Area Forecast Discussion
Issued by NWS St. Louis, MO

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537
FXUS63 KLSX 052022
AFDLSX

Area Forecast Discussion
National Weather Service Saint Louis MO
322 PM CDT THU MAY 5 2016

.SHORT TERM...  (Through Late Tonight)
Issued at 314 PM CDT Thu May 5 2016

A surface high pressure center will nudge its way into the region
tonight. The center of the high will be over northwestern AR by 12z
Fri with the ridge axis extending northeastward through eastern MO
and southwest IL. Clear skies, light northerly winds becoming
westerly, and a relatively dry air mass will create favorable
conditions for radiational cooling overnight, although not as
favorable as it would have been if the center of the high was
farther north and the winds were calm. A few locations in southeast
MO fell into the upper 30s last night, and some sheltered spots in
the eastern Ozarks might be similarly chilly tonight.

Kanofsky

.LONG TERM...  (Friday through Next Thursday)
Issued at 314 PM CDT Thu May 5 2016

(Friday and Friday night)

An omega block should have become firmly established by Fri mrng
with one low coming onshore across cntrl/sthrn CA and the other set
up over the coastal areas of the NC/VA border. This pattern means
the quiet conditions will continue thru Fri night. The CWA will be
on the downstream side of the ridge putting the FA in Nthrly flow
aloft. A weak impulse embedded in the Ntrhly flow will mean
another batch of clouds for areas E of the MS Rvr for Fri. This
scenario would normally lead to a cool day but FROPA was earlier
in the week which has allowed the airmass to modify over the past
few days. Plus the SFC ridge axis will be overhead Fri mrng with
the anticyclone centered across the AR/MO border. This puts the
area in W to SW SFC flow which will allow for a substantial warm
up after a chilly start. 850 temps of 12-16C correspond to highs
in the upper 70s to low 80s. Soundings indicate mixing to approx
that height. With return flow well underway, Fri night temps will
not be nearly as chilly as the previous couple of nights, only
dipping into the 50s. Temps should be warmer across cntrl/NE MO
and W cntrl IL where the 850 thermal ridge is closer and SFC flow
should be stronger.

(Saturday through Sunday)

The omega block is fcst to undergo an interesting evolution this prd.
Another closed low over is expected to dvlp and deepen over Hudson
Bay Fri night/Sat...eventfully absorbing the E Cst low by the end of
the wknd. A shortwave will round the base of the dvlpng low and help
push a cold front S into the Midwest Sat. The shortwave will
basically suppress the ridge between the two closed lows on the
Csts. The cold front is fcst to sag into the nthrn portions of the
FA Sat PM...reaching near I-70 by evng. Models are indicating Dps
in the 60s though not completely confident in that quality of
moisture return due to the SFC high located across the heart of
the Gulf of Mexico. With Dps fcst in the 60s by Sat aftn...models
are fcstng SFC CAPE values of generally 1000-2000 J/kg across
cntrl/nthrn MO and cntrl IL. Couple this with Bulk Shear on the
order of 40-50 kts and strong/SVR TSTMs are certainly possible.
As such...SPC has the northern half of MO and immediately adjacent
areas of W cntrl IL outlooked with a Marginal Risk on Sat.

The bndry is fcst to slowly sink S Sat night and should be
aligned by Sunday AM roughly from near MCI to near FAM before
stalling out Sun mrng. The set up on Sun aftn should be similar to
what happened on Sat. Subtle differences include slightly better
BL moisture and 5-10 kts less shear. Think sensible effects on
convective trends will be negligible with another round of strong
to potentially SVR storms possible Sunday aftn/evng. Wouldn`t be
surprised if portions of the area aren`t outlooked with a Marginal
risk on Sunday either...eventually.

There will be a decent temp gradient across the CWA this wknd. 850
temps will once again be in the low/mid teens. S of the bndry where
decent mixing occurs on Sat...highs should reach into the low 80s
again with mid 70s across NE MO and W cntrl IL where mixing will be
limited due to clouds...Erly flow and potential aftn/evng precip.
Highs will be a touch cooler across the board on Sun due to the
front being farther S with highs in the 70s along and S of the bndry
and upper 60s to the N.

(Sunday night through Thursday)

This will be an active prd. The upper low across the W is fcst to
move into the cntrl Rockies by the end of the wknd and induce lee
side cyclogenesis. The bndry begins lifting back N late Sun in
response to the SFC low lifting NE from the cntrl High Plains Sun
night to ern NE by Mon evng due to the ewrd progression of the upper
level trough. The bndry should have lifted well N of the FA by Mon
aftn putting the entire CWA in the warm sector. The position of
upper level features puts the cntrl CONUS in SW flow aloft. Several
days of low level Sthrly flow will allow decent BL moisture to be
well established by then with model consensus Dps near 60. Cold
front should be near the KS/MO border by Mon evng. TSTMs are
expected to initiate along this bndry drng the aftn/evng and
progress E. Sufficient CAPE and shear profiles appear to be in
place to allow for organized SVR storms. Hence, SPC has MO
outlooked for SVR TSTMs for Mon...with that threat being late Mon
into Mon night.

The upper trough takes on a negative tilt by Mon night. This upper
lvl feature opens and weakens as it lifts NE thru the Plains early
next week. This evolution means the cold front assoc with the lead
SFC low will be slow to clear the CWA since there is no upper lvl
support to drive it thru. There are models differences handling the
evolution of the 500mb pattern beyond Tue...but the pattern
basically becomes zonal and the front remains near the area thru the
remainder of the prd keeping daily precip chances in the fcst.

Temps should remain aoa avg thru the prd.

2%

&&

.AVIATION...  (For the 18z TAFs through 18z Friday Afternoon)
Issued at 1228 PM CDT Thu May 5 2016

VFR through the period with a few diurnal cu and occasional
cirrus. Wind gusts to around 20 kts will subside after sunset and
then become lgt/var overnight due to the passage of a surface
high pressure center. Winds will become west-southwesterly
tomorrow morning with speeds AOB 10 kts.

Kanofsky

&&

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

$$

WFO LSX



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