Scientific Forecaster Discussion
Area forecast discussion
National Weather Service Chicago/Romeoville Illinois
402 am CDT Tuesday Mar 11 2014
343 am CDT
The taste of Spring yesterday is truly in the rear view mirror as
the forecast focus was on winter storm set to impact the area
tonight into early Wednesday. Have issued a Winter Storm Warning
centered on the I-80 corridor...and that includes Chicago...for the
combination of heavy snowfall rates of up to 2 inches per
hour...snow totals around six inches...and wind whipped snow
greatly reducing visibility. Continue the watch south of there and
an advisory to the north.
The primary upper low is well-defined this morning across Wyoming
and advancing southeast with over 200m height falls at 300mb sampled
last evening. Second key short wave is dropping southward from Manitoba.
The phasing of these systems is anticipated to occur across the
western Great Lakes overnight into Wednesday morning. As often seen
with phasing systems...forcing for lift and heavy precipitation is expected
to rapidly increase tonight. Numerical guidance has had some
pronounced differences and that continued into the 11.00z and
11.06z cycles...but it is important to note that all guidance
depict all the ingredients for heavy snowfall and that are
consistent with a phasing system. Have preferred the global guidance
solution of mass fields and taken into account the MRF and local
high res models ideas of mesoscale very heavy snowfall rates and
timing. Believe the MRF is a little too robust and widespread
with its heaviest quantitative precipitation forecast...but again the idea of heavy snowfall with
amounts exceeding six inches in an area is supported.
This still looks mainly dry across the forecast area with a very
sharp middle-level stationary front sampled on last Eves radiosonde observations becoming
more active ahead of the approaching troughs. Early morning radar
mosaic indicates precipitation from South Dakota through western WI. This should only
inch southward gradually today. Frontogenesis response from even the
most south high resolution guidance would keep rain north of the
area through mid-afternoon. Still should see plenty of cloud
cover and with northeast winds increasing ahead of cyclogenesis in
the plains...expect that surface temperatures will start to level
out around the noon hour and then begin to gradually fall. Onshore
flow into Chicago off the ice-covered Lake May lead to a more rapid
Tonight and Wednesday morning...
In response to the aforementioned phasing...maximum height falls are
centered directly on the area tonight with lift rapidly increasing.
This will evolve the low-level features eastward with copious
moisture transported north of a 40-45 knots low-level jet. Precipitation from
the north is expected to quickly move south...while new precipitation
should develop just west or over the area this evening. The degree of
forcing and cool advection in the northeast quadrant of the system
will help to cool the column from north to south for a transition of
rain to snow...and this will be especially true during the middle evening
into early overnight as the most robust forcing moves in. Would
expect some areas of northern Illinois may just begin as snow or
experience a very short mix. As for areas along and south of
I-80...a couple/several hour window of rain will be likely first
before transitioning to a wet snow late this evening into early
overnight. That time period after transition and into overnight is
most favored for heavy snowfall...with upright instability forecast
by global models to reach into the central forecast area...which
lines up with the deepest convergence and focus near I-80. The
11.03z sref probabilities of over one inch per hour snowfall rates
are to 70 percent which is impressive and make sense...and would not
be surprised to see temporary two inch per hour or higher rates late
this evening into overnight. The winds look to really ramp up overnight
around the deepening low...with 45 knots of northeast flow on the NAM
and GFS at 1000-2000 feet. Even with the wet snow...expect that the
blowing of the falling snow will cause even further reduced
visibility...and even temporary white out conditions in the heaviest
of snowfall rates.
By Wednesday morning snow should be ending from west to
east...although some pretty solid rates could still be occurring in
eastern areas...including possibly Chicago into the early parts of
the morning commute. Uncertainty on end time of snowfall is fairly
high...but the winds should continue and so expect impacts...and
that may be especially true near the Lake Shore where the wind could
gust to 40-45 miles per hour. That is why run headlines through noon.
With the high moisture...as depicted by mixing ratios of 4-6 g/kg
and precipitable waters of around 0.75 in...there should not be too much of a
problem with snowfall amounts in the maximum area topping six
inches...even with the quick movement of the system and low
snow-to-liquid ratios. The highest amounts could exceed 8 inches in
isolated areas...but again limited duration should prevent that from
being too widespread.
Given rates and blowing of the falling snow...with basically a
downpour of snow with concurrent gusts of 30+ miles per hour...have opted to go
with a warning for the area with the longest duration of snow....even
though not everywhere will see six plus inches of snow.
Uncertainty of snow duration in the far south led to continuing
the watch. Further north have an advisory...and did not include
Rockford because it looks like snowfall should have ended by much
of if not the entire Wednesday morning commute. It is certainly possible
adjustments to the northern end of headlines are made today.
Wednesday night and Thursday...
Not much change made to this period. Wednesday night looks to be
pretty chilly with the fresh snow cover and diminishing winds...and
if the sky can remain clear the lows may be within five degrees of
record lows for March 13th.
Extended period...Friday through Monday...
An amplified large scale upper level weather pattern will remain the
main story during the extended period. This will likely feature
positive height anomalies across the eastern Pacific and the Pacific
coast of the Continental U.S....and below average heights across the eastern
Continental U.S.. ultimately this will result in below average temperatures
during much of the extend period.
It appears Friday will be the warmest day of the extended. A clipper
type system is expected to track eastward across the upper Great
Lakes region on Friday...and this should result in breeze...but
mild...southwesterly flow across the region. It appears temperatures
could warm into the 50s...but any remaining snow cover from the
snowfall tonight may hinder the magnitude of warming some.
This warm air will be short lived as colder air will likely begin to
spill back southward across the area by Saturday as a cold front
drops across the area. Some light snow showers may accompany this
colder air push Friday night. Guidance is hinting at decent short
wave trough digging southeastward across southern Wisconsin during
this time. In spite of this...I have chosen to leave the forecast
dry at this time.
A very active jet across the Pacific will likely continue to drive
disturbances eastward atop a building western Continental U.S. Ridge into next
week. Although the actual timing and evolution of these
disturbances is highly uncertain at this juncture...it does appear
that a northwesterly upper level pattern across the central Continental U.S.
Could produce some additional chances of snow for portions of the
region later in the weekend...depending on the actually track. With
this in mind...I have left some low end probability of precipitation in for later in the
//Ord and mdw concerns...updated 09z...
* gusty northeast winds this afternoon.
* Snow developing late this evening with a period of heavy snow
* Blowing snow overnight into Wednesday morning.
* Rapidly deteriorating visible/ceilings to LIFR/vlifr with snow.
* Strong/gusty northerly winds overnight into Wednesday morning
with gusts 30-35 kts possible. Cms
Developing low pressure over the plains today will move northeast
across southern Illinois and southern Indiana tonight. There still
remains uncertainty regarding how far north the band of heavy snow
will move but the trends over the past day have been to push this
axis further north. Thus have gone more pessimistic with this
forecast...bringing a period of moderate snow across the terminals
just after midnight when the best forcing will be moving across
the terminals. Changes should be expected with later forecasts...
including the potential for including a period of heavy snow. If
this moves across the terminals...there could heavy snow for a
2-4 hour period before snowfall intensity decreases in the 10-12z
time period. Northeast winds will steadily increase through the
evening...turning more northerly overnight. Speeds could easily
reach 20-25kts overnight with gusts into the 30-35kt range.
Initially...the snow will be somewhat wet/average but will become
drier as colder air spreads into the system from the north. Thus
blowing snow will become an issue overnight and especially into
Wednesday morning when it could still be snowing lightly.
Ceilings/visible expected to tank into LIFR/vlifr after midnight. Steep
lapse rates may also allow for an occasional lightning strike
during the period of heaviest snowfall but confidence is too low
for including in the forecast at this time.
In the shorter term...increasing high/middle clouds through the day
with some light fog through middle morning. A period of light rain is
possible across southern WI and possibly into far northern Illinois this
afternoon but expect this to remain north of the terminals through
much of the day...perhaps moving across the terminals early this
//Ord and mdw confidence...updated 09z...
* high for snow late this evening through Wednesday morning...
medium for timing and specific trends.
* High for winds through the period...medium on timing/directions.
* High for VFR through early evening...medium on ceiling/visible trends
overnight into Wednesday morning. Cms
//outlook for Ord/mdw for 12z Wednesday-12z Monday...updated 00z...
Thursday through Saturday...VFR probable.
Sunday...slight chance of -sn.
330 am...low pressure currently over the Western Plains will move
east to southern Illinois late this evening and then deepen as it
reaches New England Wednesday night. The gradient will tighten as
this low passes south of Lake Michigan with northerly winds steadily
increasing from this afternoon into early Wednesday morning. Gales
appear to begin late this evening and increase to 45 kts overnight
and continue through midday Wednesday...slowly diminishing Wednesday
afternoon into Wednesday evening. Other than tweaking wind speeds/
gusts up...no significant changes to going forecast/grids or
headlines. Winds will quickly diminish Wednesday night/early
Thursday morning as a ridge of high pressure moves across the lake.
Winds then turn southwesterly and increase to 30 kts as low pressure
moves across Ontario Thursday night and Friday...with a trailing
cold front from this low moving across the western lakes Friday
afternoon into Friday evening.
Visible satellite imagery from Monday shows some open water in the
Illinois nearshore zones and have accounted for this with wave
heights in this morning/S forecast. However...with the expected
strong northerly winds...ice in the open waters is expected to be
pushed back to the Illinois shore tonight. Cms
Illinois...Winter Storm Warning...ilz013-ilz014-ilz019-ilz020-ilz021-ilz022-
ilz023-ilz032...10 PM Tuesday to 1 PM Wednesday.
Winter Weather Advisory...ilz006-ilz008-ilz010-ilz011-
ilz012...10 PM Tuesday to 10 am Wednesday.
Winter Storm Watch...ilz033-ilz039...1 am Wednesday to 1 PM
In...Winter Storm Warning...inz001-inz002...10 PM Tuesday to 1 PM
Winter Storm Watch...inz010-inz011-inz019...1 am Wednesday to 1
lmz874-lmz876-lmz878...midnight Wednesday to 9 PM Wednesday.
lmz868...3 am Wednesday to 4 PM Wednesday.
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