Scientific Forecaster Discussion
Area forecast discussion
National Weather Service Chicago/Romeoville Illinois
328 am CDT Wednesday Sep 2 2015
328 am CDT
Primary concern in the near term is convective potential tonight.
Another very warm to almost hot day expected today with highs once
again making a run at 90 degrees and probably eclipsing 90 in many
areas. Some debris cloudiness from Iowa convection could filter
sun a bit in our western County Warning Area and perhaps keep temperatures a degree or
two cooler...but with convection likely to weaken not anticipating
a big impact.
Well defined shortwave on water vapor imagery over Minnesota and
western Iowa will continue to slowly progress eastward through
tonight. Brunt of the convection is expected to remain to our
north over Wisconsin tonight...but glancing blow from southern
extent of the shortwave should result in some weak height falls
and perhaps enough forcing to support some convection in our County Warning Area.
Various convective allowing ensembles are not overly aggressive
with convective chances tonight...so largely maintained previous
probability of precipitation with just small chances over mainly northern County Warning Area closer to
the track of the strongest portion of the vorticity.
A portion of this vorticity could begin to turn southeast as it reaches
the eastern extent of the upper ridge...lingering in the area
Thursday. Combination of lower heights and potential for low level
convergence along outflow and/or lake breeze suggests it would be
prudent to add some low chance probability of precipitation for Thursday as well. Assuming
convection and/or convective debris aren't terribly prominent then
Thursday will be another day with temperatures making a run at 90
degrees. Much of available guidance suggests a lake breeze could
form in the afternoon...which if it materializes would cause lake
adjacent areas to see temperatures drop off a bit in the afternoon.
338 am CDT
Thursday night through Tuesday...
Continued unseasonably warm and humid conditions Friday along with
slightly lower heights within a break in the upper ridge over the
region argues for some more slight chance probability of precipitation for Friday as well
with lake breeze once again a candidate to focus development.
Heading into the weekend GFS and European model (ecmwf) vary on low level wind flow
and potential for lake breezes. European model (ecmwf) would suggest little or no
lake breeze while GFS more prominently develops healthy lake
breezes both days. Away from the lake the forecast is more
straight forward...just continued unseasonably warm and rather
humid. If GFS advertised lake breeze materializes then temperatures would
likely hold in the 70s along the shore over the weekend...while
European model (ecmwf) would suggest the upper 80s/near 90 degree heat would make
it right up to the shoreline.
Monday could be one of the hottest days of the next seven
depending on debris clouds and timing of the front. For now kept
highs in the upper 80s/near 90...but with sunshine and current
timing of front panning out highs could reach low/middle 90s. Cold
front looks to approach region late Monday and could slow or stall
out nearby Monday night into Tuesday bringing a continued threat
of showers and thunderstorms.
//Ord and mdw concerns...updated 06z...
* low potential for visibility restrictions in br this morning.
* Low chance for thunderstorms and rain later today.
Light winds with VFR conditions in place early this morning...with
only some passing high clouds over head. Patchy fog is still
possible across the area this morning but these higher clouds may
limit this potential. Southwest winds and VFR conditions will
persist today...with any precipitation this morning remaining to the
south of the terminals. Precipitation potential does increase later this
evening and tonight but with lower confidence if any precipitation will
occur...have continued to leave out any mention at this time. Will
need to monitor the potential for adding showers/thunderstorms in
the tafs with later forecasts.
//Ord and mdw confidence...updated 06z...
* low confidence in MVFR visibility restrictions this morning.
* Low confidence in thunderstorms and rain potential this afternoon and evening.
* Medium high confidence with wind speed/direction.
//outlook for Ord/mdw for 12z Thursday-12z Tuesday...updated 00z...
Thursday...mainly VFR. Low chance for rain showers/thunderstorms and rain and associated
visibility impacts. East-southeast wind shift expected in afternoon.
Friday...VFR. Southeast to east winds.
Saturday and Sunday...VFR. East-southeast wind shift possible
Monday...low chance thunderstorms and rain and associated visibility impacts. Southwest
230 PM CDT
High pressure will remain over the eastern Ohio
Valley...allowing southerly to at times southwesterly flow across
Lake Michigan. A weak frontal boundary will try to push south across
far Northern Lake Michigan and turn winds northerly...but then turn
south again late in the week. The gradient will remain light...which
will allow for the development of afternoon lake breezes. Very warm
and moist air will continue to flow across the lake...and could
allow for patchy fog to redevelop late tonight. Currently winds are
just strong enough that this does not appear to be forming. So
confidence is currently low.
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