Wednesday Night: Partly cloudy. Low: 39
Thursday: Sky clearing through the morning. High: 60
Thursday Night: Moonlit sky. Low: 42
Friday: Cloudy with scattered light rain. High:65
Friday Night: Scattering clouds through the night. Low: 37
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Forecasters: McMullen, Noblitt, Cochran
Date Issued: 12/8/21 5:00 PM CST
For our forecasting period, both the NAM40 and GFS20 were compared to the current surface map produced by the NWS. Both models followed fairly well with the pressure contours as well as the locations of the two highs to our north and a low near the Gulf Coast of Texas. However, both models had difficulties with the pressure contours and pressure systems on the West Coast. In addition, both the SREF and GEFS plumes were consulted for the current surface temperatures, and both ran colder than the actual temperature at Sanborn. So, we ultimately decided to observe data from both models to see where they agreed and where some uncertainty lies. Satellite imagery was used to observe current regional cloud cover and both the NAM and GFS soundings were also utilized to forecast sky conditions.
Starting with the 250-mb plot of heights, winds, and divergence, a ridge is passing over at 21Z Wednesday. This ridge eventually gives way to strong zonal flow around 9Z Thursday, allowing a jet streak to spread to our north. Around 18Z Friday, a strong trough begins to move across the CONUS from the Rockies, forcing the downstream portion of the jet streak into the Mississippi Valley. This creates some divergence over Missouri Friday evening all the way through Saturday at 12Z, when the trough is directly over the Mid-Missouri.
As we look towards the 500-mb plot of heights and vorticity, some circulation can be anticipated as the previously mentioned ridge continues to the east. The accompanying zonal flow keeps the air calm, however some spotty circulation ahead of the incoming trough begins to push through Missouri. This culminates in strong circulation aloft as the trough from the Rockies enters the Middle Mississippi Valley around 10Z Saturday.
Following up with the 700-mb plot of heights, omega, and RH, no saturation is present over or around Missouri until 21Z Friday. During this time, a LPS and it's correlating trough moves moisture from the West Coast/Rockies across the northern Plains which saturates the northern half of the Mississippi Valley. This moisture will be crucial for cloud development and possible precipitation through the rest of the forecasting period.
Finally, the 850-mb plot of heights, winds, and temperature shows a westerly wind shift south as the noted ridge treks on. This southerly wind begins to rotate clockwise until the LPS and its trough move closer, forcing our winds to shift southwest. At around 6Z Saturday, the LPS and a cold front extending to the SW have passed far enough to our NE that the winds come clockwise to westerly before settling into northwesterly at the end of the week. WAA can be seen on the MSLP and 1000-mb to 500-mb thickness over our area until 18Z Thursday, helping to warm up our surface temperatures. The next batch of WAA, starting 9Z Friday, continues to warm us up thanks to the incoming LPS. However, with its overhead passage, some CAA could bring cooler temperatures for the rest of Friday.
Soundings indicated a slight chance of precipitation Friday. With KI in the 20s, nothing significant is expected. Since the QPF from both the GEFS and SREF plumes had quite a bit of uncertainty between their ensemble members and each other, a conservative trace amount of rain could occur Friday.
Future forecasters should be aware of the uncertainty in the models, as well as the cooler projections of both models. Pay close attention to the trough to our west Saturday as future models may provide better insight.