Area Forecast Discussion
Issued by NWS St. Louis, MO
Issued by NWS St. Louis, MO
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964 FXUS63 KLSX 172324 AFDLSX Area Forecast Discussion...Updated Aviation National Weather Service Saint Louis MO 624 PM CDT Tue Sep 17 2024 .KEY MESSAGES... - Dry conditions are expected for most areas through the end of the week. Rain chances increase this weekend, but remain low (20-40%), with confidence low (less than 10%) in rainfall capable of relieving drought conditions. - Confidence is high (90%) in above normal temperatures through the end of the week, with uncertainty in temperatures increasing this weekend and into next week. && .SHORT TERM... (Through Late Wednesday Night) Issued at 216 PM CDT Tue Sep 17 2024 Current water vapor imagery shows a cutoff low centered over the Ohio Valley beneath a ridge spanning the eastern CONUS as a trough digs equatorward over the western states. The CWA remains situated between these two features, with high pressure at the surface continuing to lead to calm and warm conditions. Temperatures in most locations are currently in the mid to upper 80s, with a majority of the area expected to top out in the upper 80s over the next hour or two. Tonight, the surface high will shift slightly eastward, returning predominantly southerly low-level flow to the CWA. This flow is expected to provide weak warm air advection and keep temperatures from getting as cool as last night, with overnight lows falling to around 60 degrees. There is a weak signal for fog across southwestern Illinois tonight, though dew points in this area are currently trending lower than forecasted lows, so it has been left out of the forecast for now. Another round of valley fog can`t be ruled out mainly in central and southeastern Missouri, but with temperatures forecast to be slightly warmer tonight than last night, this too has been left out of the forecast for now. For Wednesday, the upper-level pattern will be similar to today, though edge slightly eastward as a shortwave moves into the Northern Plains. With southerly low-level flow, mostly clear skies, and mid- level temperatures warming subtly, surface temperatures tomorrow afternoon will be around to just a couple of degrees warmer than today as highs top out around 90 degrees. Elmore && .LONG TERM... (Thursday through Next Tuesday) Issued at 216 PM CDT Tue Sep 17 2024 Starting Thursday into the weekend, guidance consensus is that an upper-level trough will be positioned over the western CONUS as a ridge edges into the Midwest. A shortwave within the trough will be moving out of the Northern Plains into Canada as a portion of the trough becomes cut off over the southwestern CONUS. As the ridge gradually builds in, mid-level temperatures will warm, correlating with a slight bump in high temperatures to end the week. As a result, ensembles are clustering around 90 degrees for highs Thursday and Friday. There is a low chance (20-40%) for rain across northeastern Missouri and west-central Illinois Friday morning as the low-level jet edges into the region, providing ample warm air advection and forcing for precipitation. However, as the jet veers into our CWA Friday morning, it will be weakening, with a downward trend in intensity and coverage of rainfall in our CWA expected. Temperatures and rain chances grow increasingly uncertain Saturday and Sunday, as both deterministic and ensemble guidance vary on the phasing of the western cutoff. The general consensus is that it will get picked up by southwesterlies over the northern half of the country, but how quickly this occurs and then how quickly it moves through the Midwest varies widely among guidance. Regardless of the exact phasing of the cutoff, moisture return over the Middle Mississippi Valley ahead of it is expected to be meager, leading to low rain chances (20-40%) Saturday and Sunday. Any rain that falls is expected to be light, as ensemble medians are around 0.1-0.2" - not nearly enough to have much effect on current drought conditions. As for temperatures, spread among ensemble members increases notably this weekend due to the differences in phasing of the cutoff among guidance, with the spread between the 25th and 75th percentile reaching 10 degrees by early next week. If temperatures remain on the warmer end of guidance, a continued stretch of warmth characterized by temperatures about 10 degrees above normal can be expected. On the cooler end, temperatures would be around seasonal normals. Confidence in either end of the spread is low, and the going forecast represents roughly the middle ground. Elmore && .AVIATION... (For the 00z TAFs through 00z Wednesday Evening) Issued at 624 PM CDT Tue Sep 17 2024 Largely dry and VFR flight conditions are expected through the TAF period with the exception of patchy river valley fog Wednesday morning. Fog should be more limited in coverage than this morning with the relatively highest chance at KSUS for a short period. Winds will remain light as well. Pfahler && .LSX WATCHES/WARNINGS/ADVISORIES... MO...None. IL...None. && $$ WFO LSX