Area Forecast Discussion
Issued by NWS St. Louis, MO

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FXUS63 KLSX 281142

Area Forecast Discussion...Updated Aviation
National Weather Service Saint Louis MO
642 AM CDT Wed Jun 28 2017

.SHORT TERM...  (Through Late Tonight)
Issued at 310 AM CDT Wed Jun 28 2017

Primary forecast challenge in the short term this morning is PoPs.
A shortwave aloft over the Great Plains has developed a surface trof
extending from the Dakotas south-southwest into New Mexico.
The thunderstorms currently over the eastern Plains ahead of the
trof are being fed by a 40-50kt low level jet and should
gradually diminish this morning as they move east and run into a
wall of low-CAPE air over Missouri.  Can`t rule out a few showers or
clap of thunder over northeast Missouri before they totally
dissipate...but don`t think anything more than slight chance PoPs
are warranted for the morning at this point.

Return flow ahead of the trof is bringing more humid low level air
back into the Mid-Mississippi Valley...but dewpoints are mostly
still in the 50s over our area.  Dewpoints do creep up into the low
to mid 60s today, but the richest moisture circles around to our
north...pooling ahead of the front across southeast Nebraska and
Iowa.  Meanwhile, warm advection aloft should push 850mb
temperatures into the mid and upper teens across much of our area.
Forecast soundings on the RAP, NAM, and GFS show this warm air aloft
as a decent capping inversion this afternoon which should inhibit
convection into the early evening.

Feeling less confident about convection overnight though.  All
guidance fires up convection along the synoptic boundary over Iowa
during the evening.  The mass fields on the GFS, NAM, and RAP
support the formation of a slow moving complex of thunderstorms over
Iowa on the nose of a 50kt low level jet.  Corfidi vectors would tend
to have this complex drift southward overnight...but forecast MUCAPE
drops off pretty quickly from northeast Missouri southward.
Interestingly, the 4km NAM NEST and the NCEP 4km WRF spread showers
and thunderstorms over most of the northern 1/2 of the CWFA.  While
I don`t want to discount the CAMs, precip that widespread isn`t
really supported by the remainder of the models.  Have therefore
just kept low to mid chance PoPs primarily over the northern 1/3 of
the CWFA tonight with some slights a bit further south into parts of
central and east central Missouri.


.LONG TERM...  (Thursday through Tuesday)
Issued at 310 AM CDT Wed Jun 28 2017

Warmer, more humid weather to move back in by Thursday. In the
meantime, stalled frontal boundary along the IA/MO border will be
focused for storms with some training likely. Depending on where the
front is, our far northern portions could see around an inch of
rain, or upwards of 4 inches. For now models have been consistent
with heaviest rain just north of forecast area. Otherwise, main
issue Thursday/Thursday night will be possible MCS development over
northeastern KS/northwestern MO sliding east towards forecast area
Thursday morning, diminishing and any kind of left over boundaries
to help redevelopment by afternoon/evening hours on Thursday. Decent
MU CAPES in excess of 2500 J/kg, lapse rates and low level jet, so
could see strong to severe storms, mainly along and north of I-70.

By Friday, surface low along boundary begins to lift northeastward
dragging cold front southeastward across forecast area. Models have
differences in timing with this boundary. NAM is faster with it
while GFS is slower with passage. Placement and timing of boundary
will be key to severe weather chances. For now will go with a blend
of the two and SPC has placed forecast area in a marginal risk for

Boundary to exit region by late Friday night, so will see a break in
the precipitation Saturday through Sunday morning, before lifting
back north as a warm front Sunday night/Monday, with increasing
chances of showers and storms once again in warm sector across
forecast area. As for the 4th of July, how stormy it may be will
depend on timing of next system as it moves through. For now have
chance pops during the day on Tuesday.

As for temperatures through early next week, will see mid 80s to low
90s on Thursday, then 80s everywhere for the rest of the forecast



.AVIATION...  (For the 12z TAFs through 12z Thursday Morning)
Issued at 628 AM CDT Wed Jun 28 2017

Expect VFR flight conditions to prevail across the area
today...and most likely for most of tonight as well. Showers and
thunderstorms over northwest Missouri should continue to weaken
and dissipate as they move east this morning. Can`t completely rule
out a little bit of precip over northern portions of the area
late this morning, but lingering showers that make it into
northeast Missouri should be very light and are not expected to
impact flight conditions. Expect wind to increase from the south-
southwest today and become gusty up to 20-25kts.

Another round of thunderstorms is expected to develop over Iowa
tonight. Some of the storms are expected to drift south into
Missouri, but confidence is not very high in what the areal
coverage will be across our area. Think at least some widely
scattered storms after 05-06Z are possible into northeast Missouri
and west central Illinois. VFR conditions are expected outside of
storms tonight.


VFR flight conditions are expected at Lambert at least through
tonight...and most likely through Thursday morning as well. Wind
will turn to the south-southwest by mid to late morning and likely
gust to 20-25kts. Storms over Iowa tonight may drift far enough
south by Thursday morning to affect the terminal, but probability
of that happening is pretty low so did not mention any thunder in
the TAF.



Saint Louis     90  74  90  75 /   5  10  10  10
Quincy          86  68  86  70 /  10  40  40  50
Columbia        88  68  89  71 /   5  10  10  30
Jefferson City  90  69  90  72 /   0  10  10  30
Salem           85  69  89  73 /   0   5  10  10
Farmington      86  68  88  71 /   0   5  10  10




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