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000
FXUS63 KLSX 170635
AFDLSX

AREA FORECAST DISCUSSION...UPDATED
NATIONAL WEATHER SERVICE ST LOUIS MO
1235 AM CST Tue Dec 17 2013

.UPDATE:
Issued at 1233 AM CST Tue Dec 17 2013

Quick update to raise POPs to likely across the northern CWA where
light snow is beginning to fall out of the echo that has been
moving southeast from Iowa this evening. KIRK is now observing
3/4SM -SN. Don`t expect more than a dusting with this band as it
quickly moves southeast during the predawn hours. Will continue to
watch radar trends though as 4KM LOCAL WRF shows this band sinking
south or a new one forming southward to the northern St. Louis metro
before sunrise.


Issued at 638 PM CST Mon Dec 16 2013

Ample boundary layer moisture and light winds this evening have
resulted in the onset of fog a bit earlier than previously
forecast, thus have introduced patchy fog generally along and
north of the Missouri River through the remainder of the evening
and overnight hours. Observations across much of the region
indicate fog with visibilities down to 3SM to 6SM, though along
and north of a line from roughly Mattoon, IL through Litchfield,
IL and to Moberly, MO, observations indicate pockets of dense fog,
reports which have been echoed from some spotter reports in both
Macoupin county IL and Pike county MO. Thus, have included mention
of patchy dense fog for northern portions of the forecast area
through about 09Z, and will continue to monitor in case a dense
fog advisory becomes warranted.

JP

&&

.SHORT TERM:  (Through Late Tonight)
Issued at 315 PM CST Mon Dec 16 2013

(Tonight)

Weak surface low over central MO with associated frontal boundary to
lift to the northeast early this evening. In the meantime, low
stratus and fog north of boundary is slowly eroding to the north.
The northward advance of the stratus will probably slow down and
maybe even begin to build back to the south, but for now kept partly
cloudy conditions overnight. Other issue is reformation of fog since
portions of area got well above freezing and snow pack has melted
some. So with light winds and partial clearing, added mention of
patchy fog for areas north of Missouri River. Then another cold
front/shortwave will slide through region on northwest flow towards
daybreak. Winds to veer to the northwest behind the frontal
boundary. As for temperatures, very large variation as of 2030z,
with low 20s in the far north to upper 50s over south central MO. So
for lows tonight some areas will see little diurnal variation, thus
steady temps overnight. Lows will be in the low 20s north to near 30
far south.

Byrd

.LONG TERM:  (Tuesday through Next Monday)
Issued at 315 PM CST Mon Dec 16 2013

A weak and ill-defined cold front is still on track to move through
the area beginning very late tonight through Tuesday morning. No
sensible weather is expected with the front, rather a brief shot of
relatively cooler air with the surface high settling into south
central MO by early evening. That said, most locations north of
I-70 will be warmer due to the lack of clouds and highs should be
seasonable for mid December. The surface high retreats on Tuesday
night with low level winds backing to more southerly in its wake
and WAA getting underway. A big snow melt day is expected on
Wednesday and anticipating there will little remaining snow cover
by evening. Strong low level WAA will be in place with veering
south-southwesterly lower trop winds, upper flow transitioning to
quasi-zonal, and h85 temps rising to +8 to +10 degC. This should
translate to at least highs in the upper 40s-upper 50s. These
above average temperatures should persist on Thursday in advance
of the next approaching cold front. The entire area will be well
within the warm sector characterized by increasing clouds and
moisture and a low chance pops confined to the pre-frontal
corridor through northeast Missouri.

The change in upper air pattern to an amplified split flow regime
will really beging to take shape in the Thursday-Thursday night-
Friday period and the increased trofing in the northern stream
will help push the cold front into and through the CWA in the late
Thursday night-Friday time frame. The vast majority of any
precipitation with this fropa should be pre-frontal, however I
can`t discount the probability some threat within the cool air.
Deeper cold air significantly lags the front so I have a mention
of freezing rain in portions of northeast MO late Thursday night
/pre-dawn Friday.

The extended portion of the model suite has come into much better
agreement within the last 24 hours with the evolution of the
aforementioned amplified split flow regime when compared to the
last few days and the handling of the northern and southern
streams. That said we will likely still see variability and
changes within the upcoming week in the NWP models. The overall
scenario we have been anticipating for the last few days still
hasn`t changed with the evolution of the pattern on a synoptic
scale favoring a widespread elongated overunning precipitation
event along/north of a strong cold front poised somewhere from the
Ohio Valley across the lower Mississippi Valley and into the
southern Plains. The exact location of this boundary and depth of
cold air remain some of the forecast details with much variability.
Somewhere within the LSX CWA there will be a threat of persistent
moderate-heavy rain, freezing rain, a wintry mix and eventually
accumulating snowfall. The new forecast has begun the trend of
ramping up these elements with a continued ensemblistic approach
and a strong weighting in the pops to the 40+ member NAEFS and
ECMWF ensemble. Deeper cold air is not forecast to arrive until
late Saturday night into Sunday when there is better interplay
between the ejecting southern trof and northern stream. The mean
of the current guidance suggests the precipitation until then
(Saturday/Saturday evening) will be liquid - with freezing rain
and potentially significant icing across the northern sections of
the CWA. As the deeper cold air arrives late Saturday night/Sunday there
should be a transition to mixed precipitation and eventually all
snow. Confidence in these exact transition zones/times remains low
at this time.

Glass

&&

.AVIATION:  (For the 06z TAFs through 06z Tuesday Night)
Issued at 959 PM CST Mon Dec 16 2013

Stratus and fog across nern MO and w cntrl IL, including UIN will
advect swd into at least a portion of the St Louis metro area
overnight. Most of the stratus clouds should remain north and east
of COU late tgt. Surface winds should increase late tgt/early
Tuesday mrng in UIN and cause some dissipation of the stratus and
fog. Winds will not pick up in the St Louis metro area until early
-mid mrng, with improving conditions expected after 12z Tuesday.
The w-nwly surface wind will veer around to a nwly direction
Tuesday mrng after the passage of a weak cold front. Little if any
cloud cover is expected for late Tuesday mrng through Tuesday
evng.

Specifics for KSTL: Haze and fog will continue late tgt with
stratus clouds developing or advecting into STL from the n-ne.
May have broken clouds around 500 ft for much of the late tgt hours.
Surface winds will strengthen by early-mid mrng on Tuesday with
the stratus and fog dissipating. Little if any cloud cover
expected for late Tuesday mrng through Tuesday ngt after passage
of a weak cold front. W-nwly surface wind will diminish Tuesday
evng.

GKS

&&

.LSX WATCHES/WARNINGS/ADVISORIES:
MO...NONE.
IL...NONE.
&&

$$

WFO LSX