Area Forecast Discussion
Issued by NWS Chicago, IL

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FXUS63 KLOT 191143

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

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 12Z TAFs...

Based on latest radar trends and model guidance, opted to start
the VCSH a bit earlier and maintain lower MVFR ceilings through
the morning. A cluster of thunderstorms is lifting toward the area
from Missouri and Iowa, and while the leading edge of convection
has been eroding as the activity encounters the cooler and drier
air in place locally, it may still become necessary to add a
period of thunderstorms or at least VCTS. It appears this activity
should move through the area by mid afternoon.



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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