Scientific Forecaster Discussion
fxus63 klot 192335
Area forecast discussion
National Weather Service Chicago/Romeoville, Illinois
535 PM CST Fri Jan 19 2018
129 PM CST
Southwest winds are occasionally gusty this afternoon as a deepening
low pressure system passes through southern Ontario. Sunshine and
warmer temperatures continue to eat away at the snowpack with
little snow left across southwestern portions of Livingston and
LaSalle counties. Deeper snowpack across Northwest Indiana is
holding temperatures down a bit more in these areas.
The low will strengthen further this evening while it will move
into Quebec. Expect winds to remain gusty and even increase this
evening area wide. Gusts to 35 mph are not out of the question.
Otherwise, expect continued higher clouds and generally mild
temperatures in the mid 20s to lower 30s.
Concern will shift to the arrival of an area of stratus currently
across the arklatex region. Initial trajectories and forecast low
level steering flow initially keeps the stratus south of I-80
through at least a portion of the morning. We do expect an
increase in coverage by noon, and then overspreading most of the
area by tomorrow evening. In spite of the clouds, temperatures
will rebound close to today's readings in the upper 30s to low
40s. Forecast soundings suggest fog/drizzle will hold off until
the evening as well.
352 PM CST
Saturday night through Friday...
Main concern for the long term forecast period will be temperature
trends through the period and rainfall amounts and associated
impacts, for Sunday night through Monday.
Latest guidance continues to trend to a rapid warm-up and increase
in deep layer moisture in advance of a system lifting out of the
south High Plains. The models have been trending a little slower
and farther south with the track of the system, which would put the
axis of heaviest rainfall over nrn Illinois/nwrn in. In advance of the
low, while high pressure remains parked over the sern conus,
persistent warm, moist advection will bring increasing pcpn chances.
However, initially, the saturated moist layer will be below 10kft
with an elevated dry layer in more swly flow aloft vs. More sly flow
at the sfc. This type of moisture profile would be more conducive
to drizzle and fog, rather than rain. Temperature trends for
Saturday night may also presents some challenges as temperatures
should still drop to below 32 f, especially for the nwrn portions of
the County Warning Area. So, there may be a chance for some light ice accumulation
or a glazing of elevated surfaces as air temps drop below freezing
for a few hours Sunday morning. The higher daytime temperatures may
help to keep the ground and Road surfaces above freezing for the
short period then air temperatures are below freezing, but a period
of freezing drizzle with a light ice glazing cannot be ruled out.
Have toned back the pcpn type mention to drizzle rather than rain as
the saturated layer should be relatively shallow through much of
the day, with the deep layer moisture not moving into the area until
later Sunday night. Through the evening, pwats will climb above
0.75 inches, indicative of the increasing deep layer moisture. By
Monday morning, pwats in excess of 1 inches will overspread the
region in the deep layer warm/moist advection ahead of the
associated approaching cold front. Through the weekend, Max temps
will gradually increase into low to middle 40s for Sunday and upper
40s to lower 50s on Monday. There is a chance for some thunder
along the cold front as it moves across the region, but the window
of opportunity for thunder should be relatively narrow and tied
closely to the timing of the frontal passage, so confidence is too
low at this time to include in the forecast. However, this is a
possibility that will need to be considered. Temperatures should
rapidly drop off following the fropa, trending back to near normal
for Tuesday morning until late in the week. The longer range
guidance is suggesting the potential for another warming trend with
chances of rain for next weekend.
352 PM CST
The increase in temperatures along with moderate rainfall will
bring an increasing chance for break-up ice jam flooding. Latest
quantitative precipitation forecast amounts for the period from 00z Monday through 00z Tuesday
suggest a corridor of 0.75 to 0.85 areal average quantitative precipitation forecast over nwrn
Indiana through central Illinois, including the Kankakee, Iroquois
and Fox River basins, with 0.6 to 0.75 inches over the remainder
of the County Warning Area. Also, snow water liquid equivalent amounts of 1/2 inch
are trapped in the snow pack as of Friday afternoon. With frozen
ground, this snow melt will translate to runoff into the rivers.
With moderate rainfall then expected, all of this should translate
to runoff as well. So, the runoff will translate to increasing
streamflow and hydraulic pressure from under the ice surface,
while the warming will contribute to ice melt from the top. This
combination will be conducive to ice breakup, with ice jams
occurring at bends in area rivers and at choke-points and
obstructions to free water flow such as bridges.
for the 00z tafs...
The main weather concerns tonight will be low level wind shear once again, but
focus will then be on the low ceilings expected to arrive across the
area Saturday afternoon and evening.
Southwest winds continue to be occasionally gust up around 22 knots
early this evening, but these winds should gradually ease through
the evening. Given that recent amdar soundings out of Ord are
already indicating around 50kt of wind around 2500 feet above ground level low level wind shear
shear will continue to be an issue at least through around 1 am
tonight when the winds aloft should ease.
Saturday should start on VFR, but later in the day expect
increasing warm moist air to allow a lower stratus deck to shift
northward over the area. While confidence on an exact time of
these lower cigs is a bit low at this time, it does appear that as
we head through the afternoon the chances for low end MVFR ceilings
will be increasing. Cigs are likely to lower into the IFR and
eventually LIFR Saturday night into Sunday. Also expect low
visibilities in fog to become an increasing issue, particularly
late Saturday night into Sunday as a surface warm front approaches
the area from the south.
352 PM CST
An active pattern will continue over the lake into next week.
A quick-moving, but fairly strong clipper-type system moving
across the upper Great Lakes has brought a tightening pressure
gradient to the upper Great Lakes, resulting in an increase winds
to require a Small Craft Advisory winds over the Illinois and
Indiana nearshore waters and a Gale Warning for the open waters.
Winds should diminish through the evening and overnight.
A strong plains low will develop Sunday afternoon and move
northeast, lifting a warm front over the lake on Monday.
Increasing temperatures and dew points over the cooler waters, as
well as added moisture from snow melt to the south of the Lake,
May increase the likelihood of fog across the southern part of the
lake late Sunday night into early Monday. Winds ahead of this
system may approach gale force Sunday night into Monday, but the
greater potential for more widespread gales will be following the
passage of the system as strong cold advection sets up following
the passage of the associate strong cold front. There is still a
bit of variance in model guidance on where this low cross will
cross the lake and the timing of the cold frontal passage, so
confidence in timing and duration of gales at this time is low.
Lm...Gale Warning...lmz777-lmz779 until 3 am Saturday.
Small Craft Advisory...nearshore waters
until 3 am Saturday.
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