Area Forecast Discussion
Issued by NWS Chicago, IL

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FXUS63 KLOT 221747

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
1247 PM CDT Thu Jun 22 2017


Warm and muggy conditions greet us this morning with dewpoints now
in the upper 60s to near 70. Heat indices will approach the mid
90s and thus expect uncomfortable conditions through the day.
HRRR/HRRRX attempt to break out some development this afternoon,
not depicted on global guidance, NAM, NAMMest, and HopWRF members.
Forecast soundings do suggest we get uncapped this afternoon as
warm and moist air continues to spread northeast behind the warm
front. The question is forcing. There is not a ton of upper air
support looking on water vapor satellite. Most of the upper
forcing from Cindy is off to the south and looks to graze our
southeast. There is some convectively enhanced energy to our our
northwest. A lake breeze is not expected to enhanced low level
convergence. Therefore no changes to the chances for showers and
storms northwest and southeast during the daytime hours, but will
monitor mesoscale details to see what these hi-res models are
keying in on and if any of this lower level moisture streaming in
from the south will amount to anything. Like the HRRRX output of
some isolated development S-N with maybe a few storms getting into
NC IL ahead of tonight`s activity.



319 AM CDT

Through tonight...

Primary short term forecast challenges include temps today then
convective chances tonight.

Warm front is lifting north across the CWA early this morning with
warmer and especially more humid air spreading into the area. In
the past 6 hours have seen dewpoints jump about 4-7F with a pool
of near 70F dewpoints now into southern CWA. As warm front
continues north into WI today, the pooling Tds along the front
should also shift north with dewpoints in our area probably mixing
out a bit this afternoon, but likely holding well into the 60s.
Temps are tricky today with temps already rising behind the warm
front in the middle of the night, however cloud cover could make
the difference between upper 80s and what otherwise could be
low/mid 90s given the 925mb temps.

Small MCS is currently ongoing over northern IA and moving
generally E to ESE in our general direction, however SPC RAP based
mesoanalysis suggests that this activity should run into strong
convective inhibition before arriving which should result in a
pretty quick decay early this morning. None the less, debris
cloudiness from this convection as well as cirrus fanning north
from land-falling Cindy opted to not making any changes to our
going high temps even though they are a bit below what is
supported by 925mb climo.

Some moisture from Cindy could be enough to allow for a shower or
storm to develop later this afternoon over central IL and did keep
a slight chance pop down south to cover that potential. More
organized, intense convection is likely to blossom farther north
across WI WSW into N IA this afternoon in advance of the
approaching cold front. This activity is likely to move generally
eastward with a gradual southward propagation allowing it to
spread into our CWA from the north later this evening and
particularly overnight. By the time the storms arrive, instability
would be weakening/inhibition strengthening, which favors storms
to be in a weakening phase as they arrive, especially give shear
profiles are pretty marginal to maintain organized severe threat.
Cannot rule out an isolated severe threat in our NW CWA late this
evening, but overall am quite unimpressed with the prospects for
severe weather tonight. High PWATs do support the idea of
efficient rain producers, but not seeing any strong signals
favoring training convection and a significant flash flood threat.

- Izzi


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


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

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.



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.



LM...Small Craft Advisory...nearshore waters
     until 7 PM Thursday.




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