Area Forecast Discussion
Issued by NWS St. Louis, MO

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000
FXUS63 KLSX 191803
AFDLSX

AREA FORECAST DISCUSSION...UPDATED
NATIONAL WEATHER SERVICE ST LOUIS MO
1203 PM CST Fri Dec 19 2014

.UPDATE:
Issued at 1155 AM CST Fri Dec 19 2014

A few tweaks to going forecast...primarily to adjust chaotic cloud
cover over our IL counties and to raise temps there a degree or
two due to a bit more sun. Other change has been to pull mention
of drizzle over mid MO for this afternoon as stratus has
temporarily lifted here. However, did continue mention of flurries
over our se MO counties as very light echoes on n fringes of s
system showing up over nw AR, and heading to the NE.

Truett

&&

.SHORT TERM:  (Through Late This Afternoon)
Issued at 300 AM CST Fri Dec 19 2014

A weak ridge of high pressure which extends from the Great Lakes
region south through the Mid-Mississippi Valley is in control of our
weather today.  Soundings are showing plenty of low level moisture
trapped under an inversion at around 850mb.  The vertical structure
of the atmosphere is much like what we had last weekend with the
moisture stratified in the low levels, so I don`t expect much
sunshine for the majority of the area...with the possible exception
of south central and extreme southwest Illinois.  Some drier air is
trying to drift in on weak easterly low level flow.  Vertical cross
sections show intermittent weak upward motion through the day,
primarily over central Missouri, and short term guidance spits out a
hundredth of QPF here and there.  Additionally, the HRRR shows some
weak simulated radar echos over parts of central and southeast
Missouri today, so have added in a little patchy drizzle to the
forecast.

Carney

.LONG TERM:  (Tonight through Thursday)
Issued at 300 AM CST Fri Dec 19 2014

(Tonight - Saturday)

The brunt of a storm system is expected to pass relatively unnoticed
to our south this evening, and if it weren`t for some interaction
with a more northerly component, would probably result in no
precipitation anywhere, but such as it is, some very light snow
looks to be possible early this evening for an area primarily south
of STL metro.  This arrangement should be short-lived, with much of
the mid and upper level moisture quickly being shunted off to the
east, leaving behind extensive low cloud cover.  At this point,
there is good model consensus on a weak WAA, convergence regime
setting up at the low levels that noses into central MO by midnight
tonight and then snaking its way northward towards the UIN area by
Saturday morning.  Have added in patchy drizzle as a result.  For
now, expecting influence of the surface RIDGE to trump this for STL
metro and areas S and E.  Temps will probably not go very far
tonight with clouds hanging tough, although where the surface RIDGE
is dominant, should see a steady but slow drop...but for areas to
the N and W of STL metro where a light S flow will persist, near
steady temps are expected.  Speaking of temps, especially to the N
and W of STL metro where drizzle is expected to develop, readings
will be within a degree or two of the freezing mark and so this will
probably manifest more as patchy FZDZ for areas in northeast MO and
west-central IL.  Prefer the lower MOS temps for Saturday maxes
given cloud cover expected, but they should still rebound into the
upper 30s and low 40s.

(Sunday - Wednesday)

A rather deep upper level TROF is expected to develop later on
Sunday over the central CONUS and then take a good part of the week
before finally shifting east.  Maintained the highest PoPs for
Monday thru Tuesday, where there is good model consensus on when the
TROF will be close enough, yet its axis still just upstream...and
then reduce to slight chances by Wednesday when the TROF axis by
this time will have shifted to the east.

While much of the precipitation will be just rain, especially on the
front-end, enough cold air starts working in on Tuesday, especially
aloft, that will allow the atmosphere to sustain a changeover to
snow.  The only limiting factor at this point will be surface and/or
boundary layer temps that will ultimately determine the rain vs.
snow question.  GFS and EC MOS surface temps suggest this will not
happen for most locales until late in the day on Tuesday with snow
then expected as the dominant p-type from that point on.  This will
also be when the upper TROF axis will be crossing thru, and so what
is left will probably not be much, but perhaps light snow accums
will be possible Tuesday night and early Wednesday.  This will be
our only realistic chance at a white Christmas, and something to
watch, as a deeper system that digs a bit more south would up the
potential.

(Christmas)

Models have been advertising for several days now a transitional
period this day which should give us a net result of quiet wx with
near seasonal temps.  Nothing has changed with the current model
runs so will let it stand.

TES

&&

.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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