Area Forecast Discussion
Issued by NWS Chicago, IL

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FXUS63 KLOT 181929
AFDLOT

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
229 PM CDT Sun Jun 18 2017

.SHORT TERM...
228 PM CDT

Through Monday...

Stratocumulus clouds in the cold advection regime have filled in behind
upper level low pressure to our northeast. West winds will
continue gusting to 25-30 kt as a fairly tight pressure gradient
sits over northeast IL with a fairly deep summer time low over
upper Michigan. There are a few showers upstream, but the cu
field will be fairly shallow and coverage should be low.

Clouds thin tonight and west winds will ease, but expect a
similar situation tomorrow though with a bit lighter west wind as
the main surface low will shift farther northeast across Ontario.
We remain in northwest flow behind upper level low pressure, a
pattern that favors diurnally driven showers, but better shower
coverage potential is expected Monday as several models depict a
modest wave in the flow. A few thunderstorms are also possible,
with these showers forming as early as mid to late morning.
Temperatures look to fall slightly cooler than today`s, with
readings generally topping out in the mid 70s.

KMD

&&

.LONG TERM...
308 AM CDT

Monday through Saturday...

Multiple periods of showers and storms are expected this week. The
upper level trough slowly shifts east early this week then zonal
flow sets up aloft.  Upper level disturbances will ride the flow
right over the region; however, the zonal flow also allows
temperature to rise back into the mid to upper 80s later this week.

Isolated to scattered daytime showers and storms are possible Monday
and Tuesday. I have limited confidence in coverage especially
Tuesday behind the upper level trough. Guidance features minor CAPE
values so kept a slight chance of thunder. Monday will be the cooler
of the two days under the heart of the trough with highs in the mid
to upper 70s.

I think high pressure will lead to a dry Wednesday, but the GFS
suggests a complex may spread over northern IL Wednesday morning. I
think the GFS is overdone especially since the other models have a
much smaller and slower complex that diminishes before reaching
northern IL.  I have higher confidence in a left over MCV and/or
upper level wave triggering showers and storms Wednesday
evening/night.  PWATs are forecast to be 1.75-2 inches, and while
shear values are around 40 kt, CAPE will be the big question.
Elevated CAPE on forecast soundings maxes out at 500-1000 J/kg so
thinking isolated strong to severe storms are possible, and could
see more high rainfall rates. Steering flow is 35-40 kt, so storms
will be moving at a decent speed, but have concerns about training.

Thursday looks like the warmest day of the week with highs in the
mid to upper 80s. I have low confidence in how much convection we
will see Thursday since the atmosphere will have to recover from the
night before. A better bet will be precip Friday.

JEE

&&

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

Concerns:
- Gusty west winds
- Isolated showers and maybe a storm this afternoon
- Scattered showers and isolated storms Monday mid morning on

Expect lower end VFR clouds to fill in a bit more this afternoon
in the cold advection regime behind upper level low pressure to
our northeast. West winds will continue gusting to 25-30 kt as a
fairly tight pressure gradient sits over northeast IL. There are a
few showers upstream, but the cu field will be fairly shallow and
coverage should be low so held off on a TAF mention.

Clouds thin tonight and west winds will ease, but expect a
similar situation tomorrow though with a bit lighter west wind,
and better shower coverage potential as we remain in northwest
flow behind upper level low pressure, a pattern that favors
diurnally driven showers. Several models depict a wave in the flow
that may trigger showers, and even a few thunderstorms, as early
as mid to late morning. Confidence on how widespread coverage will
be is only medium, but feel that there will be better coverage
during this period. Have not added thunder mention yet, but it is
possible they could be warranted in future issuances.

KMD

&&

.MARINE...
231 AM CDT

Fairly typical summer pattern with no significant synoptically
driven wind/wave events over the open lake through the upcoming
week. Does appears as though a more active pattern with periodic
thunderstorms chances will become established by middle and end of
the upcoming work week. Fairly strong westerly winds expected
along the Illinois nearshore waters this afternoon. Appears to be
a borderline SCY set-up, but given the otherwise nice weather on a
summer weekend, opting to err on the liberal side with issuing a
small craft advisory. Stable marine layer should prevent these
strong winds from making it more than a few miles offshore.

- Izzi

&&

.LOT WATCHES/WARNINGS/ADVISORIES...
IL...None.
IN...None.
LM...Small Craft Advisory...Winthrop Harbor to Gary IN until 7 PM
     Sunday.

&&

$$

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