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FXUS63 KLSX 071144 AFDLSX

Area Forecast Discussion...Updated Aviation National Weather Service Saint Louis MO 544 AM CST Thu Dec 7 2023

KEY MESSAGES

- Temperatures today and Friday will be 15 to 20 degrees above normal in the upper 50s to mid 60s today and Friday.

- Timing has shifted faster on the Friday night-Saturday storm system. This means there will be less rainfall, almost no chance for thunderstorms, and almost no chance for snow on the tail end Saturday night.

- Even though we expect less rainfall, PoPs remain high (60-80%) Friday night with the expectation of widespread showers or light rain.

- Dry and cooler weather is expected Sunday, with temperatures near normal. Temperatures are expected to warm back above normal early next week.

SHORT TERM
(Through Late Friday Afternoon)
Issued at 352 AM CST Thu Dec 7 2023

A strong upper level ridge will move east from the Great Plains into the Mississippi Valley today. At the surface, a high pressure over the southeastern US combined with strengthening low pressure over eastern Montana/western North Dakota will produce south-southwest flow across Missouri and Illinois. The combination of the strong low level warm advection, rising heights aloft, and plenty of sunshine will produce temperatures well above normal today. I find that traditional MOS guidance works better than ensemble temperatures in these circumstances due to the climatology incorporated into the MOS equations. With that said, the NBM max temps are only a couple of degrees cooler than some of the warmest MOS numbers. Additionally, the 25th and 75th percentile maxes on both the NBM and LREF are only 2 or 3 degrees apart today, lending high confidence to the high temperature forecast. I ultimately leaned on a blend of 75th percentile NBM and CONSMOS for the high temperature forecast today, which yields highs ranging from around 60 in south central Illinois up to the mid and upper 60s in central Missouri. Forecast soundings show we'll mix up to around 925mb this afternoon, and these forecast highs agree well with RAP, GFS, and NAM 925mb temperatures when mixed to the surface.

Southwest flow continues tonight and Friday as the upper level ridge translates east as the trough over the Great Plains strengthens.
The 10-15kt southwest flow is not conducive to radiational cooling, and with this afternoon's above normal temperatures tonight's lows will be pretty mild for December. Again I stuck with a blend of 75th percentile NBM and CONSMOS which yielded lows in the mid 40s to around 50. The mild temperatures will continue Friday, although the upper ridge will be well east of the area and heights will be falling ahead of the next upstream trough. This along with increasing clouds particularly during the afternoon will yield slightly cooler highs Friday afternoon though still well above normal in the upper 50s to low 60s.

Carney

LONG TERM
(Friday Night through Wednesday)
Issued at 352 AM CST Thu Dec 7 2023

Latest guidance is faster with the Friday night-Saturday system than previous runs. The consensus of deterministic guidance now has the surface front moving into central and northeast Missouri by 06Z Saturday, through the St. Louis Metro by 12Z, and out of our forecast area into far eastern Illinois and southeast Missouri before 15Z. This tracks well with the LREF mean (actually the LREF mean is a little faster). This faster solution has a few effects on the forecast for Friday night through Saturday. First, since the low-level trough moves through the Plains more quickly, there's less time for it to draw moisture from the Gulf ahead of it into the Mid Mississippi Valley. This means less precipitation ahead of and with the cold front late Friday night into Saturday morning. This also means that Saturday afternoon and Saturday night will likely be dry (though I'm still keeping a lingering 20-30 percent chance for parts of southeast Missouri and southwest/south central Illinois in case the front ultimately comes through slower). Additionally, there will be less instability ahead of the front Friday night into Saturday morning, so most of the thunder has been removed from the forecast, and with precip ending more quickly, most of the rain/snow mix Saturday night has been removed as well.

Normally I would be a little more cautious about that much change in the forecast, but as I mentioned before, the consensus of the deterministic forecast guidance supports it as does the LREF and NBM. In fact, the LREF 25th and 75th percentile temperatures on Saturday are only 3-5 degrees apart, which gives me high confidence in the timing of the front. Additionally, EOF patterns are showing less variance in the speed and amplitude of the upper trough than previously forecast for Friday night and Saturday which lends further confidence to the faster solution.

The remainder of the forecast for Sunday through Wednesday looks relatively cool and quiet. High pressure builds into the Mississippi Valley on Sunday behind the cold front, and there looks to be a brief warmup Monday and Tuesday as that high moves to our southeast brining us return flow. Another short wave trough ripples along the US/Canada border late Tuesday into Wednesday which could push another cold front into the Mid Mississippi Valley, but current indications are it will have little moisture to work with.

Carney

AVIATION
(For the 12z TAFs through 12z Friday Morning)
Issued at 538 AM CST Thu Dec 7 2023

VFR flight conditions are expected to prevail at least through this evening. Low level wind shear conditions will persist this morning through 15-16Z. Daytime heating will then mix out the low level inversion which is causing the shear, and south-southwest surface winds will become gusty. Gusty winds will persist into early evening, and possibly overnight. There are indications that MVFR ceilings will move into central Missouri from the southwest late in the period between 10-12Z Friday, although timing and ceiling heights are uncertain a this time.

Carney

LSX WATCHES/WARNINGS/ADVISORIES
MO...None.
IL...None.

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