Area Forecast Discussion
Issued by NWS Chicago, IL

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FXUS63 KLOT 251127
AFDLOT

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

.SHORT TERM...
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.

KMD

&&

.LONG TERM...
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
frame.

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
Thursday.

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.

KMD

&&

.AVIATION...
For the 12Z TAFs...

627 am...South/southeast winds will increase into the 12-16kt
range this morning with gusts into the lower 20kt range. Its
possible that wind directions may turn back more easterly at
ord/mdw this afternoon...but confidence regarding a lake breeze is
low. Speeds will diminish to around 10kts this evening and winds
are then expected to shift more southerly by Wednesday morning
when speeds/gusts will increase again.

Showers and few thunderstorms are expected to develop over eastern
IA this evening and then spread northeast overnight into Wednesday
morning...likely moving across parts of northwest IL. Much of the
model guidance keeps this activity west of the Chicago area
terminals and confidence of how widespread any convection will
become in the rfd area is low. So for now have just included
vicinity mention at rfd.

Vfr through this evening with mid/high clouds along with few/sct
cu today. Cigs will eventually lower to 4-6kft tonight and as low
level moisture increases...mvfr cigs will be possible but
confidence is too low to include more than sct mention. Patchy fog
will also be possible Wednesday morning. cms

&&

.MARINE...

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

&&

.LOT WATCHES/WARNINGS/ADVISORIES...
IL...None.
IN...None.
LM...None.
&&

$$

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