Area Forecast Discussion
Issued by NWS Chicago, IL

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FXUS63 KLOT 211750

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
1250 PM CDT Mon Aug 21 2017

1154 AM CDT

An upper shortwave moving eastward across northern Illinois
continues to help maintain some rain and isolated convection which
is moving eastward across north central Illinois. This combined
with somewhat renewed low/mid level moisture advection is expected
to keep precipitation lingering with some new development through
the afternoon. Most vigorous development is expected to focus
southeast of a roughly Gary or Valparaiso to Pontiac line and
this may be a little generous on the north end. Better instability
gradient exists in this area, especially the further south you
go. To the north, surface based instability is pretty well capped
but the higher level ascent should allow for some continued
development. In addition, there is a NNE-SSW oriented boundary
appearing on radar crossing the Mississippi River which is
focusing some minimal development as it tracks east. Thus areas of
light rain with isolated to scattered stronger cells will likely
continue through early afternoon across much of the area. Tough to
say whether there will be a noticeable break in at least isolated
to scattered precip development but if there were it would likely
be mid/late afternoon. Otherwise, cloud cover has thickened but
filtered sun is being observed when rain is not falling. Do not
expect much change in overall cloud situation in the next few



345 AM CDT

Through Tonight...

Multiple items of interest for today and tonight`s forecast. Have
added further time refinement to thunderstorm potential, which is
trending more in two time periods...1.) from mid-morning through
early afternoon and potentially more so western/southern forecast
area and 2.) mid-evening onward tonight areawide, with heaviest
rainfall totals likely across north central Illinois. Also given
heightened interest in sky cover forecasts for today`s eclipse,
spent additional time collaborating that, and it does look like
there is potential for thinning of clouds spreading from west to
east during the early afternoon.

The large, mainly unorganized area of thunderstorms across
central Iowa as of 330 am continues to creep eastward on the nose
of a 35 kt low-level jet. While there is a subtle short wave, it
is quite low amplitude, and as such this convection may struggle
to readily organize into an MCS. Even if it were, it would likely
ease slightly south of east, more so favoring the I-74 corridor of
Illinois. There is potential for an MCV to emanate eastward out
of this convection, not all that different from yesterday, except
would be passing earlier in the day. All this said, it does not
lead to the highest confidence in storms this morning, and feel
the better chance/coverage if they are able to make it into the
area is south of a Dixon-to-Joliet-to-Valparaiso line...basically
along the deeper instability gradient. A few stronger storms
through early afternoon may be seen given the modest instability
(MUCAPEs of 2000 J/kg) and very high moisture (1.70"+ PWATs). The
main threat would be isolated 50+ mph wind gusts through mainly
precipitation loading, and if any were to organize/congeal. In
addition, heavy rainfall/localized flooding would be a threat,
especially across Benton County, which saw 2-3+ inches of rainfall
early this morning.

Regarding cloud cover, the Iowa convection has produced a huge
area of convective debris cirrus that is drifting over the CWA.
Would expect this to stick around most of the morning given the
upper level westerly flow and sustainability to the driving
convection. By eclipse time (~12pm-230pm), high and probably even
some mid clouds look likely given forecast RH/PWATs and high-res
synthetic satellite imagery. However there is certainly potential
for thinning from west to east behind the convective remnants/MCV
that inch their way east-southeast this morning. That thinning, or
even some breaks in cloudiness, could occur during the eclipse
time, especially the latter half of it. That all said, forecasting
cirrus trends is very challenging and not routine. The eclipse,
clouds or not, should support temperatures leveling off or a small
dip for a 1-2 hour early afternoon period. Highs are forecast in
the mid 80s, and if some more sun is realized, upper 80s to around
90 is attainable.

The rest of the afternoon into early evening has the forecast area
under subtle short wave ridging, so there looks to be a defined
lull/break in regional convection. By mid-evening, the approaching
upper trough and associated height falls will support forcing for
ascent in a broad zone ahead of the cold front. Convection should
result from Wisconsin southwest to Kansas/Missouri by mid-
evening. Isolated development ahead of this into northern Illinois
will become more likely by late evening as the modest to strong
low-level jet veers. Storms may organize into a forward
propogating MCS given the mid-level wind field and shear
orientation, as well as support of a couple CAM solutions. So the
SPC slight risk for mainly severe winds continues in the northern
Illinois part of the CWA. Will need to watch if any small scale
WAA wings were to develop ahead of the main activity, as training
storms would present a flooding issue given such high ambient
moisture. Even without training, rainfall rates may be enough to
cause at least localized flooding. Have mentioned heavy rainfall
possible in the forecast for north central Illinois tonight.



326 AM CDT

Tuesday through Sunday...

Tuesday will be a transition day as the pattern shifts from zonal
flow aloft to more highly amplified as a deep upper low settles into
sern Canada, broad long wave troughing develops over the ern CONUS
while upper ridging builds over the Rockies.  This will set up cool
nwly flow aloft over the midwest with the cold front pushing quickly
to the south and east of the local area and high pressure builds out
of south-central Canada and spreads across the northern plains.  By
Wednesday, sfc high pressure will spread across the region while
nwly flow persists aloft.  This will set up an extended period of
dry weather with below normal temperatures.  For Tuesday through the
weekend, high temps will generally be in the 70s across the region.
A prevailing north component  to the wind field will keep temps
along the lakefront in the lower 70s while inland locations should
climb into the middle to upper 70s.  The coolest day of the period
should be Thursday as north to northeast winds bring the coolest air
into the area and lakefront locations may see temps only in the
upper 60s while inland locations only see lower to middle 70s, which
would be about 10 degrees below normal.  As the surface high drifts
east we should see a return to south-southeast surface winds and a
gradual moderation in temps late in the week and into the weekend,
with temperatures returning back closer normal for late August.


For the 18Z TAFs...

Wind trends are the initial focus followed by thunder potential,
primarily tonight. Earlier thunderstorm complex to the west has
decayed but wind fields remain disturbed as what`s left of it
passes the terminals. Expect a period of west to southwest winds
at the Chicago area terminals early this afternoon before turning
south and even south-southeast. Rockford has already been easterly
and is trending southeast and will end up south-southeast or south
in the next hour or two. South to southwest winds will then
prevail this evening turning southwest late as a cold front
approaches. May need to bump us speeds slightly this evening and
tonight. Winds will then shift northwest late Tuesday morning
behind the front with speeds increasing somewhat with gusts to
near 20 kt.

Scattered showers and possibly an isolated storm will move across
the terminals through mid this afternoon. Some new development is
possible but the chance is low. Additional isolated activity is
possible into early this evening but the chance is also low.
Attention turns to late this evening and especially overnight as
the front approaches. Thunderstorms will become more likely with
time from about late evening onward with the best chance after
06z and probably closer to 08z as storms approach from the west.
Gusty winds may occur with the storms along with reduced
visibility which could briefly fall below the 2SM advertised in
the current TAFs. Showers may linger through daybreak but then
exit with MVFR clouds expected. Improvement to VFR is expected
through late morning.



326 AM CDT

A weak trough of low pressure extends across the upper Great Lakes
with an associated frontal boundary extending southwest to low
pressure over the western plains. The front will remain nearly
stationary through Tuesday evening. High pressure building out of
central Canada will force the front south down Lake Michigan Monday
night and pushing south of the lake by Tuesday afternoon.
Winds will turn northerly and strengthen following the frontal
passage, with winds and waves reaching Small Craft Advisory levels
tuesday and Tuesday night, especially for the Indiana nearshore
waters. at this point, it looks like the Illinois nearshore waters
may remain just under Small Craft Advisory levels.  High pressure
will then park over the upper Midwest through midweek and gradually
spread east through the rest of the week and into the weekend.  As
the high spreads across the lake, winds should diminish, but a
generally northerly component to the wind field should persist into
the weekend.





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