Area Forecast Discussion
Issued by NWS St. Louis, MO

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FXUS63 KLSX 262116
AFDLSX

Area Forecast Discussion
National Weather Service Saint Louis MO
316 PM CST Sun Jan 26 2020

.SHORT TERM...  (Through Late Monday Night)
Issued at 311 PM CST Sun Jan 26 2020

The main concern for tonight will be on the possibility of dense
fog. The setup looks quite favorable, with light/variable winds
and plenty of low-level moisture enhanced by a melting snowpack.
In addition, upstream visibilities this afternoon remain quite
low, with some still below a mile in central and west-central Iowa.
Given that visibilities should decrease nocturnally, believe at
least areas of dense fog are likely tonight, mainly across
portions of northeast Missouri. Because of the increasing
confidence in dense fog tonight, a dense fog advisory has been
issued for these areas from this evening through early Monday
morning.

Another thing we are watching for tonight into Monday is a strong
midlevel impulse that will be heading down the Missouri River
late tonight. As noted by previous shifts, moisture associated
with this system is fairly scant, but it does appear that top-down
saturation may occur very late tonight into the day on Monday.
CAMs, including the extended RAP/HRRR, suggest the best chance of
any light precipitation will be across central, east-central, and
southeastern Missouri which agrees well with the track of the
aforementioned midlevel impulse. The precipitation type will be
rain or snow, and probably will depend how fast any precipitation
arrives. However, given that wetbulb temperatures stay mostly
below freezing, did lean more toward light snow vs. light rain. In
terms of amounts, any accumulations will be tough to come by given
the very marginal setup, but cannot rule out a light dusting on
elevated and grassy surfaces.

Temperatures through the short-term forecast period should be very
close to seasonal normals, with lows tonight and Monday night in the
20s and high temperatures on Monday ranging from the low 30s to low
40s from north to south.


Gosselin


.LONG TERM...  (Tuesday through Next Sunday)
Issued at 311 PM CST Sun Jan 26 2020

(Tuesday - Tuesday Night)

There is another system of interest for the middle of this upcoming
week. Models have slowly trended toward the (more) consistent
CMC/GEM with the evolution of two separate shortwave troughs, one
moving east/southeast out of the southern Plains and another
trailing shortwave diving south/southeast out of the mid-Missouri
Valley. The trailing shortwave has trended further southwestward
over the past 24-48 hours, which helps allow a bit more poleward
transport of low-level moisture. This means light precipitation
may make it further northward toward the southern CWA late
Tuesday afternoon/Tuesday night. These more northern solutions are
also backed up by many GEFS/EPS members. In fact, both the
EPS/GEFS have accumulating snowfall focused on Tuesday night in
the southern CWA. If these current trends hold, PoPs and QPF will
need to be further adjusted northward. For now, increased PoPs in
the southern half of the CWA with both schc/chc wording.
Precipitation type should be mostly if not all snow, especially
given the timing being predominantly overnight Tuesday night.
This certainly does not look like a big system in terms of
snowfall, but one that does bear further watching, especially
given recent model trends.


(Wednesday - Next Sunday)

Mainly dry weather along with seasonable temperatures is likely for
the middle of the work week heading into next weekend beneath
zonal flow aloft. Moderating temperatures are forecast toward the
end of the forecast period as a midlevel ridge begins to move
toward the intermountain west.


Gosselin


&&

.AVIATION...  (For the 18z TAFs through 18z Monday Afternoon)
Issued at 1139 AM CST Sun Jan 26 2020

IFR stratus and fog is expected to slowly advect southeastward
this afternoon, primarily affecting KUIN. The stratus should
expand as well this evening and move into the rest of the
terminals. In addition, visibilities should drop toward or below
1SM across northeastern Missouri and western Illinois, which
includes Quincy. Elsewhere, 1-3SM visibilites are expected
overnight tonight into early Monday morning. A slow improvement is
expected through the morning hours on Monday.

Winds will be light and variable through the period. There is a
low chance of light precipitation (likely in the form of snow)
mainly on Monday morning, but chances appear too low to mention
anything but a VCSH group at KSUS and KCOU currently.


SPECIFICS FOR KSTL:

VFR conditions are expected this afternoon but conditions will
deteriorate starting late this evening. IFR stratus is likely
thereafter, along with some fog reducing visibilities to about
3SM. Slow improvement is likely on Monday, but low MVFR conditions
will still prevail during the afternoon hours.

Winds will be light and variable for the most part through the
period. There is also a low chance for some light precipitation on
Monday, but the better chances at this time appear to be just west
of the terminal so no mention in the TAF at this time.


Gosselin

&&

.LSX WATCHES/WARNINGS/ADVISORIES...
MO...Dense Fog Advisory from 9 PM this evening to 10 AM CST Monday
     for Audrain MO-Knox MO-Lewis MO-Marion MO-Monroe MO-Pike MO-
     Ralls MO-Shelby MO.

IL...None.
&&

$$

WFO LSX


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