124 
FXUS63 KLSX 210420
AFDLSX

Area Forecast Discussion...Updated Aviation
National Weather Service Saint Louis MO
1120 PM CDT Fri Aug 20 2021

.UPDATE...
Issued at 955 PM CDT Fri Aug 20 2021

The forecast looks generally on track for the rest of the night.
The MCS over western Missouri appears to be weakening a bit and
WFO EAX has dropped their severe thunderstorm warnings at the time
I'm writing this. RADAR is showing the outflow well ahead of the
echomass over northwest Missouri indicating it's now outflow 
dominant and losing strength. Can't rule out some restrengthening
and/or surges and short lived bows as there is still 3000+ MUCAPE
ahead of the system. However, effective bulk shear is an 
unimpressive 20-25kts, which is not conducive to maintaining
severe storms. Additionally, instability falls of somewhat the
farther east the storms move, and there is weak upper level
ridging over the Mid Mississippi Valley which will likely aid in
weakening storms. All that said, the MCS still seems to have a
good head of steam as it's chugging east, so I've bumped up PoPs
for much of the area, and tried to adjust timing a bit using RADAR
trends for the rest of tonight and into Saturday morning.

Carney

&&

.SHORT TERM...  (Through Late Saturday Night)
Issued at 253 PM CDT Fri Aug 20 2021

While temperatures throughout the area will remain near or slightly 
below seasonal averages today, it certainly doesn't feel that way 
thanks to the high humidity in place. Surface observations across 
the mid-Mississippi valley continue to indicate dewpoints reaching 
into the lower to mid 70s early this afternoon, bringing heat index 
values well into the mid to upper 90s. Heat index values will likely 
remain above 90 degrees in many areas until early evening before 
finally cooling. 

Meanwhile, shortwave ridging aloft has not completely suppressed the 
development of scattered showers across the Missouri Ozarks, as 
afternoon heating and very high moisture content has proven to be 
sufficient for convective development. These showers remain 
generally weak thanks to a lack of appreciable wind shear, although 
a few lightning strikes have been observed. This activity is likely 
to percolate through the remainder of the afternoon, and should 
diminish during the early evening with the loss of diurnal heating. 
Bursts of heavy rain and occasional lightning strikes are all that 
is expected from these showers. 

After a quiet evening, more widespread showers and thunderstorms are 
expected to move into the region overnight, thanks to a passing 
shortwave to our north and a slowly approaching cold front. While 
these storms may become strong or severe well to our northwest this 
afternoon and evening, they are expected to notably weaken by the 
time they drift into central and northeast Missouri as wind shear 
weakens and they lose support from the previously mentioned 
shortwave. Still, at least marginal instability (~1000-1500 J/kg 
MUCAPE) and very high moisture content (2+" PWAT) will remain in 
place through the night, which will likely maintain convection as it 
moves through the area between roughly midnight and 9 am. Bursts of 
heavy rain is likely to be the primary threat with this activity, 
but we can't rule out some occasionally gusty winds as well.

This convection is likely to exit the area by mid morning tomorrow, 
but the cold front itself will lag behind, providing an
opportunity for redeveloping showers and thunderstorms in a few 
areas during the afternoon. This will be most likely across west-
central and southwest Illinois, but once again strong or severe 
storms are not expected. Meanwhile, warm and humid conditions will
persist, although some relief from the humidity can be expected 
across northeast Missouri thanks to the passing cold front. By 
tomorrow evening, the front will finally pass southeast of the 
area, and shower chances will diminish.

BRC

.LONG TERM...  (Sunday through Next Friday)
Issued at 253 PM CDT Fri Aug 20 2021

While temperatures Sunday afternoon will once again warm to near 
seasonal averages, the weak cold frontal passage will at least 
provide some modest relief from the high humidity. As such, Sunday 
is expected to be generally pleasant and much more comfortable, with 
temperatures in the mid to upper 80s and no precipitation to speak 
of.

This break from the heat and humidity will be short-lived, however, 
as a building ridge aloft will become centered over the region, and 
southerly flow will bring a quick return of Gulf moisture. As such, 
temperatures will quickly rebound well into the 90s area wide, and 
likely into the mid to upper 90s along and south of I-70. Meanwhile, 
the return of low to mid-70s dewpoints will also contribute to high 
humidity, and heat index values will rebound into decidedly 
uncomfortable territory. On both Monday and Tuesday afternoons, 
maximum heat index values of at least 100 degrees can be expected 
area-wide, with values of near and even above 105 degrees likely in 
the warmest areas, including the St. Louis metro. Confidence is high 
regarding the early week temperature forecast, as ensemble members 
exhibit very minimal temperature spreads, and have been rather 
consistent in this depiction. 

As for precipitation, while forcing will be quite weak underneath 
the center of the upper ridge, isolated afternoon showers and 
perhaps a thunderstorm or two can't be ruled out on either day 
thanks to the very high moisture content, strong heating, and the 
resulting instability that will be present. 

Heat will likely continue into Wednesday, but model trends suggest 
that the upper ridge will begin to weaken slightly, beginning a very 
gradual cooling trend that will continue into mid week. Meanwhile, 
precipitation chances also gradually increase beyond Tuesday, but 
confidence is much lower regarding this prospect.

BRC

&&

.AVIATION...  (For the 06z TAFs through 06z Saturday Night)
Issued at 1110 PM CDT Fri Aug 20 2021

Complex of thunderstorms over northwest/west central missouri will
continue move east-southeast tonight. The storms look like they
are continuing to weaken as they move toward our area, but gusty
winds to 30kts primarily over central Missouri will still be
possible. MVFR ceilings and IFR visibility will also be likely
with these storms. Also, some low MVFR/IFR ceilings have cropped
up over the past couple of hours across parts of southeast
Missouri and southwest Illinois, along with patchy dense fog.
These ceilings and fog should eventually be washed out by the
thunderstorms as they move through the area tonight. Some MVFR
ceilings are likely along with lingering showers in the wake of
the main body of storms on Saturday morning, but low clouds should
lift to VFR by late morning/early afternoon. Some scattered storms
are again possible during the late afternoon, mainly over
southeast Missouri and south central Illinois.

Carney

&&

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

$$

WFO LSX