Area Forecast Discussion
Issued by NWS St. Louis, MO

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FXUS63 KLSX 251155
AFDLSX

Area Forecast Discussion...Updated Aviation
National Weather Service Saint Louis MO
655 AM CDT Wed Apr 25 2018

.SHORT TERM...  (Through Late Tonight)
Issued at 330 AM CDT Wed Apr 25 2018

Surface observations depicts a boundary draped from north to south,
running almost parallel to the Mississippi River. The boundary marks
a sharp divide of higher surface moisture to the east and much drier
air to the west. Skies have cleared in most areas and just east of
the boundary, winds have become calm/light and variable. Combined
with a strong and shallow inversion, patchy fog has developed. Very
localized and brief episodes of denser fog (less than 1 mile) have
been observed. However, this is exception and not the rule, as the
boundary will slowly drift east overnight and anything presenting
lower visibility will be rather progressive in nature. Otherwise,
expect a large majority of location to remain above 2 miles.

The dry air across Eastern Missouri is wedged between this boundary
and another system currently over the plains. This will press
southeast through the area during the day Wednesday with a
thickening mid-level layer of cloud cover. Moisture progs are better
to the south and west of the St. Louis area. High res solutions are
a bit more aggressive about popping off late afternoon showers
associated with the mid-level lift, as far north as St. Louis.
However, the shallow dry layer at the surface will likely hold
through most of the afternoon with any window for rain being very
short lived late in the day. Expect much of any rainfall, especially
that of which is measurable, to remain south of the metro area.

The system exits by early Thursday morning, high pressure builds in
on the back side with drier air filtering in. Temperatures still a
tad below normal, but comfortable given context to recent cool
than normal conditions.

Maples

.LONG TERM...  (Thursday through Tuesday)
Issued at 330 AM CDT Wed Apr 25 2018

Low pressure just south of the boot heel on Thursday morning will
continue moving east and away from the mid Mississippi Valley
through the day.  There may be a few sprinkles wrapped around the
northwest side of the low into our southeast Missouri counties on
Thursday morning, but the last of the precip should be out of our
forecast area by Noon.  Another shortwave will race southeast across
Missouri and Illinois Thursday night.  The surface trof will move
through the forecast area overnight.  Guidance has been trending
drier with the passage of this feature over the past couple of
days...and I`m now hard pressed to find any QPF in our area.  Will
therefore go ahead and remove any remaining mention of precip on
Thursday night.

The aforementioned shortwave will amplify the longwave trof over the
eastern CONUS Friday and all guidance is holding the high amplitude
weather pattern at least through the weekend.  Another weak
disturbance will move through the Upper Midwest Friday/Friday night
and the surface front associated with it is forecast to push through
central Illinois and northern Missouri late Friday night or early
Saturday morning.  There will be very little moisture for this
system to work with so it should come through dry with little to
mark its passage through our area but some scattered clouds, a wind
shift, and slightly cooler temperatures along and east of the
Mississippi.  Stuck pretty close to ensemble guidance temperatures
for this period, with highs mainly in the mid and upper 60s and lows
in the 40s.

Medium range guidance begins shifting the pattern eastward Sunday
and Monday which will push high pressure from the Great Lakes and
Ohio Valley on Sunday over to the Eastern Seaboard on Monday.
Southerly flow along with rising heights aloft should bring us a
significant warm up Monday and Tuesday on the western periphery of
the surface high. Highs will likely rise into the upper 70s on
Monday and into the 80s on Tuesday.

Carney

&&

.AVIATION...  (For the 12z TAFs through 12z Thursday Morning)
Issued at 635 AM CDT Wed Apr 25 2018

Relatively little concern through the period with VFR conditions
and light winds in the forecast. Showers will develop over Central
Missouri during the afternoon, which would reach as far east of
the Mississippi River. KCOU has the greatest chance at seeing any
rain and even that should remain light and brief. Site further
east (KSUS, KSTL and KCPS) only have about a 4 hour window late
this afternoon/evening. KUIN will remain dry.

SPECIFICS FOR KSTL:
VFR conditions forecast through the period. Lowest cigs come in
late this afternoon and evening at about 5k-6k ft. Wind remains
light at or less than 10 kts. Showers may skim KSTL during the
late afternoon/evening. If so, much of it should remain light with
better moisture staying much further south and west.

Maples

&&

.LSX WATCHES/WARNINGS/ADVISORIES...
MO...None.
IL...None.
&&

$$

WFO LSX


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