Current Conditions at Sanborn Field
Wednesday Night - Clear skies. Low: 46-50
Thursday - Mostly sunny. Breezy. High: 68-72
Thursday Night - Becoming cloudy. Low: 52-56
Friday - Cloudy skies. Light rain beginning before noon. High: 60-64
Saturday - Cloudy. Rain continuing throughout the day. High: 46-50
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Discussion:
Mid-Missouri's warmup continues into Thursday. Strong winds out of the south will make for windy conditions and help the temperatures reach almost 20 degrees above average for mid-November. Clouds return Thursday night ahead of a frontal system sagging into the region from the northwest. Light showers and drizzle is possible by noon on Friday, but the heavier and more steady rain will hold off until Saturday and continue all day. This front will be slow to move over the Midwest on Friday and keeping skies cloudy into Saturday. Once the front passes Friday afternoon, northeasterly winds will steadily bring town temperatures to near average by Saturday.
Forecaster: Travis
Issued: 5:30 PM CST 18 November 2020
Technical Discussion (The nerdy stuff we are discussing in class)
The main issue being looked at for this forecast period is the strangely shaped front that will pass over Columbia Friday afternoon, bringing with it a prolonged period of clouds and rain. The GFS was primarily used for this forecast with consultation of ensemble guidance (GEFS and SREF plumes) for temperatures, rain timing, and QPF.
Upper-level analysis shows the central CONUS underneath a ridge and wedged between a mature longwave exiting to the northeast of Missouri and an amplifying wave moving over the western CONUS. The upper-level ridging and zonal flow regime over the Midwest has been responsible for the crystal clear skies of Tuesday and Wednesday.
As the wave to the west approaches, the GFS suggests Missouri will see one more day of clear skies before the flow switches meridional out ahead of the trough. GFS prognostic plots of 250-hPa winds show the polar jet stream stays well to the north for the entirety of the forecast period (keeping the coldest air locked up behind it). By Friday evening and into Saturday, Missouri sits under the inflection point of the trough-ridge pattern across the CONUS. A polar jet streak to the northeast and a subtropical jet streak to the southwest will result increased divergence propagating eastward and placing itself over Missouri by Saturday afternoon. This gives rise to a question as old as time: is the left exit region of the subtropical jet streak coupling with the right entrance region of the polar jet streak? Well, if I have to ask myself the question, the answer is likely yes!
This pattern reflects itself well at 500 hPa. Quiet, laminar flow remains the story over the central CONUS until the above mentioned upper-level trough moves off of the Rockies. GFS maps of 500-hPa height and absolute vorticity depict increased circulation associated with the trough co-located in time with the coupled divergence aloft. This will provide ample dynamic support for a rain event this weekend should lower level moisture exist. A quick look at the GFS' 700-hPa maps of height and RH suggest that more than enough widespread moisture will be present over the Mid-Mississippi River Valley. This comes as no surprise after looking at GFS 850-hPa guidance. Prognostics of 850-hPa height and winds indicate a strong >50kt LLJ will be chugging across the Midwest, originating from the moisture-rich Gulf of Mexico. This persistent LLJ will also be responsible for strong WAA bringing Thursday high temperatures near 20 degrees above average.
This comes as two 850-hPa highs parks themselves within fairly close proximity to Missouri. The first sits over Nebraska, resulting in NE winds behind a frontal boundary that will set up by Friday afternoon. The second is really just a far westward extension of the Bermuda high and is responsible for the SW dominant low-level flow to the south of the frontal boundary. Looking at GFS plots of MSLP, the surface boundary will be slow to move to the southeast. The GFS hints at FROPA occurring before midnight on Friday night. Breezy surface winds (>20kts) will give another windy day Thursday, dying down post frontal passage. After which temperatures will be on a steady decline into Saturday. That being said, winds at 850 hPa will remain out of the south for the remainder of the forecast period will keep advecting in enough moisture to support a continuous rain event that persists beyond the scope of this forecast.
Model soundings reveal more to the story that plan-view maps overlooked. Saturation ahead of the approaching system begins in the lower levels of the atmosphere. Skies will cloud up before sunrise on Friday as full saturation below 700 hPa occurs by 18z on Friday. Some UVM collocated with the low-level saturation points to early onset of light showers Friday afternoon before the main rain event ensues on Saturday. However, little to no upper-level dynamic support, and moisture of only 150 hPa deep, will likely only result in isolated to scattered sprinkles after noon on Friday. As the day turns into night, saturation will continue from the ground up and the column will remain fully saturated until the end of the forecast period. GFS soundings show computed K-indices getting into the upper 20s and lower 30s and PWATs continuously staying above 1 inch by Saturday--another hint at a measurable amount of precip. 3-hourly QPF plumes from both the SREF and GEFS hint at the aforementioned possibility of light showers on Friday, but both models hold off the heaviest, most steady rain until Saturday afternoon. Total amounts differ between the two ensembles. The SREF mean sits at about 0.5 inches by Sunday at 06z while the GEFS mean at about 0.80 inches of rain.
Future shifts will need to watch the what this system does into Sunday as Skew-Ts by the end of this forecast period (Sunday 06z) are still fully saturated. Totals will likely increase further on Sunday. Needless to say, Missouri will likely receive a nice rain event that will help keep us out of drought going into late November.
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