Area Forecast Discussion
Issued by NWS Chicago, IL

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FXUS63 KLOT 281913

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
213 PM CDT SAT MAY 28 2016


Visible satellite imagery continues to show thick/lower clouds
lifting North across East Central Illinois towards Northeast
Illinois late this morning. Clouds have begun to thin further
West, but expect with additional heating of the surface clouds
will further erode and allow temps to quickly warm into the lower
to middle 80s. Winds will continue to be southerly to
southwesterly with speeds between 10 to 20 mph and occasional
gusts to 25 mph this afernoon. Most forecast guidance continues to
indicate minimal activity this afternoon with respect to
thunderstorms/rain coverage or development. Have held onto the
isolated coverage for this afternoon, but could easily see a dry
forecast going until closer to 7pm. The next wave will be arriving
in the western forecast area around 8 to 9 pm cdt, and will allow
for increased chances for thunderstorms in the western counties.
Although after this period and as activity works East, the
challenge will be limiting instability later this evening and
could diminish the coverage. A few of the stronger storms this
evening could be able to grow vertically enough to tap some of the
stronger winds and may be able to produce an isolated severe
storm, although this remains a very low confidence in any severe
storms. The more likelihood of storms will be a gust to 50 mph and
perhaps pea size hail.



306 AM CDT

Through tonight...

The main weather concerns are for the potential of some
strong thunderstorms over portions of the area...especially this

a few widely scattered showers and perhaps a thunderstorm will be
possible today. This may end up especially being the case east of I-
57 as some weak mid level disturbances shift northward over the
area. However...most areas look to be dry for much of the day...and
as a result...I have lowered POPS to just slight chances in most
areas. However...the chances of more storms will be gradually
increasing over north central and northwestern Illinois late this
afternoon...and especially this evening.

An upper level trough...currently over the Plains...will gradually
shift northeastward to the upper Mississippi Valley by this evening.
This system will likely spawn additional thunderstorms to our west
over eastern Iowa this close proximity to the
entrance region of an 80 KT upper level jet...and as diurnal
destabilization maximizes. This activity then looks to progress
eastward into northern Illinois into the early evening. The main
question with this activity is how far eastward will it progress
before weakening. Many of the Convective Allowing Models
(CAMs)suggest this activity will weaken quickly through the evening
as the storms shift towards northeastern Illinois. While this
certainly seems plausible given the poor diurnal timing...I still
have mentioned higher end chance POPS into the Chicago area later
this evening as I think some storms could still persist this far

Some stronger storms will be possible early this
evening...especially over north central and northwestern Illinois.
Farther east...the expected weakening trend with the convection
should limit the severe threat. These storms could present a hail
risk...along with some locally strong wind gusts given the higher
DCAPE values. This activity looks to wane during the late evening
and overnight as it shifts eastward.



204 PM CDT

Sunday night through Friday...

The upper low will dampen and shift east of the region Sunday night.
Broad surface high pressure will form in its wake and bring a
generally drier start to the extended period. A subtle trough/weak
cold front will move through overnight with weak energy sliding
southeast in slightly more progressive wnw flow aloft. Most
guidance is dry with its passage but we cannot rule out an
isolated shower.

Memorial Day is shaping up to at least start off fairly nice
and stay that way for most areas in spite some increasing clouds.
After the weak trough passage, a ever so slightly cooler and less
humid airmass will remain in place. To our west warm moist
advection will resume after the gulf of Mexico is temporarily cut
off by the transient surface high. Most of the convection looks to
be tied to daytime heating. Some guidance including the GFS/GEM
attempt to bring some of this into our area. The NAM shoves the
moist airmass south suggesting a better push with the front, and
the EC keeps convection to our west. Given the mixed signals but
favoring a drier solution as is currently forecast, have trended
that direction.

Model guidance does show a diurnal weakening Monday evening, but
suggests some overnight development again to our west that will
attempt to push into our area into early Tuesday, but chances
remain tempered given the weak ridging, southeasterly drier
surface flow. Better chances for showers and thunderstorms returns
Tuesday through midweek in response to a storm system that will
move east through the Dakotas Tuesday. A warm and humid airmass
will return in the southwest flow ahead of this system with an
undisturbed feed from the gulf of Mexico. Shower and thunderstorm
chances appear highest Wednesday and Wednesday night as forcing
from the low is much closer to the region. Cooler and drier
weather will eventually take hold for the latter half of the week
as a modest cold front shifts in with dewpoints dropping back
through the 50s making it feel comfortably and seasonably cool.



For the 18Z TAFs...

Sluggish low level clouds continue to show some spotty MVFR bases
across Northeast Illinois although with the surface warming the
bases have begun to lift and expect VFR conds to prevail by 20z.
Winds will continue to be breezy from the South between 10 to 15
kt with gusts between 20-25kt. To the west of the area clouds have
redeveloped with scattered showers/thunderstorms. Many of the hi-
resolution forecast guidance data suggests the earliest timing for
convection will be around 22-23z at RFD. Otherwise convection
likely will be struggling to maintain intensity or coverage as it
approaches ORD/MDW but timing for those TAF sites will be closer
to 1-2z through about 4-5z. Have opted to handle this with a VCTS
at this time as confidence remains low on coverage/intensity.

Then overnight winds will remain Southwest with a steady turn
towards West and gusty winds again for Sunday with speeds nearing



204 PM CDT

While fog has lifted across the lake, it is expected to return
again tonight into early Sunday, and the marine dense fog advisory
will be extended until tomorrow.

A warm front has pushed north of the lake this afternoon with
breezy southerly winds in place. The low will shift to western
Lake Superior by Sunday night, which will maintain southwest winds
to 25 kt today and to 20 kt Sunday with elevated waves on the
north half. The low will strengthen as it moves northeast to James
Bay Monday morning. A weak cold front will moves across the lake
monday morning which will shift winds only slightly to the west
southwest, with a secondary potentially stronger cold front will
allow for a wind shift to northeast at least for the north half.
Lighter winds remain place through Tuesday before strengthening
out of the southeast in response to low pressure that will cross
the Dakotas Tuesday and move to western Lake Superior late



LM...Dense Fog Advisory...northern two-thirds of the open waters of
Lake Michigan through Sunday mid-afternoon.




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