134 
FXUS63 KLSX 260817
AFDLSX

Area Forecast Discussion
National Weather Service Saint Louis MO
317 AM CDT Sat Oct 26 2019

.SHORT TERM...  (Through Late Sunday Afternoon)
Issued at 313 AM CDT Sat Oct 26 2019

Main focus continues to be on rain coverage and amounts through 
early tonight.  Rain has already moved into the southern CWA early 
this morning and will continue to spread north across the CWA 
throughout the rest the morning. This is occurring as deep upper 
low currently over the Red River Valley will deepen as it moves 
across Missouri and Illinois late this afternoon into this 
evening. Surface low currently along the Gulf Coast will move 
north into east central Illinois by this evening and also quickly 
strengthen. It still looks like that CWA is set up for a 
favorable rain event with deformation zone of the upper low moving
over the CWA at the same time strong moisture transport will be 
bringing around 1.5" PWATS into the area. With this in mind, will 
keep categorical PoPs over almost the entire CWA with total storm 
amounts ranging from around less than 0.25" in the northwest to 
around 2.5" in eastern CWA. Forecast mid level lapse rates will 
also be near 7 C/km, so will leave isolated thunderstorms in the 
forecast.

GFS/NAM are in reasonable agreement that the upper low will lift 
into southern Lake Michigan by tonight and shortwave ridge will move 
into the area tomorrow.  This is consistent with rain moving out of 
the region this evening and the area being dry late tonight into 
Sunday.

Temperatures today will be held down by the clouds and the 
widespread rain.  Highs on Sunday will climb back into the 60s with 
sunshine and southerly winds.

Britt

.LONG TERM...  (Sunday Night through Friday)
Issued at 313 AM CDT Sat Oct 26 2019

Main concern through next week will be the potential for additional 
precipitation chances as a long wave trough digs into the central 
CONUS.  CIPS analogs continue to show surface temperatures between 
10-15 degrees below normal by mid-late next week. Yet there is 
still some uncertainty as the operational ECMWF is slower than the
GFS in how quickly it moves an upper low into the central CONUS 
next week. In addition, while the GEFS plumes agree that 
temperatures will fall behind a cold front that will move slowly 
across the area Monday into Tuesday, there remains considerable 
uncertainty regarding the details in precipitation timing. In 
addition, the GEFS mean temperatures are lower than the operation
mean.

With that said, it still looks like the first round of rain just 
behind the front late Monday into early Tuesday as a weak shortwave 
trough moves northeast across the Midwest.  The next chance of 
precipitation will then come with the approach of the upper low 
Tuesday night into Thursday.  Thermal fields from the various models 
and the GEFS plumes support keeping a rain/snow mix over the 
northern and western parts of the CWA with this event.  Model 
uncertainty remains on how quickly this system will exit the 
area, but the GEFS are favoring a quicker motion, so will leave 
Friday dry for now.

Britt

&&

.AVIATION...  (For the 06z TAFs through 06z Saturday Night)
Issued at 1039 PM CDT Fri Oct 25 2019

Confidence remains high that rain, reduced visibilites, and low
cigs will move into the region on Saturday morning, though latest
guidance has slowed the arrival time a bit compared to the
afternoon runs. Models still depict a rapid drop from VFR into IFR
cigs at the STL terminals as the rain arrives in the mid morning.
Some signal remains showing LIFR cigs near STL, but confidence on
this remains low, so have just continued the SCT003 mention for 
the time being. Cigs are expected to remain above 1000 kt at UIN 
and especially COU, which are both further removed from the 
weather system responsible for these reduced conditions. 

The biggest uncertainty with this forecast is how quickly
conditions will improve on Saturday evening. There is fairly
overwhelming model consensus showing dry air rapidly filtering 
into the area after 00z, helping to quickly get rid of the sub-VFR
conditions. However, the evening and overnight hours in the cool 
season are not when we typically see rapid cloud dissipation, so 
I'm a little hesitant to fully buy into the model solution. I am
therefore holding onto MVFR cigs a bit longer than the models
suggest at all sites. Additionally, I've kept SCT025 through the 
end of the TAFs at COU and STL to signal some chance for MVFR cigs
through the end of the TAF period.

BSH

&&

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

$$

WFO LSX