478 
FXUS63 KLSX 191803
AFDLSX

Area Forecast Discussion...Updated Aviation
National Weather Service Saint Louis MO
1203 PM CST Tue Nov 19 2019

.SHORT TERM...  (Through Late Wednesday Afternoon)
Issued at 324 AM CST Tue Nov 19 2019

The surface low associated with the clipper system early this 
morning was located over western Illinois with a trough axis 
extending nearly due southward along the Mississippi River. Shower
activity has waned over the past couple of hours and limited 
slight chance PoPs early this morning along/east of the 
aforementioned trough axis. The remainder of the day should be 
characterized by decreasing cloudiness from west to east along 
with seasonable temperatures. Coolest readings are forecast to be 
across south-central Illinois due to lingering clouds into the 
early afternoon. High temperatures will only reach the upper 40s 
in these locations. Further southwest, an earlier clearing should
lead to highs reaching the upper 50s to near 60 degrees for 
portions of central and southeastern Missouri.

Fog should develop late tonight, mainly along/east of the 
Mississippi River. These areas are favored for the development of
fog due to the expected timing of the departure of stratus today,
as well as light and variable winds expected near a surface ridge
of high pressure. The current expectation is for visibilities to 
stay largely above 1 mile, but if clearing is delayed a few hours
than currently forecast, some patchy dense fog may be possible in
west-central and/or south-central Illinois. Outside of fog 
development, a mostly clear sky along with temperatures near to slightly
above normal for the date are forecast. Lows tonight should be 
mainly in the 30s across the bi-state region.

Increasing clouds are expected on Wednesday ahead of the next 
weather system ejecting out of the Rocky Mountains. The timing of 
the onset of rain has continued to slow down compared to a couple of 
days ago, with the daylight hours on Wednesday likely staying dry 
areawide. Increasingly southeasterly winds will help yield 
moderating temperatures compared to those of today. Look for 
highs to reach the upper 50s to mid 60s across the area which is 
fairly close to the warmer MAV MOS numbers.


Gosselin

.LONG TERM...  (Wednesday Night through Monday)
Issued at 324 AM CST Tue Nov 19 2019

(Wednesday Night - Friday Night)

Widespread shower activity remains very likely Wednesday night, 
mainly ahead of the approaching cold front in a region of strong low-
level moisture advection. Rainfall amounts do not appear too high 
due to the relatively short duration of rainfall. Amounts generally 
should be around a quarter to a half inch for most locations. Also 
cannot rule out a rumble of thunder as the most aggressive model 
guidance suggests some low MUCAPE values and Showalter indices near 
0 late Thursday night. However, still feel that the probability of 
occurrence is too low to explicitly mention with this forecast
package.

The cold front will slowly progress through the CWA on Thursday with 
diminishing chances of any shower activity behind it. Another round 
of rain appears increasingly likely Thursday night perhaps into 
early Friday north of the slow-moving cold front. Added likely to 
low categorical PoPs as far north as the I-44/I-70 corridors in 
Missouri and Illinois respectively. This area of rainfall will be 
supported by strong synoptic forcing via low-level moisture 
advection, mid/upper level diffluence, and vorticity advection 
ahead of the closed low entering the south-central Plains. 

Uncertainty in the forecast increases beginning on Friday as the 
deterministic ECMWF progresses the cold front further 
southeastward than the GFS/CMC. In addition, the closed low 
ejecting out of the Rockies into the south-central Plains is 
weaker. Consequently, very little precipitation is depicted from 
Friday through Friday night on the ECMWF. This solution was not 
favored with this forecast package, but also cannot be discounted 
at this time. The more realistic and likely scenario is for 
continued light rain early on Friday focused across southern 
sections of the forecast area. Additional precipitation may then 
spread northeastward associated with the approaching closed low. 
This precipitation would start off as rain, but as the 
temperatures cool in the lower portions of the atmosphere the rain
should mix with and changeover to snow. This should occur from 
parts of central Missouri northeastward through west-central 
Illinois. Farther southeast, the vast majority of any 
precipitation should be in the form of a cold rain. Light 
accumulations of wet snow still could be in the offing for 
northwestern sections of the forecast area if the stronger GFS/CMC
solutions are on the right track.

Temperatures will start off this period very mild ahead of the cold 
front. Lows Wednesday night should be in the upper 40s to low 50s. 
Highs on Thursday should warm nicely into the low to mid 60s as 
well. For reference, these temperatures would be about 10 degrees 
above normal for the date. Behind the cold front, a return to below 
normal temperatures is expected. Lows both Thursday and Friday night 
will mostly be in the 30s and highs on Friday are only forecast to 
be in the low to mid 40s.


(Saturday - Monday)

A fairly quiet, benign weather pattern appears in the offing for 
this upcoming weekend into early next week with northwest flow 
aloft. Dry weather is favored along with moderating temperatures. 
After initially starting off on the cool side on Saturday/Saturday 
night, return flow around a departing surface ridge of high pressure 
should lead to temperatures climbing back into the 50s for Sunday 
and Monday.


Gosselin

&&

.AVIATION...  (For the 18z TAFs through 18z Wednesday Afternoon)
Issued at 1144 AM CST Tue Nov 19 2019

Primary concerns will be centered around gusty winds today. A
northwest to southeast oriented low level jet of 30-40 kts will be
aligned approximately along the Mississippi River through the
afternoon. As dry air filters into the region from the northwest,
ceilings continue to break west to east. This will introduce winds
aloft to the surface with gusts around 20 knts through early
evening.

The next question will be the need to address potential fog
development tonight. Shallow inversions and light/variable winds
east of the Mississippi River could lead to reduced visibility
early Wednesday morning. 

Otherwise, as skies clear today, VFR conditions are expected to
extend through most of the period. 

SPECIFICS FOR KSTL: 

Much of the period is expected to be VFR with the main concerns
related to gusty winds today and fog potential tonight. 

A 30-40 kt low level jet will be positioned overhead through the
afternoon with increased mixing from resulting sunshine. This has
resulted and will continue to result in surface gusts around 20
kts through sunset. Winds will die off tonight with potential for
fog primarily east of the Mississippi River. Potential become
limited further west. Therefore, stuck with 6SM mention with the
opportunity to make later adjustments, if needed. 

Clouds will continue to work back in from the west through the
later half of Wednesday. There are signals for LLWS very late in
the period with greatest risk after the period. For now, left the
mention out to be assessed in later updates. 

Maples

&&

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

$$

WFO LSX