Area Forecast Discussion
Issued by NWS St. Louis, MO

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FXUS63 KLSX 072335

Area Forecast Discussion...Updated Aviation
National Weather Service Saint Louis MO
535 PM CST Thu Dec 7 2017

.SHORT TERM...  (Through Late Friday Afternoon)
Issued at 220 PM CST Thu Dec 7 2017

This afternoon, the upper air pattern reflects a longwave RIDGE
over coastal western North America and a TROF over much of the
middle and eastern portions of North America, with the TROF axis
extending from the western Great Lakes southwestward to southern New
Mexico.  Several imbedded shortwave disturbances existed within the
resultant N-NW flow aloft across the central and north-central
CONUS.  The earlier clouds have cleared out of most of our region
with only a few remnants in portions of southeast MO and southern
IL.  Despite the abundant sunshine at present, the air is quite
chilly with temps in the upper 20s and lower 30s.

The TROF axis aloft will work its way thru our region tonight but
uneventfully, with clear skies expected to persist.  Combined with
the presence of a weak RIDGE at the surface, should allow temps to
plunge into the teens by early Friday morning, with some single
digits possible in sheltered valley areas.  Easily the coldest night
of the young winter season and coldest since mid March.

A weak imbedded shortwave disturbance will approach and move thru on
Friday, but will be able to do little more than provide some mid and
high level cloud cover.  Thanks to SW surface winds in the wake of
the RIDGE axis, temps will recover closer to averages for early
December, but still about 5 degrees below--or in the upper 30s and
lower 40s.


.LONG TERM...  (Friday Night through Next Thursday)
Issued at 220 PM CST Thu Dec 7 2017

The overall upper pattern will persist for the foreseeable future,
with NW flow aloft for our region.  Imbedded shortwave disturbances
will also continue to be a theme, but the models` ability to
effectively resolve these will diminish sharply beyond 2-3 days.  As
it stands now, there are three potential shortwave disturbances that
could affect our region over the next week.  Otherwise, the pattern
is strongly supportive of below average temps for most days in this
period with a brief one day warmup here and there possible between
airmasses, but also more notably, no strong cold air outbreaks

The first disturbance continues to target the late Friday night and
early Saturday morning timeframe.  Little change in anything here:
the vort strength looks impressive but the track is unfavorable for
significant measurable snow chances in our area with only some far
eastern portions of our region getting any PoPs.  That said, a
period or two of flurries looks plausible for areas further to the
west to about the Mississippi River.

The second disturbance(s) continues to look set for Monday but a new
development has occurred since 24 hours ago, with both the EC and
GFS preferring a depiction involving two distinct vort centers.  The
eastern one tracks too far east to have any real impact for us and
the western one could be in a good position to affect most areas of
our forecast area.  No operational model is currently outputting non-
zero QPF for this, but a few ensemble members are.  Not surprising
that a dry forecast is currently preferred but stay tuned here, as
the models typically do not get a handle on something like this
until within 3 days, so in another day or two.  But current timing
of the system will favor liquid pcpn.

A third disturbance could occur for the middle of next week, but the
resolution on features this far out is so poor as to not warrant too
much interest at this point.

Temps look to be below average for most days except Sunday and
Monday, but the cold airmasses dropping down do not look anomalously
cold either.



.AVIATION...  (For the 00z TAFs through 00z Friday Evening)
Issued at 529 PM CST Thu Dec 7 2017

Surface high pressure system centered in southern KS will weaken
and slide south-southeast and into the lower MS Valley over the
next 24 hours. This will result in light and slowly backing
surface winds tonight. VFR flight conditions will prevail through
the period with clear skies tonight and increasing mid-high
clouds during the day on Friday.


VFR flight conditions will prevail through the period with clear
skies tonight and increasing mid-high clouds on Friday. Surface
winds will be light and slowly backing tonight, becoming
southwesterly on Friday.



Saint Louis     19  42  28  34 /   0   0  10   0
Quincy          14  38  24  31 /   0   5  10   0
Columbia        14  40  25  35 /   0   5   5   0
Jefferson City  14  42  25  35 /   0   5   5   0
Salem           16  38  26  35 /   0   0  10  10
Farmington      11  40  25  36 /   0   0  10   0




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