Area Forecast Discussion
Issued by NWS St. Louis, MO

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539
FXUS63 KLSX 220851
AFDLSX

Area Forecast Discussion
National Weather Service Saint Louis MO
351 AM CDT Sun Oct 22 2017

.SHORT TERM...  (Through Late Tonight)
Issued at 350 AM CDT Sun Oct 22 2017

Cold front as of 0800 UTC was located near a KTBN>>KMYJ line and
very slowly progressing eastward. As expected, convection along the
frontal boundary has weakened progressively overnight as instability
waned. Expecting a continued slow progress of the cold front through
the day today with a round of showers moving from west to east.
Could be a few rumbles of thunder through mid morning, more so
possible across the eastern 1/2 of the CWA. Trailing stratiform
region of the precipitation shield will likely also weaken through
the morning hours and may become more fragmented. Therefore, not
expecting a whole lot of rainfall across much of the area. Mid/upper
level shortwave lags well behind the frontal boundary so rain
chances will linger across parts of the area through the afternoon
hours and into at least early tonight. This will be particularly
true for portions of southwest and south-central Illinois as
midlevel trough axis becomes negatively tilted, with a closed low
developing. The experimental HRRR even develops a well-defined
deformation zone with light rain lingering through the overnight
hours as far west as east-central and southeastern Missouri. This is
a bit of an outlier however with pretty much all other guidance
keeping a post-frontal band of rain slowly moving out of the area.
If system does close off quick enough however, the experimental HRRR
may be onto something.

Temperatures will be much cooler across a vast majority of the area
due to the frontal passage and associated rainfall. Falling
temperatures are likely across much of Illinois with not much of a
rebound west of the Mississippi due to moderately strong cold air
advection in conjunction with an overcast sky. Some clearing is
expected late tonight across portions of central and northeastern
Missouri. This is where the coldest readings are expected tonight
where lows should drop to near 40 degrees. Further to the east,
leaned aoa warmest guidance due to expected lingering cloud cover.
Lows in the upper 40s to near 50 degrees are forecast for parts of
southwest Illinois and the St. Louis metropolitan area.


Gosselin


.LONG TERM...  (Monday through Saturday)
Issued at 350 AM CDT Sun Oct 22 2017

(Monday - Thursday)

Very amplified flow regime aloft to begin the new work week as a
strong shortwave trough amplifies as it heads into the mid-
Mississippi Valley. This shortwave will bring a secondary cold front
through the bi-state area on Monday, with chilly conditions expected
on Tuesday behind the front. Highs are only expected to be in the
50s with at least some stratocumulus expected beneath the cyclonic
flow aloft. Also could see some rain showers beneath the cold core
of temperatures aloft during the day on Tuesday, mainly along/east
of the Mississippi River.

Quickly moderating temperatures will occur Wednesday and Thursday as
850-hPa temperatures rise into the +10-15C range. Warmest day of the
week definitely appears to be Thursday with highs in the upper 60s
to low 70s.

(Thursday Night - Saturday)

Confidence is increasing that the region will see well below normal
temperatures heading into next weekend as a strong cold front moves
through the area Thursday night/early Friday. Too early for
specifics, but it appears increasingly likely that the area will see
its first hard freeze of the season ending the growing season.
Northern hemispheric charts depict cross-polar flow so expecting
airmass to be pretty potent/cA in origin. SLU CIPS analog guidance
based on last night`s GEFS showed freeze probabilities of 60-80% for
the Friday/Saturday timeframe, and it appears the coldest night will
actually likely be the following night (Saturday night/next Sunday).

Regarding precipitation chances, there probably will be a likely
period of post-frontal rain on Friday/Friday night, but
deterministic models differ some 12 hours with the frontal passage.
Because of this uncertainty, elected to cap PoPs out below likelies
for now. Believe most (if not all) of this post-frontal
precipitation will be in the form of rain, but cannot totally rule
out it ending as a few wet sloppy snowflakes Friday night. What is a
bit more interesting is the possibility of secondary cyclogenesis
along the stalled boundary to our southeast, which would lead to
more precipitation falling further back into the cold air. The 0Z
ECMWF shows such a solution, with some wet (potentially
accumulating) snow across parts of northeastern Missouri. Do not
think this is the more likely solution based on past experience and
model bias, but it may be something to keep an eye on. Regardless,
with a very amplified longwave trough centered over the area next
weekend, can envision parts of the area experiencing some snow
flurries as vorticity maxima rotate around the closed low across
the western Great Lakes.


Gosselin


&&

.AVIATION...  (For the 06z TAFs through 06z Sunday Night)
Issued at 1055 PM CDT Sat Oct 21 2017

Cold front extending from eastern MN south-southwest through
northwestern MO into south central OK will move southeastward
through the taf sites late tonight and Sunday morning. There was a
line of thunderstorms along this front which will move into UIN
and COU shortly after midnight, and eventually into the St Louis
metro area by early Sunday morning. The convection should slowly
weaken in intensity late tonight as the instability wanes. There
was also scattered showers and storms ahead of the front between
JEF and UIN. Lingering light showers may continue in UIN and COU
during at least much of Sunday morning and in the St Louis metro
area Sunday afternoon due to a slow moving upper level disturbance
behind the cold front. MVFR cigs will advect into the taf sites
late tonight and early Sunday morning along and behind the cold
front. The ceiling should gradually improve to VFR conditions
Sunday afternoon. The southerly surface winds will veer around to
a northwesterly direction after fropa in UIN and COU towards early
Sunday morning and in the St Louis metro area by early Sunday
afternoon. There will be a strong southwesterly low level jet late
tonight which will lead to some LLWS conditions at the taf sites,
but it appears that there will be enough nocturnal mixing to keep
the surface wind up so it will not likely be strong enough to
include in the tafs.

SPECIFICS FOR KSTL: Cold front extending from eastern MN
south-southwest through northwestern MO into south central OK
will move southeastward through the STL area early Sunday
morning. There was a line of thunderstorms along this front which
will move into the STL area by early Sunday morning. The
convection should slowly weaken in intensity late tonight as the
instability wanes. There was also scattered showers and storms
ahead of the front between JEF and UIN which may shift southeast
into STL late tonight. Lingering light showers may continue in
the STL area Sunday afternoon due to a slow moving upper level
disturbance behind the cold front. MVFR cigs will advect into the
STL area late tonight and early Sunday morning along and behind
the cold front. The ceiling should gradually improve to VFR
conditions Sunday afternoon. The southerly surface winds will veer
around to a northwesterly direction after fropa in STL by early
Sunday afternoon. There will be a strong southwesterly low level
jet late tonight which will lead to some LLWS conditions, but it
appears that there will be enough nocturnal mixing to keep the
surface wind up so it will not likely be strong enough to include
in the taf.

GKS

&&

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

$$

WFO LSX



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