Scientific Forecaster Discussion
Area forecast discussion
National Weather Service Chicago/Romeoville Illinois
541 am CST Monday Feb 8 2016
337 am CST
The forecast details and overall message remain similar for the
upcoming long duration light snow event within a more blustery
and colder pattern. Have honed further in on this evening through
Tuesday morning as a probable time for widespread light snow with
accumulations...with today seeing more flurries and occasional
snow showers. With the over 30 hours expected of off and on
snow...accumulations continue to look on the order of one half to
2.5 inches for most areas. In general...higher totals look
east...although snowfall totals tend to end up like a wrinkled
sheet and not nice and smooth /borrowed that from a colleague/.
Finally...Porter County Indiana still looks to have lake effect
enhancement that will bring snowfall totals up.
One of the more pronounced cold core upper lows of this winter is
rotating its way southeastward across the western Great Lakes
early this morning. Large scale water vapor imagery reveals the
upper jet maximum is still rounding the top of the western North
American ridge with the ridge continuing to build. As a
result...the Great Lakes trough will continue to dig and progress
southeast through tonight as that jet heads due south over the
plains. In addition...the deep low off the eastern Seaboard will
further keep our upper low from progressing quickly
east...assisting in the building chill and long duration light
The primary surface cold front will clear the area soon after
daybreak with temperatures inching downward a few degrees and then
likely not to recover much today...with the afternoon in the middle
20s to lower 30s for most. Steep low-level lapse rates behind
this front are assisting in widespread flurries entering northern
Illinois per radar and observations...with a scattering of
embedded higher reflectivity and more snow shower-ish activity.
Am expecting this to basically be the case today across our
area...that is flurries more often than not...with occasional true
snow showers and quickly varying visibility. The coverage of snow
showers will be modulated by a handful of factors...with vorticity
maximas probably the simplest to key in on...with others being
more mesoscale /E.G. Low-level f-gen/. These factors within some
of the high-res hourly guidance through today would point toward
slightly better true snow shower coverage over the western and
southern County Warning Area. But again tough to time exactly so a broad
consistent forecast message for today.
Stronger synoptic lift...slightly better lapse rates...and a more
favorable thermal profile for dendrites all arrive early this
evening from north to south as a vorticity lobe rotates
southward. The model guidance is in fairly good agreement with
this and have just gone light snow areawide. This for northern
areas will likely overlap at least part of the evening commute
and a persistent...though light...accumulation is likely to begin
in that time. Its more challenging to say when this period will
end...but it looks to continue into Tuesday morning. With
northwest low-level winds having increased on re-enforced cold
advection...visibility could be low to quite low at times. So with
possible two commutes likely to be dealing with light accumulating
snow and some varying visibility have gone ahead and issued an Special Weather Statement
to further the message.
Low-level winds will turn off the lake in Northwest Indiana with
lake enhancement to the snow likely into both lake and Porter
County by late tonight...though the longer fetch will be pointed
into Porter County. Equilibrium levels are forecast to be around
8000 feet though with a gradually deepening dendritic growth layer.
So this will help accumulations in this area with 2 to 3 inches
presently favored in the far northeast part of the County by
Tuesday evening...and additional after that point late Tuesday
night into Wednesday...possibly even a few more inches. Will need
to monitor if a headline is needed...particularly if rates are
more enhanced...although the Better Lake effect setup continues to
look further east...so not anticipating it at this time.
Temperatures look to not climb at all on Tuesday with wind chills
in the single digits. Cyclonic flow persists through Wednesday so
continue to lean cloudier than guidance into then. If clouds can
clear by Wednesday night...that looks to be a particularly chilly
night with some outlying areas potentially dropping to or below
340 am CST
Wednesday night through Monday...
The pattern continues to look like a cooler than normal one in
this time period. The elevated baroclinic zone on the western side
of the eastern North American trough looks to be unsettled with
several ripples supporting occasional light snow
chances...although the placement of that has large spread in
guidance at this time. An even more anomalous upper level low is
forecast to dig into eastern Canada by Friday or Saturday...with
the ec further south with this feature. This results in some
differences in the surface pressure fields between the GFS and
ec...though the general synoptic pattern with a strong Canadian
high to the west and a strong cold front pushing south through the
Great Lakes can support Better Lake effect potential for some of
Our Lake adjacent areas...at this time more so far Northwest
Indiana. Overall though because of the northwest flow...the threat
of widespread high impact weather looks low at this time.
for the 12z tafs...
MVFR ceilings are expected through the period with brief periods of
IFR possible early this morning. VFR flurries will be around
throughout the day. Scattered light to moderate snow showers are
currently on radar and they will continue through middle morning. The
heaviest snow showers will bring visibility down to below 1sm and
produce a quick dusting of snow. Only a couple tenths of an inch
of snow are expected through this afternoon.
More steady snow is expected this evening into Tuesday morning
with IFR or lower visibility. Accumulating snow is expected with totals
of 1-2 inches by Tuesday morning. Gusty northwest winds also
develop tonight with gusts around 20 knots. This may result in blowing
snow and contribute to the reduced visibilities.
High confidence in snow and snow trends...cigs...and gusty winds.
Medium confidence in how low visibility will be in snow showers
overnight...could be lower than currently forecast.
301 am CST
Issued a Small Craft Advisory for tonight through early Wednesday
night for the Illinois nearshore waters...and into Thursday morning for
the Indiana nearshore waters due to hazardous winds and waves.
A weak low over the u.P. Of Michigan will drift over the lake today and
reach the eastern Great Lakes late this afternoon. Winds will
become north behind the low and increase to 30 knots. High pressure
sinks south over the plains Tuesday night and moves over the
Mississippi Valley Wednesday evening. The tightened gradient between
the high and low will result in an extended period of 30 knots winds
through Wednesday night. Low end gale force gusts are possible late
Tuesday afternoon over the northern half of the lake...and the gales
reach the southern end of the lake Tuesday night. However...do not
have enough confidence in gales to issue a gale watch for Tuesday
night into Wednesday. Winds diminish as the high moves over the
lake Wednesday night into Thursday.
Another weak low passes over the eastern Great Lakes Thursday night
and Friday while high pressure moves over Minnesota Friday night and
Saturday. Northwest to north winds increase to 30 knots late in the
Lm...Small Craft Advisory...lmz743-lmz744-lmz745...midnight Tuesday
to 9 am Thursday.
Small Craft Advisory...lmz740-lmz741-lmz742...midnight Tuesday
to 3 am Thursday.
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