Area Forecast Discussion
Issued by NWS St. Louis, MO

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561
FXUS63 KLSX 190002
AFDLSX

Area Forecast Discussion...Updated Aviation
National Weather Service Saint Louis MO
602 PM CST Sun Feb 18 2018

.SHORT TERM...  (Through Late Monday Afternoon)
Issued at 250 PM CST Sun Feb 18 2018

Upper level flow is in the process of backing SW with a deeper lift
beginning to take hold across our region, but the main issue right
now is lack of moisture thru the column, so this is primarily
translating into thick high cloud.  Thanks to strong gusty south
winds to 25-35mph, temps have risen into the 50s in many locations,
but fallen a bit short of earlier prognostications likely due to the
thick high cloud and that it is still only February.

Thicker and lower clouds will overspread our region this evening
with the advent of deeper better moisture and lift persisting
primarily from WAA.  A low level jet of 50-60kts will also nose into
our region by midnight, and it is at this point, we should start to
see showers and a few thunderstorms develop and increase in coverage
late tonight and continuing into Monday morning.  Models generally
agree on the initial outbreak occurring to the NE of STL metro
towards central IL around midnight with some backbuilding then
occurring to the southwest with some maintenance of this into Monday
morning.  Where this axis settles, sites could see between a half
inch and an inch of rainfall, but there remains some uncertainty
where this axis will be.  Probable indications right now are showing
it mainly from a central IL thru parts of the STL metro and near I-
44 corridor and have the highest PoPs here.

Heading into Monday afternoon and beyond, there looks to be a
reorganization taking place and some decrease in rain coverage looks
likely before the next round strikes beyond this forecast period.

Temps tonight will not fall much in light of the stout south flow
and clouds and rising dewpoints, and in fact, should only drop into
the 40s this evening before a steady rise returns them into the 50s
by daybreak Monday.  Have dropped temps back a few degs from the
previous forecast with abundant clouds and rain chances thru much of
the day along with a weak February sun angle, with max temps from 65-
70.

TES

.LONG TERM...  (Monday Night through Next Sunday)
Issued at 250 PM CST Sun Feb 18 2018

(Monday Night - Wednesday Night)

Very active weather pattern will continue through Tuesday night.
Main focus for early in the period will be on what should be very
beneficial rainfall. A widespread 1-3 inches appears likely by
Wednesday night with locally higher amounts certainly possible.
Still have some mild concern for the potential of heavier rainfall
rates given deep warm cloud depths and near record precipitable
water values, but model guidance continues to suggest the focus of
the precipitation should slowly slide southeastward in time from
late Tuesday morning through Wednesday morning. This scenario makes
sense given the strength of the Canadian airmass moving into the
mid-Mississippi Valley.

Speaking of the cold airmass infiltrating the area, this will become
the primary concern beginning Tuesday night through Wednesday
night, especially for northwestern sections of the forecast area.
Unfortunately, still a lot of uncertainty with respect to how
quickly temperatures drop below freezing and how much (if any)
precipitation falls back into the subfreezing air. Deterministic
12Z GFS dries things out by the time the cold air comes in while
the end of the NAM and ECMWF look a bit more ominous. Favorable
jet dynamics combined with weak impulses embedded within deep
southwest flow aloft does suggest the possibility of precipitation
hanging back a bit more into the cold air than the GFS portrays.
Given the spread, tried to use a blend of the two model camps
which yields some light icing amounts to the northwestern 2/3 of
the area.

Model agreement has increased a bit for another round of
precipitation later in the day on Wednesday into Wednesday night,
but specifics regarding timing and placement is still unknown.
Precipitation type looks to be a combination of mostly freezing rain
and sleet due to the colder air by this time having established
itself. While amounts look light, could see some impacts especially
across portions of northeast Missouri and west-central Illinois
where temperatures are forecast to be well below the freezing mark.
A combination of sleet and freezing rain also typically more
negatively impacts road conditions.


(Thursday - Next Sunday)

Southwest flow aloft continues through the rest of the extended
portion of the forecast. Thursday thankfully looks mostly dry but
another round of rainfall appears likely Friday through Saturday
which could bring another couple of inches of rain to parts of the
area. With a big change in antecedent conditions (i.e.,
river/stream levels much higher and soils closer to saturation),
would expect a much greater percentage of rain to runoff. Will
have to keep an this period maybe more so than Tuesday/Tuesday
night for the potential of river and/or flash flooding.

Sunday looks dry and continued mild but the overall pattern remains
locked in place. A general longwave trough is still expected to be
centered across the western United States with ridging off of the
east coast. No signs yet of the warm and active pattern we are
entering to end...at least within the next 7 days.

Gosselin

&&

.AVIATION...  (For the 00z TAFs through 00z Monday Evening)
Issued at 543 PM CST Sun Feb 18 2018

VFR and dry for all sites this evening ahead of an area of MVFR
stratus that will develop after midnight. Southerly winds will
diminish this evening to around 10kts by 3z. LLWS at all TAF sites is
expected as LLJ develops overnight. Gusts of up to 25kts will
return Monday after sunrise. SHRA/TSRA will develop overnight
with incoming stratus deck and continue through Monday evening.
Periods of lower visibilities with precip are possible but went
conservative at this point. LLWS at all TAF sites is expected as
LLJ develops overnight.

SPECIFICS FOR KSTL:

VFR and dry with gusty south winds to start the TAF period. Wind
gusts will diminish with sunset. LLWS is expected as LLJ develops
overnight. MVFR ceilings move in as stratus deck builds into the
area after midnight. SHRA/TSRA will develop overnight with
advance of mid clouds and last through the remainder of the
period.


Walsh

&&

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

$$

WFO LSX



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