Scientific Forecaster Discussion
Area forecast discussion...updated
National Weather Service Chicago/Romeoville, Illinois
808 PM CDT Tue Oct 25 2016
805 PM CDT
The rest of the night forecast is in pretty good shape, with an
increasing likelihood of showers after 10 PM from west to east,
with winds also steadily increasing overnight.
An initial area of patchy light rain/sprinkles out of mid-level
frontogenesis is inching east-northeast from Rockford to Chicago.
An early report out of this near Belvidere around 4 PM actually
indicated a little bit of ice, which likely was very brief and
attributed to wet bulb cooling. The focus now turns to the
blossoming area of warm advection arcing from southern Minnesota
through western Illinois. The 00z dvn sounding just ahead of this
indicated a decent amount of dryness remaining below 5000 ft, but
the increasing isentropic ascent and more convective nature should
overcome this. The showers should work into the
western/southwestern County Warning Area between 10 PM and midnight. Some
conditional instability aloft has already supported lightning to
just east of the Mississippi River and cannot rule out some
in the western/southwestern County Warning Area later tonight. The showers could
gradually ease in coverage as they work east overnight, especially
south of I-80, but still think a good part of the Chicago area
will see some light rain overnight as a 40-45 kt low-level jet
veers over the County Warning Area.
For tomorrow, the primary time for lift supportive of showers
remains late morning into the afternoon. Regular wind gusts of
25-30 mph still look like a good bet with 35+ mph along Lake
Shore areas...mainly near/north of Chicago.
101 PM CDT
Cloudy and cool conditions are the rule this afternoon as high pressure
departs to our east and cloud cover overspreads the region ahead
of low pressure across the plains.
Mid level lift will increase this evening as the approaching system
arrives. The increasing lift is evident in increased deeper
clouds on infrared and echoes aloft on regional radar mosaics. While
some sprinkles are increasingly possible this evening, forecast
soundings maintain drier low level through this evening for most
of the area that and the forcing from the low level jet and mid
level frontogenesis is not quite strong enough to overcome the dry
air. There is increased agreement in a batch of rain showers after
midnight as the f-gen ramps up considerably in the warm advection
Wing ahead of the surface low. This batch does not appear to
impact all areas, but the I-80/88 corridor northward will be
favored during this time frame. Lapse rates aloft do not appear to
support thunder during this period in spite of the strong low
level jet and mid level forcing.
After this wave moves through, it appears there will be a lull in
precipitation (for areas that actually receive rain overnight)
early Wednesday and the NAM/GFS are in good agreement on this
general idea, but there will likely still be some lingering
rainfall. The NAM is still a northern outlier with the track of
the surface low, and have tended towards the global and ensemble
guidance which continues to support an eastward track across the
I-80 corridor. Southeast winds continue to ramp up ahead of the
eastward moving surface low as the strong departing surface high
holds across and east of the Appalachians. Then as the upper
forcing arrives, expect rain/rain showers to increase from west to
east mid to late morning and spreading over the remainder of the
area Wednesday afternoon. Even with the northern low track,
instability is still suggested to be on the lower side and
therefore just maintained a slight chance thunder mention in spite
of high chances of rainfall. North of the low expected just a
cool, rainy and windy day, with conditions being more showery in
nature south where the thunder chances are a bit higher.
244 PM CDT
Wednesday night through Tuesday...
The period will begin with the eastward exit of an area of low
pressure, which will move across northern Illinois Wednesday
evening. As a result, showers will linger over the area,
especially during the evening, before gradually abating from west
to east overnight. A period of gusty west-northwesterly winds look
possible overnight in the wake of this low, especially for areas
south of I-80 where a corridor of better pressure rises is
expected to occur. There is not really any big cold expected
behind this system on Thursday. So, while it will be cooler in the
50s, it is really not that cold for this time of year.
The main period of interest during the period then looks to focus
on the increasing potential for a period or two of well above
normal temperatures and breezy conditions, especially Friday
through Saturday. The large scale upper-level pattern across North
America will feature an upper trough over the eastern Pacific,
with a broad down stream ridge setting up over the central Continental U.S..
within this larger scale pattern, it appears that a significant
piece of upper level energy, emanating from the eastern Pacific
trough, will eject eastward across the central and northern
rockies later this week, then across the western Great Lakes
region by Saturday evening. This system will drive a area of low
pressure over the upper Midwest into Saturday, resulting in a
markable increase in a southwesterly low level jet (~50kt) over
the region Friday night into Saturday. These stronger winds will
result in the advection of 15 (20) degree celsius temperatures at
850mb (925mb) over the area for Saturday. These values are right
around the daily maximum on record for this time of year per the
spc's sounding climatology. As a result, temperatures on Saturday
may not be too far away from the record values (78 at Ord and 81
at rfd for saturday)!
While it also looks mild on Friday with highs mainly in the mid to
upper 60s, it also appears that it will become quit breezy during
the day on Friday, with gusty southwesterly winds likely
continuing Friday night into Saturday. Therefore, these stronger
winds Friday night could keep temperatures from dropping off much
overnight, with temperatures possibly remaining in the 60s.
Following a warm breezy day Saturday, the storm system should
push a cold front across the area Saturday night. This could again
set up a period of some breezy northwesterly winds into Sunday
morning as surface high pressure quickly builds in behind this
low. However, due to the Pacific nature of this storm, the air mass
in the wake of the cold front will not be that cold. In fact,
highs on Sunday could again be near 60 which is not far off our
normals. Precipitation chances also look rather low over the area
with this system, with the best chances north of the area.
Temperatures look to rebound back into the 60s for early next week
as remain in a mild and mainly dry weather pattern.
for the 00z tafs...
A band of high-based light showers/sprinkles has developed across
northern Illinois into Northwest Indiana and is expected to
gradually shift east of the terminals this evening. Do not
anticipate any impacts to cigs or vsby with this first band.
Additional showers currently across Iowa into west central
Illinois will arrive late in the evening and overnight. Expect
primarily VFR conditions overnight, though cannot rule out tempo
MVFR conditions if a terminal is directly impacted by a heavier
shower. Should be a lull in precipitation from late tonight
through mid/late Wednesday morning when additional showers will
move in ahead of an approaching area of low pressure. The low is
expected to track over northern Illinois Wednesday evening. Winds
are still a bit uncertain tomorrow evening given the proximity of
the low, but do anticipate strong southeast flow out ahead of the
low along with lowering cigs as the low nears with IFR conditions
probable Wednesday evening and night. Bmd
308 PM CDT
The main weather concern for Lake Michigan continues to focus on
a period of southeasterly gales expected on Wednesday. Overall,
expect the winds to become easterly tonight, while gradually
increasing up to 30 knots by early Wednesday morning. Winds will then
become southeasterly up to 40 knots gales during the day Wednesday as
the surface pressure gradient strengthens in response to an
approaching eastward moving surface low. The strongest winds are
still expected across the southern half of Lake Michigan,
including the Illinois and Indiana near shore waters. Winds should
then abate and become northerly in the 15 to 25 knots range for a
period late Wednesday night into Thursday as the surface low moves
towards the eastern lakes region.
Surface high pressure will then build over the region by late
Thursday. This should set up a period of light winds, generally
under 15 knots for Thursday and Thursday night. However, as this
surface high shifts east of the area on Friday, winds will
increase out of the southwest as another storm system takes shape
over the northern High Plains. A period of winds of 25 to 30 knots
will be possible over Southern Lake Michigan late Friday and
Friday night. This looks to result in hazards conditions for small
craft for the Illinois and Indiana near shores. This surface low
is finally expected to pass eastward somewhere over Lake Michigan
by Saturday evening. In its wake, there could be a period of
strong northwesterly winds into Sunday.
Lm...Gale Warning...lmz777-lmz779...7 am Wednesday to 10 PM Wednesday.
Gale Warning...lmz740-lmz741-lmz742-lmz743-lmz744-lmz745...7 am
Wednesday to 7 PM Wednesday.
Visit US at http://weather.Gov/Chicago (all lowercase)
Follow US on facebook...twitter...and youtube at: