Area Forecast Discussion
Issued by NWS Quad Cities, IA IL

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FXUS63 KDVN 150421
AFDDVN

Area Forecast Discussion
National Weather Service Quad Cities IA IL
1121 PM CDT Mon Aug 14 2017

...AVIATION UPDATE...

.SYNOPSIS...
Issued at 336 PM CDT Mon Aug 14 2017

The small convective complex that rolled through southeast IA
this morning has left a weak boundary across southeast IA into
west central IL. The main warm front was further north reaching
from far southeast MN into southern WI. A weak cold front trailed
southwest, roughly bisecting IA from SW to northeast. In the warm
sector, temperatures were in the 70s to near 80s with dewpoints in
the 60s to near 70 across the local forecast area. Aloft, a
shortwave was seen swirling on water vapor imagery into the
western Great Lakes with a trailing dry slot moving across IA.

&&

.SHORT TERM...(This Evening through Tuesday)
ISSUED AT 336 PM CDT Mon Aug 14 2017

The main challenge is the potential for showers and thunderstorms
tonight and Tuesday. Latest radar imagery indicated weak echoes
forming along the highway 20 corridor in eastern IA, closest to the
triple point of the weak boundaries mentioned in the synopsis. This
will likely be the onset of the activity for tonight and will have
low pops for what will likely be isolated to widely scattered
showers and a few thunderstorms late this afternoon and early this
evening. As indicated by a rough consensus of high-res convective
models, this convection will spread east to southeast with the
advancing weak cold frontal boundary. Storms are not likely to be
severe, considering the marginal instability in place and lack of
forcing. These should dissipate by late evening as mid level
subsidence increases and instability wanes. Will mention patchy fog
over mainly the north tonight as wind fields become light with
lingering low level moisture.

Tuesday, the passage of a shortwave ridge aloft and onset of surface
high pressure passing to the north is shown driving the front south
of the forecast area by early in the day. Will keep low chances for
showers and thunderstorms across far northeast MO into SE IA and
west central IL due to low confidence in exactly where this boundary
will stall out. Elsewhere, the big question will be how much cloud
cover can linger north of the boundary, which will be a critical
factor as to how warm temperatures can recover. For now, have stayed
close to a guidance blend with a range from the upper 70s north to
mid 80s south.

.LONG TERM...(Tuesday Night through Monday)
ISSUED AT 336 PM CDT Mon Aug 14 2017

Tuesday night...Most of the 12z run model solutions have the main
warm front draped to the south along and south of the IA/MO border
region overnight. Early indications suggest that some marginal H85
MB(LLJ) south to southwest flow and convergence of 20 KTs running
into 15 KTS up over and to the north of the sfc front, may produce
some elevated bands of warm air advection type showers and storms
mainly after midnight and south of I80. But these nocturnal
processes will have to battle temporary upper ridging up acrs the MS
RVR Valley to lee of west central plains incoming upper trof, and
much of the local area may stay dry into early Wed morning.

Wednesday...Seasonably strong upstream short wave upper trof will
look to sweep east acrs much of the Midwest this period, but it`s
prime axis will not look to clear the MS RVR until midday Thu,
according to ensemble timing of the latest medium range model runs.
As for the day, still many variables with respect to potentail
convective debris and sfc boundaries laying around, lingering out
whatever Tue night convective activity that may develop. Thus
thermodynamic/instability profiles that can build up by late Wed
afternoon still at question to fuel more significant storms by late
Wed into Wed night. A few solutions which keep showers and
embedded storms going for much of the day limit instability build
up, and paint a picture of a cloud period with occasional showers
and storms going into Wed night. Some others either decay morning
debris in time for afternoon heating and sfc DPT advection feed into
the upper 60s and 70s for MUCAPEs of 2000-3000 J/KG ahead of
incoming llvl front. Or other models like the 12z ECMWF don`t have
much overnight activity to overcome and delay the main convective
event until late afternoon and evening acrs central into eastern IA.
With projected increase of deep layer shear to 35-45+ KTs to the lee
of upper trof complex, any decent heating/air mass recovery would
support a chance for at least a few strong to severe storms by
Wednesday evening. Cloud cover, increasing column saturation and
possible debris make for a challenging high temp fcst for Wed, but
more solutions point toward more areas making it to 80 or warmer as
the afternoon progresses, especially along and south of the Hwy 30
corridor.

With a possible precipital water(PWAT) feed of 1.9 to over 2 inches
acrs the area, the more organized storm clusters will be proficient
rain producers at heavy rates. With extent of bulk shear profiles,
could see areas that get under the path of an organized storm
cluster or get hit a few times, receiving well over an inch of
rainfall by Thu morning, or even rainfall values matching the PWATs.

Thursday and Friday...With current upper trof progression speed,
lingering lift should allow for some continues areas of showers and
a few thunderstorms lasting well into mid Thu morning. Bit of a lag
in post-frontal cool air advection, wrap around clouds and lingering
precip make for another challenging temp fcst day Thursday. High
pressure may make for late Thu into Fri a precip lull period, with
temps near or a touch below normal. Low confidence for now with this
scenario, but some signs of another short wave embedded in
northwesterlies aloft, getting ushered down acrs portions of the area
for another shower chance late Fri night lingering into portions of
Sat.

Saturday through next Monday...Again some showers and a few storms
may linger well into the day Saturday, then another round of llvl
ridging sliding acrs the region for a possible precip lull window
Sat night into Sunday. Longer range progs and upper jet patterns,
suggest the re-establishment of a warm dome upper ridge somewhere
acrs the southern plains to low to mid MS RVR Valley by the second
half of the weekend. Where the prime upper ridge axis lays out, edge
of building cap/EML and short wave track will look to set up on it`s
southern periphery. Thus the region may be susceptible to storm
clusters or MCS like developments by as early as Sunday night into
Monday. But how far north or south these features lay out still very
uncertain at this time. But once it get`s established, the active
storm track may linger well into next week.     ..12..

&&

.AVIATION...(For the 06Z TAFS through 06Z Tuesday Night)
ISSUED AT 1119 PM CDT Mon Aug 14 2017

Isolated overnight showers are expected to avoid TAF sites. Fog
starting to develop north of Interstate 80 in area of clearing and
light winds. Expecting MVFR to occasionally IFR fog with periods
of IFR ceilings. By late Tuesday afternoon/evening, KBRL would be
favored for isolated showers and storms.

&&

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

$$

SYNOPSIS...Sheets
SHORT TERM...Sheets
LONG TERM...12
AVIATION...RP Kinney


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