Area Forecast Discussion
Issued by NWS St. Louis, MO

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FXUS63 KLSX 182056

Area Forecast Discussion
National Weather Service Saint Louis MO
356 PM CDT MON JUL 18 2016

.SHORT TERM...  (Through Late Tonight)
Issued at 342 PM CDT Mon Jul 18 2016

An outflow boundary was located across the area this afternoon.
Given the overall instability and weak ascent, iso-sct SHRA/TSRA
are possible during the late aftn and evening hours.


.LONG TERM...  (Tuesday through Next Monday)
Issued at 342 PM CDT Mon Jul 18 2016

Although there will be different products in effect for different
parts of the LSX CWA at different times this week, the key
message to focus on is that dangerous levels of heat and humidity
are expected across the region this week. The message is the same
whether the product for your area is a Heat Advisory or an
Excessive Heat Warning. One might even say that the difference
between the two products is a matter of degree...degrees

An upper ridge will amplify across the plains and then shift
eastward this week. This pattern shift will keep hot/humid
conditions in place across the region. Heat index values will
reach 100-110 deg F on most days this week and possibly into
early next weekend. There may be sporadic relief in some spots
due to isolated to scattered thunderstorms.

Local heat climatology studies for KSTL/KCOU/KUIN indicate that
mid to late July is the most common time period when temperature
and dew point values yield heat index readings at or above
criteria for Heat Advisories and Excessive Heat Warnings. As
such, the expected heatwave this week is normal, climatologically
speaking. Please remember that heat has a cumulative effect on
the human body, which means that the human body experiences more
and more stress on each successive day of similarly hot/humid

Heat headlines sometimes create confusion because there are two
different products (Heat Advisory and Excessive Heat Warning),
each with two sets of criteria (duration and intensity). LSX`s
criteria for heat headlines is as follows:
Heat Advisory (duration): heat index >= 100 deg F for 4+ days.
Heat Advisory (intensity): heat index 105-109 deg F for 1 day.
Excessive Heat Warning (duration): HI 105-109 for 4+ days.
Excessive Heat Warning (intensity): HI >= 110 for 1 day.

Both the St. Louis metro area and parts of central MO should meet
the criteria for an Excessive Heat Warning this week, but for
different reasons. The St. Louis area is likely to reach criteria
for duration (4+ days of HI above 105) and for intensity (two
separate days with HI near 110, on Thu and Fri). Parts of central
MO are likely to reach criteria based on duration (4+ days of HI
above 105). The rest of the CWA is likely to reach criteria for a
Heat Advisory based on duration (4+ days of HI above 100).

The areas which were upgraded to Excessive Heat Warnings with the
afternoon package are the areas where confidence is highest that
duration criteria will be met. Central MO is far enough west that
it is less likely to be affected by a synoptic boundary (possibly
reinforced by outflows) which may lower temperatures across the
northern and eastern CWA on at least one day this week. The urban
heat island of the St. Louis metro area is likely to keep air
temperatures slightly higher there than across surrounding areas.
Elsewhere, there remains enough uncertainty on convective trends
and outflow on Tuesday, the southwestward progress of the
synoptic boundary, dew points towards the end of the week, and a
potential synoptic boundary late this week or early next weekend
to preclude upgrading to an Excessive Heat Warning. Headlines will
likely be adjusted based on confidence regarding these items.



.AVIATION...  (For the 18z TAFs through 18z Tuesday Afternoon)
Issued at 134 PM CDT Mon Jul 18 2016

Tafs VFR outside of isolated storms. Kept VCTS mention in metro
area tafs through 19z. Otherwise, winds from the south becoming
light and variable.

Specifics for KSTL:
Tafs VFR outside of isolated storms. Kept VCTS mention in metro
area tafs through 19z. Otherwise, winds from the south becoming
light and variable by 05z Tuesday.



MO...Heat Advisory until 2 PM CDT Friday for Audrain MO-Callaway MO-
     Crawford MO-Franklin MO-Gasconade MO-Iron MO-Knox MO-Lewis
     MO-Lincoln MO-Madison MO-Marion MO-Monroe MO-Montgomery MO-
     Osage MO-Pike MO-Ralls MO-Reynolds MO-Saint Francois MO-
     Sainte Genevieve MO-Shelby MO-Warren MO-Washington MO.

     Excessive Heat Warning until 2 PM CDT Friday for Boone MO-Cole
     MO-Jefferson MO-Moniteau MO-Saint Charles MO-Saint Louis
     City MO-Saint Louis MO.

IL...Heat Advisory until 2 PM CDT Friday for Adams IL-Bond IL-Brown
     IL-Calhoun IL-Clinton IL-Fayette IL-Greene IL-Jersey IL-
     Macoupin IL-Marion IL-Montgomery IL-Pike IL-Randolph IL-
     Washington IL.

     Excessive Heat Warning until 2 PM CDT Friday for Madison IL-
     Monroe IL-Saint Clair IL.



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