Area Forecast Discussion
Issued by NWS St. Louis, MO

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FXUS63 KLSX 240506

Area Forecast Discussion...Updated Aviation
National Weather Service Saint Louis MO
1206 AM CDT WED AUG 24 2016

.SHORT TERM...  (Through Late Tonight)
Issued at 326 PM CDT Tue Aug 23 2016

Will have ongoing isolated/scattered activity early this evening,
mainly along and south of I-70. Then focus shifts to Iowa, northern
MO later this evening as low level moisture increases ahead of a
shortwave that will slide through overnight. So kept best chances
over northeast Missouri/west central Illinois after midnight.
Otherwise, mostly cloudy skies to prevail with lows only dipping
into the low 70s.


.LONG TERM...  (Wednesday through Next Tuesday)
Issued at 326 PM CDT Tue Aug 23 2016

(Wednesday - Wednesday Night)

Showers and thunderstorms may be ongoing on Wednesday
morning...particularly for areas along/east of the Mississippi
River. As alluded to by the midshift...lesser chances for
showers/thunderstorms exist from the late morning into early
afternoon hours as shortwave ridging builds into the mid-Mississippi
Valley. By late afternoon...expect an area of thunderstorms to
blossom near the MO/KS border in the proximity of the slowly
progressing cold front to the south and east. MLCAPE values above
2000 J/kg and 0-6 km bulk shear of 30-35 knots suggest some
organized severe thunderstorms are possible very late Wednesday
afternoon through the mid evening hours before instability wanes
nocturnally. Based on soundings...both damaging winds and large hail
appear as possible severe threats.

Regardless of expect widespread showers and
thunderstorms to affect portions of northeast Missouri and west-
central Illinois early on Wednesday night. Slow movement of
front...strong forcing for large-scale ascent...high PWAT and warm
cloud depths all suggest the possibility of locally heavy rainfall.
A couple of mitigating factors include possibility in waning of
activity late night after passage of vort max aloft as well as
relative dry antecedent conditions/near normal soil moisture
percentages. Will hold off on any heavy rainfall mention for now but
did hit the PoPs harder boosting them Wednesday evening into the
categorical category.

Turning to temperatures...Wednesday looks like a return to
uncomfortable conditions for most as the combination of highs in the
low to mid 80s and dewpoints above 70 degrees yield peak heat index
values of 100-105. Leaned toward the warmer guidance for
temperatures already so while heat index criteria (105+) cannot
totally be ruled out...coverage would very likely be isolated and
brief in nature. Lows on Wednesday night will be slightly above
normal with most of the CWA still residing within the warm sector.
Lows may dip into the mid 60s across far NW sections of the area due
to heavy precipitation rates. Elsewhere...lows between 70 and 75
degrees are expected.

(Thursday - Next Tuesday)

Cold front will continue its slow movement to the south and east
during the day on Thursday. Accompanying the front...scattered
showers and thunderstorms will remain possible across the bi-state
region. Highest chances will reside in close proximity to the front
where low-level convergence is maximized. Elsewhere...showers and
thunderstorms are still possible due to lack of any cap within an
unstable atmosphere. Dearth of any organized focusing mechanisms
aloft to aid in forcing for large-scale ascent preclude higher
chances of showers/storms on Thursday.

Still appears that a wet and active pattern will continue across the
region through the remainder of the extended. While cold front is
still expected to clear the CWA by Thursday night...that boundary
will quickly stall just south of the CWA before returning north and
eastward as a warm front on Friday night. Similar to what was stated
in yesterday`s AFD...probably will be a relatively quiet and dry 12-
24 hours. However...with front already returning northeastward on
Friday night...chances of showers and storms will also be on the
increase heading through Saturday.

The CWA will remain draped on the northeastern periphery of the very
strong and persistent mid/upper level ridge across the southeast
CONUS. Combination of impulses ejecting out of the desert SW and
frontal boundary in the vicinity of the area...chances of showers
and storms look to remain through at least the middle of next week
for portions of the area each forecast period. Expect temperatures
to be near normal during the day and slightly above normal each



.AVIATION...  (For the 06z TAFs through 06z Wednesday Night)
Issued at 1200 AM CDT Wed Aug 24 2016

Scattered thunderstorms are possible overnight, but the highest
thunderstorm chances appear to be after 24/18z when a cold front
moves into the area, particularly at KUIN and KCOU. Although
scattered afternoon thunderstorms are also possible for
KSTL/KSUS/KCPS, confidence was too low to include in the TAFs
attm. Some transient stratus was noted on 04z METARs which was
primarily affecting KUIN at TAF issuance. Winds will become
southwesterly as the front approaches, then back and become
southerly after 25/00z.





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