Area Forecast Discussion
Issued by NWS St. Louis, MO

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FXUS63 KLSX 111206

Area Forecast Discussion...Updated Aviation
National Weather Service Saint Louis MO
606 AM CST Sun Dec 11 2016

.SHORT TERM...  (Through Late Tonight)
Issued at 347 AM CST Sun Dec 11 2016

The primary forecast issues for today/tonight involve precipitation
chances and precipitation types.

A compact upper disturbance will zip through ND/MN today in tandem
with a secondary area of vorticity located to its south. At the
surface, a developing area of low pressure over OK/KS caused by lee
cyclogenesis will expand and lift northeastward in response to the
approaching upper trough axis and vorticity maxima. A new surface
low eventually develops near the IA/MO/IL border by 21z and
continues lifting northeastward into WI. The net result will be a
surface baroclinic zone oriented SW-NE which develops across MO
today and then moves southeastward tonight as a cold front. Large
scale ascent ahead of the upper trough and frontogenesis between H9-
H7 should be sufficient to support precipitation today, especially
during the afternoon. Precipitation should end by midnight across
most of the CWA.

Attention then turns to ptypes. Temperatures were steady or slowly
rising during the pre-dawn hours early this morning, and the entire
CWA was already several degrees above freezing by 09z. This suggests
that the initial ptype would be rain. However, surface dew points
have not kept pace with the rise in temperature, which leaves a
significant dew point depression and room for dynamic effects at
pcpn onset. Evaporative cooling could cause a brief period of mixed
precip (rain/snow or rain/sleet based on BUFKIT soundings) at pcpn
onset, especially before noon. Any periods of mixed precip are
expected to be brief and limited to pcpn onset. In a classic
demonstration of the Schmocker rule, precipitation should have
already ended by the time the colder air temperatures move back into
the CWA after fropa.


.LONG TERM...  (Monday through Saturday)
Issued at 347 AM CST Sun Dec 11 2016

The synoptic pattern at the start of the long term period will
feature a general trough across the north-central CONUS.  This
regime will persist through the end of the upcoming week, when a
pattern shift occurs and the trough shifts off the East Coast, with
another trough building into the western CONUS by the end of the
period which will place the Midwest more within a southwest flow

The period will start dry as Canadian high pressure will be situated
across the Northern Plains on Monday.  This location will keep the
coldest air off to the northwest on Monday, so highs should still
manage to climb into the mid 30s to low 40s.  However, the surface
ridge will shift east Monday night into Tuesday, allowing a dry cold
front to sweep through the region late Tuesday.  This will bring
much cooler temperatures for Tuesday especially across northern MO
and western IL, where high temperatures likely won`t make it out of
the low 20s!

A weak PV anomaly will slide southeast across the northern Plains
into the Midwest on Wednesday.  Ahead of it, confluence in the mid-
levels will lead to an area of mid-level frontogenesis.  However,
similar to the (non)event last Wednesday, it appears that the
Canadian ridge will nudge enough into the region by then to keep the
low-levels very dry.  So despite the mid and upper-level forcing, it
appears the dry near-surface airmass will largely win out.  Given
guidance flip-flopping on this system over the past couple of days,
will continue to leave in a slight chance of snow but this may need
to be taken out of the forecast soon if the latest trends hold.

The strong Canadian surface ridge (1036-1040mb) will settle into the
region late Wednesday into Thursday.  This will likely bring our
coldest air yet this season, with temperatures in the low teens to
upper single digits on Thursday morning and highs Thursday not
getting out of the low to mid 20s in most areas.

The pattern will amplify substantially on Friday as the western
trough ejects out into the Central Plains, causing the upper-level
flow to back to the southwest over our region.  A low-level jet will
increase atop this cold airmass during the day on Friday causing a
classic overrunning scenario.  The latest guidance, however, is not
overly bullish on QPF Friday within this warm air advection regime.
There are also differences noted in both deterministic and ensemble
data in timing and thermal profiles.  Using an ensemble approach, it
appears that the precipitation will generally start as some light
snow early Friday, then perhaps transition to more of a wintry mix
Friday afternoon into Friday night as the warm nose nudges into the
region.  Guidance has trended stronger with the surface low
associated with this ejecting Plains` trough, which allows surface
warm-air advection to win out, changing precip over to mostly rain
late Friday night into Saturday.  It appears precip will mostly come
to an end by Saturday afternoon, a quicker solution than previous
model runs.  Given the uncertainty and this being 6 to 7 days away,
will continue with simple rain/snow wording.  If trends hold, light
mixed precipitation will likely be needed in the forecast on Friday
before things change to mostly rain Friday night into Saturday.



.AVIATION...  (For the 12z TAFs through 12z Monday Morning)
Issued at 530 AM CST Sun Dec 11 2016

Cloud bases will gradually lower to MVFR by 11/18z due to
increasing saturation as precipitation evaporates before reaching
the ground. Precipitation will start to reach the ground after the
low levels have moistened. There may be a brief period of
rain/sleet or rain/snow when precipitation begins, but it should
quickly change over to all rain based on BUFKIT soundings and
surface temperatures. Initially southerly to southeasterly winds
will become southwesterly this afternoon ahead of a cold front,
then turn northwesterly after the front has moved across the area
tonight. Rain will end by 12/06z at all TAF sites.





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