Area Forecast Discussion
Issued by NWS St. Louis, MO

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FXUS63 KLSX 202031
AFDLSX

Area Forecast Discussion
National Weather Service Saint Louis MO
331 PM CDT Tue Jun 20 2017

.SHORT TERM...  (Through Late Wednesday Afternoon)
Issued at 330 PM CDT Tue Jun 20 2017

A cdfnt continues to drop south twd the region this afternoon. Still
expect isod to sct TSRA to impact mainly the ern half of the CWA
thru the evening hours. Elevated CAPE remains limited with better
thermal profiles further east. Still, can not rule out an isod
threat, mainly for damaging winds due to the steep sub-cloud lapse
rates. If storms become outflow dominate and become rooted in the
boundary layer, much more CAPE will be available increasing the hail
threat.

The fnt will sink thru about half of the CWA before stalling and
lifting back nwd as a wrmfnt on Wed. Mdls prog warmer 850mb temps on
Wed and have adjusted max temps upward slightly across all but far
nern portions of the CWA. The sfc fnt may stall across this area
during the afternoon hours as an MCS drops sewd twd the region
during the day Wed. Given the NW flow aloft, this slower nwd
progression of the fnt seems more likely compared to the GFS/ECMWF
solns. This fnt shud provide a focus for isod to perhaps sct TSRA
again during the late afternoon hours. With the greater uncertainty,
have kept dry PoPs going for now, but updates may need add
mentionable precip.

Tilly


.LONG TERM...  (Wednesday Night through Next Tuesday)
Issued at 330 PM CDT Tue Jun 20 2017

(Wednesday Night - Friday Night)

Seasonably hot and humid weather is expected to continue to begin
this forecast period. Temperatures should be several degrees warmer
than normal both Wednesday night into Thursday along with little to
no chance of any widespread convective activity. Max heat index
values on Thursday should be in the mid 90s for most areas.

Shortwave trough will help bring a cold front southward into the
region very late Thursday night and move through the rest of the
area early on Friday. Expect convection to weaken late Thursday
night/early Friday morning as it enters portions of northeast
Missouri and west-central Illinois as low-level jet weakens and
sharply veers. It does appear coverage of storms will ramp back up
as frontal boundary interacts with quasi-stationary shear axis
aloft. Temperatures Friday afternoon will likely be several degrees
cooler behind the front with high temperatures in the low to mid 80s
for most areas or right near normal for mid/late June.

Will continue to keep an eye on the post-tropical remnants of
Tropical Storm Cindy (currently impacting the Gulf coast) for
mid/late week. In absence of very pronounced mid/upper level
anticyclone across the southeastern CONUS and shortwave trough
digging into the Upper Midwest Thursday night helping to keep
Cindy`s remnants further to the south, do not see a whole lot of
impact across our CWA. Main area of precipitation should be
further to the south and east across portions of the lower
Mississippi Valley and Ohio River Valley as cold front interacts
with shear axis aloft.


(Saturday Night - Next Tuesday)

A secondary shortwave trough will cross the northern Plains this
weekend and push another cold front through the bi-state area.
GFS/ECMWF differ with respect to timing, but it appears rain chances
may increase sometime this weekend but lots of temporal uncertainty
preclude higher PoPs at this time.

Regardless, well below normal temperatures are likely on Sunday into
early next week as upper-level trough amplifies across the eastern
2/3 of the CONUS, and 850-hPa temperatures plummet below +10C. Very
anomalous Canadian high pressure settling into the mid-Mississippi
Valley as well on Sunday/Sunday night. Look for mostly dry weather
conditions with highs in the mid 70s to near 80 degrees and lows
mostly in the 50s.

Mid/upper level heights rise heading into midweek next week combined
with return flow around periphery of departing low-level anticyclone
should yield moderating temperatures by Tuesday afternoon.


Gosselin


&&

.AVIATION...  (For the 18z TAFs through 18z Wednesday Afternoon)
Issued at 1218 PM CDT Tue Jun 20 2017

An approaching cdfnt will help initiate TSRA late this afternoon
and thru the evening hours. Expect most of the storms to remain
east of the terminals, but some may develop further west. Some of
the stronger storms may be capable of producing large hail and
damaging winds. Otherwise, winds will be light and somewhat vrb
tonight behind the weak fnt. Winds shud become swly again Wed thru
the morning as the weak fnt lifts back nwd as a wrmfnt.

SPECIFICS FOR KSTL: Added VCTS early this evening to account for
TSRA potential. Otherwise, predominately swly winds.

Tilly

&&

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

$$

WFO LSX



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