Area Forecast Discussion
Issued by NWS Chicago, IL

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FXUS63 KLOT 191128

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
628 AM CDT Wed Oct 19 2016

334 AM CDT

Today through Friday Night...

Quiet weather across the area today, before a rain-producing low
pressure system tracks through the Ohio Valley tonight through
Thursday, and eventually pulls colder air into the region with
blustery north winds and lake effect shower potential Thursday
night into Friday.

The cold front which pushed through the area Tuesday has settled
just north of the Ohio River as of early this morning, with the
western portion extending to a weak area of low pressure over
northern Texas. Weak high pressure ridging has pushed into
northern IL, with clear skies and calm winds allowing temperatures
to fall off into the 40`s and lower 50`s across the cwa. Residual
low-level moisture over the southeast third or so of the forecast
area has resulted in patchy fog development, with a few locally
dense patches over northwest Indiana. Expect fog will continue
there through sunrise, and will need to monitor the extent of
lower visibilities which might require a headline. Fog should burn
off within a few hours of sunrise.

Attention then turns to the west, where a moderately strong mid-
level short wave was noted propagating across the central Rockies.
This wave is progged to continue east across the Plains while
amplifying this afternoon, with the weak surface wave rippling
along the stationary front in the southern Plains, before
consolidating and deepening across the Ohio Valley early Thursday
as the mid-level wave digs and amplifies. As this process develops
later today, frontogenetic forcing increases within the elevated
baroclinic zone north of the surface front, with saturation noted
in the 850-700 mb layer mainly south of the cwa by late this
afternoon, but eventually spreading north into the forecast area
this evening.

This looks to bring rain to areas generally along/south of a Peru
to Chicago line tonight into Thursday, though with mainly lighter
precip along that axis with drier low levels and mainly mid-level
saturation noted in various model time sections. More widespread
rain (thus highest pops) will be carried mainly southeast of a
Pontiac to Valparaiso line, where 0.25-0.35 inches of QPF are
indicated. GFS appears to be producing too much QPF farther north
into northeast IL in mid-level f-gen axis, where soundings
indicate substantial dry air will exist below 700-750 mb.

Low level northerly winds ramp up on Thursday as the surface low
deepens and moves off to the southeast, allowing colder air to
spread south into the region. 850 mb temperatures approach 0 deg C
by 12z Friday, setting up deep lake-induced instability and a
decent setup for lake effect rain showers Thursday night and
Friday. WRF and GFS forecast soundings over the south end of the
lake at 12z Friday depict AOA 500 J/kg of lake-induced SBCAPE,
with equilibrium levels of 15-20 KFT. Taking these thermodynamic
profiles into account, have added a slight chance thunder mention
for parts of eastern Cook, Lake and Porter counties near the lake
Thursday night and Friday, and have added a mention of waterspouts
over the lake. Wind profiles then tend to back more northwesterly
Friday night, which should help shift lake effect convergent axis
east of the cwa, as low pressure deepens well to our east off the
east coast.

Temperature wise, sunshine starts off today with a gradual
increase in patchy mid-clouds during the day. Model thermal fields
support above average highs from the upper 60s north to the lower
70s south. Weak northeast winds will likely limit lakeshore areas
to the mid-60`s. Cooler weather moves in for Thursday and
especially Friday with gusty north winds, and highs only in the
low-mid 50`s expected Friday. Lows are expected to dip into the
30`s across western parts of the cwa late Thursday night where
clouds and winds should diminish, away from lake-effect issues
farther east. Surface high pressure ridge edges in from the west
late Friday night, setting up the potential for freezing temps
across the cooler parts of northern IL away from the Chicago urban
heat island.



334 AM CDT

Saturday through Tuesday...

Broad upper ridging develops eastward across much of the central
CONUS over the weekend, kicking the deep upper trough to the east
across New England and off the Atlantic Coast. Surface ridge
across the area Saturday is expected to provide dry weather, along
with the start of moderation in temps which continues into Sunday
as the ridge moves east and winds turn southerly. Confidence
decreases a bit in the details beyond Sunday, with guidance
generally indicating strengthening of the upper ridge across the
Plains and Rockies, while allowing a weak-amplitude trough to slip
across the Great Lakes region Monday. New ECMWF is noticeably
quicker with this disturbance, with a weak cold front moving
through Sunday night. Better agreement exists in model forecasts
with bringing the upper ridge east mid-week, which should support
additional warming and the next chance for organized precip with
better southerly return flow Tuesday night and Wednesday.



For the 12Z TAFs...

The primary forecast focus/concern today is northeast winds and
associated speeds at ORD/MDW. Some patchy MVFR BR remains possible
at GYY this morning. Winds are starting the day calm and on the
larger scale will become light north and northeast by mid day.
This should enable modest lake enhancement at ORD/MDW/GYY this
afternoon, though speeds will remain primarily below 9-10 kt with
medium confidence in this. Occasional sprinkles or spotty light
rain are possible this evening and overnight, but the lower levels
of the atmosphere will remain quite dry. Thus, have continued to
leave rain mention out of the TAFs and still anticipate little to
no impacts from the rain if it does reach the surface. CIGs will
be VFR through tonight.

A cold front will then move south across the area early Thursday,
shifting winds to gusty northerly (~350-010 direction), with
sustained speeds of 10-15 kt and gusts of 15-20 kt. Forecast
guidance is indicating that lower CIGs will accompany the gusty
northerly winds, with lower MVFR included in the ORD 30-hr TAF.
High-end IFR cannot be ruled out. Have medium-high confidence in
the wind forecast and medium confidence in the lower CIGs Thursday
morning, with low confidence in specific CIG details.



240 AM CDT

The primary concerns are a prolonged period of brisk northerly
winds tonight through Friday night and the potential for
waterspouts Thursday night and Friday. A cold front will move
down the lake tonight, with winds turning northerly and increasing
to up to 25 kt. Speeds/gusts will further increase to 30 kt on
Thursday on the south half of the lake and continue into Thursday
night. Winds will stay elevated with speeds/gusts up to 25 kt
through Friday night. Much colder air will move over the lake
Thursday night and Friday, creating a favorable setup for lake
effect showers and even isolated thunderstorms. With this,
waterspouts are also possible. Have added a slight chance of
waterspouts to the forecast, and the parameters appear favorable
for at least isolated waterspout potential.

Will need a Small Craft Advisory for the near shore waters
Thursday morning through Friday night for the strong northerly
winds and building waves. It`s possible the advisory will be
needed longest for the Indiana waters, as direction will turn more
northwesterly on Friday.






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