Area Forecast Discussion
Issued by NWS St. Louis, MO

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000
FXUS63 KLSX 010542
AFDLSX

AREA FORECAST DISCUSSION...UPDATED AVIATION
NATIONAL WEATHER SERVICE ST LOUIS MO
1142 PM CST Sat Jan 31 2015

.UPDATE:
Issued at 936 PM CST Sat Jan 31 2015

Band of moderate to heavy snow has developed over the past couple
of hours yielding 2-4+" across portions of north-central and
northeast Missouri...including around 2 inches in Edina and
Novelty in Knox County as of 900 PM. Based on radar
imagery...believe this to persist through at least 0600 UTC. After
that time frame...did not deviate too much at all from previous
forecast as fear that lighter precipitation rates may yield areas
currently receiving snow to mix with if not outright change back
over to rain. However...long fetch of dry low-level air will
continue to advect into the region which may keep things all snow.
If this does indeed occur...snowfall totals may need to be further
boosted. Further to the south and east...dropped PoPs
significantly due to aforementioned dry low levels and have PoPs
ramping up as shield of precipitation slowly moves in from western
Missouri. Updated zones out and WSW shortly.

Gosselin

&&

.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 albeit 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 06z TAFs through 06z Sunday Night)
Issued at 1121 PM CST Sat Jan 31 2015

Specifics for KUIN: An air temperature of around 34 degrees has been
sufficient for the transition from rain to snow, and various
processes in the atmosphere continue to keep the temperature
fluctuating through the low to mid 30s, which is causing the ptype
to unpredictably switch back and forth between RA and SN. With
time, the ptype should become predominantly SN. I have attempted
to convey this complexity in the TAF. Several inches of
accumulating snow are expected at the terminal, but some of the
snow could melt during periods of rain. Winds will swing around to
the northwest and become quite gusty during the afternoon and
evening hours.

Specifics for KCOU: Precipitation was starting to reach the ground
at KCOU at TAF issuance. An air temperature of around 34 degrees
has been sufficient for the transition from rain to snow over the
last few hours around the region. Various processes are causing
the temperature to fluctuate through the low to mid 30s, therefore
the ptype can switch back and forth between RA and SN
unpredictably. It is possible that a solid changeover to SN at
KCOU may not occur until the colder air arrives with the cold
front on Sunday. Winds will swing around to the northwest and
become quite gusty during the afternoon and evening hours.

Specifics for KSTL, KSUS, KCPS: Dry air has been holding its own
over the last several hours (as per the donut hole on radar over
KLSX and the 20 degree dewpoint depressions at KSTL/KSUS/KCPS).
Precipitation has been slower to reach the ground at St. Louis
metro area TAF sites than previously anticipated, so I backed off
on the rain by a couple of hours. The overall thinking is that
precipitation will be predominantly rain overnight and for most of
Sunday. It will take the arrival of the cold front on Sunday
afternoon to start changing the rain over to snow. Winds will
swing around to the northwest and become quite gusty between
02/00z and 02/03z.

Kanofsky

&&

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

     WINTER STORM WARNING until 9 PM CST Sunday FOR Knox MO-Lewis MO.

IL...WINTER WEATHER ADVISORY until 9 PM CST Sunday FOR Pike IL.

     WINTER STORM WARNING until 9 PM CST Sunday FOR Adams IL-Brown IL.

&&

$$

WFO LSX





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