Area Forecast Discussion
Issued by NWS St. Louis, MO

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FXUS63 KLSX 180455

Area Forecast Discussion...Updated Aviation
National Weather Service Saint Louis MO
1155 PM CDT Wed May 17 2017

.SHORT TERM...  (Through Late Thursday Afternoon)
Issued at 324 PM CDT Wed May 17 2017

Surface low over northeast Nebraska to continue lifting to the
northeast into the Great Lakes region by Thursday morning. In the
meantime, prefrontal trof to continue tracking east into Illinois
this evening. Main activity to stay to our north, but could see
isolated/scattered storms through midnight.

On Thursday as surface low moves into Great Lakes, main cold front
to slide south into forecast area, stalling out along I-70 corridor.
Decent CAPES by Thursday afternoon as well as little or no cap.
However, not much in the way of a triggering mechanism, but if
storms develop, they will develop quickly. For now have slight
chance pops along and south of I-70.

Gusty south winds to diminish by sunset this evening, so 7 PM
expiration time for wind advisory still looks good.  As for
temperatures, will see lows in the 60s tonight and highs on Thursday
in the upper 70s to upper 80s.


.LONG TERM...  (Thursday Night through Next Wednesday)
Issued at 324 PM CDT Wed May 17 2017

(Thursday Night - Saturday Night)

Old cold frontal boundary will become quasi-stationary somewhere
across the central portion of the CWA Thursday night. While
mid/upper level ridging Thursday evening will try and suppress any
convection from developing (or maintaining itself from earlier in
the afternoon), cannot rule out a stray storm or two where sfc
convergence is strongest right near the cold front. Any storm that
does mature will have the chance at being strong to severe given
the very unstable atmosphere in place Thursday evening. Probably
will be at least a brief period of little to no convective
activity late Thursday evening into the early overnight hours
before a more organized round of thunderstorms develops mainly on
the cool side of the boundary late Thursday night. This activity
will be aided by strong forcing for ascent downstream of a
midlevel vorticity maxima and increasing moisture convergence at
low levels.

Thunderstorms will continue through the mid-morning hours on Friday
likely helping push the quasi-stationary front further to the south
toward portions of southeast Missouri and southwest Illinois.
Appears there will be a break or at least a downturn in convective
activity late Friday morning into the early afternoon hours as low-
level jet abates and midlevel shortwave trough advects northeast of
the area.

The closed midlevel low begins to slowly move eastward late Friday
afternoon and Friday evening. Cooling aloft and strengthening
divergence at upper levels will likely increase the threat for
thunderstorms once again. Favorable deep-layer shear and instability
will likely exist across much of the CWA through Friday night so
strong to severe thunderstorms definitely appear a possibility.

Threat for showers and thunderstorms (and strong to severe
thunderstorms most likely) will continue through much of the day on
Saturday ahead of a cold front trailing from a surface low near the
Quad Cities early Saturday afternoon. Timing of cold frontal passage
is pretty good within the model suite with the front clearing the
far eastern CWA Saturday night.

(Sunday - Next Wednesday)

Much cooler weather continues to look like the main weather story in
the extended as a deep closed low semi-anchors itself near the Great
Lakes. A period of prolonged northwest flow aloft is becoming
increasingly likely with below normal temperatures. Chances of
showers, and perhaps a few thunderstorms, increases on Monday night
and Tuesday ahead of a strong shortwave trough. Not surprisingly,
especially this far out, there is a high degree of timing
uncertainty with the ECMWF about 6-12 hours faster than the
operational GFS.



.AVIATION...  (For the 06z TAFs through 06z Thursday Night)
Issued at 1155 PM CDT Wed May 17 2017

VFR conditions are expected through the TAF period. Skies will
stay mostly clear to partly cloudy overnight. While winds will
continue to die down from earlier values, still could see some
gusts through the overnight hours near 20 knots. A cold front will
sag towards the I-70 corridor Thursday afternoon. This front will
focus some increasing cumulus development and could even spark an
isolated storm, but coverage is expected to be too low to include
at this time. Otherwise, the nearby front will make for rather
variable winds at COU/SUS/STL/CPS late tomorrow afternoon through
tomorrow evening, but speeds near the boundary will be fairly

VFR conditions expected through the TAF period. SSW gusts near 20
knots will continue to be possible overnight, but winds should go
lighter by Thursday afternoon as a cold front settles near the
terminal. Given the proximity of the front, wind direction may be
rather erratic Thursday afternoon into Thursday evening. Can`t
completely rule out an isolated storm along the boundary tomorrow
afternoon, but coverage will be too low to include any mention at
this time.





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