Area Forecast Discussion
Issued by NWS St. Louis, MO

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FXUS63 KLSX 190957
AFDLSX

Area Forecast Discussion
National Weather Service Saint Louis MO
357 AM CST Mon Feb 19 2018

.SHORT TERM...  (Through Late Tonight)
Issued at 357 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...  (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 06z TAFs through 06z Monday Night)
Issued at 1144 PM CST Sun Feb 18 2018

MVFR cigs have both spread northward and developed across the area
this evening. Current thinking is they will persist through at
least mid morning on Monday. Meanwhile a strong south-
southwesterly LLJ will also produce LLWS conditions overnight and
through mid-morning, after mixing with commence and increasing and
gusty southerly winds will be the rule. Little has changed on the
thinking with regards to precipitation. Showers and a few
thunderstorms will develop overnight, especially between 09-12Z,
and will largely be focused from southwest to northeast through
the St. Louis area. Longevity is a bit harder to gage and current
thinking is through at least the morning although there may be
diminishing coverage in the afternoon. The best coverage of
showers and thunderstorms will also shift northward during the
afternoon and persist into the evening.

SPECIFICS FOR KSTL:

MVFR cigs/flight conditions will persist through at least mid
morning on Monday. A strong south-southwesterly LLJ will also
produce LLWS conditions overnight and through mid-morning, after
mixing with commence and increasing and gusty southerly winds will
be the rule. Showers and a few thunderstorms will develop
overnight, especially between 09-12Z, and will largely be focused
from southwest to northeast through the St. Louis area. Longevity
is a bit harder to gage and current thinking is through at least
the morning with diminishing coverage in the afternoon. Flight
conditions could certainly be lower with the heavier bouts of
precipitation on Monday morning. Current thinking is from late
afternoon into mid-evening there will be a minimum/lull in the
precipitation coverage with an upswing late evening and overnight
into Tuesday morning.

Glass

&&

.PRELIMINARY POINT TEMPS/POPS...
Saint Louis     72  63  73  33 /  90  80  90 100
Quincy          66  58  65  24 /  70  80  90  80
Columbia        70  60  67  26 /  70  70  90  90
Jefferson City  70  62  69  27 /  70  70  90  90
Salem           69  62  72  38 /  70  70  90 100
Farmington      69  62  73  35 /  60  80  90 100

&&

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

$$

WFO LSX



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