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Area forecast discussion...updated aviation
National Weather Service Saint Louis MO
1139 PM CST Wed Nov 20 2019

Update...
issued at 923 PM CST Wed Nov 20 2019

Large area of showers is moving across Missouri this evening in
strong low level warm air advection ahead of upper trough
currently over the Central Plains. Have adjusted the forecast to
bring these showers slightly faster as they progress to the
northeast during the late evening and early overnight hours.
Additional showers currently entering southwest Missouri will move
up into the southern and east parts of the County Warning Area later tonight. It
still looks like we will get a thin line of showers and possibly a
thunderstorm as the cold front moves southeast across the area
tomorrow morning into early afternoon based on the latest runs of
the rap/NAM. Made some minor changes to temperatures tonight
based on temperatures trends.

Britt

&&

Short term... (through late Thursday afternoon)
issued at 257 PM CST Wed Nov 20 2019

The mid-Mississippi Valley is currently situated beneath an upper
ridge, with surface high pressure to our east and developing low
pressure to our west. The surface low will slide to the northeast
overnight as a shortwave embedded in SW flow pushes through the
Midwest. The forcing provided by the wave, coupled with strong low
level warm, moist advection, will be sufficient to generate
widespread precip beginning this evening in western MO before
moving east of the Mississippi late tonight. Precip will taper off
tomorrow morning beneath weak mid level height rises, though
another round of more scattered showers are expected tomorrow as
the cold front trailing the previously mentioned low sweeps
through the area. Temperatures will remain quite warm ahead of the
front thanks to the strong southerly flow, and some guidance is
even showing weak insatiability building in between the initial
widespread showers and more scattered precip along the front.
Therefore have introduced a period of low thunder chances tomorrow
morning and into the early afternoon.

Bsh

Long term... (thursday night through next wednesday)
issued at 257 PM CST Wed Nov 20 2019

Another embedded shortwave will move through the region on
Thursday night as the upper level jet intensifies. These features,
coupled with continued moisture advection along the baroclinic
zone, which at this time will be situated just south of the I-44
corridor, will result in another round of widespread rain across
southern Missouri. Rain activity in this area will again persist
through much of the night until the wave moves out of the area and
frontogenetical forcing weakens/moves out on Friday morning.
Colder air will be moving into the region at this time, but precip
should be confined to areas which will only support liquid precip.

Friday looks to be relatively dry until a weakening closed upper
low pushes toward the region in the afternoon. This low will help
deepen a surface trough over OK, which in turn will shunt the
baroclinic zone back into southern MO. The coupled forcing along
the front and passing upper trough may be enough to support light
precip in southeast MO and southern Illinois into Saturday morning. In fact,
most deterministic guidance, along with gefs and wpc ensemble
means do show at least a few hundredths in these areas. Cold air
will continue to filter into the region through the day on Friday
and into Saturday, and temps may be cool enough to support a brief
period of light snow on Saturday morning.

Dry northwest flow will begin to take over on Saturday, with high
pressure passing to the south of the region on Sunday. This will
result in a period of dry weather, with cool and cloudy
conditions on Saturday turning warmer more sunny by Sunday and
Monday. Meanwhile, a long wave trough will build into the western
conus, which will shift US into a more active pattern toward
mid week. While a storm is likely to impact the central portion of
the country during this time, model consensus is poor and forecast
details remain murky.

Bsh

&&

Aviation... (for the 06z tafs through 06z Thursday night)
issued at 1128 PM CST Wed Nov 20 2019

Showers will affect all of the terminals through at least midday
on Thursday when a cold front will move southeast across the area.
Additional showers will move into the area from the southwest
during the afternoon affecting mainly the St. Louis area
terminals. Ceilings will drop below 3000ft above ground level between 12-16z as
the cold front moves through the region and remain MVFR through at
least 00z. There will also be 45kt winds between 1000-2000ft above ground level
tonight, though surface winds will remain high enough to keep low level wind shear
too marginal to mention in the tafs.

Specifics for kstl:

Showers have moved into the terminal and should persist at least
periodically until the cold front moves through during the late
morning. Ceilings will initially be VFR, but will drop below
3000ft by 15z and remain MVFR through at least 06z on Thursday
night. There will also be 45kt winds between 1000-2000ft above ground level
tonight, though surface winds will remain high enough to keep low level wind shear
too marginal to mention in the tafs.

Britt

&&

Lsx watches/warnings/advisories...
MO...none.
Illinois...none.
&&

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