Area Forecast Discussion
Issued by NWS Quad Cities, IA IL

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FXUS63 KDVN 180440

Area Forecast Discussion
National Weather Service Quad Cities IA IL
1140 PM CDT Mon Jul 17 2017


Issued at 336 PM CDT Mon Jul 17 2017

GOES satellite imagery showing mainly clear skies across E Iowa/NW
Illinois and a narrow north-south ribbon of wildfire smoke over
north-central Illinois. The smoke is elevated above the sfc but it
may cause a bit of haze in the sky - most noticeable during the
mornings and evenings. This smoke plume was located at the Quad
Cities longitude around sunrise this morning and should exit the
eastern CWA later this evening. Temps and dewpoints were
comfortable  - near 80 F and in the 60s, respectfully.


.SHORT TERM...(This Evening through Tuesday)
ISSUED AT 336 PM CDT Mon Jul 17 2017

This Evening and Tonight

Clear and seasonable tonight. Cooler in the upper 50s across the
east where sfc winds may decouple at times. Light but steady
winds across the west will hold temps in the mid 60s. Otherwise,
dry with no chance for rain during this period.


Hot and humid air mass returns. Peak heat indices will quickly
rise through the morning, peaking in the mid to upper 90s during
the afternoon. Upstream dewpoints across central Iowa into north-
central Missouri are in the lower 70s on avg. This muggy air mass
will advect to the east, causing the forecast spike in heat
indices. Also, thinking we`ll be dry and capped through the
afternoon with plenty of solar insolation to heat sfc temps into
the upper 80s to lower 90s.

In coordination with surrounding WFOs, did not begin the Excessive
Heat Watch for the south and southwest sections of the CWA until
Wednesday. However, peak heat indices in the mid to upper 90s (and
possibly near 100 F in the counties along and south of highway 34
to the west of Galesburg, IL) will be an impact to those outdoors
or without access to air conditioning. So on Tuesday, we`ll
likely handle messaging of the heat impacts with social media,
weather stories, and a special weather statement if necessary.

.LONG TERM...(Tuesday Night through Monday)
ISSUED AT 336 PM CDT Mon Jul 17 2017

Tuesday night through Saturday...Primary concern and most widespread
weather threat this period/week will be the building upstream
thermal dome acrs the plains an MO RVR Valley, shifting this way and
bringing with it oppressive heat and humidity. Main warm front will
try to lift up north/retreat through most of the area Wed into Thu,
with high sfc DPTs in the mid to upper 70s pooling around it and a
bit south. South to southwesterly LLVL flow advecting these high
values from a more juiced source region, and also enhanced by crop
evapo-transpiration may even produce some very high DPTs around 80
along the front or lingering storm outflow boundaries. This
combined with ambient temps warming into the upper 80s to mid 90s
(away from storm debris clouds and rain-cooled outflow pools) will
make for Heat Index readings from 100, to possibly near 110 degrees
most of this period.

But the big "fly in the ointment", as has been touched upon by
recent discussions and what occurred last week, is the local CWA
lying close to projected "Ring of Fire" storm track from the
Dakotas, MN/IA border region into WI/GRT LKS on periphery of the
building heat dome. Ensemble 19-23c H85 thermal MCS track is also
dangerously close, laying out from SD to far southern MN, into
WI/lower MI almost every night through Sat morning. Thus at least
the northern to northeastern third or more of the DVN CWA will be at
risk of being impacted convective debris, storm outflow and even
portions of the MCS/organized storm clusters themselves. The storms
possibly clipping these areas overnight into early morning almost
every night, with associated debris an outflow lingering in the
north well into the day or even afternoon, squashing the ambient
temp warm up potentail. But these same areas may have the highest
sfc DPTs for very humid conditions. But without the higher ambient
temps, heat worthy of headlines will not really be attained. Thus
low confidence that these northern areas will reach Heat Advisory
conditions, let alone excessive Heat Warning Criteria.

All in all, it appears that areas south of I80, then maybe angling
northwestward up through the Iowa City and Williamsburg areas, will
have the best chance of reaching at least advisory criteria daily
through Sat. 4 days compounded duration make for Excessive Heat build
up impacts, thus will issue an Excessive Heat Watch for these areas
starting Noon Wednesday and lasting through Sat evening. Could also
see the scenario where some areas along and north of I80 are handled
by a daily heat headline depending on convective evolution and
debris. The southern third may need an Excessive Heat Warning for
almost 4 days of Heat Index readings of 105+ through Sat. But
Saturday right now looks more at question with a southward digging
short wave and some elevated cap/EML erosion, enough for widespread
showers/storms and clouds limiting ambient temp heat up potential
and the need for heat headlines except maybe for the far south. Right
now Thu looks like the hottest day if convection and debris stay
just off to the north.

As for the storms, any organized cluster or mature portion of an MCS
that can make it down into portions of the local fcst area will
bring the primary threats of damaging winds and flooding rains,
utilizing the high fuel content of a loaded atmosphere just to the

Sunday through next Monday...Some long range signals for this
period, that the digging short wave trof Sat into early Sunday may
allow for some relief behind it in the form of Canadian high
pressure dumping down the plains and upper Midwest/western GRT LKS
to scour out the high heat and humidity. But depends on the Model,
the GFS doesn`t clear things out until next Tuesday, while the new
12z ECMWF does so by late Sunday or Sunday night. ..12..


.AVIATION...(For the 06Z TAFS through 06Z Tuesday Night)
ISSUED AT 1125 PM CDT Mon Jul 17 2017

There is a chance of storms late in the day Tuesday ahead of a
weakening upper level disturbance, and also Tuesday night as
convection over MN/WI and an attendant outflow enhanced
boundary shifts e/se. Best chances for showers and storms
appear to reside over northeast IA, therefore have PROB30 groups
at CID and DBQ for Tuesday evening while no mention at MLI and BRL.
Outside of this convection potential expect VFR conditions with
light e/se winds turning southerly at around 10 kts on Tuesday.


IA...Excessive Heat Watch from Wednesday afternoon through Saturday
     evening for Des Moines-Henry IA-Iowa-Jefferson-Johnson-
     Keokuk-Lee-Louisa-Van Buren-Washington.

IL...Excessive Heat Watch from Wednesday afternoon through Saturday
     evening for Hancock-Henderson-McDonough-Mercer-Warren.

MO...Excessive Heat Watch from Wednesday afternoon through Saturday
     evening for Clark-Scotland.



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