Area Forecast Discussion
Issued by NWS St. Louis, MO

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FXUS63 KLSX 190348
AFDLSX

Area Forecast Discussion...Updated Aviation
National Weather Service Saint Louis MO
1048 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.

CVKING

.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.

(Tuesday)

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
rain.

(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.

TES

&&

.AVIATION...  (For the 06z TAFs through 06z Saturday Night)
Issued at 1026 PM CDT Fri Aug 18 2017

Weakening, spotty convection will be moving through parts of the
St Louis metro area around 06Z late tonight, but should shift
southeast of this area by 07Z followed by decreasing cloudiness.
May need to include VCSH in the St Louis metro area tafs til 07Z
depending on the latest radar trends. South-southwest surface
winds will become light late tonight, and this along with a
clearing sky may lead to some light patchy fog early Saturday
morning. A weak westerly surface wind can be expected on Saturday
with scattered diurnal cumulus clouds possible during the late
morning and afternoon along with some high level cloudiness.
Little if any cloud cover is expected Saturday night with light
surface winds.

SPECIFICS FOR KSTL: Weakening, spotty convection will be moving
through the STL area around 06Z late tonight, but should shift
southeast of this area by 07Z followed by decreasing cloudiness.
May need to include VCSH in the STL taf til 07Z depending on the
latest radar trends. Southwest surface winds will become light
late tonight, then veer around to a west-northwesterly direction
Saturday morning, albeit still quite weak. Scattered diurnal
cumulus clouds will be possible during the late morning and
afternoon on Saturday along with some high level cloudiness.
Little if any cloud cover is expected Saturday night with light
surface winds.

GKS

&&

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

$$

WFO LSX



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