Area Forecast Discussion
Issued by NWS St. Louis, MO

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730
FXUS63 KLSX 220904
AFDLSX

Area Forecast Discussion
National Weather Service Saint Louis MO
404 AM CDT Thu Jun 22 2017

.SHORT TERM...  (Through Late Tonight)
Issued at 403 AM CDT Thu Jun 22 2017

Flat zonal upper level flow prevailed over the northern CONUS early
this morning with a RIDGE across the southern CONUS.  Tropical Storm
Cindy appears to have come onshore just west of the Louisiana-Texas
border.  Cindy`s influence extends towards our region, with richer
moisture and cloud cover inching northward thru southern MO and IL.
A weak surface RIDGE hangs on over our region for now, with a weak
boundary across central IL kicking off a few SHRA.  It is yet
another mild summer night with temps in the 70s for most locations
with a few pockets of 60s.

Hi-res discrete models suggest the isolated shower activity in
central IL should be short lived and dissipate/exit by mid-morning.
With little else to keep it going and convective processes switching
to something more diurnal by then, this makes sense.

An upper TROF will begin to develop and deepen over the north-
central CONUS later today and continue to deepen as it slides east
into the upper MS valley tonight.  Cindy is expected to track north
and then northeast, reaching eastern AR by late tonight.  A series
of disturbances and rich moisture is forecast to stream out ahead of
Cindy`s track into the lower OH valley beginning already by late
this morning.  This will place our forecast area along the northwest
edge of these features and where there will be a PoP gradient.
For most areas for much of today and this evening, we are expecting
isolated to scattered showers to move in or develop later this
morning and linger until around sunset--with better rain chances
heading towards southeast MO and far southern IL.  During peak
heating, CINH weakens enough everywhere with the best instability
actually across areas N and W of STL Metro and this is where the
main thunder chances will be although expected coverage should
remain isolated.

Most of this activity away from the moisture and disturbance stream
just to our southeast should be diurnally driven and largely
dissipate around sunset with showers over southeast MO and far
southern IL likely persisting.

TSRA is anticipated to develop along a cold front this afternoon
well to our NW and this activity will follow the front into our
region late tonight.  While this will be weakening at that time,
there is a small chance for a strong storm capable of strong gusty
winds.

Clouds will keep temps in check today for most areas, especially S
and E of STL metro, ranging from upper 70s for maxes in southeast MO
to the upper 80s/near 90 in northeast MO where sunshine will linger
the longest.  Min temps tonight not expected to be much different
than persistence due to high moisture content of airmass and clouds,
with the only changes being in northeast MO where the cold front is
expected to move thru and temps should slip back into the 60s.

TES

.LONG TERM...  (Friday through Wednesday)
Issued at 403 AM CDT Thu Jun 22 2017

Showers and thunderstorms can be expected Friday, especially during
the morning and especially across southeast MO and southwest IL as
the remnants of Tropical Storm Cindy moves from AR into TN and a
cold front moves southeastward through the forecast area.  The
convection should be confined to southeast MO and southwest IL,
south-southeast of STL by Friday afternoon.  The precipitation will
move south-southeast of the forecast area by Friday evening as the
cold front shifts southeast of the region, while the remnants of
Cindy moves eastward through the Tennessee Valley region.  A surface
ridge will build into the region with northwesterly winds bringing
cooler and less humid air into our area.  Temperatures will be
cooler Friday night due to low level cold air advection, a clearing
sky, and surface dew points lowering into the 50s.  Lows will be
about 5 to 10 degrees below normal.  A relatively cool weekend is
forecast with northwest upper level flow as an upper level trough
centered over the Great Lakes region deepens and sends a secondary
reinforcing cold front southeastward through our forecast area
Saturday night. It appears that this frontal passage will be dry.  A
northwest flow shortwave may bring some showers to parts of central
and southeast MO late Sunday night.  The relatively cool and mostly
dry weather pattern will continue early next week with continued
northwest upper level flow, and as a large surface ridge moves
slowly through the region.  Should see warmer temperatures by
Wednesday as the Great Lakes upper level trough moves well east of
the region, while southerly low level flow returns as the surface
ridge shifts east of the area.  Increasing low level warm air and
moisture advection may lead to a threat of convection across parts
of northeast and central MO by Wednesday.

GKS

&&

.AVIATION...  (For the 06z TAFs through 06z Thursday Night)
Issued at 1148 PM CDT Wed Jun 21 2017

High clouds will increase tonight, but VFR conditions will prevail
overnight. Low-level moisture will increase through the day on
Thursday, which will help MVFR cigs develop and move northward.
Latest indications are that the line between VFR cigs and MVFR
cigs will be very close to the St. Louis metro sites. Have highest
confidence in MVFR cigs at CPS thus have introduced them there,
but will continue to leave out MVFR cigs at STL/SUS. COU/UIN will
remain VFR through the period. Cannot rule out a few showers near
the St. Louis metro sites through the day on Thursday, but the
bulk of the activity should remain across southeast MO and IL.
Winds through the period will be out of the south around 10 knots,
with some higher gusts around 15 knots possible.

SPECIFICS FOR KSTL:
High clouds will increase tonight, but VFR conditions will prevail
overnight. Low-level moisture will increase through the day on
Thursday, which will help MVFR cigs develop and move northward.
Latest indications are that the line between VFR cigs and MVFR
cigs will be very close to STL. Given the main slug of moisture
should remain east of the terminal, will leave out any cig
restrictions for now and let the midnight shift re-evaluate.
Cannot rule out a stray shower through the day on Thursday, but
the bulk of the activity should remain across southeast MO and IL.
Winds through the period will be out of the south around 10
knots, with some higher gusts around 15 knots possible.

KD

&&

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

$$

WFO LSX



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