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Area forecast discussion 
National Weather Service Chicago/Romeoville Illinois
1232 PM CDT Sat Mar 28 2015

1207 PM CDT

High pressure spanning the region providing quiet weather
today...but with chilly conditions in place. Temperatures still in the
low 30s for most locations at this time with onshore flow limiting
temperatures in the 20s still for locations near the lake. Sunny skies
will prevail today with temperatures generally in the middle to upper
30s...although some locations could top at around 40 degrees this



Short term...
306 am CDT

Through tonight...

Surface high pressure under dry northwest upper flow today will
offer a sunny sky and little in the way of sensible weather. A lake
breeze is expected to develop and spread inland a couple tiers of
counties early to middle afternoon. This will stunt warming some and
maybe even lead to a drop by a couple degrees for places near the
Lake Shore.

The low-level air mass will modify some today and without as strong
of a lake advective component in the wind speed...temperatures
should result around 5f higher. The light onshore flow during late
March on average results in about a 6-7 degree difference between
the far downtown Chicago and the far outlying metropolitan and am near that
in the forecast for today...about 38-39 outlying to 34 downtown.
Tagging 40 is expected toward the I-39 corridor.

The dry air mass today away from the lake modified air could result
in dew points falling into the single digits during peak mixing
early to middle afternoon.



Long term...

Sunday through Sunday night...

Main concerns are with strong southwest winds and rain showers
ahead of an approaching cold front. Part of strong upper wave/vorticity
maximum moving onto Pacific northwest coast as noted on west/v imagery this morning
will dive southeastward toward the area. In addition...robust low
level jet of 50+ knots will focus a narrow axis of strong moisture
transport...with warm air advection/isentropic ascent also aiding in precipitation
blossoming. Fairly steep lapse rates aloft could result in brief
periods of moderate intensity showers. There is some concern that
precipitation could start as light snow or a mix at onset in parts of the
northern County Warning Area...aided by wet bulbing of initially dry column...but
rapid warming of the boundary layer should cause any initial
frozen ptype to go over to rain. Departure of expansive high
pressure to the southeast and approach of cold frontal trough
trailing from low pressure near or north of Lake Superior will
result in fairly impressive isallobaric pressure falls. Thus
south-southwest winds will become quite strong...with gust speeds
up to ~40 miles per hour supported by model soundings. The strong winds will
counteract highs in the middle to upper 40s to make for a raw day.

Have bumped up probability of precipitation a bit more within the categorical range from
the late morning through early afternoon...then a trailing upper
wave should keep some shower activity going into the northeast
half especially of the County Warning Area in the late afternoon and possibly
early evening depending on path of the short wave. Winds will turn
west and then diminish behind the cold frontal passage...with
clearing skies and lows falling into the lower 30s.

Monday through Friday...

Upper level pattern will deamplify some...which will keep the cold
air locked to the north in Canada. Overall...this will result in a
much milder week...though there is uncertainty in the guidance in
how warm certain days will become. Thus stayed close to a guidance
smart blend for temperatures. Monday should be a fairly nice day
in the middle to upper 50s and no lake cooling anticipated.
Differences also continue on handling of a clipper type system
Monday night...though 00z European model (ecmwf) did trend north and 00z
operational GFS also remain well north. Maintained lower end
chance and slight probability of precipitation for now north of the Kankakee River. Warmer
end of guidance would yield at least middle to upper 60s by
Wednesday...then next frontal system could arrive Wednesday
night...with a slowing trend on last few model cycles. Added
slight chance thunder wording in GFS and some ensemble
members do bring an impressive surge of higher dewpoints
northward. Uncertainty then really grows later in the week...with
some potential for a stronger low pressure area as per 00z
operational European model (ecmwf).




//Ord and mdw concerns...updated 18z...

* lake breeze influenced east-northeast winds this afternoon
generally 8-10 knots range.
* Precipitation developing middle-morning Sunday...could include a
mix of sleet/snow for 1-2 hours at onset before changing to all
rain with temperatures above freezing.
* Very gusty southwest winds Sunday...sustained 20 knots with gusts
over 30 knots.


//discussion...updated 18z...

Surface high pressure moving across the terminals today...with
weak gradient and light winds allowing lake breeze to develop and
push inland with east-northeast wind near 10 knots eventually
becoming light southeast this evening. May briefly see winds 10kt+
but should be sporadic/short lived.

Low pressure will move across southern Canada tonight and
Sunday...allowing winds to become south and increase especially
toward morning. Just prior to sunrise winds in the 1000-1500 feet
layer increase to 40+ kts...though gusty surface winds also
develop which should result more in a mechanical turbulence
scenario rather than a sharp low level wind shear layer. Southwest winds will
increase during the morning...with gusts reaching/topping 30 kts
by late morning and continuing into the afternoon.

Precipitation is expected to develop from the west Sunday moisture lifts north toward the Canadian low. Column
looks to initially be cool least through evaporative support a mix of snow/sleet during the onset of
precipitation before changing to all rain as warm advection
overwhelms column and saturation occurs. Surface temperatures
will be rising into the middle-upper 30s during the morning and
around 40 by early afternoon.


//Ord and mdw confidence...updated 18z...

* medium-high in lake breeze wind trends this afternoon.
* High in precipitation developing by middle-late morning Sunday. Low
in precipitation type at onset.
* High in strong gusty SW winds Sunday. Low in true low level wind shear Sunday


//outlook for Ord/mdw for 00z Monday-12z Saturday...updated 12z...

Sunday night...mainly VFR. West winds.
Monday...chance of rain and MVFR at night. Southwest winds.
Tuesday...VFR northeast winds.
Wednesday...chance for rain showers at night.
Thursday-Friday...chance of rain. West winds.



425 am CDT

Main marine concern continues to be with south-southwest gales on
Sunday. Have issued a Gale Warning for the entire lake...including
the nearshore waters. Strong low pressure will track near or north
of Lake Superior and expansive high pressure will depart to the
southeast US...with tight pressure gradient and strong pressure
falls ahead of cold front trailing from the low. South-southwest
winds will quickly ramp up and become strongest on the northern
portion of the lake...where there remains some potential for gusts
approaching storm force for a few hour window middle to late Sunday
morning. Have stayed with 45 knots speed mention in Gulf...but may
need to add occasional storm gusts in subsequent issuances. In the
nearshore and south half...expecting speeds of 35 to 40 knots. Winds
will shift west behind the cold front Sunday evening...with speeds
remaining elevated around 30 knots across the open waters. Will need
to watch for a renewed period of gales on the north half closest
to the departing low pressure area.

A clipper type system will then move across the lake Monday
night...with uncertainty in the path of this system...followed by
high pressure middle week and then a cold frontal passage Wednesday
night. There is some potential for a stronger system approaching
the lake later in the week...but plenty of uncertainty due to
significant differences in forecast guidance.



Lot watches/warnings/advisories...
Lm...Small Craft Advisory...lmz740-lmz741-lmz742...4 am Sunday to 10
am Sunday.

Gale Warning...lmz740-lmz741-lmz742-lmz743-lmz744-lmz745...10 am
Sunday to 4 PM Sunday.

Gale Warning...lmz777-lmz779...4 am Sunday to 7 PM Sunday.



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