Area Forecast Discussion
Issued by NWS Quad Cities, IA IL

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FXUS63 KDVN 271723

Area Forecast Discussion
National Weather Service Quad Cities IA IL
1223 PM CDT Tue Jun 27 2017


Issued at 339 AM CDT Tue Jun 27 2017

06Z surface data has high pressure from the upper Midwest into the
lower Ohio Valley. Dew points were in the 40s from the Great Lakes
into the northern Plains with 50s and 60s from the Ohio Valley and
southern Plains to the Gulf Coast.


.SHORT TERM...(Today and Tonight)
ISSUED AT 339 AM CDT Tue Jun 27 2017

Temperatures this morning are again close to records. Refer to the
climate section for specific numbers.

Quiet and pleasant weather will be seen through sunset. Although
temperatures will be warmer, they will still be slightly below
normal. Attention then turns to tonight.

The quiet conditions will continue through midnight with an increase
in clouds as convection develops over western Iowa.

After midnight, the atmosphere aloft will become unstable as warm
air moves into area. Where the thunderstorm complex organizes across
Iowa and moves will be important in terms of the sensible weather
across the area prior to sunrise.

The implied signal from the models is that the thunderstorm complex
will generally organize across western Iowa and move toward the
northeast. Boundaries put out by the complex would provide the focus
for new convection to develop to the east and south. Going on this
idea, showers and some thunderstorms should move into areas west of
a Dubuque, IA to Memphis, MO line through sunrise Wednesday.

However, if the thunderstorm complex organizes across central
instead of western Iowa, then the risk of showers and thunderstorms
would be as far east as the Mississippi by sunrise Wednesday.

.LONG TERM...(Wednesday through Monday)
ISSUED AT 339 AM CDT Tue Jun 27 2017

The weather pattern will be busy for the next week as numerous waves
of energy progress through the zonal flow pattern. The first potent
shortwave trough of interest will move through Wednesday, with a set
of shortwaves moving across the forecast area Thursday afternoon and
overnight. Both of these systems will be notable for chances of
severe storms and extreme heavy rainfall.

For Wednesday, a shortwave trough in the mid-levels will pass across
the region. This will be broken into 2 sets of energy. The first
will push through during the morning hours which is expected to
cause the development of storms overnight, which could push east of
the Mississippi River by 18Z. The second batch of energy is about a
state behind to the west which is tied to the surface low and cold
front which will be over northwest Minnesota and western Iowa
respectively at mid day. The thinking is that the overnight activity
will weaken during the morning hours, but the low level jet doesn`t
really diminish much so there could continue to be enough forcing to
keep this activity going through the morning hours before it exits
the forecast area.

While moisture increases overnight Tuesday into Wednesday due to a
40-50kt low level jet, it will continue to increase through the day
Wednesday ahead of the cold front. This will make for increased
instability with dewpoints rising into the upper 60s and PW`s
increasing. The NAM goes a little overboard with its moisture and
does bring in some areas of 2.25 inches of PW and dewpoints in the
mid 70s by late afternoon. Thoughts are that this is about 5 degrees
too high and will keep the forecast with upper 60s to near 70
dewpoints. Regardless, even with PW`s approaching 2 inches, which
looks very possible, that is near records for this time of year and
almost 2 standard deviations above normal. Thinking that additional
thunderstorms will develop ahead of the cold front in the afternoon
somewhere across western or central Iowa which should develop into a
large line or cluster of thunderstorms to impact much of the
forecast area. Deep layer shear will be at 40 to 50 kts throughout
the day so rotating storms are likely, however as it appears
initiation will be west of the forecast area the threat for discrete
cells may be low. Therefore, looking for potential severe supercells
embedded in a line as it moves through with all hazards possible.
Something to think about is that as the PW`s increase through the
day, so does the freezing level. By late afternoon, the warm cloud
depth is 4000m or more which is a good indicator of efficient heavy
rain producers. Seeing this could indicate very heavy rainfall from
storms with the potential for an inch or more in a short amount of
time. This could also limit hail potential except for in the
stronger storms.

One of the questions for Wednesday will be the timing of
precipitation. While there are questions to how fast showers and
storms will move east, it is likely there will be convection ongoing
somewhere over central Iowa and moving into the forecast area at
12Z. Probabilities will increase through the morning, especially
west of the Mississippi River.

Moving to Thursday, the front will lay up somewhere in the
vicinity from northern Missouri into central Illinois. As
mentioned earlier, another mid level shortwave trough will move
towards the region later in the day and overnight. This will kick
up the southerly flow again with storms expected to develop along
and north of the front. Models are indicating this activity will
slowly move southward, and while severe storms can`t be ruled out,
training storms and heavy rainfall may be the larger story. Still
watching how this will play out, but attention should be kept for
the southern half of the forecast area again Thursday afternoon
and overnight.

Looking beyond Thursday, the active pattern continues with nearly
daily chances for showers and storms as numerous shortwaves move
through the upper level flow. The zonal pattern will also keep
temperatures near to below normal for much of the week.


.AVIATION...(For the 18Z TAFS through 18Z Wednesday Afternoon)
ISSUED AT 1223 PM CDT Tue Jun 27 2017

VFR conditions will be the rule today through tonight as high
pressure slowly retreats eastward. Overnight, strengthening south
to southwest winds aloft above much lighter south winds at the
surface will likely result in low level wind shear conditions,
which has been added to the forecasts. A thunderstorm complex is
expected to develop overnight well to the west, that may reach
into eastern IA as early as sunrise. For now, this will be
mentioned as prob30 groups for mid to late morning, which is the
period most likely to be impacted. There is the potential for at
least isolated showers and thunderstorms before this time.




LONG TERM...Brooks
AVIATION...Sheets is the U.S. government's official web portal to all federal, state and local government web resources and services.