Area Forecast Discussion
Issued by NWS Quad Cities, IA IL

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FXUS63 KDVN 242048 CCA
AFDDVN

Area Forecast Discussion
National Weather Service Quad Cities IA IL
348 PM CDT WED AUG 24 2016

.SYNOPSIS...
Issued at 309 PM CDT Wed Aug 24 2016

The main synoptic front is located from northeast Iowa to north
central Kansas as of 2 pm today. Along this boundary last night,
several areas of very heavy rainfall took place, with training
storms and efficient rainfall processes. This resulted in swaths of
5 to 8 inches of rain in northeast Iowa, as well as 1 to 3 inch
amounts over our central and southeast CWA. The PWAT values remain
around 2 inches along the front today, and this sets up the stage
for another round of potential flooding rains tonight.

&&

.SHORT TERM...(This Evening through Thursday)
ISSUED AT 309 PM CDT Wed Aug 24 2016

Earlier on today, I issued a flash flood watch for locations
generally along Interstate 80 where heavy rains fell last night, and
the ground is now fully saturated. Tonight, it appears models may be
too far south with the axis of heaviest rain.  It appears a band or
at least a secondary band of heavy rain should be found along the
surface front as it slowly progresses east through eastern Iowa. The
short wave in northeast KS/southeast NE will bring a new round of
widespread thunderstorms with torrential rainfall, as PWAT values
hover near 2 to 2.25 inches and synoptic forcing increases with the
passage of a 125 KT jet streak through southern Minnesota tonight.
The flow is southwesterly ahead of the boundary, which is parallel
to it, and training storms have recently formed in southeast NE
showing that effect of the wind field. Earlier CAMS runs suggest
that storms along the front will move northeast, into a CAPE rich
environment over eastern Iowa. This would bring a threat of training
thunderstorms and flash flooding by mid evening over counties near
Interstate 80, and possibly south. Overnight, a large MCS should
also develop in northern Missouri, possibly directly affecting our
southern counties. This MCS supported by high PWAT values and strong
upper diffluence from the 125 KT jet streak, will likely contain a
large stratiform rain shield with efficient rainfall processes. That
rain shield will most certainly affect our CWA and pops overnight
are forecast categorical for that reason.  I do not think we will
have much severe weather, with the surface winds veered along the
front and little dry air aloft. However, should the bowing segment
lift into our southern CWA tonight, some stronger winds may be
possible.

Tomorrow morning, the front should sweep moisture and remaining
convergence east, ending rains early. Clouds and weak cold advection
should help limit highs to the upper 70s to lower 80s.
ERVIN

.LONG TERM...(Thursday Night through Wednesday)
ISSUED AT 320 PM CDT Wed Aug 24 2016

Thursday Night...Midwest remains beneath long-fetched southwest
mid-level flow, right entrance region of an upper-level jet, and
within the 850mb baroclinic zone. The good news is that models
have the low-level convergent boundary pushing southward through
most of the forecast area, taking the anomalously high PWATs
greater than 1.50" with it.

Rain Chances: Have slight chances for showers and storms for the far
SW, negligible forcing for ascent should preclude anything
significant. The NAM brings heavy precip into the SE CWA, but this
is the outlier solution and was mostly ignored. For low temps,
looking at mid 50s NW to lower 60s I-80 and south.

Friday...Increasing 850-500mb SW flow forecast to advect the higher
PWATs right back into E Iowa/NW Illinois. But, the strength of an
embedded 500mb vorticity max is in question. And for that reason,
POPs are low - at or below 20%. Sfc dewpoints will take some time to
respond to the SE sfc winds. Expect comfortable humidity levels with
dewpoints in the upper 50s to lower 60s, pushing the mid 60s far
south.

Friday Night through Saturday...Possibility for a convectively
induced vorticity max to roll through the Upper Mississippi Valley,
especially Friday night into Saturday. The main mid-level trough
axis is forecast to slide through North Dakota, South Dakota, and
into Minnesota. For E Iowa/NW Illinois, convective chances seem to
be more dependent on convective complexes forming within the
greater instability over Nebraska and Kansas Friday afternoon. We
will then have to watch how this possible MCS evolves to the ENE.

Thunderstorm Threats: Similar to the past several events, heavy
rainfall rates and localized flash flooding are the main threats.
Anomalous PWATs approaching 1.75" are near the 90th percentile for
the end of August according to the NAEFS.

Saturday Night through Wednesday...Continued warm and humid.
Temps in the lower 80s for highs, dewpoints well into the 60s or
occasionally lower 70s. Have lower end POPs of 20-40% in the
forecast. However, there will be plenty of prolonged dry periods.
At this time, models are forecasting an absence of shortwave
troughs across the Midwest; therefore rain chances are likely to
be contingent on mesoscale boundaries or instability gradients.
Uttech

&&

.AVIATION...(For the 18Z TAFS through 18Z Thursday Afternoon)
ISSUED AT 1224 PM CDT Wed Aug 24 2016

Skies will be improving to VFR around 18Z if not already in place
at that time as the last remnants of the widespread thunderstorms
move east. After a clear to scattered afternoon, more showers and
thunderstorms will redevelop over central and southern Iowa,
moving into eastern Iowa by mid to late evening. These storms will
bring torrential rainfall rates, and poor visibilities as they
encounter terminals. That said, we are forecasting a broad period
of MVFR with thunderstorms until we can time in the stronger
periods. Late tonight, rains may continue to be widespread, but
thunder will gradually become less frequent, and MVFR cigs will
continue, possibly IFR. A cold front arriving early Thursday
morning will bring VFR CIGS and good visibilities.

&&

.HYDROLOGY...
Issued at 309 PM CDT Wed Aug 24 2016

Flash flooding will be possible in the counties generally along and
south of Interstate 80 tonight as efficient rainfall process and
storm training allow for areas of 1 to 4 inches of rainfall. This
may bring the threat of significant rises and flooding to area
tributary rivers but nothing we can specifically forecast at this
early hour. The Mississippi river is forecast to rise to action
stage at many sites especially from the Quad Cities downstream. This
is primary due to flash flooding rains last night upstream.
Additional heavy rain may change these forecasts but the placement
north or south of the heavy rain band will greatly determine this
impact.
ERVIN

&&

.DVN Watches/Warnings/Advisories...
IA...Flash Flood Watch from 7 PM CDT this evening through Thursday
     morning for Cedar-Des Moines-Henry IA-Jefferson-Johnson-
     Keokuk-Lee-Louisa-Muscatine-Scott-Van Buren-Washington.

IL...Flash Flood Watch from 7 PM CDT this evening through Thursday
     morning for Bureau-Hancock-Henderson-Henry IL-McDonough-
     Mercer-Putnam-Rock Island-Warren-Whiteside.

MO...Flash Flood Watch from 7 PM CDT this evening through Thursday
     morning for Clark-Scotland.

&&

$$

SYNOPSIS...Ervin
SHORT TERM...Ervin
LONG TERM...Uttech
AVIATION...Ervin



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