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Area forecast discussion...updated
National Weather Service Chicago/Romeoville, Illinois
852 PM CDT Sat Oct 19 2019

852 PM CDT

No major changes to the forecast for the rest of tonight into
early Sunday morning. The dense fog potential for especially north
central Illinois early Sunday morning continues and will be
primarily a race in time; that is will the back edge of stratus
pass that area prior to daybreak. If so, the likelihood of dense
fog increases sharply.

Scattered light showers and patches of sprinkles are still dotting
the area this evening but will end by midnight as the sheared
short wave and its associated saturated, isentropic ascent ahead
of it departs northeast. The deeper moisture into the mid-levels
will also shift away. The 00z dvn sounding showed a saturated
sounding for most of the lowest 3.5 km, but as that higher
moisture clears the saturation remains in the near-surface area as
seen by low clouds on satellite. The defined back edge to these
is only inching eastward in far eastern Iowa. Pure extrapolation
would have it into far north central Illinois around 4-5 a.M.
Immediately behind this back edge, dense fog has readily developed
across parts of eastern Iowa in a nearly calm wind regime and dew
points anomalously high for a mid October night in the lower to
mid 50s.

While all the ingredients are there, there remains uncertainty on
specific timing of the back edge of stratus to reach the area. On
the side favoring fog, there are some holes developing in the
stratus, meaning there could be an earlier arrival of favorable
conditions. High-resolution guidance is in fair agreement on a
gradient in visibility forecasts between light/no fog and dense
fog near or just west of the Rock River. All of this points toward
just not enough confidence to hoist a dense fog advisory
along/west of I-39 at this time, but have included mention of
areas of dense fog in that area's gridded forecast. Would give the
chance of dense fog advisory conditions for Lee/Ogle/Winnebago
counties about 60 percent right now.

Otherwise, the cold front that made decent progress into the area
late this afternoon has weakened quickly, and temperatures are
now only gradually dropping under the stratus. Temperature/dew
point spreads in northeast Illinois have closed to a few degrees
and should continue to, so pockets of shallow thicker fog are
possible in these areas too early Sunday morning, but may be more
dependent on holes developing in the stratus.



Short term...
307 PM CDT

Through Sunday...

We've managed to eke out a really respectable day with
temperatures this afternoon in the mid and upper 60s (to near 70
degrees across our south). Visible satellite loops reveal that
cloud cover will continue to incrementally invade the sky from the
west through the rest of the afternoon, however with the approach
of our latest weather disturbance. The incoming rain shield looks
pretty ragged, with one more cohesive area of light to moderate
shower activity lifting into southwestern Wisconsin at this hour,
within a plume of deeper mid-level moisture and closer to the
attendant 700 mb sheared vort Max. Think that this area of showers
will become even splotchier with eastward extent into this
evening, and pops have generally been capped at 30-40% near the
I-39 corridor, to around 20% or less closer to Chicago and points

Once this latest system peels east of the area later tonight,
skies look to clear--most notably across our westernmost counties
where partly cloudy to mostly clear conditions are expected to
develop after midnight. With no good impetus to shove lingering
low-level moisture out of the region coupled with what is expected
to be a favorable environment for outgoing longwave radiation, it
looks like a fairly widespread area of low stratus will begin to
develop late tonight, with some potential for this to build down
into at least patches of fog (some perhaps locally dense at
times). The most likely area for fog development will be near and
west of a La Salle to Crystal Lake line where The Spine of a
surface ridge will allow surface winds to go slack or calm at
times. Can't discount the need for a targeted dense fog advisory
very late tonight and into the wee hours of Sunday morning, but
we'll leave this decision up to future shifts as near term trends
can be more easily elucidated. Farther east, winds immediately
off the surface look to remain in the 10-15 kt range which should
help curtail the potential for widespread fog formation.

Sunday will feature decreasing cloud cover as any morning stratus
quickly scatters. Winds will turn to favor a more east or
northeasterly direction through the afternoon. The onshore
component off the cool waters of Lake Michigan should hold high
temperatures Lakeside in the low to mid 50s while inland locales
should manage to warm into the low to even mid 60s.



Long term...
307 PM CDT

Sunday night through Saturday...

The system that will be the next significant system to impact the
local area has now moved onshore into the Pacific northwest and is
being well sampled by the upper air network.

Peering across the Pacific northwest early this morning on the
satellite we see the beginning of what will become our weather maker
for early next week. The upper level pattern is transitioning from a
short wavelength to more of a high amplitude, long wavelength
quickly progressive upper level pattern. A strong upper level jet
will carve out a broad upper trough, with a deep closed moving out
of the northern rockies and into the northern plains. The system
should be vertically stacked, with the associated sfc low closely
tied to the path of the upper low as it lifts to the upper
Mississippi Valley by Monday evening. The longer range guidance is
in relatively good agreement on handling the path and strength of
the system and confidence is relatively high in the sensible weather
elements associated with the system.

The initial impacts will be strengthening southeast to southerly
winds strong warm/moist advection in advance of the associated cold
front. The remnants of tc Nestor are expected to track to the mid-
Atlantic coast by Sunday evening as the approaching sfc low deepens,
which will, in turn, open up the western Gulf of Mexico. Strong
warm/moist advection will set up across the Midwest, by the time the
cold front crosses the middle Mississippi Valley Monday morning,
pwats should increase to 1.0-1.25 inches with a southerly low-level
jet of 35-40 kt. Increasing deep layer moisture and both sfc and
upper level forcing should allow for a period of moderate rain. The
main question that remains is thunder potential. Latest guidance
indicates adequate dynamics, but instability may be limited by
increasing dense cloud cover in advance of and during the frontal
passage. Given the concerns over limited instability, will maintain
the slight chance thunder mention as any thunder will likely be more
isolated/embedded than discrete.

With the system expected to be very quickly progressive, the back
edge of the pcpn shield should be exiting off the the east as the
region becomes dry slotted through Monday afternoon. Impacts will
then shift from rain/ts potential, to strong, gusty winds. The
potential for strong wind gusts will be maximized from Monday
afternoon through Tuesday as a broad zone of 35-40 kt winds at the
top of the mixed layer overspreads the region. Wind gusts could
also be enhanced by strong pressure rises over the region as the
frontal trough quickly pushes to the east and sfc ridging builds
across the Lower/Middle Mississippi Valley. Potential for wind
gusts approaching Wind Advisory criteria will be also be aided by
strong cold advection as indicated in 850 mb temps dropping from +11
c ahead of the front Monday morning to -1 c Monday evening following
the fropa. So, will still carry gusts in the 35-40 mph range, which
is just under Wind Advisory criteria, but conditions will need to be
monitored closely should Wind Advisory issuance be needed. Either
way looks like a period of windy conditions to start off next week.

The wrap around precip shield back in the cold side of the system
will largely shift through Wisconsin, but it looks like we could get
clipped with some light pcpn precipitation late Monday night into
Tuesday, mainly for northern Illinois.

While the leading portion of this system will shift to the east
quickly, the upper low will lift north toward Hudson Bay and provide
a persistent fetch of colder, drier air from the northwest. While
drier weather would generally be favored, but broad cyclonic flow
aloft on the srn/swrn periphery of the upper low could allow for
some scattered showers which would be diurnally driven for the
middle to latter portions of next week.


for the 00z tafs...

VFR and mainly dry conditions still in place this evening,
however, isolated light showers approaching the terminals at this
time. No real impact with these showers, other than a few hour
window of an occasional light shower without any vis restriction.
This precip exits later this evening, with VFR conditions still
continuing for much of the night. By late tonight into early
Sunday morning, will see VFR conditions lower and likely
significantly lower for the rfd area where a period of IFR
ceilings and reduced vis around one mile are expected. At this
time, it appears that rfd will mainly be impacted but continue to
monitor these lower conditions to reach the remaining sites.
Given the setup and expected trends later tonight, it does look
that a small window of at least low end MVFR ceilings could reach
these remaining sites and have included this in the latest
forecast. Conditions improve by mid morning, with VFR conditions
once again expected for the remainder of the period. A weak front
is moving across the area this evening and shifting the winds to a
southwest to west direction. Do expect this direction for much of
the night, with a trend towards a light northwest wind late
tonight. Light and variable winds Sunday morning will gradually
increase through midday, and become oriented out of an easterly



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