Area Forecast Discussion
Issued by NWS St. Louis, MO

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FXUS63 KLSX 191156
AFDLSX

Area Forecast Discussion...Updated Aviation
National Weather Service Saint Louis MO
556 AM CST Mon Feb 19 2018

.SHORT TERM...  (Through Late Tonight)
Issued at 329 AM CST Mon Feb 19 2018

Focus thru tonight will be precip chances.

Area is under deep swly flow with an upper trof digging into the
Rockies and upper ridge over the SE U.S. With a strong LLJ over
the area, ample moisture will be advected into the region. A
wrmfnt stretching across the area is expected to continue to be
north of the CWA around sunrise or shortly thereafter. This will
allow strong WAA across the area with gusty swly winds. At this
point, believe winds will remain below advisory levels with
limited mixing due to lack of insolation.

A 50-70 kts LLJ will remain in place as well as an ULJ
strengthening over the region thru tonight. This will provide more
broad scale lift, resulting in a need for fairly high PoPs across
much of the region. However, mdls are in general agreement with
the best moisture convergence roughly along I-44 in MO and I-70 in
IL. This shud be the focus for more widespread rain thru at least
the morning hours and into early afternoon. Still believe there
will be a relative lull in activity during the afternoon before
another round begins in the evening. Exactly when that lull will
begin and end remains more uncertain. With the front approaching
the CWA, believe one area of focus during the evening will be the
nrn portions of the CWA, perhaps as far south as KCOU to roughly
KPPQ. Believe this activity will wane during the evening hours and
refocus once again along I-44 in MO and I-70 in IL.

As for temps, trended twd the warmer guidance with the wrmfnt
lifting north of the area and strong WAA thru tonight.

Tilly

.LONG TERM...  (Tuesday through Sunday)
Issued at 329 AM CST Mon Feb 19 2018

A series of disturbances will move across MO/IL within a
southwest flow pattern aloft, bringing periods of active weather
to the area from Tuesday through Sunday. Although there are
several forecast periods with likely or higher PoPs, there will
probably be some dry periods during the week.

Widespread precipitation is expected on Tue and Tue night. The
precipitation type on Tuesday will begin as rain showers with
isolated thunderstorms possible depending on instability. A slow-
moving cold front then moves through the region on Tuesday night,
and the resulting thermal profiles in BUFKIT are typical profiles
for freezing rain if the surface temperatures are below freezing.
However, there is significant disagreement between models
regarding the speed of the cold front and therefore the speed and
intensity of the cold air which will be responsible for the change
in ptypes. For example, the NAM is quicker and colder (more
freezing rain and higher ice accumulations) while the GFS is
slower and warmer (less freezing rain and lower ice
accumulations). There are no strong signals yet to suggest that
either the coldest or the warmest solution should be preferred,
therefore this forecast package continues to follow a middle
ground by including a wintry mix of precipitation types on late
Tue night into Wed morning to account for the arrival of the
colder air. A light glaze of ice along with minor snow/sleet
accumulations are possible (less than 1/4" each), and these ptypes
and amounts will be refined during subsequent forecast package
updates.

The cold front stalls south of the region on Wed, bringing a few
days of cooler temperatures to MO/IL during the middle of the
week. By the end of the week, the stalled boundary lifts back
northward as as warm front and provides a focus for additional
precipitation on Friday and through the weekend as an upper trough
approaches from the west.

One item to note is that forecast PW values of 1.2-1.6" during the
week are near or above the max values for Feb. Given the
prolonged drought across most of the area, initial thoughts are
that this week`s rainfall will be beneficial rainfall. However,
some models do show a narrow corridor of prolonged rainfall. If
this occurs, then the rainfall could contribute to minor flooding.
Later shifts may need to consider a Flood Watch or a Flash Flood
Watch depending on observed rainfall coverage and amounts during
the early part of the week.

Kanofsky

&&

.AVIATION...  (For the 12z TAFs through 12z Tuesday Morning)
Issued at 540 AM CST Mon Feb 19 2018

For KCOU/KUIN...Showers and a few thunderstorms have developed
this morning. A few showers may impact the terminals throughout
the day. However, believe the more prolonged period of rain will
be later this afternoon into the overnight hours. Otherwise,
expect cigs at UIN to lift into MVFR as the warm front lifts north
of the terminal around 14z or so. Believe that cigs will remain in
the MVFR category thru the period and may drop back into the IFR
range later tonight.

SPECIFICS FOR KSTL/KSUS/KCPS: Showers and a few thunderstorms
have developed this morning. These will continue to move NE today.
In general, expect the ongoing showers to continue, then a brief
lull late this morning before another round this afternoon. Expect
another lull in activity later this afternoon and into early
evening. Otherwise, cigs shud improve this morning, eventually
into the low VFR range. Expect cigs to drop into MVFR range again
this evening. Uncertainty remains how low cigs will drop and may
not be going low enuf soon enuf for tonight.

Tilly

&&

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

$$

WFO LSX


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