Scientific Forecaster Discussion
Area forecast discussion
National Weather Service Chicago/Romeoville Illinois
1236 PM CDT Monday Aug 31 2015
1117 am CDT
Main questions for today pertain to possibility of fog along
portions of the Lakeshore...and then degree of afternoon heating
elsewhere. Overall the temperature trends appear to be on
track...so no changes are currently planned for afternoon highs
and would expect to only need a tweak of a couple of degrees at
most. The question of fog is more uncertain. Haze in several
locations is keeping the visibility in the 5 to 7 mile range...but
webcams and satellite pics are hinting at the presence of
lingering fog over the lake in some areas. If this should persist
and wander onshore...locations near the lake could see visibility
drop closer to the 1 mile range...though this would still be above
320 am CDT
Big forecast challenge through tonight is fog early this
morning...then smoke and its potential impact on temperatures. Dense smoke
was apparent Sunday afternoon to our west and northwest over
Wisconsin and Iowa. To see a map showing smoke location and density
check out our twitter (@nwschicago) and facebook (/nwschicago)
including a link to where that map and other information can be
For today expect fog to burn off rather quickly this morning leaving
mostly sunny skies (though likely hazy thanks to the smoke) in its
wake. Have leaned toward the cool side of guidance today due to
smoke...though it is unclear how dense smoke will be today. Less
smoke means temperatures could be a degree or two warmer and denser smoke a
bit cooler perhaps. Gradient looks to be light enough to allow a
lake breeze today which should keep temperatures in the 70s along
the immediate Lakefront. Some patchy fog could develop again
tonight...though as low levels slowly dry out a bit will probably
see less fog than this morning.
320 am CDT
Tuesday through Sunday...
Today may be the last day of meteorological Summer...but it would
appear that mother nature failed to get the memo as a stretch of
very warm to almost hot weather through next weekend with
unusually strong upper level ridge over the region. Lingering
slight weakness in the upper ridge from Indiana south to the lower
Ohio Valley could allow for a slight chance of mainly
afternoon/early evening isolated storms through Tuesday...though
probability of precipitation of 20 percent or less look appropriate. Medium range guidance
hints at possible convection with very subtle disturbances late in
the week...but again kept probability of precipitation below mentionable levels given the
weak forcing and inability to really pinpoint any organized
forcing at this distance.
Big story this week will be the well advertised heat wave that
should get going in full force tomorrow. European model (ecmwf)/NAM/GFS all forecast
925mb temperatures around 25-26c which is pushing the limits of climatology for
early September. Using the new Storm Prediction Center sounding climatology tool
(www.Spc.NOAA.Gov/exper/soundingclimo) the 7 day running average
925mb for this week is around 19-20c with 24c the 90th percentile.
Our 925mb temperatures are prognosticated to be close to 3 Standard deviations
above average with climatology supporting highs in the low/middle 90s with
925mb temperatures of 25c...however dewpoints look to be a bit higher
than normal for a September heat wave and also there is the big
smoke wildcard...so kept temperatures a bit below what 925mb temperature climatology
Surface gradient could be strong enough to prevent any significant
lake breeze Tuesday and Wednesday allowing the heat to reach right up to
the beaches...weaker gradient looks to allow some lake cooling
late in the week and possibly over the weekend. Outside of any
lake cooling temperatures look likely to remain in the upper 80s/near 90
through the upcoming weekend.
//Ord and mdw concerns...updated 18z...
* lake breeze this afternoon to switch winds east but below 8kt.
* MVFR fog development tonight.
Current radar depicts lake breeze making inroads in Cook County
with time of arrival to Ord/mdw in the 20-21z hour. Network observation
indicate winds in the 090-120 range with speeds at 4-8kt behind
the lake breeze. Otherwise...high pressure over the Tennessee Valley will
keep hazy/scattered cloud conditions presenting VFR weather through
Conditions remain favorable for fog tonight...but not nearly as
dense as last night. A slightly tighter pressure gradient will
keep winds moving from the SW at 4kt...just enough to prevent
settling in the boundary layer. Tonight will also lack a dry
layer in the 2-5 kft range which normally would maximize fog
potential. Given the inversion...stagnant airmass...and low level
moisture...will include high end MVFR fog in the tafs for Tuesday
morning. Daytime Tuesday will be similar to today but a 15-20kt
elevated SW wind will likely pin the lake breeze close to shore.
//Ord and mdw confidence...updated 18z...
* high on lake breeze direction and speed today...medium high on
* Medium high on fog remaining MVFR tonight.
//outlook for Ord/mdw for 00z Wednesday-12z Monday...updated 12z...
Thursday...slight chance of thunderstorms and rain.
Sunday...slight chance of thunderstorms and rain.
406 am CDT
As a large ridge of high pressure remains nearly stationary over
the eastern third of the country through this week while any low
pressure systems track to the north of the lake...southerly winds
will be observed over the lake. Areas of fog with patchy dense fog
has developed once again across the lake this morning with the
more dense fog being observed over the nearshore waters. This
dense fog will likely persist through this morning...with areas of
fog still likely later today and tonight.
Lm...dense fog advisory...nearshore Winthrop Harbor to northerly is until 10 PM Monday.
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