Area Forecast Discussion
Issued by NWS St. Louis, MO

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FXUS63 KLSX 181702
AFDLSX

Area Forecast Discussion...Updated Aviation
National Weather Service Saint Louis MO
1202 PM CDT Mon Jun 18 2018

.SHORT TERM...  (Through Late Tonight)
Issued at 245 AM CDT Mon Jun 18 2018

Upper level RIDGE prevailed over our region early this morning
with a light southerly flow at the surface. A cold front was well
to our north over the upper Midwest. Skies were mainly clear with
temperatures ranging from the lower 80s in the Saint Louis urban
heat island to the mid 70s in outlying areas.

Little to no change is expected today in our overall setup with
upper RIDGE remaining dominant overhead and a frontal boundary
making little progress southward, still well to our north by day`s
end. A similar starting point to morning temperatures and
moisture levels from 24 hours ago are anticipated. As a result, we
should see max temperatures within a degree or two of
persistence, with mid 90s for most areas, some upper 90s in the
STL metro area, and leaning towards lower 90s in southeast MO.
This is expected to yield peak heat index values around 105 in the
STL metro area and between 100-103 elsewhere and mandates leaving
all heat headlines as-is.

Considering we are expecting a similar airmass as in previous
days, it will simply be hard to not put a small token PoP in
during peak heating in the afternoon for much of the area. We are
looking at CAPEs of 1000-2000 J/kg in a very low shear environment
with virtually no CINH during peak heating. It will then be all
up to intangible factors to force a short-lived locally driven
convective cell. Organized severe weather is not expected and
usually in a pulse storm environment which this is, we`d like to
see CAPE higher than 2000 J/kg for isolated severe potential.
Anything that does develop should quickly weaken/dissipate by
sunset.

For tonight, attention turns northward towards the very slowly
southward moving cold front. Models in recent days have really
slowed down the southward progress and any convection that
develops along it should struggle to push south with all mean wind
vectors pointing northeastward. Have edged PoPs downward and
limited to just northern MO and central IL.

TES

.LONG TERM...  (Tuesday through Sunday)
Issued at 245 AM CDT Mon Jun 18 2018

The upper ridge will start to break down on Tue and then become
suppressed to the southeast on Wed due to an approaching closed
upper low which becomes an open wave and rejoins the prevailing
flow. The approaching upper low induces surface cyclogenesis over
the plains, which then acts to reorient existing surface baroclinic
zones across the region. Models have been trending farther north
with one of these boundaries, and it is now expected to remain
across IA on Tue instead of reaching the LSX CWA during the day.
With the active frontal zone located farther to the north, there
should be less cloud cover and less precipitation across the
central and southern CWA than previously expected. Although
temperatures should be a few degrees cooler on Tue than on Mon,
highs in the low/mid 90s with dew points in the 70s should still
be sufficient to produce heat index values around 100 degrees
across the central CWA.

The cumulative effect of prolonged heat exposure is known to be
an important factor in heat stress. Given that the overnight lows
on Monday night will still be in the upper 70s to around 80
degrees in the St. Louis area, the Excessive Heat Warning in the
St. Louis metropolitan area will be extended through Tuesday even
though the peak heat index values may only reach the low 100s.
The Heat Advisory will also be extended within an east-west band
roughly centered along the I-70 corridor where heat index values
should reach 100 deg. Farther north, increased cloud cover should
keep temperatures cool enough to avoid the need for extended heat
headlines. Farther south, both temperatures and dew points have
been lower compared to surrounding areas in recent days, and this
trend is expected to hold on Tue. The next two shifts will have
an opportunity to re-evaluate the need to extend the Heat
Advisory in these areas based on the latest observations and model
guidance.

By Wed, increasing clouds and precipitation near the active
frontal boundaries and ahead of the approaching low pressure
system should keep temperatures several degrees cooler across the
area, bringing a relief from the recent oppressively hot and
humid conditions.

There will be a chance of SHRA/TSRA through the end of the week
across the area until the low pressure system moves away from the
region. Precipitation trends for the weekend are unclear, although
some models show a second surface low developing in the TX/OK
panhandles along the trailing end of one of the baroclinic zones
associated with the departing low pressure system. If this occurs,
it would lift the boundary back into the region, bringing
additional SHRA/TSRA chances to the area for the weekend.

Kanofsky

&&

.AVIATION...  (For the 18z TAFs through 18z Tuesday Afternoon)
Issued at 1144 AM CDT Mon Jun 18 2018

Scattered diurnal cumulus clouds will develop across the area
this afternoon with daytime heating. There will be isolated,
mainly late afternoon showers/storms, but not enough coverage to
warrant including in the tafs at this time. Most of the cumulus
clouds will dissipate early this evening, but will develop again
late Tuesday morning, particularly in UIN as a cold front tries
to sag southward towards northern MO and central IL. Southwest
surface winds will persist through the period.

SPECIFICS FOR KSTL: Scattered diurnal cumulus clouds will develop
across the area this afternoon with daytime heating. There will
be isolated, mainly late afternoon showers/storms, but not enough
coverage to warrant including in the STL taf at this time. Most
of the cumulus clouds will dissipate early this evening, but will
develop again late Tuesday morning and afternoon along with a few
afternoon showers and storms. Southwest surface winds will
persist through the period.

GKS

&&

.CLIMATE...

Record Highs and Warmest Lows

6/18

STL 100 (1953) 79 (2009)

COU 100 (1953) 75 (2014)

UIN 99 (1913) 77 (1944)


&&

.LSX WATCHES/WARNINGS/ADVISORIES...
MO...Heat Advisory until 8 PM CDT this evening for Audrain MO-
     Crawford MO-Iron MO-Knox MO-Lewis MO-Madison MO-Marion MO-
     Monroe MO-Pike MO-Ralls MO-Reynolds MO-Saint Francois MO-
     Sainte Genevieve MO-Shelby MO-Washington MO.

     Heat Advisory until 8 PM CDT Tuesday for Boone MO-Callaway MO-
     Cole MO-Franklin MO-Gasconade MO-Lincoln MO-Moniteau MO-
     Montgomery MO-Osage MO-Warren MO.

     Excessive Heat Warning until 8 PM CDT Tuesday for Jefferson MO-
     Saint Charles MO-Saint Louis City MO-Saint Louis MO.

IL...Heat Advisory until 8 PM CDT this evening for Adams IL-Brown IL-
     Pike IL-Randolph IL.

     Heat Advisory until 8 PM CDT Tuesday for Bond IL-Calhoun IL-
     Clinton IL-Fayette IL-Greene IL-Jersey IL-Macoupin IL-
     Marion IL-Montgomery IL-Washington IL.

     Excessive Heat Warning until 8 PM CDT Tuesday for Madison IL-
     Monroe IL-Saint Clair IL.

&&

$$

WFO LSX



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