Area Forecast Discussion
Issued by NWS St. Louis, MO

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FXUS63 KLSX 210446
AFDLSX

Area Forecast Discussion...Updated Aviation
National Weather Service Saint Louis MO
1046 PM CST Mon Feb 20 2017

.UPDATE...
Issued at 914 PM CST Mon Feb 20 2017

A line or narrow band of showers and isolated thunderstorms
extending from near VIH to UIN was advancing slowly eastward,
with lighter more spotty showers west of the line. This activity
was ahead of an upper level trough with embedded shortwaves and
ahead of an associated weak cold front. This precipitation will
continue shifting eastward, moving through the St Louis metro area
during the late evening and overnight hours. The few embedded
storms and heavier showers may weaken by late tonight with more of
a broader band of light showers or rain moving through southeast
MO and southwest IL late tonight into early Tuesday morning. Lows
tonight will be unseasonably warm again due to the cloud cover and
relatively high surface dew points.

GKS

&&

.SHORT TERM...  (Through Late Tuesday Afternoon)
Issued at 322 PM CST Mon Feb 20 2017

Showers associated with weak trof over central Missouri are fairly
sparse as they try to track slowly to the east into drier air. Also,
mid level lapse rates and decent MU CAPES have failed to materialize
so far. So have slowed down onset of precipitation most locations
for rest of this afternoon with better chances by early this evening
over central/northeast MO and west central Illinois. Then as trof
slides east, activity to spread across rest of forecast area. In the
meantime, gusty south winds to diminish towards sunset and mid/high
clouds to thicken up. Another mild night is in store despite main
cold front moving through towards daybreak, will see lows in the
upper 40s to mid 50s.

Precipitation to taper off from west to east on Tuesday, coming to
an end by midday. Not a lot of CAA behind this system, so will still
see well above normal highs in the upper 60s to low 70s, with clouds
slowly scattering out from northwest to southeast through out the
day. If it clears out quicker tomorrow, high temps will have to be
raised accordingly.

Byrd

.LONG TERM...  (Tuesday Night through Next Monday)
Issued at 322 PM CST Mon Feb 20 2017

In the wake of Tuesday`s system, heights aloft will gradually lower
while winds back to more west-southwesterly Tuesday Night-Wednesday.
This will occur in conjunction with the onshore movement of the next
upstream Pacific trof, as well as a series of low-amplitude short
wave trofs tracking from the Northern Rockies into the Upper MS
Valley. Weak surface high pressure present on Tuesday night will
shift southeast and weaken, and pronounced low level WAA will be
well underway by midday Wednesday. There will be a fair amount of
high clouds, however current thinking is that record highs are still
possible...again.

Continued movement of the northern stream short-wave train into the
Great Lakes region will bring a cold front into the northern CWA
late Wednesday night, with the front becoming quasi-stationary
across northern MO into central IL during the afternoon. Above
average temperatures will persist despite the front, and south of
the front it will be once again be unseasonably warm with records
possible depending on cloud cover. Steep mid level lapse rates
combined with increasing low level moisture via a strengthening/backing
southerly LLJ, will result in elevated instability to the north
of the west-east front, and support the potential for cool sector
thunderstorms Thursday afternoon and night. The warm sector at
this time still appears to be quite capped owing to limited
boundary layer moisture and a stout EML.

The models remain consistent despite some timing/speed issues for
Thursday night into Friday, ejecting a strengthening upper trof
across the Plains and into the MS Valley. In response a deepening
surface low will track from KS into NW MO by 12Z Friday, and then
into the western Great Lakes region by Friday evening. This track
will allow the west-east boundary to retreat northward on Thursday
night, expanding the warm sector across the region. A dryline
followed by the cold front will then sweep across the CWA on
Friday, attendant with the movement of the surface low. The warm
sector ahead of the advancing dryline still appears to remain
capped until afternoon when the front nears and moves east of the
MS River. At that time, strong height falls/cooling aloft will
weaken the cap which should allow for isolated to scattered
thunderstorms across eastern portions of the CWA. Ahead of the
dryline/cold front on Friday it will once again be rather warm,
while in the wake of the cold front, gusty northwest winds and
good CAA will bring falling temps.

The models continue to flip-flop on potential for precipitation back
in the cold air on Friday night into early Saturday. The GFS at this
time is a bit more agressive owing to a bit deeper upper trof and
vorticity maxima. Confidence is low, but enough to mention a chance
of light snow for northeast MO and west central IL.

A fast and more quasi-zonal flow aloft evolves during the weekend.
Seasonably cold temperatures are then expected Saturday. Confidence
wanes Saturday night into early next week. The models are showing
differences resolving fast-moving low-amplitude short waves in the
aforementioned quasi-zonal flow. There could be one or two periods
of precipitation in this time frame, including snow for northern
parts of the CWA.

Glass

&&

.AVIATION...  (For the 06z TAFs through 06z Tuesday Night)
Issued at 1023 PM CST Mon Feb 20 2017

A band of showers will continue to advance eastward through the St
Louis metro area overnight.  This activity was ahead of an
approaching upper level disturbance and associated weak cold
front. The light showers will likely shift east of UIN and COU by
08Z Tuesday and east of the St Louis metro area by 12Z Tuesday.
The prevailing cloud ceilings will lower late tonight and early
Tuesday morning, likely down to the MVFR catagory. The clouds will
clear out Tuesday afternoon in UIN and COU and in the St Louis
metro area Tuesday evening. The surface wind will become light
late tonight, then remain weak on Tuesday with weak surface
ridging over northwest MO and a weak surface low over AR.

SPECIFICS FOR KSTL: A band of showers will continue to advance
eastward through the STL area overnight. This activity was ahead
of an approaching upper level disturbance and associated weak cold
front. The light showers will likely shift east of the STL area by
12Z Tuesday. The prevailing cloud ceiling will lower late tonight
and early Tuesday morning, likely down to the MVFR catagory. The
clouds will clear out Tuesday evening. The surface wind will
become light late tonight, then remain weak on Tuesday with weak
surface ridging over northwest MO and a weak surface low over AR.
A clear sky and light surface wind will likely lead to fog
development late Tuesday night.

GKS

&&

.CLIMATE...
Record Highs

      2/20    2/21    2/22    2/23

STL: 78/2016 79/1935 78/1995 77/1996
COU: 76/2016 76/1935 76/1995 73/1933
UIN: 72/2016 69/1930 70/1922 70/1922


&&

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

$$

WFO LSX



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