Area Forecast Discussion
Issued by NWS St. Louis, MO

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FXUS63 KLSX 231037

Area Forecast Discussion...Updated Aviation
National Weather Service Saint Louis MO
537 AM CDT Tue Oct 23 2018

.SHORT TERM...  (Through Late Tonight)
Issued at 332 AM CDT Tue Oct 23 2018

Northwest upper level flow has taken hold across the region in the
wake of a trough that passed well to our north. The surface cyclone
associated with the trough is currently over the Great Lakes with a
weak surface cold front extending back into the Mid Mississippi
Valley. The cold front will continue to slide southeast today as
high pressure builds into the Upper Midwest beneath upper level
height rises. This will result in another day of calm weather, with
high temperatures across all but the far southern portions of our
area about 5-10 degrees cooler than what we saw on Monday.

.LONG TERM...  (Wednesday through Monday)
Issued at 332 AM CDT Tue Oct 23 2018

The surface high will gradually shift east on Wednesday as the
cool, dry air mass builds into the northeastern CONUS. Therefore
another largely cloud-free day is expected on Wednesday. Temps
will be similar to Tuesday across our north, but for areas along
and south of I-70, temperatures will be another 5-10 degrees
cooler than on Tuesday.

On Thursday, the upper level ridging will break down as a trough
pushes into the region. Meanwhile, a plume of mid level moisture
associated with Hurricane Willa will push into the region. The
cyclonic vorticity advection associated with the shortwave,
coupled with the moisture from Willa, has much of the forecast
guidance painting various amounts of precip across the region on
Thursday evening/night. The NAM/GEM saturate the airmass quite a
bit faster than the GFS/Euro, and therefore put down about twice
as much QPF as the more conservative solutions. Given the very dry
airmass currently in place and the fact that the dryness will be
reinforced behind today`s fropa, it stands to reason that the
atmosphere will take a while to saturate on Thursday night.
Therefore I`m inclined to lean on the GFS/Euro a bit more than the
NAM/GEM. Regardless, we should see at least some spotty light
showers before the shortwave clears the area on Friday morning.

After a quiet day on Friday, another shortwave embedded in the
northwest upper flow will race through the region late Friday and
Saturday. With nothing to scour the moisture behind Thursday night`s
precip, there will likely be sufficient mid and low level moisture
in place to support another round of light rain on Saturday as the
short wave moves through. Beyond that, we will remain entrenched
in northwest upper level flow. All forecast guidance shows
embedded shortwaves intermittently passing through the region,
though there is little consensus on timing this far out.
Regardless, these passing waves have resulted in a few periods of
low precipitation chances through early next week.



.AVIATION...  (For the 12z TAFs through 12z Wednesday Morning)
Issued at 531 AM CDT Tue Oct 23 2018

Patchy fog in low lying areas should dissipate quickly as the sun
comes up this morning. Afterward, skies will stay mostly clear
save for a few high clouds streaming into the area from the west.
Otherwise, a weak cold front will move south out of the area,
leaving northerly flow in its wake.





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