Area Forecast Discussion
Issued by NWS Chicago, IL

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FXUS63 KLOT 250808

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
308 AM CDT Tue Apr 25 2017

259 AM CDT

Through Wednesday morning...

Dry colors on water vapor characteristic of our pleasant spring
weather of late are slowly being replaced by the cool moisture
laden colors upstream of the midwest, with expansive upper level
low pressure making its way into the heart of the western United
States. Moisture return is also increasing across the Ohio valley
and advecting northeastward. Meanwhile, there is still quite a bit
of lower level dry air in place in our area to support another
fairly nice and warm spring day today along with weak ridging
still holding in our region due to the slow moving closed upper
low along the east coast. 925/850 temps do not change dramatically
today, but with the higher starting point we could add a few
degrees even with some more clouds. Southeast winds will again
confine lake cooling to the northeast Illinois shoreline where
highs reach the 60s instead of the inland 70s. With the upper
level moisture encroaching with a wave currently across Iowa this
morning, expect a bit thicker higher cloud shield this afternoon,
but no real precipitation even though hi-res reflectivity depicts
some decaying echoes.

Low level moisture transport will increase this evening ahead of
the low on a southwesterly low level jet. Shortwave energy will
track through the region and likely clip the western portion of
our area tonight into early Wednesday. There will be some weak
elevated instability across north central Illinois where some
storms are possible, and while the thunder potential farther east
is non-zero, with weaker instability/forcing farther east these
may just be some showers farther east Wednesday morning.



259 AM CDT

Wednesday afternoon through Monday...

The upper jet feeding into and out ahead of the deep upstream low
should gradually increase the lift out ahead of the system`s cold
front. Most guidance has been a little slower with any development
ahead of the front tying any precipitation closer to the front.
Instability will likely increase ahead of the front with dewpoints
getting to near 60 in our southern zones, though guidance does
show some decrease farther north. The forcing is still stronger
to our west in the afternoon, but still some shortwaves moving
northeast, and there could be some lingering outflow boundaries
from any morning activity to our west which will warrant a
scattered storm mention in the afternoon. If instability is
greater, there would be a severe threat given the shear profiles
with damaging winds and possibly some large hail with any discrete
storms. And while the tornado threat is probably better south of
the area, the threat may not be zero in our southern zones. This
evolution is the general idea depicted by the NAM/GEM/NAMNest/EC,
while the GFS is a bit more progressive with the front which
could suggest greater coverage of storms in the afternoon time

In addition to any discrete storms, several hi resolution sources
suggest the development of a line of thunderstorms in the late
afternoon, and more so in the evening time frame. With instability
weakening this would suggest heavy rain would be the greater
threat as the upper flow becomes more parallel to the front and if
this later time frame suggested by numerous sources plays out,
but there would be a wind damage threat given any earlier onset to
storms and depending on how organized the line is this far north.
Some lingering cloudiness would then result with the upper low
still to move through, but moisture will decrease somewhat
quickly, and we will be left with a much cooler day area wide on

The upper flow will re-amplify behind this first low later
Thursday into Friday. Friday and the weekend still appear to have
periods of shower activity, but the trend of guidance has been to
keep the cold frontal boundary positioned to our south most of
the period, but at times shortwave features in the southwest flow
ahead of a deep upper level low will attempt to bring surges of
moisture farther northward. With the main front to our south, it
appears the best moisture transport will have a harder time
getting this far north, other than occasionally through the
weekend, but there will be several waves of precipitation, with
the most widespread and more moderate to potentially heavy rains
coming Saturday night to Sunday night with the approach of the big
upper low and the northward shifting warm front, and then the
system`s cold front which will pass through Sunday night. It is
this Saturday night-Sunday evening time frame that poses the best
thunder chances as the theta-e surge increases significantly.

While model discrepancies certainly exist, the current consensus
model suite would suggest a wave of precipitation Friday
afternoon and diminishing in the evening, and this looks to be
light. Then there will likely be a lull or much lighter rain
Friday night into sometime Saturday as the main front shifts
north. GFS says as early as morning, EC is more so afternoon time
frame. Then comes another surge late Saturday night into Sunday.
Then the last hurrah comes with the front late Sunday, with the
best thunder chances Sunday.



For the 06Z TAFs...

1228 am...Southeast winds will remain around 10kts for much of
the early morning hours and then increase into the 12-16kt range
later this morning with higher gusts developing. Winds will turn a
bit more to the south/southeast later this morning as well. Gusts
will diminish this evening with speeds remaining around 10kts
tonight into Wednesday morning.

Scattered high clouds to start will likely lower to a bkn mid deck
later today with few/sct cu possible. Cigs are expected to lower/
develop into the 4-6kft range tonight. While vfr is expected
through the period...some patchy/shallow ground fog will be
possible in the usual locations through sunrise this morning and
again early Wednesday morning.

A few showers may be approaching northwest IL/rfd toward the end
of the current forecast and then possibly spread across northwest
IL early Wednesday morning...after the end of the current taf. cms



216 am...Low pressure over MN this morning will slowly dissipate
today leaving a trough extending south to a second stronger low
over the southern plains. This low will move northeast to the
western lakes region Wednesday evening and then slowly lift
northeast into Ontario Thursday. Southeast winds today will turn
more southerly Wednesday...though there may remain easterly winds
over far northern portions of the lake. As the low departs...there
could be a period of southwesterly winds to 30 kts on Thursday
and continuing Thursday night over northern portions of the lake.
A warm frontal boundary will move into the southern lakes region
Friday and remain stationary into Saturday. Low pressure is then
expected to develop over MO Saturday night and move across the
lakes region Sunday and Sunday night...lifting the warm front
north. cms





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