Area Forecast Discussion
Issued by NWS Chicago, IL

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FXUS63 KLOT 170839

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

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.



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.



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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