Area Forecast Discussion
Issued by NWS St. Louis, MO

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840
FXUS63 KLSX 110548
AFDLSX

Area Forecast Discussion...Updated Aviation
National Weather Service Saint Louis MO
1148 PM CST Sat Dec 10 2016

.UPDATE...
Issued at 946 PM CST Sat Dec 10 2016

Warmer conditions this evening due to an overcast sky across most
of the forecast area, strong southwesterly low level winds, and
southeasterly surface winds. Temperatures have become nearly
steady or have even risen slightly this evening. It appears that
any measureable precipitation will remain north of the forecast
area for the rest of the night. The surface/boundary layer is
still quite dry as reflected by the relatively low surface dew
points. There may be some virga with any light precipitation
evaporating before reaching the ground. Could not totally rule
out a few flurries across northeast MO and west central IL late
tonight and sprinkles or a little sleet for central MO with
evaporative cooling.

GKS

&&

.SHORT TERM...  (Through Late Sunday Afternoon)
Issued at 334 PM CST Sat Dec 10 2016

Weak warm front continues to lift northward through region this
afternoon. So will see southeast winds veer to the south this
evening. Otherwise, mid and high clouds to prevail tonight which
will keep temperatures from dropping much after sunset. Will
actually see steady or slowly rising temps in some locations.

By early Sunday morning, main cold front to begin moving eastward
across the forecast area. Low level moisture will be on the
increase, but qpf amounts should be on the low side with this
system. With warm southerly winds and warm air aloft, most of
precipitation for our area will be in the form of rain. Will briefly
see isolated areas, mainly along and north of I-70, of light
freezing rain, but no ice accumulation is expected. By 15z Sunday,
temperatures will be warm enough for rain through the rest of the
daytime period. Despite clouds and precipitation, temperatures will
warm up into the upper 30s to mid 40s.

Byrd

.LONG TERM...  (Sunday Night through Next Saturday)
Issued at 334 PM CST Sat Dec 10 2016

Broad fast cyclonic flow aloft will be present across the CONUS late
Sunday and this will transition to an increasingly west-northwest
flow aloft by midweek as a large upper vortex drops southeastward to
Quebec. A series of fast-moving low amplitude short waves within
the flow and associated surface boundaries will be responsible for
any sensible weather.

The first of these features and the associated cold front will move
across the region late Sunday afternoon and Sunday night. All
indications are that the precipitation with this system will be pre-
frontal driven by low-level WAA and large scale ascent with the
upper wave. Surface temperatures should all be above freezing at the
start of Sunday evening in advance of the front where the precipitation
will reside, and little if any precipitation is expected in the
wake of the cold front which would bring in deeper cold air to
account for any precipitation type change. Shallow cold air will
spread into the area through the night Sunday in the wake of the
cold front as the accompanying high pressure system builds into
the area. The high pressure system will dominate the area on
Monday with seasonably cold temperatures. Yet another fast-moving
short wave and attendant cold front will move across the region on
Monday night. This will bring both deeper and an increasingly cold
air mass into the area as another high pressure system builds into
the region. The expansive surface high will dominate the area
Tuesday into Tuesday night with colder temps than the previous 36
hours. Yet another short wave and accompanying cold air surge will
move across the area on Wednesday. The GFS and ECMWF are at odds
on any snow with this system and have flipped solutions advertised
24 hours ago. The GFS is dry while the ECMWF suggests a west-east
band of light snow driven largely by mid-level frontogenetical
forcing could impact northern portions of the CWA on Wednesday
morning. Confidence is low and the forecast maintains some low
pops given this uncertainty. This later reinforcing cold surge is
quite formidable with well below average temps Wed-Thurs.

A big pattern change will occur from midweek into next weekend as
the large vortex in southeast Canada and broad cyclonic flow/trof
shift well into the Atlantic, with a new deep trof carving out in
the western U.S. and upper ridging in the eastern U.S.. For our area
this results in a period of rising heights aloft and backing upper
flow to southwesterly by the end of the week. There are some rather
significant differences for a potent system forecast to impact the
area Friday into Saturday, both in terms of speed/longevity in
precipitation and also the depth of the cold air and p-type. Both
the latest GFS and ECMWF are warmer than previous runs and suggest
that this precipitation event will unfold on Friday morning with
snow or a wintry mix at the onset and then a transition through
p-types as strong WAA and the elevated warm layer spread
north/northeast in advance of the low level cyclone. Confidence is
low on the details with lots of spread in the GEFS ensemble
members and due to the pattern shift. The forecast at this time
will hedge from mentioning alot of mixed precipitation, and will be
primarily snow. However we will need to transition to this more
wintry mix scenario if more consistency in solutions becomes
evident.

Glass

&&

.AVIATION...  (For the 06z TAFs through 06z Sunday Night)
Issued at 1139 PM CST Sat Dec 10 2016

Southeast surface winds will continue late tonight along with
VFR, low level clouds as a surface ridge over the TN Valley region
and Mid Atlantic region shifts eastward with surface low pressure
moving into the Plains. A southwest low level jet will bring
increasing low level moisture into the area late tonight into
Sunday ahead of an approaching cold front. Rain will spread into
the taf sites late Sunday morning and afternoon along with the
cloud ceiling dropping into the MVFR catagory as the low levels of
the atmosphere saturate. The cig heights may at least briefly drop
into the IFR catagory Sunday afternoon or evening. Surface wind
will be gusty Sunday morning and veer around to a south-southwest
direction Sunday afternoon. The rain should shift southeast of the
taf sites Sunday evening with the surface wind veering around to a
west-northwest direction after fropa.

SPECIFICS FOR KSTL: Southeast surface winds will continue late tonight
along with VFR, low level clouds as a surface ridge over the TN
Valley region and Mid Atlantic region shifts eastward with surface
low pressure moving into the Plains. A southwest low level jet
will bring increasing low level moisture into the area late
tonight into Sunday ahead of an approaching cold front. Rain will
spread into STL Sunday afternoon along with the cloud ceiling
dropping into the MVFR catagory as the low levels of the
atmosphere saturate. The cig heights may at least briefly drop
into the IFR catagory Sunday evening. Surface wind will be gusty
Sunday morning and veer around to a south-southwest direction
Sunday afternoon. The rain should shift southeast of STL Sunday
evening with the surface wind veering around to a west-northwest
direction after fropa late Sunday evening.

GKS

&&

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

$$

WFO LSX



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