Area Forecast Discussion
Issued by NWS Quad Cities, IA IL

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FXUS63 KDVN 210857
AFDDVN

Area Forecast Discussion
National Weather Service Quad Cities IA IL
357 AM CDT Mon Aug 21 2017

.SYNOPSIS...
Issued at 355 AM CDT Mon Aug 21 2017

A line of elevated, heavy rain producing storms has recently formed
from Fairfield up to Sigourney. Further west, the MCS over central
Iowa produced localized heavy rainfall and flash flooding in the
Denison, IA area and wind gusts over 50 mph near Atlantic, IA.

Across the remainder of E Iowa/NW Illinois there has been areas of
fog through the night, the worst of which occurred in the counties
along I-80 and highway 30. A few sites - Moline, IL and Savanna,
IL - have briefly dropped to 1/4 of a mile. However, dense fog
has not been persistent or widespread enough to consider an
advisory. Conditions should gradually improve as mid-level clouds
continue to increase downstream of the convection.

&&

.SHORT TERM...(Today and Tonight)
ISSUED AT 355 AM CDT Mon Aug 21 2017

Through Mid Morning

Hi-res convective allowing models (HRRR/NMM/RAP) in good agreement
on bringing elevated, warm air advection driven showers and
thunderstorms into the west-central and NW CWA early this morning.
They are also showing an expansion of scattered showers and storms
through or slightly south of the Quad Cities into central Illinois
between 7-10 AM. These storms are not expected to be severe. Heavy
rainfall rates of 1-2"/hr and lightning strikes are the main
threats.

There is more forecast uncertainty to the SW. This region is in line
for the sfc-based part of the MCS that is moving southeastward into
a high DCAPE environment (>1000 J/kg). Furthermore, latest SPC
Mesoanalysis shows increasing 0-1 km bulk shear to over 20 kts,
which would help to balance and sustain the convective cold pool.
The limiting factors are minimal SBCAPE and low effective deep layer
shear.

The 06Z NAM is aggressive in tracking the sfc-based portion of the
MCS through the SW CWA. This seems overdone because of the
aforementioned limiting factors. It is more likely to either fall
apart or move in during a weakening phase. Uttech


Mid morning onward: Any convection should be waning by late
morning with the diurnally dissipating low level jet and
increasing suppression from a period of shortwave ridging shown by
00z models in this timeframe. This is also along the lines of what
is indicated by most high res convective model. For now, will have
slight to low chance pops going from late morning through early
afternoon due to low confidence in the timing. Extensive cloud
cover lingering from the upstream nocturnal complex will likely be
decreasing by this time, but there is low confidence in how
trends with low to mid level clouds due to the low level boundary
that will gradually weaken and lift north during the day. Thus,
for now, will continue to go with mostly cloudy conditions, but
there could be sufficient breaks in the cloud cover by early
afternoon to permit viewing of the partial solar eclipse at
midday in some locations.

Temperatures will be largely dependent on cloud trends and for now
have lower 80s north to upper 80s south, counting on at least
partial clearing before late day convection develops.

Late this afternoon and tonight: thunderstorms will likely develop
along boundaries in the afternoon, especially to the west and
northwest of the forecast area. Tonight, with more widespread
thunderstorms are expected with the front dropping southeast along
with strong upper level support. SPC has a slight risk for severe
storms over the entire forecast area. Based on frontal timing and
convective allowing models, the threat for severe storms
(including wind, hail and tornadoes) will likely be well west and
northwest of the forecast area in the afternoon, but possibly
reach into eastern IA by early evening. Later in the evening,
more organized storms are likely as the front approaches and a low
level inflow sets up. This will lead to a heavy rainfall threat
over east central IA into northwest IL and possible damaging winds
early on. Overnight, as the low level jet becomes more west to
northwest, the heavy rainfall threat will diminish with the
complex of storms advancing southeast across the forecast area.
Instability will also be waning by this time, diminishing the
threat of damaging winds after midnight. Widespread rainfall
amounts well over 1 inch will be possible, but the progressive
nature of the system will not be conducive to storm training or
excessive rainfall.

.LONG TERM...(Tuesday through Sunday)
ISSUED AT 355 AM CDT Mon Aug 21 2017

Tuesday, current timing of the cold front indicates post frontal
showers and isolated thunderstorms will be ending across the
southeast early in the day. Clearing skies and north winds then
follow with an incoming drier airmass. Model blend highs from the
mid 70s north to lower 80s south may be too warm.

High pressure then provides seasonably cool and dry weather
through the rest of the week into the weekend. A warming trend
then develops on a return flow over the weekend, returning highs
to the 80s over at least the south, with low chances for showers
over mainly the north by late Sunday.

&&

.AVIATION...(For the 06Z TAFS through 06Z Monday Night)
ISSUED AT 1131 PM CDT Sun Aug 20 2017

Patchy to areas of fog expected into the overnight. The fog will
be most favored and prevalent near to north of I-80 where
visibilities will be variable from MVFR to LIFR, with pockets of
VLIFR. Storms over western IA will track into eastern IA in the
pre-dawn hours before weakening through mid to late morning.
Expect periods of MVFR to IFR conditions in the storms, with a
threat for hail in the strongest cells. Models are indicating
additional storms possible mainly for Monday night.

&&

.DVN WATCHES/WARNINGS/ADVISORIES...
IA...NONE.
IL...NONE.
MO...NONE.
&&

$$

SYNOPSIS...Uttech
SHORT TERM...Uttech/Sheets
LONG TERM...Sheets
AVIATION...McClure


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