Area Forecast Discussion
Issued by NWS St. Louis, MO
FXUS63 KLSX 081734
Area Forecast Discussion...Updated Aviation
National Weather Service Saint Louis MO
1134 AM CST Thu Dec 8 2016
.SHORT TERM... (Through Late Tonight)
Issued at 319 AM CST Thu Dec 8 2016
Water vapor imagery early this morning is showing that a confluent
upper trough is moving across the area. The RAP is showing that the
trough axis will move quickly off to the east this morning. The
forecast will remain dry today and tonight as subsidence sets in
behind the trough axis. Expect skies to remain mainly clear over
the area as the RAP is showing the low level flow gradually
straightening out, and then becoming slightly anticyclonic by this
evening as high pressure moves in.
With strong cold air advection today, will go slightly cooler than
MOS highs and stay closer to SREF mean temps in most areas.
Northwest winds will stay up in the 5-10mph range tonight, so GFS
MOS lows look good in most areas.
.LONG TERM... (Friday through Wednesday)
Issued at 319 AM CST Thu Dec 8 2016
The ridge axis will be over the eastern Plains on Friday morning.
Highs will struggle to reach the mid 20s to low 30s across the
area due to the cold start to the day. The ridge will will move
east of the Mississippi by 12Z Saturday which will turn the flow
back to the south. This should allow for a modest warm up on
Saturday...finally bringing temperatures above freezing during the
Models are coming into better agreement with the Saturday night-
Sunday night system. GFS is trending to a less amplified shortwave
solution...more like the ECMWF. This is producing less precip over
our CWFA late Saturday night and Sunday. The best chance for precip
now looks to be Sunday night as the shortwave rolls through the
Mississippi Valley. Precip types will be a little problematic.
Forecast soundings for Saturday night and early Sunday morning show
enough dry air in the column for any precip that reaches the ground
to fall as snow. Warm advection ahead of the trof should eventually
turn any snow/mixed precip to rain by late Sunday morning/afternoon
except over northern portions of the CWFA where it may remain cold
enough for wintry precip. Colder air moves in behind the trof so
rain will probably change back to snow Sunday night before ending.
Models diverge somewhat again toward the end of the forecast period.
Monday looks like a break in the weather with weak high pressure in
control...but another Arctic front will be headed south into the
Mississippi Valley on Tuesday. There are differences on the speed
and strength of the front as it moves through our CWFA on Tuesday.
The GFS shows a more amplified upper level wave over the Great
Plains on Tuesday and the attendant cold front is also sharper and
the cold air advection behind the front is stronger than the
ECMWF. GFS also has a lot more precip than the ECMWF. Chance PoPs
still look appropriate given the uncertainties, but if the EC
trends toward the more amplified GFS chance PoPs won`t be high
enough. Both models build cold high pressure over the area on
Wednesday with ensemble guidance struggling to get highs into the
mid and upper 20s. If the GFS is right, 850mb temps will drop into
the -20 to -25C range. This would put lows in the low single
digits and highs in the teens for Wednesday. Not quite ready to
jump on that so am sticking close to the ensembles.
.AVIATION... (For the 18z TAFs through 18z Friday Afternoon)
Issued at 1129 AM CST Thu Dec 8 2016
Deep cyclonic flow persists across the region and at the surface
is dominated by a surface low in southeast Canada and expansive
high pressure system across the Plains. This flow regime will be
responsible for gusty west-northwest surface winds this afternoon
as well as stratocu. The most extensive stratocu resuling in MVFR
cigs is expected to remain from IA into central and northern IL
and removed from the terminals. VFR flight conditions are expected
through the forecast period.
SPECIFICS FOR KSTL:
VFR flight conditions expected through the period with some cold
air stratocu this afternoon. Gusty west-northwest winds will
continue this afternoon then diminish this evening.