Scientific Forecaster Discussion
fxus63 klot 181128
Area forecast discussion
National Weather Service Chicago/Romeoville, Illinois
528 am CST Thu Jan 18 2018
336 am CST
Today through Friday night...
Dry and gradually milder weather will persist through week's end.
Moderation will be slowed however, due to expansive and only
slowly moderating nature of the upstream Arctic air mass, snow
cover and shallow boundary layer mixed depths.
Primary surface weather feature on the map this morning is an
expansive area of Arctic high pressure centered across the lower
Mississippi Valley. Farther north, low pressure was analyzed just
north of International Falls Minnesota, associated with a pretty healthy
looking clipper which will pass well to our north across the
northern Great Lakes today. In between, the main influence on our
weather will continue to be a relatively tight pressure gradient,
which will allow blustery west-southwest winds to persist across
the forecast area. With the exception of a few passing high clouds
along the southern periphery of the clipper to our north, skies
will mostly sunny today. This abundant sunshine and warm advection
flow from the west will help nudge temperatures up today, though
Arctic air entrenched upstream across the plains and our recent
snow cover will work to limit surface warming, which is depicted
in model forecast soundings only mixing to around 950 mb. Thus
have again undercut MOS guidance for highs today by blending
colder raw model surface temps. Still, afternoon readings should
approach 30 degrees or exceed it by a degree or two. Very similar
conditions persist tonight into Friday, though with continued
moderation anticipated as broad upper ridging spreads across the
area. Similarly undercut temps Friday, though 35-40 degrees looks
reasonable across the area by the afternoon. The blustery winds,
gusting around 20 mph at times both today and tomorrow, will keep
it feeling cooler however, with wind chills generally in the teens
to lower 20's today and in the upper 20's and low 30's Friday.
336 am CST
Saturday through Wednesday...
The main forecast focus in the long term period is on a deep low
pressure system which is expected to emerge from the plains this
weekend, and track into the western Great Lakes region through
Monday. Medium range model guidance remains in general agreement
in tracking the surface low just northwest of the forecast area,
keeping the weather forecast office lot cwa in the warm sector, and resulting in
primarily snow melt, drizzle/fog and rain event until colder air
wraps in late Monday behind the departing low.
A deep upper trough is evident off the West Coast of the Continental U.S.
Early this morning in GOES vapor imagery. Several aircraft obs
depict a strong jet streak, of greater than 140 kts, translating
into the base of this trough over the Pacific. Model guidance
amplify this trough as it moves ashore over California and baja
early Saturday, with the trough further deepening and forming a
closed upper low as it emerges from the south-central rockies
Sunday. Global models are in pretty good agreement with the
evolution, though some spread in speed and placement develop as
the system lifts into the western Great Lakes region during the
Monday-Monday night time frame. Given the strength of the upper
trough, have some preference toward the slower European model (ecmwf) and Gem
depictions, which are clustered closer together than the faster
GFS. Surface low development is indicated in the Lee of The
Rockies Saturday night, with the low then deepening on a general
track across Iowa by early Monday, and passing near or just north
of northwest Illinois later in the day/evening. This is generally along
the location of a low level baroclinic zone which lays out along
that axis in association with a cold front which settles south
from a northern stream disturbance Friday night.
Based on the slightly slower model representations, and the
aforementioned surface low track, low level flow looks to remain
primarily southwest across the forecast area Saturday, with a
gradual increase in low level moisture as the western Gulf gets
tapped. A surge of above freezing dew point air is progged to
spread into the cwa Saturday night, with forecast soundings
suggesting the development of low clouds, fog and drizzle as the
milder, moist air spreads over our lingering snow cover. Surface
temps should remain above freezing Saturday night, though it may
be close along the Illinois/WI border for a time which could support
some brief patchy freezing drizzle there. The warm front lifts to
the Illinois/WI border Sunday, with the entire area expected to warm
into the 40's, before more widespread rain develops Sunday night
into early Monday as large scale forcing increases ahead of the
approaching low. Forecast soundings even depict some weak elevated
instability associated with the low-level Theta-east axis ahead of
the cold front Monday, which could even support an isolated rumble
or two of thunder. Temps, which hold in the 40's Sunday night,
could reach/exceed 50 degrees Monday ahead of the front. With the
slower European model (ecmwf)/Gem progression, colder air slowly wraps in behind
the cold front late Monday, which would support a transition from
rain to snow or snow showers, which would persist in some fashion
into early Tuesday before the system finally exits the area. No
real punch of real cold air is depicted however, with temps around
average through mid-week.
Will have to monitor the potential for flood issues as this
system unfolds as melting of our snow pack (which may have up to
0.50 inches of water equivalent in places), and model qpf of
0.50-1.00 inches, could cause some issues with frozen ground.
for the 12z tafs...
Southwest winds and VFR conditions will prevail through the
forecast period. Winds are expected to gust into the low 20kt
range much of the day, then gusts should taper off this evening
and overnight. Depending on how quickly surface winds drop off
this evening, there may be a short window with winds just aloft
of 40-45kt that could produce low level wind shear. Wind gusts
should pick up again mid morning Friday but not quite as strong as
300 am CST
A broad ridge will remain in place over the deep south through the
remainder of the week and into the weekend while a series of lows
move east across Canada. Expect a west to southwest gradient to
prevail over Lake Michigan through at least Saturday. Low pressure
will move from Alberta this evening to near James Bay Friday
evening. Winds will pick back up to gale force across the lake
Friday afternoon through Friday night, and should ease back below
gale force during the predawn hours Saturday. Will hoist a gale
watch this morning for the open waters to cover this time frame.
Another low will take shape over the central Great Plains over the
weekend then lift over or near Lake Michigan on Monday. Models
continue to have modest differences with timing, strength, and
the exact track of the low so the details are still a bit unclear.
In general though, winds are expected to turn south to southeast
on Sunday ahead of the low. A more northerly low track will also
favor high dew point air lifting into the region resulting in
stable conditions over the lake and likely fog. Gales or near
gales are possible Sunday and Sunday night. As the low shifts east
of the lake on Monday, gales then appear likely and could
potentially be strong as cold air spreads back over the region
behind the deepening low.
Lm...Small Craft Advisory...nearshore waters
until 4 PM Thursday.
Gale watch...lmz777-lmz779...noon Friday to 3 am Saturday.
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