Area Forecast Discussion
Issued by NWS St. Louis, MO

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FXUS63 KLSX 192115
AFDLSX

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE ST LOUIS MO
315 PM CST Fri Dec 19 2014

.SHORT TERM:  (Through Late Tonight)
Issued at 309 PM CST Fri Dec 19 2014

Shortwave energizing southern stream system over the Gulf Coast is
spreading some mid and high level cloudiness across our region
this afternoon. Regional radar mosaic is indicating some very
light echoes associated with this cloud deck slowly working their
way from n AR into se MO, so will continue the mention of flurries
in our far southeast counties into the early evening hours.

Otherwise, broad area of low level moisture remains locked on SW
periphery of the surface ridge that stretches from the Great Lakes
into the lower Ohio Valley, and this low level moisture should
keep skies cloudy over most of the region even after the passage
of the aforementioned shortwave. A secondary shortwave is progged
to drop from the mid MO Valley after midnight tonight, producing
some weak low level WAA advection over western sections of the
CWA. However, forecast sounding suggest that moisture will remain
stratified in the lowest few thousand feet of the AMS with very
little moisture in layer that would allow for ice nuclei to form.
Therefore, any precip associated with the broad area of lift and
WAA would likely be in the form of drizzle, much as suggested by
going forecasts. Temperatures aren`t expected to drop more than a
few degrees from their current levels, but latest forecast of
hourly temp trends suggest that some freezing drizzle will be
possible in northern parts of the FA, primarily along and north of
a Shelbina-Bowling Green line. Relatively warm ground temps (MO Ag
site at Novelty has 2-4" soil temps between 32-35) and road
treatments from yesterday`s light snowfall should mitigate most
problems if freezing drizzle does form, but obviously we will be
monitoring this threat.

Truett

.LONG TERM:  (Saturday through Next Friday)
Issued at 309 PM CST Fri Dec 19 2014

Expect quiet weather this weekend with a gradual warming trend
from Saturday through Monday. A compact vort max moving from IA
into IL on Saturday might support light drizzle during the morning
hours.

Precipitation chances increase on Sunday night into Monday ahead of
a clipper-like system which will be developing over the northern
plains and eventually closing off over the central or eastern CONUS.
This system will probably bring periods of rain to the area from
Monday through Tuesday, especially on Monday (central MO) and
Monday night (eastern MO and western IL). Cooler temperatures are
expected behind Monday night`s cold front, and this may cause the
rain to mix with snow at times. Light snow could linger into
Wednesday, depending on how quickly the system moves away from the
area.

A brief period of shortwave ridging is anticipated during the middle
to late part of the week ahead of a developing low pressure system
which could bring widespread rain/snow to the area next
Friday/Friday night. The model solutions which depict low pressure
systems at these extended time ranges often undergo several
changes, therefore there is low confidence in the timing/location
of any precipitation for late next week.

Kanofsky

&&

.AVIATION:  (For the 18z TAFs through 18z Saturday Afternoon)
Issued at 1155 AM CST Fri Dec 19 2014

Pocket of low level dry air on the IL side of the Mississippi
River continue to make for tricky aviation forecasts. Will
primarily go with MVFR cigs from 1.5-3kft this afternoon, although
its possible that sites along the river (KUIN and STL metro TAFs)
may be on the fringes of this cloud deck. Wedge of dry air is
forecast to hold firm tonight, and while it appears likely that
IFR fog and stratus will once again form over COU in the deeper
low level moisture, trend is less clear cut for sites along the
river. It`s interesting that most guidance isn`t as bullish as
yesterday with IFR development at UIN and STL metro, in spite of
the fact that low level flow does become a bit more southerly
during the predawn hours (albeit weakly so). So, for these
locations have headed towards lower end MVFR cigs/vsbys during the
predawn hours, with the expectation that the primary IFR deck will
remain just west of the MSRVR.

Specifics for KSTL: As mentioned above, area will be on the
western fringes of a pocket of drier low level air over IL. Have
generally gone with bkn MVFR cloud deck through 06z to try to
reflect this, but a slight shift in the edge of this moisture
could mean more or less cloudiness in the MVFR range. Forecast
hints at some lower vsbys and cloudiness during the predawn hours,
but based on the preponderence of guidance have maintained
prevailing ceiling and vsbys in the MVFR range.

Truett

&&

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

$$

WFO LSX






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