Area Forecast Discussion
Issued by NWS Chicago, IL

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878
FXUS63 KLOT 290827
AFDLOT

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


.SHORT TERM...
326 AM CDT

Through Sunday night...

The primary concerns continue to focus on a very dynamic and slow
moving storm system expected to impact the region through the
weekend. The main changes to the going forecast was to tone down
total QPF amounts some over the area, as well as to push off the
start time of the flash flood watch till this evening.

The center of the closed upper low is currently over the four
corners region. This system will continue to dig southeastward
over central New Mexico through the morning. Thereafter, as the
attendant upper jet begins to develop along the southeastern
periphery of the upper low, the storm system will begin to track
slowly to the north-northeast into Kansas by Sunday morning.
Strong dynamics associated with this process will allow surface
low pressure to quickly develop into eastern OK and KS later today
into this evening. The surface low then quickly become occluded
over Iowa by late Sunday and develop into a vertically stacked
system.

While the center of the storm system will remain well to our
southwest today, its impacts are already apparent over southern
Illinois and Missouri at this time. Convection has been ongoing in
this region for much of the night in response to strong moisture
convergence along the nose of 50+ KT low-level jet. This activity
is expected to continue shifting northeastward early this morning,
likely impacting my southern counties by daybreak as some light
showers and perhaps a few storms. The rest of the area will have
to wait until additional rain develops and arrives from the west-
southwest. A lead convectively enhanced mid-level disturbance is
expected to shift northeastward towards the area by this
afternoon. So, it will likely be dry across northern Illinois for
several hours this morning before rain begins to shift into the
area this afternoon. Given the fact that the rainfall during the
day is not likely to be overly heavy, we have pushed off the
start time of the flash flood watch until 7 pm this evening.

With the main instability axis remaining to our south in closer
proximity to the surface warm frontal boundary, there are some
concerns that convection developing to the south could act to slow
the northward progress of the better moisture transport along the
low-level jet. As a result of this, there remains questions as to
the magnitude of the precipitation rates over northern Illinois
into tonight. While the entire area is likely to get good amounts
of rainfall tonight into Saturday, it appears the better focus for
the heaviest rain rates and amounts may remain just south of my
area, or possibly into my southern counties in closer proximity to
the better instability gradient just north of the warm frontal
boundary. This would also seem to fit better with the conceptual
model for heavy rain events. Therefore, farther north, the lack of
instability may result in less efficient rain rates with more
stratiform type rains, especially today into this evening.

With time, the surface warm front is expected to make northward
progress into northern Illinois on Sunday This should allow for
much better chances for thunderstorms across the area, with a
better potential for very heavy rain rates. There is also some
concern for severe thunderstorms Sunday afternoon and/or evening
as a cold front sweeps eastward over the area. While there are
questions as to the extent of the instability with cloud cover and
morning thunderstorms over the area, the strong wind field with
the storm system could easily support severe thunderstorms should
they develop ahead of the approaching cold front. This potential
will have to be watched.

The threat of heavy rain and any severe threat should come to an
end from west to east Sunday evening. As a result, the flash flood
watch may be allow to end earlier than the 12Z Monday time.

Temperatures today will be quiet cool north of the warm front to
our south. Expect 40s north and possibly near 60 far southeast.
Surface winds are also likely to be quite gusty out of the
northeast through the day. Expect temperatures to warm on Sunday
as the warm front lifts northward over the area. There may end up
being quit a temperature gradient over the area, with 70s south
and 50s far north.

KJB

&&

.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...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...4 PM Saturday to 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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