Area Forecast Discussion
Issued by NWS Chicago, IL

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FXUS63 KLOT 211943

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

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.



241 PM CDT

Tuesday night through Monday...

Transition to drier and much cooler weather occurs mid-week and
lingers through the end of the week before a gradual warm-up
brings temps back to near normal for late August.

Surface cold front clears the forecast area to the southeast by
Tuesday evening, with breezy northwest winds bringing a cooler and
less humid air mass into the region. This trend persists through
the end of the week, as a long-wave upper trough deepens across
the eastern CONUS, and keeps us in deep northwest flow. Surface
high pressure spreads across the region during the period, though
models have been fairly consistent in depicting a mid-level short
wave which digs across the Upper Midwest and Great Lakes region
late Wednesday night into early Thursday, with a reinforcing push
of cooler air and a surface cold front passage. Guidance continues
to indicate the potential for some spotty showers early Thursday
as 850 mb temps drop to about +6C over the warm late-August waters
of Lake Michigan. Otherwise, dry conditions are expected to
persist into the weekend as the surface high strengthens across
the western Lakes.

925 mb temps support daytime highs in the low-mid 70`s, with
overnight lows dropping into the low-mid 50`s. A few upper 40`s
are possible in our typical cool spots away from the immediate
Chicago metro area and warmer lake front areas.

The eastern CONUS upper trough eventually is progged to drift off
to the east this weekend, with the surface high pressure ridge
also moving off and allowing winds to become more southeasterly.
Medium range solutions of the ECMWF and GFS both indicate a short
wave trough propagating from the Northern Plains southeast into
the Midwest Saturday night into Sunday, though there are
differences with the evolution beyond that time. The EC develops a
deep closed upper low over the area which lingers into the
following Monday, while the GFS is much less amplified. In general
however, a gradual moderation in low-level temps and a modest
increase in low-level moisture is likely.



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.



241 PM CDT

Main marine forecast concerns continue to be on a period of
northerly winds down the length of the lake Tuesday night into
Wednesday. While not overly strong, the long fetch will likely
result in small craft advisory conditions for the shores of
southern Lake Michigan.

In the near term, a cold front has become nearly stationary near
the north shore of the Lake in the Upper Peninsula. Low pressure
is expected to develop along this front over Wisconsin tonight,
moving northeast over far northern Lake Michigan early Tuesday
morning and trailing a cold front which will push down the lake
during the day. The low is expected to deepen later in the day as
it pulls away into Quebec, setting the stage for breezy northwest
winds across the lake. These north-northwest winds will continue
into Wednesday before weakening as high pressure spreads into the
region. The high eventually late Thursday, allowing winds to
diminish and waves to subside. The strongest winds, 20-25 kts, are
expected late Tuesday and Tuesday night, with small craft
advisory conditions expected along the southern shores of the lake
from Tuesday afternoon through Wednesday.






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