Area Forecast Discussion
Issued by NWS Chicago, IL

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FXUS63 KLOT 292005
AFDLOT

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
305 PM CDT Sat Apr 29 2017

.SHORT TERM...
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
possible.

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
Sunday.

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.

JEE

&&

.LONG TERM...
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.

MTF

&&

.AVIATION...
For the 18Z TAFs...

Showers have reached all of the terminals and will persist through
the majority of the forecast period. Could hear a few rumbles of
thunder at the eastern terminals in the late aftn and early
evening. Have medium confidence in thunder occurring and low
confidence in coverage and location. Gusty northeast winds also
continue and cigs will fall to IFR late this aftn. Thinking cigs
will stay above LIFR although some models suggest LIFR will occur.
Showers diminish to sprinkles late this evening, but another round
of showers is expected Sunday morning. Thunderstorms may also
occur in the afternoon, but have concerns about how much energy
will be available for storms. East winds at arnd 15 kt are
expected Sunday.

JEE

&&

.MARINE...
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.

JEE

&&

.LOT WATCHES/WARNINGS/ADVISORIES...
IL...Flash Flood Watch...ILZ014-ILZ021-ILZ022-ILZ023-ILZ032-ILZ033-
     ILZ039...7 PM Saturday to 7 AM Monday.

IN...Flash Flood Watch...INZ001-INZ002-INZ010-INZ011-INZ019...7 PM
     Saturday to 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 Saturday.

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

&&

$$

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