Area Forecast Discussion
Issued by NWS Chicago, IL

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FXUS63 KLOT 291556

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
1056 AM CDT Sat Apr 29 2017

1040 AM CDT

Showers are starting to form and spread along and south of I-80
late this morning. The warm front lies over central IL and IN,
and the latest RAP analysis shows that PWAT values are slowly
increasing. However, as suspected, the convection to our south is
stunting moisture transport into the region.

Thinking showers and a few thunderstorms will form and cover the
region by the mid afternoon. Have low confidence in how widespread
thunderstorms will be, but with the wave moving through and
guidance continuing to show elevated CAPE, kept at least a chance
of thunderstorms across the region. The best chance for
thunderstorms will be along and south of I-80. Many models feature
convective reflectivity in their forecasts so I went ahead and
bumped the QPF up slightly as a convective line shifts east. To be
fair, the RAP is not as excited about convection as many other
models are, but it seems to be the odd man out. Evening convection
may limit precip overnight, and will be addressing that through
this afternoon.



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

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.



343 AM CDT

Monday through Friday...

The large upper low and the attendant surface low will continue
gradually shift towards the Upper Great Lakes on Monday into
Monday night. Rap around moisture will likely result in some
additional light showers over the area on Monday before ending
Monday evening as the storm system finally shifts out of the
region. It appears that much of the extended period will be mainly
dry, though another storm system could try to produce
precipitation in the area around mid week. But at this time we
have maintained a primarily dry forecast. Otherwise expect
temperatures to be on the cool side, mainly in the lower 60s.



For the 12Z 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.

Dry conditions continue across the terminals, and are expected to
continue through late morning. Based on latest radar trends, start
time for showers still on track later today. Precip to the south
will continue to lift north northeast through this afternoon,
with showers becoming possible by midday. Should see an increase
to more moderate rain by late this afternoon into the early
evening and its during this time that there could be some thunder
around the terminals. If thunder were to occur today, its during
this time frame that it would have the highest possibility.
Confidence still low at this time though, and have excluded
thunder in the tafs. Light rain or drizzle then likely to continue
for much of tonight. Additional chances for showers and maybe a
thunderstorm will occur Sunday morning. Scattered MVFR ceilings
remain and will likely periodically be observed today, slowly
lowering this afternoon and evening with IFR becoming possible. No
change in wind direction for much of the period, with north
northeast winds continuing and increasing in speed today.



437 AM CDT

A period of strong winds is expected across the lake today
through Sunday night, with gales appearing likely over much of the
lake and nearshore waters. In the near term, high pressure is
building just to the north/northwest of the lake, while low
pressure develops across the southern Plains. A tightening
gradient along with cooler air over the lake is allowing winds to
steadily increase this morning. Increasing speeds towards 15 to 25
KT and to 30 KT are occurring, and expect to 30 KT winds to be in
place over much of the lake here this morning. This increasing
trend will continue to occur throughout the day, with higher
confidence now in place with gale potential this afternoon through
tonight across the south half and nearshore waters. With this
higher confidence, did issue a Gale Warning for the IL/IN
nearshore waters and south half for this period. Although winds
will remain elevated tonight, speeds should fall below gales late
tonight. However, as soon as these winds diminish in these
locations, expected speeds to possibly reach gales over the north
half on Sunday through Sunday night. Have not made any changes to
the current headline in place, keeping the Gale Watch going for
this potential.



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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