Area Forecast Discussion
Issued by NWS Chicago, IL

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FXUS63 KLOT 221928
AFDLOT

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
228 PM CDT Thu Jun 22 2017

.SHORT TERM...
225 PM CDT

Through Friday...

Concerns center around isolated thunderstorm development this
afternoon, and then with a line of storms late this evening and
overnight.

Inhibition gradually continues to erode from the south generally
driven by low level moisture advection soundings. AMDAR soundings
still depict some decent capping thanks to fairly warm
temperatures in the 800-700 mb layer, at least closer to Chicago
The HRRR now has backed off a bit on the coverage for the
afternoonand early evening, consistent with very little/weak
convergence in place and a lack of a larger scale forcing
mechanism. But there is some weak upper forcing pushing in from
the south, thus we maintain an isolated shower and storm mention
for the afternoon/early evening. Shear profiles are still fairly
weak during this period thus severe threat is low.

We will hang onto instability through the evening ahead of the
cold front currently draped across the northern plains/upper
midwest. Stronger upper level winds will approach the area along
the southern fringes of the upper low across Manitoba. Clusters of
storms will continue to develop along and ahead of the front. Any
storms that area able to get going this evening ahead of the front
will form in an axis of 1000+ J/Kg of MLCAPE and shear in excess
of 30 kt, which keep the threat of hail or locally damaging winds,
mainly across north central IL this evening, especially if
anything forms upstream and develops a cold pool while pushing
ahead into the continued unstable airmass. The cold front will
pass through the Chicago area after midnight, with the line of
storms passing southeast with it. There are some waves in the
southwest flow aloft that may initially slow the progression of
the front before it gets shoved southeastward late tonight, and
may prevent a solid line, but confidence is pretty high in many
areas seeing some activity. Moisture pooling ahead of the front
would suggest at least some brief heavy downpours in this
scenario, while the more significant flood threat is largely just
to our north/northwest.

Quite the airmass change will filter air on Friday, with much
drier air across the upper Midwest (note the 40s Tds in MN). These
will not get here that quickly but eventually will get in here
this weekend. Still humidity levels will be much lower, and temps
look to top off in the upper 70s to near 80. It may be a bit
cooler along the IN shore. Showers and storms look to still be
ongoing tomorrow morning, but guidance is in good agreement of
getting the front through in the afternoon.

&&

.LONG TERM...
319 AM CDT

Friday through Wednesday...
Cold front should be in the process of moving across the area
daybreak Friday with showers and t-storms probably lingering into
the morning hours over about the southeast 1/3 to 1/2 of the area.
In the wake of the front, look for noticeably less humid air to
begin advecting from the NW in Friday afternoon. As is typical
with summer cold front, the cooler air lags pretty far behind the
front, so temps should still be seasonably warm Friday with many
areas likely still reaching into the 80s, albeit mostly lower 80s.

Over the weekend, an anomalously deep/chilly upper trough is
forecast to dig into the Great Lakes region. the cold air aloft
should allow for a very healthy afternoon cumulus field to blossom
both Saturday and Sunday, but rain prospects don`t look too
impressive given the drier air mass in place. The latest version
of the GFS convective parameterization has a tendency to break out
widespread light convective precip in these type of scenarios and
am favoring the drier ECMWF solution. Certainly a brief light
shower or sprinkle is possible this weekend, but vast majority of
the weekend looks to be dry and seasonably cool with highs only in
the lower end of the 70s spectrum.

Upper trough is forecast to eventually loosen its grip and allow
for a moderation trend next week, but latest ECMWF is less
progressive that prior runs, so we could be in store for a couple
more days with sub-80F highs in late June to kick off next week.
Overnight low overs the weekend and into next week should fall
into the 50s outside of the Chicago urban heat island, so some
very comfortable temps/humidity levels are coming after today`s
brief taste of moderate heat/humidity.

- Izzi

&&

.AVIATION...
For the 18Z TAFs...

Concerns center around isolated thunderstorm development this
afternoon, and then with a line of storms late this evening and
overnight. MVFR/Low VFR clouds will also spread in from the south
through the afternoon and evening. Concern regarding winds
shifting to NNW-N, to possibly NNE at ORD/MDW just behind the
front.

A continuous stream of MVFR or lower VFR clouds will advect
northward through the afternoon and evening. Guidance suggests
this may briefly come in MVFR and settle to VFR before becoming
predominant MVFR ahead of the cold front tonight.

Convection continues to weaken off to the northwest. There is a
weak convergence axis over northeast IL that may trigger a shower
or two that may become a storm as inhibition erodes. Confidence is
too low on coverage to include in the TAF set, as the forcing is
subtle, but will need to monitor. Better chances storms comes
tonight. High confidence in a line of storms moving with the cold
front, with some weakening trend likely after passing the Chicago
terminals. Current RAP/HRRR timing is just slightly later than
previous runs, but 6-7z timing in an around the Chicago terminals
seems reasonable. There could be some prefrontal showers or
thunderstorms after 3-4z as well, but confidence is low on this.

Winds may abruptly shift to N or even NE with the front at ORD/MDW
but look to settle back to NW through the morning. MVFR clouds
lift behind the front.

KMD

&&

.MARINE...
217 AM CDT

Southwest winds will be gusting around 25 kt today so issued a small
craft advisory through this afternoon.

The low over western Manitoba will weaken as it reaches Ontario
tonight and then continues to Quebec. Its cold front will pass over
the lake tonight into early Friday morning with winds becoming west
to northwest behind the front. Westerly winds are then forecast
through early next week.

The remnants of Tropical Storm Cindy will move over the southern
Mississippi Valley today and then the low will fill as it shifts
east over the Ohio and Tennessee River valleys late this week.  High
pressure builds over the plains Sunday and then builds over the
western Great Lakes early next week.  Winds become southerly behind
the high Wednesday.

JEE

&&

.LOT WATCHES/WARNINGS/ADVISORIES...
IL...None.
IN...None.
LM...Small Craft Advisory...nearshore waters
     until 7 PM Thursday.

&&

$$

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