Area Forecast Discussion
Issued by NWS Chicago, IL

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FXUS63 KLOT 192003
AFDLOT

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
303 PM CDT Fri May 19 2017


.SHORT TERM...
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.

KJB

&&

.LONG TERM...
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.

KMD

&&

.AVIATION...
For the 18Z TAFs...

A rather active weather period is expected across the area over
the next 30 to 36 hours as a storm system impacts the region. The
main aviation weather concerns are:
1.) The CIGs, with low MVFR through this afternoon, but IFR
conditions of 600-800 feet likely on Saturday.
2.) The threat for a period of thunderstorms Saturday morning, and
again possibly late Saturday afternoon or evening.
3.) The timing of the passage of the warm frontal boundary, and
associated wind shift Saturday afternoon.

Another 1 to 3 hour period of light showers can be expected at
the eastern terminals early this afternoon. Otherwise, expect low
clouds of 1,000-1500 feet AGL to be the rule throughout the rest
of the day.

An area of low pressure is forecast to shift north-northeastward
from the Plains this afternoon, to far eastern Iowa and
northwestern Illinois by late Saturday afternoon. As this occurs,
an attendant surface warm frontal boundary will be forced
northward into northern Illinois by late Saturday afternoon. There
are questions as to how quickly this warm frontal boundary may
move across the Chicago area terminals, and hence when the wind
shift from an easterly to a southerly direction will occur. There
is a possibility of this frontal passage not occurring until
sometime Saturday evening. As a result, easterly winds may be
favored for much of the day Saturday, with low CIGs again likely.
CIGs will likely fall into the IFR range Saturday morning, and
these conditions are likely not to improve much until the warm
frontal passage occurs.

As the storm system begins to approach the region tonight into
Saturday, thunderstorms are likely to develop to our southwest
over Missouri tonight. Some of these thunderstorms appear likely
to move across the terminal sites during the early to mid morning
hours on Saturday. For this reason, I added a tempo for such from
13 to 16 Z. Otherwise, it appears there will be a dry period
following this morning activity, with only low clouds being the
issue into the afternoon. Addition scattered thunderstorms may
develop again over the area late Saturday afternoon or evening,
and these storms could impact the terminals. However, confidence
at this time is a bit low with these later afternoon storms being
far enough north to impact the terminals.

KJB

&&

.MARINE...
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
levels.

KMD

&&

.LOT WATCHES/WARNINGS/ADVISORIES...
IL...None.
IN...None.
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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