Area Forecast Discussion
Issued by NWS St. Louis, MO

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FXUS63 KLSX 091205

Area Forecast Discussion...Updated Aviation
National Weather Service Saint Louis MO
605 AM CST Fri Dec 9 2016

.SHORT TERM...  (Through Late Tonight)
Issued at 334 AM CST Fri Dec 9 2016

A surface high pressure center will move across the area today. With
such a brisk start to the morning (early morning lows are on track
to be in the teens areawide) and the relatively low sun angle at
this time of year, it will be a struggle for most of the area to see
daytime high temperatures rise above freezing. BUFKIT soundings show
that T92 is around -10 deg C by mid-afternoon which yields highs in
the mid to upper 20s or perhaps very low 30s after accounting for
the strength of the surface high pressure center, and local
conditional climatology charts (light west or north winds in
December with no clouds below 3kft) show a 10 degree diurnal rise
from the early morning low. Both the slightly deeper mixing (between
925hPa and 900hPa) and the strength of the surface high (~1035hPa)
support adding a few degrees onto the default values from the
charts, which yields highs in the upper 20s or perhaps very low 30s.
Winds become calm early tonight then gradually turn southeasterly
after the surface high shifts eastward. Overnight lows are expected
to fall into the teens again.


.LONG TERM...  (Saturday through Thursday)
Issued at 334 AM CST Fri Dec 9 2016

The synoptic pattern at the start of the long term period will
feature swift northwest flow dominating much of the northern CONUS.
This regime will generally persist through the period, bringing
continued cold conditions along with a fairly active weather pattern
as a couple systems impact the region.

Saturday may end up as one of the better days in the period.  A
quick-moving PV anomaly approaching the Rockies will help induce a
lee cyclone across eastern CO/southeast WY.  This will turn the flow
more southerly on Saturday.  Despite the southerly flow, increasing
clouds and this fairly cold and dense airmass will only allow
temperatures to warm into the mid and upper 30s.

The main focus of the long term period will be on this low pressure
system as it lifts out into the Plains ahead of the approaching PV
anomaly. Overall, dprog/dt plots suggest that guidance has trended
warmer with this system. The NAM continues to remain a cool outlier,
with the GFS/GEM/ECMWF/SREF all substantially warmer.  With no snow
cover to overcome and a day on Saturday to help scour out this cold
airmass, think the warmer consensus is the way to go at this point.
That being said, still think precipitation could start out as a mix
early Sunday morning as the warm air advection wing pushes over the
area. In fact, forecast cross-sections are suggesting that
saturation will be fairly shallow initially and only into the -5C
range, suggesting ice crystals are not likely. With surface
temperatures near freezing, this could lead to a brief period of
freezing drizzle at precipitation onset Sunday morning. The best
potential for this would be along/north of I-70. As the day goes on,
the maturing surface cyclone should push into north-central MO and
southeast Iowa, placing the region firmly in the warm sector and
turning any mixed precipitation into all rain Sunday afternoon into
early Sunday evening. As the cyclone moves out, there could be
enough lingering moisture in the cold air to get some light snow on
the backside, with only a minor accumulation possible across far
northeast MO and west-central IL.  Bottom line, this does not look
to be a major winter weather producer for our area.

Guidance begins to diverge as we head into the middle of next week.
The latest ECMWF/GFS solutions are now dry for Tuesday, but the GEM
continues to show a system riding along a strong arctic front. The
GFS/ECMWF are slower with bringing an arctic front through the
region, moving it through on Wednesday with varying amounts of
precipitation along it (Euro drier, GFS wetter).  Given that
guidance struggles in swift northwest flow regimes like the one we
will see this week, have gone with an ensemble approach which keeps
small pops in Tuesday and Wednesday.  There is general agreement
that a very cold airmass will attempt to build in from the north by
the middle and end of next week.  Have kept temperatures in the 20s
for highs and in the teens for lows, but these may not be cold
enough if trends continue.



.AVIATION...  (For the 12z TAFs through 12z Saturday Morning)
Issued at 541 AM CST Fri Dec 9 2016

Specifics for KCOU, KUIN: MVFR stratus deck was affecting KUIN at
TAF issuance, but cigs should begin rising within a few hours and
the cloud deck will eventually slide eastward. KCOU is far enough
west that it will not be affected by the stratus deck near the
Mississippi River. Winds will become light and variable during
the passage of a surface high pressure center, then become
southeasterly to southerly around the back side of the high.

SPECIFICS FOR KSTL, KSUS, KCPS: A stratus deck near the MO/IA
border will continue advecting southeastward this morning and may
bring scattered VFR stratus to St. Louis metro area TAF sites.
Initially west to northwest surface winds will become light and
variable today during the passage of a surface high pressure
center, then become southeasterly to southerly around the back
side of the high after 10/12z.



Saint Louis     29  19  36  31 /   0   0   0  10
Quincy          25  16  33  28 /   5   5  10  10
Columbia        28  16  38  30 /   0   0   0  10
Jefferson City  29  17  39  31 /   0   0   0  10
Salem           28  18  34  29 /   0   0   0  10
Farmington      30  17  36  29 /   0   0   0   5




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