Current Conditions at Sanborn Field
Friday Night - Persistent rain after sunset. Low: 46-50
Saturday- Clouds and rain. High: 50-54.
Saturday Night - Gradually clearing skies by sunrise. Low: 40-44.
Sunday - Mostly sunny. High: 64-68.
Monday - Cloudy with a slight chance of rain. High: 58-62.
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Discussion:
The weekend will get off to a soggy start as rain moves into central Missouri tonight after a glorious sunny Friday. This precipitation is associated with a frontal system pushing through the Midwest this evening and Saturday morning. Rain will persist through much of the day tomorrow though thunderstorm activity is not anticipated at this time for either Friday night or Saturday. Any precipitation that falls after mid-morning Saturday will be steady as the wraparound moisture associated with the system pushes through Missouri. Rainfall accumulations from this system look to fall between 1.5 and 2 inches by Sunday morning. Saturday's high temperatures will be rain cooled back into the mid to upper 50's. Sunday will be a rebound back to fair weather and slightly warmer before a shortwave moves into the region Monday returning cloudy skies and a chance for rain to begin the next work week.
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Forecaster: Bongard, Travis
Issued: 5:00 PM CST 09 April 2021
Technical Discussion (The nerdy stuff we are discussing in class):
A textbook, active, springlike pattern is in place over North America. This will keep our weather volatile for the remainder of the forecast period. Though there is good consensus between the GFS and NAM in regard to the placement of the low in the Southern Plains and associated moisture, the NAM’s typical bullish take on warm temperatures has pushed towards primarily GFS guidance. The main focus for this period is yet another burst of upper-level energy that will bring rain into this weekend.
Northern hemispheric plots of the 500-hPa large scale flow indicate a very active period with 5 (arguably 6) long wave troughs propagating along the Northern Hemisphere polar jet stream. According to the GFS, one of these longwave troughs currently sits over the Northern Plains and will remain so for the remainder of the forecast period. This longwave has been and will continue to ensure mid-Missouri weather changes frequently as shortwave disturbances will continue to propagate down from the Northwest out of Central Canada, reinforcing the longwave as cyclogenesis continues. The GFS gives no indication that hemispheric flow will de-amplify for the remainder of this period.
Looking at synoptic scale plots, GFS 250-hPa shows explosive upper-level divergence centered on the Lower Mississippi Valley region moving into southern- and mid-Missouri Friday night and into Saturday morning. Enhanced divergence is associated with the left exit region of a developing jet streak ¼ wavelength downstream of the shortwave trough mentioned above. Enhanced upper-level circulation at 500-hPa to the Southwest of Missouri ahead of the negatively tilted shortwave trough will increase cyclonic vorticity advection into the region. All of these factors will favor cyclogenesis. As the wave passes, it will once again put mid-Missouri under an upper-level shortwave ridge by late Sunday night into early Monday morning.
Moisture will not be in a deficit either as GFS plots of 700-hPa RH show saturated air advecting into the region from the southwest, following the mid-level atmospheric flow. Plentiful moisture is also indicated by model soundings as PWATs max out at just over 1” at 06z Saturday. The warm front at both 850-hPa and MSL struggles to reach mid-Missouri as the center of the 850 low itself will track just to the south. If this was January with near freezing temperatures, the track according to the Brown and Younkin method would favor a major snowstorm, but thankfully we are much too warm for that. Even so, as the low passes, post-frontal rain will stick around as mid-Missouri will remain under the precip shield until Sunday around 03z. To aid in precip development is the placement of the two surface highs to Missouri’s NW and SE and the two surface lows to the SW and NE. A brief look at GFS the MSLP map shows Missouri situated in the saddle-point between these systems as moisture moves into the area. Frontogenetic forcing with the saddle-point will yield more than enough UVM to get efficient precipitation going. Again, if this was a winter-time event, this would be looking very nice.
As for specific timing of precip, GFS soundings indicate a gradually saturating column throughout the day. As evening approaches, the ‘zipper’ will close and the ceiling will descend until around 00z Saturday when the GFS favors full saturation of the column. Soundings remain saturated in the lower-levels until 00z Sunday. In between that, the GFS pushes the front through the area by 12z Saturday (a whole 12 hours before precip is expected to taper off!!). Ensemble plumes favor rainfall accumulations to reach 1.5 inches when all is said and done on Saturday night. With the 2 inches of rain that Sanborn Field recorded on Wednesday and widespread >1 inches elsewhere around mid-Missouri increases the potential for flooding issues. What this system lacks, however is any semblance of instability. GFS soundings never indicate more than 50 J/Kg of MUCAPE. As such, not expecting any thunderstorms.
As mentioned above, once this system clears out Saturday night, a shortwave ridge will move in, but seeing as how the polar jet stream does not de-amplify, Monday morning’s shift should look closely at another period of unsettled weather for next week.
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