Area Forecast Discussion
Issued by NWS Chicago, IL

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FXUS63 KLOT 191957

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
157 PM CST Thu Jan 19 2017

156 PM CST

Through Friday...

Rain...fog...and drizzle, maybe a thunderstorm are all on the docket
for the short term. The deep/cutoff upper low across the plains
will slowly lift north tonight. The low will take on a negative
tilt which should keep forcing strong ahead of the low. In
addition to the strong height falls ahead of the low, an elevated
warm front will be lifting northward as well but will never
completely lift north through the area. Isentropic lift with the
moist southerly flow ahead of the low will lead to rain increasing
through the afternoon and continuing for at least a good portion
of the night. Guidance is not all that excited about thunder
chances, but have left a small thunder mention as there could be a
few storms with elevated lapse rates ahead of the low and modest
warm advection on the nose of the low level jet.

Once the lead wave moves through overnight, we will lose the deeper
moisture layer, but continued warm/moist advection ahead of the
low and over the somewhat stalled out warm front across Central
Illinois, in conjunction with continued waves rotating around and
ahead of the upper low would suggest continued clouds, fog, and
drizzle. The pattern would suggest fog/drizzle could easily lead
to at least localized dense fog, but certainly expect lingering
low cloudiness most if not all day. In spite of clouds being in
place to limit warming, dewpoints will be coming up through the
afternoon which should allow temperatures to get into the 40s in
most areas.



255 AM CST

Friday through Wednesday...

Active weather continues throughout the extended forecast with a
series of waves moving across the country resulting in periodic
chances for precipitation. We should see a brief lull in rain
during the day Friday as our upper low departs northeast of the
area and weak mid/upper ridging builds overhead. Meanwhile,
another wave is progged to round the base of the longwave trough
over the Central Plains early Friday and lift into the Upper
Midwest Friday night into Saturday. Continue to carry a chance for
rain with this wave but the better chances and amounts are still
focused to our west. 850 mb temps are still expected to peak in
the 6-8C range early Saturday out ahead of the trough and
afternoon highs are still expected to top out in the mid 50s. A
stronger upper wave/cutoff low is progged to undercut the region
late in the weekend and lift across the Appalachians early next
week. Temperatures are expected to moderate some as northwest flow
develops behind this low and there will be periodic chances for
precip as an inverted trough slowly rotates through the area.
Models seem to be in better agreement at least on the track of the
500 mb low through Monday, though timing differences persist, and
solutions begin to diverge more significantly beyond Monday.



For the 18Z TAFs...

Main TAF concerns are timing of IFR/LIFR cig/vis trends through
the period, primarily early this afternoon for KRFD, then tonight
through Friday morning for all sites. Showers/isolated embedded
thunder expected to spread across from the south area late this
afternoon, continuing past midnight CST before diminishing to
drizzle which will linger Friday morning.

Weak surface high pressure ridge across the forecast area this
morning has allowed extensive low stratus/fog to develop, though
obs trends over the past few hours continue to indicate a gradual
improvement in conditions with lower cigs scattering across
portions of the Chicago metro area. Low level moisture remains
trapped beneath shallow (but fairly strong) inversion north of an
area of weak low pressure and a warm front currently over the
lower mid-Mississippi and lower Ohio river valleys. A band of
showers from eastern Iowa to downstate IL/lower Ohio Valley will
spread northeast and increase in coverage across the terminals
later this afternoon, with a period of rain/showers expected to
persist through the evening hours in advance of a mid-level
disturbance approaching from the southwest. Forecast soundings and
RAP model analysis does depict an axis of weak elevated
instability coincident with this band of precipitation, which
would support the potential for isolated embedded thunderstorms
especially late this evening through midnight when strongest large
scale ascent is present. Though greatest instability and thunder
threat appears to remain just south of the terminals, did include
a VCTS mention late this evening to express the overall trend.

LIFR/VLIFR conditions are then expected after midnight through
Friday morning, as surface low pressure trough lifts across the
region. Forecast models are in good agreement in depicting
saturated boundary layer up through about 850 mb, which will
likely support an extended period of low cigs/fog and drizzle.
LIFR conditions could persist into Friday afternoon.



401 AM CST

Generally quiet conditions expected across the lake at this time.
High pressure will exit to the east today, and allow for a weak
low to lift up the Mississippi river today. However, it is
expected to dissipate as it moves north. Another low is forecast
to form over the Plains Friday night and reach Minnesota Saturday
night, with a stronger low expected to form over the southern
Plains Saturday night. However, neither systems expected to bring
any significant winds across the lake during this time.






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