Area Forecast Discussion
Issued by NWS St. Louis, MO

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FXUS63 KLSX 060339

Area Forecast Discussion...Updated
National Weather Service Saint Louis MO
939 PM CST Mon Dec 5 2016

Issued at 930 PM CST Mon Dec 5 2016

The upper low currently across eastern TX is on the move and will
eject east-northeast tonight across the lower MS Valley and into
the lower TN Valley by daybreak Tuesday. The main thrust of large
scale forcing/ascent will remain south of the area, however we
should still see some rain in the southern part of the CWA. There
is a growing southwest-northeast oriented rain band currently
stretching from eastern OK into northwest AR associated with
deformation/mid level frontogenetical forcing. This band of rain
should progress northeast tonight into early Monday morning
impacting southeast MO and parts of southwest IL. There may be a
few narrow bands of light rain also to the north of this main area
that could be as far north as St. Louis. I have increase the POPS
tonight, especially in the southern CWA. Present indications from
T/TD values and associated advections would suggest this should be
all rain. Elsewhere stratus will expand/develop across the area
through the overnight hours. The best potential for fog will be
across northeast MO and west central IL and possibly into central
MO, more removed from the clouds associated with the southern
system. This area would also have the best possibility for any dense
fog as well. The cold front will come through on Monday scouring
out the lower clouds and bringing the first thrust of colder air
into the the region.



.SHORT TERM...  (Through Late Tuesday Afternoon)
Issued at 217 PM CST Mon Dec 5 2016

Models are pretty close on moving out the closed low over sw Texas
and bringing an open wave across the area. They are also
contining the trend of moving the precipitation futher south and
east. Thus the forecast reflects this trend in the pops. Fog
could be an issue in some places again: Quincy and Columbia. The
wind and quickly advancing clouds may keep that down, but have put
patchy fog in the forecast. Temps pretty close to previous
forecast so have only made minor adjustments.


.LONG TERM...  (Tuesday Night through Next Monday)
Issued at 234 PM CST Mon Dec 5 2016

The synoptic pattern at the start of the long term period will
feature a broad trough across much of the CONUS, which will amplify
and shift eastward through the end of the week.  Despite the main
trough shifting east, active zonal to northwest flow will persist
through the period keeping cool conditions and a relatively active
pattern in store.

The latest guidance continues to trend towards less QPF with a PV
anomaly slated to move through the Midwest Wednesday afternoon into
Wednesday evening.  The reason for this drier trend appears to be a
quicker arrival of the cold, very dry airmass in the low-levels.
Forecast cross-sections continue to show a decent mid-level
frontogenetical circulation, albeit of fairly shallow slope.  But
given the quicker arrival of the very dry airmass, it appears most
snow falling from this circulation will sublimate before reaching
the ground.  Typically in these situations there are a few spots
where the precip rate is able to overcome the dry near-surface
airmass, thus a quick dusting to a couple tenths of snow continues
to look possible in a few spots, but this does not look to bring
widespread accumulations at this time. We will continue to monitor
this system given the late afternoon/evening arrival could lead to
some isolated travel issues even with only a dusting of snow.

The main story of the period continues to center on the cold airmass
which will arrive Wednesday night through the end of the workweek.
Highs Thursday and Friday will be confined in the mid to upper 20s
(with a few locations around 30), while lows will dip into the
teens.  Couple these temperatures with a light breeze, and wind
chill values will approach 0 at times.

Guidance begins to really diverge this weekend into early next week.
The GFS is stronger/quicker with the next PV anomaly crossing the
country, whereas the Euro is a bit slower and weaker.  Looking at
both the GEFS ensemble and the European ensemble members, the spread
during this timeframe continues to be large.  Therefore, will
continue with just a general guidance blend which keeps pops
generally in the 20-40 range.  Will go with a rain/snow mix in this
time as well given quite a range in thermal profiles with the
varying solutions.



.AVIATION...  (For the 00z TAFs through 00z Tuesday Evening)
Issued at 550 PM CST Mon Dec 5 2016

Another night with a good deal of uncertainty in the specifics.
Most problematic area is from west-central IL through northeast
and central MO where datasets conflict on scenario and the
synoptic pattern is not clear cut which is prefered.

The scenario followed is stratus/fog will redevelop this evening
across northeast MO/west-central IL with KUIN developing IFR
flight conditions. Some of the guidance once again wants to
develop very low visibilities BLO 1SM but my confidence is low on
that scenario at this time. Not sure if the stratus and lower
visibility fog will develop into central MO and impact KCOU. At
this time I just have MVFR conditions and will continue to
evaluate through the evening. Otherwise a storm system to the
south will spread increasing clouds/stratus into the area from
the south tonight bringing MVFR to IFR conditins. Light rain will
spread into the area as well - Some spotty light rain may be
possible in the St. Louis region but the better probability is
across southeast MO and southwest IL. A cold front will move
through the region on Tuesday bringing increasing northwest winds
and resulting in diminishing clouds and a return to VFR flight


VFR flight conditions are expected this evening with stratus/fog
overnight as a new storm system moves to the south and impacts the
area. Predominately MVFR flight conditions are expected overnight
into Tuesday morning. I can`t rule out some sprinkles or a brief
period of light rain in the 07-10Z time frame. A cold front will
move through the region on Tuesday bringing increasing northwest
winds and resulting in diminishing clouds and a return to VFR
flight conditions around midday.





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