Area Forecast Discussion
Issued by NWS Chicago, IL

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FXUS63 KLOT 231911

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
211 PM CDT MON MAY 23 2016

211 PM CDT

Through Tonight...

With a dry airmass and ample sunshine, temperatures across the
region are expected to rise into the lower to middle 80s this
afternoon.  A lake breeze boundary is slowly pushing inland this
afternoon and the IL lakefront never had the opportunity to warm up,
with the temperature at Northerly Island only reaching 64 F.  High
pressure centered over the eastern Great Lakes will continue to
slowly move to the east tonight as a cold front and associated sfc
trough approach the Middle and Upper Mississippi Valleys.  Low level
synoptic scale flow will gradually veer from SELY to SWLY
overnight...bringing slightly higher sfc dewpoints and
increasing deep layer moisture into the WRN parts of the CWA by early
tomorrow morning, with the moister air reaching the I-39 corridor.
The increase in deep layer moisture will inhibit cooling overnight
tonight and min temps tomorrow morning should range from around 60 F
along the I-39 corridor to the lower to middle 50s over NWRN IN.
Showers and thunderstorms have developed over WRN IA/MO this
afternoon and should continue to slowly move to the east this
evening. A gradual increase elevated instability in the environment
ahead of the approaching sfc trough will keep the potential for some
convective activity to reach the Rockford area during the early
morning hours.  However...some of the guidance is suggesting that
there could be a decreasing trend to the convection durg the pre-
dawn hours and then increase through the day tomorrow.  So, have
maintained the increasing PoP trend during the late night hours over
the WRN portions of the CWA, but will cap PoPs in the slight chance
to chance range with a slight chance of thunde



250 AM CDT

Tuesday Night through Sunday...

The pattern remains active during mid to late week and possibly
into the beginning of the holiday weekend. The better chances for
storms appear centered on Wednesday and Wednesday night and then
again on Friday. The potential for scattered severe storms does
exist on Wednesday and Wednesday night.

The surface cool front is forecast to advance south into far
northern Illinois late Tuesday night and likely have some lake
enhancement to it. This front and potentially low clouds
along/north of it will keep northern locations cooler...though
confidence is low on just how much so. Moist advection aloft and
potential propagation of upstream convection will bring increasing
storm chances by Wednesday morning and growing into the afternoon
and evening as instability south of the boundary increases. Dew
points are forecast to reach the mid to upper 60s south of the
boundary providing modest instability. In addition...the boundary
looks to become more warm frontal by afternoon and begin to lift
north in response to a 1006 mb surface low lifting northeast over
the Upper Midwest. Mid-level winds of 30-40 kt are expected to
bring decent deep layer shear that working with the instability
could bring scattered severe storms in an environment that should
be favoring at least storms. This is most true for along/south of
the warm the southern CWA. So have likely
thunderstorms mentioned Wednesday into Wednesday night and more
precise timing will be refined as we draw closer.

Presently Thursday looks like a day in-between with less
convective chances though still quite mild with southwest winds
capable of bringing 80s all the way to the lakeshore. The upper
low off the northern CA shore this morning is forecast by the GFS
and EC to track northeast across the Midwest as a fairly strong
wave on Friday and Friday Night. Some slowing potential exists
with this into Saturday given the recent pattern. Above normal
heights and southerly low-level flow look to continue supporting
80s into the holiday weekend.




Main concern through this evening remains wind direction and the
impact of a lake breeze. Guidance has been very inconsistent on
the development of a lake breeze and the resultant wind shift.
Much of the guidance is backing off on the idea that winds will
turn ELY and is settling into a more SELY direction and that wind
speeds may be a bit lower. Latest radar imagery and coastal obs
showing the lake breeze just inland into far NERN IL amd making
very slow progress. So, will continue to monitor lake breeze
progress, but currently thinking that there is less of a chance
for winds to shift ely. Winds should diminish to arnd 5 kt at

Thunderstorms are developing across parts of IA and MO this
afternoon and are expected to slowly move east this evening and
tonight. There is still a chance of this activity reaching RFD
shortly before 12z tomorrow morning and then possibly across the
rest of the terminals mid/late morning. Given the continued
uncertainty...with some guidance suggesting that the activity
could diminish in the morning hours and then refiring in the
afternoon, have opted to just go with a prob30 mention.




110 am...High pressure centered over the eastern lakes this
morning will move southeast to the Carolinas Tuesday morning. The
gradient will tighten this afternoon into tonight ahead of cold
front with southerly winds increasing to 15 to 25 kt. Speeds will
begin to diminish Tuesday afternoon with the cold front moving
across northern parts of Lake Michigan Tuesday evening and then
moving to the southern part of Lake Michigan by Wednesday morning.
Winds will shift northerly behind the front but the gradient is
not particularly strong so not expecting more then 10-20
most. As low pressure moves across the northern plains Wednesday
night into Thursday...this frontal boundary will lift back north
across the lake as a warm front with southerly winds then expected
into this weekend. cms





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