Area Forecast Discussion
Issued by NWS Quad Cities, IA IL

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FXUS63 KDVN 260255
AFDDVN

Area Forecast Discussion
National Weather Service Quad Cities IA IL
955 PM CDT Tue Jul 25 2017

...HYDROLOGY UPDATE...

.SYNOPSIS...
Issued at 330 PM CDT Tue Jul 25 2017

Western GRT LKS high pressure to continue move off to the east,
allowing warmer an more moist air to seep back toward the area from
the plains and lower MO RVR Valley. Active westerlies along and
north of this modifying airmass will usher a main wave, seen now
ridge-riding acrs the central Rockies/UT into western WY, down acrs
the local area by late Wed. This will set the stage for another
round of storms, some with heavy rain and potentially strong to
severe, acrs the area Wed afternoon and night, before digging
northwest flow behind the passing wave brings down not as warm and
drier air into the weekend.

&&

.SHORT TERM...(This Evening through Wednesday)
ISSUED AT 330 PM CDT Tue Jul 25 2017

Tonight...Generally a fair wx night as high pressure continues to
slide east, and organizing frontal zone is the convective focal
point from the west central plains, up acrs MN and into the
northwestern GRT LKS. Our area of concern may just get some high
debris clouds spilling in off that activity at times tonight, while
light south to southeasterly sfc winds maintain at 4 to 8 MPH
overnight. The winds and cirrus may limit much of any dense fog
threat, but there could still be lighter fog as well as some valley
occurrence with the sfc DPT increase and over areas of recent heavy
rainfall. Lows generally in the low to mid 60s east of the MS RVR,
and in the mid to upper 60s to the west.

Wednesday...rather seasonably strong short wave noted above, will
look to dig down acrs IA by afternoon. Increasing flow velocity in
the low to mid levels, especially noted is west-southwest flow
increasing to 50 KTS at around H5 MB, will make for strong deep
shear profiles by late afternoon/Wed evening. The challenge will be
how much heat and humidity(thus instability) will be allowed to build
up as convective debris from overnight activity to the west/northwest
or renewed showers/storms move acrs the area as the day progresses.
Forecast highs may be too warm if this builds in and thickens by
late morning and midday, and could squash CAPE build up except maybe
the far south.But with uncertainties, will go with highs in the
lower 80s in the far northwest, to the upper 80s-around 90 south of
I80. With DPTS in the low to mid 70s, CAPEs could approach 2500-
3500+ in the southern areas by mid afternoon along with heat index
readings in the upper 90s to near 100...again if cloud cover doesn`t
thicken acrs these areas sooner.

Lift from the approaching wave and ahead of the incoming frontal
system, should bring about an increase in coverage and intensity
even amongst any lingering debris as the afternoon progresses. Where
the CAPEs are allowed to build up, such as latest thinking south of
I80, this is where some of the stronger late afternoon storms may
fire with some chance of severe/winds and hail, as well as very
heavy rainfall.  Even toning down the bullish NAM`s PWATs of 2.5 to
2.7 inch feed up acrs the area by 20-25 KT H85 flow, 1.9 to 2.4
still very possible and would help produce localized swaths of heavy
rainfall where storms pass. For now, will advertise 0.40 to 0.80 of
an inch by 00z Thu in the far west where storms move across, but how
far north or south the prime axis is still uncertain, unless the
propagation toward the higher CAPES again supports to south...Iowa
City to Burlington and points west.    ..12..

.LONG TERM...(Wednesday Night through next Tuesday)
ISSUED AT 330 PM CDT Tue Jul 25 2017

Forecast focus on the potential for severe weather and heavy
rainfall Wednesday evening into the overnight. Then mainly dry and
rather pleasant for much of the remainder of the extended.

Wednesday night: Thunderstorms that develop Wednesday afternoon will
continue to push across the cwa as an MCS. There will be the
potential for damaging winds and torrential rainfall. The NAM is the
most aggressive with PWAT`s of over 2.5 inches, and this amount of
moisture will be capable of dumping a couple of inches of rain in
less than an hour. This may cause flash flooding at some locations.
Low confidence in where the heaviest axis of rain will occur as the
NAM/ECMWF says our northern cwa, while the GFS suggests our southern
cwa. The Canadian model indicates along I-80. There is still no
evidence of back-building or training in the cwa as the low level
jet shifts southward of the dvn cwa overnight. In other words, this
is a progressive system with the cold front pushing south and east
of the cwa after midnight, diminishing the thunderstorm activity.

Thursday: The consensus models indicate lingering showers and a few
thunderstorms in our southern and eastern counties. However, believe
the models are hanging onto the pcpn too long for later shifts to
decide. Highs will be in the 80 to 85 range. Despite a north wind
behind the cold front, drier air will lag a while and it will be
Thursday night before the noticeably comfortable humidity arrives.

Friday through early next week: A rather comfortable stretch of
weather is expected with little if any rainfall. The storm track
will have shifted well up in Canada with the southwest monsoon quite
active. Assuming the models are correct, the dry weather should
continue for much of next week. Temperatures will be somewhat below
normal for late July/early August with highs in the upper 70s to
lower 80s at most locations. Lows should generally be in the upper
50s to lower 60s.

Haase

&&

.AVIATION...(For the 00Z TAFS through 00Z Wednesday Evening)
ISSUED AT 645 PM CDT Tue Jul 25 2017

High level clouds will increase over the area tonight. This will
limit the potential for shallow fog, which is included only as 5sm
to 6sm visibilities in the forecasts for early Wed morning.
Otherwise, conditions will remain VFR with light south to
southeasterly surface winds. Scattered thunderstorms are expected
by late Wednesday afternoon ahead of an approaching cold front.
This is included as prob30 groups at all sites at the end of the
forecast period.

&&

.HYDROLOGY...
Issued at 955 PM CDT Tue Jul 25 2017

Updated forecasts this evening have lowered expected crests along
the Mississippi River at Illinois City, Muscatine, New Boston,
and Keithsburg, resulting in a category change from Moderate
Flooding to Minor Flooding. Downstream crests, anticipated to just
reach Moderate Flooding at both Gladstone and Burlington remain
unchanged outside minor timing adjustments.

Otherwise, only minor adjustments made to on-going flooding
impacting the Rock and Pecatonica Rivers. A flood warning
continues along the lower end of Yellow Creek in Stephenson
County, where Emergency Management reported continued road
closures this evening, but water levels continuing to recede.

&&

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

$$

SYNOPSIS...12
SHORT TERM...12
LONG TERM...Haase
AVIATION...Sheets
HYDROLOGY...Sheets



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