721 
FXUS63 KLSX 070838
AFDLSX

Area Forecast Discussion
National Weather Service Saint Louis MO
338 AM CDT Fri Oct 7 2022

.SHORT TERM... (Through Late Saturday Night) Issued at 330 AM CDT
Fri Oct 7 2022

A series of shortwaves continue to dig through an upper level trough 
over the eastern CONUS. One of these shortwaves brought last 
evening's cold front through the area, while another disturbance is 
shunting a reinforcing shot of cool and dry air through the Northern 
Plains and Upper Midwest this morning. This airmass will continue 
southward today and tonight, helping to clear morning clouds and will
keep afternoon temperatures cool. 

Tonight, high pressure associated with this cooler airmass will 
settle over the region, resulting clear skies, light winds, and by 
tomorrow morning, the coolest temperatures of the season thus far. 
Even the 75th percentile of NBM minimum temperatures (min Ts) on
Saturday morning fall to or below 32F across much of northeast MO
and portions of west-central IL, meaning that at least 75% of 
model guidance is forecasting freezing temperatures in these 
areas. The combination of the above meteorological setup typical 
of very cool nights and strong model consensus for freezing 
temperatures has bolstered confidence that many areas in our north
will see their first freeze of the season. We've therefore gone 
ahead and upgraded the Freeze Watch to a Freeze Warning in these 
areas. Further south, the 75th percentile of NBM min Ts drops 
below 36 for most areas in our Missouri counties outside of St. 
Louis, and just a degree or two warmer across Illinois. These 
temperatures, along with light winds, will lead to conditions 
favorable for frost. With widespread frost expected across our 
rural Missouri areas and more patchy frost expected around St. 
Louis and into Illinois, we've issued a Frost Advisory for the 
rest of the forecast area.

We'll remain beneath northwesterly flow aloft on Saturday while the 
center of the surface high slides east across the region. Clear 
skies will allow temperatures to rebound 25-30 degrees from the 
morning lows, but given the cool starting point, temps will remain 
roughly 10 degrees below normal. Temperatures will again be quite 
cool from Saturday night into Sunday morning, though more westerly 
winds on the back side of the high will keep temperatures a bit 
warmer than what we're expecting tonight. Another freeze warning is 
unlikely, but an additional frost advisory may be needed, especially 
over our SW IL counties where the light northerly winds will hang on 
a bit longer.

BSH

.LONG TERM... (Sunday through Thursday) Issued at 330 AM CDT Fri 
Oct 7 2022

The eastern CONUS trough will persist into Sunday and Monday, 
keeping northwest flow in place a top the region. The surface high 
will continue to very slowly drift further to our east, keeping weak 
west to southwest winds in place over the Mid-Mississippi Valley. 
With little in the way of moisture return, skies will remain 
mostly clear through Monday. The combination of clear skies and 
S-SW winds will push temperatures back to normal by Sunday and 
above normal by Monday. 

On Tuesday, an upper trough will begin to dive into the Canadian
Plains and northern High Plains, with the southern periphery of 
the trough reaching the Mid-Mississippi Valley on Wednesday. In 
response, low level low pressure/troughing will develop to our 
west and support increasing SSW flow across the region. This will 
further boost temperatures before a cold front moves into the 
region on Wednesday, perhaps stalling across the region on 
Thursday. The front will bring precipitation chances back into the
region, though this portion of the forecast is far from certain. 
First of all, there are differences in frontal timing and 
evolution amongst the guidance. Secondly, moisture return does not
appear to be all that impressive ahead of the front, limiting 
precipitation potential. Nevertheless, the majority of members in 
the latest run of the NBM do show at least measurable precipitation
across our northern CWA at times on Wednesday and Thursday. 
Chances for 0.1", 0.25", and 0.5" fall off from there, topping out
across our north at around 40%, 25%, and 15% respectively. That's
all to say that while some light precipitation is likely midweek,
especially in NE MO and west-central IL, it seems unlikely that 
we'll see enough precipitation to put much of a dent in the 
increasingly dry conditions we've got across the region. 

BSH

&&

.AVIATION...  (For the 06z TAFs through 06z Friday Night)
Issued at 1031 PM CDT Thu Oct 6 2022

VFR flight conditions are expected to continue through the
remainder of the night. The cold front has made it south of the
St. Louis and Columbia metro terminals, and winds are expected to
remain elevated overnight. Gusts have dropped off after the
initial frontal passage and upstream obs do not indicate that
significant gusts will continue overnight, though a stray gust in
the upper teens cannot be ruled out.

Cloud cover associated with the front has been limited and
remained at VFR flight conditions. Ceilings are expected to be at
or above 25,000 ft across all terminals once some of the 
straggling thunderstorm cloud debris near the St. Louis metro 
terminals diminishes or exits the terminal space.

As previously mentioned, winds are expected to remain elevated
overnight, even in the sheltered and "calmer" TAF sites like 
KJEF, KSUS, and KCPS. Fog development at these sites is therefore 
unlikely, but if a site were to drop to 3 kt winds or less for
several hours overnight, the risk for fog at KJEF, KSUS, and KCPS
would increase. Given the large amount of uncertainty, have kept 
fog and visby reduction out of the TAF for now.

MRM

&&

.LSX WATCHES/WARNINGS/ADVISORIES...
MO...Frost Advisory from 1 AM to 9 AM CDT Saturday for Boone MO-
     Callaway MO-Cole MO-Crawford MO-Franklin MO-Gasconade MO-
     Iron MO-Jefferson MO-Lincoln MO-Madison MO-Moniteau MO-
     Montgomery MO-Osage MO-Reynolds MO-Saint Charles MO-Saint 
     Francois MO-Saint Louis City MO-Saint Louis MO-Sainte 
     Genevieve MO-Warren MO-Washington MO.

     Freeze Warning from 1 AM to 9 AM CDT Saturday for Audrain MO-
     Knox MO-Lewis MO-Marion MO-Monroe MO-Pike MO-Ralls MO-
     Shelby MO.

IL...Frost Advisory from 1 AM to 9 AM CDT Saturday for Bond IL-
     Calhoun IL-Clinton IL-Fayette IL-Greene IL-Jersey IL-
     Macoupin IL-Madison IL-Marion IL-Monroe IL-Montgomery IL-
     Randolph IL-Saint Clair-Washington IL.

     Freeze Warning from 1 AM to 9 AM CDT Saturday for Adams IL-Brown 
     IL-Pike IL.

&&

$$

WFO LSX