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Area forecast discussion...updated aviation
National Weather Service Saint Louis MO
651 am CDT Thu Jun 29 2017

Short term... (through late this afternoon)
issued at 335 am CDT Thu Jun 29 2017

Primary concern today is convective trends. Most models are not
handling the current convection over Missouri very well. Trends
over the past 60-90 minutes show a definite weakening with cloud
tops warming on GOES-16 infrared and less organization to the individual
storms on radar. However, most guidance looks far too fast in
shutting the convection down over the next few hours. The low level
jet should be weakening and veering slightly over the next few hours
according to guidance which should continue the weakening trend.
However, the jet will continue to interact with the outflow boundary
as it moves through central and east central Missouri producing
additional showers and storms at least through 12-13z.

Lingering showers and storms should dissipate by 14-15z this morning
and there should be some dry time from mid morning into the early
afternoon. There will likely be a lot of convective debris over the
area which could initially limit insolation somewhat. However,
expect much of the cloud cover to thin out and clear by mid-late
morning. This combined with southerly flow should allow for a good
warm up into the upper 80s to low 90s. Growing instability this
afternoon with MLCAPE values in excess of 2000 j/kg over northern
into central Missouri combined with 40-50kts of 0-6km shear will be
more than enough to support severe thunderstorms. However...since
the models aren't doing so well with the current convection, am not
very confident in how they're handling initiation for this
afternoon. Think most likely scenario right now is that storms will
develop over northern/northwestern Missouri this afternoon and grow
upscale into another mesoscale convective system.

Carney

Long term... (tonight through wednesday)
issued at 335 am CDT Thu Jun 29 2017

Tonight, timing and placement of showers/storms will depend on where
activity fires up once again along stalled boundary near
Iowa/Missouri border and track east along it. For now best chances
will be along and north of Interstate 70. As for rainfall, could see
another inch or two over northeast Missouri and into west central
Illinois.

By Friday, frontal boundary begins to sink south across forecast
area. Models continue to have differences on timing which would
affect when and where new activity would fire up, due to ongoing
storms and outflows. Will side more with the slower GFS/European model (ecmwf) which
seems more reasonable, with decent mu convective available potential energy between 2500 and 3500
j/kg, as well as lapse rates and convergence along the boundary. So
Storm Prediction Center is keeping forecast area in marginal risk with slight risk over
portions of central and southwestern MO.

The front will push south of forecast area after 06z Saturday, but
will see lingering activity just behind boundary through Saturday
afternoon, before precipitation comes to a end.

Dry conditions expected for last half of weekend, though some
isolated elevated storms not question Sunday afternoon, especially
north of I-70. Then by late Sunday night as next system approaches
region from the Central Plains, it will lift stalled frontal
boundary back north through forecast area. Will see increasing
chances of showers and storms through the day Monday and persist
into the 4th of July Holiday. Extended models have differences on
strength and timing of this system, so at this time can not
guarantee a rain free Holiday.

As for temperatures through the forecast period, to remain near
normal, in the 80s with overnight lows in the 60s to low 70s.

Byrd

&&

Aviation... (for the 12z tafs through 12z Friday morning)
issued at 636 am CDT Thu Jun 29 2017

Primary concern for this morning is precipitation trends.
Expecting the remaining showers over the area to continue to
diminish through 14z. However...the northern fringes of another
line of thunderstorms now over eastern Kansas may affect parts of
central Missouri later this morning. Model guidance is not
handling any of this precipitation very well, so confidence is low
at this time. Elsewhere and otherwise...expect VFR flight
conditions and south-southwest flow today outside of any
precipitation. Expecting another round of thunderstorms tonight as
a surface front moves into Missouri. Timing and coverage of
thunderstorms is still uncertain since model guidance is not doing
well at this time. There does seem to be some agreement in the
guidance that MVFR ceilings will develop over southern Missouri
late tonight and expand northward toward the I-70 corridor by 12z.

Specifics for kstl:

Showers in the vicinity of the terminal are dissipating and are
expected to be gone by 13z. Expect VFR flight conditions to
prevail thereafter through the day with west-southwest flow. Wind
could be gusty at times. Another round of thunderstorms is
possible tonight, though timing and coverage of the storms is
uncertain at this time. Model guidance is not performing very well
at this time which lowers confidence in the forecast for tonight.
There is some agreement that there will be MVFR ceilings
developing over southern Missouri late tonight...spreading north
into the terminal by early Friday morning.

Carney

&&

Preliminary point temps/pops...
Saint Louis 92 74 87 70 / 20 40 50 40
Quincy 86 69 83 64 / 40 60 40 10
Columbia 90 71 83 65 / 30 50 40 40
Jefferson City 92 72 85 66 / 20 50 50 50
Salem 88 72 86 69 / 10 20 50 40
Farmington 88 71 85 67 / 10 10 50 60

&&

Lsx watches/warnings/advisories...
MO...none.
Illinois...none.
&&

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