Area Forecast Discussion
Issued by NWS St. Louis, MO

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FXUS63 KLSX 232012
AFDLSX

Area Forecast Discussion
National Weather Service Saint Louis MO
312 PM CDT Wed Aug 23 2017

.SHORT TERM...  (Through Late Thursday Afternoon)
Issued at 310 PM CDT Wed Aug 23 2017

The synoptic pattern early this afternoon features a trough across
the northeastern CONUS, which will place the Midwest within cyclonic
northwesterly flow through the period.

Pretty spectacular weather is on tap through tomorrow as surface
high pressure currently over southwest IA/northwest MO continues to
influence our sensible weather.  This high will shift east across
the region tonight, allowing for calm winds and clear skies. With
dewpoints having mixed out into the low 50s this afternoon,
expect a cool night. Went below guidance for temps tonight which
places them in the mid to upper 50s. Would not be surprised to see
some of the cooler valley locations dip into the low 50s or even
upper 40s. Given these cool temperatures over relatively warm
river waters, can`t rule out some steam fog developing early
Thursday morning.

Thursday looks like a carbon copy of today.  Expect mostly sunny
skies (perhaps a bit more high-level cloudiness over northern
MO/western IL) with temperatures in the upper 70s to low 80s.

KD

.LONG TERM...  (Thursday Night through Next Wednesday)
Issued at 310 PM CDT Wed Aug 23 2017

The synoptic pattern at the start of the long term period will
feature continued northwesterly flow aloft, a pattern which will
shift to more northerly through the period as a PV anomaly digs
southeast out of central Canada. This regime will make for continued
seasonably cool conditions along with increasing shower/storm
chances this weekend into early next week.

Friday and Saturday will continue the stretch of very pleasant late
August weather as surface high pressure over the Great Lakes
continues to influence our sensible weather. Highs each day will
continue to run below normal, with highs in the upper 70s/low 80s.

Forecast confidence begins to decline Saturday night into early next
week.  There are a couple factors at play leading to this lowered
confidence: (1) A PV anomaly and associated area of low pressure
diving southeast and (2) the remnants of what is expected to be
Hurricane Harvey in the western Gulf of Mexico.  In general,
guidance has sped up the arrival of PV anomaly and associated
surface cold front. The GEM still is a quick outlier, thus a blend
of the ECMWF/GFS/GEFS is preferred. This brings a threat of
showers/storms to the area as early as Saturday night which looks to
last into early next week as the PV anomaly helps to close off an
upper-level low near the region.

Just how quickly that upper low pulls out will likely be influenced
by the remnants of Hurricane Harvey. The consensus is for the upper-
level low to be shunted to the east by the middle of next week as
Harvey lifts northeast into the TN Valley. However, some outlying
guidance wants to lift Harvey more due north which could would then
affect portions of the area mid-late next week. Still plenty of time
to figure out the details, but thinking is that much of the moisture
associated with Harvey will remain southeast of the region.

KD

&&

.AVIATION...  (For the 18z TAFs through 18z Thursday Afternoon)
Issued at 1209 PM CDT Wed Aug 23 2017

VFR conditions are expected through the period as a surface ridge
provides fairly light winds and mostly clear skies. Can`t rule out
some river fog near SUS/CPS early Thursday morning, but don`t have
high enough confidence in its occurence to include at this time.

KD

&&

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

$$

WFO LSX



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