Area Forecast Discussion
Issued by NWS St. Louis, MO

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580
FXUS63 KLSX 150558
AFDLSX

Area Forecast Discussion...Updated Aviation
National Weather Service Saint Louis MO
1158 PM CST Sat Dec 14 2019

.UPDATE...
Issued at 919 PM CST Sat Dec 14 2019

Only some slight changes needed to the tonight portion of the
forecast to account for southeastward clearing of stratus. However
high and mid clouds from the Plains will spread back over the area
tonight in the wake of the diminishing lower clouds. I have
slightly tweaked lows down. Tonight is tranquil.

The anticipated winter storm is still on track for Sunday into
Monday. The new deterministic 00Z NAM and latest RAP are still on
target with snow spreading west to east across the area on Sunday,
beginning in central MO from 8-10 am, far eastern MO from 10 am to
noon, and across western IL during the afternoon. These
deterministic model runs have also shifted the axis of heaviest
snow a bit further south centered along the I-70 corridor.
Impressive isentropic lift and frontogenetic forcing is expected
to support a rather wide snow band with rates at times from 0.5 to
1 inch per hour. If these solutions are in fact correct with the
intensity and location then are current snowfall amounts during
the afternoon would need to be doubled, and closer to 4-5 inches
along this axis. I hesitate making alot of changes to the forecast
at this time based on 2 deterministic models when a closer
evaluation of the full model suite and ensembles may not fully
support this decided south shift. None the less, hazardous travel
is expected on Sunday and into Sunday night. The current winter
storm watch still is reasonable and will be converted overnight to
a series of winter storm warnings and advisories.

It still appears that the first wave of snow will exit by Sunday
evening and many areas may lose ice nuclei thus light freezing
rain or drizzle would occur. Another round of snow would then
impact the region on Monday/Monday evening.

Glass

&&

.SHORT TERM... (Through Late Monday Night)
Issued at 337 PM CST Sat Dec 14 2019

Early this afternoon, a very broad-based TROF extended over much of
the CONUS with a W-SW flow aloft over our region.  The strong storm
system that we have been watching carefully has come onshore and was
located over northern Arizona.  At the surface, a cold front was to
the southeast of our region with colder air spilling in on NW winds.
 An extensive area of clouds existed over much of the Midwest with
clearing in Iowa but additional cloud cover in Kansas already
sliding east towards our region that will likely limit any clearing
over our way.  Temperatures were in the 30s for most of the area.

Quiet weather is expected tonight as we will be in-between systems.
In the meantime, colder air will settle in from a 1025mb area of
high pressure, resulting in min temps early Sunday morning mainly
in the lower to mid 20s.  This will be key for what happens next.

The well-advertised next storm system continues to look set for
Sunday through Monday night.  Confidence continues to increase on
the general characteristics:

(1)  Two main waves of precipitation:  one focused on Sunday
afternoon-evening and the other one on Monday, lingering into Monday
night.  This longer duration into Monday night is a key change.

(2)  The first wave on Sunday will primarily be in the form of snow
and will be driven by frontogenesis in a weak but slowly
strengthening broad WAA regime.  The trend for this area seems to
have settled on a region near the Interstate 70 corridor, especially
for what could be focused bands of snow.  If this materializes,
these bands of snow will be capable of heavy bursts of snow with
rates in excess of 1"/hour.  The cold surfaces will allow snow to
accumulate on any untreated surfaces and result in hazardous travel.
 This episode is expected to exit by late Sunday evening.  Where it
lingers the longest will be capable of several inches of snow.

(3)  The interlude between the two main waves of precipitation
will begin moving in from the southwest Sunday evening and
continue into early Monday morning and will feature intermittent
freezing drizzle or light freezing rain and a resultant ice
accretion for most areas.  Only where cloud ice can hang on better
in far northern MO will precipitation types of snow linger.

(4)  Cloud ice will be re-introduced enough with the concluding wave
of precipitation after sunrise on Monday and looks to now
linger into Monday night with the upper level system moving slower,
but with the broad moderate WAA regime continuing.  Two upper level
impulses, one lead one Monday morning will bring cloud ice back into
the equation for much of the area near and north of I-70, should
transition the freezing drizzle back over to snow, with the main
system set for Monday afternoon and evening, expanding coverage of
snow at the expense of freezing drizzle into the southern portions
of the forecast area (southeast MO and far southern IL).  This
second wave of precipitation, especially where it looks to
transition back to and continue as snow looks now to intersect much
of the area that will benefit most from the first wave of snow on
Sunday.

(5)  Adding it all up, there continues to be a threat for several
inches of snow for areas near and north of I-70.  Much of this
area, in fact, is flirting with Warning (6") criteria and with the
trends shaping up, this threat for higher snow totals is edging
closer to the I-70 corridor.  For this reason, we are continuing the
Watch, but beginning it sooner and extending it longer in time.  The
southern portion of the Watch will be converted to an Advisory where
lower snow totals and icing will be an issue.

TES

.LONG TERM... (Tuesday through Next Saturday)
Issued at 337 PM CST Sat Dec 14 2019

Medium range guidance has been very consistent in keeping the mid
Mississippi Valley in cold, dry northwest flow Tuesday and into
early Wednesday behind the storm.  It`s unlikely that temperatures
will rise above freezing across northeast Missouri and west central
and south central Illinois with fresh snow on the ground, however
some mid and upper 30s look likely across parts of southeast
Missouri.  Wednesday morning continues to look like the coldest of
the week due to a clear sky, light wind, and the snow cover.  Lows
in the low to mid teens across most of the forecast area still looks
probable. Models show the low level ridge moving east of the
Mississippi Valley Wednesday as the upper level ridge builds into
the Great Plains.  The combination of increasing southerly low level
flow and rising heights will bring warmer air back to Missouri and
Illinois for the latter half of the week.  Areas with snow on the
ground will still be chilly Wednesday with heights struggling up to
near freezing, but parts of southeast and central Missouri should
warm up to the low to mid 40s.

The upper level ridge axis is forecast to move east of the
Mississippi River on Thursday.  Low level warm advection will
continue with southwest flow Thursday and Friday and highs are
expected to climb above normal values for Thursday and Friday. ECMWF
has trended toward the GFS which brings enough low level moisture
out of the Gulf to produce some light rain across Arkansas and
southern Missouri ahead of a short wave trough on Friday. There`s
still little if any precip indicated in our area, so have kept
Friday`s forecast dry for the time being.  Saturday looks dry as
well with the trough to our east...though it`s a low confidence
temperature forecast with GFS showing slightly different surface
patterns with the GFS being the cooler of the two.

Carney

&&

.AVIATION...  (For the 06z TAFs through 06z Sunday Night)
Issued at 1142 PM CST Sat Dec 14 2019

Stratus has largely cleared south of all but KCPS at 0530z. The
more solid edge should clear south of KCPS within the next few
hours although some patchy MVFR stratus could linger at KCOU and
the KSTL/KSUS as well through 07-08z. VFR flight conditions are
then expected to persist until around mid-morning Sunday.
Thereafter snow will develop/spread from western MO across the
region during the later part of the morning and through the
afternoon. Flight conditions will deteriorate after the onset of
snow eventually lowering to IFR with moderate snow possible at the
St. Louis terminals and KCOU. Snow will eventually change to
light freezing rain or drizzle during the evening.

SPECIFICS FOR KSTL:

Some patchy MVFR stratus will be possible through 07-08z,
otherwise VFR flight conditions are then expected until late
morning Sunday. Snow will develop/spread into the terminal from
the west near midday, with intensity increasing during the
afternoon. Flight conditions will deteriorate after the onset of
snow eventually lowering to IFR during the afternoon, with
moderate snow possible. Snow will eventually change to
light freezing rain or drizzle during the evening.

Glass

&&

.LSX WATCHES/WARNINGS/ADVISORIES...
MO...Winter Weather Advisory from 9 AM Sunday to midnight CST Monday
     night for Crawford MO-Iron MO-Madison MO-Reynolds MO-Saint
     Francois MO-Sainte Genevieve MO-Washington MO.

     Winter Storm Watch from 9 AM CST Sunday through Monday evening
     for Audrain MO-Boone MO-Callaway MO-Cole MO-Franklin MO-
     Gasconade MO-Jefferson MO-Knox MO-Lewis MO-Lincoln MO-
     Marion MO-Moniteau MO-Monroe MO-Montgomery MO-Osage MO-Pike
     MO-Ralls MO-Saint Charles MO-Saint Louis City MO-Saint
     Louis MO-Shelby MO-Warren MO.

IL...Winter Weather Advisory from 9 AM Sunday to midnight CST Monday
     night for Randolph IL.

     Winter Storm Watch from 9 AM CST Sunday through Monday evening
     for Adams IL-Bond IL-Brown IL-Calhoun IL-Clinton IL-Fayette
     IL-Greene IL-Jersey IL-Macoupin IL-Madison IL-Marion IL-
     Monroe IL-Montgomery IL-Pike IL-Saint Clair IL-Washington
     IL.

&&

$$

WFO LSX



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