Area Forecast Discussion
Issued by NWS Quad Cities, IA IL

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FXUS63 KDVN 200421
AFDDVN

Area Forecast Discussion
National Weather Service Quad Cities IA IL
1121 PM CDT Wed Jul 19 2017

...AVIATION UPDATE...

.SYNOPSIS...
Issued at 300 PM CDT Wed Jul 19 2017

This afternoon temperatures ranged across the area from the upper
80s to the mid 90s.  This resulted in heat indices ranging from near
100 to 110 across the area.  A warm front extended across the area
and aided in pooling low level moisture across the area.  To the NNW
a thunderstorm complex was slowly moving east southeast.  This
complex is expected to be the main forecast concern in the short
term.

&&

.SHORT TERM...(This Evening through Thursday)
ISSUED AT 300 PM CDT Wed Jul 19 2017

Next 12 to 18 hours are forecast to be active with a couple rounds
of thunderstorms, some of which could be severe.

Main forecast concern in the short term are the chances for
thunderstorms this afternoon and overnight across the area.  The
northern CWA is under an enhanced risk for severe weather.  The
ongoing thunderstorm complex has accelerated with time and is
currently rooted in the boundary layer.  The overall gradient in
CAPE and cap suggests that these storms will eventually turn south
and into our area late this afternoon.  WPC QPF depicts this quite
well.  The CAMs have been slow to catch onto this trend, but appears
to be trending that way as well.  As these storms approach the area,
an upper level vort max is expected to accompany this storm.  This
wave will increase deep layer shear and set the stage for severe
weather.  Damaging winds and heavy rain will be the main threat for
the area.  Low level shear is expected to increase tonight and could
lend itself to a tornado threat across the area.

Another wave is expected to initiate thunderstorms across the Dakotas
this afternoon and evening.  As these storms grow upscale they are
expected to track ese as well.  These storms could affect the area
near sunrise and could pose a severe wind, heavy rain and tornado
threat.

Thursday, the warm front is expected to remain across the area.
Showers and storms could be ongoing at the beginning of the day. Any
cloud cover from these storms could affect temperatures across the
area tomorrow.  This could also affect overall heat index values
tomorrow.  Regardless, it will be hot and humid across the area.
With the warm front across the area there is a chance for afternoon
thunderstorms. At this time, it appears that the limiting factor to
any thunderstorms would be a midlevel capping inversion in place.

.LONG TERM...(Thursday Night through Wednesday)
ISSUED AT 300 PM CDT Wed Jul 19 2017

Thursday Night through Saturday

Heat Headlines:

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 or
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.

Thunderstorm Chances:

No significant changes to the extended forecast regarding threat
for storms. E Iowa/NW Illinois is situated along the edge of the
capping inversion aloft that inhibits convective development, and
near the favorable 850mb baroclinic zone of upper teens to lower
20s (Celsius) for occasional rounds of mesoscale convective
systems. We continue to forecast a tight NE to SW PoP gradient
from 40-60% across the far N/NE to only 20% over the
south/southwest. However, the final track of these storm clusters
will depend on where convection forms on previous days, because
they will layout a low-level convergent boundary in their wake. It
is too early to know exactly where the sfc boundary will be
located from Thursday night through Saturday. As mentioned, the
best chances for storms are north of I-80, probably Friday and
Friday night.

High instability with +4000 J/kg MUCAPE and deep layer shear over
35 kts are supportive of a damaging wind threat, especially from
organized bowing structures - if they form. Additionally, high
PWATs near 2.00-2.25 inches translates to torrential rainfall
potential with 2-3 inches/hr possible and a risk for flash
flooding within the heaviest cores. Uttech

Sunday through Wednesday

ECMWF/GFS still similar in bringing a cold front across the CWA
on Saturday night and Sunday, accompanied by a slight chance for
showers or thunderstorms along and south of I-80. In the wake of
the front, much cooler temperatures and much less humidity will be
a welcomed relief early next week. The consensus model has highs
in the upper 70s far north to mid 80s south with lows in the mid
50s to lower 60s. Haase

&&

.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.
ERVIN

&&

.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.

&&

$$

SYNOPSIS...Gibbs
SHORT TERM...Gibbs
LONG TERM...Uttech/Haase
AVIATION...Ervin



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