Area Forecast Discussion
Issued by NWS Quad Cities, IA IL

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648
FXUS63 KDVN 261817
AFDDVN

Area Forecast Discussion
National Weather Service Quad Cities IA IL
117 PM CDT Wed Jul 26 2017

.UPDATE...
Issued at 1247 PM CDT Wed Jul 26 2017

MCV and associated large area of showers and some embedded
thunderstorms currently moving acrs western into central IA.
Plenty of debris clouds associated with this system already
cloaking the area and tempering warm up potentail some, but Vis
satellite also showing some thinning allowing for some mid 80s.
Thus still see CAPEs growing to 2000-3000 J/KG in some spots
locally, and expect some embedded convective flare-ups as the
upstream area of precip moves in from the west this afternoon.
Low to mid level lapse rates are marginal, and some delay in
strengthening low to mid level flow make for a marginal severe
storm threat, but still will have to be on guard for
strengthening smaller scale bow or storm cluster able to produce
damaging winds late this afternoon into early evening. Portion of
MO RVR Valley 2 to 2.4 inch PWAT plume upstream will also make it
this way east on nose of LLJ and fuel locally heavy rain with any
storm cluster or passing embedded isolated storm. Will keep the
flash flood watch ongoing, but it may not be a widespread threat
this afternoon and evening if stronger storms stay isolated to
smaller clusters embedded in the passing system.

More and more signs, as well as recent convective allowing model
runs, suggest the larger scale heavy rain threat/MCS potential may
flare later tonight up on the southern remnants of the passing
afternoon/early evening system, which would be mainly along and
south of our area of concern from the far northwest/north central
MO/IA border region, down toward the STL area. Will continue to
watch trends through afternoon package issuance time.   ..12..

&&

.SYNOPSIS...
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.

Heat...

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 18Z TAFS through 18Z Thursday Afternoon)
ISSUED AT 1247 PM CDT Wed Jul 26 2017

Besides some lower MVFR clouds around the DBQ terminal, VFR until
the system moves in from the west with showers and embedded
strengthening thunderstorms. Thus low VFR to MVFR conditions mid
afternoon through mid evening with this passing system, and
embedded IFR with stronger storms. All terminals will be subject
to this system and varying rounds of flight conditions. Then
tonight after the rain moves off to the east and a frontal system
sags through the region, some chance of low MVFR clouds or even
IFR decks in the northern TAF sites, as well as MVFR to IFR fog
into early Thu morning. Some signs that the more widespread
storm/rain threat will shift just to the south of the area acrs
northern MO later tonight into Thu morning. Increasing northeast
sfc flow by late morning will hopefully scoop up CIGs and improve
fogged up VSBYs toward the end of the TAF cycle.   ..12..

&&

.HYDROLOGY...
Issued at 1246 PM CDT Wed Jul 26 2017

The flood wave continues to move down the Wapsipinicon River,
with the crest approaching Anamosa where it should crest Thursday.
Downstream near the DeWitt gage there remains a bit of uncertainty
as to how high the water will get. With levels cresting a bit
below the original forecasts it is not out of the question that
it may crest below 12.5 feet at DEWI4. However, uncertainty with
the rain event for this afternoon and evening leaves enough
concern to not make any adjustments to the forecast at this time.

The Pecatonica River continues to rise upstream of Stephenson
County, however looks to be nearing the crest at Martintown, WI
early this afternoon. Also, reports from Emergency Management
indicate the river is spreading significantly across farmland in
the floodplain so considerable amounts water is still making
its way into and through the river system. While the timing is
uncertain, feel quite confident that the river will begin to rise
again soon in the Freeport area, and could approach the 16.0 ft
mark again, which the ongoing forecast shows. With all the water
yet to move through the Pecatonica and into the Rock River, will
continue to see high levels along the Rock River through its
confluence with the Mississippi River. However, the primary crest
is working through the lower reaches of the Rock, where Moline
appears to have crested last night while maintaining a nearly
steady height through today.

The Mississippi River at the Quad Cities is receding and will
likely fall below flood stage yet today. The crest has made it to
the Keithsburg area, with areas upstream receding and downstream
still rising. No areas are currently forecast to hit moderate
flood stage.

Today`s rain continues to be a concern. It still looks like the
heaviest widespread rains will fall in areas near I-80 and
southward, though any thunderstorm will have the potential for
torrential rainfall and 1 to 2 inches in a short amount of time.
This will be monitored closely as any shift in the expected
heavier rainfall corridor could make a big difference in the river
forecasts.

&&

.DVN WATCHES/WARNINGS/ADVISORIES...
IA...Flash Flood Watch from 4 PM CDT 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 4 PM CDT this afternoon through Thursday
     morning for Bureau-Hancock-Henderson-Henry IL-McDonough-
     Mercer-Putnam-Rock Island-Warren-Whiteside.

MO...Flash Flood Watch from 4 PM CDT this afternoon through Thursday
     morning for Clark-Scotland.

&&

$$

UPDATE...12
SYNOPSIS...McClure
SHORT TERM...McClure
LONG TERM...08
AVIATION...12
HYDROLOGY...Brooks



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