Area Forecast Discussion
Issued by NWS St. Louis, MO
FXUS63 KLSX 020603
AREA FORECAST DISCUSSION...UPDATED AVIATION
NATIONAL WEATHER SERVICE ST LOUIS MO
1203 AM CST Mon Dec 2 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 06z TAFs through 06z Monday Night)
Issued at 1144 PM CST Sun Dec 1 2013
Weak surface frontal boundary extending from southern Ohio through
southern Missouri continues to weaken with tme. Surface High over
Great Lakes region continues to bring weak easterly flow over
area. Satellite imagery over southwest Missouri appears to expand
northward and may affect COU area. Will amend current TAF for SC
deck. Visibilities may drop to 3 to 5 miles at several TAF sites
mainly after 1000 UTC and continue to 1400 UTC. Scattered clouds
mainly around 030 to 040 expected with patchy mid-level clouds.
Surface winds will become southeasterly 6 to 8 kts then south-
southwest 7 to 9 kts during Monday afternoon.
Specifics for KSTL:Scattered mid to high clouds will begin after
0800 UTC continue through Monday morning. Visibilities will may
drop to 4 to 5 SM in light fog after 1000 UTC and may persist
through 1400 UTC. After 1400 UTC scattered clouds 040 and patchy
mid-level deck will continue through much of Monday. Surface winds
will become southeast 5 to 7 kts towards daybreak and then
gradually shift to SSW direction 7 to 9 kts by afternoon.