Area Forecast Discussion
Issued by NWS St. Louis, MO

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FXUS63 KLSX 101204

Area Forecast Discussion...Updated Aviation
National Weather Service Saint Louis MO
604 AM CST Sat Dec 10 2016

.SHORT TERM...  (Through Late Tonight)
Issued at 336 AM CST Sat Dec 10 2016

Persistent mid-level cloud cover has kept early morning lows
slightly warmer across most of the area than was previously
forecast, and many locations may see early morning lows bottom out
in the 20s instead of the teens. The slightly warmer start to the
day will probably not provide much of a boost for daytime highs
compared to the previous forecast due to the presence of mid-level
clouds, but today`s highs in the 30s will still be around 10 degrees
warmer than yesterday.

Initially northwest flow aloft today then transitions to quasizonal
flow aloft tonight ahead of an approaching upper disturbance. At the
surface, southeasterly surface winds around the back side of a high
pressure center will gradually increase late this afternoon and
tonight in response to a tightening pressure gradient. The
combination of steady WAA due to SSE/SE winds, persistent cloud
cover, and mechanical mixing due to wind speeds AOA 7-8kts should
prevent overnight lows from falling too far tonight. For most of the
area, temperatures should remain nearly steady or perhaps even
slowly rise after midnight.


.LONG TERM...  (Sunday through Friday)
Issued at 336 AM CST Sat Dec 10 2016

(Sunday thru Sunday Night)

A short wave embedded in quasi-zonal flow will pass Sun PM. At
the SFC...low pressure is fcst to move from the sthrn High Plains
on Sun mrng to Georgian Bay of Lake Huron by Mon mrng. This will
drag a cold front thru the area Sun PM. There should be a band of
SHRA`s that accompany FROPA. There is some question as to P-types
at the beginning and ending of the precip. The question with onset
is timing...will it begin before temps rise above freezing. At
this would appear that strong WAA will cause lows to
occur this evng with steady or slowly rising temps overnight.
Temps should be near or above freezing to start the prd...rising
above freezing across the entire FA by 15Z. So...current fcst
indicates precip starting as rain in all areas. Precip should end
as rain too with the cold air not in place by the time the precip
ends...classic Schmocker Rule.

(Monday thru Thursday night)

The week should feature a mainly zonal flow with a broad very low
amplitude trof across the cntrl CONUS that moves off the E Coast
Thu...with weak ridging mvng in for Thu night. A weak SFC ridge
builds in late Sun night and Mon with sthrly winds by Mon PM. A
secondary cold front moves thru the area late Mon night and early
Tue mrng...introducing another Arctic airmass to the region Tue thru
Thu. A 1030+mb SFC ridge will build into the region on Wed
introducing the coldest air mass to date so far this year. 850 temps
are fcst to be in the range of -8C to -12C for most of the FA by Thu
mrng. The SFC ridge should be settled near the CWA making this
likely the coldest mrng of the week with lows in the mid single
digits N to mid teens S. Although guidance is in good agreement
about the next cold air intrusion...they are not in sync wrt ripples
in the zonal flow. The GFS is dry on Wed while the ECMWF has a band
of QPF N of I-70. Then they swap on Thu with the Euro having a dry
solution and the GFS has a band of light QPF...again along and N of
I-70. Low level moisture will be lacking due to the dry nature of
the Arctic air mass so not confident that either of these systems
will produce measurable precip...but do warrant slight/chance
PoPs...for now.

Mon temps should be seasonal with highs in the mid 30s N to mid 40s
S. Due to the timing of the Arctic cold front late Mon night into
early Tue mrng...expect a non-diurnal temp trend on Tue. Temps will
likely be steady or slowly fall across the steady along the I-
70 corridor and then have a mrng bump across the S with falling temps
drng the aftn. Highs will mainly be in the 20s Wed and Thu. If any
snow cover is able to dvlp drng the week...then these temps will
likely be too high.

(Friday thru Friday night)

A significant trof is beginning to dvlp across the wrn CONUS towards
the end of the week...which puts our area in SW flow aloft. And
while the GFS and ECMWF agree on the general upper air
pattern...they differ significantly wrt details of the sensible wx.
Regardless...both solutions depict a storm system impacting the
cntrl CONUS with wintry precip...though far too early for any



.AVIATION...  (For the 12z TAFs through 12z Sunday Morning)
Issued at 533 AM CST Sat Dec 10 2016

Mid/high VFR clouds are expected to persist through the TAF pd
ahead of a developing disturbance over the plains. Initially light
winds will become southeasterly today around the back side of a
high pressure center, and wind speeds will increase after 18-21z
due to a tightening surface pressure gradient. Light rain is
expected shortly beyond the end of the valid TAF pd.





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