Area Forecast Discussion
Issued by NWS Chicago, IL

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FXUS63 KLOT 171815
AFDLOT

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
1215 PM CST Fri Nov 17 2017

.UPDATE...
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.

Rodriguez

&&

.SHORT TERM...
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.

KMD

&&

.LONG TERM...
238 AM CST

Saturday afternoon/night through Thursday...

The focus after this will be a quick cold shot, but more
importantly a significant increase in winds that will ensue
through the evening Saturday and make it feel somewhat
uncomfortable not only beginning Saturday late morning, but also
for the remainder of the weekend. Winds may reach 35-40 mph midday
Saturday which will add some bite to falling temps through the
afternoon. There may be some weak precipitation possibly
freezing drizzle or very light snow with a trailing wave in the
evening/overnight but otherwise most guidance is dry. After cold
lows in the 20s, northwest winds will maintain quite cold highs on
Sunday in the low to mid 30s as a surface ridge expands to our
west, while the still deepening low will make headway to the
eastern Great Lakes.

We get a good bounce back on Monday as another low enters the
Canadian prairies, with highs nearing 50, but the roller coaster
continues with another cold front very late Monday or Tuesday
giving Tuesday highs a bit of a dip. This front is still dry on
most guidance, other than lake effect snow on the east side of
Lake Michigan. Sprawling high pressure will continue dry
conditions Wednesday into the Thanksgiving holiday, but it will
remain on the colder side with high in the 30s and lows in the 20s
Wednesday, a bit warm on Thanksgiving Day.

KMD

&&

.AVIATION...
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.

Rodriguez

&&

.MARINE...

143 am...Southerly gales are expected today as high pressure
moves east and a tight pressure gradient moves across Lake
Michigan. Winds will diminish this evening ahead of a cold front
and then shift northerly by Saturday morning behind the cold
front. Another period of gales is expected from late Saturday
morning through Sunday morning...likely to 45kts on the south end
of the lake. These gales will slowly diminish from north to south
Sunday morning with a ridge of high pressure moving across the
lake Sunday afternoon. Winds will shift southwesterly Sunday night
into Monday as low pressure moves across Ontario. Another period
of gales is possible from Monday afternoon into Tuesday morning
and then a large area of high pressure is expected to move across
the region for the middle of next week. cms

&&

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