Scientific Forecaster Discussion
Area forecast discussion
National Weather Service Chicago/Romeoville Illinois
712 PM CDT Friday Mar 14 2014
issued by National Weather Service Milwaukee/Sullivan WI
at 337pm CDT Friday Mar 14 2014
Forecast confidence medium.
Water vapor imagery shows a fairly compact middle-level short wave
moving across Northern Lake Michigan...with surface low pressure
over Lake Huron and a trailing surface cold front moving southeast
through southern WI into northwest Illinois. The surface cold front will sweep across
the rest of the area late this afternoon/evening.
A quiet night expected with some lower clouds in the cold advection
behind front. 850mb temperatures dip to around -2c to -4c by 12z
Saturday leading to lows in the middle 20s inland north...with upper
20s in the southern forecast area and from the city and south along
the lake. Gusty west winds will ease somewhat this evening as the
surface low over Lake Huron moves away to the east...but will remain
strong enough along with some lower cold advection clouds to prevent
temperatures from getting even colder.
Next short wave approaches from the northwest Saturday. 12z runs of the GFS
and Canadian Gem bring precipitation to the northern counties by 00z
Sunday...while 12z NAM and European model (ecmwf) keep the forecast area dry...in
spite of strengthening 850-700 mb frontogenesis. Lack of precipitation looks
to be tied to dry layer below 5k feet on cross sections and forecast
soundings that is reflected as well in high condensation pressure
deficits on isentropic surfaces. A blended solution brings slight
chance/low chance probability of precipitation in the late afternoon to northern County Warning Area...with the
better chance coming Saturday night. Top down method indicates it
will be all snow...though warm air advection ahead of the wave and
associated surface low brings a tight temperature gradient across
the County Warning Area with highs in the upper 30s north to around 50 south.
Long term...(saturday night through friday)
issued at 337 PM CDT Friday Mar 14 2014
Saturday night...forecast confidence medium.
High pressure singking into the Great Lakes region will push Arctic
air into northern Illinois beginning Saturday afternoon through Sunday.
There is increasing confidence for snow to spread into northern Illinois
early Saturday evening as a narrow zone of strong...sloped 925-700mb
frontogenesis slips southward. This will be a wet snow over a fairly
short period of time and amount to around one inch total. Northeast
winds will increase and become gusty overnight with the cold air
Sunday through Monday...forecast confidence high.
Sunday will be a bitter day with brisk northwest winds...scattered
low stratocumulus clouds and high temperatures in the 20s. With
skies clearing and winds tapering off Sunday night under high
pressure...low temperatures will drop into the teens.
A middle level ridge over the northern plains will move into the Great
Lakes on Monday as a middle level trough deepens over the Pacific
northwest. Warmer air will gradually return to the Midwest on Monday
with highs rising into the 30s.
Tuesday and Wednesday...forecast confidence low.
The middle level trough will dig into the northern plains Tuesday to
bring our next chance for precipitation. The GFS and European model (ecmwf) are in
agreement that northern Illinois will be in the warm sector as the surface
low tracks across northern Iowa/southern Minnesota Tuesday and into central
Michigan Tuesday night...so any precipitation will likely be rain.
Timing is uncertain for when the cold front will move through the
lot forecast area. The European model (ecmwf) is about 12 hours slower with this
front than the GFS. Either way...it looks like rain will transition
to snow with this front...with precipitation ending shortly after the
frontal passage so not much accumulation is expected.
Thursday and Friday...forecast confidence medium.
High pressure will take control of the center of the country for the
latter half of the work week. Maximum temperatures will be in the lower
to middle 40s...a little below normal for middle March. Models are showing
another push of warm air approaching Illinois on Friday with a weak
//Ord and mdw concerns...updated 00z...
* gusty winds from the west/northwest of 20-30kt will persist
through 2z...then continue to diminish to around 10kt by 6z.
* Clear skies overnight...then skies thicken and steadily lower
tomorrow afternoon. But should remain VFR until after 00z sun.
Low pressure continues to lift northeast across Lake Huron...with
the tight gradient still holding over northern Illinois/northwest in.
High pressure will build into the area this evening...which will
aid in bringing much lighter winds to the taf sites tonight along
with near cloudless skies. Winds will be light at less than 7kt
overnight from the west...then slowly turning northwest...and
eventually northeast by daybreak Saturday with speeds generally
around 5-8kt. The atmosphere remains very progressive as another
low pressure system is expected to develop across the Southern
Plains and lift northeast through the Ohio Valley Saturday
night/early Sunday. Prior to this...a frontal boundary will be
dropping south across the taf sites Sat evening. This will be
joined with some moisture and should see area of light snow
develop after 00z. Ceilings will be steadily lowering throughout the
afternoon hours Sat...but should remain VFR until after 02z sun. A tighter
gradient will be developing along with the frontal boundary...and
suggests winds will be on the increase yet again Sat night from the
northeast...with a few gusts approaching the 15 to 20 knots range.
Guidance has demonstrated some uncertainty with Saturday night
timing/placement/intensity of snow...thus at this time we have
held onto only carrying light snow and keeping VFR visibilities. Ceilings should
be hovering around MVFR conds.
//Ord and mdw confidence...updated 00z...
* high confidence in all forecast elements through midday Sat.
* Medium confidence in timing of snow/MVFR ceilings after 00z sun.
//outlook for Ord/mdw for 12z Sunday-12z Friday...updated 00z...
Sunday...VFR. Gusty northeast winds.
Tuesday...chance of rain Tuesday afternoon & night with VFR becoming MVFR.
Wednesday...slight chance of ra/sn. MVFR possible.
issued at 337 PM CDT Friday Mar 14 2014
Will let Small Craft Advisory expire in the nearshore waters at
10 PM with surface pressure gradient relaxing as low moves east of
Lake Huron and across SW Quebec overnight. A second short wave
will bring a chance for rain or snow...or a mix...to Northern Lake
Michigan tonight. High pressure building in behind this wave will
turn northwest winds to the northeast and increasing to near gale
force in the south Saturday night and Sunday. Just borderline at
this time so no watch...but later shifts will have to monitor.
Lm...Small Craft Advisory...nearshore waters
until 10 PM Friday.
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