Area Forecast Discussion
Issued by NWS Chicago, IL

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FXUS63 KLOT 301157

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
657 AM CDT Sun Apr 30 2017

446 AM CDT

We made a last minute decision to expand the flash flood watch a
county west. Given this was the area that got the heaviest
rainfall on Saturday, additional heavy rainfall today could result
in a flash flooding threat in these areas as well.



325 AM CDT

Through tonight...

The main forecast concerns continue to focus on the impacts of a
very dynamic and slow moving storm system expected to impact the
sensible weather across the region through Monday. However, of
most concern is the threat for more moderate to briefly heavy
rainfall today, and also the possibility for severe storms late
this afternoon into the evening.

The upper level low is centered over the Texas Panhandle early
this morning with surface low pressure taking shape over
northeastern OK into eastern KS. While round one of the heavier
rain ended across the area several hours ago, expect rain to shift
northward back over the region early this morning as another
convectively enhanced disturbance shifts northward over northern
Illinois. This will result in another period of light to
moderate rainfall through late morning. Some embedded
thunderstorms will also be possible with this activity.

With this morning convective activity in the region, it will
likely aid in keeping the effective warm frontal boundary south of
much of my CWA until at least mid day. Thereafter, the warm front
should begin to gradually shift over my southern counties this
afternoon as the parent surface low shifts northeastward towards
northeastward into Iowa while becoming occluded. As this occurs,
a tipple point is likely to set up over western Illinois later
this afternoon. Farther east, it appears the warm front may
struggle to get north of the I-80 corridor through the afternoon.
As a result, another chilly day in the 40s is likely over much of
northern Illinois with east-northeasterly flow off of the lake
continuing. To the south of the front, however, temperatures are
likely to warm into the lower 70s.

The main concerns for later today into this evening with be the
threat of severe thunderstorms as a cold front approaches from the
west. There remains questions as to the extent of surface based
convection over the area during this period due to extensive cloud
cover and periods of showers at times into the afternoon.
Nevertheless, there will be a conditional threat for severe
thunderstorms this afternoon, and this looks to be especially the
case along and really south of I-80. While buoyancy is not
expected to become overly impressive tied to the relatively
unimpressive mid-level lapse rates and little to no insolation
this afternoon, the wind field over the area, associated with the
larger scale storm system, is expected to be very impressive. In
fact, given that the near surface flow is likely to remain backed
south- southeastward near the surface warm front beneath south-
southwesterly 850 MB flow around 50 KT, low level shear/helicity
will also be very impressive and could support some tornados.
While this is a conditional threat on there being enough
instability to generate convection, it does appear at this time
the main threats would be south of I80. This situation will need
to be monitored closely through the day.

Very heavy rainfall and possible flash flooding could also become
an issue later today into this evening given the rain that has
already fallen, and the likely high rain rates expected with any
thunderstorms. The good news is that these afternoon and evening
storms should be moving quite quickly, which would limit the heavy
rain rates over any given location, unless any training convection
develops. Otherwise, the thunderstorm and heavy rain threat
should wane from west to east through the evening as the cold
front moves across the area.



336 AM CDT

Monday through Saturday...

The large upper low and the attendant surface low will continue
gradually shift towards the Upper Great Lakes on Monday into
Monday night. Rap around moisture will continue to result in some
additional light showers over the area on Monday and possibly into
Monday night before ending. It appears that much of the extended
period will be mainly dry. However, another upper trough is
expected to dig across the central portion of the country around
midweek and this may drive another period of rain over portions of
Illinois late Wednesday into Wednesday night. At this time it
appears the best chances may remain south of, or just clip my
southern counties. Otherwise expect temperatures to remain on the
cool side, mainly in the 50s to lower 60s.



For the 12Z TAFs...

Main forecast concerns continue with periodic showers and
thunderstorms today through this evening, IFR/LIFR ceilings, and
a east northeast wind today.

Rain has returned to the terminals this morning and will likely
remain in place for much of the morning before becoming more
scattered by midday and early afternoon. Any thunder this morning
is well to the south of the terminals and will likely continue
this way for much of the morning. Although thunder potential does
slightly increase by midday, overall threat appears low and have
excluded it in the taf. The highest chance for thunderstorms
arrive late this afternoon into the early evening ahead of an
approaching front, with any thunder threat then diminishing into
the overnight hours. IFR/LIFR ceilings are in place and don`t see
any improvement to these lower ceilings until late tonight behind
frontal boundary. East northeast winds persist, and will continue
through this evening, but with a trend to a southeast and then
south direction expected later tonight into early Monday morning.



237 PM CDT

Headlines...Will go with a gale warning for the south half and
nearshore waters this afternoon through most of tonight. While gales
will subside, small craft conditions will likely continue through
Sunday. Gales are then expected over the northern half of the lake
Sunday afternoon and night, and upgraded the previous gale watch to
a warning.

A weak high is over western Ontario and the western Great Lakes
while a low is forming over Texas.  The tightened pressure gradient
will keep northeast winds at 30 kt and they will increase to 35 kt
gales over the south half late this afternoon.  Both the low over
Texas and the high over Ontario will lift north through Monday and
gales will form over the northern half of the lake Sunday afternoon.

The low will pass over the western Great Lakes Monday night and
reach Quebec Tuesday.  The low`s cold front swings through late
Monday night/early Tuesday morning. Southwest to west winds may
increase to gales over the southern half Monday night into Tuesday.
High pressure moves over the lake Wednesday and then shifts
southeast of the lake late in the week.



IL...Flash Flood Watch...ILZ012-ILZ013-ILZ014-ILZ019-ILZ020-ILZ021-
     ILZ022-ILZ023-ILZ032-ILZ033-ILZ039 until 7 AM Monday.

IN...Flash Flood Watch...INZ001-INZ002-INZ010-INZ011-INZ019 until 7
     AM Monday.

LM...Small Craft Advisory...nearshore waters
     until 4 PM Tuesday.




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