Area Forecast Discussion
Issued by NWS St. Louis, MO

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FXUS63 KLSX 191708
AFDLSX

Area Forecast Discussion...Updated Aviation
National Weather Service Saint Louis MO
1108 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 18z TAFs through 18z Tuesday Afternoon)
Issued at 1025 AM CST Mon Feb 19 2018

Cold front extending from a surface low in south central IA
southwest through northwestern MO and into the TX panhandle will
remain northwest of the taf sites this period, although it will
be close to UIN and COU by 18Z Tuesday. A strong southwesterly low
level jet will continue to bring abundant low level moisture into
the region with periods of showers and a few storms. Showers
continued to move through the St Louis metro area late this
morning. Another area of showers and storms moving northeastward
through southwestern MO will impact COU and the St Louis metro
area this afternoon, and possibly into UIN as well. Waves of
showers and embedded storms will continue to impact the taf sites
tonight and Tuesday. The MVFR ceilings in UIN will continue,
possibly dropping into the IFR catagory tonight. The MVFR ceilings
will move into COU early this afternoon and into the St Louis
metro area by late afternoon. South-southwesterly surface winds
will continue through the period. There will be some LLWS tonight
but with boundary layer mixing it does not look quite strong
enough to include in the tafs.

SPECIFICS FOR KSTL: Cold front extending from a surface low in south
central IA southwest through northwestern MO and into the TX
panhandle will remain northwest of STL this period, although it
will be close to STL by 00Z Wednesday. A strong southwesterly low
level jet will continue to bring abundant low level moisture into
the region with periods of showers and a few storms. Showers
continued to move through the STL area late this morning. Another
area of showers and storms moving northeastward through
southwestern MO will impact STL this afternoon. Waves of showers
and embedded storms will continue to impact the STL area tonight
and Tuesday. MVFR ceilings will move into the STL area by late
afternoon. South-southwesterly surface winds will continue
through the period. There will be some LLWS tonight but with
boundary layer mixing it does not look quite strong enough to
include in the STL taf.

GKS

&&

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

$$

WFO LSX



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