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fxus63 klsx 200554 

Area forecast discussion...updated aviation
National Weather Service Saint Louis MO
1154 PM CST Tue Nov 19 2019

Short term... (through late Wednesday night)
issued at 324 PM CST Tue Nov 19 2019

Low clouds continue to move east this afternoon with drier air
eroding the back side of the cloud deck. This has resulted ample
sunshine along and west of the Mississippi River through much of the
morning. Temperatures have recovered quite well as a result with
widespread 50s and some 60 degree readings on the Missouri side of
the river. Further east, clouds have remained broken/overcast, but
will should continue to gradually break. Where sunshine has broken
out, ample mixing has led to gusty winds to around 20 kts.

Winds are expected to become light and variable under surface high
pressure overnight. Shallow, steep inversions and 1-3 degree
dewpoint depressions, generally along and east of the Mississippi
River, could lead to fog development late tonight. While widespread
dense fog isn't certain, localized areas of sub-1 mile visibility is

Surface high pressure begins to move east into Wednesday as an upper
ridge builds overhead. This will result in return flow with milder
air working northward into the region. While temperatures will
likely warm well into the 50s and even lower 60s, I believe the one
caveat to the mid-60s will be related to how quickly cloud cover
attempts to move back in from the west. Wednesday temps are already
in the upper 75th percentile and therefore have not been adjusted

The next system will stage west of the area over the Central Plains
by late Wednesday evening. The low level jet begins to strengthen
over sections of western Missouri, which coincides with a pre-
frontal trough that extends southeast of the main low. Though
precipitation will likely hold off until after sunset, the west rim
of the County Warning Area will see precipitation chances increase between 00z and
06z Thursday. Showers become more numerous overnight as the low
level jet focuses overhead with the northeast progress of the surface
low. Expect rain potential to extend into Thursday morning,
especially along and north of I-70.


Long term... (thursday through next tuesday)
issued at 324 PM CST Tue Nov 19 2019

Thursday begins a period of uncertainty, where relatively small
differences in model guidance result in starkly different forecasts
of sensible weather in the mid-Mississippi Valley. In the big
picture, models are in relative consensus. By Thursday morning, a
surface low will push through the upper Midwest, dragging a cold
front into the mid-Mississippi Valley. An additional shot of
light precip is likely on Thursday along the front before the
front stalls somewhere in the vicinity of the Ohio River.
Meanwhile, another surface low will develop in response to a
closed low over the SW Continental U.S. On Thursday night. This will enhance
return flow along the stalled front resulting in more precip in
the vicinity of the front. Precip chances will continue into
Saturday morning as the upper low pushes through the region.

All that said, the devil is in the details; specifically frontal
positioning from Thursday night through Saturday morning. The
envelope of guidance currently shows frontal positions ranging
from roughly the I-44 corridor to the MO/Arkansas border. Should the
more northern solutions play out, a la the Gem/NAM, we're likely
to see precip through the day on Friday, especially in our south.
Moreover, with precip extending farther north into the cooler air,
it's possible to see a little snow mix into the rain on Friday
morning, though the NAM is the only model showing precip extending
far enough north to result in snow. If the front is indeed
stalled farther to our south, like in the European model (ecmwf)/GFS/gefs, precip
in our region will be more limited on Friday and it would all fall
as rain. Regardless of frontal position, precip appears likely
across our south on into Friday morning, with a chance lingering
through the day.

The last shot at precip, associated with the passing upper low on
Fri night/Sat morning, looks equally uncertain. The Gem and NAM
are unsurprisingly wet into Sat morning due to the more northerly
frontal position. The operational GFS and about half of the gefs
members even bring light precip as far north as St. Louis on
Friday night as the upper low pulls the front further north before
exiting the area. The European is a good bit drier than the other
solutions, and only shows very weak signal for quantitative precipitation forecast into Sat
morning. Accordingly, have limited precip chances below 50% during
this period. Cooler air will be filtering into the region at this
time, and it's likely that snow will mix into any rain that falls
on Friday night and Saturday morning.

The pattern turns quiet by Saturday afternoon as dry northwest upper flow
and surface high pressure take hold. The high will gradually slide
to our east by the end of the weekend, helping surface temperatures
climb back to or above normal. Much of the guidance then shows a
potent cyclone moving through the middle portion of the country next
Monday night or Tuesday, likely to bring precipitation to the mid-
Mississippi Valley. However, significant differences remain in
system timing and track, resulting in considerable uncertainty at
this time.



Aviation... (for the 06z tafs through 06z Wednesday night)
issued at 1136 PM CST Tue Nov 19 2019

The main concern tonight into tomorrow morning is the fog along
and east of the Mississippi River with a high stalled over the
region. IFR fog has developed across south central Illinois, and
there is a possibility that this dense fog will further lower vsby
at uin Tuesday morning. Other terminals west of the Mississippi
River should remain VFR overnight and through the period, though
fog may move into cps and sus. Updates will be made as conditions

Specifics for kstl:

VFR conditions should prevail overnight at Lambert. However, there
is a chance for fog to develop at the terminal before sunrise.
Currently, confidence is low that the fog would lead to MVFR
conditions. Light rain moves in late Tuesday night/early Wednesday
morning near the end of the taf period, and cigs should fall to
near MVFR.



Lsx watches/warnings/advisories...
Illinois...dense fog advisory until 9 am CST Wednesday for Bond Illinois-Brown Illinois-
Calhoun Illinois-Clinton Illinois-Fayette Illinois-Greene Illinois-Jersey Illinois-
Macoupin Illinois-Madison Illinois-Marion Illinois-Montgomery Illinois-Pike Illinois-
Washington Illinois.

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