Scientific Forecaster Discussion
Area forecast discussion
National Weather Service Chicago/Romeoville Illinois
1002 PM CDT sun Mar 9 2014
322 PM CDT
Warmer temperatures in the near term...and precipitation type/snow potential for
especially southern County Warning Area later Tuesday-Wednesday are main forecast concerns
Surface low pressure continued to deepen over Ontario this afternoon
with a large area of 5+ mb three hourly pressure falls spreading
across the upper Midwest and western Great Lakes. Strong warm
advection was occurring aloft...though surface temperatures have been slow
to rise due to shallow mixing of this morning's strong inversion.
Winds will increase late this afternoon and evening as gradient
continues to tighten however...with temperatures expected to continue to
rise into the 40s through early evening before becoming steady or
dropping off a few degrees later tonight as winds ease a bit. Low
level thermal ridge is prognosticated across the forecast area Monday
morning/mid-day with h9 temperatures around +10 c...supporting maintenance
of going forecast temperatures above warmest guidance with highs from the
lower 50s north to the low 60s where little/no snow cover impact
will exist. Weak cold front settles across area Monday afternoon...
though with still relatively mild air mass of westerly Pacific
origin behind the front.
Attention then turns to precipitation development Tuesday as
amplified upper trough currently over the eastern Pacific eventually
arrives and phases to some degree with another short wave trough
digging from central Canada. Models in good agreement in developing
west-east band of precipitation in association with strong middle-level
frontogenesis and beneath divergent right entrance of strong upper
jet streak during the day...focused mainly north/northwest of the
County Warning Area. Forecast thermal profiles over the forecast area above freezing
below 850 mb with surface temperatures above 40 thus relatively low probability of precipitation in
place along the Illinois/WI border for rain before evening. Surface low
pressure develops into lower Missouri/Ohio Valley Tuesday night with
low level northeast flow and cold advection cooling the column as
precipitation develops across the region. Decreasing low level thickness
and cooling soundings support a change over to wet snow Tuesday
evening...with the potential for some accumulating wet snow
particularly across our south/southeast County Warning Area where strongest forcing
aligns late Tuesday night into Wednesday morning. Fairly low
snow/liquid ratio expected...though it appears that perhaps several
inches possible across our east central Illinois/northwest in counties
south of I-80. Upper trough axis then passes across area Wednesday
as low continues off to the east through Ohio Valley...with
potential for some light snow/snow showers continuing as Canadian
vorticity digs into developing eastern North American long wave trough
into Wednesday afternoon. Height rises/subsidence and surface high
pressure ridge then build quickly in from the northwest Wednesday
//Ord and mdw concerns...updated 03z...
* gusty south-southwest (210-230 deg) winds through late evening.
* Low level wind shear through late evening.
VFR conditions are in store for the next 24-30 hours so the only
concerns are with winds. South-southwest winds of 210-230 degrees
will gust into the low to occasionally middle 20s range this evening.
Low level wind shear will peak at 50-55 knots in the 1500-2000 feet above ground level layer over the
next several hours. Winds will veer slightly with gusts and low level wind shear
cutting off 7-8z tonight as a weak frontal trough approaches. Then
on Monday...some models are struggling with existing snowpack and
very mild air mass moving over the area...but guidance with a
better handle indicates potential for west-southwest winds to gust to the
middle-high teens/possibly to around 20 knots during the afternoon.
Another weak cold front will approach late tomorrow afternoon/early
evening along with a relaxation of the pressure gradient causing
winds to veer to west-northwest or northwest and diminish. Have some concern that
with mild land and very Cold Lake...a lake breeze could form late
in the afternoon and push inland. However no explicit forecast
guidance portrays this scenario and confidence is very low in a
lake breeze occurring...so not yet Worth a mention in Ord/mdw
//Ord and mdw confidence...updated 03z...
* medium-high confidence in wind and gust trends through the
* High confidence in low level wind shear through late evening.
//outlook for Ord/mdw for 12z Tuesday-12z Sunday...updated 00z...
Tuesday...chance of rain during day with a chance of rain or snow in
evening and chance snow overnight. IFR possible.
Wednesday...slight chance of snow. Gusty northerly winds.
206 PM CDT
Winds have been slow to ramp up this afternoon over the lake but
with strongest winds aloft forecast to move in this evening am
reluctant to back down from the forecast of gales despite what
appears to be somewhat diminished chances. Winds should ease a bit
Monday as gradient gets a bit baggier in advance of a couple of
cold front slated to move south across the lake. Secondary front
moves across the lake Tuesday with Colorado low prognosticated to deepen
as it moves east into the Ohio Valley Tuesday night into
Wednesday. Result should be strengthening cold air advection and
strong northerly winds over the lake...probably nearing gale force
if current model projections of the strength of the low hold.
Given its still 48-72 hours out no gale watch will be issued at
this time...but later shifts may need to consider a gale watch if
current trends hold.
Lm...Small Craft Advisory...nearshore waters
until 4 am Monday.
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