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fxus63 klsx 212057 
afdlsx

Area forecast discussion
National Weather Service Saint Louis MO
357 PM CDT Wed Aug 21 2019

Short term... (through late Thursday afternoon)
issued at 332 PM CDT Wed Aug 21 2019

The main issue tonight and into the first part of Thursday morning
will be the potential for heavy rain and flash flooding. The
deterministic models are in rather good agreement with the
synoptic and mesoscale features indicating a favorable set-up
evolving this evening. Thunderstorms are expected to Blossom by
mid-late evening along the MO portion of the I-70 corridor in
response to lift associated with the migrating short wave trof
currently stretching across southeast Nebraska, and an increasing
west-southwesterly low level jet. The low level jet will result in a broad region of
lift along a slowly advancing west-east front and will be
augmented by divergence aloft associated with the entrance region
of an ulj attendant with the upper trof stretching through the
Great Lakes. This set-up will be favorable for waves of thunderstorms
training in an axis stretching from eastern Kansas through central
and eastern MO into southwest Illinois. High pw's in excess of 2 inches
and good inflow cape will support high rainfall rates with the
strongest storms. Given the persistence expected to be centered in
the 02-12z time frame, heavy rainfall is expected along with
potential for flash flooding. Ballpark rainfall amounts of 2-4
inches are anticipated with locally higher totals. The focus and
corridor of showers and storms is expected to sag into southern MO
and southern Illinois by mid morning on Thursday associated with the
sagging front and/or effective boundary. Overall forcing by mid
morning should also be weakening. A broad Flash Flood Watch has
been issued within the aforementioned time frame 00-15z
encompassing the region with the highest heavy rain and flood
potential centered along the I-70 corridor.

As far as severe weather - there remains a threat mainly this
evening and probably within the first few hours of the above
described scenario as storms tap the strongest cape. I don't think
the threat will be widespread but a few large hail and wind damage
events will be possible between 02-06z.

Glass

Long term... (thursday night through next wednesday)
issued at 332 PM CDT Wed Aug 21 2019

By Thursday evening the front has moved out of the County Warning Area. The main
band of showers and thunderstorms are slowly ushered out of the
area Friday morning by a surface high building in over the Great
Lakes region.

Through Friday the upper level low associated with the
departing cold front lifts out and the upper flow becomes dominated
by a short wave ridge. The models show little consensus in resolving
the location of the short wave ridge, yet there is confidence that
small disturbances will propagate across the region on Friday. While
the main axis of precipitation associated with the front will be
well south of the cwa, a stray shower or thunderstorm cannot be
ruled out on Friday in our southern County Warning Area where the column remains
moist. Post cold frontal temperatures will be cooler than normal
Friday through Sunday, though how much cooler will depend on the
persistence of widespread cloud cover.

The surface high persists through Sunday, keeping the Missouri-
Illinois area mostly free from precipitation over the weekend. By
the end of Sunday an upper level trough begins to dig into the
region. As this trough moves across the Great Plains and into the
upper Mississippi River valley Monday into Tuesday, it brings
another frontal passage and our next best chance of rain. The
pattern shift back to an approaching trough also brings a return of
seasonal temperatures.

Mrm

&&

Aviation... (for the 18z tafs through 18z Thursday afternoon)
issued at 1245 PM CDT Wed Aug 21 2019

An outflow boundary from showers and thunderstorms over western MO
this morning has pushed well east and through the St. Louis
terminals. Winds in the wake are a bit chaotic at this time and
gusty but should settle down and have a west to northwest
component by mid afternoon. KUIN could see a few bouts of showers
through mid afternoon but should remain VFR. Isolated to scattered
showers and thunderstorms will be possible across the region
through the afternoon but I think they will remain away from any
terminals and VFR flight conditions are expected. Present thinking
is that we will see a big increase in showers and thunderstorms
across the area mid-late evening, continuing overnight as a cold
front sags south. I think MVFR flight conditions will be more
prevalent but flight conditions could certainly lower to IFR at
times with heavier rainfall. MVFR flight conditions are expected
to persist on Thursday morning even after the rainfall ends and
shifts into southern MO and southern Illinois.

Specifics for kstl:

An outflow boundary from showers and thunderstorms over western MO
this morning has pushed well east through kstl. Winds in the wake
are a bit chaotic at this time from the north and gusty, but
should settle down and have a west to northwest component by mid
afternoon. While there could be isolated showers and thunderstorms
across the area this afternoon I think they will remain away the
terminal and VFR flight conditions are expected. Present thinking
is that we will see a big increase in showers and thunderstorms
mid-late evening, continuing overnight as a cold front sags south.
I think MVFR flight conditions will be more prevalent but flight
conditions could certainly lower to IFR at times with heavier
rainfall. MVFR flight conditions are expected to persist on
Thursday morning even after the rainfall ends and shifts into
southern MO and southern Illinois.

Glass

&&

Lsx watches/warnings/advisories...
MO...Flash Flood Watch from 7 PM CDT this evening through Thursday
morning for Audrain MO-Boone MO-Callaway MO-Cole MO-
Crawford MO-Franklin MO-Gasconade MO-Jefferson MO-Lincoln
MO-Moniteau MO-Montgomery MO-Osage MO-Pike MO-Saint Charles
MO-Saint Francois MO-Saint Louis City MO-Saint Louis MO-
Sainte Genevieve MO-Warren MO-Washington MO.

Illinois...Flash Flood Watch from 7 PM CDT this evening through Thursday
morning for Bond Illinois-Calhoun Illinois-Clinton Illinois-Fayette Illinois-Greene
Illinois-Jersey Illinois-Macoupin Illinois-Madison Illinois-Marion Illinois-Monroe Illinois-
Montgomery Illinois-Randolph Illinois-Saint Clair Illinois-Washington Illinois.

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