Area Forecast Discussion
Issued by NWS Chicago, IL

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FXUS63 KLOT 200552

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
1252 AM CDT Sat May 20 2017

303 PM CDT

Through Saturday...

The main focus in the short term is with thunderstorm chances and
timing on Saturday and also with temperatures on Saturday as we
await the arrival of the surface warm front.

The main change to the going forecast for Saturday was to lower
temperatures over far northern Illinois (roughly along and north
of I-80). Outflow from early morning thunderstorms (discussed
below) may help slow/push the effect warm frontal boundary a bit
farther south early Saturday morning. This may then result in a
slower northward shift of the warm frontal boundary into northern
Illinois Saturday afternoon. As a result, the front may not make
it into northern Illinois until Saturday evening. With this in
mind, it is likely that many areas in northern Illinois will
continue with low clouds and a cool east-southeasterly wind off
the lake. High temperatures may therefore struggle to get into the
low 60s far north. However, south of the front, expect
temperatures to climb well into the 70s to near 80 over far
southern portions of the area. So, there may end up being a tight
gradient in temperatures across the area Saturday afternoon.
However, this gradient may need to be tightened up more than the
updated forecast shows.

As for thunderstorm chances it appears that convection will
develop tonight along the nose of a developing low-level jet to
our southwest over portions of Missouri. This activity is likely
to shift northeastward and impact much of northern Illinois and
northwestern Indiana during the morning on Saturday. Given the
high moisture content to the airmass shifting over the region
Saturday morning (PWAT`s around 1.5"), these storms could be some
effect heavy rainfall producers for a period in the morning. As a
result, rainfall amounts over a half inch will be possible, with
isolated areas likely to exceed an inch.

The thunderstorm activity should gradually shift east-northeast
of the area by late morning, likely resulting in a period of
precipitation free weather into the afternoon, albeit cloudy. As
the surface warm front shifts northward into my southern counties,
some breaks/erosion of the low clouds will be possible, which
would help warm things up and destabilize the atmosphere in my
south. This may result in additional thunderstorm development
during the mid to late afternoon hours, and into Saturday evening.
Some of these storms could have a threat of becoming severe,
especially across my southeastern counties. A lot of this severe
potential, however, will depend on the amount of scattering cloud
cover and the associated destabilization that occurs within the
warm sector. As a result, there is still uncertainty on the
extent of severe storms in the region.



221 PM CDT

Saturday night through Friday...

Ongoing shower and thunderstorm activity is the leading focus,
with strong to severe thunderstorms possible during the evening
hours. The severe potential hinges on the northward extent of the
warm frontal boundary.

There is a secondary theta-e surge during the evening hours which
will likely lead to a reinvigoration of showers and thunderstorms
ahead of multiple frontal boundaries. At this point the better
chances are along and east of interstate 57 with a collocation of
strong deep layer shear and a better chance to tap into available
instability with more guidance getting these areas into the warm
sector. A more marginal threat exists farther east with
instability concerns, but is non zero given the strength in the
shear parameters.

The big closed upper low will pull away from the region to the
north overnight into Sunday, though its impacts will be felt again
later in the extended period. Guidance is in good agreement with
showers and thunderstorms ahead of the systems cold front to exit
east of the area overnight and into Sunday morning.

A strong mid level dry slot will spread overhead into Sunday which
will keep shower chances very low, and we should begin to see the
sunshine return. But the roller coaster temperature ride will
take another dip downward as cold advection continues in wake of
the front, and with high pressure across Missouri and a modest
surface low still in place across the northern Great Lakes, expect
breezy west winds and readings back in the 60s.

Downstream blocking will prevent the upper low from departing the
Great Lakes region into mid week. Expect some temperature
recoveries on Monday with a brief period of southerly flow as a
secondary low will likely develop in the region. Additional
energy will wrap around the back side of the low bringing several
chances for showers as early as late Monday, with additional
chances Tuesday where thunderstorms are ahead of a cold front. The
upper low will pass through on Wednesday for what would be a cool
intermittently showery day.

Beyond that time frame, low level warm advection behind the
finally departing low coupled with northwest flow aloft suggest a
pattern favorable for additional precip chances late week, but
with temperatures slowly headed back near normal, though some
uncertainties here as well as the ridge to our west does flatten
some late week.



For the 06Z TAFs...

Complex forecast the next 24-30 hours as slow moving storm system
affects the region with at least periodic showers and perhaps
isolated thunderstorms along with low CIGS. One batch of showers
over central IL just before 0600 UTC was weakening fairly quickly
as it moves northeast into the drier and more stable airmass. Not
out of the question there could be a brief period of -RA at the
terminals overnight, but given the rather quick decay of this
precip felt the VCSH more than covered the potential.

CIGS have been steadily lowering from the south with MVFR CIGS
over Chicago terminals likely to gradually lower to IFR. IFR CIGS
are close (as near as KJOT and KC09), but northeast winds blowing
in drier air may delay the arrival of the IFR until after sunrise.
Confidence is low, but felt previous TAFs solution looked
reasonable and certainly in the ballpark wrt to timing, so didn`t
make any big changes.

Large batch of showers and thunderstorms over MO and the southern
plains will likely continue to march NE toward the area. Simple
extrapolation has the leading edge reaching the terminals in the
1300-1400 UTC time frame, but guidance is slower and given the
somewhat drier/more stable air mass the slower solution could pan
out. Nudged the timing of the steady rain back just a hair, but
ultimately this timing could need to be nudged a bit later in
future updates. This batch of organized rain should push east of
the terminals during the early afternoon, but IFR CIGS and perhaps
some drizzle/spotty showers likely to persist in the afternoon as
warm front lurks just south of the terminals.

Seems likely that the early morning rain/isolated t-storms should
lessen instability and makes the prospects of additional
thunderstorms development in the afternoon quite low. The best
chances would be east and maintained the PROB30 for thunder at
GYY, but suspect any afternoon storms would probably develop even
farther east than GYY.

Warm occluded front should move across the terminals early
Saturday evening resulting in a wind shift to southerly and
scouring of low CIGS and VSBY.

- Izzi


221 PM CDT

North-northeasterly full fetch winds will veer more
easterly tonight as high pressure across Lake Superior will shift
east across Ontario, while low pressure from southern plains will
lift northeast across Iowa and into Minnesota Saturday. Therefore
the highest waves across southern lake Michigan will ease some
later tonight into Saturday, though remain elevated through the
day. The low will lift a warm front across the lake across
portions of the lake and allow winds to shift more southerly, at
least for the southern half. This low will only slowly kick off to
the east, with a secondary low expected to approach the lake on
Tuesday. This low will send another sharp cold front south down
Lake Michigan through the day and evening Tuesday, bringing in
breezy north-northwest winds which could approach gale force



LM...Small Craft Advisory...IN nearshore waters until 3 PM Saturday.

     Small Craft Advisory...IL nearshore waters until 9 PM Saturday.




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