467 
FXUS63 KLSX 172027
AFDLSX

Area Forecast Discussion
National Weather Service Saint Louis MO
327 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.

Byrd


.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.


Gosselin


&&

.AVIATION...  (For the 18z TAFs through 18z Thursday Afternoon)
Issued at 1233 PM CDT Wed May 17 2017

As system over eastern Nebraska lifts northeast, it will drag a
prefrontal trof through region. Main activity will be further 
north with surface low. So timing and coverage hard to pin down 
for forecast area. Kept VFR conditions with vicinity ts mention 
through 02z-03z Thursday. Otherwise, winds to remain gusty to 
near 35 kts at times, diminishing after sunset this evening.

SPECIFICS FOR KSTL: 
As system over eastern Nebraska lifts northeast, it will drag a
prefrontal trof through region. Main activity will be further
north with surface low. So timing and coverage hard to pin down
for forecast area. Kept VFR conditions with vicinity ts mention
through 02z-03z Thursday. Otherwise, winds to remain gusty to near
35 kts at times, diminishing after sunset this evening.

Byrd

&&

.LSX WATCHES/WARNINGS/ADVISORIES...
MO...Wind Advisory until 7 PM CDT this evening for Audrain MO-Boone 
     MO-Callaway MO-Cole MO-Crawford MO-Franklin MO-Gasconade MO-
     Iron MO-Jefferson MO-Knox MO-Lewis MO-Lincoln MO-Madison MO-
     Marion MO-Moniteau MO-Monroe MO-Montgomery MO-Osage MO-Pike 
     MO-Ralls MO-Reynolds MO-Saint Charles MO-Saint Francois MO-
     Saint Louis City MO-Saint Louis MO-Sainte Genevieve MO-
     Shelby MO-Warren MO-Washington MO.

IL...Wind Advisory until 7 PM CDT this evening for Adams IL-Bond IL-
     Brown IL-Calhoun IL-Clinton IL-Fayette IL-Greene IL-Jersey 
     IL-Macoupin IL-Madison IL-Marion IL-Monroe IL-Montgomery IL-
     Pike IL-Randolph IL-Saint Clair IL-Washington IL.

&&

$$

WFO LSX