Scientific Forecaster Discussion
Area forecast discussion
National Weather Service Chicago/Romeoville Illinois
412 PM CST Friday Dec 13 2013
319 PM CST
Short term...tonight through Saturday night...
Not much has changed with the current thinking for tonight and
Saturday regarding snowfall amounts and placement. The current
water vapor imagery continues to indicate a decent disturbance
near the Texas Panhandle early this afternoon...with several lead
disturbances across OK and into western MO. Profiler and vwp data
across the Southern Plains and middle Mississippi Valley indicates
that around 30kt of southwesterly flow around 850 mb has developed
in association with this system. This is allowing for some decent
moisture transport and isentropic ascent across the Missouri and
west central Illinois this afternoon. This has produced a good
area of precipitation across Missouri...especially along the
leading edge of the better moisture transport.
I expect this area of precipitation to continue to move northward and to
overspread most areas along and south of Interstate 80 through the
evening as the better moisture gets drawn northward in association
with the approaching southern stream system. It appears likely that
a band of moderate to heavy snow will set up this evening across my
southern counties...in association with a developing band of
frontogenesis. The enhanced forcing for ascent within the
frontogenetic circulation will allow for some decent ascent within
the dendritic growth zone. This along with the fact that steep lapse
rates and -epv exits within and above this zone also suggests that
their will be some fairly efficient snowfall production within this
band of snow. With this in mind...it appears that snowfall rates
could approach 1 inch per hour for a couple hour period this
evening...especially to the south of Interstate 80 within and near
the current Winter Storm Warning.
A second area of snow will be favored...mainly across southern
Wisconsin this evening as a secondary axis of frontogenesis develops
along a baroclinic ribbon...and combines with increasing flow and low
level convergence off of Lake Michigan. However...most high res
guidance continues to favor areas north of the Illinois boarder.
Therefore...it appears most of northern Illinois will remain snow
free...through at least middle to late evening.
The snow should begin to spread northward by late this evening...and
begin impacting most of northern Illinois by or shortly after
midnight as the surface inverted trough becomes oriented north to
south across the area overnight. The snow will continue into
Saturday as some larger scale support for ascent overspreads the
area in association with the approaching northern stream short wave.
This could even lead to a bit in an uptick to the snowfall
intensity...particularly across eastern Illinois and northwestern
Indiana...where middle level deformation will continue in association
with the middle level circulation. The snow will gradually end across
northern Illinois as the flow turns northwesterly. Some lingering
lake effect snow showers are possible across northwestern Indiana
into Saturday evening.
There are some possibilities for some lake enhancement for areas
near the lake late tonight and into Saturday as the flow turns east
northeastward. However...thermodynamics over the lake do not appear
to be very favorable for any significant lake enhancement potential.
Delta t's look to remain around 12 degrees celsius and equilibrium
levels never appear to reach into the dendritic growth zone.
So...overall...it still appears snowfall amounts will range from
about 5 to 8 inches across the warning area...3 to 6 inches in the
advisory areas...and 2 to 3 inches around northeastern
Illinois...and an inch or less across north central Illinois.
Once the snow ends...the northwesterly flow will allow colder air
to spill back across the region setting up another cold Sunday
Medium-long range...Sunday through next Friday...
Highlights of the medium-long range forecast period are continued
northwest upper flow with a parade of weak clipper systems into
mid-week...then a shift toward southwest flow aloft middle/late next
week as upper ridge builds across the eastern Pacific and Gulf of
Alaska and induces downstream long wave trough across western North
America. This will support a transition from below normal temperatures
sun-Wed...with the potential for a brief warm-up Thursday-early Friday
ahead of western trough.
Sunday through Wednesday...
Middle-level short wave trough axis associated with early weekend
winter storm will be well east of the region by Sunday morning.
Middle/upper level low over develops over Ontario and the northern
lakes however...with region of enhanced northwest flow aloft from
the northern plains to the Midwest. Several minor amplitude ripples
are indicated in model guidance which propagate along low level
baroclinic zone which stretches northwest-southeast across the
region. Some timing differences with the more subtle disturbances
between models...though it appears a weak wave will traverse the
area Sunday with a somewhat better defined vorticity moving over the area
Sunday night and early Monday. Forecast soundings and time/height
depictions indicate generally dry moisture profiles...with only a
pocket of deeper middle-level moisture Sunday night-early Monday with
isentropic forcing ahead of the stronger wave. Thus the best
potential for a little light snow appears be Sunday night...though
with little accumulation expected (a dusting to perhaps a half inch)
as lift appears generally weak and well below the dendritic growth
zone. Weak ascent lingers into Monday...especially across the
northeast/east parts of the County Warning Area with the left exit region of the
upper jet streak helping to maintain some large scale support during
the day. A more amplified short wave is prognosticated to dig across the
western Great Lakes region on Tuesday...though the best warm
advection and isentropic upglide appears to remain just to the
north/northeast of the County Warning Area...though a reinforcing push of colder air
does arrive Tuesday night with a cold front.
Coldest conditions look to occur Sunday behind the early weekend
winter storm...with highs in the teens and lows in the single digits
to around 10 above. Some moderation occurs Monday-Tuesday with the weak but
nearly persistent warm advection associated with the aforementioned
clippers. A little cooler Tuesday night-Wednesday behind Tuesday
nights cold front.
As indicated above...developing upper ridging over the east Pacific
and trough inducement over the western Continental U.S. Wednesday-Thursday
results in surface low pressure development over the plains by early
Thursday. Fairly decent model agreement in this low deepening and
lifting northeast across Minnesota and the northern Great Lakes
Thursday and Thursday night...placing weather forecast office lot County Warning Area in the warm sector
as warm front lifts north. Shallow cold surface layer may result in
a little mixed precipitation Thursday morning especially north as
warming occurs more vigorously aloft...though GFS/European model (ecmwf) runs
indicate that low levels warm enough to produce mainly rain by
Thursday evening/night. Some timing differences do arise with the
new 12z European model (ecmwf) (the 00z run was in better agreement with the 00/12z
GFS and ensemble members)...with the European model (ecmwf) faster with the low
lifting across the northern lakes early Friday and bringing the
trailing cold front through faster. Continue to favor a slightly
slower solution similar to the 00z ec/12z GFS and gefs runs. This
would support temperatures potentially warming into the 35-40 f range
Thursday and potentially into the 40s/even 50 across the southeast
Friday prior to cold frontal passage. Precipitation potential appears to be
greatest along/ahead of the cold front into Friday. Decreasing
thicknesses in colder air behind the front portend a change back to
snow from the west Friday.
Things look potentially interesting just beyond the end of day
7...as southwest-northeast oriented baroclinic zone could become the
focus for a couple of rounds of wintry precipitation somewhere from here
into the Ohio Valley...while high pressure and a return to
sub-average temperatures build back across the upper Midwest.
//Ord and mdw concerns...updated 22z...
* a chance for MVFR ceilings prior to onset of snow.
* Snow spreading northward over airports between 02z-04z.
Increasing in intensity into overnight with IFR conditions.
* East winds around or slightly above 10 knots tonight.
Significant upper disturbance over the southern rockies and
southern High Plains late this morning is forecast to continue to
move east across the plains to the central and lower MO valley by
12z Sat...and then on to the upper Midwest and middle MS valley by
15/00z. Associated surface low to track southeast from Kansas to the lower
MS valley by Sat morning and then east across the deep S during the
day. Meanwhile...a second low is to develop in the plains and
reach the middle MS valley by 12z...continuing up the Ohio Valley
during the day.
Per 12z radiosonde observations a strong baroclinic zone was from the Canadian
rockies east-southeast across the far northern plains and the upper Midwest
and through the Great Lakes into the NE states /850hpa temperature of
-23c at inl and +7c at sgf/. Models indicate that as the storm
system moves east today and tonight that the baroclinic zone will
shift S as cold air advection continues over the Great Lakes and
the northern portion of the upper Midwest while strong warm air
advection spreads from the lower MO valley to the middle MS valley
and then across the Ohio Valley and Midwest.
Middle level moisture as well as lower level moisture /12z 700hpa
dew point depression of 1 degree c at oun and forward...850hpa
depression of 0-1 degree c at crp and forward/ is prognosticated to rapidly
stream north and NE from the Southern Plains ahead of the approaching
system. MVFR ceilings had already spread as far NE as irk and
qun at midday. As this moisture continues to move into and
overspread the local area this evening ceilings are anticipated to
drop from VFR through MVFR and IFR to LIFR during the middle and late
Along with the arrival of deep moisture will be upward motion
increasing in both depth and strength through the column as
ins entropic upglide/q-vector convergence/frontogenesis increases
and expect light snow to begin locally around 03z and intensify by
06z with a period of moderate snowfall occurring overnight causing
increasing reduction in visibility. With the surface low to track
from central MO NE to northern in overnight and Sat morning the
local area looks to be more on the northern edge of the snowfall
with the band of heavier snow across central Illinois to central and
There may be some lake enhancement to the snowfall overnight
tonight into early Sat afternoon but any significant lake effect
snow is expected to be occurring in northwest in as the low level flow
veers from NE to north.
Conditions to improve Sat during the later morning and the
afternoon as the surface and 850hpa lows move east to Ohio...and the
upper trough axis reaches Lake Michigan and lower Ohio Valley by 15/00z.
Winds locally do not look to be particularly strong with this
system...generally in the 8-12kt range rest of today through the
day Sat while winds back from out of the east to the NE.
//Ord and mdw confidence...updated 20z...
* high confidence in wind direction and that speed will remain
around to above 10 knots tonight.
* Medium confidence in timing of snow onset...period of peak
* Medium confidence in timing of deterioration of ceilings.
//outlook for Ord/mdw for 00z Sunday-12z Friday...updated 12z...
Saturday night...mainly VFR. Period of MVFR ceilings early evening
with light snow/flurries ending.
Sunday-Monday...slight chance of snow and MVFR.
Thursday...VFR. Possible period of light wintry mix.
144 PM CST
A large area of high pressure is settling over the southeastern Continental U.S. As weak
low pressure moving across northern New England dags a cold front Down
Lake Michigan. The front has stalled out over the southern tip of
the lake leading to light and variable winds over the southern tip
of the lake. North winds are strengthening over the northern
portions of the lake as another area of Arctic high pressure builds
out of Canada and spreads across the northern plains and upper Great
Lakes. The front over the southern portions of the lake will wash out as
a double barreled low pressure system develops over the southern and
Central Plains. The complex low pressure system will track eastward this
evening with one circulation center tracking through central Illinois and
another tracking through the lower Mississippi Valley. A weak
inverted surface trough axis associated with the more northern of the
two lows will move across the lake early tomorrow. As the high
builds across the upper Great Lakes this evening...winds over the
entire lake will gradually turn easterly...but as the inverted
trough axis crosses the lake tomorrow...winds over the lake will
gradually back through northeasterly to northerly tomorrow evening and then
then continue to back to northwesterly by late Saturday night and early
Sunday morning. As colder air associated with the Arctic high is
drawn over the lake...the strong cold advection and strengthening
pressure gradient will lead to winds increasing to around 30kt
through the day on Sunday. While gales are not expected at this
time...the strong winds and colder air will lead to a chance for
light to moderate freezing spray on Sunday. The brisk northwesterly winds
will persist through Sunday night...and then another high will build
across Lake Michigan on Monday...allowing winds to temporarily
subside. As the high moves off to the east on Tuesday...another low
will track across scntrl Canada...leading to increasing southwesterly-westerly
winds by Tuesday...which should persist through the middle of next
Illinois...Winter Storm Warning...ilz033-ilz039 until noon Saturday.
Winter Weather Advisory...ilz019-ilz021-ilz022-ilz023-ilz032
until noon Saturday.
Winter Weather Advisory...ilz014...10 PM Friday to 6 PM Saturday.
In...Winter Weather Advisory...inz001-inz002 until noon Saturday.
Winter Storm Warning...inz010-inz011-inz019 until noon Saturday.
Lm...Small Craft Advisory...lmz740...midnight Saturday to 3 PM
Small Craft Advisory...lmz741-lmz742-lmz743...3 am Saturday to 3
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