Area Forecast Discussion
Issued by NWS Chicago, IL

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FXUS63 KLOT 172015

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
215 PM CST Fri Nov 17 2017

957 AM CST

No significant changes to the current forecast this morning into
this afternoon. As was mentioned previously, any clearing of the
low stratus will be replaced by mid and high clouds this morning.
Any sunny skies this morning in east central IL and northwest IN,
will be replaced with this cloud cover by midday. Only weak and
isolated returns noted on radar at this time, and with forcing
expected to remain initially weak in the near term, expect this to
continue. Focus/forcing still looking to ramp up later this
afternoon into the evening, with rain increasing in coverage
across the area. While instability still anticipated to increase
overhead, embedded thunder will remain a possibility during this
time. However, severe weather is not expected.



238 AM CST

Through tonight...

High pressure overhead last evening has shifted to the eastern
Great Lakes early this morning, which will make way for a bit more
active fall weather in the short term. But once we get through
the next few days, quieter and cool weather will prevail for the
upcoming holiday week.

The upper air pattern is characterized by a trough in the east,
one in the west, a compact shortwave across Manitoba, with a
ridge over Mexico. A fast flow pattern aloft between the high and
the western low will be aimed at portions of the midwest Friday
into Saturday. The upper jet will steer the western low to the
central Rockies tonight. The warm advection pattern behind the
departing high will begin earnest today as a surface low will
organize. 9-10 mb 6 hr pressure falls will allow winds to pick up
fairly quickly. Even with the clearing lower clouds, we will still
have some higher clouds shifting in. Given the strength of the
WAA and the stronger upstream upper forcing, expected precip to
develop to the west today, then with increased wind/moisture
convergence, coverage should grow later in the day. SE winds will
maintain drier low levels initially, but better coverage is
expected by early evening hours with the approach of the upper jet
and strengthening surface low. There is still a decent signal for
isolated thunderstorms with elevated CAPE above a lower level
inversion, and the strong WAA may be able to tap into the
increased instability.

NAM shifted south from its previous runs, somewhat considerably,
but is considered an outlier by WPC given that the GFS ensemble
mean is farther north of the remaining model consensus. Developing
deformation should lead to at least a brief snow mix in the cold
advection regime ahead the strengthening low. There appears to be
a secondary surge in precipitation as the upper trough approaches,
and the deformation/frontogenetical circulation sets up. Initial
concerns when we have the stronger precip rates Saturday morning
that snow could become dominant for a time. Model QPF Saturday
morning is between 0.2 and 0.6 inches across north central IL,
with the exception of the NAM. At this time the thermal profiles,
and the low level energies do suggest more rain the snow, but this
will be a period to watch for some brief wet accums on grassy
surfaces. The lift eases in the afternoon as the colder air gets
pulled in from the northwest where there could be some snow
falling, but accums would be negligible if you believe most
guidance, but there may be a little window for some light accums
again in the afternoon as temperatures continue to fall, with the
surface temps beginning an initial deterrent to accumulation.



211 PM CST

Saturday night through Friday...

Primarily dry conditions are expected throughout the extended time
frame, but temperatures will be quite variable as a series of lows
move across the northern Great Lakes region.

Saturday night, deep area of low pressure will be over the central
and eastern Great Lakes region with precipitation now east of the
local area but cold air arriving behind the cold front. 850mb
temperatures will bottom out around -10C Sunday morning but the
thermal trough will quickly shift off to the east with
temperatures moderating through the day Sunday. Strong subsidence
behind departing 500mb trough axis will overspread the area
through the day. Forecast soundings indicate stratus will be in
place early in the day Sunday, but subsidence helping to dry the
column and surface ridge axis building in from the west may allow
some sunshine later in the day.

Monday, another vort is progged to dig across the Canadian
Prairies and reach the Great Lakes region midday Tuesday.
Attendant surface low will stay over Canada, and precipitation
remains favored north of the local area so will maintain a dry
forecast through this time frame. Southwest flow out ahead of the
wave will help temperatures further moderate, reaching back into
the mid to upper 40s Monday and Tuesday. Expect a reinforcing
shot of cold air to push across the region Tuesday evening and
Tuesday night putting the area back into a deep freeze Tuesday
night with colder air remaining in place through early Friday.
Another clipper-like system will take a similar though slightly
more southerly track across the region Friday into Saturday with
temperatures undulating once more. This system may also provide
the next decent chance for precipitation in the area.



For the 18Z TAFs...

Complicated forecast, as approaching system will provide several
periods of rain that could include thunder late this
afternoon/early evening, as well as a possible rain/snow mix
Saturday morning. Varying winds and speeds are likely, as well as
IFR/LIFR ceilings and reduced vis under drizzle/fog tonight.

In the near term, overall dry conditions are in place. However,
beginning to see an increase in coverage of light rain over
portions of north central and northeast Illinois. Although
coverage will likely remain lower, this steady progression of the
current rain will likely move across all the terminals this
afternoon. As this rain moves overhead, would not be out of the
question to observe a very brief period of sleet, however, don`t
anticipate any impacts. As southerly winds remain elevated this
afternoon, VFR ceilings will lower to MVFR. This lowering will
continue this evening as ceilings fall to IFR, especially with the
arrival of more widespread rain. Late this afternoon and early
evening, some embedded thunder will be possible but with
confidence low, have not included it in the TAF. Low ceilings and
possible lower vis under rain/drizzle/fog are likely tonight. As
the system pushes through late tonight, winds will turn to the
northwest and then north. A lull in precip is then possible but
expect precip to move back overhead by mid morning. While colder
air is pushing overhead, its possible for a rain snow mix before
this precip departs during the day. At this time, highest chances
of this occurring are for RFD and have included a PROB30. Will
need to monitor for possible rain/snow for the other terminals.



211 PM CST

A strong and deepening low will lift across the Great Lakes region
resulting in a period of southerly gales today followed by stronger
high-end northerly gales Saturday into Saturday night. Winds look
to peak at around 45kt midday Saturday as the surface low tracks
east across the central Great Lakes. Gales should end across the
lake sometime early Sunday morning as the low pulls away across
Quebec and high pressure builds across the Great Plains. Surface
ridge will settle over the southeastern states Monday while
another low deepens over the Canadian Prairies. The low will track
across Ontario into Quebec on Tuesday with gales once again
possible across Lake Michigan Monday into Tuesday.



LM...Gale Warning...LMZ777-LMZ779 until 9 PM Friday.

     Gale Warning...LMZ740-LMZ741-LMZ742-LMZ743-LMZ744-LMZ745-LMZ777-
     LMZ779...9 AM Saturday to 6 AM Sunday.

     Small Craft Advisory...nearshore waters
     until 3 AM Saturday.




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