Area Forecast Discussion
Issued by NWS St. Louis, MO
FXUS63 KLSX 012130
AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE ST LOUIS MO
330 PM CST Sun Dec 1 2013
.SHORT TERM: (Through Late Tonight)
Issued at 231 PM CST Sun Dec 1 2013
Very weak cold front has settled into srn portions of MO and IL this
aftn, while surface ridge over the northern Plains was shifting ewd
into the Great Lakes region. Some of the mid-high level cloudiness
across the nrn Plains, ahead of weak shortwaves will likely advect
e-sewd into our forecast area tgt, especially the nrn half. Also
expecting the development of patchy stratus and fog late tgt, mainly
over the srn portion of the forecast area where the sfc/boundary
layer will be more moist as evident by higher sfc dew points and the
development of a little diurnal cumulus clouds this aftn. Surface
winds will also be light or calm tgt aiding in fog formation.
.LONG TERM: (Monday through Next Sunday)
Issued at 330 PM CST Sun Dec 1 2013
(Monday - Tuesday)
Upper flow will be in the process of transitioning from a NW flow to
SW, with a series of weak upper level disturbances edging the area.
The first will pass just to the S early Monday morning, with another
slated to pass just to the N and E later on Monday, and yet another
late Tuesday. Available moisture will be at or just above seasonal
normals and these weak disturbances, at least in our area, will
struggle to get any kind of measurable pcpn out. For sake of
consistency, have maintained lo PoPs.
One thing that does look likely is the mild temps will stick around
with highs 55-65 Monday and in the 60s on Tuesday.
(Wednesday - Friday Night)
In this latest episode of the models, the GFS which had been the
fastest with the cold front has slowed down just a bit for FROPA
mainly Wed morning, while the EC and GEM dig their heels in for a
slower solution Wed ngt. The NAM, which does tend to do better for
Arctic air releases of which this will be, is the fastest with a
FROPA on Tuesday night. Needless to say, there is still a lot of
disagreement on when the surface front is moving thru and this will
have a lot to say in what form pcpn will take. For now, the more
compromise GFS seems to be the way to go and will work off of that.
That said, we will be in a SW upper level flow throughout this
period with waves of disturbances riding up, resulting in
overrunning pcpn events. As the shallow Arctic air drives SE thru
our area, the SW flow aloft that will funnel these disturbances will
also cause the air aloft to cool at a much slower rate than that at
the surface and will create situations where mixed wintry pcpn will
The timing of the disturbances, and thus the main episodes of pcpn,
is still the same: mainly Thursday thru Friday. The BIG difference
from 24hrs ago is that the cold air will have an extra 12hrs to work
its way in and deepen, resulting in a snowier forecast for most
areas than what it looked like. A colder solution will favor a more
southerly track for the pcpn with UIN perhaps too far N now, COU and
STL on the northern edge but looking like all snow now. The mixed
pcpn, which will likely feature PL, is shunted to areas of SE MO and
S IL. FZRA is looking like less of a possibility. Early
indications are that up to a few inches of snow are possible near
the I-70 corridor and areas SE with some variation in the model
solutions. Best to not put too much stock in specifics at this time
but focus on trends instead as we expect additional adjustments in
the timing and track which will affect type and amount.
A cold period expected if frontal timing is correct, as Arctic air
builds in, with high temps in the 20s and mins in the single digits
by period`s end if it all plays out.
(Saturday - Next Sunday)
The cold continues with the Arctic hi in the neighborhood. EC shows
yet another system making a push on Sunday, with frozen pcpn, but
surprise--model disagreements exist once again and so cannot place
too much confidence on this just yet.
.AVIATION: (For the 18z TAFs through 18z Monday Afternoon)
Issued at 1050 AM CST Sun Dec 1 2013
Very weak cold front has just pushed se of CPS late this mrng.
Only expect light nely sfc winds at UIN and COU this aftn, and
nwly in the St Louis metro area veering around to a nely direction
by early evng. The sfc wind will remain light tgt and continue to
veer around to an e-sely direction by late tgt as the sfc ridge
over the nrn Plains moves ewd through the Great Lakes region. The
sfc wind will strengthen some from a sely direction Monday mrng.
Mid-high level clouds will advect into the region tgt. It appears
that there will be patchy fog and stratus clouds redeveloping late
tgt. The best potential for stratus clouds may be south of the taf
sites, but there will likely be at least light fog impacting the
COU and St Louis areas and possibly also UIN as well.
Specifics for KSTL: Light nwly sfc wind this aftn will gradually
veer around to a nely direction by this evng, e-sely direction by
late tgt, then strengthen to 7-9 kts from a s-sely direction Monday
aftn. Mid-high level cloudiness will advect into STL by late tgt.
May also have some light fog late tgt/early Monday mrng as well as
patchy stratus clouds mainly Monday mrng.