National Weather Service Weather Forecast Office

• Home
• Site Map
• News
• Organization
• Search for: NWS All NOAA 

000
FXUS63 KLSX 130317
AFDLSX

AREA FORECAST DISCUSSION...UPDATED
NATIONAL WEATHER SERVICE ST LOUIS MO
917 PM CST Thu Dec 12 2013

.UPDATE:
Issued at 904 PM CST Thu Dec 12 2013

Temperature trends have proven to be a bit tricky this evening
thanks to areas with snow cover, and thus have made some tweaks to
the hourly temperature trends to reflect observations. Have also
gone ahead and bumped mins up a bit over northeastern Missouri,
though still feel that the coolest locations will be across the
eastern Ozarks and into southwest Illinois where snow cover still
lingers tonight. Lows in the aforementioned locations are expected
to be in the mid to upper teens, with generally low to mid 20s
elsewhere as warm advection and increasing cloud cover tonight
prevent lows from dropping off too much.

JP

&&

.SHORT TERM:  (Through Late Tonight)
Issued at 320 PM CST Thu Dec 12 2013

A tranquil night appears to be on tap for the area before the next
storm system on Friday-Friday Night. Remaining chilly will be the
main story for tonight but nothing like this morning or recent
nights. The large high pressure system has retreated south-southeast
of the area and low level warm air advection is in full swing. This
regime will continue tonight and lead to continued warming of the
low-mid troposphere but nocturnal cooling and decoupling of the
boundary layer. The coolest readings will be found from the eastern
Ozarks into southwest and south central Illinois where the snow pack
persists and lows will be in the teens.

Glass

.LONG TERM:  (Friday through Next Thursday)
Issued at 320 PM CST Thu Dec 12 2013

(Friday through Sunday)

Weather system has finally come on shore into southern CA this
morning, but models still have some uncertainty on timing and pcpn
type with this system as it moves into region. Precipitation will
develop and spread northeastward into central MO between 15z and 18z
Friday, and into remainder of forecast area during the afternoon
hours. Models continue to be warmer than runs earlier this week. So
could start off as a liquid most locations, so have rain or freezing
rain in grids. With such dry air over region currently, would not be
surprised to see a bit of sleet when the pcpn begins as evaporative
cooling cools 850mb temps down just a bit. In far northern portions
of forecast area they will see a mix of snow and some sleet when the
precipitation moves in.

Then as inverted trof slides east through region, colder air to
slowly filter in with precipitation changing over to all snow after
06z Sat. The heaviest snows will be over northeast MO/west central
IL from mid afternoon through the evening hours with 4 to 6 inches
of snow possible, so kept winter storm watch in affect for this area
until we have a better handle on exact snowfall amounts expected for
this area. Some concern about rain/freezing rain for areas that
still have snow pack over southeast MO/southwestern IL. Despite
above freezing temperatures, the very cold surfaces could become
hazardous on Friday, so have issued a winter weather advisory for
this area.  Also, issued a winter weather advisory for areas along
I-70 because of such uncertainty with pcpn type tomorrow. These
areas could see a light glazing, then 1 to 3 inches of snow as the
colder air filters in.

The precipitation to taper off by midday Saturday. Could see some
lingering light snow/flurries as low level moisture lingers
overhead as a second upper level shortwave slides through the
region.

As for temperatures through the period, expect highs in the mid to
upper 30s Friday and lows Friday night in the low 20s far north to
around 30 far southeast. Temperatures on Saturday will be in the
upper 20s to upper 30s.

Dry weather to persist through the rest of the weekend with lows
Saturday night ranging from around 10 degrees to low 20s far
southeast. Highs on Sunday will be in the mid 20s to low 30s.

Byrd

(Sunday Night through Next Thursday)

Kept with a dry forecast with near normal temperatures through
midweek before introducing a chance of rain or snow to parts of the
area next Thursday.  There is a certain amount of uncertainty with
this forecast as the GFS and ECMWF are each different with how far
south the cold air can move next week, and how deep a trough will
dig over the Rockies next Thursday.  Both models do suggest a chance
for some precipitation over the area on Thursday with southerly flow
ahead of deepening trough, so went ahead with the chance of
precipitation on Thursday.

Britt

&&

.AVIATION:  (For the 00z TAFs through 00z Friday Evening)
Issued at 550 PM CST Thu Dec 12 2013

VFR for the first 6-12 hours of the TAF period, although a
developing low level jet will produce LLWS conditions at
KCOU/KCPS/KSUS. Attention then turns to the upcoming winter
weather for Friday and Friday night. Conditions will deteriorate
throughout the day. The primary issue here is that the ptype will
vary depending on what the temperature is when it begins to
precipitate. At KCOU, temperatures will still be below freezing
when precipitation begins during the morning, therefore a period
of FZRA is expected before temperatures warm up enough for a
changeover to RA. The later onset of precip at KSUS/KCPS means
that there will be more time for the air to warm up above freezing
before it begins to rain. Although this would keep the initial
ptype as pure rain, elevated surfaces may still be cold enough to
pick up a light glaze. Precipitation will change over to snow
during the late afternoon with snow continuing beyond the end of
the 24-hr TAF period.

Specifics for KSTL: VFR for the first 6-12 hours of the TAF
period, although a developing low level jet will produce LLWS
conditions overnight and during the morning hours. Attention then
turns to the upcoming winter weather for Friday and Friday night.
Precipitation is expected to begin at KSTL after 18z. At this
time, it looks like air temperatures will have warmed up enough at
the surface for precipitation to initially fall as pure rain, but
elevated surfaces may still be cold enough to pick up a light
glaze. There may be a period of freezing rain or sleet before the
rain changes over to pure snow during the late afternoon and
evening hours. Snow will continue beyond the end of the 30-hr TAF
period.

Kanofsky

&&

.LSX WATCHES/WARNINGS/ADVISORIES:
MO...WINTER WEATHER ADVISORY from 6 AM Friday to 6 AM CST Saturday
     FOR Boone MO-Callaway MO-Cole MO-Crawford MO-Franklin MO-
     Gasconade MO-Iron MO-Moniteau MO-Montgomery MO-Osage MO-
     Reynolds MO-Warren MO-Washington MO.

     WINTER WEATHER ADVISORY from Noon Friday to 6 AM CST Saturday
     FOR Jefferson MO-Madison MO-St. Charles MO-St. Francois MO-
     St. Louis City MO-St. Louis MO-Ste. Genevieve MO.

     WINTER STORM WATCH from 6 AM CST Friday through late Friday
     night FOR Audrain MO-Knox MO-Lewis MO-Lincoln MO-Marion MO-
     Monroe MO-Pike MO-Ralls MO-Shelby MO.

IL...WINTER WEATHER ADVISORY from Noon Friday to 6 AM CST Saturday
     FOR Bond IL-Clinton IL-Fayette IL-Madison IL-Marion IL-
     Monroe IL-Randolph IL-St. Clair IL-Washington IL.

     WINTER STORM WATCH from Friday afternoon through late Friday
     night FOR Adams IL-Brown IL-Calhoun IL-Greene IL-Jersey IL-
     Macoupin IL-Montgomery IL-Pike IL.

&&

$$

WFO LSX