Area Forecast Discussion
Issued by NWS St. Louis, MO

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FXUS63 KLSX 010342
AFDLSX

AREA FORECAST DISCUSSION...UPDATED
NATIONAL WEATHER SERVICE ST LOUIS MO
942 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 recieving 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 00z TAFs through 00z Sunday Evening)
Issued at 519 PM CST Sat Jan 31 2015

Specifics for KUIN: A winter storm system will affect west central
IL through Sunday. As the colder air arrives overnight, rain will
change over to snow and then start to accumulate at KUIN. The most
intense snowfall is expected sometime between 15-21z. Winds will
become northwesterly and gust to 20-25 kts on the back side of the
system.

Specifics for KCOU: KCOU will be on the edge of a winter storm
system which is affecting the region. Some light snow could
occasionally mix in with the rain, but rain should be the dominant
precipitation type until after the cold front moves through on
late Sunday afternoon, but precipitation will also be winding down
around the time that it becomes cold enough for snow. Winds will
become northwesterly and gust to 20-25 kts on the back side of the
system. Although there is initially some dry air to overcome,
ceilings should deteriorate to IFR by 06z.

Specifics for KSTL, KSUS, KCPS: St. Louis metro area TAF sites
will be on the southern edge of a winter storm system which is
affecting the region. The dominant precipitation type should
remain pure rain until colder air arrives with the cold front on
Sunday afternoon. Winds will become northerly and gust to 20-25
kts on the back side of the system. Although there is initially
some dry air to overcome, ceilings should deteriorate to IFR
overnight.

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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