Area Forecast Discussion
Issued by NWS St. Louis, MO

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345
FXUS63 KLSX 141708
AFDLSX

Area Forecast Discussion...Updated Aviation
National Weather Service Saint Louis MO
1208 PM 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.

Carney

.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.

TES

&&

.AVIATION...  (For the 18z TAFs through 18z Sunday Afternoon)
Issued at 1142 AM CDT Sat Oct 14 2017

A cold front will drop southeastward through the taf sites
tonight. There may be scattered showers or thunderstorms in UIN
this afternoon ahead of the front. A line of thunderstorms will
move southeastward along the front through UIN and COU this
evening and through the St Louis metro area late tonight. There
may be strong winds with the storms in UIN and COU. A strong
southwesterly low level jet will lead to LLWS conditions in the
taf sites late this evening and overnight. The showers and storms
will shift southeast of UIN and COU by early Sunday morning, and
southeast of the St Louis metro area by late morning. There will
be a band of MVFR cigs which will move through the taf sites late
tonight and early Sunday morning along and just behind the cold
front. Gusty south-southwest surface winds this afternoon will
veer around to a northwesterly direction late tonight after fropa
and will be strong and gusty through Sunday morning due to a tight
surface pressure gradient.

SPECIFICS FOR KSTL: A cold front will drop southeastward through
STL late tonight. A line of thunderstorms will move southeastward
along the front through STL late tonight, beginning around 07Z
Sunday. A strong southwesterly low level jet will lead to LLWS
conditions in the STL area late this evening and overnight.
Lingering showers will shift southeast of the St Louis metro area
by late morning. There will be a band of MVFR cigs which will
move through the STL area late tonight and early Sunday morning
along and just behind the cold front. Gusty south-southwest
surface winds this afternoon and this evening will veer around to
a northwesterly direction late tonight and early Sunday morning
after fropa and will be strong and gusty through Sunday morning
due to a tight surface pressure gradient.

GKS

&&

.CLIMATE...
Record max temperatures for Saturday, 14 October...

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


&&

.LSX WATCHES/WARNINGS/ADVISORIES...
MO...None.
IL...None.
&&

$$

WFO LSX



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