Area Forecast Discussion

000
FXUS63 KLSX 101734
AFDLSX

Area Forecast Discussion...Updated Aviation
National Weather Service Saint Louis MO
1234 PM CDT Fri Jul 10 2020

.SHORT TERM...  (Through Late Tonight)
Issued at 359 AM CDT Fri Jul 10 2020

A cold front is moving through southeast Missouri and southern
Illinois this morning.  This front will bring the area somewhat
cooler air today, but the real change in the weather will be much
less humid air.  Dew point temperatures are falling into the mid 60s
behind the front, and with some mixing this afternoon the dew point
looks to drop into the low 60s in many locations.  This will bring
relief from the 100+ heat index values.  Heat index looks to top out
at or below 90 degrees today.

Short range guidance is showing an MCS developing over northern
Nebraska or southern South Dakota late this afternoon or early this
evening.  Northwest flow aloft will drive the MCS southeast through
the night.  Nearly every model, including the ensemble means, keep
the MCS west of our forecast area through 12Z Saturday.  There`s one
member of the SPC HREF (the HRW ARW) that brings storms into
northeast/central Missouri around 12Z, but it`s the outlier.  Have
therefore kept the forecast dry tonight.

Carney

.LONG TERM...  (Saturday through Thursday)
Issued at 359 AM CDT Fri Jul 10 2020

The vast majority of model guidance keeps the MCS west of our
forecast area from western Missouri south through south central
Missouri.  Even the SPC HREF 0.01 qpf probs barely make it to 40
percent over a couple of central Missouri Counties.  Will keep some
low chance PoPs in on Saturday morning to cover the small potential
for the northeastern end of the MCS scraping our area.  Most of the
day Saturday should be dry in the wake of the MCS, with northwest
flow prevailing across the area.  However, deterministic guidance
does ramp up instability over the area during the afternoon, with
MLCAPE approaching 2500 J/Kg over parts of northern Missouri and
west central Illinois.  However, there seems to be little in the way
of a triggering mechanism to take advantage of that instability.
There is a short wave aloft that drops into the Mid Mississippi
Valley between 00Z and 06Z Sunday, but most of the low level forcing
associated with the wave is over Illinois.  Will keep chance PoPs in
the forecast for Saturday night, primarily along and east of the
Mississippi in case there is some development further to the
southwest of the short wave.  The surface reflection of the short
wave aloft pushes a cold front through Missouri and Illinois by
early Sunday morning.  Even though it will be a little cooler and
drier behind the front on Sunday, the northwest flow aloft brings
another vortmax across the Mid Mississippi Valley which could be
enough to pop a few storms across eastern portions of the forecast
area.  Temperatures this weekend will range from near normal in the
upper 80s to low 90s on Saturday, dropping into the mid 80s on
Sunday behind the front.

Attention turns to the ridge that is building across the southwest
U.S.  The long wave pattern with a trough over the eastern U.S.
begins shifting on Monday and the ridge builds into the Mid
Mississippi Valley on Tuesday.  Although there is about 10 degrees
difference between the 25th and 75th percentile for highs Monday,
ensemble guidance does indicate a solidly warmer day on Monday with
highs rising back into the upper 80s and low 90s.  As the ridge
continues to strengthen, temperatures will continue to rise.  Medium
range guidance shows a short wave moving across the Midwest which
attenuates the ridge and drags a front down into southern Iowa. This
may bring northern portions of the area relief from the heat through
Thursday, but areas along and south of I-70 will likely remain in
the mid to upper 90s.  Max heat index values for Tuesday through
Thursday look likely to reach the 100-107 degree range.

Carney
&&

.AVIATION...  (For the 18z TAFs through 18z Saturday Afternoon)
Issued at 1230 PM CDT Fri Jul 10 2020

VFR flight conditions will persist through the forecast period.
Otherwise, will have wind gusts near 20 kts this afternoon at KUIN
and KSTL, before diminishing early this evening. Still could see a
MCS complex slide southeast across western Missouri that will clip
portions of central Missouri, so have vicinity thunderstorms at
KCOU between 13z and 16z Saturday.

Byrd
&&

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

$$
WFO LSX

NWS LSX Office Area Forecast Discussion