Area Forecast Discussion
Issued by NWS St. Louis, MO

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FXUS63 KLSX 271149
AFDLSX

Area Forecast Discussion...Updated Aviation
National Weather Service Saint Louis MO
649 AM CDT Wed May 27 2020

.SHORT TERM...  (Through Late Thursday Afternoon)
Issued at 354 AM CDT Wed May 27 2020

A broad upper level remains closed off and centered over southeast
Oklahoma early this morning. Water vapor imagery continues to
show strong southerly flow out of the gulf and into the Midwest.
Surface analysis depicts that a warm, moist air mass remains in
control with surface dewpoints and temperatures in the mid to
upper 60s across the area.

A weak surface low and mid-level trough will continue to push
north out of Arkansas, around the eastern periphery of this upper
level feature this morning. Rain chances will increase through
the morning as radar trends shower more numerous shower activity
over the last couple of hours. CAMs show this trend continuing,
especially over southeast MO and southwest IL with a generally
broad area of showers and potentially an embedded thunderstorm.
So far, there has been little/no observed lightning with limited
instability.

This initial push will continue northeast through this afternoon
with the area of better mid-level ascent. Behind this, showers
will likely become more widely scattered with the potential for an
afternoon thunderstorm. The best potential might be realized
should we see some breaks in the clouds. Otherwise, instability
remains somewhat inhibited by extensive cloud cover with HREF
probs floating around 1000 J/kg MUCape the height of the
afternoon. For the same reason, today`s highs will run about 10
degrees cooler than Tuesday`s. We`ll see a lull in overall
shower/thunderstorm activity with the loss of diurnal instability.
However, given the broad area of upper lift overhead, hit-or-miss
showers will be possible through tonight. Lows will only drop
into the 60s with ample extend of clouds.

It looks like we`ll finally see some progress in the overall
pattern by Thursday afternoon. The closed upper wave begins to
pull northeast, along with the approach of a surface cold front
out of the northwest. Ahead of the front, scattered showers and
thunderstorms will develop with more numerous activity assigned to
the frontal boundary. The best potential for any widespread
showers and storms looks to be across the northern sections of
Missouri, as the front slowly move southeast through the day. Much
of this will hold off until later in the afternoon/evening.

Maples

.LONG TERM...  (Thursday Night through Tuesday)
Issued at 354 AM CDT Wed May 27 2020

The upper low is pulled into the Great Lakes within the northern
stream Thursday night. A mid-level trough and surface cold front
will extend southwest of the upper system, which will push northwest
to southeast through Friday morning. Given late timing, this will
likely involve a broken line of weakening showers/thunderstorms left
over from better diurnal instability from earlier in the day. The
frontal progress should have a bulk of the activity through by mid-
morning Friday with showers potentially lingering through the east
half of the area through late morning.

Moisture-rich air ahead of the front will be replaced by dewpoints
that range from the mid-40 to low-50s by Friday afternoon. Upper
ridging amplifies over the central plains with a building surface
high over the Upper Midwest and northwest flow aloft. GEFS ensemble
members show little spread through at least May 31st, lending to
some confidence that this airmass will hold through at least Sunday.
This should provide comfortable conditions with temperatures near
seasonal norms.

By early next week, longwave ridging over the plains nudges slightly
east with strengthening surface high centering over the eastern
Great Lakes. This begins to result in return flow as soon as late
Sunday, giving way to moderating temperatures Monday and Tuesday.
Though there is increasing spread in ensembles, few outliers lean in
favor of cooler conditions. Above normal temperatures are favored
beyond Monday with much of any activity focused north


Maples

&&

.AVIATION...  (For the 12z TAFs through 12z Thursday Morning)
Issued at 635 AM CDT Wed May 27 2020

Scattered showers have become more numerous early this morning and
are expected to impact most terminal sites. This activity has
been accompanied by IFR ceilings and minor reductions to
visibility in the 3SM to 5SM range. Few spots have locally dipped
near 2SM, but this will be the exception rather than the rule.

This initial push of shower activity is expected to lift north
through the early afternoon, become isolated to widely scattered
through the remainder of the day. Ceilings will gradually lift
along with patchy visibility reductions. A few thunderstorms could
be in the vicinity of terminals through the afternoon and
afternoon, but confidence is too low to carry it as a prevailing
condition.

There will be a lull in overall activity overnight with isolated
showers possible. Any persistent rain/impacts should hold off
until after the end of the period.

SPECIFICS FOR KSTL:

The main concern this morning is the IFR ceilings and variable
reductions to visibility. The great impacts seem to be running
along and ahead of an area of showers currently pushing northeast
out of southeast Missouri.

Showers will be possible through the day, along with a couple of
thunderstorms. Instability looks to be inhibited by morning cloud
cover, leaving some question to the extent of thunderstorm
potential. Therefore, VCTS suits the going forecast with the best
potential after 18z and up through dusk or so.

We will at least see some relief with the ceilings a clouds lift
through the afternoon and evening. Gradual improvement is expected
behind the initial line of showers as overall spread of activity
become isolated to widely scattered from late this afternoon into
tonight. Additional thunderstorm potential may return late in the
period (Thursday).

Maples

&&

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

$$

WFO LSX



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