625 
FXUS63 KLSX 200530
AFDLSX

Area Forecast Discussion...Updated Aviation
National Weather Service Saint Louis MO
1130 PM CST Tue Dec 19 2017

.SHORT TERM...  (Through Late Wednesday Afternoon)
Issued at 352 PM CST Tue Dec 19 2017

Weak cold front is making progress to the south across the forecast 
area this afternoon. As of 20z, the front was along and just south 
of the I-70 corridor. Front should exit the forecast area by 00z. In 
the meantime, low sc is eroding from north to south, but have high 
overcast layer advecting north across region, so will remain mostly 
cloudy initially then slowly thin out for northern portions late 
this evening.

In the meantime, the upper trof over western TX will continue to 
lift northeastward towards forecast area tonight. Some differences 
in model solutions, but ultimately have some overrunning rain 
developing towards midnight tonight and slowly make it's way into 
southeast Missouri and southern Illinois. Shortwave to weaken and 
exit region by midday on Wednesday with rain coming to an end. 
Otherwise, will have partly to mostly sunny skies over northern half 
of forecast area, while southern half to start off cloudy, then see 
some sunshine late in the day. Highs will remain mild and about 10 
degrees above normal, in the mid 40s to low 50s.

Byrd

.LONG TERM...  (Wednesday Night through Next Tuesday)
Issued at 352 PM CST Tue Dec 19 2017

Focus remains precip with system Fri into Sat, turning to temps over 
the weekend and into next week. 

Overall, not much has changed regarding the uncertainty with the 
system on Fri or timing/placement of any subsequent s/w that may 
bring snow to the area on Sun. Period begins with zonal flow over 
the area ahead of trof digging into the srn Rockies. A fair amount 
of uncertainty still exists regarding this trof, which currently 
remains just off the wrn U.S. coast. While this trof wasn't 
necessarily completely sampled this morning, it shud have been 
partially resolved. Still, timing of this trof being ejected into 
the Plains differs among mdls. Similar to yesterday, the GEM remains 
the slower soln, the GFS faster with the ECMWF between them. GEFS 
members do vary with some slower solns, but not as slow as the GEM 
and have generally disregarded this soln. Differences in 
placement/timing of the secondary trof axis on Sun also exist with 
lower confidence in this feature as well. 

The 12z/19 GFS now has a band of precip well back into the cold air 
Fri afternoon/evening which may start as a mix, wud quickly change 
to snow as cold air rushes in. This feature wud rely on the phasing 
of the nrn and srn trofs and chances of this happening as-is in this 
soln are somewhat low. However, the chance of snow will still exist 
as the ECMWF soln is similar with much lighter precip amounts 
spreading nwd. Have therefore kept PoPs in the lower chance range 
for now with many questions remaining. 

Did raise PoPs across srn and sern portions of the CWA where all 
mdls suggest precip. Granted, even uncertainty in this remains due 
to cycle to cycle changes among mdls, which is why PoPs were capped 
in the low likely range. 

Colder air will be moving into the region behind this system, which 
is one item mdl solns agree on. However, this first shot of cold air 
will only lower temps back to seasonal avg on Sat. The much colder 
air shud arrive sometime Sun as the second trof axis rotates thru 
the area. 

Some light snow may also accompany this colder air on Sun into Sun 
night. Of course, mdls differ on placement of any precip. Have 
therefore kept PoPs in the low chance range for now. Regardless, 
this round of precip, if it does occur, is expected to be light.

Tilly

&&

.AVIATION...  (For the 06z TAFs through 06z Wednesday Night)
Issued at 1130 PM CST Tue Dec 19 2017

VFR conditions and dry weather are expected to prevail at the TAF
sites thru much of the valid period as light NE surface winds veer
E-SE by the end of the valid period. Only real issue is where
thick high clouds thin out, some sites have been able to develop a
quick mist event, such as SUS and CPS. As the cloud hole fills in,
this should reduce or eliminate VSBY reductions in fog, with a
more final clearing of these thick high clouds not set to occur
until later Wednesday morning. 

TES

&&

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

$$

WFO LSX