Area Forecast Discussion
Issued by NWS Chicago, IL

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FXUS63 KLOT 280724

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
224 AM CDT Wed Sep 28 2016

218 PM CDT

Through Wednesday...

Round 1 of the airmass change occurred Sunday night as we
transitioned from our maritime tropical to our continental polar
air mass, but conditions have been fairly benign from a cloud and
precipitation perspective. Up to this point we have felt the wind
impacts and a cool down to more seasonal temperatures, but round 2
is on its way as it will now look and feel a bit more fall-like
Wednesday. Deep upper level low pressure continues to spin across
Lake Superior with an extensive cloud shield associated with it.
Most of the precipitation with the low is still in northern and
central Wisconsin.

Model guidance suggests the cloud shield will fill in this
evening, with precipitation increasing more so late this evening
and overnight as the upper low moves overhead. It is during this
time that rain/drizzle will become a bit more widespread.

On Wednesday the upper low will pull to the east which will,
generally, decrease shower coverage, but expect a generally cloudy
day all in all. Have high temperatures near 60/low 60s. Upstream
in Wisconsin highs have barely gotten out of the 50s. We will not
be that cold as the core of cold temperatures will not be on top
of us during the day, but some highs in the upper 50s are not
unreasonable. The caveat to the decrease in rain is with the low
in the area, coupled with a cold airmass moving over the warm
lake, rain showers will likely pivot around the back side of low
with an uptick in the afternoon as low level winds/convergence
favor increase lake effect parameters across northwest Indiana and
northeast Illinois. Have maintained the waterspout mention along
with some chance of thunderstorms close to the lake as lake
induced equilibrium levels allow parcels to cross the charge
separation region.



322 PM CDT

Wednesday night through Tuesday...

The pesky upper level low pressure will continue to be the main
driver of our sensible weather into the weekend before finally
pivoting northeast away from the region on Sunday. This will mean
cool and periodically showery weather, especially for northeast
and east central Illinois and northwest Indiana. The upper low
will drive fairly far south into the Ohio Valley on Wednesday
night into Thursday, keeping shower chances focused farther east
(mainly along/east of I-55), including the lakeshore where
impressive lake induced thermodynamics will be in place much of
the time. Kept a slight chance of thunder near the lake into
Wednesday evening. With cold temperatures aloft and resulting
steep low level lapse rates and minor SB CAPE Thursday afternoon,
added a slight chance of thunder. The upper low will then
retrograde north-northwest Friday into Friday night, spreading the
chance for scattered showers farther west, with again a slight
chance for isolated thunder for areas mainly east of I-55.

With the lake effect parameters remaining quite favorable and
persistent onshore flow into Saturday, will need to monitor for
enhanced low level convergence and potentially heavy showers at
times near the lake.

Afternoon temperatures will be several degrees warmer on Thursday
given the farther south upper low position, then cool at least a
few degrees on Friday. For now, stuck with the blended
initialization on Saturday, but the most recent guidance suggests
potentially similar conditions to Friday, suggesting cooler
temperatures and slightly higher PoPs than in current forecast
could be in store.

For Sunday through Tuesday, the departure of the upper low Sunday
will enable temperatures to moderate with lower shower chances.
Monday and Tuesday will be primarily dry with warm advection and
mid-level height rises, ahead of the next mid/upper trough and
associated surface system that may affect the area Tuesday night
into Wednesday, beyond Day 7.



For the 06Z TAFs...

1233 am...Forecast concerns include wind directions...cigs and
potential for showers.

Northerly winds early this morning may turn light northwest or
even variable for a time overnight before the gradient begins to
tighten toward dawn when wind directions will become northerly.
With time winds should be north/northeast mid/late morning and
then likely remain north/northeast for the rest of the period.
However...its possible winds could stay north or turn back to
north/northwest at some variability in wind directions
is possible. Speeds will increase to 10-15kts later this morning
and then continue to increase this afternoon with gusts into the
mid 20kt range possible...but confidence regarding how strong wind
gusts will become is low.

Light showers from southeast WI back toward the Quad Cities will
rotate across the terminals overnight into the morning hours...but
this precip should remain light and possible if it remains more
scattered...some areas could stay dry. Opted to continue mention
of light showers in this forecast later this morning for now.
Additional more convective showers will possible this afternoon
into early this evening but coverage is uncertain and for now just
vicinity mention.

Cigs will be problematic as they are currently vfr and some
guidance is now suggesting nothing more than high mvfr for the
Chicago terminals. Cigs to the north are generally mvfr and with
shower activity not expected to be too heavy...removed ifr
mention but trends will need to be monitored...especially once
winds strengthen and come in from the lake. Eventually...any lower
cigs should lift back to vfr by late afternoon/early evening. cms



224 am...An area of low pressure over northern Lake Michigan will
move south and merge with a second area of low pressure over
central Illinois. This combined low will slowly move southeast to
the Ohio Valley tonight into Thursday. A strong ridge of high
pressure will build across the upper midwest and Ontario tonight
and Thursday and this will tighten the gradient across the lake...
especially southern Lake Michigan tonight when gales are possible.
The low is expected to move back west Friday and Saturday...
centered over Indiana. This will maintain a strong gradient across
the lake with northeast winds to 30 kt expected for a prolonged
period...likely through Friday night. The gradient will slowly
weaken as the low drifts northeast across the eastern lakes
Saturday night into Sunday.

This low pressure combined with colder air aloft will allow the
potential for waterspouts to develop through Thursday morning...
ending from north to south this afternoon through Thursday
morning. cms


     UNTIL 7 PM Wednesday.

     Gale Warning...LMZ740-LMZ741-LMZ742-LMZ743-LMZ744-LMZ745-LMZ777-
     LMZ779...7 PM Wednesday TO 3 AM Thursday.

     Small Craft Advisory...LMZ740-LMZ741-LMZ742-LMZ743...1 PM
     Wednesday TO 7 PM Wednesday.




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