Area Forecast Discussion
Issued by NWS Chicago, IL

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FXUS63 KLOT 281952
AFDLOT

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
252 PM CDT Fri Apr 28 2017

.UPDATE...
1057 AM CDT

Have slowed initial light shower mention a tad today along with
lowering temperatures a few degrees across parts of north central
Illinois.

Initial surface warm front is moving slowly northward in the CWA,
located near I-80 at present and should not move much further
north. Broad, mainly weak uplift of lower 50 dew points up to
this boundary will help to gradually saturate a fairly dry
atmosphere as seen on the 12Z DVN sounding. A mid-level short wave
moving across the IA/MO border region will concentrate lift,
although this wave is weakening as it progresses east. Should see
light showers creep into the western and southern forecast area
early this afternoon. The I-80 to I-88 stretch, or immediately
north of the warm front/surface wind shift (including the city of
Chicago), may be the area that struggles the most to fill in with
rain, but still should see scattered light showers mid-late
afternoon.

The northeast winds north of the boundary, while light, will
likely keep temperatures down somewhat. This is especially true
toward Rockford where the clouds have filled in. In addition, flow
off the lake likely will result in temperatures dropping a few
degrees this afternoon in lakeside northeast Illinois.

MTF

&&

.SHORT TERM...
251 PM CDT

An updated short term discussion will be posted shortly.

&&

.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 18Z TAFs...

Concerns with the aviation forecast are a period of light showers
mid-afternoon through early evening, northeast winds increasing
later tonight and more so Saturday, and then more solid rain
spreading back in by Saturday afternoon.

Moisture return has started over the area with light rain showers
creeping northeastward into north central Illinois. These should
see an expansion this afternoon, though low-level dry air will
likely hinder widespread rain and low cloud coverage. After dark,
MVFR should seen an expansion with potential for IFR. Confidence
in cigs not dropping to IFR tonight is low-medium at the Chicago
area airports.

A lake breeze pushing inland this afternoon across ORD and MDW will
turn winds more easterly, though a sharp increase in speed is not
expected. Northeast winds will steadily increase tonight with
occasional gusts favored overnight.

Looking ahead to Saturday, low pressure will be taking shape
across the Ozarks region with an extending frontal boundary and
much more moisture lifting north on Saturday. Rain is expected to
inch northward during the morning and then a more rapid expansion
in the afternoon. IFR conditions are likely, including temporary
visibility to around 1SM. Instability for thunderstorms slowly
increases late in the day and a little more so after dark.

MTF

&&

.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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