Area Forecast Discussion
Issued by NWS St. Louis, MO

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FXUS63 KLSX 312125
AFDLSX

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE ST LOUIS MO
325 PM CST Sat Jan 31 2015

.SHORT TERM:  (Through Sunday night)
Issued at 324 PM CST Sat Jan 31 2015

No changes to current headlines. Focus continues to be the system
approaching the CWA thru Sun.

Mdl solns continue to trend warmer tonight and into Sun. The
RAP/HRRR solns are the warmest, suggesting the entire CWA remains
warm enuf for RA thru at least Midnight tonight. The GEM being the
coldest with the NAM/local WRF/ECMWF only slightly warmer. These
cooler solns also suggest the 850 mb 0 degree C isotherm remains S
of the 850mb low track. Believe this is the result of two features.
First, adiabatic cooling due to dynamic lift across the region and
second, evaporative cooling, albeit not much, at the onset of
precip. The column quickly becomes saturated this evening with the
current precip shield moving newd across thru the area, eliminating
the second issue.

Question then turns to if the very strong WAA/frontogenetical
forcing can overcome the cooling due to lift and cause precip to
either turn back to or mix with RA late tonight. Forecast soundings
across the nrn portions of the CWA continue to show the lowest 2kft
to be at or just below 0C. The BL will remain as-is thru the morning
hours as the 850 mb low. By 18z Sun, CAA quickly takes over the nrn
third to half of the CWA. However, as the main trof digs and the
system deepens, much of the CWA is dry-slotted and bulk of the
remaining precip is due to the def zone.

Some questions remain with this system increasing the uncertainty.
Mdls have come in with more QPF, esp later tonight. If these higher
QPF amnts and colder solns verify, the going forecast may be too
low. With a few minor tweaks, overall, snowfall forecast is very
close to the prev forecast.


.LONG TERM: (Monday through Next Saturday)
Issued at 324 PM CST Sat Jan 31 2015

(Monday-Tuesday)

The work week will start off on a cold note featuring much below
average temps with cyclonic flow and lots of remnant stratus early
Monday morning. The low clouds however should clear southward during
the morning as high pressure builds into the area. By afternoon warm
advection aloft is already underway and heights are rising in the
wake of the departing upper trof. This will be accompanied by lots
of high clouds overspreading the region from afternoon and
continuing into Monday night as the surface high retreats. The pick
day of the week should be Tuesday with a strong warm advection
regime forecast. This should result in a brief bump in highs to
above normal readings. The exception is possibly across far
northeast Missouri into west central Illinois where there will be
snow cover and a bit of high cloudiness.

(Wednesday-Saturday)

Cold weather and below average temperatures will once again take
hold during the later part of the upcoming week. The flow aloft
initially across much of the Conus will be broadly cyclonic anchored
by an upper vortex in the northern Hudson bay region. A pronounced
shortwave migrating through this flow aloft will send a rather
strong cold front through the area late Tuesday night-early
Wednesday. The biggest uncertainties during the extended forecast
time frame deal with the Wednesday-Wednesday night period in the
wake of this front. The GFS and CMC GEM both have another
progressive disturbance lagging in the cyclonic flow. They both
suggest that this disturbance and the associated forcing and
resultant frontogenesis will produce a post-frontal wnw-ese oriented
band of accumulating snow. The ECMWF on the other hand lacks this
scenario and has high pressure dominating, however suggests a warm
advection snow band may occur Thursday. At this point I have a
little more confidence in the GFS/GEM solution albiet low, and will
continue with some low pops.

By the beginning of the next weekend, heights aloft are rising as
the mean longwave trof axis shifts off the eastern seaboard and warm
advection is underway signaling moderating temperatures.

Glass

&&

.AVIATION:  (For the 18z TAFs through 18z Sunday Afternoon)
Issued at 1140 AM CST Sat Jan 31 2015

A large rain shield currently stretching from western MO into the
Plains was moving east-northeast at midday. The rain will continue
this motion and spread into KUIN and KCOU where flight conditions
will gradually lower during the afternoon to MVFR and to IFR during
the evening. Further east in the St. Louis metro area, the
precipitation will be encountering antecedant dry air which will
delay the onset of steadier rain until almost midnight, when
flight conditions will also then lower. At this point the only
location which will see snow during the valid TAF period is KUIN
when there should be a changeover sometime in the 04-07z time
frame and there will be accumulations at the terminal.

Specifics for KSTL:

VFR flight conditions are expected to persist this afternoon and
evening with some short periods of light rain/sprinkles. Steadier rain
should not commence until close to or after midnight/06z Sat. when
flight conditions will then lower steadily to MVFR, with IFR
anticipated between 10-12z. A changeover to snow will occur late
in the TAF period, after 21z when the surface low and cold front
pass through.

Glass

&&

.PRELIMINARY POINT TEMPS/POPS:
Saint Louis     36  42  15  27 / 100 100  40   0
Quincy          32  34   7  20 / 100 100  60   0
Columbia        36  39  11  27 / 100  90  50   0
Jefferson City  37  40  13  29 / 100  90  50   0
Salem           36  41  18  27 /  90 100  30   5
Farmington      38  45  17  31 /  90 100  20   0

&&

.LSX WATCHES/WARNINGS/ADVISORIES:
MO...WINTER WEATHER ADVISORY from 9 PM this evening to 9 PM CST
     Sunday FOR Marion MO-Monroe MO-Pike MO-Ralls MO-Shelby MO.

     WINTER STORM WARNING from 9 PM this evening to 9 PM CST Sunday
     FOR Knox MO-Lewis MO.

IL...WINTER WEATHER ADVISORY from 9 PM this evening to 9 PM CST
     Sunday FOR Pike IL.

     WINTER STORM WARNING from 9 PM this evening to 9 PM CST Sunday
     FOR Adams IL-Brown IL.

&&

$$

WFO LSX





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