Area Forecast Discussion
Issued by NWS Quad Cities, IA IL

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000
FXUS63 KDVN 181731
AFDDVN

Area Forecast Discussion
National Weather Service Quad Cities IA IL
1231 PM CDT Tue Jul 18 2017

...AVIATION UPDATE...

.SYNOPSIS...
Issued at 327 AM CDT Tue Jul 18 2017

A relatively dry airmass remains over the forecast area early
this morning as the region was under the influence of high
pressure over Lake MI into northern Illinois. A much more humid
airmass was poised not far away, west of a stationary front
reaching across central IA through northeast MO into southern IL.
Southwest of this boundary, temperatures and dewpoints were in
the 70s, while dewpoints in the upper 50s to lower 60s prevailed
over the local area. An upper level ridge remained over the
Central Plains with shortwaves along the northern periphery
triggering thunderstorm complexes, with the most organized
complexes over over southern NE and the western Dakotas.

&&

.SHORT TERM...(Today and Tonight)
ISSUED AT 327 AM CDT Tue Jul 18 2017

Challenges center around the long advertised onset of heat and
humidity and the potential for thunderstorms. High pressure over
the western Great Lakes gets nudged east today as upstream
shortwaves flatten the weak ridge aloft into a more zonal flow.
This will lead to the breakdown of the boundary to the west,
opening the door for the airmass with dewpoints in the lower to
mid 70s over MO and western IA to spread eastward today. This
airmass will support highs in the lower 90s west and south, while
the far north and northeast reach the upper 80s. This would result
in heat index values peaking in the low 90s north to possibly
near 100 in the far south this afternoon, below heat advisory
criteria.

There is lower confidence with thunderstorm development and
coverage with typical variations among models with convective
trends. Looking at the big picture, the shortwave triggering
storms in NE tonight will be arriving into eastern IA this
afternoon, most likely in a weakening stage. With the weakening
low level boundary and associated moisture convergence over
central IA also shifting east, cannot rule out thunderstorm
development, as suggested by some convective allowing models, this
afternoon over eastern IA, tracking through the area this evening and
have slight chances for this possibility. Overall, models have
the main focus for a more organized convective complex over
southern MN into northern IA late this afternoon and evening,
which may then spread southeast along an outflow boundary into an
initially high CAPE environment with 0-3 km possibly over 30 kts
over the northern half of the forecast area. SPC maintains a
marginal threat roughly northwest of a line from Sigourney to
Freeport, where damaging winds and heavy rainfall appear to be the
greatest threats. Chance pops are maintained overnight, for
mainly areas north of Interstate 80. Higher dewpoints will limit
overnight lows to the upper 60s to lower 70s.

.LONG TERM...(Wednesday through Monday)
ISSUED AT 327 AM CDT Tue Jul 18 2017

Forecast focus continues to be the heat in our southern cwa with the
storm track in or near our north.

Wednesday through Saturday: Models continue the theme of hot and
humid conditions for locations near and south of Interstate 80,
where excessive heat watch continues. Later shifts will have to
decide when the watch gets replaced with a warning, as this has been
a consistent signal. This still looks to be a 4-day heat wave with
afternoon highs in the mid to upper 90s and dewpoints in the lower
70s. This will soar the heat index values to around 105, perhaps
somewhat higher.

In our northern cwa, this area will be closer to the storm track of
periodic MCS`s, debris clouds, outflow boundaries/cold pools. The
tricky part will be trying to forecast ahead of time if/when any of
these thunderstorm complexes will dive southeast into the dvn cwa.
Some of the models indicate the storms remaining north of the cwa
for much of this period. However, not convinced that we will remain
dry the entire time, and my gut says we will have to deal with these
storms but low confidence in timing/location. With the high CAPE,
PWAT`s over 2 inches, and enough shear at least close by the area,
that some of these storms may produce damaging winds, large hail and
torrential rainfall. The forecast highs in our north of mid 80s to
lower 90s is based on the area being in or close to thunderstorm
complexes to keep temperatures in check. However, if storms manage
to stay well to our north (and more sunshine) then these highs, and
heat index values, will be much higher than currently forecast.

Sunday through early next week: Models seem to be coming to a
similar solution in sending a rather strong cold front southward
through the area, as an upper level trough digs southeast across the
Great Lakes region. This front would arrive later Sunday or Sunday
night, and then push south of the dvn cwa by Monday. This would
bring refreshingly cooler temperatures, on northeast winds, and much
less humidity for early next week. The current grids would suggest
highs in the 70s north to lower 80s south, with lows dipping into
the mid 50s to lower 60s.

Haase

&&

.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

&&

.DVN WATCHES/WARNINGS/ADVISORIES...
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.

&&

$$

SYNOPSIS...Sheets
SHORT TERM...Sheets
LONG TERM...Haase
AVIATION...Uttech



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