Area Forecast Discussion
Issued by NWS Chicago, IL

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FXUS63 KLOT 282127
AFDLOT

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

.MESOSCALE DISCUSSION...
424 PM CDT

Broken line of thunderstorms moving across the Mississippi River
this hour will approach the western boundary of the forecast area
around or shortly before 600 pm CDT. Temperatures in the low to mid
80s with dewpoints into the low to mid 60s are contributing to a
corridor of around 1000 J/kg MLCAPE across northwest and north
central Illinois...however 700-500mb lapse rates are marginal around
6-6.5 C/km which will help limit the large hail threat for our area.
Deep layer shear is also on the marginal side around 30kt per latest
SPC meso-analysis. In order to see any appreciable severe weather
threat reach portions of north central Illinois would likely need to
see better cold pool development occurring within the broken line
of thunderstorms...but thus far there is no strong indication
this is occurring with upstream storms struggling due to some of
the marginal conditions. Still...DCAPE axis of 1100-1200 indicates
at least isolated microburst potential barring any better linear
organization as we head into the evening.

Deubelbeiss

&&

.SHORT TERM...
217 PM CDT

Through Sunday...

Low clouds have finally lifted Northeast of Chicago, and has
allowed for the surface to received direct light/heating. This
has pushed surface temps into the lower 80s, the abundance of low
level moisture and dewpoints in the mid 60s, will likely struggle
to allow temps to warm much more then the lower 80s. The far urban
heat island effect from the South/Southwest winds could see a few
85-86 degree values this afternoon near Chicago. Otherwise the
main concern will be on the potential for isolated severe storms
mainly along and west of interstate 39.

A decent cumulus field has developed in the wake of the lower
clouds from this morning, but vertical growth has been slow given
the weak warm layer aloft. But expect as the afternoon progresses
the vertical structure will increase allowing parcels to quickly
produce greater depth to the cumulus clouds. Further west across
Iowa is a mid-lvl wave that is helping to agitate the environment
more and producing some larger ascent and a few thunderstorms are
beginning to initiate. Most hi-resolution guidance suggests a
narrow channel of vorticity will elongate as it slips East and
become oriented from near LaCrosse WI south through West Central
IL, but as the vort lobe pushes East the depleting issue could be
the lack of direct heating and we begin to lose the diurnal
component. This could limit the severe threat further East this
evening. That being said, there is some concern that given the dew
points being anomalously high that a broken line of storms could
develop and ride East along the moist layer towards the Chicago
metro between 2-3z. The main threats/hazards for Northern Il from
any storms will likely be damaging wind gusts up to 70mph and
perhaps an isolated hail to quarter size.

Convection should quickly diminish around 4-6z, with much of the
overnight expected to be quiet and dry as a weak diffluent zone
lifts overhead. The 500mb trough axis will be steadily pivoting
East overnight, arriving overhead closer to daybreak but becoming
negatively tilted. Lapse rates will steepen by midday Sunday,
however the lack of forcing and some drier air arriving could
limit precip/convection to areas North/Northeast of Il. Have opted
to go with a dry Sunday to account for this, but could see the
need to bring back a 15-19 POP if the dry air is stubborn to
arrive. Highs will likely push back into the low 80s again Sunday.

Beachler

&&

.LONG TERM...
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.

KMD

&&

.AVIATION...
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
20-25kt.

Beachler

&&

.MARINE...
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
Wednesday.

KMD/DLB

&&

.LOT WATCHES/WARNINGS/ADVISORIES...
IL...NONE.
IN...NONE.
LM...NONE.
&&

$$

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