Area Forecast Discussion
Issued by NWS Quad Cities, IA IL

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FXUS63 KDVN 200853

Area Forecast Discussion
National Weather Service Quad Cities IA IL
353 AM CDT Thu Jul 20 2017

Issued at 352 AM CDT Thu Jul 20 2017

Very warm and humid airmass remains over the area under a
typical summer "ring of fire pattern". Early this morning,
a thunderstorm complex with heavy rainfall and possibly damaging
winds was ongoing across far SE MN, southern MN into northern IL.
Further upstream, a much weaker system was over SW SD and
Northern NEB.


.SHORT TERM...(Today and Tonight)
ISSUED AT 352 AM CDT Thu Jul 20 2017

With strong storms on-going early this morning, will keep
discussion short. No changes to heat headlines this morning, with
highs back into the upper 80s to mid 90s and heat index readings
today well into the 100 to around 105 range this afternoon,
especially south of highway 30.

Convective trends remain low confidence. Current complex in the
northeast will move out by early morning, and there could
potentially be storms developing and possibly becoming strong
along the CAPE gradient oriented NE to SW across the forecast area
this morning, for which low chance pops are maintained. Later
today, the synoptic boundary is shown setting up roughly along the
I-80 corridor for a possible focus for additional thunderstorm
development in the afternoon and evening. This will be dependent
on the among of capping and timing of shortwave/MCVs, which will
hopefully be more apparent later today.

.LONG TERM...(Friday through Wednesday)
ISSUED AT 352 AM CDT Thu Jul 20 2017

Forecast focus on continued hot and humid conditions through
Saturday, along with increasing chances for thunderstorms. Cooler
and much less humid early next week before the heat builds again.

Friday through Saturday: The excessive heat warning will continue
through Saturday evening, along and south of Interstate 80. Highs
will be well into the 90s with dewpoints well into the 70s to near
80. The heat index values will soar to 105 to 110.

In our northern cwa, this area looks to be very stormy with several
rounds of convection. Most of the models are keying in on Friday
night into Saturday morning to be of particular concern. This is
when a nearly stationary frontal boundary in our north will combine
with a strengthening southwest low level jet. Low pressure and a
cold front will be approaching from the west. PWAT`s are forecast to
increase to 2.4 inches which is at the top of the charts. Corfidi
vectors indicate the potential for back-building and training of
storms. If this materializes then rainfall in some locations would
exceed 3 to 5 inches, which would lead to flash flooding. However,
this far out there is still not enough confidence to the exact
locations to receive the heaviest rainfall.

Meanwhile, with the high CAPE values, sufficient shear and gradually
the heights falling, severe weather will also be a concern. SPC has
a slight risk over portions of the cwa during the Friday through
Saturday time frame. Damaging winds and large hail is expected to be
the main threat, along with frequent lightning and torrential
rainfall. Severe weather is possible anytime during this period, but
the most favored time would be the afternoon and evening hours with
the heat of the day. The cold front should come through by Sunday
ending the rain potential.

Sunday through Wednesday: Upper level trough will be over the Great
Lakes vicinity with a northwest flow across the cwa. This will bring
comfortable temperatures and humidity especially early next week.
However, by Wednesday the ridge builds again with warmer and more
humid conditions returning.



.AVIATION...(For the 06Z TAFS through 06Z Thursday Night)
ISSUED AT 1118 PM CDT Wed Jul 19 2017

Hot, humid, and mainly VFR weather will continue through Thursday
evening, with a nearly continual threat that thunderstorm
clusters may form upstream and spread into the area. These are not
well forecast by any means of model data, and with most hours dry,
I will not put in endless hours of low thunderstorm threats. In
any case, expect a slight chance for strong storms through the
period, with possible small round of storms toward sunrise today,
and again toward late afternoon/evening. This may miss our area
completely, or cause widespread thunderstorms with strong winds.
It is too early to determine in this hot, humid, and chaotic
convective pattern.


IA...Heat Advisory until 8 PM CDT Friday for Benton-Clinton-Jones-

     Excessive Heat Warning until 8 PM CDT Saturday for Cedar-Des
     Moines-Henry IA-Iowa-Jefferson-Johnson-Keokuk-Lee-Louisa-
     Muscatine-Scott-Van Buren-Washington.

IL...Heat Advisory until 8 PM CDT Friday for Whiteside.

     Excessive Heat Warning until 8 PM CDT Saturday for Hancock-
     Henderson-Henry IL-McDonough-Mercer-Rock Island-Warren.

     Heat Advisory until 8 PM CDT Saturday for Bureau-Putnam.

MO...Excessive Heat Warning until 8 PM CDT Saturday for Clark-



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