Area Forecast Discussion
Issued by NWS Quad Cities, IA IL

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FXUS63 KDVN 190554

Area Forecast Discussion
National Weather Service Quad Cities IA IL
1254 AM CDT Sat Aug 19 2017

Issued at 1240 AM CDT Sat Aug 19 2017

Increased PoPs significantly over the west-central CWA as complex
of showers and storms associated with PVA from mid-level
vorticity max continues to propagate further east than the hi-
res model guidance is showing. Expect scattered storms to impact
portions of Benton and Iowa Counties over the next 1-2 hours.
Although the storms should be in a weakening phase, they may also
make it into parts of Linn and Johnson Counties between 2-4 AM.
PoPs are not as high here, but may increase them if radar trends
support it.

The main threats are wind gusts over 30 mph, brief heavy rain, and


Issued at 330 PM CDT Fri Aug 18 2017

The latest sfc analysis was indicating a weak sfc low in the VCNTY
of the DSM area, with lobe of current pressure falls suggesting it
to follow east-southeastward generally along and south of the I80
corridor though this evening. This system a sfc reflection of a lead
vort max aloft acrs central IA, but a more vigorous vort max was
seen on water vapor imagery currently dropping southeastward acrs
southwestern MN into eastern SD. After these systems for tonight, it
still appears a warming weekend on tap with storm chances increasing
by late Sunday and into early next week.


.SHORT TERM...(This Evening through Saturday)
ISSUED AT 330 PM CDT Fri Aug 18 2017

Tonight...Watching the moisture convergent and sfc DPT gradient
extending from along the western IA/MO border region, east-
northeastward up to just southwest of the Quad Cities. Some cumulus
trying to tower in theses areas looking at latest satellite Vis
loop. With lead vort max aloft, this will be the area to watch for
sctrd thunderstorms development later this afternoon into early
evening, with the resultant activity then moving acrs areas along and
south of I80 through mid evening before shifting south and
east(especially south) toward more optimum CAPE build up. According
to latest mesoscale analysis, these areas currently being affected
by an EML/CAP. But with increasing deeper layer shear and CAPE build
up of 1000-2000 J/KG, if the CAP goes allowing for sctrd storms in
these areas(more favored along and south of a Fairfield IA to
Princeton IL line) there will be a chance for a few severe storms
with main threats large hail and damaging winds/downbursts.

Then  eyes will turn to the upstream strong to severe storm clusters
to the northwest acrs northwest into central IA, being driven by
strong vort lobe aloft dropping out of SW MN. Some chance for this
activity to make it into the west central and southwestern CWA after
10-11 PM CDT tonight. Several earlier run CAMs decay this convection
as it enters the western doorstep of the DVN CWA, but a few later
runs and looking a ongoing lift of digging wave aloft, allow some of
them to maintain and move acrs the southwestern half of the fcst
area through 3-4 AM, if in a weakening fashion. Will cover both of
these two scenarios with appropriate POPS, but the later night POPs
may have to be raised after monitoring incoming trends of this
upstream activity out of central IA.

Then as late night ridge axis lays out acrs central IA by Sat
morning, some indicators suggest fog may occur after 3 AM in light
sfc wind fields. Crossover temps neared during that window. Will
keep patchy fog going, and really see the potentail of valley/river
fog. Sfc moisture profiles and late night BL layering suggesting
shallow dense fog in some spots, especially the northwestern to
western third of the DVN CWA and will go with areas of fog, as
opposed to patchy, in those locations. Lows in the low 60s to mid
50s most locations by early Sat morning.

Saturday...Probably the best day of the weekend, with ongoing light
sfc wind regime, sunshine under high pressure, and temps warming
into the low to mid 80s in most locations with manageable humidity
levels.    ..12..

.LONG TERM...(Saturday Night through next Friday)
ISSUED AT 330 PM CDT Fri Aug 18 2017

Expect building heat and humidity through early next week with
potential for several rounds of showers and storms, followed by a
dry and cooler period through the rest of the week.

Saturday night/Sunday will be mainly dry with a flattening upper
ridge in place across the area. By Sunday night into Monday, expect
showers and storms as mid-level energy moves in from the west and a
frontal boundary stalls out in the region. Dewpoints in the 70s
feeding into this boundary could produce some widespread heavy rain.
Cloud cover in the area does not look favorable for ideal eclipse
viewing conditions. For Monday, deep layer shear should be
sufficient for severe storms, but limited insolation may help lessen
the threat.

For Monday night and Tuesday, the main frontal system and upper
trough will push through the region, accompanied by widespread heavy
rain. The overall severe threat will be largely dependent on the
timing of the front. Average 1 to 2 inch rainfall amounts are
possible through the period, with some pockets of higher totals.

Afternoon heat indices could push into the 90s Sunday and Monday,
with some relief on the way behind the front beginning Tuesday. For
Wednesday through Thursday, expect dry weather with highs mainly in
the 70s.


.AVIATION...(For the 06Z TAFS through 06Z Saturday Night)
ISSUED AT 1132 PM CDT Fri Aug 18 2017

Skies will be fair through the period with mostly VFR conditions and
light winds. Overnight we will see patchy fog develop with periods
of MVFR conditions possible toward morning and possibly briefly IFR
conditions. After 19/13Z...fair skies and VFR conditions return with
light winds through Saturday evening.




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