Area Forecast Discussion
Issued by NWS Chicago, IL

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FXUS63 KLOT 210216

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
916 PM CDT WED JUL 20 2016

915 PM CDT

For Evening Update...

Changes made to remove slight chance pops from the cwa overnight.

Late afternoon and evening satellite/radar data depicts a weak
short wave propagating east-southeast from central Minnesota across
central Wisconsin, where a weak area of elevated convection has
now dissipated. Another area of very weak convection across north
central Iowa has continued to struggle despite better low level
convergence, likely due to weak low level lapse rates beneath
capping inversion aloft. That said, 925-850MB jet/moisture
transport axis is progged to linger from the mid-Missouri Valley
to the upper Mississippi Valley and northern Lakes overnight. Not
out of the question that elevated convection could perk up a bit
from northern Iowa into Wisconsin overnight, though the strongest
forcing remains into northern MN/WI/MI through early Thursday
morning. Available CAM guidance supports this with forecast
convection focused well north of our forecast area. Any potential
farther south looks to be rather isolated and likely holding off
until Thursday morning. Based on both observed and model forecast
trends have removed slight chance pops for the overnight hours.
Have maintained slight chance across northeast parts of the cwa
and Lake Michigan after sunrise, in association with advancing
warm front. Heat headlines look good Thursday, with 90-95 degree
air temps and surface dew points in the mid-upper 70s upstream and
little in the way of convective debris clouds expected.

Updated grids/zfp/lfp already available.



325 PM CDT

Through Thursday...

Main forecast concerns/challenges are with the hot/humid
conditions expected over the next couple of days, with an
Excessive Heat Warning and Heat Advisory issued across the CWA
this afternoon.

In the near term, small area of diminishing precip still slowly
pushing southeast through northern Illinois, primarily impacting
areas along the I-39 corridor. This area of precip will continue
to drift southeast over the next couple of hours while also
decreasing in coverage. Lightning has been limited over the last
hour, and although still possible over the next couple of hours,
think lightning will continue to be less with time. Then
anticipate dry conditions across the CWA for most of tonight.
Increasing LLJ could provide an isolated shower/thunderstorm late
tonight into early Thursday morning, but with the greater
forcing/focus expected to be well north of the area, am leaning
more towards a dry forecast. If any precip does develop late
tonight into Thursday morning, it will exit/diminish through mid

Had made a midday update to the heat headlines in place,
replacing the Excessive Heat Watch with a Warning and then issuing
a Heat Advisory across northwest Indiana. These updates were made
with increasing confidence for a long duration of hot and humid
conditions beginning late morning Thursday and persisting into
Friday. Guidance consistent with swinging much warmer air across
the CWA throughout the day on Thursday, with 850mb temps
approaching 24C. Lack of cloud cover and strong insolation in this
environment supportive of mid 90s and with very humid air in
place, expect dangerous heat conditions across the CWA with heat
index values ranging from 103 to as high as 115 degrees to be in
place. At this time, expect the highest heat indices to be across
north central Illinois where thermal ridge and higher dewpoint
axis will reside. However, do think there is a possibility that
temps could be slightly higher than currently forecast as well as
with slightly higher dewpoints. This is especially the case with
dewpoints, as dewpoints are around 80 degrees at this point across
Iowa. If this were to occur, the lower end of the heat index range
could be higher, even across the Advisory area in northwest



331 PM CDT

Thursday night through Wednesday...The early portions of the long
term forecast period will continue to focus on the excessive heat
potential, though there will be some concerns about thunderstorms
Thursday night and into Friday.

For Thursday night, after a day with temperatures in the middle 90s
and with dewpoints in the middle to upper 70s, overnight
temperatures will have little opportunity to drop off very much with
conditions remaining rather muggy.  Temperatures Friday morning
should only drop into the middle 70s over much of the CWA and only
dropping to near 80F overnight.  Pcpn and thunder potential will
largely be dependent on the track of shortwave energy tracking along
the northern periphery of the strong upper ridge centered over the
central plains.  At this point there is still some uncertainty as to
how far northeast shortwaves will track, with some guidance
suggesting a ridge axis extending across the western Great Lakes and
other guidance suggesting that shortwave energy can turn southeast
through Wisconsin and across nrn IL/IN.  Given the uncertainty at
this point, will continue to carry chance PoPs for now.  However,
should the signal for convective activity continue to favor
thunderstorms over the local area, there will be a significant
chance that any thunderstorms could produce heavy rainfall as there
is good model agreement on pwats well in excess of 2 inches over the
area at that time.  For the remainder of Friday, there will still be
concern for excessive heat as the large scale upper level pattern
will change little.  The center of the upper ridge will remain
parked over the central plains and upper heights will change little
from Thursday to Friday.  The potential for temperatures to be in
the middle 90s will continue, however, there are a few question
marks that could have a major impact on temperatures.  The first
will be any possible remnants of overnight convection and residual
cloudiness.  Also, a sfc cold front moving down the length of Lake
Michigan associated with a low pressure system moving through
central Canada may reach the south end of the lake as early as late
Friday afternoon which would turn winds off of the lake and limiting
warming, especially for nern IL.  Given the uncertainties in the
speed with which the front will move down the lake and even how far
south the front will push before washing out, will maintain
temperatures across the cwa in the lower to middle 90s with
dewpoints in the middle to upper 70s.  Temperatures and dewpoints
should be a bit lower over nwrn IN than nern/ncntrl IL, so max heat
index over nwrn IN should remain under 105 F while heat indices over
ncntrl/nern IL should be in the 105-110F range.

Over the weekend, the upper level pattern should begin to break
down, with a series of strong nrn stream shortwaves tracking along
the Canadian border and suppressing upper level heights.  This
series of shortwaves will also bring periodic chances for showers
and thunderstorms over the weekend.  The strongest forcing should
remain north of the CWA closer to the shortwave track, but there
should still be enough residual moisture and instability to keep
chance PoPs over the CWA through the weekend.  Max Temps over the
weekend should be a bit lower than the next couple days, topping out
around 90F.  Looking into early next week, temperatures should be
back closer to seasonal normal levels.  The longer range guidance is
indicating a low pressure system tracking across the upper
Mississippi Valley Sunday and dragging a cold front across the
region Sunday night.  This should bring temperatures back into the
middle 80s for Monday through Wednesday.


For the 00Z TAFs...

623 pm...Forecast concerns include gusty southwesterly winds
Thursday and small chance for thunderstorms overnight and again
Thursday evening.

Southerly winds generally under 10kts this evening will diminish
a bit more overnight and then turn more southwesterly Thursday
morning with speeds increasing into the 10-15kt range with gusts
as high as 20kts during the late morning and afternoon.

There is little focus for thunderstorm development during the
period. If any convection developed the best timing would be
toward sunrise into the mid morning Thursday and then again
Thursday evening or Thursday night. However...with the upper
ridge building into the area...expect mainly dry conditions for
the terminals through Thursday evening and maintained dry tafs.

Moist airmass/low levels may allow some light fog to develop
overnight through sunrise...mainly away from the urban/metro area
and will monitor trends later this evening. cms


335 PM CDT

High pressure will slowly move from the eastern Great Lakes to the
Mid-atlantic region while an elongated trough of low pressure
develops over south central Canada into the northern plains. The
northern portion of the trough will quickly move east across the
upper Great Lakes Thursday night and Friday while the southern
portion of the trough hangs back over the central plains. This
will keep brisk, southerly to southwesterly winds over the lake
into Thursday night. a weak cold front will finally cross the
lake late Thursday night or Friday morning, briefly turning winds
to northwesterly on Friday. High pressure will build across the
Great Lakes Friday night and Saturday, leading to a period of
relatively light and variable winds. By Saturday night, another
low is expected to develop over the central plains as the high
pushes east of the lakes. This will set up another period of brisk
generally southerly winds on Sunday.


IL...Excessive Heat Warning...ILZ003-ILZ004-ILZ005-ILZ006-ILZ008-
     ILZ022-ILZ023-ILZ032-ILZ033-ILZ039...NOON Thursday TO 7 PM

     Excessive Heat Watch...ILZ008-ILZ010-ILZ019-ILZ021-ILZ023-ILZ032-
     ILZ033-ILZ039...7 PM Friday TO 3 PM Saturday.

IN...Heat Advisory...INZ001-INZ002-INZ010-INZ011-INZ019...NOON
     Thursday TO 7 PM Friday.




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