Scientific Forecaster Discussion
Area forecast discussion...updated
National Weather Service Chicago/Romeoville Illinois
819 PM CST Thursday Dec 12 2013
819 PM CST
Went ahead and issued a Winter Weather Advisory for areas mainly
south of Interstate 80 for Friday evening through Saturday morning
as confidence remains high on accumulating snow of 3 to 6 inches
for this area...including a potential good burst of snow during
Friday evening within the first several hours after onset. All in
all...much of the meteorological reasoning from the previous shift
still stands very true.
The upper low across Arizona this evening is beginning to tap into the
subtropical jet and seeing quickly cooling cloud tops across the
Southern Plains. All guidance seems to have a good handle on this
position and lead vorticity entering the Texas Panhandle...which actually
looks to be the instigator to the snowfall Friday afternoon in
western/central Illinois. The quick progression of this wave and broad
moist advection surge northward occurs into the forecast area
mainly during the evening...although do expect some snow likely to
have begun around the Pontiac area prior to 6 PM...even given
slower solutions. Snowfall looks to spread north across the South
Chicago metropolitan during the early evening and likely across much of the
metropolitan by midnight...but the heavier rates are favored more so
across the south. The enhanced convergence and modest tightening
of the middle-level thermal gradient in place allow for the
likelihood of some evening banding in the advisory area where
snowfall rates would exceed one half inch per hour for a period.
The 00z NAM has trended slightly slower and that leads to a
little less total quantitative precipitation forecast further north...such as into Chicago. All in
all it puts it closer to the mean solution of the NCEP
models/ec/Gem. There should be a tight gradient in snow through
middle Friday evening...maybe even tighter than guidance is indicating...and
even seeing that hinted at within the simulated reflectivity of
high res models that have solutions that far out. This should
become more diffuse into overnight as the middle-level wave of the
northern stream interacts with the southern system and the area
fills in with lift and saturation.
Thought about including a few counties further north in the
advisory including Cook...but have opted to wait at this time.
There will in all likelihood be a gradient in snowfall from north-
to-south across Cook County...plus there could be some lake
enhancement Saturday morning and maybe even lake effect snow
showers into Saturday afternoon. So would rather give the midnight
shift some leeway to not only better refine where the synoptic
gradient of snowfall will be and just how tight it will be...but
also duration of snow.
304 PM CST
Short term...tonight through Saturday night
The main forecast concerns during the short term period continue
to focus on the good potentially for accumulating snow across
portions of the area late Friday through Saturday as a complex
system impacts the area.
After another extremely cold morning...southwesterly winds have
allowed temperatures to recover nicely into the lower 20s in most
areas. Warmer tempers will continue into Friday and Saturday...but
at the cost of getting snow.
As mentioned above the main attention early this afternoon revolves
around the evolution of system currently noted in the water vapor
imagery across Southern California. Model guidance continues in good
agreement with a second...northern stream system wave...currently
noted just off the Pacific northwest coast...digging southeastward
across the western Continental U.S. And ejecting the southwest system out
across the Southern Plains Friday. This should induce an area of low
pressure/inverted Gulf Coast surface trough across the lower and
middle Mississippi Valley into early Saturday. Moisture return
should be decent ahead of this feature across the middle Mississippi
Valley on Friday as a 35 knots southerly enhanced lower level wind
field develops. This increased low level wind field will also help
set up some decent isentropic ascent during the day...particularly
across southern and central Illinois and Missouri...likely leading
to saturation and snowfall develop by late afternoon...especially
across Missouri and portions of west central sections of
Illinois...possibly including some of my southwestern counties. The
snow is then expected to overspread most areas along and south of
Interstate 80 during the evening...then across portions of
northeastern Illinois overnight.
Although uncertainty in the larger scale aspects of this system is
gradually lowering...the uncertainty in the mesoscale features of
the system remains somewhat elevated. It does...however...appear
that the main axis of heavier snow may remain to the south of
Interstate 80. These are the areas where it appears the lower level
southerly wind field will nose...setting up the best placement of
moister convergence and band of frontogenesis. The heaviest snow
band may only be 50 or so miles wide...so the exact placement of
this area of better mesoscale forcing remains uncertain.
However...considering the fact that the middle level short wave itself
prognosticated to track across southern Missouri and Illinois...and hence
producing the best larger scale forcing for ascent (q-vector
convergence) to the south...I think there are better chances for
heavier snow south of Interstate 80. All in all...I am thinking a
band of moderate to possibly a short period of heavy snow will set
up across my southern counties...roughly along and south of a line
from near Grand Ridge through Gardner to near Valparaiso Indiana.
This should occur during the evening...with lighter snow continuing
It appears that snow rates will be the greatest during
the evening as this mesoscale forcing coincides with some decent middle
level lapse rates...per forecast soundings. Snow rates could exceed
a 0.5 inch per hour within any heavier bands of snow. Accumulating
snow will continue overnight...but should overall remain lighter.
These areas will likely need to be upgraded to an advisory.
Currently anticipating 3 to 6 inches south of Interstate 80. I will
hold off on any headlines at this time to allow the overnight shift
to get another look at things to see if things trends try to shift
north or south of the currently anticipated area. Although it
appears most areas will remain just under 6 inches...there could
be some areas down south that could come very close to 6 inch
amounts. The Garcia method continues to suggest that amounts
could be as high as 6 inches.
Farther north across the Chicago area uncertainty in the exact snow
amounts is higher. At the present time...it appears lighter snow
will overspread much of northern Illinois later in the evening.
Surface flow is expected to become easterly off of Lake Michigan and
this could result in some lake enhanced showers. However...it
appears the better thermodynamic setup for lake effect snow will
remain north across the shores of Wisconsin...where there will also
be better support from a northern stream middle level short wave moving
across that area. Current projections are for around -8 to -10 850
mb temperatures across Southern Lake Michigan. And this suggests only 12-13
degree 850-surface water temperature differences. Additionally...model
soundings across Southern Lake Michigan suggest that lake induced
equilibrium levels will remain below the dendritic growth zone.
So...all in all it appears snow amounts north of I-80 will remain
lighter...likely in the 2 to 3 inch range...with even lighter
amounts of an inch or less across north central Illinois.
The snow should wind down from west to east by Saturday afternoon as
the system shift eastward towards the Atlantic coast. The flow will
become northwesterly once again with colder air spilling back across
the region setting up another cold Sunday morning.
Long term...Sunday night through Thursday
Synopsis...very low confidence in the extended period due to large
spread amongst operational and ensemble models. Late next week...a
very strong upper jet stream will carve out a deep trough across the
western Continental U.S. With a fresh Arctic discharge. Coupled with strong
upper ridging and warmth in the southeast Continental U.S....this pattern could
result in the development of a strong storm system somewhere over
the Midwest. The devil is in the details though...which will take
some time for guidance to resolve and to have any confidence in
those details. Beyond this...there is pretty good agreement in very
cold air pushing back into the western lakes region as ridging near
Alaska strengthens and results in cross-polar flow. Thus...a cold
Christmas Holiday period is certainly possible.
Sunday night through Monday night...
There is continued variance in the track of a clipper system Sunday
night on the operational models...with the European model (ecmwf) and Gem still
indicating potential for a swath of light snow across parts of the
County Warning Area into early Monday...while the GFS remains farther north. Kept
probability of precipitation in the slight chance range as per blended initialization. The
European model (ecmwf) and Gem even have another clipper skirting the northern County Warning Area on
the heels of the first late Monday. The northern track of the
clippers on operational and ensemble mean of the GFS would result in
a bit of a warming trend into Monday...while the further south
baroclinic zone on the European model (ecmwf) would keep US colder. Snow cover will
play a role in temperatures regardless of the thermal profiles.
Tuesday through Thursday...
The disagreement further grows in the middle to late week period...so
continued to rely on the blended initialization. The GFS is in
reasonable agreement with its ensemble mean in a shot of modified
Arctic air Tuesday into Wednesday...while the European model (ecmwf) continues to
insist on a brief but possibly significant warm up Wednesday into
Thursday with 850 mb temperatures warming to +6 to +8c. At this time...the
European model (ecmwf) scenario of a strong lead northern wave leading to deep
cyclogenesis over the northern plains and upper Midwest doesnt seem
to have much support and prefer the idea of the energy in the
western trough being slower to eject as is typically the case. With
even the GFS though showing pretty strong warm air advection over a low level cold
air mass by later Thursday...have indicated a slight chance of snow
or sleet in the grids.
Confidence in temperatures in the extended period...low.
Confidence in precipitation trends in the extended period...low.
//Ord and mdw concerns...updated 00z...
* east southeast winds gradually increasing in speed Friday
afternoon...nearing 10 knots by early evening.
* IFR ceilings possible in advance of snow by 22z Friday.
* Snow developing Friday evening...most likely sometime between 01z and
06z...with IFR conditions likely.
A cold front across the northern Great Lakes will inch southward
through tonight and basically become a stationary/warm front
across the area on Friday. This will lead to variable winds at
times late tonight and Friday morning...before turning easterly in
the afternoon. In addition...gradual saturation along this front
Friday afternoon will bring lowering ceilings...along with at least
marginally unstable flow off of Lake Michigan. So by middle to late
afternoon across taf sites a lowering in ceilings is anticipated.
This hold up of the front will be in response to low pressure
developing across the Southern Plains...and that will bring snow
our way Friday evening. The snow will approach from the south/southwest.
While the northward expansion in snow should be fairly rapid
during the late afternoon and early evening it is expected to slow
thereafter...which makes it a little challenging to get a specific
start time for Ord/mdw. But middle Friday evening seems most probable at
this time. Snow should continue through Sat morning...being
mainly light with at times moderate. At this time...one to
three inches of snow is forecast across most of the taf
sites by Sat morning...with gyy more in the two to four inch
range. Some lake enhancement of snow is possible late Friday night
into Sat morning.
//Ord and mdw confidence...updated 00z...
* high in easterly wind speeds staying below 10 knots prior to 00z
* Medium on arrival time of IFR ceilings Friday evening...though could end
up being a few hours prior to snow arrival.
* Medium on arrival time of snow Friday evening.
//Outlook for Ord/mdw for 12z Saturday-12z Thursday...updated 00z...
Saturday...snow and IFR in the morning with a chance of snow in the
Sunday-Monday...slight chance of snow and MVFR.
349 PM CST
High pressure entered over far western Tennessee moves east to the Carolinas
overnight while a high centered over the northern portions of
Alberta and Saskatchewan builds southeast over Manitoba...western Ontario
and the upper Midwest. This allows a cold front draped across Lake
Superior to move S Down Lake Michigan...reaching the S end of the lake by
daybreak Friday. Winds will become northwest tonight behind the front
with a few near gale or low end gale force gusts on the far north part
of the lake before winds diminish overnight.
The front becomes stationary across northern Illinois...the S tip of Lake
Michigan and far Southern Lower Michigan during Friday as the southern anticyclone
drifts east to the Carolina coast and the northern anticyclone settles
over Lake Winnipeg. Meanwhile...low pressure in the Central High
plains deepens as it moves to the Texas Panhandle. The low then moves east
across the lower MS valley by Sat morning while a second low
develops and moves NE to the middle MS valley.
In response to the high to the north and the low to the SW winds on Lake
Michigan will become easterly and freshen during Friday and Friday night...and
then northeast to north and diminishing overnight Friday and Sat as
the pair of lows move further off to the east.
A low is then forecast to develop over or near the Minnesota arrowhead Sat
and move east to Eastern Lake Superior Sat night. This low is expected
to weaken as it moves east over the upper Great Lakes during sun and
Sun night while low pressure moves east across the north portions of
Alberta...Saskatchewan and Manitoba. A cold front associated with
this system will move east across Lake Michigan late Monday and Monday night.
With moderately strong east to NE winds on Sat wave conditions will
likely equal or exceed Small Craft Advisory criteria.
Illinois...Winter Weather Advisory...ilz019-ilz021-ilz022-ilz023-ilz032-
ilz033-ilz039...6 PM Friday to noon Saturday.
In...Winter Weather Advisory...inz001-inz002-inz010-inz011-inz019...6
PM Friday to noon Saturday.
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