Area Forecast Discussion
Issued by NWS Chicago, IL

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FXUS63 KLOT 290527

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
1227 AM CDT SUN MAY 29 2016

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 06Z TAFs...

Only aviation weather concerns through Sunday night is a brief
period of showers and possibly an isolated embedded thunderstorm in
the next 1-2 hours for Chicago terminals...then breezy west-
southwest winds through the day Sunday.

Low pressure was over the upper Midwest at midnight...with a cold
front trailing southward across east-central Iowa and into
Missouri. A decaying area of showers and a few lingering
thunderstorms was well ahead of the front across far eastern WI
and eastern IL. Activity has shown a marked downward trend in
lightning over the past couple of hours...with most of the
remaining thunder located in the vicinity of KBMI. An area of
showers roughly from KARR-KJOT-KIKK was lifting northeast...and
will likely bring a brief period of light rain to the Chicago area
terminals prior to about 07z. Once this passes...dry weather is
expected to persist through the remainder of the taf period.

South winds near 10 kts expected for the remainder of the night...
becoming west and gusting 20-25 kt by mid-morning as cold front
moves east across the area and boundary layer mixing deepens after
sunrise. Breezy westerly winds to continue through the 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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