Scientific Forecaster Discussion
Area forecast discussion
National Weather Service Chicago/Romeoville Illinois
1243 am CDT Wednesday Mar 12 2014
905 PM CDT
No changes planned with the current headlines. The low pressure
has pushed east into central Missouri this evening...and continues
to slowly slide east to northeast. Precipitation has taken just a
little longer to develop...however local radar indicates several
areas beginning to develop. In addition some convection has
developed this evening as well across the forecast area...mainly
south of I-88 in northern Illinois. Due to the low topped nature of
these thunderstorms...reports of pea size hail. Temperatures will
begin to fall...which will likely allow a transition towards snow
later this evening. One item that has been updated for tonight was
to include the mention of slt chance thunder for areas along and
south of I-80 in northern Illinois/northwest in. It is possible that
once the steady heavy snow gets going...that given how unstable
the environment remains...that thundersnow may occur in a few
locations across northeast Illinois/northwest in...just south of
355 PM CDT
Headlines...upgraded the southeastern counties from a Winter Storm
Watch to a Winter Storm Warning. Otherwise no changes were made to
Rest of today through Wednesday afternoon...
an upper level trough axis stretches from Hudson Bay through the
northern rockies...with another weak shortwave is moving into North
Dakota and northern Minnesota. The surface low is over southeastern
Kansas and is beginning to deepen.
A stationary front is over the far southern section of the County Warning Area...and
stretching from northern MO through Logansport Indiana. Temperatures have
been rather impressive today given the cloud cover. Everywhere
jumped to at least 40 with areas south of I-88 reaching into the
upper 40s to around 60 in central Illinois. Even seeing 40 degree temperatures
throughout southern WI despite the northerly winds. As winds become
north across the County Warning Area will see temperatures start to fall this afternoon.
May have too quick of a trend in the grids...but expecting temperatures to
fall pretty quickly.
The low will continue to deepen as it crosses over southern Illinois and
continues east into the upper Ohio River valley tomorrow afternoon.
High pressure is centered over Manitoba and it will move over SW
Ontario by tomorrow morning. As such the pressure gradient will
tighten overnight resulting in strong winds. In general winds will
be 20-30 miles per hour with gusts up to 40 miles per hour. The strongest gusts...up to 45
miles per hour...will be along the Lakeshore.
Right now there is a band of light rain over southern WI and a few
stray echoes over northwest Illinois. Expecting that band to slowly shift
south into northern Illinois late this afternoon and initially fall as
rain. Expecting snow to spread across the central and southwestern
portions of the County Warning Area this evening as the low moves over southern Illinois.
At the same time...the main trough axis and associated vorticity streamer
will be moving over the Mississippi River bringing impressive
forcing to the region. Guidance indicates there will be a band of
frontogenesis associated with negative epv roughly along the I-55
corridor. Additional forcing will be provided by the upper level
jet. We will be under the left exit region of the jet streak
associated with the deepening low...and then at the nose of the jet
pushing in from the north. All of this forcing is over the
dgz...which varies from 100mb to 200mb deep. Therefore...kept the
same idea as the previous forecast with the majority of snow falling
overnight tonight. Rates of 1-2 inches per hour are expected along
and east of I-55.
Left thundersnow out of the forecast because do not have enough
confidence in it. Forecast soundings feature minimum cape
values...less than 50 j/kg. May see a random strike of lightning
associated with the heavy snow...but not expecting an abundance of
Therefore the Winter Storm Warning is for periods of heavy snow and
near blizzard like conditions with the gusty winds. Visibilities at
or below a quarter of a mile are possible overnight and into the
pre-dawn hours Wednesday morning. The morning commute will probably
be impacted by the lingering snow and gusty winds. Even if it is
not snowing that hard during the commute...the gusty winds will
likely be blowing snow around and reducing visibility.
Snow intensity dwindles tomorrow morning as forcing and saturation
in the dgz both diminish. As the trough moves through...the snow
ends from west to east. May see some light snow persist over
Northwest Indiana after the warning expires...but thinking the main
show will be over. Snow totals will vary from 2 to 4 inches across
the Illinois/WI state line to 4 to 8 inches along and south of I-88. May
see a few locally higher totals especially if we see some
Winds will be due north down the lake and may see a few light snow
showers primarily over Northwest Indiana in the afternoon. Do not
think the showers will amount to much with the extensive ice cover
over the lake. Also may see a few stray light snow showers over the
western County Warning Area late Wednesday morning into early Wednesday afternoon as
another vorticity streamer moves through the region. Confidence is low
with these snow showers as the forcing and moisture do not look
overly impressive at this time.
As mentioned above...temperatures will fall later this afternoon through
this evening with overnight lows in the 20s. With the strong
winds...wind chills will be in the single digits tonight. Temperatures will
not rebound much tomorrow with the high temperature being reached in the
morning. The cold air associated with the trough moves over the
region during the day...so high temperatures will be in the 20s. Wind
chills will make it feel like +10 or lower.
Wednesday night and Thursday...
the trough shifts east Wednesday night and upper level temperatures begin
to rise. Overnight lows will be in the single digits...except 10-15
degrees over the city. Wind chills will be around zero with some
outlying areas getting as low as -5.
There is a slight chance of light snow north of I-88 Thursday
morning as a weak warm front moves into the region. Forcing does
not look impressive with the front so have low confidence in this
chance of snow.
High temperatures on Thursday will be in the 30s due to warm air advection associated with
the front. The warming trend continues overnight with lows around
//Ord and mdw concerns...updated 06z...
* heavy snow through 10z-11z with vlifr cigs/vis.
* Strong/gusty northerly winds through middle/late afternoon.
* MVFR ceilings after snow ends through middle afternoon. Cms
Precipitation has transitioned to a wet snow across the Chicago
terminals with a mix of rain still possible along a gyy/jot line.
Large area of snow over southeast Iowa/northern MO and western Illinois
will continue east/northeast across the terminals through the
early morning hours with prevailing moderate snow expected.
Periods/bursts of heavy snow also possible and maintained previous
tempo. With a sharp edge expected along the northern edge of the
snow...prevailing visibilities at Ord/dpa could be a bit higher
than advertised and will need to monitor trends over the next few
hours. Still a few lightning strikes...at times...which will be
possible in the bursts of heavier snow but expected coverage too
low for mention in the forecast. Short term/hi res guidance in
good agreement with the heaviest snow shifting east of the Chicago
terminals 10z-11z with rapid improvement in both ceilings and visible
expected in the 11z-13z time period.
Further northwest at rfd...period of light snow still possible for
a few hours...but heavier snow and lower conditions appear to
remain south of rfd.
North/northeast winds have been gusting into the 20-25kt range at
times but one minute data at Ord/mdw have been relatively gust
free since precipitation has changed over to snow. Winds will increase
overnight...but not quite as fast or strong as previously
expected. Strongest winds likely to occur after snow ends and
continue for much of the day with northerly directions. Snow is
fairly wet with temperatures still in the lower 30s and with expected
lower speeds/gusts during much of the falling snow...have removed
blowing snow...which may still be possible with any lighter/drier
snow that falls as snowfall is ending toward daybreak. Cms
//Ord and mdw confidence...updated 06z...
* high for snow/cig/vis trends.
* Medium for wind speeds/gusts/directions and timing.
* Medium for MVFR ceilings into this afternoon. Cms
//outlook for Ord/mdw for 12z Thursday-12z Tuesday...updated 00z...
* Thursday...dry/VFR. Gusty southwest winds.
* Friday...dry/VFR. Gusty westerly winds.
* Sunday...slight chance of snow.
210 PM CDT
Low pressure over the Central Plains will continue to deepen as it
tracks east across southern Illinois tonight and into the Ohio
Valley Wednesday. As the low tracks south and then southeast of the
lake...high pressure will also build across the upper Great Lakes
region...setting up strong north winds. The combination of a strong
gradient and strong cold advection will lead to winds quickly
increasing to gales by late this evening. The latest guidance has
been relatively consistent in indicating the potential for high end
gales by late Wednesday morning and early afternoon...with the
strongest gales over the southern portions of the lake...including
the nearshore waters. By Wednesday evening...the high pressure will
begin to spread across Lake Michigan allowing winds to subside below
gale force. By Thursday night...the high will push to the southeast
of the lake as a clipper drops out of northern Alberta and swings
just north of the upper Great Lakes on Friday. With the surface high
sagging off the southeastern coast...a period of southwesterly winds will develop
ahead of an approaching cold front trailing from the clipper.
Anticipate a period of brisk southerly to southwesterly winds for Thursday night
through Friday...but more gales are not expected at this time. As
the clipper turns northeastward into Quebec...it will drag a weak cold front
down the lake with another Arctic high building in behind the
front. 25-30kt north winds will set up following the frontal passage for
Saturday night and Sunday...but as the high spreads across the Great
Lakes Sunday night...winds should become more light and variable.
Illinois...Winter Storm Warning...ilz013-ilz014-ilz019-ilz020-ilz021-ilz022-
ilz023-ilz032 until 1 PM Wednesday.
Winter Storm Warning...ilz033-ilz039 until 1 PM Wednesday.
Winter Weather Advisory...ilz006-ilz008-ilz010-ilz011-ilz012
until 10 am Wednesday.
In...Winter Storm Warning...inz001-inz002 until 1 PM Wednesday.
Winter Storm Warning...inz010-inz011-inz019 until 1 PM Wednesday.
lmz874-lmz876-lmz878 until 10 PM Wednesday.
lmz868 until 7 PM Wednesday.
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