Area Forecast Discussion
Issued by NWS St. Louis, MO
FXUS63 KLSX 252140
AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE ST LOUIS MO
340 PM CST WED NOV 25 2015
.SHORT TERM: (Through Late Tonight)
Issued at 340 PM CST Wed Nov 25 2015
First weak shortwave trough is currently exiting to the northeast
but area will stay in persistent southwesterly flow aloft through
tonight. There will be some weak vort maxes that will move through
the flow aloft that will provide some weak ascent above increasing
moisture convergence because of the low level jet. Rain chances
will increase through the late evening and overnight over central
and northeast Missouri as well as west central Illinois where the
best low level moisture convergence will be.
Temperatures will not drop too much tonight with strong warm air
advection and dewpoints rising through tonight. Expect lows to be
closer to the warmer MAV MOS guidance.
.LONG TERM: (Thursday through Next Wednesday)
Issued at 340 PM CST Wed Nov 25 2015
(Thanksgiving Day - Saturday Night)
A strong wound-up storm system over the Great Basin region of the
western CONUS is expected to move little during this period,
maintaining an active southwest flow aloft over our region, and
shooting out pieces of energy or distrbances our way as well.
The first of these disturbances is expected to arrive late Thursday
afternoon, with essentially a near continuous train thru late
Saturday after that. This will be further augmented by a slow
moving cold front and rich moisture from the Gulf of Mexico. The
cold front will begin moving thru late Thursday afternoon in
northeast MO and will take all of Friday and much of Friday night
to move thru the remainder of the forecast area.
Despite much cooler air ushering in behind the cold front,
temperatures are expected to stay above freezing for most areas thru
this entire period and will keep pcpn-types as all rain.
The one exception could be in parts of northeast MO and west-central
IL late Friday night and early Saturday morning, where temps will be
around the freezing mark. Fortunately, much of the pcpn by that
time should be to the south with only light spotty pcpn currently
Otherwise, the main concern will be the prolonged period of heavy
rainfall--especially for parts of southeast MO and far southern IL--
where rainfall totals may top 4 inches by Saturday morning. A Flood
Watch has already been issued and considering that the rainfall
forecast has changed little since then, if not the heavy QPF axis
sliding a bit further south, no change was needed to the current
area. The only adjustment seriously considered was an extension in
time into Saturday for southeast MO and far southern IL, but wanted
to take another look or two at that to see if the southern trend
Temps will be quite mild for late November ahead of the front, but
will rapidly drop to below average for daytime maxes behind it, with
readings in some areas not getting out of the 30s. Fortunately,
plenty of warm air aloft should keep the pcpn as rain unless surface
temps can slip below 32F.
(Sunday - Next Wednesday)
Model solutions continue to vary quite a bit out in this period but
the general theme continues to be the storm out west gets nudged
into the main flow and tracks generally east, passing mainly to our
north by Tuesday sliding a surface TROF thru on Monday. Looks like
another round of primarily rain on Monday with this TROF. Temps
look to be around average for this time of year, with clear absence
of any significant cold air surges from the north, but also absent
any strong influence from either the moist Gulf of Mexico or the
much milder air to the south.
.AVIATION: (For the 18z TAFs through 18z Thursday Afternoon)
Issued at 1159 AM CST Wed Nov 25 2015
A few sprinkles may still affect the St. Louis metro TAF sites early
in the TAF period, otherwise expect most of the afternoon to be dry.
Expect clouds to move back into the area this afternoon and ceilings
to be either high MVFR or low VFR during the period. There will be
some potential for increasing rain chances, particularly at KUIN and
KCOU on Thursday morning. A low level jet will develop this evening
and have added LLWS this at all of the terminals.
Specifics for KSTL: May still be a few sprinkles early this afternoon
at the terminal along with the possibility of high MVFR ceilings, but
currently think that ceilings will stay in the low VFR range. A low
level jet will develop over the area by early this evening, so have
added LLWS this evening. There will be the potential for scattered
rain showers to develop over the area by late tonight or during the
day tomorrow, but have kept them out of the forecast for now as I do
not have enough confidence on timing. There also is the possibility
of high MVFR ceilings developing at the terminal at times tonight
MO...Flood Watch from Thursday evening through late Friday night FOR
Boone MO-Callaway MO-Cole MO-Crawford MO-Franklin MO-
Gasconade MO-Iron MO-Jefferson MO-Madison MO-Moniteau MO-
Montgomery MO-Osage MO-Reynolds MO-St. Charles MO-St.
Francois MO-St. Louis City MO-St. Louis MO-Ste. Genevieve
MO-Warren MO-Washington MO.
IL...Flood Watch from Thursday evening through late Friday night FOR
Clinton IL-Madison IL-Monroe IL-Randolph IL-St. Clair IL-