Scientific Forecaster Discussion

Return to Local Conditions & Forecast

Without Abbreviations
With Abbreviations

Area forecast discussion 
National Weather Service Chicago/Romeoville, Illinois
319 PM CDT Thu Sep 29 2016

Short term...
240 PM CDT

Through tonight...

Similar to the past couple days, and upcoming ones for that
matter, tonight will continue to be dominated by the large
closed upper low centered over Kentucky this afternoon. Mid-
afternoon visible satellite imagery has indicated some breaks with
more cumuliform clouds across northern Indiana into far eastern
Illinois. Isolated showers are likely to fill in there and cannot
100% rule out a lightning strike or two. Further to the north, a
ribbon of scattered to numerous showers will continue within the
wraparound moist conveyor belt of the occluded system. This axis
will persist into this evening with likely an uptick in showers
later this evening from lift of a left entrance region of the
upper jet wrapping around the low center.

Clouds overall will continue tonight, even if some areas see a
break late in the day into early evening. This along with the
continued winds will prevent temperatures from falling much at all
with lows in the mid to upper 50s, and likely around or above 60
near the Lake Shore.

Speaking of the Lake Shore, continued battering waves of 7 to 11
ft through tonight will probably cause some minor flooding in
typical vulnerable areas immediately along the lake front. The
upstream mid-lake south buoy has been steady around 7 ft today,
and would expect to see around 115-125% of that within our
northeast Illinois nearshore given the flow pattern. The Wilmette
buoy has been near that, with 8-9 ft a good part of today and
webcams and pictures supporting that. While that is borderline
for what usually causes flooding, have seen around a 15 inch
increase in lake levels the past 48 hours from the constant
northeast flow and that could inch up the next 12-18 hours.

Mtf

&&

Long term...
317 PM CDT

Friday through Thursday...

Friday will be another blustery fall day as the upper level low
pushes north. Occasional rain showers as we've been seeing for the
past few days will continue, and the best chance of showers will be
east of a McHenry to Pontiac line closer to the main low.
However, showers are possible across the warning area. Models
feature minimal cape along and south of I-55, but think a couple
embedded thunderstorms will be possible Fri afternoon. Severe
weather is not expected.

The tight pressure gradient will result in gusty winds. Gusts will
vary from up to 25 mph away from the lake to up to 30 mph along the
Lakeshore. The winds and damp conditions will likely make it feel
cooler than the mid to upper 60 degree high temps.

The low continues it slow path north resulting in much of the same
Saturday. Periodic showers are once again expected, but winds will
be lighter and high temps will be slightly warmer. The low shifts
northeast Saturday night. Guidance suggests lake effect showers will
be possible over Northwest Indiana into early Sunday morning.
Guidance differs on how strong of a vort streamer will shift south
through the region Sunday. Decided to go dry but damp conditions and
perhaps a few sprinkles are possible. Sunday will be drier than it
has been lately.

An upper level ridge and surface high move overhead Monday leading
to much drier and warmer conditions. Highs should be in the low to
mid 70s by Tuesday. The next system arrives mid to late next week
with the European model (ecmwf) having a faster solution than the GFS. Kept thunder
out of the forecast for now since the cold front precip looks to
arrive Wednesday night. If the cold front arrives at a more
favorable time, we could see more thunder than currently
forecast.

Jee

&&

Aviation...
for the 18z tafs...

The concerns with the aviation forecast this afternoon through
Friday are periodic showers and possible MVFR visibility, along
with continued MVFR cigs and increasing potential for IFR cigs by
daybreak Friday.

Low pressure across the Ohio River valley continues to have a far
reaching extent with gusty northeast winds continuing over the
region through the period. Gusts exceeding 25 kt will
occasionally be seen this afternoon at Chicago area airports.
Forcing for showers will continue, some of which will be lake
enhanced. Temporary reduction in visibility is likely and had
been observed this morning as low as briefly 3sm at Ord. While
showers should undulate in coverage some through Friday morning,
the overall support for them will continue.

Steady MVFR cigs should be seen this afternoon and into early
evening. Thereafter, a slow lowering of cloud bases is likely
into the overnight. There is IFR cloud cover upstream in lower
Michigan and this should expand westward. The slightly warmer air
off the Lake May result in cloud bases more at 1000 ft than below
that at Ord and mdw, but tough to say for sure and would be
surprised if not even temporary IFR during the morning rush. Not
expecting much improvement in clouds through the entire day
Friday.

Mtf

&&

Marine...
240 PM CDT

Strong northeast winds will continue across the lake through
Friday before easing. Waves have been steady around 7 ft at the
south buoy and 8-9 ft at the Wilmette buoy. Would expect these to
hold steady or undulate slightly, maybe even coming up some later
tonight. The occasional gale force gusts also cannot be ruled out
tonight and early Friday morning. The threat for waterspouts is
less than last night, but still present over the south given the
instability and sizable lake-induced cloud depths.

Waves will be slow to subside into Saturday and it is possible
the current Small Craft Advisory for Illinois and Indiana needs to
be extended through Saturday morning.

Mtf

&&

Lot watches/warnings/advisories...
Illinois...Lakeshore Flood Advisory...ilz006-ilz014 until noon Friday.

In...Lakeshore Flood Advisory...inz001 until noon Friday.

Lm...Small Craft Advisory...nearshore waters
until 4 am Saturday.

&&

$$

Visit US at http://weather.Gov/Chicago (all lowercase)

Follow US on facebook...twitter...and youtube at:
www.Facebook.Com/nwschicago
www.Twitter.Com/nwschicago
www.Youtube.Com/nwschicago

National Weather Service Glossary of Abbreviations