Area Forecast Discussion
Issued by NWS Chicago, IL

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FXUS63 KLOT 170743

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

819 PM CST

Evening Update...

No significant changes to going forecast this evening. Only minor
concern is with some partial clearing of low clouds across east
central IL which may allow slightly colder temps overnight in
southern parts of our cwa.

Surface high pressure ridge was aligned north-south from Lake
Michigan down the IL/IN border this evening. Light winds and a
stout subsidence inversion associated with the ridge has allowed
low stratus to hang tough across the area through the day, though
have noted a few holes along the southern periphery of the cloud
deck over east central IL in the past couple of hours. Low level
flow will turn from east to southeast overnight and gradually
increase, which may result in some decrease in low level clouds
across southern counties of the forecast area overnight. 00Z ILX
sounding shows rapid drying above the strong inversion, suggesting
low level moisture/clouds is likely quite shallow and easily
eroded. While mostly cloud skies are expected to persist due to
patchy mid-high clouds passing overhead, these areas may see a bit
more of a temp drop overnight. Thus have lowered overnight mins
there to around the 30 degree mark as opposed to the lower 30s
where low clouds are expected to linger.

Otherwise, going forecast appears on track with southeast winds
gradually increasing through morning, and mostly cloudy skies.



131 PM CST

Through Friday...

Cloudy and dry tonight with rain spreading from west to east Friday

Stratus is rapidly clearing over eastern Iowa and central Wisconsin,
but I`m unsure how quickly stratus will clear over northern Illinois
and Indiana. The areas with the better chance of seeing sunshine are
west of a Waukegan to Peru, IL line. I decided to maintain a mostly
cloudy forecast with high temps this afternoon only in the upper
30s.  High pressure over Illinois will continue to shift east
tonight with winds becoming south by Friday morning behind a warm

I based low temperatures off of a cloudier forecast and raised lows
to around freezing away from the city.  If clouds clear out, actual
low temps may be cooler than currently forecast.

A surface low will form over Kansas Friday afternoon/evening.  The
pressure gradient will tighten over the midwest leading to south
winds gusting to 30 MPH.  Warm air advection will usher in warmer
air, but with thick cloud cover overhead, I`m unsure how much we
will warm. I made minor adjustments to the previous high temps which
has low to mid 40s across the forecast area.

Guidance is consistent with showers spreading across the forecast
area Friday afternoon. Coverage and intensity increase late Friday
afternoon into Friday night.



209 PM CST

Friday night through Thursday...

Primary challenges within the extended forecast are dealing with
the low tracking across the region Friday night into Saturday
and the cold air that overspreads the region in its wake.

A strong Pacific wave is progged to move onshore over northern
California early Friday and quickly push east across the mid
Mississippi Valley by around midday Saturday. At the surface, low
pressure will deepen in response and track from near Kansas City
Friday night to the local area Saturday morning. Southerly flow
and WAA in place out ahead of the low track Friday night will
result in widespread rainfall across the region with the potential
for a few embedded thunderstorms due to modest instability rooted
between 800-900mb. In addition, expect non-diurnal temperature
trends Friday night.

By midday Saturday, surface low will shift off to our east with
northerly flow spreading across the Upper Midwest and strong CAA
driving 850mb temps from around +10C to -2 to -4C locally. A
deformation axis and corridor of strong f-gen will then pivot
across the CWA late morning through the afternoon supporting
continued chances for precipitation. P-type becomes a concern at
this point as the column cools enough to support a transition over
to wet snow, particularly over western half of the CWA. There
looks to be a pretty narrow window of time that would support wet
snow or a rain snow mix before the better forcing shifts east of
the area, so do not anticipate much if anything in the way of
accumulating snowfall but cannot rule out some spots getting
dusting. High temperatures on Saturday will likely occur at
midnight (Friday night) or early in the morning with falling
temperatures during the day behind the low. Temps will eventually
bottom out in the 20s Saturday night into Sunday morning.

Second vort max is progged to dig across the western Great Lakes
early Sunday but by this point moisture is fairly meager over the
region and outside of an uptick in lake effect snow mainly over
MI/IN do not anticipate any precipitation with this wave. Surface
ridge axis builds overhead later in the day Sunday. Temperatures
will be cool, topping out in the low to mid 30s, but otherwise
quiet weather expected through the day.

Ridge axis shifts to our east Monday and Tuesday allowing
southerly flow to return as a clipper system takes aim on the
Great Lakes region. For now, models favor a track that keeps
precipitation well to our north, but do expect temperatures to
rebound back to near 50 Monday afternoon with non-diurnal trends
(falling daytime temps) possible again Tuesday as a trailing front
sweeps across the region. Dry but cool conditions are expected
Wednesday and Thursday as high pressure builds back across the
central U.S.



For the 06Z TAFs...

The concerns with the Chicago TAFs through Friday are:

- MVFR ceilings persisting into overnight and returning Friday
  afternoon, with IFR arriving at some point Friday evening

- Gusty south-southeast winds much of Friday gradually veering to
  due south during Friday evening

- Strong southwest winds of 45-55 kt around 2000 ft late Friday
  afternoon and evening

- Scattered showers Friday afternoon with more of a steady
  rain/drizzle Friday evening and MVFR (possibly IFR) visibility

- Isolated storms possible Friday evening and early overnight

The MVFR clouds trapped under a strong inversion are starting to
advect northward, with some holes developing in the layer as well.
Confidence is low if the MVFR fully clears out by Friday morning,
and it is possible some 1000-2000 ft ceilings remain through
Friday morning.

As low pressure strengthens across the central Plains, south-
southeast winds will increase quickly Friday morning. Confidence
in speeds and gusts is medium-high, and sustained could even reach
around 18 kt. As for direction, confidence is high in the winds
staying slightly east of due south through the day Friday. The
timing of 190 degrees in the evening is low-medium.

A strengthening low-level jet of 45+ kt around 2000 ft during
late Friday afternoon will steer in greater moisture for rain
shower development. Rain is likely to become more widespread by
the early evening rush, and some of this could briefly be heavy
including with thunder. There is potential for IFR ceilings by as
early as 00Z, with these ceilings becoming much more likely by
03Z-06Z. LIFR is likely by overnight Friday, especially as the
low passes over the region.




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


LM...Gale Warning...LMZ777-LMZ779...9 AM Friday to 9 PM Friday.

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




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