Scientific Forecaster Discussion
Area forecast discussion
National Weather Service Chicago/Romeoville Illinois
1031 am CST sun Dec 8 2013
547 am CST
Middle levels have been saturating faster than expected per vwp
early this morning and have begun to seed low level lake effect
cloud cover which has already expanded west into portions of far
northeastern Cook County and far Eastern Lake County. As
isentropic ascent continues through the morning...lake effect snow
should also continue to pick up in intensity. The best convergence
associated with the mesolow over Lake Michigan is still expected
to focus snowfall over Eastern Lake County and areas north this
morning. Have nudged up timing of synoptic snow later this morning
by a couple hours as virga continues to encroach into the
area...and is already reaching as low as 3000-3500ft.
314 am CST
Main forecast concern will be snowfall across particularly the
northern portions of the forecast area today.
Today and tonight...
high pressure is in place across the western lakes region this
morning and will shift east through the day with return flow
developing across the area. Very cold airmass remains in place
though early this morning with temperatures bottoming out in the single
digits for many areas outside the urban heat Island. Lake Michigan
water temperatures estimated in the low to middle 40s has led to
offshore flow overnight with a mesolow beginning to develop across
the southern end of Lake Michigan. Local 8km WRF shows this feature
shifting northwest into southeast WI by middle morning as a short southeast
fetch develops. Very difficult to pin down how widespread the lake
effect impacts will be...but based on the high res guidance which is
best handling the situation...appears impacts will mainly be limited
to portions of Lake County Illinois which may see around an inch by early
afternoon...though cannot rule out light accumulations south into
northern Cook County.
A southern stream 500 mb wave is noted over the Great Basin and will
race east...lifting across the Midwest tonight. There will only be
a weak surface reflection as a low will move near dbq this evening
then near Sheboygan tonight. This is not a favorable track for
seeing heavy snow...though still expect light accumulations as we
are well entrenched in the cold air for warm advection precipitation to
fall as snow. Moisture availability the big concern...especially
as you head into the southern County Warning Area where the moisture may be too
shallow for introduction of ice into the column. Biggest concern
for light freezing drizzle is across east central Illinois and
Northwest Indiana. Across the northern portions of the
County Warning Area...expect fairly efficient dendrite production...especially
towards the beginning of the event when a deep dgz is noted in the
soundings. Could be looking at 20-25:1 slr across north/northwest
counties in the County Warning Area. Blended towards NAM/GFS quantitative precipitation forecast and with expected
slr...did not really come out with any big changes in expected
snow totals from previous forecast. Still appears 1 to 3 inches
possible north of the I-88 corridor...up to 2 inches south to the
I-80 corridor. South of I-80...a dusting if any accumulation
expected... with the main concern being freezing drizzle.
Monday and Tuesday...
high pressure will build across the middle Mississippi Valley which
will result in dry weather across the area...but a clipper system
pushing across the northern lakes region will result in breezy
conditions as a tight gradient develops between these features and a
reinforcing shot of cold air clips the area. 850 mb temperatures around -16c
will push into northern Illinois Monday night. With overnight
lows in the low to middle single digits...wind chills Monday night
are expected to fall well below zero...with around -15 possible
west of the Fox River valley...just shy of advisory criteria.
Wednesday through Friday...
northwest flow in place Wednesday and models in decent agreement
advertising a potent upper level wave digging into the middle to upper
Mississippi Valley. Models develop very light quantitative precipitation forecast with this
system...but tend to struggle in such cold airmasses. Bumped up probability of precipitation
slightly Wednesday as this type of setup can often produce
"surprise" quick but light snow accumulations. Behind this
system...high pressure builds back across the area with no relief in
sight from the well below normal temperatures until possibly late in
the week. GFS shows a modest upper level ridge building over the
area allowing temperatures to rebound...but doesnt have much support from
other models or gefs mean which keeps the ridge axis farther
west...so while temperatures may moderate some...still looking at
//Ord and mdw concerns...updated 16z...
* -sn developing by middle day...intensisty increasing into the
* MVFR ceilings.
* Several hour period of steadier snow likely late afternoon into
early evening...with IFR/snow.
Virga (snow sublimating/evaporating before reaching the ground)
has been in full swing across northern Illinois the past few
hours. Monitoring radar VAD wind profiler data shows decided trend
toward snow penetrating closer and closer to the surface...reaching as
low as 6000ft mean sea level as of 1123z. Observational trends would suggest
that model forecast soundings are too dry and snow may begin to
reach the ground sooner than forecast this morning. Would appear
as though one wave of light snow could impact the terminals
middle/late morning into early afternoon with a possible lull before
another steadier batch of snow arrives late afternoon early
evening. Hard to definitively Pin Point snow vs no snow hours so
had to blanket much of period with tempo and prevailing -sn...but
still anticipate best chance of highest impact snow to be 22/23z
through about 03z. North winds early this morning should veer to
just south of due east by middle to late morning as mesoscale low over
Lake Michigan lifts northwest into southeast WI and dissipates. Does
appear as though we will ice crystals in the cloudiness this
evening but lift should be mostly gone by then too and ceilings fairly
high...so think fzdz potential is low with precipitation likely to end as
flurries...particularly with winds turning westerly and drier air
//Ord and mdw confidence...updated 16z...
* medium confidence in snow timing and intensity through
* High confidence with MVFR ceilings this morning.
* Medium confidence in period of steadier snow late afternoon into
//outlook for Ord/mdw for 00z Tuesday-12z Sunday...updated 12z...
Monday night and Tuesday...generally VFR.
Wednesday...chance of -sn and MVFR ceilings.
New model output coming in has backed off on how quick winds
increase this afternoon as the low moves NE to the middle-lower MO
valley today and also on the strength of the winds as the low
continues NE to the eastern u.P. Of Michigan overnight and to James Bay
during Monday with a cold front crossing Lake Michigan. Have left the gale
watch in effect for Monday though probability of occurrence is
less based on new model guidance.
Have also delayed the beginning of the Small Craft Advisory for
the Illinois nearshore waters until late afternoon as winds are
expected to increase later that previously thought.
420 am CST
Cyclonic circulation with mesolow over the far south end of the
lake this morning will result in somewhat variable winds early
before east winds ramp up quickly later this morning. Weak surface low
will lift north across the lake tonight with moderately strong
west winds developing in its wake spreading south to north late
tonight. Surface low begins to quickly deepen as it lifts north to
James Bay Monday night resulting in a period of west gales
northern 2/3rd of the lake Monday into Monday evening. Narrow/weak
surface ridge moves across the lake late Monday night resulting in a
brief relative lull in winds before next fast moving surface low zips
east to Lake Superior with tight pressure gradient leading to
rapid increase to southwest gales Tuesday morning through Tuesday
evening. Air temperatures over the lake look cold enough to support heavy
freezing spray on Tuesday into Tuesday evening. Have opted to just
issue a gale watch for Monday/Monday evening gales for now...but day
shift will need to consider gale and heavy freezing spray watches
for Tuesday. Gale and heavy freezing spray threat begins to
subside Tuesday night.
Lm...Small Craft Advisory...lmz740-lmz741-lmz742...4 PM Sunday to 10
Small Craft Advisory...lmz743-lmz744-lmz745...midnight Monday to
10 PM Monday.
lmz669-lmz868-lmz870...10 am Monday to 10 PM Monday.
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