Area Forecast Discussion
Issued by NWS Chicago, IL

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FXUS63 KLOT 300533

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
1233 AM CDT Sun Apr 30 2017

610 PM CDT

Early this evening, regional radar composite showed a well
defined MCV over central IL enhancing a region of warm air
advection driven rain over about the NW 2/3rd of our CWA.
An area of moderate to nearly heavy rain extends from Peoria to
Lacon then northeast across LaSalle, southern DeKalb, southern
Kane, Grundy, and Kendall counties, beginning to spread into
heart of Chicago metro area. Rainfall rates of 0.2 to 0.4" per
hour have been observed in this area.

Over the next several hours anticipate this MCV to continue
tracking ENE toward our extreme southeast CWA. Intense line of
convection across central IL will continue to lift northeast and
could clip our southeastern most counties. While it would be well
north of the warm front, it is possible that some strong winds
could penetrate through the stable boundary layer with this line,
as has already been observed in central IL. Better severe threat
should remain south of our area however.

In addition, the area of moderate to heavy rain should continue
spreading northeast across the heart of the Chicago metro area.
There will probably be a maximum swath of an inch to possibly as
much as an inch and a half of rain with this first wave through
early this evening from LaSalle County ENE into Chicago. Radar and
satellite trends suggest that in the wake of this MCV we will see
a dramatic decrease in precip intensity or precip ending
altogether for a potentially extended period of time.

This MCV and associated squall line to it south will re-enforce
the synoptic front well south of our area and should focus
convective develop later this evening and overnight well south of
our CWA. Will work on re-fining pop/qpf/wx grids to reflect these
anticipated trends. Flooding threat with this first wave expected
to be low, but it will prime things if any additional heavy rain
falls Sunday.

- Izzi


237 PM CDT

Through Tonight...

Overall, will continue the flash flood watch as heavy rain is
possible this evening. The precip doesn`t look like it will evolve
as I imagined yesterday, but we are still looking at a lot of rain
over the next 48 hours that will result in river flooding and
some areal flooding.

The surface low is forming over Texas and its warm front lies from
central Missouri through central Indiana.  The front will lift over
our far southern counties by Sunday morning.  Showers have blossomed
nicely across the forecast area, and it seems like the majority of
convective activity will occur south of I-80 especially where rain
has briefly ended.  Still have at least a slight chance of
thunderstorms across the region though as embedded thunder is

Followed the HRRR and NAMNest guidance that feature a line of
convective showers/storms forming from Rockford south over the next
couple of hours and then marching east. This convective line should
produce the most precip tonight and raised QPF values slightly. Have
0.5 to 1.0 inches of rain between 00 and 06Z Sunday. However, we
could see a break in precip or just light showers/drizzle after the
line. Lowered precip chances to likely behind the line because I did
not have enough confidence to go with straight drizzle.  Lowered QPF
using a blend of the previous forecast and GFS between 06 and 12Z

Temperatures are also a bit tricky. Temps will continue to fall over
the northwestern half of the CWA while temps will warm over the
southeast CWA.  That temp trend should continue into this evening.
Then temps over the northwest will hold steady or warm slightly in
the low 40s.  Temps over the southeast will also hold steady around
50 or in the low 50s.



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...LMZ740-LMZ741-LMZ742-LMZ743-LMZ744-
     LMZ745...4 AM Sunday to 4 PM Sunday.




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