Area Forecast Discussion
Issued by NWS Quad Cities, IA IL

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922
FXUS63 KDVN 191752 AAC
AFDDVN

Area Forecast Discussion...UPDATED
National Weather Service Quad Cities IA IL
1252 PM CDT Wed Jul 19 2017

...AVIATION UPDATE...

.UPDATE...
Issued at 1245 PM CDT Wed Jul 19 2017

Expanded Excessive Heat Warning and Heat Advisory further north
approximately a tier of counties. Current sfc dewpoints are in
the lower to middle 70s across E Iowa/NW Illinois and could
occasionally reach the upper 70s through this week, which will
push afternoon heat indices over 100 F.

The Excessive Heat Warning now also includes Iowa City and the
Quad Cities Metro to name a few of the most populated areas. For
counties in the warning, we are either anticipating 4-5
days of peak heat indices of +100 F, or 2-3 consecutive days of
near or over 105 F. It`s the long duration that increases the
susceptibility to heat related illnesses for people working,
spending time outside, or without access to air conditioning. The
cumulative effects of the heat begin to wear on the body.

The northern tier of the Heat Advisory (not including Bureau or
Putnam Counties) is set to expire Friday evening due to lower
confidence on upper 90s or near 100 F heat indices on Saturday as
a front begins working southward through E Iowa/NW Illinois. If
the forecast or conditions change, the advisory could simply be
extended. Uttech

&&

.SYNOPSIS...
Issued at 343 AM CDT Wed Jul 19 2017

The thunderstorm complex that rolled through in the evening has
dissipated to a few lingering showers over Bureau county, while
lone cells that were seen over the past several hours ahead of a
weak boundary along the eastern IA highway 20 corridor have since
dissipated. Otherwise, lingering low level moisture and light
winds were leading to some patchy light fog early this morning.

&&

.SHORT TERM...(Today and Tonight)
ISSUED AT 343 AM CDT Wed Jul 19 2017

Forecast focus remains on the heat and thunderstorm chances.

Regarding the heat: A similar airmass in place as yesterday,
although with less cloud cover based on regional satellite and
convective trends, should lead to afternoon highs a few degrees
warmer than yesterday. Have gone with highs from the upper 80s
north to near mid 90s south. This combined with dewpoints
returning to widespread lower to mid 70s will result in afternoon
heat index peaking in the lower 90s north to around 100 central
and south. This will be just into the heat advisory threshold for
much of the central and southern 2/3rds of our forecast area,
roughly covered by headlines already scheduled to go in effect
today. In coordination with surrounding offices, will be rolling
our excessive heat watch area over a heat advisory for the same
period as the warning in place across our south, beginning noon
today and continuing until 8 pm Saturday.

A complex of thunderstorms is expected to develop over the
northern plains today and dive south, possibly into the northern
half of the forecast area a linear complex with severe wind and
possibly heavy rainfall, either late evening or after midnight,
based on the timing of divergent convective models. SPC currently
has a slight risk for this scenario for east central Iowa into
northwest Illinois and an enhanced risk further north, touching
the highway 20 corridor west of Dubuque. Will keep low chances for
thunderstorms into eastern IA in the forecast for late this
evening, then likely along the highway 20 corridor after midnight,
with chance pops as far south as the I-80 corridor. Overall
confidence in the timing and track of this system is low based on
model data, which suggest it could potentially stay north of the
area, or pass just to the northeast into southern WI and northeast
IL toward Thursday morning.

.LONG TERM...(Thursday through Tuesday)
ISSUED AT 343 AM CDT Wed Jul 19 2017

Forecast focus on hot and humid conditions through Saturday, along
with periodic thunderstorm complexes, then cooler and much more
comfortable by early next week.

Thursday through Saturday: The theme continues with the heat dome
keeping much of the cwa hot and very humid, especially along and
south of Interstate 80. Therefore, the excessive heat warnings and
heat advisories are in effect. Highs will be well into the 90s, with
dewpoints well into the 70s to around 80. Heat index values will be
up to around 106. In our far northern cwa from about Highway 30
northward, temperatures should not be as hot. This area will be
closer to any thunderstorm complexes, debris clouds, and outflow
boundaries/cold pools.

As in previous discussions, the pattern favors periodic thunderstorm
clusters tracking near or across the dvn cwa, especially north of I-
80. Due to the high CAPE values, PWAT`s well over 2 inches,and
sufficient shear, any of the complexes would produce mainly damaging
winds/hail and several inches of rain in a short time. The tricky
part is trying to forecast the exact locations these storms will
impact and the timing. Therefore, the consensus models keep pops
generally in the 30-60 percent range until confidence is higher.
As of this writing, SPC has a marginal risk for severe storms in our
north on Thursday (day 2), and a slight risk on Friday (day 3).

Sunday through Tuesday: ECMWF/GFS still similar in bringing a rather
strong cold front across the cwa on Sunday, accompanied by at least
a chance for thunderstorms. In the wake of the front, much cooler
temperatures and much less humidity will be a welcome relief early
next week. The consensus model has highs in the mid 70s to lower 80s
with lows in the mid 50s to lower 60s.

Haase

&&

.AVIATION...(For the 18Z TAFS through 18Z Thursday Afternoon)
ISSUED AT 1252 PM CDT Wed Jul 19 2017

Difficult aviation forecast for the next 12 to 18 hours with
chances for thunderstorms are likely at most of the TAF sites.
Confidence is low where storms would develop, at this time the
best chance is MLI and DBQ. Otherwise, VFR conditions are expected
through most of the period.

&&

.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...Uttech
SYNOPSIS...Sheets
SHORT TERM...Sheets
LONG TERM...Haase
AVIATION...Gibbs



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