is warm and moist (with dew points above 60 F at this time), while the air ahead of the warm front is cool, with less moisture. The air behind the dry line is extremely dry and warm (dew point temperatures in the teens and 20s F).
9-km ECMWF Global Wave United States Simulated WV Satellite 108
European Model forecast simulated upper-atmospheric water vapor imagery for 7 pm CDT Wednesday. The computer model simulates what the weather satellite water vapor channel may see Wednesday night as far as cloud structures. Water vapor imagery is a special type of infrared imagery where water vapor concentration in the upper atmosphere can be detected based on its “brightness temperature” (upper atmosphere is moist, it appears bright, upper atmosphere is dry, it appears dark, meaning water vapor is warm, located at lower level of the atmosphere). Here, the water vapor imagery is enhanced with colors to better interpret the temperature of the condensed water (clouds). We can see intense thunderstorm formation over Kansas at this time (likely supercells) with some further development into Oklahoma as well. This is NOT LIKELY how it will look exactly Wednesday evening, only a general idea of the storm structures based on the larger-scale flow pattern and expected ingredients for storm formation.

So stay tuned early next week. The weather will definitely be news yet again this Spring! Stay safe and be ready this week in these regions!

–Meteorologist Nick Humphrey

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Meteorological Analysis of January 22, 2017 Great Plains/Midwest Blizzard As It Happened

Here is a meteorological analysis I did on Monday of the blizzard which raked much of the Central Plains and upper Midwest from Nebraska and Kansas to Minnesota and Wisconsin with heavy snow and high winds. It was recorded around 11:30 am CST Monday. Those who follow me on my Facebook page (also see my feed on the sidebar) were able to see it right after it was uploaded, but I’m posting it here for those interested in hearing me discuss the event as it happened. Peak snow totals up to a foot and a half resulted in parts of Nebraska, Iowa and Minnesota. Lincoln and surrounding parts of the town only saw 2-3 inches but the totals increased dramatically not far west and north. We got off easy compared to the one foot and greater totals in northeast Nebraska. Winds gusts throughout the region peaked 45-60 mph. You can hear the noise of the high wind through my door in the video.

By the way, my son Bruce makes a guest appearance as he tries to turn off my computer while recording. Haha.