Area Forecast Discussion
Issued by NWS Chicago, IL

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FXUS63 KLOT 191802

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

1027 AM CDT

A convectively enhanced mid-level disturbance, currently noted in
the water vapor and radar imagery entering my southwestern CWA,
is driving an area of rain and some storms across the area this
morning. A majority of the thunderstorms have confined to
southern portions of the area. Overall, expect this band of
showers/storms to continue to move east-northeastward into far
northern Illinois and southern Wisconsin though the late morning.
A gradual break in the precipitation is likely in most areas
south of I88 late this morning. While some occasional showers may
shift back over the area for a period early this afternoon, it
appears that shower chances will diminish across the area late
this afternoon as the parent mid-level disturbance/MCV shifts
east of the area.

Otherwise, expect cloudy, breezy and very cool conditions through
the rest of the day.



236 AM CDT

Through tonight...

Primary short term challenge is with precip chances through
tonight as well as temps today.

Classic lake charged cold front came barreling through much of
the CWA Thursday evening accompanied by some pretty dramatic
temperature drops, particularly closer to Lake Michigan. Marine
enhanced chilly air mass is establishing itself across the area
early this morning and is likely to set the stage for an
unseasonably chilly day today, particularly north of I-80. Given
the anticipated cloud cover, likelihood of some rain, and the
brisk northeast winds, not anticipating much recovery in temps
today north of I-80, where much of the day could be spent around
or even a bit below 50 degrees. Far southern and southeastern CWA
stand should make it into the 60s.

Precipitation chances today are tricky and result in below average
forecast confidence. Shortwave trough with multiple apparent MCVs
is quite apparent on regional radar mosaic moving across IA and MO
early this morning. This shortwave is expected to continue moving
northeast this morning but is progged to weaken as it runs into
narrow NW-SE oriented upper ridge axis. Large convective complex
extending from IA south through MO has been steadily moving east,
though recent GOES-16 IR imagery and lightning data suggest that
convective complex is begin to weaken over IA and northern MO.
Given the anticipated quick dampening of the shortwave as it moves
into ridge axis and stout northeast winds ushering in dry low
level air mass, it is clear that a continued weakening of the
convective complex will take place as it moves into northern IL.
What is unclear, is how quickly it will weaken and how much (if
any) rain will make it into the eastern CWA. Have largely
maintained pops similar to previous forecast though did bump them
up western and southern CWA where the chances of rain appear
pretty good later this morning.

In the wake of this shortwave, look for mainly dry conditions
overspreading the CWA from the southwest this afternoon with at
least the first half of tonight looking primarily dry over most of
the CWA. Next, likely convectively enhanced, shortwave is expected
to bring an increasing threat of showers and perhaps some t-storms
back into the CWA later tonight.

- Izzi


236 AM CDT

Saturday through Thursday...

Intermittent showers and some thunderstorms look likely Saturday
as the upper low over the Rockies lifts out with the entire upper
trough becoming negatively tilted Saturday. Result should be a
strengthening surface low tracking north into IA Saturday which
should result in warm front being drug north across our CWA with
MUCH warmer and more humid air spreading north in the wake of the
front. If atmosphere were to recover from prior convection and
destabilize, then wind fields would certainly support a threat of
strong to severe t-storms Saturday. Best chance of sufficient
instability materializing for a severe threat looks to be over our
SE CWA, which is well depicted in the latest SWODY2.

Sunday looks to be the drier of the weekend days, thought gusty
westerly winds and cooler temperatures are expected in the wake of
the cold front. Monday should be windy and milder ahead of another
system progged to drop in from the northwest Monday night as
strong shortwave is progged to pivot SE around the western
periphery of the upper low. Somewhat chilly (by late May
standards) and unsettled conditions are then expected to set Tues
through Thurs with at least periodic shower chances and highs only
in the 60s.

- Izzi


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.



256 AM CDT

High pressure building over the lakes will support north to
northeasterly flow across Lake Michigan today. Waves along the
southerly shores at the end of this long fetch will pose hazards
for small craft through the early part of the weekend but should
begin to subside late Saturday as winds turn east and then
southeast. A front pushing across the lake Saturday night will
turn winds southwesterly for the remainder of the weekend.



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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