Area Forecast Discussion
Issued by NWS St. Louis, MO

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FXUS63 KLSX 141145

Area Forecast Discussion...Updated Aviation
National Weather Service Saint Louis MO
645 AM CDT Sat Oct 14 2017

.SHORT TERM...  (Through Late Tonight)
Issued at 350 AM CDT Sat Oct 14 2017

The shortwave currently digging across Idaho into Wyoming and
northern Utah will force low level cyclogenesis over the central
Plains today.  The surface low is forecast to move rapidly northeast
into Iowa today, dragging a cold front into northwest Missouri by
around 00Z.  Expecting strong warm advection in the warm sector
ahead of the front today...with temperatures well above normal in
the mid and upper 80s.  Can`t rule out tying or even breaking some
records in the lower 90s.  Not only will it be hot, it will be humid
too.  Dew point temperatures will be pushing the mid and upper 60s.
All this combined will make it feel more like late August or early
September than mid October.  Most models print out some light QPF
today across parts of northeast Missouri into west central Illinois
in the warm advection ahead of the front.  Convection-allowing
models also develop scattered showers and thunderstorms ahead of the
front. Am therefore keeping chance PoPs today stretching from
Columbia Missouri up through northeast Missouri and into west
central Illinois.

Coverage of thunderstorms should increase late this afternoon into
this evening as the front dives southeast into Missouri and
Illinois. The front should be through central and northeast Missouri
and west central Illinois by 06Z...east central Missouri by
09Z...and through the remainder of the forecast area by 12Z.  Strong
cold advection behind the front will cause temperatures to turn
sharply colder with lows Sunday morning ranging from the mid 40s in
northeast Missouri to the mid 50s in southeast Missouri and south
central Illinois.

SPC`s severe weather threat for late this afternoon and tonight
looks to be in the right place across northeast Missouri and west
central Illinois.  Instability 1000+ J/Kg of MLCAPE ahead of the
front and 50+ kts of deep layer shear should be sufficient for
rotating storms.  Damaging wind will be the primary threat...and
with 0-1km shear of 45kts and 0-1km helicity of over 200 m2/s2 will
have to watch the tornado threat closely.  Luckily, it looks like
instability drops off pretty rapidly during the evening.  GFS has
the instability down to less than 500 J/Kg by 06Z.  Therefore think
we`ll see the coverage and intensity of the storms diminish pretty
rapidly between 04-06Z...though showers will likely continue along
and ahead of the front as it pushes through.


.LONG TERM...  (Sunday through Friday)
Issued at 350 AM CDT Sat Oct 14 2017

An upper TROF will swing thru the MS River Valley on Sunday with a
pair of embedded shortwaves. The first and more notable shortwave
will push thru Sunday morning and will rapidly end any rain
chances across our region. The airmass behind this will be
relatively quite chilly versus what preceded it on Saturday, being
accompanied by blustery northwesterly winds gusting to 30-35mph
and with most locales struggling to get out of the 50s despite
decent sunshine.

The second shortwave embedded in the upper TROF will track much
further to the north and largely miss our region on Sunday night,
besides acting on a much drier column by this point.  Clear skies
are expected as the surface RIDGE settles in.  Min temps by early
Monday morning should drop into the 35-40 range for many areas and
with the anticipated near calm winds, should see patchy frost for
portions of the forecast area.

Already surface flow will become southerly by late Monday and will
probably continue thru the remainder of the work week, especially
under the consideration that there is a better model consensus
regarding the late week upper level pattern, which is has better
support for flow becoming southwest aloft.  The dry column of the
air should also persist, and will result in a dry forecast after
Sunday with mostly clear skies and temperatures returning to above
average by Tuesday with 80s possible in places for the latter half
of next week.



.AVIATION...  (For the 12z TAFs through 12z Sunday Morning)
Issued at 634 AM CDT Sat Oct 14 2017

Expect VFR flight conditions to prevail today south of a warm
front over southern Iowa. Widely scattered showers and possibly a
few thunderstorms will affect northern portions of the forecast
area over northeast Missouri and west central Illinois through
the morning. Precipitation will become more widespread late this
afternoon into the night time hours as a cold front sweeps into
the area. Strong to severe thunderstorms are possible with winds
in excess of 50kts and possibly a few tornadoes across parts of
central and northeast Missouri and west central Illinois. Storms
will likely weaken as they move east-southeast overnight ahead of
the front. There will probably be some MVFR along the cold front
as it moves across the area as well. Gusty southwest wind will
prevail today...and the wind will turn sharply to the west-
northwest after the front passes.


VFR flight conditions and gusty southwest flow is expected at
Lambert today into the evening. Thunderstorms will develop ahead
of a cold front over northwest Missouri this afternoon and will
move east this evening. Current thinking is that these storms will
not be in the vicinity of the terminal until 05-06Z tonight.
While gusty winds associated with the storms are likely, it looks
like the storms will be weakening as they move into the STL Metro
area. Could see some MVFR ceilings as the front moves through the
area late tonight. Precipitation should end between 10-12Z with
VFR conditions and gusty northwest flow thereafter.



Record max temperatures for Saturday, 14 October...

Saint Louis:  90/1899
Columbia:     92/1899
Quincy:       91/1938




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