Area Forecast Discussion
Issued by NWS St. Louis, MO

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FXUS63 KLSX 040557
AFDLSX

Area Forecast Discussion...Updated Aviation
National Weather Service Saint Louis MO
1157 PM CST Sat Dec 3 2016

.UPDATE...
Issued at 932 PM CST Sat Dec 3 2016

The lower tropospheric ridge centered in the Ohio Valley and
attendant dry air has been impeding the north/northeastward
spread of rain thus far this evening. However radar is showing an
increasing amount of echo aloft across southern and central MO
within the last hour or so. The ridge will continue to depart to
the east tonight with lower trop flow above the PBL becoming
southerly resulting in increasing moisture transport into the
region as well as increasing isentropic ascent. Additionally
during the overnight hours, large scale ascent associated with the
migratory upper trof moving through the Plains will begin to
overspread the area from the west. The net result should be an
increase in coverage of rain actually reaching the ground as
saturation and lift ramp-up, especially during the overnight
hours. The strongest lift will occur on Sunday morning, and be
centered primarily along north of I-70. The latest short term
model guidance continues to support enough evaporative cooling to
erode the elevated warm layer and lower freezing levels close
enough to the surface for a rain/snow mix, and for some areas in
northeast MO/west-central IL perhaps a period of all snow. This
should occur gradually from the predawn hours into Sunday morning.
Present indications are that surface temperatures will be marginal,
despite the aforementioned cooling. Current thinking is minor
accumulations of wet snow, primarily on elevated and grassy
surfaces still looks reasonable for northeast MO and west-central
IL.

Glass

&&

.SHORT TERM...  (Through Late Sunday Afternoon)
Issued at 322 PM CST Sat Dec 3 2016

Light rain was trying to spread northward into southeastern MO late
this afternoon, well north of a low pressure system along the Gulf
Coast region.  An initially dry atmosphere in the lower layers
around 850 mb, on the western periphery of the surface ridge
centered over northern IL will slow the onset of precipitation this
evening, but parts of southeast MO and southwest IL should
eventually experience some light rain later this evening as the
boundary layer saturates. More significant will be a band of
precipitation which will move through much of the area, particularly
northeast MO and west central IL late tonight and Sunday morning
ahead of a northern stream shortwave. Most of the forecast soundings
indicate that much of the precipitation will be in the form of
liquid rain, but some of the model soundings do drop the freezing
level low enough late tonight and early Sunday morning that the rain
may mix with or even change over to a brief period of light snow,
mainly across northeast MO and west central IL.  May see very minor
snow accumulation, less than one inch in this region, mainly on
grassy areas. Roads should be warm enough that any snow would melt
on impact and not accumulate or create hazardous driving conditions.
Any snow should change over to liquid rain by late Sunday morning as
the boundary layer warms.  This rain will shift east of the forecast
area by late afternoon.

GKS

.LONG TERM...  (Sunday Night through Next Saturday)
Issued at 322 PM CST Sat Dec 3 2016

The synoptic pattern at the start of the long term period will
feature split flow aloft, with the progressive/zonal northern stream
located across the north-central CONUS, and an upper-level low
lifting out of the Southern Plains.  This regime will transition into
a broad trough across much of the CONUS by the end of next week,
allowing significantly colder temperatures to enter the region.

There will be a couple systems to focus on this period.  The first
will be associated with the ejecting southern stream trough which
will help induce a surface low across the AR/LA/TX region by Monday
night.  Despite a favorable surface low track, mid-level deformation
and associated frontogenetical vertical circulation noted on forecast
cross-sections, it appears that the low-levels are going to remain
too warm for much in the way of wintry precip. Can`t completely
rule out a few flakes trying to mix in across central and northeast
MO Monday night into Tuesday morning, but this looks mostly to be a
cold rain event.

A strong PV anomaly will then dive from the eastern Pacific into the
Central Plains Wednesday into Wednesday night, digging out a trough
across much of the central CONUS.  With the PNA finally going
negative, it appears this digging trough will have access to plenty
of cold air that has been bottled up across northwestern Canada
and northern Alaska. The surface front will have pushed well south of
the region by the time the upper-level ascent from the PV anomaly
arrives, thus expect a light band of precipitation to set up in the
postfrontal cold airmass.  This will bring a band of light snow,
possibly mixed with a few rain drops initially, to portions of
central/eastern MO and western IL.  Too early to speculate on any
light accumulations (if any), but QPF with this system looks rather
light 0.1"-0.2".

The coldest airmass of the season will arrive behind this system for
Thursday into Friday.  Highs will likely not make it out of the 20s
Thursday afternoon, with temperatures Friday morning in the 10-15
degree range.  These temperatures coupled with a northwest breeze
will cause wind chills to dip to around 0 at times late Thursday
into early Friday.

KD

&&

.AVIATION...  (For the 06z TAFs through 06z Sunday Night)
Issued at 1144 PM CST Sat Dec 3 2016

Rain has developed across central and southern MO and will
continue to increase across the entire area overnight and spread
to the east and northeast. Widespread precipitation is expected on
Sunday morning impacting all the terminals, with the precipitation
moving to the east of all the TAF sites by early afternoon.

Flight conditions are initially VFR but will begin trending
downward overnight as the precipitation continues and becomes more
widespread with MVFR conditions developing at KUIN and KCOU and
then lowering to IFR early Sunday morning, while the St. Louis
terminals will see MVFR conditions in the predawn hours lowering
to IFR around mid-morning. Rain is expected in the St. Louis
region while present indications are the rain could mix with or
change to snow at times at KUIN and possibly KCOU. Flight
conditions will gradually improve on Sunday afternoon as
precipitation exits east and low-level flow becomes more
westerly. Fog development looks like a good possibility between
06-12z Monday.

SPECIFICS FOR KSTL:

Light rain will spread into the terminal overnight with flight
conditions initially VFR then lowering to MVFR in the predawn
hours. Flight conditions will lower to IFR around mid-morning with
the steadier rain continuing until around midday. Flight
conditions will gradually improve late Sunday afternoon as
precipitation exits east and low-level flow becomes more westerly.
Fog development looks like a good possibility between 06-12z
Monday and have added a TEMPO group to account for this scenario.

Glass

&&

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

$$

WFO LSX



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