Area Forecast Discussion
Issued by NWS Quad Cities, IA IL

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FXUS63 KDVN 260925

Area Forecast Discussion
National Weather Service Quad Cities IA IL
425 AM CDT Wed Jul 26 2017

Issued at 341 AM CDT Wed Jul 26 2017

High pressure over the Great Lakes still governing the weather
across the area early this morning with dry and seasonable
conditions. High clouds continue to be thick at times streaming
into the cwa off convection to our north/west from WI and MN
to NE. This convection is being aided by frontal boundary and
moisture transport from nocturnal LLJ along with ascent from
MCV near the NE/SD border. Observational data shows axis of
unseasonably high moisture levels pooling right ahead of the
surface front with PWATs of 2.21 inches at MPX, 229% of normal.


.SHORT TERM...(Today and Tonight)
ISSUED AT 341 AM CDT Wed Jul 26 2017

As the surface cold front pushes south and across the area later
today and tonight, pooling moisture along with convergence aided
by LLJ plus additional lift from approaching MCV and attendant
low pressure system will lead to widespread showers and storms.
This will bring a risk of heavy rain/flash flooding to some areas
along with the potential for a few strong to possibly severe storms.

Heavy rain and flash flooding potential:

Dprog/dt or trends of majority of 00z model guidance continue to
support a southward shift to axis of heaviest rain potential
next 24 hours and generally suggest southern cwa at greatest risk
for widespread heaviest rain axis. This also has support from
progged location of 40+ kt veered LLJ and axis of unseasonably
high moisture levels. PWATs are generally shown to surge to around
2.5+ inches along/s of I-80, which is 2 to 3 standard deviations
above normal. This coupled with deep warm cloud depths of 14-16kft
agl and slowing storm motion vectors more pronounced this evening
with influence of LLJ suggests potential for impressive rainfall
rates and some backbuilding or repeated development for a risk of
flash flooding thus have hoisted a flash flood watch for near/south
of I-80 late this afternoon and tonight. Localized rainfall amounts
of 2 to 4+ inches very possible in the watch area, especially
heading further south in the watch area. Do have concerns for flash
flooding over recently hard hit areas of northeast IA into northern
IL. However, heavy rainfall looks to be more localized with the
better signal for widespread heavy rain further south. For this
reason and after coordination with neighboring offices I have left
northern cwa out of a flash flood watch for now. Will be watching
these trends carefully today, and should the signal for widespread
heavy rain shift or develop over northern cwa then may need to expand
the flash flood watch. The good news is there is time to assess the
trends with bulk of heavy rain occurring tonight.

Severe weather potential:

Bulk shear of 25-40 kts and potentially higher with aid from MCV
combined with MLCAPE of 1500-2500 J/KG will support the risk
for a few severe storms late this afternoon and evening. Feel
the main threat will be with damaging winds with wet microburst
potential and bowing segments with a few clusters as storms ramp up
in coverage. But, some concerns exist with rapid saturation and
weaker low to mid level lapse rates along with warm cloud depths that
could limit or mitigate damaging wind setup. A factor will be how
much heating/destabilization occurs today. Can`t rule out large hail
but should be tough to come by and favored with any initial discrete
storms. Also a very low tornado potential exists, but just where
will be dependent on track/location of surface low and backing of
surface winds, or any boundary storm interaction. Overall, any
severe threat should be waning mid to late evening as nocturnal LLJ
strengthens and shifts focus to heavy rain/flash flooding.


A lot of uncertainty on high temps today due to anticipation of
ample convective debris cloudiness. Forecast is geared toward
more high cloudiness persisting today and therefore highs on cooler
side of guidance blend and generally mid 80s N to lower 90s S.
These forecast highs combined with dew points in the 70s yield PM
heat indices just below criteria and in the range of mid to upper 90s
with possibly a few sites around 100 far south. Plan to highlight in
HWO, WX story and issue SPS, but hold off on advisory given below
criteria most if not all areas and uncertainty with convective debris.

.LONG TERM...(Thursday through Tuesday)
ISSUED AT 341 AM CDT Wed Jul 26 2017

On Thursday, rain will end from northwest to southeast during the
day. Most models suggest a fairly steady southeastward movement of
the rain. Thus the possibility does exist that most if not all of
the area may be dry during the afternoon. Temperatures Thursday
should be at or slightly below normal.

Thursday night through Friday night quiet and dry conditions will be
seen across the area as high pressure builds into the Midwest.
Temperatures should average slightly below normal.

Saturday on...

The quiet and dry conditions will continue Saturday through Sunday
night as high pressure moves slowly into the Ohio Valley.
Temperatures should be close to or slightly below normal.

On Monday, the next cold front will move through the area. Moisture
is quite limited and the overall forcing is weak. The model
consensus has slight chance pops across roughly the north half of
the area during the afternoon.

Monday night through Tuesday quiet and dry conditions will be seen
across the area as the next Canadian high builds into the Midwest.


.AVIATION...(For the 06Z TAFS through 06Z Wednesday Night)
ISSUED AT 1152 PM CDT Tue Jul 25 2017

High level clouds from storms over the plains will cover the
region tonight, preventing ground fog from becoming more than an
MVFR issue. Otherwise, winds will be light from the south to
southeast overnight into Wednesday. Widespread thunderstorms are
expected to accompany a cold front from late afternoon through
evening. Prevailing thunderstorms and at least MVFR conditions were
brought in at all sites at 00z, which may be overdone by mid to
late evening as storms gradually pass southeast of the region,
which will be fine tuned in later forecasts.


IA...Flash Flood Watch from late this afternoon through Thursday morning
     for Cedar-Clinton-Des Moines-Henry IA-Iowa-Jefferson-
     Johnson-Keokuk-Lee-Louisa-Muscatine-Scott-Van Buren-Washington.

IL...Flash Flood Watch from late this afternoon through Thursday morning
     for Bureau-Hancock-Henderson-Henry IL-McDonough-Mercer-
     Putnam-Rock Island-Warren-Whiteside.

MO...Flash Flood Watch from late this afternoon through Thursday morning
     for Clark-Scotland.



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