Scientific Forecaster Discussion

Return to Local Conditions & Forecast

Without Abbreviations
With Abbreviations

Area forecast discussion 
National Weather Service Chicago/Romeoville Illinois
1015 am CST Friday Feb 27 2015

908 am CST

Wind Chill Advisory allowed to expire this morning as lowest
values are around 15 below...and are isolated at this time. Full
sunshine under high pressure will allow temperatures to continue rising
back above zero for all areas through late morning...with highs
in the teens still on track this afternoon.



Short term...
312 am CST

Through Saturday...

High pressure centered over the middle Missouri Valley early this
morning will bring relatively quiet weather over the next day
or so as it builds east over central Illinois by this evening. This
morning...a few lingering lake effect snow showers persist near the
Lake/Porter County line. Drier air and lowering inversion heights
should allow precipitation to end by middle morning. Low temperatures this
morning bottomed out 10 to 20 below west of the Fox River valley
with much of the rest of the County Warning Area...outside downtown Chicago and
Northwest Indiana which saw clouds from lake effect...falling into
the single digits below zero. Winds are fairly light this morning
and some sites are even reporting calm conditions. Even so...a few
sensors are reporting 20 to 25 below zero wind chills. A Wind Chill
Advisory will remain in effect through middle morning for north central
and northeast Illinois. Expect temperature to top out in the middle
teens today with plenty of sunshine...and tonight temperatures will
radiate below zero once again under mostly clear skies and calm



Long term...
312 am CST

Saturday night through Thursday...

Shortwave trough is prognosticated to dig down the West Coast Saturday...
closing off over California Saturday night into Sunday. Alberta clipper is prognosticated to dig from Alberta
Saturday evening into the upper Midwest by Sunday evening. With
surface high pressure settling off the southeastern coast and
troughing in the Gulf moisture and broad ascent is
expected to set up across the middle Mississippi and Ohio Valley
persisting through the day Sunday. With little organization at the
surface...moisture is somewhat slow to push north across the Ohio
Valley...but precipitable waters are expected to rise to around 0.70 inches in
central Illinois midday Sunday...around +2 sigma per ilx radiosonde observation
climatology...though the precipitable waters quickly taper to less than 0.5 inches
across the northern counties. Precipitation should begin as warm air advection
ramps up over the area late Saturday evening through midday
Sunday. Warm air advection then shifts to the east while DPVA and broad middle level
height falls overspread the region from the northwest allowing
snow to continue Sunday afternoon into the evening. Middle level
mixing ratios south of I-80 average out to around 3.0 g/kg
suggesting the potential for a high end advisory event in our
south. There are some limiting factors too...including a
relatively shallow dgz though forcing does seem to be maximized
within this layer. And while there does appear to be decent f-general
over the area...models arent too crazy about any potential
instability so the banding potential right now doesnt appear
great. Not sure we would reach 6 inch/12 hour or 8 inch/24 hour
criteria for a watch/warning but with an extended period of
snow...far southern counties could certainly push 6 inches of snow
storm total. With snow starting fourth period...will not be
issuing any headlines yet. Bumped up snow totals south of I-80
slightly but made less changes north where totals may end up being
high...especially if we see a sharp cutoff on the north side as we
have in other events this season.

Transient area of high pressure will bring a brief respite from the
precipitation Monday...though attention turns back to the west as
sharp troughing over the West Coast inches east across the
intermountain west with downstream amplification of the middle level
ridge. Warm/moist advection ramps up into the middle Mississippi
Valley...especially Monday night into early Tuesday. While some
discrepancies in the details...thermal ridge is prognosticated to push
across northern Illinois on Tuesday with 850 mb temperatures warming to
around +5-7c. Strengthening jet stream and a middle level wave
ejecting ahead of the main upper trough will result in a deepening
surface low over the plains into the Midwest. A 50-60kt low level
jet ahead of the low will transport 1.0+ inch precipitable waters //99th
percentile// into central and northern Illinois. Precipitation
will begin very early Tuesday...and while there may be p-type
issues at the onset...expect much of it will fall as rain which
could result in Hydro issues with melting snow pack. This is a
very dynamic system and could even support some upright
instability rooted around 850mb. Should the current track verify
may have to add thunder mention in the south. Despite the brief
warm-up into the upper 30s to lower 40s Tuesday...colder air will
start to filter back in behind the system midweek with single
digit lows once again Thursday and Friday mornings.




Temperatures are forecast once again to be close or break the
following daily records through the end of the month...including
one on Friday in Rockford that was set just last winter:


Friday Feb 27th low maximum: 15 in 2014
Saturday Feb 28th low: -7 in 1962
Saturday Feb 28th low maximum: 17 in 1962


Friday Feb 27th low: -6 in 1897
Saturday Feb 28th low: 0 in 1884




//Ord and mdw concerns...updated 16z...

* none.


//discussion...updated 12z...

High pressure will spread east across the Midwest and southern
Great Lakes region through tonight. Dry Arctic air mass will
provide mainly clear skies through the taf period. West-
northwest winds generally under 10 knots will gradually become
southwest 5 knots or less this evening...and south-southeast early
Saturday morning.


//Ord and mdw confidence...updated 16z...

* high for all elements.


//outlook for Ord/mdw for 00z Sunday-12z Friday...updated 12z...

Saturday night...snow developing overnight. IFR likely by morning.

Sunday...periods of snow accumulating snow ending by evening. IFR

Monday...VFR then chance of wintry mix and IFR Monday night.

Tuesday...wintry mix changing to rain by afternoon...then possibly
ending as light snow Tuesday night. IFR.

Wednesday...MVFR possible.

Thursday...primarily VFR.



219 am CST

Main period of concern for gusty 20-30 knots winds looks to be Sunday
night into early Monday...and potentially again on Wednesday. Much
of lake remains ice covered...with only the central and southern
portions well away from shore appearing open in recent satellite

High pressure centered over the middle-Missouri River valley will move
east-southeast across the Midwest south of the lakes through tonight
and to the middle-Atlantic region by late Saturday. Meanwhile...low
pressure will move across Hudson Bay today...with a deeper trough
developing north of the lakes Saturday. This will result in winds
becoming westerly over Lake Michigan today...increasing a bit into
the 20 knots range tonight as the gradient tightens up between the high
passing south of the lakes and low pressure farther to the north.
Saturday into Sunday...winds will shift to the south-southwest as a
cold frontal trough approaches from the northwest. The front will
pass across the lake Sunday with a shift back to the west...becoming
northwest and becoming gusty 25-30 knots Sunday night into Monday
morning as high pressure moves in from the west and briefly tightens
the gradient once again. The high will move across the southern
lakes and Midwest Monday with northwest winds quickly diminishing
and eventually becoming southeasterly Monday night as the high
slides off to the east.

Medium-range model runs indicate low pressure will move near
Southern Lake Michigan Tuesday or Tuesday night. Current indications
are that southeast winds will increase Tuesday into the 20 knots range
or so...before briefly decreasing and veering north-northwest by
early Wednesday and increasing into the 25-30 knots range as colder air
once again surges in.



Lot watches/warnings/advisories...


Visit US at http://weather.Gov/Chicago (all lowercase)

Follow US on facebook...twitter...and youtube at:

National Weather Service Glossary of Abbreviations