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Area forecast discussion...updated aviation
National Weather Service Saint Louis MO
644 PM CDT Thu Aug 22 2019

Short term... (through late Friday afternoon)
issued at 325 PM CDT Thu Aug 22 2019

The west-east cold front continues to move slowly southward late
this afternoon and stretched from south central Illinois to just north
of St. Louis into central MO. Frontal lift along with an
eastward-tracking vort Max currently centered in south central MO
will continue to support scattered showers and tstorms along and
ahead of the cold front as it advances southward tonight. I would
think the coverage will be greatest through mid-evening owing to
diurnal instability and then coverage will wane. There is a signal
of some mid level frontogenesis across southern MO and southern
Illinois from late evening into the overnight hours attendant with
another weather impulse, so even after the diurnal weakening there
will continue to be a chance of showers and storms through the
night.

Otherwise drier and cooler air will settle into the area tonight
as surface high pressure builds into the area in the wake of the
front. Clouds tonight are a bit tricky. There will be quite a bit
of cloudiness into the evening along and ahead of the front. While
some of the diurnally driven clouds will dissipate, there is a
strong signal in the low level moisture progs that stratus will
redevelop overnight and into early Friday morning across the
southern half of the County Warning Area.

Progression of the upper flow will bring a ridge aloft into the
upper MS valley on Friday while the positively tilted upper trof
currently over the Great Lakes shifts eastward. A weak impulse
originating from the Central Plains will undercut the ridge and
could bring scattered showers and thunderstorms to parts of
southeast MO and southern Illinois. Otherwise the expansive surface high
will dominate the area. It will be cooler due to the exchange in
air mass and also due to clouds. The overnight stratus should
become more cumuliform with daytime mixing and diurnal cu will
also ensue in areas not covered by early morning stratus.

Glass

Long term... (friday night through next thursday)
issued at 325 PM CDT Thu Aug 22 2019

Saturday looks like the driest and coolest day of the period. The
models continue to show an upper level short wave ridge
persisting over the upper Mississippi River valley region. This
short wave ridge and the weak flow associated with it over the bi-
state region combined with the large surface high will help keep
precipitation at Bay Saturday and temperatures below normal.

By midday Sunday the short wave ridge moves out and an upper level
low begins to dig into the region. All models show an axis of the
upper low moving through the Missouri-Illinois area during the day
on Sunday. The GFS, NAM, and Euro show a short wave coming down from
the trough and impacting the County Warning Area on Sunday, though there is little
consensus on timing. Given the uncertainty surrounding the potential
development of this feature, pops have been left at a slight chance
in our southern County Warning Area. Our best chance of thunderstorms will come
Monday into Tuesday when the models show good consensus that a cold
front will track through the bi-state region.

Temperatures Saturday and Sunday will remain cool for August in the
wake of thursday's cold front. High temperatures are expected to
reach into the upper 70s and low 80s over the weekend. Monday will
be the warmest day of the week in advance of the cold front. Tuesday
and beyond after the cold front passage, temperatures will be
slightly cooler than normal.

Mrm

&&

Aviation... (for the 00z tafs through 00z Friday evening)
issued at 632 PM CDT Thu Aug 22 2019

A cold front is slowly moving southward away from the terminals
this evening which should also the threat for showers and
thunderstorms. The main concern for late tonight into the day on
Friday will be the development of MVFR stratus at kcou and the
Metro terminals. Timing wise, it appears it should develop just
before sunrise, with ceilings slowly lifting back to VFR
thresholds around midday. Wind speeds should remain fairly light
through the period (under 10 knots) out of the northeast behind
the aforementioned frontal boundary.



Specifics for kstl:

A cold front has recently moved through the terminal, which should
bring an end to the threat for showers and thunderstorms as well
as a wind shift to the northeast. MVFR ceilings are expected to
affect the terminal immediately before sunrise on Friday before
slowly lifting through the day. VFR conditions are favored by
midday or early afternoon and continue through the evening hours.



Gosselin



&&

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

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