Area Forecast Discussion
Issued by NWS Chicago, IL

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FXUS63 KLOT 290014

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


714 PM CDT

Northern Ogle & Winnebago County storm did finally exhibit a
better defined wind signature, necessitating first SVR issuance.
Also monitoring Lee County segment for potential intensification
and severe issuance. Given potential for isolated/scattered severe
gusts to reach counties just to east/northeast, opted to locally
expand the watch to include Boone, DeKalb and McHenry counties.


Previous update from 641 PM CDT

Limiting factors mentioned in previous meso AFD seem to be
preventing more robust updrafts (including marginal effective
shear and fairly weak midlevel lapse rates). As broken line of
storms progresses east northeast across western CWA cores have
been pulsy but generally remaining below severe limits. With DCAPE
values at or above 1000 j/kg according to SPC mesoanalysis, still
cannot rule out isolated gusts to 50 to 60 mph from the strongest
cores. Given very pulsy nature of the updrafts, threat for any
severe criteria hail appears very isolated. Unless some signs of
congealing and cold pool growth are seen shortly, overall strong
to severe threat appears it will remain limited and isolated. With
MLCAPE around 1000 j/kg likely to wane fairly quickly toward and
beyond sunset, thunderstorms should show a downward trend
intensity wise with time this evening. For all these reasons, not
anticipating needing to expand the going watch eastward.



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.



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 air mass 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 air mass 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 air mass
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 00Z TAFs...

Broken line of TSRA over western IL expected to track eastward
this evening...affecting the Chicago area terminals by mid-late
evening. Anticipating this band of storms to begin weakening with confidence in TSRA vs just SHRA decreases into the
Chicago area. Once that band moves through look for VFR conditions
the remainder of the TAF cycle with gusty southwest winds veering
to west Sunday afternoon.



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






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