Scientific Forecaster Discussion
Area forecast discussion...updated aviation
National Weather Service St Louis MO
1140 PM CST Wednesday 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 Continental U.S. 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
Illinois 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 Illinois--
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 quantitative precipitation forecast 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 trough thru on Monday. Looks like
another round of primarily rain on Monday with this trough. 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 06z tafs through 06z Thursday night)
issued at 1140 PM CST Wed Nov 25 2015
Ceilings across the forecast area have increased throughout the evening as
some lower level dry air as worked north across the region.
However, both rap and NAM low level relative humidity progs are forecasting
renewed saturation during the predawn hours. Since sref MVFR cig
probabilities are also coming up from southwest MO into west
central Illinois during the morning hours on Thursday, have introduced a
tempo for MVFR ceilings (1500-2000 ft) during the morning at both
kcou and KUIN. Some lowering of ceilings are also expected in the
stl Metro area, but believe here the bases will remain above 3kft.
Strong southerly low level flow also persists, so will maintain
low level wind shear in the forecast until gusts come up early Thanksgiving
Precip is going to remain a hit and miss proposition. Spotty
showers currently extending from mid MO to central Illinois will likely
persist for several hours, and it's certainly possible that
additional spotty, light, warm air advection-driven showers will reform over the
northwest half of the County Warning Area throughout the predawn and morning hours
on Thursday. However, it certainly appears that the main rain
threat will be entering the area tomorrow evening, and
especially after 06z, as cold front finally pushes into western
sections of the County Warning Area. Not only will the rainfall become heavier
and widespread, but ceilings and visbilibites will likely take a
nose dive with frontal passage at both kcou and KUIN after 06z.
Specifics for kstl: VFR ceilings aoa 4kft are expected overnight
and into most of Thanksgiving day. Strong low level jet will
maintain low level wind shear threat until mixing commences and transfers some of
the higher winds to the surface..in the 12-15z time frame.
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.
Illinois...Flood Watch from Thursday evening through late Friday night for
Clinton Illinois-Madison Illinois-Monroe Illinois-Randolph IL-St. Clair Illinois-
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