016 
FXUS63 KLSX 062339
AFDLSX

Area Forecast Discussion...Updated Aviation
National Weather Service Saint Louis MO
639 PM CDT Mon Sep 6 2021

.SHORT TERM...  (Through Late Tuesday Night)
Issued at 227 PM CDT Mon Sep 6 2021

High pressure continues to slowly drift to the east of the 
Mississippi River, which has kept skies largely clear and veered low 
level winds to the SW. This combination has allowed temperatures to 
climb into mid/upper 80s area wide, and a few areas may yet top 90 
before the day is through. With the surface high sliding further 
east and a cold front pushing into the Plains tonight, winds will be 
a bit more elevated and should result in less fog than we've seen
the last few nights. The exception will be in areas closer to the
center of the high, in SE MO and southern Illinois, where river 
fog will be possible.

Tomorrow, a shortwave will ripple through the Upper Midwest, pushing 
the aforementioned cold front through the Mid-Mississippi Valley
in the afternoon and evening, opening the door for showers and 
storm chances. However, with the shortwave passing to our north, 
forcing in our area will be shallow and limited to the lower 
levels along the front. Additionally, surface based convection 
appears unlikely given a robust capping inversion ahead of the 
front. CIN will be less prohibitive for elevated parcels just 
behind the front, possibly supportive of post-frontal showers of 
storms. The strength of any convection will be modulated by the 
quality of moisture across the region. The two most recent HREF 
runs push mean dewpoints to nearly 70 as moisture pools along the 
front tomorrow afternoon. I'm not certain these dewpoints will be 
achieved given current obs in the low 60s, deeper moisture well to
the south of the region, and a limited period for moisture 
return. If the dewpoints are indeed overdone in the guidance, 
modeled instability is also too high.

Even with limited instability, at least isolated elevated convection
remains possible beginning tomorrow afternoon as the front moves 
into NE MO and west-central IL. This is reflected in the guidance,
with the majority of large scale and convective allowing guidance
painting at least light QPF just behind the front beginning in 
the afternoon. Should these storms develop, a few may become 
briefly organized given roughly 30kts of deep layer shear in 
central IL and could result in an isolated occurrence of severe 
wind/hail. However, with only marginally supportive shear and 
questions about storm development, any mention of severe storms 
will be limited to this discussion. As the front slides south 
through the region in the evening, convection will do the same, 
exiting the area around midnight.

BSH

.LONG TERM...  (Wednesday through Next Monday)
Issued at 227 PM CDT Mon Sep 6 2021

The Middle Mississippi Valley will remain beneath NW upper level 
flow through the remainder of the workweek as a ridge builds over 
the western CONUS. In the lower levels, high pressure will move 
into the region in the wake of Tuesday's cold front, slowly drifting
to the SE through the week. Deep northwesterly flow on Wednesday 
and Thursday will keep conditions cool and dry, resulting in a very
pleasant couple of days. 

By Friday, the low level high will begin to shift east of the 
region, veering winds to the SW and boosting temperatures back to
just above normal. Meanwhile, guidance is in consensus that a 
shortwave will zip from the crest of the ridge to our west into 
the Great Lakes sometime in the late week, though considerable 
differences remain in shortwave timing and intensity. This may 
push a cold front into the region at some point over the weekend, 
which coupled with brief return flow, could result in limited 
precipitation chances. However model consensus and continuity on 
these finer scaler details remains poor at this time, so I've left
the dry forecast initialization as is.

BSH
&&

.AVIATION...  (For the 00z TAFs through 00z Tuesday Evening)
Issued at 639 PM CDT Mon Sep 6 2021

Surface ridge will continue to slide off to the southeast through
Tuesday morning with winds gradually veering to the southwest by 
mid morning on Tuesday. Breezy winds aloft will mix down to the 
surface on Tuesday with gusts to 20 kts at times. Otherwise, a 
cold front will move into the region during the afternoon and 
slowly slide south. Winds will veer to the northwest at KUIN and 
KCOU by 21z Tuesday as front moves through. Still some uncertainty
about development of showers and storms along the boundary, 
especially near the TAF sites late afternoon through the evening 
hours. For now added vicinity storm mention at KSTL between 00z 
and 03z Wednesday, with rest of the TAF sites dry through 00z 
Wednesday.

Byrd
&&

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

$$
WFO LSX