Area Forecast Discussion
Issued by NWS St. Louis, MO

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449
FXUS63 KLSX 261744
AFDLSX

Area Forecast Discussion...Updated Aviation
National Weather Service Saint Louis MO
1244 PM CDT Wed Jul 26 2017

.SHORT TERM...  (Through Late Tonight)
Issued at 328 AM CDT Wed Jul 26 2017

Upper level RIDGE dominated the south-central CONUS early this
morning, extending from the southern High Plains into the Mid-
Mississippi River Valley.  A MCS was rolling thru eastern Nebraska
with lots of the high cirrus shield spilling southeastward towards
and into our region around the eastern periphery of the upper RIDGE.
At the surface, the old frontal boundary that had been across
southern MO the past couple of days is now buckling northward as a
warm front in western MO, but for now, most of our region is on the
cool side of this front.  Temps were in the 70s for most sites in
our region.

Two big, yet paradoxical issues to deal with in the short-term: heat
and heavy rainfall.

First up is heat.  As has been advertised well the past several
days, a good setup for a short lived heat event remains in place
today but with a few more complications than originally envisioned.
Max temps from Tuesday clearly show a lot of heat potential near and
just south of the surface frontal boundary, with most sites reaching
the upper 90s just to the south across southern MO.  The surface
warm front is expected to push just east of the MS river by midday
and thru the remainder of the CWA by mid-afternoon.  Throughout the
day, cirrus clouds will continue to stream southeastward from the
MCS well to the northwest and as the MCS tracks east across IA and
weakens, it will continue to effectively kick out the cirrus, with
thicker cloud expected to be introduced from midday and afternoon
from north to south.  This will leave a tricky and small window
between warm frontal passage and when the cirrus shield gets thick
enough to have an impact.  Based on this, the heat should have the
greatest impact on STL metro and areas to the south in southeast MO
where combo of delayed arrival of thick cirrus and earlier warm
FROPA will be best and should result in max temps of upper 90s to
around 100.  Central MO will see an earlier arrival of thicker
clouds where southwest IL will see delayed FROPA of warm front and
have tapered max temps a bit as a result, into the mid 90s. Moisture
pooling near surface warm front will help, especially in IL where it
will be in the afternoon during peak heating and should allow max
heat index values to reach around 105 for much of the current Heat
Advisory area, with 105 to 110 for the STL metro heat island and
some areas in southeast MO.

Isolated TSRA will again be possible but more favored in southwest
IL due to warm front location and to a lesser extent in the eastern
Ozarks of southeast MO.  Increasing chances for rain should also be
realized late this afternoon in northeast MO and west-central IL
from weakening approaching MCS edging to the north.

Second is heavy rainfall.  Model guidance is depicting a classic
heavy rainfall setup from late this evening thru overnight for
northern MO into central IL, with PWs of 2 to 2.5" and warm cloud
depths of close to 5km with shear vectors that will be parallel to
the anticipated orientation of the convection that fires.  As a
result, will issue a Flash Flood Watch for parts of northeast MO and
west-central IL.  This new MCS that develops tonight should mainly
track E-SE and edge the I-70 area initially but likely firing new
TSRA on its southern periphery heading into Thursday.

TES

.LONG TERM...  (Thursday through Tuesday)
Issued at 328 AM CDT Wed Jul 26 2017

The models have come into better agreement with the position of the
advancing cold front on Thursday and Thursday night. Present
indications are that the primary cold front will stretch from NW IL
across northern MO and then trail southwest into southeast KS at 12Z
Thursday. A convectively enhanced short wave is progged to be
located from east central IL through east central MO. Forcing via
the short wave and westerly LLJ should be maintaining an ongoing
elongated MCS stretching from the western OH Valley back into
central MO. Along or south of the I-70 corridor there will also
likely be a convective outflow boundary. Current thinking is the MCS
will shift east/southeast during the morning and weaken owing to a
weakening LLJ and the short wave outrunning the convection. However
by mid afternoon I would expect new development along either the
remnant boundary or advancing cold front. The coverage and
convective organization should increase through the later afternoon
owing to increasing deep layer shear and diurnally enhanced
instability, and is expected to be greatest across the southeast
half of the CWA where a few severe storms would be possible as well.

The cold front should be located from the OH Valley across southeast
MO into eastern OK by early Thursday evening, with showers and
thunderstorms located along/ahead of the advancing cold front. By
daybreak Friday the cold front will be well south of the CWA with
surface high pressure settling into the area, and northeast low
level flow bringing cooler and drier conditions.

The upper air regime across the CONUS will feature a mean trof in
the eastern U.S. and building upper ridge in the western U.S.
from the weekend into early next week. This will maintain surface
high pressure across the mid-upper MS Valley into the OH Valley
and lead to an extended period of below average temps and low
humidity for late July.

Glass

&&

.AVIATION...  (For the 18z TAFs through 18z Thursday Afternoon)
Issued at 1237 PM CDT Wed Jul 26 2017

Few diurnal showers and thunderstorms just starting to develop
across portions of southeast Missouri and southwest Illinois. Also
watching slowly decaying showers approaching from northwestern
Missouri. Main threat of showers and storms however continues to
look like late evening through the overnight hours, first
affecting KUIN, then KCOU, followed by metro terminals. Activity
should weaken/wane Thursday morning before additional
showers/storms form along cold front. MVFR to near IFR ceilings
likely late tonight/early Thursday behind this front for KUIN.
Winds will also swing from the southwest to the northwest with
this frontal passage.


SPECIFICS FOR KSTL:

Watching some diurnal convection this afternoon into early this
evening but believe coverage too low with brunt of activity
staying south of the terminal. Higher chance of storms overnight
tonight. IFR visibilities possible in any heavier cores. Should be
a lull in activity late Thursday morning before additional storms
fire along cold front in vicinity of KSTL early Thursday
afternoon. Front should pass through with drier weather and a wind
shift to the northwest by mid afternoon.


Gosselin

&&

.LSX WATCHES/WARNINGS/ADVISORIES...
MO...Heat Advisory until 8 PM CDT this evening for Boone MO-Callaway
     MO-Cole MO-Crawford MO-Franklin MO-Gasconade MO-Iron MO-
     Jefferson MO-Lincoln MO-Madison MO-Moniteau MO-Montgomery
     MO-Osage MO-Reynolds MO-Saint Charles MO-Saint Francois MO-
     Saint Louis City MO-Saint Louis MO-Sainte Genevieve MO-
     Warren MO-Washington MO.

     Flash Flood Watch from 7 PM CDT this evening through Thursday
     morning for Knox MO-Lewis MO-Marion MO-Shelby MO.

IL...Heat Advisory until 8 PM CDT this evening for Bond IL-Calhoun IL-
     Clinton IL-Fayette IL-Greene IL-Jersey IL-Macoupin IL-
     Madison IL-Marion IL-Monroe IL-Montgomery IL-Randolph IL-
     Saint Clair IL-Washington IL.

     Flash Flood Watch from 7 PM CDT this evening through Thursday
     morning for Adams IL-Brown IL-Pike IL.

&&

$$

WFO LSX



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