Area Forecast Discussion
Issued by NWS Quad Cities, IA IL

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FXUS63 KDVN 182234

Area Forecast Discussion
National Weather Service Quad Cities IA IL
534 PM CDT Tue Jul 18 2017

Issued at 532 PM CDT Tue Jul 18 2017

The MCV is convectively active now over our Iowa counties. I have
increased pops to likely for this activity, and with the
relatively slow movement, an inch of rain seems likely with these
storms. They will continue to lift east to northeast, with the
strongest updraft/storms on the east to southeast flank of the
MVC. Some gusty winds are possible near 40 to 50 mph with the
stronger storms.


Issued at 349 PM CDT Tue Jul 18 2017

The hot and humid air mass has advected into E Iowa/NW Illinois
today. As sfc winds turned to the SSW, dewpoints jumped into the
lower to middle 70s. The muggy air together with sfc ambient temps
in the mid 80s to around 90 F was creating heat indices this
afternoon in the lower to upper 90s for most areas. Have the
impacts of upper 90s to near 100 F heat indices handled with a
Special Weather Statement for locations roughly along and south of
a Benton County (Iowa) to Clinton County to Whiteside County

Regional and local radar depicted a small cluster of showers and
storms forming over south-central Iowa between Altoona and
Oskaloosa. A weak low to mid-level vorticity max was exposed by this
area of convection via weak cyclonic rotation in recent radar loops


.SHORT TERM...(This Evening through Wednesday)
ISSUED AT 349 PM CDT Tue Jul 18 2017

This Evening and Tonight

Best chances for scattered showers and thunderstorms will likely be
associated with the vort. max mentioned in the synopsis. The
forcing/PVA attendant to this feature is quite not
foresee widespread coverage of rain. The track of the small
cluster of convection will likely be along the counties in the
I-80 corridor. However, even in this zone, there will be many
locations that receive little to no rainfall. Weak effective bulk
shear under 25 kts should preclude a severe weather threat.

The next area of concern is with convection developing off to the
north across NE Iowa and SE Minnesota. 18Z run of the 3 km hi-res
NAM flipped back to sustaining a broken line of convection dropping
into the northern CWA after 9-10 PM. Confidence on this solution is
low and PoPs in the 20-40% message that. It`s in this area where the
Storm Prediction Center has a marginal risk for severe storms
capable of producing scattered damaging wind gusts due to moderate
DCAPE values near 1000 J/kg. However, the severe threat is low and
depends on the convection remaining organized as it propagates to
the SE tonight. Thinking due to lack of sufficient deep layer
shear, most storms that impact the CWA will stay below severe


Excessive heat warnings and watches are in effect for portions of
the area. The watch was expanded to include several more counties to
the north and now includes the Quad Cities Metro area. Please refer
to the heat headlines for location specific details. Peak heat
indices will be highest south of I-80 where the warning is in
effect. The bottom line is we are entering a prolonged period of
heat indices near or over 100 F that may last through Saturday of
this upcoming weekend. It`s important to take the necessary
precautions to prevent heat related illnesses.

Continued hot and humid with peak heat indices a few degrees higher
than today (Tuesday). Warming 850mb temps should effectively cap the
lower atmosphere, preventing convection through the afternoon.
Although, there is a possibility for a convectively induced sfc
boundary to be draped somewhere across the central to northern CWA.
So there is a low chance for an isolated shower or storm. Uttech

.LONG TERM...(Wednesday Night through Tuesday)
ISSUED AT 349 PM CDT Tue Jul 18 2017

Thursday through Saturday Heat mentioned above, have
coordinated an Excessive Heat Warning through Sat evening generally
south of I80, and continue/expanded Heat Watch to the north along
the I80 corridor including the Quad City area through Princeton IL.
Away from convective debris and storm outflow, expect average
afternoon ambient temps in the low to mid 90s and sfc DPTs in the
mid 70s if not higher/pooled along boundaries. The "away from
convective debris/clouds" is a big one, and see the challenging day-
by-day need to adjust additional Heat Advisory and Warning areas
north or south depending on recent or projected convective activity
zones. But the southern third or a bit more of the DVN CWA has the
best chance for 2-4 day accumulative effect of 105+ Heat Index
readings, thus the issuance of the long duration warning. But even
here, there could be a southward push of the main front or outflow
boundary by nocturnal storms(further south than any model suggests),
focusing convective debris or even new storm development into the
warning areas and thus putting a lid on hitting true excessive heat
criteria at times.

Wednesday night through Saturday precip/storm chances...12z GFS and
18z NAM are more bullish in bringing down a portion of an MCS into
the northern third or even more of the DVN CWA after Midnight Wed
night into Thu morning, while the 12z ECMWF propagates a convective
system along and just to the north of the DVN CWA during that time,
maybe scraping the Hwy 20 corridor. See the further south push of a
portion of this system if it is mature and using sfc cool pool
mechanics. With deeper layer shear and lingering CAPEs, could see a
bowing segment producing damaging winds even late night, and
torrential rainfall at high rates where these system passes. Some
areas getting a quick half inch to over an inch by Thu morning.

Nocturnal LLJ impinging on lingering boundary may even produce
localized flash flood potentail under back-building storms lingering
into mid thu morning. The same scenario may unfold for the following
two nights Thu night and Fri night being so close to the increased
mid layer westerly steering flow and edge of more enhanced elevated
mixed layer to the south. Thus late night thunderstorm chances into
the start of the weekend. High CAPES, shear, and PWATs of 1.8 to
well over 2 inches continue to suggest the main threats being
damaging wind gusts and very heavy, possibly flooding rains
especially in areas of repeated storm tracks. The medium range models
vary on what night or nights is more primed for the area the get
clipped or more widespread impacted by these systems, and will have
to carry nightly POPS with higher values in the north for now. But
again, by Thursday or especially Friday, can`t rule out a southward
shunted boundary "lighting up" along and south of I80 in the late
afternoon or evening. Low confidence for Saturday and it appears
muddled with storms chances and storm strength potentail.

Sunday through next Tuesday...A digging short wave trof aloft,
opening up northwesterly steering flow for relief in the form of
Candian high pressure, still appears to be on track according to
longer range upper jet trends/pattern changes. Currently progged
incoming airmass trajectories suggest highs only in the upper 70s to
lower 80s for much of the area both Monday and Tuesday, with Sunday
being a bit warmer transition day. the humidity will be scored out
as well by increased drying northwesterly flow early next week, but
will have to battle crop evapo-transpiration and area swaths that get
hit by heavy rain the previous 2-4 days. ..12..


.AVIATION...(For the 18Z TAFS through 18Z Wednesday Afternoon)
ISSUED AT 1221 PM CDT Tue Jul 18 2017

VFR will prevail through this TAF cycle. There is a low chance
for a shower or brief storm to impact KCID/KDBQ late this
afternoon into tonight, but did not mention in TAFs at this time
due to low confidence on occurrence. Will continue to monitor and
amend if necessary. Uttech


IA...Excessive Heat Warning from noon Wednesday to 8 PM CDT Saturday
     for Des Moines-Henry IA-Jefferson-Keokuk-Lee-Louisa-Van

     Excessive Heat Watch from Wednesday afternoon through Saturday
     evening for Cedar-Iowa-Johnson-Muscatine-Scott.

IL...Excessive Heat Warning from noon Wednesday to 8 PM CDT Saturday
     for Hancock-Henderson-McDonough-Mercer-Warren.

     Excessive Heat Watch from Wednesday afternoon through Saturday
     evening for Bureau-Henry IL-Putnam-Rock Island.

MO...Excessive Heat Warning from noon Wednesday to 8 PM CDT Saturday
     for Clark-Scotland.



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