Area Forecast Discussion
Issued by NWS St. Louis, MO

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FXUS63 KLSX 230335
AFDLSX

Area Forecast Discussion...Updated
National Weather Service Saint Louis MO
935 PM CST Wed Feb 22 2017

.UPDATE...
Issued at 919 PM CST Wed Feb 22 2017

A weak cold front has just moved southeastward through IRK and
MCI this evening. This front will continue moving southeastward
into central MO and southern IL by Thursday morning. There was
only some high level clouds this evening, but it still appears
that patchy fog and stratus clouds will develop late tonight and
early Thursday morning along and south of the front where high
surface dew points exist and as the surface wind becomes light.
The fog will take longer to develop than the previous night and is
not expected to be widespread or dense. It will be another
unseasonably warm night. Low temperatures will be slightly cooler
across northeast MO behind the cold front.

GKS

&&

.SHORT TERM...  (Through Late Thursday Afternoon)
Issued at 256 PM CST Wed Feb 22 2017

Low stratus continues to burn off this afternoon. These low clouds
lingered in many areas this morning longer than expected, therefore
high temps have been lowered in affected areas. Winds remain 5 to 10
kts from the south to southwest.

Cold front associated with surface low in WI slides into the CWA
from the NW this evening. The front stalls near the I70 corridor as
it becomes caught in the parallel upper flow before transitioning to
warm front early tomorrow. This warm front is associated with
surface low over central KS. As the warm front progress north on
Thursday afternoon slight POPs for the northern CWA are included.

Overnight temps in the 40s north of I70 and the 50s to the south.
Temps on Thurs also center on I70 with low to mid 60s north and low
70 to the south.

Walsh

.LONG TERM...  (Thursday Night through Next Wednesday)
Issued at 256 PM CST Wed Feb 22 2017

(Thursday Night - Friday)

Surface low early Thursday evening is forecast to be in central
Kansas with a warm front draped eastward likely just north of the I-
70 corridor in Missouri. Highest threat of precipitation will be
along and north of this boundary where low-level moisture advection
is maximized. Scattered showers and thunderstorms are possible for
our far northern counties. Otherwise...expect a very warm late
February night within the warm sector of the storm system across the
south-central Plains. Low temperatures in the 50s to near 60 degrees
are forecast along with a partly to mostly cloudy sky and
a moderate southerly breeze.

One more unseasonably warm day is on tap for majority of the CWA on
Friday with near-record breaking highs possible again for KSTL.
Warm, dry windy day will be the story for most of the area as
dryline moves through ahead of the actual cold front. Best chance of
thunderstorms along the dry line will be across far eastern IL
counties Friday afternoon as DCPVA ahead of deepening system
aloft helps erode stout capping inversion from 800 and 850 hPa.
Limited moisture return and weak instability look to limit the
severe threat though cannot rule out a strong wind gust or two
associated with a thunderstorm downdraft due to the pronounced
dry air aloft and high DCAPE associated with this feature.

Other concern Friday afternoon and evening will be on strong
west/southwest wind gusts due to mixing down of strong winds from
the top of the mixed layer. Wind gusts Friday afternoon and
evening will likely be between 25 and 35 knots with wind advisory
gusts not out of the question. For more information on how this
will affect fire weather conditions, please see fire weather
discussion below.

(Friday Night - Saturday Night)

Much colder air will be ushered in behind the cold frontal passage
Friday night. Some wraparound light snow may clip far northern
sections of the area with a light dusting of snow possible. Flurries
may also occur further to the south within the cyclonic flow
aloft. Temperatures should bottom out below freezing for most of
the northwestern 1/2 of the CWA with mid 20s likely across
portions of northeastern Missouri. These cold temperatures
combined with the strong NW winds will make it feel like it is in
the teens to twenties...certainly a shock compared to the 60s and
70s of just 12 hours prior.

Lingering flurries or light snow is possible Saturday morning mainly
across portions of west-central Illinois. Much cooler and cloudy
conditions will start the weekend with slowly decreasing cloudiness
from northwest to southeast during the day on Saturday. Highs are
expected to range from the mid 30s to low 40s...some 25-35 degrees
colder than Friday and slightly below normal for a change.

Surface ridge of high pressure will slide eastward across the CWA
Saturday night. A seasonably cold night (though the coldest areawide
in some 2 weeks) is forecast with lows in the low to mid 20s.

(Sunday - Next Wednesday)

Medium-range guidance diverges a bit beginning late this weekend and
heading into the next work week. Zonal flow aloft with embedded
disturbances are always difficult when it comes down to pinning down
timing/strength of these systems. Signs of some light rain possibly
mixed with snow late Sunday/Sunday night where chance PoPs are in
the forecast currently.

Stronger system with widespread showers and possibly some
thunderstorms still appears on tap for Tuesday/Tuesday night as the
longwave trough ejects out of the desert southwest. Temperatures
both day and night should moderate back to above normal for
early to mid next week.

Gosselin

&&

.AVIATION...  (For the 00z TAFs through 00z Thursday Evening)
Issued at 549 PM CST Wed Feb 22 2017

A cold front extending from a surface low over WI southwest
through northwestern MO will move southeastward into central MO
Thursday morning, then lift back north Thursday afternoon and
evening as a surface low moves through KS and into northwest MO by
late Thursday evening. Stratus and fog will develop late tonight
and early Thursday morning along and south of this front where
surface dew points are still high. MVFR conditions can be expected
late tonight and Thursday morning, then the fog will dissipate
late Thursday morning and the stratus will lift. The surface wind
will become northwesterly late tonight in UIN and COU after fropa,
then veer around to an easterly direction and strengthen on
Thursday. Light surface winds can be expected in the St Louis
metro area tonight and Thursday morning, then strengthen from a
southeast direction Thursday afternoon into the evening.

SPECIFICS FOR KSTL: A cold front extending from a surface low over
WI southwest through northwestern MO will move southeastward into
central MO Thursday morning, then lift back north Thursday
afternoon and evening as a surface low moves through KS and into
northwest MO by late Thursday evening. Stratus and fog will
develop late tonight and early Thursday morning along and south of
this front where surface dew points are still high. MVFR
conditions can be expected late tonight and Thursday morning, then
the fog will dissipate late Thursday morning and the stratus will
lift. Light surface winds can be expected tonight and Thursday
morning, then strengthen from a southeast direction Thursday
afternoon into the evening.

GKS

&&

.FIRE WEATHER...
Issued at 256 PM CST Wed Feb 22 2017

Very deep mixing Friday afternoon and evening is likely with
winds at the top of the mixed layer of 35-50 knots. These winds
aloft should mix down fairly efficiently so wind gusts at the sfc
in the 30-35+ knot range are expected. Combination of strong WSW
surface winds and the warm/dry airmass behind dryline should yield
very low relative humidity values as well of 20-30 percent. These
two factors in conjunction with dry fuels should lead to at least
elevated fire danger for portions of the area with the possibility
of red flag warning criteria possible.

Gosselin


&&

.CLIMATE...
Record Highs

       2/22    2/23    2/24

STL: 78/1995 77/1996 81/1930
COU: 76/1995 73/1933 81/1930
UIN: 70/1922 70/1922 73/1930

Record High Minimums

       2/22    2/23    2/24

STL: 55/1985 55/1930 58/2000
COU: 53/1922 53/2000 61/1930
UIN: 57/1922 49/2000 55/1930


&&

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

$$

WFO LSX



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