Area Forecast Discussion
Issued by NWS Quad Cities, IA IL

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FXUS63 KDVN 220448

Area Forecast Discussion
National Weather Service Quad Cities IA IL
1148 PM CDT THU JUL 21 2016


Issued at 940 PM CDT Thu Jul 21 2016

A fairly significant update to the forecast has been done. Pops
have been increased and expanded into areas that were previously
projected to be dry.

MCS across Wisconsin/Illinois will slowly build to the south and
west the remainder of the night. The outflow boundary put out the
mcs will provide the focus for new storm development later

There are questions as to how strong the cap is and whether or not
it will kill off the convection as it moves to the south and east.
Although not certain, storms could re-develop and move over the
same areas. If this occurs, excessive rainfall will be possible
that could result in flooding.


Issued at 308 PM CDT Thu Jul 21 2016

18z surface data has a frontal boundary from western Lake Superior
to the South Dakota/Nebraska border. Dew points were in the 70s
across the Midwest with pockets of dew points in the low 80s.


.SHORT TERM...(This Evening through Friday)
ISSUED AT 308 PM CDT Thu Jul 21 2016

The excessive heat warning will continue through Friday.

Very hot and humid conditions will continue through sunset with no
rain expected.

After sunset, the forecast becomes a conditional IF...THEN
situation. IF a thunderstorm complex develops along the IA/MN border
THEN thunderstorms will likely overspread at least the eastern two
thirds of the area. IF thunderstorms occur THEN overnight lows will
be lower than forecast.

IF no thunderstorm complex develops THEN very warm and humid
conditions will be seen through sunrise Friday with low temperatures
potentially higher than forecast.

Right now the current forecast is for isolated to low end scattered
convection developing based on an old rule of thumb about morning
accas over the area.

What happens tonight will influence Friday. IF no thunderstorms
occur Thursday night THEN very hot and humid conditions will be seen
Friday with high temperatures potentially warmer than forecast.

IF thunderstorms occur Thursday night into Friday THEN clouds and
convection may persist keeping temperatures much cooler than

.LONG TERM...(Friday Night through Thursday)
ISSUED AT 308 PM CDT Thu Jul 21 2016

The period of excessive heat will continue synoptically through
Saturday, if not Sunday. However, as we get past Friday, the
probability of MCS contamination seems to become more likely and I
will not yet extend the headline through Sunday. Over all, our
situation remains very much steady state as it is tonight, we are
playing with fire.  The warm side of the nearby boundary has every
seasonal variable supporting exceptional heat, from duration to crop
transpiration, to frontal convergence just north of deep
southwesterly flow, we are in the hot zone. Along the edge of the
heat, like today, both progressive MCS and flash flood producing
back building MCS are likely each day. The potential for any of
these to back build into the CWA are strong, and if they do, a short
respite from heat may occur. We will need to continue with the most
likely hazard of heat, through Saturday, and if now storms occur
Saturday night, they Sunday as well. Highs in the lower to mid 90s
are expected both Saturday and Sunday, though dewpoints Sunday may
be a few degrees lower than now, in the lower to mid 70s.

Monday onward, the models build high pressure into the upper
Midwest, and with northwest flow aloft, this should bring dewpoints
back to the 60s for much of next week. The dry weather should begin
to get active again by Tuesday night again, as a mid week short wave
crosses the plains. The GFS  brings another prolonged period of MCS
with this return flow, while the ECMWF has a much deeper trof and
cooler air mass in place beyond Wednesday.  Given the active
monsoonal flow, and weaker synoptic July pattern, it seems the GFS
may be most accurate unfortunately.


.AVIATION...(For the 06Z TAFS through 06Z Friday Night)
ISSUED AT 1125 PM CDT Thu Jul 21 2016

Mostly VFR conditions to continue with possible episodes of convection
lasting into late overnight and possibly lasting well into Friday
morning from ongoing convective complex. These storms could produce
brief intervals of MVFR to IFR conditions in heavy rain. Have a tempo
group at terminals next several hours and then VFR conditions by sunrise.
Then fair skies suggested Wind generally easterly becoming southerly
at 5 to 15 Kts by late afternoon. Possibly another round of convection
may develop late PM into the evening hours.


.DVN Watches/Warnings/Advisories...
IA...Excessive Heat Warning until 7 PM CDT Saturday for Benton-
     Buchanan-Cedar-Clinton-Delaware-Des Moines-Dubuque-Henry IA-
     Muscatine-Scott-Van Buren-Washington.

IL...Excessive Heat Warning until 7 PM CDT Saturday for Bureau-
     Carroll-Hancock-Henderson-Henry IL-Jo Daviess-McDonough-
     Mercer-Putnam-Rock Island-Stephenson-Warren-Whiteside.

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



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