Area Forecast Discussion
Issued by NWS St. Louis, MO
FXUS63 KLSX 140320
AREA FORECAST DISCUSSION...UPDATED
NATIONAL WEATHER SERVICE ST LOUIS MO
920 PM CST Fri Dec 13 2013
Issued at 904 PM CST Fri Dec 13 2013
The ongoing winter storm continues to evolve over the region, with
some locations currently experiencing a lull in precipitation.
There is another area of precipitation currently over northeastern
Oklahoma, northwestern Arkansas, and southwestern Missouri which
is moving northeastward toward the area ahead of an approaching
shortwave, currently located over central Oklahoma. This is
expected to be the main driving force behind the continued
snowfall overnight tonight, especially over central and southern
portions of the forecast area.
Overall, the north-to-south trend in changing over from rain to
snow continues, with the changeover line roughly along I-44 at
this time and advancing south. While there have been a few reports
of sleet, the predominant p-types continue to be rain and snow,
with some pockets of freezing rain possible over the eastern
Ozarks for another hour or so. Forecast soundings over the
southern tier of the CWFA indicate that evaporative cooling
continues to overwhelm warm advection, and thus expect a
changeover to snow for the entire CWFA within the next 1-2 hours.
Snowfall totals over northeastern MO and west central IL so far
this evening are on the order of 2 to 3 inches, with lesser
amounts to the south where the changeover from rain to snow has
more recently occurred. Thus, still feel that widespread storm
totals generally in the 3-5 inch range look good for this event.
.SHORT TERM: (Through Late Tonight)
Issued at 312 PM CST Fri Dec 13 2013
(Tonight through Saturday)
Storm is ongoing, first wave of precipitation is moving northeast
through forecast area this afternoon. Mostly rain as of 19z with
temperatures in the low to mid 30s, though there are pockets of
freezing rain and some sleet/snow. Only a light glazing, mainly over
central MO. As for northeast MO/west central IL, still could see an
axis of higher snows in this area through tonight, but warmer air
has prevailed this afternoon with it being mostly rain, though in
the past half hour sites in Adams Co. IL are reporting mostly snow.
Snow totals could be slightly less when all is said and done in this
area, but still between 3 and 5 inches.
850MB low to lift northeast through area tonight, mainly along I44
corridor. CAA to filter in behind system, with rain changing over to
snow from north to south, with snow everywhere by 06z Saturday. As
for STL metro area, will see rain transition to snow between 7 and 9
pm, just after rush hour, but could still see some isolated slick
spots from the rain during the rush hour. Secondary upper level
shortwave to also slide northeast just south of forecast area, so
could see some enhanced lift thus higher snowfall totals especially
along I44 corridor. So raised snowfall totals in this area a bit,
but still between 3 and 5 inches possible. Because of the lingering
light snow on Saturday morning in the east, with keep winter weather
advisory going for areas along and east of a Calhoun Co. IL to
Crawford Co. MO line through 18z.
Then snow to taper off to light snow/flurries by midday on Saturday
as system exits region. Lows tonight will be in the mid 20s to low
30s. Highs on Saturday will be in the upper 20s to upper 30s.
.LONG TERM: (Saturday through Next Friday)
Issued at 312 PM CST Fri Dec 13 2013
(Saturday Night through Monday)
Will continue with a dry forecast late weekend into early next work
week as subsidence sets in behind the exiting upper trough that will
move east of MO/IL by 00Z Saturday evening. The NAM/GFS/ECMWF are
all showing northwesterly flow aloft behind the trough, though with
slightly different timing of weak vort maxes that will be passing
just north of the area. Will likely be some low level clouds still
around the the area early on Saturday night that will move out of
the area by Sunday morning. Then expect some mid-high clouds to
linger into Monday which will be caused by the vort maxes in the
northwesterly flow aloft.
Kept with the below guidance lows Saturday night because of the
expect fresh snow cover, and leaned toward the cooler NAM MOS Sunday
and Sunday night for temperatures for the same reason.
(Tuesday through next Friday)
GFS and ECMWF are in fairly good agreement that longwave trough
over eastern North America will move east on Tuesday. Another
trough will then dig through western North America and extend from
Manitoba into the central Rockies by Friday morning. In the
meantime, expect dry conditions through at least Wednesday Night
as a surface high moves across the area. Temperatures Wednesday
and Thursday look to be above normal as 850mb temperatures will be
The ECMWF is about 6 hours faster than the GFS in showing another
sharp cold front moving across the area next Friday that is
associated with the central Rockies trough. Like a week ago,
there is still some uncertainty on the timing of this front, so it
exact precipitation types are not clear. Will go with a mix of
rain and snow at this point.
.AVIATION: (For the 00z TAFs through 00z Saturday Evening)
Issued at 530 PM CST Fri Dec 13 2013
Overall trend is deteriorating conditions for the rest of the
night at all TAF sites. Areas of rain will change over to snow
this evening with snow then continuing overnight. Based on BUFKIT
soundings and recent spotter reports, expect a chaotic transition
period with ptypes switching back and forth at times (including
brief occurrences of sleet) before the atmosphere cools enough for
precipitation to fall as pure snow. The low sun angle, lack of dry
air at low levels, and model forecasts of ample moisture support
the idea of little improvement in ceiling heights tomorrow even
after precipitation has ended.
Specifics for KSTL: Overall trend is deteriorating conditions for
the rest of the night. VFR cigs at the start of the TAF period
should be very short lived as various sources of cooling combine
to cause lowering cloud bases. Visibilities within areas of rain
may vary for the next few hours until the rain has completely
switched over to snow. The loss of ice crystals as precipitation
is ending tomorrow morning is expected to produce a few hours of
freezing drizzle. The low sun angle, lack of dry air at low
levels, and model forecasts of ample moisture support the idea of
little improvement in ceiling heights tomorrow even after
precipitation has ended.
MO...WINTER WEATHER ADVISORY until Noon CST Saturday FOR Crawford MO-
Franklin MO-Iron MO-Jefferson MO-Lincoln MO-Madison MO-
Reynolds MO-St. Charles MO-St. Francois MO-St. Louis City
MO-St. Louis MO-Ste. Genevieve MO-Warren MO-Washington MO.
WINTER WEATHER ADVISORY until 6 AM CST Saturday FOR Audrain MO-
Boone MO-Callaway MO-Cole MO-Gasconade MO-Knox MO-Lewis MO-
Marion MO-Moniteau MO-Monroe MO-Montgomery MO-Osage MO-Pike
MO-Ralls MO-Shelby MO.
IL...WINTER WEATHER ADVISORY until Noon CST Saturday FOR Bond IL-
Calhoun IL-Clinton IL-Fayette IL-Greene IL-Jersey IL-
Macoupin IL-Madison IL-Marion IL-Monroe IL-Montgomery IL-
Randolph IL-St. Clair IL-Washington IL.
WINTER WEATHER ADVISORY until 6 AM CST Saturday FOR Adams IL-
Brown IL-Pike IL.