Area Forecast Discussion
Issued by NWS Quad Cities, IA IL

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FXUS63 KDVN 201815
AFDDVN

Area Forecast Discussion
National Weather Service Quad Cities IA IL
115 PM CDT Thu Jul 20 2017

...18z AVIATION UPDATE...

.UPDATE...
Issued at 643 AM CDT Thu Jul 20 2017

Storms feeding off an elevated feed of moisture, and along an
axis of much higher MUCAPES to the west, continued across
northwest IL at sunrise. These were situated in a couple lines
dropping south across NW IL around 40 mph. These storms, have had
an overall weakening trend, but still exhibiting very frequent
lightning and capable of wind gusts to 40 mph and heavy rainfall
rates. More isolated showers were popping up south of these storms
along the IL I-80 corridor and also over far eastern IA, with a
few stronger cells recently popping up between Marshalltown and
Vinton.

This activity is likely to dissipate over the next couple hours,
but will leave behind outflow boundaries that will be watched for
redevelopment later today. Will have to watch portions of northern
Jo Daviess and Stephenson counties, where 24 rainfall totals may
have exceeded 3 inches, leaving the area vulnerable to flash
flooding from any additional heavy rainfall this week.

&&

.SYNOPSIS...
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.

Haase

&&

.AVIATION...(For the 18Z TAFS through 18Z Friday Afternoon)
ISSUED AT 100 PM CDT Thu Jul 20 2017

Challenging fcst with respect to what boundary possibly lights up
with sctrd showers/storms later this afternoon and evening, and
if any late night convection moves in or develops overhead. Some
of the latest HiRes models suggest a boundary along and just south
of I80 possibly the focus for late afternoon and evening storms,
that may impact the MLI and north BRL sites, then will have to
watch for late night/after midnight into early Fri morning storms
further north possibly getting into the VCNTY of CID and DBQ. The
stronger storms will produce temporary MVFR to IFR conditions
from heavy rain reduced VSBYs and lowered CIGs, as well a possible
strong gusty winds at variable directions(but most likely from
the west or northwest). Otherwise mainly VFR conditions until late
tonight when some MVFR fog may develop as winds go light. Even a
chance for patchy dense fog in some spots...but low confidence in
this potentail and where it may occur.   ..12..

&&

.DVN WATCHES/WARNINGS/ADVISORIES...
IA...Heat Advisory until 8 PM CDT Friday for Benton-Clinton-Jones-
     Linn.

     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-
     Scotland.

&&

$$

UPDATE...Sheets
SYNOPSIS...Sheets
SHORT TERM...Sheets
LONG TERM...Haase
AVIATION...12



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