Area Forecast Discussion
Issued by NWS Chicago, IL

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FXUS63 KLOT 251909

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

208 PM CDT

Through Wednesday...

For the remainder of the afternoon and overnight, conditions will be
relatively quiet.  Main forecast concern will be timing and
intensity of tsra expected to move into the region tomorrow.

With low pressure tracking north along the Atlantic coast, a ridge
of high pressure will remain parked from New England through the ern
Gulf Coast.  With deepening low pressure over the central plains and
a trough extending north into the upper Mississippi Valley, low level
flow will be off of the Gulf of Mexico and deep layer moisture will
be on the increase with sfc dewpoints climbing into the lower 60s,
temperatures in the mid to upper 70s and pwats approaching 1.5
inches. With the increasing moisture and instability as well as a
wind profile with a 50 kt low level jet extending into nrn IL ahead
of the approaching cold front, severe thunderstorms will be a
possibility and SPC has extended the Slight Risk for severe
thunderstorms through central IL into the far southern portions of
the Chicago Metro area with a Marginal Risk over the remainder of
the CWA, including the Rockford area and northwestern Indiana.  At
this point, the main severe threat should be for large hail and
strong straight line winds. Through the day, isolated severe storms
could evolve upscale into more linear bow echoes and strong winds
becoming the main severe threat.  However, the progression of the
pattern is looking to become less progressive through the day
tomorrow and heavy rainfall and flooding may become an increasing
concern, especially into the evening hours.


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 18Z TAFs...

Forecast concern initially will focus on winds for the remainder
of the afternoon and through the night and then shift to pcpn/tsra
chances for tomorrow.

With a ridge of high pressure extending from New England through
the central Gulf coast and a trough of low pressure extending from
the central plains to the upper Mississippi valley, winds across
the region have been generally southerly through much of the day.
As the low pressure deepens over the plains, wind direction will
gradually back to sely this afternoon and overnight tonight. With
filtered sunshine, modest mixing will allow for ocnl gusts to arnd
20-25 kt through sunset. The pressure gradient should strengthen
overnight, keeping winds in the 10 to 15 kt range overnight and
then becoming gusty again tomorrow, to arnd 25 kt.

Warm/moist advection will increase through the morning hours and
isentropic lift in the warm sector ahead of an approaching cold
front will bring increasing chances for pcpn/tsra. The window of
opportunity for the greatest amount of ts coverage and intensity
will be through the afternoon and evening hours though there will
be a chance for at least some sct shra/tsra by late morning.
Progression of the frontal system is expected to slow durg the
afternoon and into the evening, so locally heavy rainfall may
contribute to lower visibility than currently indicated in the
TAFs. Also, while widespread winds should be in the 15g25kt range,
localized winds invof tsra will likely be much stronger and of
variable direction. Ocnl mvfr cigs are likely invof tsra, but
should otherwise gradually lower to lower end vfr as moisture
streams into the region.



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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