Scientific Forecaster Discussion
Area forecast discussion
National Weather Service Chicago/Romeoville Illinois
1146 am CST Tuesday Jan 27 2015
330 am CST
Infrared and water vapor satellite loops early this morning have
shown the last in a series of sheared vorticity maxima to have moved
southeast of the area. Light snow associated with these has mainly
ended. Low-level winds have also shifted more northerly as a
surface induced trough axis pulls away. This has ended the low-
level isentropic ascent and Omega which helped to induce much of
the freezing drizzle last night. So with that have cancelled the
advisory as the widespread threat has ended going into the rush
hour. One hopefully only small fly in the ointment to this is the
chance for freezing drizzle lingering in mainly lake and Cook
counties...including Chicago...as winds turn off the lake early
this morning steering in some marginal lake-induced instability.
The mkx radar is indicating light echoes moving westward that
dual-pol suggests is freezing drizzle. Rap profiles are only
saturated about as cool as -8c adding support to that. With not
expecting a situation such as last evening given the treatment on
roads and the different more mesoscale/localized induced
lift...feel better going with an Special Weather Statement if needed. Expect the chances
to end by 10 am or so as drier air continues to advect westward
over the lake seen by dew points near 10f on the Michigan shore
along with an east wind.
Clouds should be slow to clear today given the current expansiveness
to the north and the cyclonic flow continuing through this
morning...as well as aforementioned lake-oriented flow into
northeast Illinois. Mesoscale model moisture/condensate fields
indicate clouds lingering the longest across the Illinois part of
the County Warning Area with Indiana experiencing northeasterly dry advection.
Have highs only a few degrees warmer than current early morning
values given this thought process. Confidence is low on whether
clearing occurs areawide late this afternoon and evening as
guidance would suggest...plus there will likely be cirrus
spreading in too. This introduces bust potential in low
temperatures over north central Illinois where low teens to middle
20s could occur depending on clouds. Feel more confident Northwest
Indiana will be mostly clear and touch some of those lower to middle
During the day Wednesday...a thermal ridge builds into the area and
expecting at least some sunshine as southwesterly low-level flow
will be enough to advect out any residual low clouds. The warmest
part of the thermal ridge reaches US more so Wednesday night and
climatology for forecast 925mb temperatures would keep
surface readings in the 30s for most of the County Warning Area especially with
some snow cover albeit minimal.
330 am CST
Wednesday night through Monday...
The next chance for precipitation is Wednesday night and
Thursday...and confidence has further increased that this will be a
light precipitation event...as well as one with only liquid
Wednesday night if precipitation develops over our area. A closed
upper low near Los Angeles early this morning is still forecast to
get caught in the westerlies and dampen the strong upper ridge in
place over The Rockies. A gradual phasing with a northern stream
progressive wave is expected Wednesday night through Thursday
over the Great Lakes region. While the 27.00 GFS is more
aggressive with this phasing than other 27.00 guidance...it only
brings a little more oomph for precipitation here. The thermal ridge
mentioned in the short term section for Wednesday is 2c-6c at
850-925mb and surface temperatures are not likely to drop given
the southern flow and increasing clouds...so have chances of rain
and drizzle Wednesday night as moisture transport develops. On
Thursday...transition to a light rain and snow mix and at this
point wpc and ourselves really have no accumulations forecast.
Have still tried to take into account some possible slowing in the
forecast given the current significant closed system off the
northeast u... kept precipitation chances into Thursday afternoon.
The progressive active flow pattern is forecast to continue into
the weekend...with strong high pressure on Friday giving way to
precipitation chances this weekend. Another closed low over the
far southwestern U.S. Is forecast to move gradually eastward and
looks to be able to tap into Gulf and eastern Pacific moisture.
With a cut-off feature this could end up slowing and certainly
changing in evolution. Guidance disagrees how any northern stream
disturbance may aid to pull some of this moisture over our area.
At this time this looks to be a possibly long duration but light
intensity quantitative precipitation forecast event at this latitude. Given the features at play
and current pattern we are in...its just way too early as
potential could go either way from that solution. Confidence is a
little higher in that it would probably be a mainly snow event if
it were to occur. Guidance is in better agreement than 24 hours
ago with a Canadian/modified Arctic air mass being tapped in the
wake of this system with possibly single digit nighttime
temperatures depending on if any fresh snow cover can occur.
//Ord and mdw concerns...updated 18z...
* MVFR ceilings clearing the terminals early this afternoon...but
likely remain nearby this evening and tonight.
Broad MVFR stratus deck blankets the region today but has been
steadily eroding from the northeast...with clearing skies making
progress Down Lake Michigan this morning and into the Chicago area
early this afternoon. Cloud bearing flow out of the northeast this
morning will back to the north this afternoon and evening so expect
the progress of the clearing to slow some...and may even reverse
tonight as a surface ridge builds across Indiana with flow becoming
more southerly. Surface winds will generally decrease from middle
afternoon on as the surface high nears...and go light and
variable for a period this evening and overnight...before turning
south/south-southeast tomorrow. Gusty winds pushing 20 knots are
expected tomorrow afternoon.
//Ord and mdw confidence...updated 18z...
* medium-high confidence in periods of VFR this afternoon...low-
medium confidence in ceiling trends overnight.
//outlook for Ord/mdw for 00z Thursday-12z Tuesday...updated 12z...
Wednesday night...chance of rain/drizzle. MVFR/IFR possible.
Thursday...chance of rain/snow. MVFR likely/IFR possible.
Friday and Saturday...primarily VFR. Chance of snow Saturday
Sunday...chance of light snow. IFR possible.
Monday...MVFR ceilings possible.
213 am CST
Marine concerns on Lake Michigan are a period of strong south winds
Wednesday afternoon and night...then a shift to strong north to
northwest winds Thursday afternoon into early Friday...with the
potential for a period of northwest gales especially Thursday
afternoon and Thursday evening.
In the near term...relatively quiet weather across the lake early
this morning as a weakening surface trough over Illinois and the
lower Ohio Valley continues to move off to the southeast...allowing
weak high pressure to settle across the area from the north today.
Looking to the west...an area of low pressure developing in the Lee
of the Canadian rockies will slide southeast into the plains
through tonight...then will move slowly east toward Lake Michigan
Wednesday and Wednesday night. In response to pressure falls ahead
of the low...the gradient will tighten across Lake Michigan
Wednesday with south winds ramping up into the 30 knots range by late
afternoon and evening. Winds will diminish briefly early Thursday
morning as the low pressure center and cold frontal trough cross the
lake...before winds shift to the north-northwest and increase
quickly back into the 30-35 knots range Thursday afternoon. Forecast
soundings indicate the potential for gales Thursday afternoon and
evening in the strong pressure rises and cold advection behind the
departing low and cold front. These strong winds will continue into
early Friday...before diminishing as a broad area of high pressure
spreads into the area by Friday evening. The high will slowly move
off to the east on Saturday...with a weak low expected to pass
across the northern lakes Saturday night and trail a cold front
across Lake Michigan through Sunday.
For the Illinois and Northwest Indiana nearshore waters Small Craft
Advisory conditions look to develop Wednesday afternoon and continue
through Thursday morning...with a brief break before high wind/wave
conditions develop again by early Thursday afternoon. High waves
would likely linger through much of Friday...especially along the
Indiana shore east of Gary.
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