Scientific Forecaster Discussion
Area forecast discussion
National Weather Service Chicago/Romeoville Illinois
1007 am CST Sat Feb 28 2015
320 am CST
Main forecast concern over the weekend will be dealing with snow
chances and amounts tonight through the day Sunday.
Today...high pressure is centered over the upper Ohio Valley and
will shift east across the middle Atlantic coast while a potent trough
digs south across the West Coast and closes off later today.
Meanwhile...a northern stream clipper is prognosticated to dive into the
northern plains this evening and across the upper Midwest Sunday.
Ahead of these systems...the baroclinic zone will wave north with
broad and persistent warm advection overspreading the forecast area
this afternoon with increasing cloud cover. A lead wave will result
in precipitation overspreading the region late this evening and
early overnight producing light accumulations across the County Warning Area...but
especially south of I-80 where 1-3 inches is possible by around
daybreak. North of I-80 would only expect a dusting. While much of
the area may see a lull in precipitation very early Sunday before
the northern stream wave moves across the region...the far southern
tier of counties in particular could see more steady snow continuing
just north of the baroclinic zone. The main uncertainty resides in
exactly where this will set up...with models trending south over the
past 24 hours. Based on the latest projections/model consensus...an
advisory may still be needed for the southern tier of counties in
the forecast area...however should southward trends continue then we
may skate by without any headlines. Even with the uncertainties in
the location...there remain other limiting factors with this
system...including a relatively shallow dgz...no strong signal for
mesoscale banding this far north...and being a more prolonged light
snow should help mitigate any impacts. The latest storm total
forecast calls for up to 2 inches north of I-80...and 2 to 5 inches
south...though southern portions of Ford/Iroquois/Benton counties
may see 4 to 6 inches. Bottom line is that we continue to taper snow
totals in the north...and will wait another cycle before we issue
headlines for the south...mainly due to uncertainties in the track.
320 am CST
Sunday night through Friday...
High pressure will allow clouds to clear out Sunday night and for
some sunshine on Monday...but a more potent system continues to bear
watching for Tuesday. Deep troughing over the West Coast will inch
east across the intermountain west early next week with a lead wave
ejecting over the plains resulting in a deepening surface low
prognosticated to track from the Central Plains Monday night and across the
middle/upper Mississippi Valley Tuesday. Many of the forecast details
hinge on the track...but so far models seem to be focused on a track
across or just west/north of the County Warning Area. This should result in the warm
sector lifting north across Illinois. Models continue to hint at
50-60kt low level jet advecting around +5c 850 mb thermal ridge and 1.0
inch precipitable waters across central/northern Illinois Tuesday afternoon. At
the surface...warm/moist advection will fight against our snowpack
but could see temperatures pushing the 40 mark. As previously
mentioned...rapid melting/runoff remains a concern Tuesday given the
anomalously high precipitable water air/high quantitative precipitation forecast and middle 30 dewpoints which should
eat away at the snow. P-type may be a concern mainly at the onset
with the warm air overriding our below freezing surface temperatures...but
typically find that models are too slow and under-do the strength of
the warm advection down low so will continue to monitor but freezing
may be somewhat localized. Otherwise any frozen precipitation types early
should transition to all rain from middle morning to early afternoon.
Precipitation may change back to snow on the backside Tuesday evening/night
but do not anticipate any accumulations. Cold high pressure makes
its way back into the region Wednesday with 850 mb temperatures bottoming out
around -20c Wednesday night when sub zero lows are once again
possible. Models do hint at another brief warmup Friday into
Temperatures are forecast once again to be close or break the
following daily records through the end of the month.
Saturday Feb 28th low: -7 in 1962
Saturday Feb 28th low maximum: 17 in 1962
Saturday Feb 28th low: 0 in 1884
//Ord and mdw concerns...updated 16z...
* a period of light snow expected to develop this evening.
MVFR/IFR likely. Lull in precipitation expected early Sunday morning
with another period of light snow middle morning into the
Surface high pressure is sliding off to the east of the area this
morning...with light south winds less than 10 knots gradually
becoming southeast by this evening. Patchy middle-level clouds across
Missouri and central Illinois will eventually spread across the
terminals from the south this afternoon...with lower VFR ceilings
developing by evening.
Middle-level disturbance approaches this evening...with a period of
light snow expected to develop during the evening and persist
until after midnight. MVFR ceilings/visible likely...possibly brief IFR
visible in light snow. Appears there will be a lull in precipitation during
the early morning hours of Sunday...before another period of light
snow develops middle-late Sunday morning with a return of MVFR/IFR
conditions. Terminals will be along northern edge of snow...so
somewhat lower confidence in impacts on visible/ceilings especially
Sunday. Light south winds become light southwest Sunday morning.
//Ord and mdw confidence...updated 16z...
* medium on snow timing and impact to cig/vis.
* High all other elements.
//outlook for Ord/mdw for 00z Monday-12z Saturday...updated 12z...
Sunday night...MVFR ceilings mainly early.
Monday...VFR. Chance of snow after midnight.
Tuesday...rain...freezing rain...or snow. IFR probable.
Wednesday...MVFR ceilings possible.
Thursday and Friday...mainly VFR.
205 am CST
Marine concerns focus mainly on a couple of periods of winds in the
25-30 knots range...Sunday night...and again with a greater potential
for a period of gales Tuesday into Wednesday.
High pressure was centered across Illinois...Indiana and Ohio early
this morning. This high will develop eastward across the northern
Appalachians today...and to the Jersey Shore on Sunday. Farther
north...a deep low will move across far northern Ontario and Hudson
Bay but will trail a cold front and associated trough southward
across the western lakes Sunday. Colder air returning behind this
front and high pressure building east from the plains will result in
northwest winds increasing into the 25-30 knots range Sunday night.
Winds will diminish fairly quickly Monday however...as the high
moves quickly across the Midwest and Great Lakes Monday afternoon.
Winds will veer back around to the south-southeast Monday night as
the high continues off to the east...and an area of low pressure
takes shape over the Central High plains. This low is expected to
deepen while tracking east across Iowa and Illinois on Tuesday...and
northeast across lower Michigan and southwestern Ontario Tuesday
night. South-southeast winds will increase into the 20-25 knots range
ahead of the low Tuesday...then likely into the 30-35 knots range
Tuesday night into Wednesday as cold air again surges in behind the
departing low. The low will continue to deepen off to the northeast
into Quebec Wednesday...as high pressure builds southeast across the
plains...helping to maintain gusty northwest winds into Wednesday
Some model differences remain with the strength and timing of the
passage of the low across the area Tuesday and Tuesday night...
though the scenario is similar amongst guidance solutions. Gale
headlines may need to be considered as the time approaches and
guidance solutions converge on the timing of the greatest threat.
Visit US at http://weather.Gov/Chicago (all lowercase)
Follow US on facebook...twitter...and youtube at: