Area Forecast Discussion
Issued by NWS St. Louis, MO

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FXUS63 KLSX 030841

341 AM CDT Sun May 3 2015

.SHORT TERM: (Today and Tonight)
Issued at 325 AM CDT Sun May 3 2015

There is quite a bit of cloud cover early this morning across the
region courtesy of two weak short waves in the westerly flow
aloft. The first of these currently centered from northern IL into
southeast MO has been generating some spotty high-based showers
from the associated mid clouds/moisture. Present indications are
this wave and the mid level moisture will continue to push east
this morning with a few sprinkles still possible after 12z in
south central IL. The other short wave is currently centered near the
Nebraska/Iowa border into northeast KS and is more problematic. The
associated convection has generated a sizeable cloud canopy which
has not been resolved by the models and will overspread a good
portion of the CWA this morning. Unclear is how it will evolve
with time - i.e. thinning, but the RAP has this wave centered through
far eastern IA at midday and the current thought is to have quite
a bit of clouds the first half of this morning then thinning and
moving east by midday into early afternoon. There are also
suggestions in the RAP, ECMWF, and a few of the hi-res WRFs that
the southwest stretching tail of this short wave could generate a
few showers across northeast MO and west central IL this afternoon
and early this evening. I have added a slight chance as a result.
Otherwise a warm day is on tap with well above normal temperatures
expected. While the morning clouds will hamper the temp rise a
bit, increasing south-southwesterly low level flow and rising low-
level temperatures should compensate and will result in highs in
the upper 70s to mid 80s across the area.

South-southwest lower trop flow will maintain a low-level warm
advection region tonight and moisture will gradually be improving.
The thermal and moisture advection combined with a plume of
relatively steep mid level lapse rates will result in a swath of
weak instability across northern parts of the CWA in advance of
the southward sinking cold front. While the front will remain well
north of the CWA at daybreak Monday, northwest steering flow will
help push at least scattered showers and thunderstorms or
thunderstorm clusters into northeast MO and west central IL late
tonight. We could also see spotty WAA generated showers well in
advance of the activity that has moved off the front, hence the low
pops all the way down to St. Louis.


.LONG TERM:  (Monday through Saturday)
Issued at 325 AM CDT Sun May 3 2015

The west-east oriented cold front will ooze southward out of IA
and northern IL into northern sections of the CWA by early Monday
evening. Ongoing showers and thunderstorms at daybreak should
dissipate by midday. Heating and continued low-level moistening
in their wake will result in weak-moderate instability along the
frontal zone and the combo of a migrating impulse and good low
level convergence along the boundary should result in extensive
convective development during the later half of the afternoon
spanning from KS across northern MO/southern IA into central IL.
A broad veering southwesterly LLJ will maintain numberous
thunderstorms/clusters during the evening, with the activity
shunting a bit northward overnight in response to increasing
ridging aloft and a slight northward shift in the frontal zone.

The front will continue to shift northward on Tuesday as ridging
aloft continues to build from the lower TN Valley into the upper
MS Valley. There are some suggestions of air mass type convection
across southern portions of the CWA but I`m not sold on it and
have kept pops in the slight range and confined the coverage.
Otherwise Tuesday will be another day of above normal temps with
highs in the 80s. This warmth will persist on Wednesday and even
into Thursday with the chance of showers and thunderstorms
slowly increasing west-east over the 2 day period as the ridging
aloft shunts east and southwest flow aloft becomes more dominant
with time. The greatest threat of showers and thunderstorms in the
extended period will be Friday-Saturday with an active southwest
flow aloft, moist unstable warm sector, and slowly encroaching
cold front.



.AVIATION:  (For the 06z TAFs through 06z Sunday Night)
Issued at 1128 PM CDT Sat May 2 2015

VFR flight conditions are expected to continue through Sunday.
Very light showers continue to drift southeast over parts of the
area. Shower activity should continue to diminish slowly through
the rest of the night. Showers are so light that there should be
no impact to operations if one does move over an airport. Low
level wind shear is a good possibility especially over central and
northeast Missouri as well as west central Illinois. Models are
indicating that the wind will increase to around 30kts between
400-800 ft providing +/- 20-25 kts of shear from the surface to
those altitudes. Shear values look to fall off further east/southeast.
Expect increasing south-southwest wind on Sunday morning with
gusts to around 20kts during the afternoon.

Specifics for KSTL:

VFR flight conditions are expected to continue through Sunday.
There may be a sprinkle or two through the rest of the night, but
airport operations are not expected to be affected. Some low level
wind shear still looks possible at Lambert, but values do not look
high enough to warrant a mention in the TAF at this time. Wind
will increase from the south-southwest Sunday morning with gusts
to around 20kts during the afternoon.





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