937 FXUS63 KLSX 082009 AFDLSX Area Forecast Discussion National Weather Service Saint Louis MO 309 PM CDT Tue Sep 8 2020 .SHORT TERM... (Through Late Wednesday Afternoon) Issued at 309 PM CDT Tue Sep 8 2020 Primary concern for the rest of this afternoon into the evening is convective trends. Scattered storms have already formed and will be ongoing for the remainder of the afternoon into the evening. A few of the storms will likely be strong to severe as a decent amount of MLCAPE has built up this afternoon. Latest SPC mesoanalysis shows 2500+ J/Kg MLCAPE just south of the cold front, just north of I-44 in Missouri and I-70 in Illinois, though effective shear is pretty weak. Respectable DCAPE values in excess of 1000 J/Kg in this environment is supportive of pulse severe storms with microbursts. The RAP shows the instability weakening considerably after sunset (though MUCAPE remains stout enough for thunder through the night. Convection allowing models suggest the thunderstorm coverage should decrease significantly through late evening and will be generally confined to northern portions of the area by early Wednesday morning. The cold front will continue to drift south, though deterministic guidance keeps the coolest air bottled up across northern Missouri and west central Illinois. This will translate into a warm and humid day again across east central and southeast Missouri into southwest Illinois. There may be some lingering convection on Wednesday morning north of the front in northeast Missouri and west central Illinois...though the low level forcing is weak, and the primary area of moisture convergence is well north of the forecast area over central Iowa. Carney .LONG TERM... (Wednesday Night through Next Tuesday) Issued at 309 PM CDT Tue Sep 8 2020 Guidance is in general agreement that the mid Mississippi Valley will see somewhat cooler and drier weather for Thursday and Friday, although the details are a bit sketchy. The GFS is more aggressive in driving the cold front south into the lower Mississippi Valley than other models, and it looks like it's trying to keep more low clouds around than the EC or NAM. This translates into 2m temperatures that only get to the mid 60s to low 70s on the GFS while the NAM and EC get back into the low 80s along and south of I- 70...keeping the cooler temperatures in the mid 60s and 70s bottled up in northern Missouri and west central Illinois. Ensemble guidance is leaning more toward the warmer solutions which does lend confidence to the EC/NAM for Thursday into Friday. the large cut off low over the Four Corners Region begins lifting and filling in on Friday, and by 00Z Saturday the GFS and ECMWF have it moving east across the Great Plains as a (mostly) open wave. The resulting surface low drags another cold front into the Mid Mississippi Valley Saturday afternoon. Southerly flow ahead of the front will should be enough to get most of, if not the entire area into the 80s again, with some scattered convection most likely in the afternoon and evening. The airmass behind the front doesn't look particularly cold at this juncture, however deterministic guidance does show a large and persistent high pressure system building into the Midwest and Great Lakes behind the front Sunday and Monday. The high produces cool and dry northerly flow both days which keep highs in the mid and upper 70s with little chance of precip. Tuesday may be a little warmer as the high drifts east and the flow turns back around to the east/southeast. Carney && .AVIATION... (For the 18z TAFs through 18z Wednesday Afternoon) Issued at 1236 PM CDT Tue Sep 8 2020 Low MVFR and IFR flight conditions will prevail north of a cold front which is currently stretching from near KCOU to KSPI. This cold front will drift southward over the next 24 hours and the low ceilings will drift south along with it to overspread the entire forecast area by 08-10Z. The low ceilings are expected to lift slowly through the morning. Additionally, scattered showers and thunderstorms will affect the area, mainly along and north of I-44 in Missouri, and I-70 in Illinois. A few storms could produce wind gusts between 40-50kts this afternoon. SPECIFICS FOR KSTL: Expect showers and thunderstorms in the vicinity of the terminal this afternoon. Latest model guidance suggests that the thunderstorm activity could continue beyond 22Z as forecast in the TAF. The cold front which is helping to drive these storms is expected to drift through the terminal this evening, and this will bring the wind around to the north-northeast and bring the lower ceilings which are now in northeast Missouri down into the STL Metro. Low MVFR and IFR flight conditions are likely tonight, and will probably be slow to improve on Wednesday. Carney && .PRELIMINARY POINT TEMPS/POPS... Saint Louis 66 86 65 80 / 40 10 5 5 Quincy 55 71 58 73 / 60 20 10 20 Columbia 58 79 60 75 / 60 10 10 20 Jefferson City 61 80 62 78 / 70 10 10 20 Salem 64 88 64 83 / 10 10 0 5 Farmington 63 85 64 83 / 10 10 0 5 && .LSX WATCHES/WARNINGS/ADVISORIES... MO...None. IL...None. && $$ WFO LSX