Area Forecast Discussion
Issued by NWS Chicago, IL

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FXUS63 KLOT 241356

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
856 AM CDT Sat Mar 24 2018

855 AM CDT

A strong band of snow/sleet has setup across
Livingston/Ford/Iroquois counties, and also right along far
western LaSalle counties, with a very sharp cutoff in where
precipitation is able to get to the ground to a continued feed of
dry air on brisk easterly winds. The story is basically told by
the fact that in Pontiac the dewpoint is 32 degrees and just to
the northeast in Morris (Grundy county) the dewpoint is 19. Expect
this to remain the case today. Therefore the forecast
coummunication challenge and message remains that there should be
some slight north and eastward expansion as better forcing will
expand into ou area, though amounts will cutoff significantly and
impacts will vary greatly even across these warned counties. But
for areas in the snow band expect significant heavy wet snow or
wintry mix, and even some rain mixed in, with the best rates
through mid afternoon. Will attempt to tighten this gradient and
amounts with the morning update.

For those of you in the north/eastward portions of the warned
counties you will likely experience vastly different impacts from
the snow then those to the southwest, where we have had reports of
3-4" inches of snow in some locations thus far.



355 AM CDT

Today through Sunday night...

Ongoing winter storm remains the focus of the near term forecast
this morning. Have Made a few changes to going headlines, adding
Iroquois county to the Winter Storm Warning, and extending times
for the Winter Weather Advisory to 00Z/7 pm and the warning
counties (plus Benton Co`s advisory) to 03Z/10 pm this evening.
Very tight northern gradient remains a concern, as it will likely
split several of the advisory counties with more impacts in the
south/southwest parts, and little impact in the north.

GOES water vapor imagery depicts vigorous mid-level short wave
trough traversing the mid-Missouri Valley into Iowa early this
morning, with a strong elevated warm frontal zone extending east-
southeastward across portions of central IL/IN. Strong moisture
advection/convergence within the warm frontal zone, and resulting
strong frontogenetic forcing along the thermal gradient, was
producing a band of moderate to heavy precipitation (mostly in the
form of snow observed at the surface) across about the
southwestern third of the WFO LOT cwa, generally along/south of a
Sterling to Pontiac and Danville IL line as of 330 am. RAP
soundings depict steep (7.5-8.0 C/km) lapse rates above 650 mb,
and occasional lightning has been detected within the stronger
portions of the frontogenetic precip band overnight. Reports of
2-3" of snowfall have been received from portions of the DVN and
ILX cwa`s adjacent to ours.

High-res guidance continues to focus precip along this same axis
today, with that very sharp northern cut-off keeping areas from
Rockford southeast through the south/southeast suburbs of Chicago
generally precip free, owing to dry easterly low level flow
preventing saturation. Forecast soundings are generally below
freezing during the event, though do depict a deep isothermal
layer right around the freezing mark far south. This would support
wet snow for the most part, though with the potential for some
mixing with sleet or rain/freezing rain at times. Liquid
equivalent QPF amounts remain around or just over an inch across
the far southwestern portion of the cwa, with persistent forcing
for moist ascent through much of the day. Precip should tend to
begin to shift south-southeastward this afternoon, though most
guidance lingers precip across the southern tier or so of counties
into early evening. Thus, have extended end times for headlines
as outlined above. Total snowfall amounts in the warning area are
expected to range from 6-8+ inches, with the greatest amounts
likely over parts of far southwest Lee, La Salle and
Livingston/Ford counties. Snow should taper off far south quickly
this evening, with everything done before midnight. Expected
impacts in the warning area include heavy, wet snow accumulation,
gusty east winds of 30-35 mph, and the potential for light glazing
from any freezing rain.

For the advisory counties, impacts will vary from accumulating
wet snow across mainly the south/southwest parts of included
counties, to little or no impacts in the far north/northeast. Many
areas north of the advisory may see little or no snow/precip.

The mid-level wave responsible for the winter storm moves out of
the area Sunday, with mid-level ridging building into the area
downstream of an amplifying west coast trough. This will allow
decreasing cloudiness, though forecast soundings indicate patchy
high clouds may linger for a time. Breezy east winds will persist,
in the lingering tight gradient within the southwestern periphery
of strong surface high pressure over southeastern Canada. This
will maintain cooling off of the Great Lakes region, with temps
expected to range from the upper 30`s to lower 40`s - coolest
along the Lake Michigan shore, and over snow covered areas of the
southwestern cwa. Have also lowered min temps across the snow
cover tonight and a bit Sunday night as well.




Monday through Friday...

324 am...Primary forecast concern is periods of rain and rainfall
amounts Monday afternoon into Tuesday night.

High pressure will be departing to the east on Monday as a trough
of low pressure slowly moves from the central Plains to the
western Great Lakes through midweek. Periods of rain will move
across the area ahead of this trough. Precipitable water values
have come up just a bit into the 1 to 1.25 inch range centered on
Monday night/Tuesday morning. Instability has also increased just
a bit. Though there remains some differences in locations of the
heaviest axis of qpf with the gfs/gem in the 1.5 inch range across
the cwa and the ecmwf further south. There will likely be some
variability across the area with the convective nature of the
precip but given the precipitable water values...amounts around/
above an inch certainly seem possible. With the expected snowfall
across mainly the southwest third or so of the cwa today...this
heavier rain early next week may lead to some flooding concerns
but to what extent remains uncertain. Despite clouds/precip...
high temps on Tuesday have the potential to get into the 60s but
also dependent on the speed of the associated cold front which may
arrive in the afternoon.

A weak ridge of high pressure will move across the area Wednesday
into Wednesday night with another cold front moving across the
area Thursday. Still some timing differences regarding how much
precip will develop with this front as another system lifts
northeast from the Ohio Valley into the Northeast Thursday night
and Friday. Gfs/ecmwf are then, currently, in good agreement with
another cold front moving across the region for next weekend. cms


For the 12Z TAFs...

649 am...Only forecast concern this period are the winds.

Easterly winds have steadily increased in the past few hours and
will remain strong/gusty into this evening with gusts 25-30kts.
Wind directions may turn a bit more to the east/northeast by this
afternoon. While gusts will likely diminish at many locations by
early/mid evening...its likely gusts will continue at ord/mdw/gyy
and trends will need to be monitored later today. Speeds/gusts
will increase again Sunday morning.

Snow is expected to continue from northwest IL through east/
central IL into this afternoon with a sharp cutoff to this precip
and confidence is increasing this cutoff line will be southwest of
a rfd/jot/rzl line. Ifr/lifr cigs/vis can be expected where snow
occurs. cms



416 am...Low pressure over Kansas early this morning will weaken
as it moves to Tennessee tonight. Strong high pressure over James
Bay will move into Quebec tonight with a weaker ridge building
back into the upper midwest and western great lakes region. This
will maintain a strong easterly gradient across Lake Michigan with
a prolonged period of 30kts into Sunday morning. There will also
be the potential for some gale gusts over southern portions of the
lake this afternoon into this evening. Another low will develop
over the plains Monday and slowly move across the western lakes
region through midweek. Southeasterly winds will increase Sunday
night with 30kts possible. The low and its trough/cold front will
move across the lake Tuesday night shifting winds to the
northwest. Weak high pressure will cross the region Wednesday with
a cold front moving across the region Thursday. cms


IL...Winter Storm Warning...ILZ010-ILZ019-ILZ021-ILZ032-ILZ033-ILZ039
     until 10 PM Saturday.

     Winter Weather Advisory...ILZ008-ILZ011-ILZ020-ILZ022-ILZ023
     until 7 PM Saturday.

IN...Winter Weather Advisory...INZ010-INZ011 until 7 PM Saturday.

     Winter Weather Advisory...INZ019 until 10 PM Saturday.

LM...Small Craft Advisory...IL nearshore waters until 4 PM Sunday.

     Small Craft Advisory...IN nearshore waters until 10 AM Sunday.




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