Area Forecast Discussion
Issued by NWS St. Louis, MO

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FXUS63 KLSX 251746
AFDLSX

AREA FORECAST DISCUSSION...UPDATED AVIATION
NATIONAL WEATHER SERVICE ST LOUIS MO
1146 AM CST Thu Dec 25 2014

.SHORT TERM:  (Through Late This Afternoon)
Issued at 301 AM CST Thu Dec 25 2014

A weak ridge aloft was over the Plains early this morning with a
strong storm system digging thru the inter-mountain West. The only
effect of this right now was a large area of high clouds being
ejected out into the Plains. At the surface, a ridge of high
pressure was overhead with winds already backing from the SW over
much of our area, and from the S in western MO. A large area of
low clouds still prevailed over much of the forecast area, but as
of 08z, the back edge extended from near KIRK to KPOF, leaving much
of central MO clear. Temps were in the 30s area wide, but the
lowest values near the freezing mark were found where the clouds
have cleared out, although the southerly winds were preventing
them from dropping too much. The radar was pcpn echo free,
although if there was any, it was very light and likely over in
parts of southern IL.

The weak upper level ridge will move to our east this afternoon
tilting the flow from the SW aloft, while southerly flow will
strengthen at the surface. Given the current rate of clearing, we
should see the low clouds get thru the UIN area around 11z, the
STL metro area between 11-13z and the SLO area around 15z. This
will leave just some thin hi clouds to deal with the remainder of
the day and the long advertised sunny Christmas day will come to
fruition.

Given the highly anticipated decent levels of sunshine with strong
southerly flow will usually give us a diurnal swing of about 20
degrees this time of year, and so should yield upper 40s to mid
50s for max temps...from central IL to central MO. This comes
close to the higher MAV MOS numbers, although in some cases, went
above the MAV.

TES

.LONG TERM:  (Tonight through Wednesday)
Issued at 301 AM CST Thu Dec 25 2014

Winds will continue backing tonight and become southerly to
south-southwesterly ahead of a strong low pressure system in the
southwestern CONUS. Continued WAA at low levels will produce
another warm December day with daytime highs in the lower 50s on
Friday, which is 10-15 degrees above average. A vort max will drop
down the back side of the upper trough on Friday, leading to two
separate upper disturbances. The first one will lift through the
central CONUS on Friday night and Saturday, bringing a chance of
rain to most of the CWA. This upper disturbance and its surface
reflection will send a cold front through the area on Friday night
and Saturday. It wouldn`t be surprising if a few snowflakes mixed
in with the rain on Saturday and Saturday night in the colder air,
but little to no accumulations are expected due to the brief
duration of any snow as well as the warm ground temperatures.

The second upper disturbance becomes more of an elongated and
sheared vorticity axis for a while as it moves through northern
Mexico and lifts into the southern CONUS. This system is currently
forecast to pass well south of the area on Sunday night and
Monday with no effects on sensible wx for the LSX CWA.

Meanwhile, a new trough will be developing over the western CONUS
during the early part of the week. ECMWF/GFS/GEM all show the H5
trough becoming nearly zonally oriented and closing off, although
there are differences regarding the trough axis orientation and
location of the closed H5 low. 25/00z ECMWF and GFS have very
different depictions of how this system eventually progresses
eastward late next week, which is very common for model forecasts
at extended ranges. An Arctic air mass is still expected to become
dislodged and move into the northern and central CONUS next week,
bringing well-below normal temperatures to the area. The
significance of the Arctic air mass is that it would provide
pre-existing cold air (especially at low levels) into which any
precipitation might fall once the large and complex low pressure
center finally does lift out of the southwestern CONUS late next
week and early next weekend.

Kanofsky

&&

.AVIATION:  (For the 18z TAFs through 18z Friday Afternoon)
Issued at 1132 AM CST Thu Dec 25 2014

Quiet conditions with a VFR fcst thru tonight with high pressure
dominating the area. Return flow on the backside of the high will
allow for potential LLWS tonight. The set up looks marginal attm
with sfc winds around 10kts increasing to 30kts around 1kft to
40-50kts near 2kft. Will let later shifts determine the need to
include it in the fcst. MVFR CIGs are expected to move into the
terminals late Friday mrng as better moisture works back into the
area.

Specifics for KSTL:

VFR fcst thru tomorrow mrng with high pressure in control before
MVFR CIGS move into the terminal around 18Z. There is the
potential for marginal LLWS tonight but will late later shifts
address the need to include it in the fcst.

2%

&&

.LSX WATCHES/WARNINGS/ADVISORIES:
MO...NONE.
IL...NONE.
&&

$$

WFO LSX





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