Area Forecast Discussion
Issued by NWS Chicago, IL

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FXUS63 KLOT 290525
AFDLOT

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
1225 AM CDT Sat Apr 29 2017

.UPDATE...
859 PM CDT

For Evening Update:

Minor adjustments to going forecast this evening, mainly to refine
pops based on radar trends. Generally, only light showers over
far north/northwest and far south/southeast parts of of the FA
expected.

Evening surface analysis depicts a stationary frontal boundary
from southern IL/IN...extending southwest to low pressure over
west Texas. Scattered thunderstorm clusters were occurring along
the front well south of the area this evening. Farther north,
925-850 mb obs indicate an elevated frontal zone from KS into
northwest IL and southern WI. Showers were occurring with a pair
of low-amplitude short wave disturbances, mainly north of and
southeast of the cwa, with a minimal in precip across the forecast
area in a region of weak warm/moist advection and isentropic
ascent though with little forcing. 3km WRF and HRRR seem to be
capturing the general character of the precip fields and trends,
which suggest maintaining some low-chance pops for some spotty
light rain across the far north and far south parts of the area
this evening. Guidance does depict a sheared mid-level wave
approaching from the southwest toward morning which looks to
increase precip potential across the southern tier or two of WFO
LOT counties toward sunrise, but otherwise little in the way of
significant rainfall is expected overnight.

Most guidance is in good agreement in spreading rainfall northeast
across the forecast area mid-late Saturday morning, with another
low-amplitude short wave rippling along the elevated baroclinic
zone across the area. Will have to watch convective development
north of the surface front overnight however, which may tend to
block some of the stronger moisture transport into northern IL and
IN. Stout northeast winds develop by early Saturday as well, which
will also work to maintain a feed of drier low level air into the
region. While periods of rain on Saturday are a good bet, these
factors may help initially reduce heavy rainfall threat somewhat.

Ratzer

&&

.SHORT TERM...
305 PM CDT

Through Saturday...

Concerns through Saturday are with moderate to potentially heavy
rainfall beginning Saturday afternoon. A Flash Flood Watch has
been issued from Saturday afternoon through the rest of the
weekend.

Light rain has struggled to become widespread across the area
with this initial mid-level disturbance and start to isentropic
ascent, due to weak forcing for lift and inherent low-level dry
air. Spotty light showers should continue through early evening,
with likely a wedge in the central CWA with little to no rain.

As gradual cyclogenesis occurs across the southern Plains tonight,
the synoptic boundary (initial warm front) that is located just
north of I-80, will start to collapse southward with northeast
winds slowly increasing. While a spotty light shower or two
through the rest of tonight cannot be ruled out, a general lull in
forcing is expected with focus well to the southwest of the
immediate area.

As the frontal zone to the south develops warm frontal
characteristics on Saturday morning, it will start to slowly
progress northward. The low-level northeast flow will initially be
problematic for rain, but this should be overcome from south-to-
north during the afternoon. Just how quickly this occurs will
hinge heavily on amount of convection to the south robbing
moisture transport. If slower, rain may not begin in the far north
until late day or even early evening. Confidence in rainfall
amounts for Saturday afternoon is somewhat low, especially the
northern half of the CWA, and it could end up being lower during
that period.

MTF

&&

.LONG TERM...
251 PM CDT

Saturday night through Friday...

The potential for copious amounts of rain still exists for Saturday
night in particular, and continuing into Sunday. However, still have
concerns that convection to our south will result in less moisture
and rainfall than the models are currently predicting.  While models
may be too high on specific precipitation forecasts, still have
enough confidence to go with a Flash Flood Watch for areas along and
east of the I-55 corridor Saturday afternoon through Sunday night.

Saturday night...The surface low organizes over southeast Missouri
with its warm front remaining just south of the forecast area.
Expecting moisture to pool across central and northern IL and
northwest IN, mainly along and east of I-55. However, have serious
concerns about how much moisture will actually pool in this area.
Convection to the south may hog some of the forecasted moisture
leading to less moisture/rainfall than forecast.  Current forecasts
feature 1.5 to around 1.8 inches of PWAT, well above normal for this
time of year. Given my concerns, went with lower forecasted rainfall
than many of the models would suggest. Either way, expecting
widespread rainfall across the region Saturday night.  Will keep a
chance of embedded thunder in the forecast since the saturated
soundings still feature a little elevated CAPE.  Thinking the better
coverage of thunderstorms will be also along and east of I-55, but
will keep a chance along and south of I-88.

Sunday...Rain continues as the warm front lifts north to either the
I-80 to I-88 corridor. PWAT values are still forecast to remain over
1.5 inches so the threat of heavy rain will continue.  Agree with
SPC`s Day 3 outlook for our area. Have low confidence in severe
storm coverage and strength because I am unsure how well and quickly
the atmosphere will recover from Saturday night`s rainfall.  In
addition, cloud cover may limit CAPE values.  If we get get breaks
in sun and are able to recover, strong to severe storms are
expected.  Shear values will be 60-80 kt. CAPE values will likely be
highest south of I-80 and forecast values range from 500-1000 J/kg
in the GFS to 1000-2000 J/kg in the typically overly aggressive NAM.
 Therefore, will lean more toward the GFS` solution.

If strong to severe storms form, expecting discrete storms. Severity
of threats will depend on how much CAPE we have to work with, but
all severe types and flooding are possible with the storms.

Have low confidence in high temps Sunday as any breaks in clouds
will likely lead to higher temperatures than forecast especially
south of the warm front.

Sunday night....

The low`s cold front moves through Sunday evening and night and
expecting storms to consolidate along the front leading to
additional rainfall.

Monday through Friday...

Cooler air arrives with the upper level trough early next week
leading to highs in the 50s most of next week.  Light wrap around
showers are expected  Monday as the upper level low rotates over the
region.  While we do not need more rain, rainfall amounts should be
minimal.  Monday also looks gusty with southwest winds gusting to
around 30-35 MPH.

Rain finally moves out and high pressure moves in Tuesday, and we
may even see a little sunshine!  The high should keep rainfall
associated with the next low off to our southeast, but the GFS has
the precip clipping the southeast corner of the forecast area.  Have
low confidence in whether the showers will reach the forecast area
or not.

JEE

&&

.AVIATION...
For the 06Z TAFs...

Concerns over the forecast period are with MVFR and then IFR
ceilings, strong north northeast winds/gusts, and periodic showers
with thunder becoming a possibility towards the end of the period.

In the near term, dry conditions across the terminals, which are
expected to continue early this morning through mid/late morning.
Begin time for shower development by midday still looks to be on
track at this point, with scattered development likely continuing
into the afternoon. Still some uncertainty with additional
development later this evening and overnight but at this point,
continued light drizzle or rain still seems reasonable. Will need
to monitor the evening time frame for possible stronger shower
development and then more so in the overnight hours, the
possibility for isolated thunder. Low confidence with either of
these solutions at this time, so have maintained current forecast.
MVFR ceilings have developed across the terminals and do look like
they will will be in place for the remainder of the day, lowering
with time this afternoon and evening. The IFR which is occurring
at RFD at this time, should remain isolated and brief this
morning, but will need to continue to monitor for these lower
ceilings to remain in place. No change in wind direction through
the period, with north northeast winds continuing and increasing
in speed through the period.

Rodriguez

&&

.MARINE...
251 PM CDT

Headlines...Will continue the Small Craft Advisory for late tonight
into Sunday afternoon. While gales are possible over the southern
half of the lake Saturday evening, I think it will be a brief 5 hour
window that will not need a watch at this point. However, I did have
enough confidence to issue a gale watch for the northern half of the
lake Sunday afternoon into Monday morning.

Weak high pressure shifts southeast this evening and winds become
north to northeast. Winds increase as the gradient tightens between
a high building over south central Canada and low pressure taking
shape over the southern plains. Guidance differs on whether the
gradient will be strong enough to support low end gales over the
southern end of the lake Saturday afternoon and evening.

As the low shifts north to Iowa Sunday afternoon, the area of
stronger winds also shifts north. Have higher confidence in gales
occurring over the northern end of the lake Sunday afternoon into
Monday morning, so decided to go with a gale watch.  The low moves
over the northern end of the lake late Monday night/early Tuesday
morning along with the low`s cold front.  Winds become west behind
the front with at least 30 kt expected. Another Small Craft Advisory
will likely be needed Monday and Tuesday. There is a chance that low
end gales may occur over the southern half of the lake but
confidence is medium-low given differences in model solutions.

The low continues to Quebec Tuesday night followed by high pressure
building over the lake Wednesday night into Thursday.

JEE

&&

.LOT WATCHES/WARNINGS/ADVISORIES...
IL...Flash Flood Watch...ILZ014-ILZ021-ILZ022-ILZ023-ILZ032-ILZ033-
     ILZ039...1 PM Saturday to 7 AM Monday.

IN...Flash Flood Watch...INZ001-INZ002-INZ010-INZ011-INZ019...1 PM
     Saturday to 7 AM Monday.

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

&&

$$

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