Area Forecast Discussion
Issued by NWS Chicago, IL

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FXUS63 KLOT 231135

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
635 AM CDT Sun Apr 23 2017

303 AM CDT

Through Monday night...

Big bubble, no trouble. With low pressure across the Ohio valley,
water vapor satellite depicting moisture aloft is nice and dry
across our region clear back to Colorado, with surface high
pressure sprawled out from plains of Colorado clear across the
the Great Lakes region. A cold front is found across the upper
Great Lakes with a very dry low level airmass ahead of it.

The high will only slowly migrate east today with the upper level
block still in place, which will deliver another dry and mostly sunny
day. Lower level warming will also support temperatures back up
above normal today, upper 60s to near 70 west. With the weak
pressure gradient, expect a lake breeze this afternoon which will
keep temps at or below 60 lakeside. The cold front will cruise
south down Lake Michigan today with some high cloud cover
spreading ahead of it into our area, but the cooling and thicker cloud
cover will largely remain north of the area. We will still have to
watch as the front will likely sneak into northern/northeast
Illinois across McHenry/Lake counties late this afternoon and
evening. The front will not make much headway south as low level
flow begins to turn southwesterly as the ridge shifts east and
more progressive flow develops upstream. It will be dry and mostly clear
again tonight, with slightly warmer overnight temps.

The first in a series of lows will eject from the northern Rockies
Monday, but the upper flow remains less progressive wit the upper
low over the southeast. More pronounced southerly low level flow
(southeast at the surface) will bring some additional warming, but
still cooler at the lakefront in northeast Illinois. The day will
start sunny with some increase in higher clouds late in the day.



303 AM CDT

Monday through Sunday...

Dewpoints remain very low into the extended, which will limit
precipitation impacts through at least Tuesday evening, but a
significant pattern change will take place with a series of low
pressure systems tracking from the west toward our region, while a
big area of high pressure will take shape across the southeast

Decent model agreement on the low for Tuesday night from the
southern plains midday Tuesday and shifting into central
Wisconsin by daybreak Wednesday, which will bring our first chance
for rain after midnight. There is some weak elevated instability
to warrant keeping a very low t-storm mention, but not a great
thunder setup. Guidance is in better agreement with the cold
frontal timing for Wednesday, bringing it through the heart of the
area in the afternoon. There is decent strengthening of an upper
level jet, actually several jet segments during this time with a
southern and northern stream system, and therefore leading to a
strengthening of a series of lows into one across southern or
central lower Michigan. These forcing mechanisms suggest an uptick
in shower activity, and with a little better instability this also
warrants a little higher chance t-storm mention. Surface based
instability may be hard to come by given the cloudy pattern.

The cold front will push through the area Wednesday night. There
could be some lingering lighter rain as the upper trough axis will
still be shifting through. Weak high pressure will build in on
Thursday which should dry things out, but some lingering clouds
and onshore flow/possible lake breeze will make for cooler
conditions Thursday. The cold front will get shoved well south to
the Ohio valley during this time. With high pressure stretched
out across the northern Great lakes later Thursday into Friday,
much of the region will remain in a cool onshore flow pattern,
possibly with lingering clouds. While there still remain
significant model differences regarding the eventual evolution
and progression of a deep upper low across the west. Elevated
showers and storms can be expected at times during the weekend
with the GFS/GEM having a bit more precip with a potential first
round Friday afternoon. There is better agreement with the EC in
at a least a portion of the sloped warm frontal boundary lifting
through Friday night into Saturday morning where a better chance
of showers and maybe a storm would result. These same models and
the GEM show the main upper low not clearing the region into late
Sunday if not early Monday. Therefore Saturday night into Sunday
is another window where rain is expected, with a slightly better
chance of thunderstorms as well. Model blend pops are bit wishy-
washy given the inconsistencies in timing from the extended
guidance, so tried at least nudge them a bit more in periods of
concern. Expect additional model shifting given the evolution of
deep upper lows can certainly give the models fits at this



For the 12Z TAFs...

635 am...Only concern this period is lake breeze this afternoon.

High pressure over the area this morning will slowly move south
today and tonight. Light/calm winds to start will become light
westerly this morning. The weak gradient will allow a lake breeze
to develop which will likely move through gyy late this morning
and then through ord/mdw early this afternoon. Confidence on
timing is only medium and trends will need to be monitored as the
lake breeze may arrive sooner than currently expected. Speeds will
also need to be monitored as there could be a period of 10kts with
the lake breeze passage.

A cold front will move south across Lake Michigan today and may
reach far northeast IL late this afternoon or early this evening.
Confidence regarding how far south this front will move is low but
if it were to reach the Chicago would shift an
east/southeast wind behind the lake breeze more to the northeast.

There were a few areas of shallow ground fog early this morning
and that may occur again early Monday morning in the usual
locations. Otherwise...dry/vfr through the period with few/sct
high clouds. cms



213 am...Weak low pressure near James Bay early this morning will
move into northern Quebec today. A trailing cold front will move
south across northern Lake Michigan this morning and then into
central portions of the lake this afternoon. There remains some
uncertainty as to how far south this front may push. Its possible
this front may move as far south as the IL/WI state line by late
this afternoon...on the west side of the lake. A ridge of high
pressure over the southern lakes region will slowly sag south
today and tonight.

An area of low pressure will move from the western plains tonight
to Lake Superior it dissipates. A trailing cold front
will extend from the western lakes to the a second area of low
pressure over the southern plains Tuesday. This low will lift
northeast across the western lakes region Wednesday into Wednesday
night. cms





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