Area Forecast Discussion
Issued by NWS Chicago, IL

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FXUS63 KLOT 181815

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
115 PM CDT Tue Jul 18 2017

315 AM CDT

Through tonight...

The near term weather today will remain quiet though turning more
humid. Southerly winds kick in behind departing high pressure
gradually advecting dew points in the mid to upper 60s northward.
The exception for southerly winds will be lakeside counties where
an afternoon lake breeze will occur, dropping temperatures a bit
after it passes. Elsewhere, highs will be in the upper 80s
correlating to 850mb temperatures around 19C.

The upper ridge across the central/southern Plains will build
gradually through tonight, with a belt of strong westerlies
to its north beginning to arc further eastward. A subtle short
wave, probably enhanced a bit by convection, will be moving east
across the Upper Mississippi Valley into Wisconsin tonight. The
trailing southwest influence of this short wave is expected to
extend into northern Illinois. Ongoing convection this evening to
the northwest of the area should start a gradual decline in
intensity overnight due to easing convective maintenance
parameters. Nonetheless, at least isolated showers/storms are
likely to make it to the Illinois/Wisconsin state line region late
evening into the overnight, possibly as a decaying small scale
MCS. The severe weather and heavy rain threats with this look



315 AM CDT

Wednesday through Monday...

Some showers and maybe a couple storms could be ongoing at
daybreak Wednesday in northeast Illinois/far northwest Indiana,
but should be fading soon after. The key influence of the late
tonight showers will be where they modulate a surface trough,
that will evolve into a warm front by Wednesday evening. While
cannot rule out an isolated afternoon storm on this trough, it
seems a low chance. Temperatures on Wednesday (and going forward)
will heavily depend on the trough placement. Model guidance shows
considerable spread for this feature just 24-36 hours out, owing
to different handling of convective evolution tonight. To the
south of this boundary, lower 90s for highs Wednesday are a good
bet with dew points having climbed into the lower 70s, yielding
heat index readings nearing 100. To the north, mid 80s for highs
are more likely with lakeside locations being cooler.

This starts the beginning of an active period through the rest of
the week being on the northeast side of a stout mid-summer ridge,
with concerns for heavy rain producing storms and humid and hot
weather, especially in areas that remains south of storm-induced
outflows. Dew points look to be in the 70s much of this time
outside of any convective impacts.

The uncertainty on this boundary placement provides a challenge in
how convection, possibly organized/strong and likely heavy rain
producing, will evolve Wednesday night from the Upper Mississippi
Valley. This should ride on the northern side of the boundary
due to the support of a stout upper level westerly speed max over
110 kt. Parameters for maintenance into overnight are much better
than tonight, which is why convection should make it to our CWA
longitude, the question is whether it will evolve south into our
area. While 00Z global guidance generally favors more into
Wisconsin, the thickness fields and propagation vectors are
conceptually not entirely in sync with that, showing potential for
storms to turn southeast into northeast Illinois overnight
Wednesday or early Thursday morning, which the 00Z NAM indicates.
Have collaborated with NWS MKX and inched up blended guidance PoPs
near the state line and into the Chicago metro in that time
period. Will have to watch for heavy rain, especially in and near
Lake County where conditions are already quite wet.

The thermal ridge looks to lean over into our area on Thursday in
the wake of the morning short wave. If no low-level influence of
cool convective outflows, Thursday has the potential to see 90
degrees across the entire area, with mid 90s for much of the south
and possibly west. Forecast highs are a little below that given
uncertainties on convection and outflows, but the
southern/southwestern CWA right now has better potential to see
heat index approach 105.

As for storms, the convectively-reinforced boundary should have a
lull that might last all of Thursday, before gradually becoming
more active again during the night and into Friday in the right
entrance region of the belt of westerlies. A period or two of
severe weather concern may arise given the instability/shear
parameter space forecast, but more so may be an enhanced heavy
rain threat immediately north of the boundary. Given the concerns
already on Wednesday night/early Thursday and then this period, as
well as some wet antecedent conditions in northeast Illinois,
have issued a Hydrologic Outlook for the northern half of the CWA
(ESF). See Hydrology discussion below for more and uncertainties.

The southern CWA continues through Saturday to be most favored for
temperatures exceeding 90, again with uncertainty if storms
progress further south. Heat index readings again Friday and
Saturday are forecast to be near 105. Confidence is particularly
low on Sunday, but by the tail end of the weekend and especially
Monday, an air mass change is forecast with drier weather
presently forecast to start next week.



315 AM CDT

Have issued a Hydrologic Outlook (ESF) for concerns of multiple
rounds of storms in an atmosphere that favors heavy rainfall, and
wet antecedent conditions particularly over the Des Plaines and
Fox River basins. Precipitable waters will be consistently above
1.60" for much of Wednesday night-Saturday night and so the
moisture will be present with fairly high confidence. The
uncertainty resides in placement and timing of storms, which
given the forecast weather pattern, is a particularly high
uncertainty. Nonetheless, computer models have indicated that the
general southern Wisconsin/northern Illinois region should see
storms late Wednesday night into early Thursday, and then possibly
additional multiple rounds Thursday night into Friday. These
could yield quite a gradient in rainfall amounts with much lower
south (south of I-88) to higher north.

The bottom line is that there are a lot of storm chances coming
up in an environment that can produce copious rainfall, with a
few periods of likely storms within that probable as the time
period draws nearer.



For the 18Z TAFs...

Forecast concerns include timing of a lake breeze this
afternoon, timing and coverage of thunderstorms overnight and then
a northeast wind shift Wednesday morning.

Light and variable winds have been observed this morning as weak
high pressure covers the region. Latest radar imagery indicates
that a lake breeze boundary has already formed and is steadily
moving inland. Had initially timed the lake breeze passage to be
arnd 21z, but latest trends suggest that the boundary could push
through MDW as early as 19z, but the portion of the boundary to
the north of MDW appears to be moving a bit slower, but will still
need to monitor trends should the lake breeze speed up and reach
the terminals earlier than anticipated.

Scattered thunderstorms are developing over the upper Mississippi
Valley this afternoon in a zone of warm/moist advection in advance
of a cold front extending from western Lake Superior into the
Upper Missouri Valley. Expect that the front will slowly sag
southeast, approaching the Wisconsin-Illinois border close to or
shortly after 06z tonight. Confidence is moderate on the timing,
but much of the guidance is suggesting that the thunderstorm
activity could be on a weakening trend as the front moves into nrn
IL. So, will keep the prob30 going for the late night into early
morning hours, with low confidence in coverage. As the cold front
pushes south of the terminals, winds should becm nely, though with
weak push to the front, expect that wind speeds should be less
than 10kt. As some cool, moist air moves inland in the nely flow
behind the front, some mvfr cigs may be possible, but confidence
is low in coverage, so will only go with sct015 for now, though
some bkn015 is possible.



225 am...High pressure over the southern Great Lakes will slowly
move south today as a cold front moves south down Lake Michigan
late tonight into Wednesday morning. Ahead of this front...the
gradient will tighten with southerly winds 15-25kts expected. The
cold front is expected to move south of the lake by Wednesday
afternoon with another area of high pressure building across the
region. A broad area of low pressure is expected to form from
southern Canada to the central plains late in the week and this
will lift the front back north as a warm front with southerly
winds returning Thursday. This area of low pressure is expected to
consolidate as it moves across the Great Lakes region Sunday...
with winds shifting north behind its passage. cms





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