Area Forecast Discussion
Issued by NWS Chicago, IL
FXUS63 KLOT 271739
Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
1239 PM CDT Thu Oct 27 2016
304 AM CDT
The bulk of the precip has ended for most locations as the surface
low continues to shift to the east early this morning. However,
convergent flow off the lake on the backside of this system is
allowing for some light showers or drizzle to persist. This is
mainly impacting locations in far northeast Illinois and northwest
Indiana, and will likely continue to only impact these locations
for the next 2-3 hours. Focus will shift more into northwest
Indiana over these next several hours before diminishing as a more
subsident environment moves overhead. Cloud cover and temps will
likely be the main issues for today and while high pressure
shifts to the east today, think this low cloud deck will likely
linger slightly longer than what guidance is indicating. Did
increase cloud cover for most of the day, with cloudy skies
expected into the afternoon. CAA and these clouds will likely
affect high temps today, and did make minor changes to reflect
latest trends. Skies should then see a clearing trend later this
evening/overnight and with this anticipated clearing, patchy fog
will be possible.
246 AM CDT
Friday through Wednesday...
Fairly interesting pattern in the long term with strong, largely
zonal, Pacific jet stream setting up across the northern tier of
CONUS providing for a temp roller coaster over the next week. The
peaks of the thermal roller coaster will feature highs WAY above
average (perhaps not far from records) while during the dips in
the temp roller coaster temps will fall back to normal or even a
bit above still.
Friday morning, first northern stream system will be moving across
the northern plains with strengthening southerly flow locally.
Could see some lingering cloudiness Friday morning from the
current stratus deck, but the southerly flow should advect
clearing north into our CWA during the morning. Very impressive
low level warming is forecast to take place Friday, with 925mb
temps progged to warm from 7-12C at 12z Fri to 14-19C by 00z Sat.
Opted to go a bit above guidance for highs Friday given the
magnitude of WAA.
Friday evening, potential exists for a period of very strong
winds. Surface low is not forecast to be particularly deep, but
will be moving very quickly (almost 50 mph) going from northern MN
at 21z Fri to to southwestern Quebec by 12z Saturday morning. The
very fast movement will result in an impressive isallobaric
couplet over the northern Great Lakes Friday evening, helping
maximize the efficiency of the already stout pressure gradient
over the area. Guidance continues to suggest temps Friday evening
will remain nearly steady or perhaps even rise a bit, which should
limit the typical wind dampening effects of nocturnal decoupling.
Forecast soundings keep the majority of the 50-60kt LLJ momentum
trapped above a shallow inversion around 1000ft AGL, but my
confidence in the models` handling of the boundary layer thermal
and wind profiles isn`t terribly high. For now, bumped winds up to
the highest forecast guidance which is just below advisory
criteria, but am concerned that given the set up we could tap into
the higher momentum air with the LLJ and see wind gusts exceeding
wind advisory criteria (sustained 30+ mph and/or gusts 45+ mph) for
a few hours Friday evening. Interestingly, the 00z 4km NAM does
forecast sfc wind gusts of 40-50kt at 03z from roughly I-80 north
into southern WI. While that model is on the upper end of the
guidance spectrum, it is well within the realm of possibilities.
Next topic is temps. As referenced above, temps should remain
unseasonably warm Fri night providing a nice more spring board
from which temps could really take off Saturday. The array of
models have all slowed the timing of cold front Saturday, which
opens the door to another very warm day. Models vary on how
extensive cloud cover will be Saturday, which makes the difference
between very warm temps and record warm temps ahead of the front.
There is still a bit of variability on the timing of FROPA
northern CWA, which could also play a role in keeping temps from
getting out of hand. For now, raised high temps some over the
blended model guidance Saturday, especially southern CWA. Worth
noting that I think these forecast highs (upper 60s north/mid 70s
south) are probably too conservative. If slower FROPA pans out and
there is even mixed sunshine, then I could easily see highs in the
mid 70s to lower 80s, but given a couple forecast uncertainties,
opted to remain conservative with highs for now.
Aforementioned cold front will drop south across northern IL by
early Sat evening and into central IL by later Sat night. Models
are coming into better agreement in depicting a rather stout axis
of low-mid level frontogenesis to the north of this boundary
supporting a band of showers and perhaps some thunderstorms
Saturday night. Still some placement/timing discrepancies, so pops
are conservative for now, but will probably be increasing pops for
Sat night over the next day or so as models hone in on
Sunday, dry with seasonable temps.
Monday, we do it all over again with another strong sfc low
tracking across the northern tier of states. ECMWF/GFS are a good
10-15mb deeper with this sfc low compared with Friday`s. Stage is
set for another day of unseasonable warmth Monday, with the big
question being whether low level thermal ridge arrives in time
Monday afternoon to support near record highs. By 00z Tues, GFS &
ECMWF both forecast 925mb temps to be approaching records for the
date. Again, nudged high temps up over blended model guidance, but
think we are still on the lower end of the spectrum of
possibilities for high temps Monday. Monday night looks to be very
windy (possibly advisory worthy) and remaining unseasonably warm,
possibly not dropping below 60F in many areas. Again, inherent
uncertainties this far out, so forecast this morning will not
reflect what current models suggest is the most likely scenario
Cold front looks to move through early Tuesday, knocking temps
back down to more seasonably reasonable levels (though probably
above average still) through midweek. As boundary stalls out,
medium range models suggest we could potentially enter into a
wetter pattern for the middle and end of next week, but medium
range predictability isn`t great and recent model volatility has
been pretty high run to run in the longer range.
For the 18Z TAFs...
Stratus cloud shield across the Great Lakes is showing no sign of
breaking up except over small portions of the open waters of Lake
Michigan. Observations show widespread MVFR in the region, and the
morning soundings support the stratus staying put into the evening.
We may see some scattering and lifting of the clouds as high
pressure nudges into the region tonight. Do expect VFR ceilings
around 23z, but will not be surprised if high MVFR conditions
persist. Winds turn southerly on Friday as high pressure departs to
the east. As warm air advects into the area, expecting better mixing
to allow 20-25kt winds through the afternoon. There is potential for
higher gusts beyond the TAF period.
300 AM CDT
Active period on the lake over the next week. A brief period of
fresh northerly winds expected today in the wake of departing low
over southern portions of the lake. Winds should be enough to kick
of small craft advisory worthy waves for our NSH waters into at
least early this afternoon. Brief lull in active pattern tonight
as ridge moves by, but even by late tonight southerly winds will
be ramping back up. Low will track quickly from northern plain
into Quebec Fri into Sat. While low will not be particularly deep,
the fast movement will result in a period of southerly gales over
the lake Fri afternoon into at least the first half of Friday
night. Some concerns about stability issues keeping winds down
some, but even with some low level stability near the lake sfc due
to the warm air mass, forecast soundings still support a period of
high end gales over northern portions of the lake. Issued a gale
watch for open lake to cover this threat.
For the IL & IN nearshore waters, the potential for gales looks
briefer and confined mainly to Friday evening. Strongly considered
a gale watch for IL nearshore where potential looks greatest, but
after collaboration with MKX opted to hold off and let day shift
take another look. Southwest winds ease Saturday and shift north
as a front drops down the lake Saturday and could be a period of
moderate north winds behind the front late sat into Sat night,
which could push waves close to small craft advisory levels for
IL/IN NSH. Front will surge back north Monday with another period
of southerly gales likely Monday into Monday night. Low is
forecast to be even stronger and still fast moving, have
introduced gales over the open lake, but if low is as deep as
currently forecast then a period of storm force southerly winds
are possible, mainly northern portions of the lake.
LM...Small Craft Advisory...INDIANA NEARSHORE WATERS UNTIL 3 PM Thursday.
Gale Watch...LMZ777-LMZ779...4 PM Friday TO 3 AM Saturday.
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