Area Forecast Discussion
Issued by NWS Chicago, IL

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FXUS63 KLOT 300825

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
325 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 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.



302 PM CDT

Sunday through Saturday...

The challenging forecast continues through the end of the weekend,
with the closed low deepening across the central Plains Sunday and
moving northeast Sunday night. As the system matures, the warm
and moist conveyor belts will be spreading over the region, but
similar to this morning, ample ongoing storms are favored to our
south. This will be in response to constant storm development
modulating the primary effective surface warm front and keeping
it over Missouri and southern Illinois into Sunday a.m. The
effects of the robbed moisture transport may temper rainfall in
the a.m. into early p.m. hours.

Guidance continues to show spread in boundary placement and QPF
magnitudes through the rest of Sunday. The main surface low is
presently favored to be a little east of Kansas City and at around
994mb by mid-afternoon. A secondary low/triple point is favored
to be somewhere near the Mississippi River/western Illinois by
late afternoon as the system dry slot moves in. The northward
progression of this surface feature will be affected by just how
widespread convection and its cooling footprint is through the
day. In addition, there is favored to be convective development
further south along the cold front across the lower Mississippi
Valley. These will be inhibiting factors to at least some degree
of warm air and instability return north of the boundary for
convection. In the presence of 0-6km shear at or above 60 kt, a
conditional threat for quick-moving, mainly low-topped supercells
and severe weather will exist if enough low-level CAPE can track
northward. This looks to be more so over the southeast CWA though
some storms are certainly possible areawide on the nose of the dry
slot given the strong low-level convergence. Overall have been
leaning a little more toward the high-res NAM-WRF and a further
south placement of the most effective air, but will need to
monitor closely.

Overall clusters of showers and some storms in the afternoon
into the early evening are expected, and continued moderate to
briefly heavy rainfall rates are likely. Progression of
showers/storms will be fairly quick Sunday afternoon and evening,
so that may ease the true flash flooding threat with more of an
areal flooding concern due to the overall wet 30-36 hour period
from this afternoon through Sunday evening. While the dry slot
should bring much of the rain threat to an end by late Sunday
evening, have continued the watch end time as is given that any
flooding would persist after the rain.

Sunday temperatures are tricky. The northern CWA is likely to
remain within northeast/east winds the longest amount of time,
and should not climb out of the 40s most of the day if at all.
Temperatures will be on the climb south, but how quickly and how
far north the warm air reaches will again depend on the primary
warm front. Have highs in the lower 70s forecast in the far
southeast but these could extend further north, even up to the
southern Chicago metro especially based on global guidance
solutions. Behind the cold front Sunday evening winds will turn
southwest and breezy.

The surface low is expected to only slowly occlude into Monday,
with fairly stout cold advection pegged into the CWA on southwest
to west winds gusting to 35 mph or so wrapping around the system.
With low-level clouds favored, especially north and west CWA,
temperatures may not go anywhere on Monday. Have collaborated and
undercut some on guidance, but could be too warm in northern
locations. How far rain showers extend south and east on the
commahead of the system will depend on how quickly it moves
northeast, but do have likely mention in northwest locations.

Beyond, high pressure will generally prevail much of the rest of
the workweek on global guidance solutions. There could be a frost
threat Tuesday night in outlying areas.



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.



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...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...Gale Warning...LMZ740-LMZ741-LMZ742-LMZ743-LMZ744-LMZ745-LMZ777-
     LMZ779 until 4 AM Sunday.

     Small Craft Advisory...nearshore waters
     until 4 PM Sunday.




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