Area Forecast Discussion
Issued by NWS St. Louis, MO

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FXUS63 KLSX 161130

Area Forecast Discussion...Updated Aviation
National Weather Service Saint Louis MO
630 AM CDT Tue May 16 2017

.SHORT TERM...  (Through Late Tonight)
Issued at 358 AM CDT Tue May 16 2017

An upper level RIDGE axis is now to our east, resulting in SW flow
aloft for our region.  A pair of strong storm systems are well to
our west, with one near Las Vegas and the other just coming onshore
near British Columbia.  At the surface, our region was in the warm
sector with a light southerly flow, with a frontal boundary to our
north extending across far northern IA.  Temperatures were in the
60s with some thin high cirrus clouds approaching from the west.

SW flow aloft will continue thru tonight with the strong storm
system over the southwestern CONUS ejecting out into the central
Plains by late tonight while the second strong storm system from
British Columbia settling into southern Idaho.  The warm sector
setup at the surface will continue thru late tonight with southerly
flow and the absence of any fronts nearby.  While convective
development should be very limited at best thru early this evening
in our region, the approach of the strong storm system from the
southwestern CONUS and its assocated broadscale lift ahead of it
should initiate convection well to our west during the evening hours
with this then propagating northeast and weakening overnight.  Low
PoPs were maintained for areas north and west of STL metro but even
this looks to not be too concerning until after 09z tonight with a
dry forecast elsewhere.

Thin cirrus and scattered cumulus clouds will be the main cloud
cover into the evening, and should still result in decent sunshine
across the region.  With similar temps aloft to mix down from
persistence, the main variables in play are a bit better mixed lower
atmosphere and somewhat more cloud cover, but we will be starting a
few degrees warmer as well.  All of this should largely cancel out
with a persistence forecast looking awfully hard to beat today,
which is a couple degs higher than MOS:  from 85-90.  MOS min temps
used for tonight, with readings expected to drop into the 60s most
places with around 70 in STL metro.


.LONG TERM...  (Wednesday through Monday)
Issued at 358 AM CDT Tue May 16 2017

Showers and storms will spread into our forecast area on
Wednesday, particularly across northeast MO and west central IL
ahead of a negatively tilted upper level trough moving through
northwest MO.  The models depict relatively strong upper level
divergence across this area ahead of this upper level trough.  A few
strong to severe storms are possible due to strong 0-6 mb deep layer
shear mainly Wednesday afternoon as a strong mid level wind max
moves through our area ahead of the upper level trough.  Daytime
highs on Wednesday will not be as warm as the past couple of days
due to cloud cover and scattered convection, albeit still above
normal.  A cold front will sag southward into our forecast area
Thursday afternoon and night. This boundary will be the focus for
convection late Thursday night and Friday as low-mid level warm air
advection increases over this front, well ahead of an upper level
low moving into the Plains. This upper level trough/low will move
into the northern Plains and an associated surface low and trailing
cold front will move through our forecast area Saturday and Saturday
night. The operational GFS and ECMWF models have come into better
agreement with the timing of this cold front, although the GFS is
still a little quicker.  Convection is expected along and ahead of
this front.  Most of the showers and storms should shift southeast
of our forecast area by Sunday afternoon or evening, depending on
the exact timing and strength of the upper level trough moving
through the northern Plains and the progression of the cold front.
Cooler temperatures can be expected Saturday night and Sunday after
the passage of the cold front. Temperatures Sunday and Sunday night
should be slightly below normal.



.AVIATION...  (For the 12z TAFs through 12z Wednesday Morning)
Issued at 630 AM CDT Tue May 16 2017

VFR conditions, dry wx, and southerly surface winds will prevail
thru much of the valid period. Small rain chances will enter the
picture for UIN and COU very late in the valid period, but look to
be low enough to preclude mention at this time. South winds will
become gusty by late morning and continue until early evening,
with a moderately strong wind continuing thru tonight. Marginal
LLWS conditions may set in tonight but will let next shift take
another look before adding to TAF.





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