Area Forecast Discussion
Issued by NWS St. Louis, MO
FXUS63 KLSX 021753
Area Forecast Discussion...Updated Aviation
National Weather Service Saint Louis MO
1153 AM CST Fri Dec 2 2016
.SHORT TERM... (Through Late Tonight)
Issued at 326 AM CST Fri Dec 2 2016
Primary forecast challenge today will be clouds and temperatures.
There are two areas of clouds affecting Missouri and Illinois today.
From the southwest, an area of mid-level clouds around 10,000 FT is
spreading east-northeast across southern portions of the CWFA. This
area of clouds is expected to cover most of the CWFA that is along
and south of the Missouri River and I-64 in Illinois by sunrise. A
second are of lower clouds around 2500-4000 FT with some higher
clouds over top. RAP seems to be handling these clouds well at the
moment so am using it as the basis for the sky forecast today. A
weak low and mid level trof will swing through the area today which
will turn the flow to the northwest. Mid level ceilings over the
eastern Ozarks and southern Illinois should erode and dissipate as
drier air at those levels moves in on the northwest flow.
Conversely, the northwest flow at lower levels will likely continue
to bring more clouds down out of Iowa across northern and
northeastern portions of the CWFA today. This will lead to a tricky
temperature forecast. Stuck pretty close to the short-term ensemble
guidance for temperatures today. Will likely have to adjust as
ceiling trends play out today.
Low level northwest flow continues tonight which will likely keep
ceilings close to northern and northeast zones. The southern extent
of the ceilings is uncertain at this time. Higher clouds will be
overspreading the area from the southwest late tonight ahead of the
next storm system. This will cut off the best radiational cooling
before the end of the night so lows won`t be as cool as they
otherwise could be. MOS guidance in the upper 20s to low 30s looks
.LONG TERM... (Saturday through Thursday)
Issued at 326 AM CST Fri Dec 2 2016
Active pattern will bring a series of storm systems across the area
over the next week and temperatures that will fall well below normal
by next Thursday.
The GFS is more aggressive than the NAM/ECMWF in bringing rain into
southeast Missouri on Saturday afternoon as large scale ascent
begins to increase ahead of an upper trough moving through the
Central Plains. This ascent will increase on Saturday night as the
trough moves closer and low-mid level moisture convergence increases
in the atmosphere. The upper trough will move east of the area on
Sunday causing the precipitation to end from west to east. There
will be just enough cold air in this system and nocturnal cooling
for rain on Saturday night to mix with or changeover to snow at
times before changing back to rain on Sunday. Could be some minor
snow accumulations on grass, mainly along and north of I-70.
The GFS and ECMWF shows a break precipitation Sunday night and early
Monday before a southern stream system brings a chance of rain
Monday, with some a mix of rain and snow mixing in on the back side
of the system on Monday night.
A cold front will move across the area on Wednesday bringing much
colder air into the area along with another chance of rain and snow.
By Thursday, temperatures will be the coldest of the season so far.
.AVIATION... (For the 18z TAFs through 18z Saturday Afternoon)
Issued at 1151 AM CST Fri Dec 2 2016
MVFR conditions expected at UIN this afternoon into portions of
tonight, with VFR conditions expected elsewhere. MVFR strato-cu deck
has arrived ahead of schedule at UIN. UIN will remain on the edge
of the better low-level moisture through this evening, thus it
will be a battle of just enough shallow mixing to scatter/lift
cloud deck to VFR or UIN remaining high-end MVFR this afternoon
all the way through tonight. Will split the difference with an
MVFR TEMPO group this afternoon, mixing out to VFR this evening
and bringing the MVFR deck back in overnight as the 850-925mb flow
turns more northerly. Otherwise, all other sites will remain VFR
through the period with generally light northwesterly winds today
turning to more easterly (but remaining light) through the day on
Saturday. Mid-level clouds will increase through the day on
Saturday as a storm system organizes out west.
SPECIFICS FOR KSTL:
Mid/high clouds will be on the increase this evening into tonight
as a storm system organizes out to the west. Winds will remain
generally light out of the northwest this afternoon into tonight,
turning more to the east and remaining light through the day
Saturday. Expect increasing mid/high cloudiness through the
period, but all cigs will remain VFR and all precip will hold off
until after the period.