Wednesday, March 18, 2020

Wednesday Night - Cloudy. Possible showers and Thunderstorms p.m. Low: 54-58

Thursday - Mostly cloudy. Thunderstorms possible. High: 72-76

Thursday Night - Clearing skies. Low: 34-38

Friday- Mostly cloudy. High: 40-44

 Saturday - Mostly sunny High: 42-46

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Clouds will stick around overnight, with showers and thunderstorms possible towards daybreak. For Thursday morning, mid-Missouri will see its greatest chance of widespread rain in the early morning hours, primarily between 7 am and 10 am. These showers and storms will be caused by an approaching warm front, which will usher in lots of moisture, lift, and instability. There is the potential for strong storms Thursday morning, though the threat will be primarily confined to heavy rain, small hail, and gusty winds. During the day on Thursday, strong southerly winds behind the warm front will drive temperatures upward. The second round of thunderstorms is possible Thursday evening, with large hail, damaging winds, and isolated tornadoes possible with any storms that do form. Early Friday morning, a low-pressure system and its associated cold front will push its way into central Missouri. Temperatures will not be as warm for Friday and Saturday, but clouds will start to move out of the area Friday night, leading to sunny skies on Saturday. The main concern there is temperatures, with readings 5-10 degrees below freezing likely early Saturday morning. 

Forecasters: Vanderpool,Pauley,Munley,Ritter
Issued:  5:00pm: March 16, 2020
Technical Discussion (The nerdy stuff we are discussing in class)

Model diagnostics from the WPC recommends the 12z GFS for deterministic guidance in the short-range. Therefore, we have gone with that for the majority of our guidance. However, the HRRR was also used for the prognosis of the early-morning wave of convection anticipated on Thursday. 09z SREF and 12z GEFS were also used to provide temperature ranges, as well as to provide an idea as to the confidence of our forecast. This was especially pertinent when looking at CAPE and shear for tomorrow’s dynamic setup. 

Currently, a rather active pattern exists across the CONUS. Working from the top down, the jet stream exists across an extremely broad area of the nation, with a large positively-tilted trough noted in the west at 250-mb (axis roughly from western Montana to central California) and a large ridge at that same level parked over the southeastern United States. At 500-mb, two closed geopotential height contours were noted directly beneath the 250-mb trough, with the vorticity maximum associated with the southernmost closed contour over far western Arizona. Extremely broad moisture return at 700-mb and 850-mb was occurring out ahead of the approaching trough, which will help to set the stage for widespread showers and storms beginning shortly before daybreak tomorrow. 

Tomorrow, things will start off interesting. Warm, humid air surging northward from the Gulf of Mexico in the lower levels of the atmosphere will drive a warm front into central Missouri. With short-range hi-res models and SREF plumes showing the potential for over 1000 J/kg of elevated CAPE, it looks like elevated thunderstorms will be very likely in mid-Missouri as this warm front provides lift and moisture convergence. The best time for these storms to occur looks to be between 11z and 15z. Shear profiles don’t look overly impressive for that round, though elevated CAPE and lots of moisture influx could provide for heavy rain, marginally-severe hail, and brief gusty wind threat. 

Following tomorrow’s morning round of thunderstorms, the warm front will push northward into Iowa and we will lose forcing and moisture convergence. This will place mid-Missouri firmly in the warm sector of a rapidly-approaching surface low. 12z GFS and 09z SREF runs both agree that 0-6 km shear behind this warm front will be on the order of 60 to 75 knots. Low-level helicity values also look very impressive, on the order of 150 - 300 m^2 / s^2. Overall, this is shaping up to be a very dynamic storm system. The main limiting factors for severe storms will be A) the lack of a focused forcing mechanism when CAPE and shear are maximized, B) lack of sunshine, which would cut down on instability, and C) the fact that shear will be largely unidirectional. Thus, IF convection fires again tomorrow afternoon and evening in mid-Missouri, the main threats will be gusty winds, large hail, and a brief tornado. Most short-range convection-allowing models seem to indicate that the favored region for convective initiation will be in far northeastern KS, northwestern MO, and southern IA, where forcing will be strongest. Thus, evening storms in mid-Missouri will probably be fairly scattered in nature (as they move in from the west) and most likely to occur between ~5 pm and ~10 pm. Storm motion will be very quick, which should cut down on the flash-flooding threat for the afternoon and evening hours. Overall, severe weather is possible, though storm coverage for the evening will likely be quite limited. Again, this will be tied into the overall amount of sunshine we receive, as well as if storms that form to our west and north can make it as far east as Columbia. 

Thursday night and early Friday morning, a powerful cold front will swing through mid-Missouri. The most likely timing for this frontal passage will be between 09z and 12z Friday morning. A much colder air mass will be ushered in behind this front, with temperatures Friday likely to reach only the lower 40s. Residual cloud cover will serve to keep us even chillier. This cold surge will reach its peak early Saturday morning when lows will likely drop 5-10 degrees below freezing. This may damage any sensitive vegetation currently outside. 

Saturday, the overall flow will return to a more zonal pattern. The strong low pressure set to affect the Midwest Thursday evening will have moved all the way to New York, leaving central Missouri in a ridge-like pattern as the center of the high-pressure system behind the cold front builds into the area. Winds will continue to flow in from the north, keeping us in cooler temperatures, though the air mass behind the front will likely modify a bit due to plentiful sunshine. GFS model guidance indicates that the atmosphere overhead will remain relatively dry, with no precipitation and fewer clouds. 

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