Area Forecast Discussion
Issued by NWS Chicago, IL

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FXUS63 KLOT 300947

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

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 will 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 06Z TAFs...

Main forecast concerns continue with periodic showers and
thunderstorms throughout the entire period, IFR and possible LIFR
ceilings, and continued northeast wind today.

In the near term, much of the moderate rain has exited to the
northeast with only light rain or drizzle in place. There appears
to be a several hour window when this lighter precip will occur,
with even some dry conditions possible. Am monitoring additional
precip south of the terminals which will likely lift north later
this morning. With this returning, more widespread light to maybe
even moderate showers will occur for a good portion of the
morning. At this time, thunder should stay just to the south. This
then exits by late morning but with scattered showers and even
thunderstorms likely returning during early afternoon. After this
period of precip, additional shower and thunderstorm chances will
remain Sunday evening. Ceilings have come down recently, and are
in the IFR range. This IFR will likely remain in place for much of
the period, and am monitor for ceilings to possibly drop further
through early this morning, maybe to the 300-500 FT range.



438 AM CDT

As low pressure over the PLains continues to lift north through
the region today into tonight, expect stronger east northeast
winds to continue over much of the lake. At this time, highest
speeds are up to 30 KT over the entire lake and expect this to
continue for much of this morning. While the nearshore waters and
south half generally stay in this speed range, will see the north
half increase to gales by this afternoon with these gales
continuing into tonight. By later today into this evening, surface
pattern does get complicated with this low approaching and surface
warm front trying to lift north near the southern end of the lake.
This does add some level of uncertainty with regards to wind
direction and speed over the south half and nearshore waters.
Nonetheless, speeds do look to remain elevated. This will continue
to provide hazardous conditions for small craft today and tonight,
with these conditions likely persisting through early this week.



IL...Flash Flood Watch...ILZ014-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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