Area Forecast Discussion
Issued by NWS St. Louis, MO

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FXUS63 KLSX 182353

Area Forecast Discussion...Updated Aviation
National Weather Service Saint Louis MO
653 PM CDT Fri Aug 18 2017

.SHORT TERM...  (Through Late Saturday Afternoon)
Issued at 310 PM CDT Fri Aug 18 2017

Low level moisture continues to stream north into western Missouri
ahead of the approaching cold front characterized by dewpoints in
the lower to middle 70s. Latest convection allowing models continue
to advertise a line of thunderstorms developing across north central
Missouri and then advecting south toward I-70 around midnight.
Greatest risk of severe thunderstorms appears to be across central
Missouri where best instability will reside.

Wind will become light and variable behind the precipitation and
there is a chance that some fog will form from central Missouri to
west central Illinois toward daybreak. Otherwise, a mostly sunny day
is in store for Saturday with temperatures near seasonal normals.


.LONG TERM...  (Saturday Night through Next Friday)
Issued at 310 PM CDT Fri Aug 18 2017

(Saturday night - Sunday)

Models remain on track with advertising a NW flow aloft at the start
of this period, with rising heights aloft due to a retrograding
RIDGE from the SE CONUS on Sunday.  At the surface, the weak area
of high pressure will already be off to our east, and will result in
a light southerly flow during much of this period.  With a
relatively dry column with some recovery late to the west and no
disturbances to focus off of, this should be a dry period for most
areas.  Models continue to show some hint of MCS development in the
Mid-MO river valley Saturday night that may track east into northern
portions of our area on Sunday.  Confidence has edged a bit higher
than it was 24hrs ago, but overall rain chances remain low as its
eastward extent remains questionable.

While temperatures could be impacted by downstream cloudiness posed
by a possible MCS in northeast MO/west-central IL, it still looks
like an above average temp day for much of the rest of the forecast
area with max temps in the low-mid 90s.  Dewpoints should remain in
the 60s and will preclude any heat issues.

(Monday - ECLIPSE DAY)

The overall pattern for this day has changed little since 24hrs
ago, with a good consensus on a retrograded RIDGE aloft dominating
areas just to our south and the primary MCS track being well to our
north in IA just ahead of the synoptic surface cold front.  Models
continue to have a largely due west to due east flow at H250-300
which should keep any significant thick hi level cloud from any MCS
that does develop to the north.

However, there continues to be support for some minor/isolated TSRA
development near and ahead of a weak warm front that is anticipated
to extend north-south thru western MO Monday morning and will track
slowly eastward during the day.  While this convection could
dissipate by midday, as is rather typical for this situation, it is
the leftover convective debris cloud found at the mid and hi levels
that could ultimately squash good views of the eclipse.  The
satellite imagery from today looks very similar to what could happen
on Monday--with the clouds thick enough to have an impact on viewing
and the clouds hi enough to not be affected by any change of surface
temps versus convective temps.

Putting it together, the best eclipse viewing continues to appear to
be in southern IL which should be farthest away from any origin
areas for convection, with central MO looking like the poorest
potentially given its proximity to the weak warm front and less time
to recover if debris clouds do move overhead.

Despite a brief cooldown around midday and early afternoon due to
the eclipse, a rapid recovery during the mid-late afternoon should
allow temps to reach the upper 80s and low 90s.


A significant upper shortwave TROF still looks set to move thru on
Tuesday along with a surface cold front and should be the next good
chance of widespread rain.  The slightly earlier favored onset makes
the possibility of temps rising well into the 90s less and stuck to
values more near climo with mid 80s to low 90s.  High dewpoints will
exist in the 70s, but the heat headline possibility looking low with
temps appearing more and more to be held in check by clouds and

(Wednesday - Friday)

NW flow aloft will prevail this period while a cool high pressure at
the surface will dominate much of the period as well.  Max temps in
the upper 70s and low 80s look likely with nighttime mins in the
50s.  Rain chances look minimal.



.AVIATION...  (For the 00z TAFs through 00z Saturday Evening)
Issued at 535 PM CDT Fri Aug 18 2017

Convection developing across northwest and north central MO ahead
of a weak cold front will move southeastward through the taf
sites tonight. This convection should shift southeast of the St
Louis metro area by early Saturday morning. South-southwest
surface winds will become light later this evening, and this
along with a clearing sky late tonight/early Saturday morning
behind the convection, plus possible evening rainfall may lead to
some fog mainly for UIN and COU. A weak westerly surface wind can
be expected on Saturday with little if any cloud cover.

SPECIFICS FOR KSTL: Convection developing across northwest and
north central MO ahead of a weak cold front will move
southeastward through the STL area tonight, around 05-08Z. This
convection should shift southeast of the STL area by early
Saturday morning. Southwest surface winds will become light later
this evening, then veer around to a west-northwest direction
Saturday morning, albeit still weak. Little if any cloud cover is
expected for Saturday and Saturday night.





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