Area Forecast Discussion
Issued by NWS Chicago, IL

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FXUS63 KLOT 221233

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
733 AM CDT Thu Mar 22 2018

339 AM CDT

Through Friday...

The meteorological attention is on Friday night into Saturday,
and ahead of that the weather is quiet with high pressure moving
over through Friday morning. A lake breeze will turn this
afternoon dropping temperatures lakeside, while areas away from
the lake should warm to around 50. Afternoon mixing and ambient
dry air should allow humidity in many places away from the lake
to drop to 20-30 percent.

Friday will see strengthening easterly flow which should keep
temperatures slightly cooler along with increasing afternoon
clouds. Some light precipitation, as rain, could begin in the far
western CWA prior to sunset.



339 AM CDT

Friday Night through Wednesday...

Late season snow -- likely heavy at times in the western and
southern CWA -- continues to increase in confidence for Friday
night and Saturday. Have collaborated and issued a Winter Storm
Watch for later Friday evening through mid-afternoon Saturday for
the most favored area. The persistent challenge with this event of
a sharp gradient in snowfall continues but there is modest
confidence in that gradient being over or just north of the
northern watch counties, including the western/southern Chicago
metro. For north of this area including cities of Rockford and
Chicago, some snow is likely including minor accumulation, but
more limited duration of the snow keeps forecast amounts lower
than six plus inches. Mixed precipitation, including possibly
some ice, remains a threat in mainly the far south Friday night
into early Saturday.

The Pacific system of interest is about 500 miles off the
California shore early this morning, and is forecast to ride over
and break down the Rockies/Plains upper ridge. The 850-500mb
baroclinic zone draped from the Dakotas through the Mid-
Mississippi River Valley presently will lift northeast and sharpen
Friday night and serve as the focus for moderate to heavy snow
from the Upper Midwest through our forecast area and
southeastward. The low path on a blend of guidance is from just
south of Kansas City to Evansville. For Pacific systems this is
ideal with all other elements in place for the southern half of
the CWA (an ideal Chicago path would be northern MO into central

Model guidance is in solid synoptic agreement with the track of
the system and its high moisture replenishment into the region,
with global guidance favoring a tad further south especially the
EC than the 00Z NAM and 03Z SREF. Given the sharpness in gradient
expected between light and heavier snowfall, it is not a surprise
that details in the gradient vary substantially and are still
very much prone to shifts. What does increase confidence for the
heart of the watch area is the sustained high QPF (0.60"+) falling
as snow in all guidance over the past several solutions. Have
leaned a little more on the global guidance for placement and
tighter snowfall gradient. As for temperature profiles for
precipitation type and ability to accumulate snow, these are all
fairly similar. Accounting for the fact warm advection sometimes
can get further north still would not change things much, and the
trend actually has been to retard the northward advance of the
warm nose aloft.

Precipitation is expected to be advancing northeastward through
Friday evening into the overnight, with wet bulb and dynamic
cooling helping to favor a profile of snow not long after onset,
apart from the far south where rain will be seen for awhile at
first before a probable mix. The specific timing of snow onset has
lower confidence given the drier air and stiffening easterly
flow, but west/southwest areas should begin prior to midnight.
Synoptic and mesoscale lift and forcing overlap nicely for heavier
snow overnight in western and southwestern areas and then
expanding across most of the southern CWA Saturday morning as the
upper wave dives southeast over the region.

A 40-45 kt low-level jet is forecast to develop Friday night
transporting mixing ratios in the layer of deepest ascent to a
high 4.5-5 g/kg, and PWATs to just under one inch. The locked-in
warm air advection pattern supports vertically sloped
frontogenesis and likelihood of banding. The 800-700mb f-gen was
keyed in on for where the heaviest precipitation will likely be
favored, with some instability (negative EPV) shown in the layer
above this. Now these are mesoscale features which can be prone
to large errors beyond 24-hours, but the synoptic pattern supports
these developing and being somewhat persistent, and it is just
specific placement that will have to be watched and possibly

This will be a wetter snow with lower ratios, although large
clumped aggregates at times look favored given the deeper thermal
profiles centered around -4C. Have used snow ratios of around
10:1. While some areas are forecast to receive over an inch of
precipitation, possibly even one and a half inches in the south,
some of this looks to fall as not snow or with marginal
temperatures at the surface at first leading to melting.

Again the uncertainty resides in how far north as the
precipitation shield battles the low-level dry conveyor belt from
the east. Ensemble output such as from the 00Z GFS Ensemble
(GEFS) really captures this well, with fairly stout clustering at
ORD (0.15" mean) and Pontiac (1.25" mean), but in-between large
spread. These details will continue to be refined in time.

Easterly winds gusting during Saturday will likely plaster the
wetter snow on some surfaces and could lead to very sharply
reduced visibility in the pouring, heavier snow. There could be
some utility issues given the wet nature of the snow and the gusty
winds. Will also note in messaging the possible health concerns
with shoveling heavier snow content.

Snow should end from north to south Saturday afternoon into early
evening. It is likely snow intensity is light for the last several
hours and impacts likely waning.

For beyond this event, the concern may become with the hydro
aspect given how quickly this high water content snow may melt. A
warm-up including into the 50s with high dew points is forecast
Tuesday within a warm sector. Throw on that rain which looks
likely for some period Monday night-early Wednesday, and we could
have rising rivers and possibly some flooding depending on
rainfall amounts.



For the 12Z TAFs...

Little change necessary to the going forecast.  Quiet weather
expected for the period with the only forecast concern being
winds, and particular, timing of a possible lake breeze passage.

As high pressure moves across the region, winds will becm lgt/vrbl
to light nwly for much of the day. Light winds and little or no
significant cloud cover will keep conditions favorable for lake
breeze formation, with the main questions remaining, will the
lake breeze push inland far enough to cause a wind shift to ely
and an increase in wind speeds to close to 10 kt. The higher
resolution guidance still indicates that the lake breeze will
push inland, with a lake breeze passage timing arnd 23z at
ORD/MDW. Confidence in these details medium. After sunset, high
pressure will still remain in place, so winds should drop off
again the lgt/vrbl with no other significant weather impacts.


410 AM CDT

High pressure extends from Hudson Bay through the western Great
lakes to the Central Gulf Coast.  The high pressure will remain
nearly stationary into the weekend while low pressure tracks
across the Great Plains.  Northerly flow will prevail ahead of
the ridge axis through Thursday and much of Friday, then winds
should begin to veer to the east later Friday into Saturday as low
pressure deepens and tracks across the Central Plains. East winds
should peak around 25 to 30 kt Saturday morning. The low will then
track across the mid Mississippi Valley Saturday and weaken.
Another low will form over the plains late in the weekend and move
across Lake Michigan around midweek.


IL...Winter Storm Watch...ILZ008-ILZ010-ILZ011-ILZ019-ILZ020-
     ILZ021...10 PM Friday to 4 PM Saturday.

     Winter Storm Watch...ILZ022-ILZ023-ILZ032-ILZ033-ILZ039...1 AM
     Saturday to 4 PM Saturday.

IN...Winter Storm Watch...INZ010-INZ011-INZ019...1 AM Saturday to 4
     PM Saturday.




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