Area Forecast Discussion
Issued by NWS Quad Cities, IA IL

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FXUS63 KDVN 202030

Area Forecast Discussion
National Weather Service Quad Cities IA IL
330 PM CDT WED JUL 20 2016

Issued at 330 PM CDT Wed Jul 20 2016

The back edge of the convection in IL fits nearly perfectly with
the 10C 700 mb temperature isotherm. At 3 pm the SPC meso analysis
showed this isotherm extended from about Rockford to Peoria with
the 12C isotherm from about Dubque to Burlington. Shower and
thunderstorm activity has diminished rapidly in nw IL the past
couple of hours as this strengthening cap arrives, suppressing the
convection. Heat index values were around 100 in our sw cwa.

To our west across central IA and into the Plains current
temperatures were in the lower to mid 90s with dewpoints well into
the 70s to lower 80s. SBCAPES in central IA were 6500 J/KG but
this was under a strong capping inversion.


.SHORT TERM...(Late This Afternoon and Tonight)
ISSUED AT 330 PM CDT Wed Jul 20 2016

Headlines: An excessive heat warning is in effect.

Late this afternoon: We Will have only slight pops in nw IL as
any lingering showers should end rather quickly.

Tonight: Strengthening capping inversion will prevent any
thunderstorm development so a dry night expected. The NAM 4km and
SPC 4km WRF-NMM are dry but the HRRR wants to rapidly develop
thunderstorms just west of the cwa this afternoon. This looks
unreasonable due to the strong cap and besides, the HRRR has not
been reliable at all lately to say the least. Instead the main
message will be a very warm and muggy night ahead. Minimum
temperatures should be in the mid 70s with a south wind about 5 to
10 mph. In our sw cwa heat index values will not drop below 90
until about 9 or 10 pm.  Haase

.LONG TERM...(Thursday through Wednesday)
ISSUED AT 330 PM CDT Wed Jul 20 2016

Overview...Excessive Heat Warning in effect for the entire forecast
area Thursday through Saturday for peak heat indices around 110 F.
People are strongly encouraged to limit time outdoors, especially
from the late morning into the early evening. The oppressive
humidity will make outdoor activities very strenuous thereby greatly
increasing the risk for heat exhaustion or heat stroke.

Aside from the heat, the trend will be toward drier conditions, but
cannot ignore threat for scattered storms Thursday Night through
Saturday with a washed out boundary from a remnant MCS that is
forecast to track through Wisconsin and Michigan, possibly into NE
Illinois. Any storms would provide a brief reprieve from the
oppressive heat for localized areas.

NAEFS 850mb temps to 24 C are close to 2 standard deviation above
the mean for mid July, but are not terribly unusual as this
translates to a return interval of once a year or once every other
year, again during the middle of July.

Thursday...300mb jet streak traverses far south-central Canada in
concert with mid to upper-level height rises over E Iowa/W Illinois
and increasing 1000-500mb thicknesses to ~586 dam. Closer to the
sfc, 30 kt 850mb winds out of the southwest will lead to a breezy
day as the boundary layer height increases into the aftn. Expect sfc
gusts of 20-25 mph. This appreciable mixing is manifest as a dry
adiabatic layer below 800mb - an environmental setup favoring max
temps in the mid to upper 90s. With above average rain and soil
moisture for most of the CWA in July and mature crops, there should
be no problem holding dewpoints in the mid 70s or higher through the

Thursday Night into Friday Morning...Shortwave moving through
Northern Great Lakes to ignite an MCS over the Eastern U.P. The
brunt of this system should miss E Iowa/NW Illinois to the east.
However, a very weak sfc boundary/convergence zone is forecast to
slowly drop in from the north probably more into Friday morning.
Isolated to scattered showers and storms are possible along the
boundary. But absence of even moderate 0-1 km convergence and an
upper disturbance should preclude widespread convection.

Friday...Pooling of moisture along the sfc boundary, light sfc
winds, and little in the way of diurnal mixing raises confidence in
dewpoints hovering in the upper 70s to near 80 F. Even if temps are
a few degrees lower than Thursday, the incredibly high dewpoints
will compensate, making for another day of oppressive heat indices.
Airmass storms with isolated to scattered coverage are possible
through the day (at this time do not see anything widespread).
Torrential rain, isolated gusty winds, and lightning strikes are the
main threats.

Saturday...Previous day`s sfc boundary to respond to developing low
pressure in the Northern Plains by lifting northward. Sfc winds are
forecast to become a little steadier, out of the south but not as
strong as Thursday. E Iowa/W Illinois remains under the 850mb
thermal ridge emanating from the Central Plains which does not
change until Saturday evening/night when storm chances increase
ahead of a cold front. Depending on location, this will be the third
or fourth consecutive day of heat indices reaching or exceeding
100 F.

Sunday through Wednesday...Consensus blend brings a cold front
through, knocking temps into the middle 80s on avg, giving us some
relief from the extreme heat. At this time, there are low chances
for storms Sunday and then Tuesday through Wednesday. It is too
early to determine the risk for severe storms. Uttech


.AVIATION...(For the 18Z TAFS through 18Z Thursday Afternoon)
ISSUED AT 330 PM CDT Wed Jul 20 2016

Pockets of MVFR cigs this afternoon into early this evening
otherwise VFR conds expected through this taf cycle. South to
southeast winds around 10 knots through tonight, then south 10 to
20 knots Thursday.


.DVN Watches/Warnings/Advisories...
IA...Excessive Heat Warning from 7 AM Thursday to 7 PM CDT Saturday
     for Buchanan-Cedar-Clinton-Delaware-Dubuque-Jackson-Jones-

     Excessive Heat Warning until 7 PM CDT Saturday for Benton-Des
     Moines-Henry IA-Iowa-Jefferson-Johnson-Keokuk-Lee-Louisa-
     Van Buren-Washington.

IL...Excessive Heat Warning from 7 AM Thursday to 7 PM CDT Saturday
     for Bureau-Carroll-Henry IL-Jo Daviess-McDonough-Mercer-
     Putnam-Rock Island-Stephenson-Warren-Whiteside.

     Excessive Heat Warning until 7 PM CDT Saturday for Hancock-

MO...Excessive Heat Warning until 7 PM CDT Saturday for Clark-



LONG TERM...Uttech
AVIATION...Haase is the U.S. government's official web portal to all federal, state and local government web resources and services.