Area Forecast Discussion
Issued by NWS Chicago, IL

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FXUS63 KLOT 151916

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
216 PM CDT Thu Mar 15 2018

215 PM CDT

Tonight through Friday...

Little of concern in the near term portion of the forecast this
afternoon, with the only item of note being the passage of a weak
cold front/lake breeze combo sagging through northeast IL and
northwest IN. This will produce cooler (mid-upper 30`s) northeast
winds for counties adjacent to the Lake, while areas farther
inland continue to enjoy 45-50 degree weather through the rest of
the afternoon.

Surface high pressure across Manitoba and western Ontario will
drift southeast to the upper Midwest and northern Lakes tonight,
with weaker surface ridging pushing east across the forecast
area overnight. East winds persist and will keep things a little
cooler Friday, with temperatures only in the upper 30`s again over
far northeast IL, and lower 40`s for the most part well inland and
south of I-80. High clouds will increase during the day as a low
pressure system develops across the Plains to our west, and will
slowly lower and thicken from the west by evening. Precip
potential looks to hold off until Friday night for most of the
cwa, though have included some slight chance pops late in the
afternoon across the far southwest where a few showers could
sneak in toward evening.



321 AM CDT

Friday through Wednesday...

The main focus during the long term period continues to focus on
the increasing potential for a wintry mix of snow, sleet and
freezing rain over the area Friday night into early Saturday
morning, then the potential for another round of active weather
early next week.

While conditions will be quiet across the area during the day on
Friday, clouds will be on the increase through the day and
temperatures will likely be several degrees cooler as a
northeasterly wind drags in some cooler air. The main focus for
wintry precipitation over the area is Friday night into early
Saturday morning. During this period, a weakening storm system is
expected to shift from the Plains to the Mid-Mississippi Valley.
As this occurs, model guidance continues to be in agreement that
this will result in the development of precipitation across
portions of the region Friday night as the mid-level disturbance
approaches. The precipitation looks like it will have to fight
drier air near the surface over much of northern IL Friday
evening. As a result, this may delay the start time of
precipitation into northeastern IL until after midnight.

With east-northeasterly surface winds and subfreezing web bulbs
near the surface beneath a melting layer aloft it appears the area
could be in for a mix of freezing rain and sleet, with snow also
possible over far northern IL Friday night. There still remains
questions as to the extent of the freezing rain potential across
my far southern areas (well south of I-80) as surface temperatures
could become marginal, or even remain just above freezing through
the night. If this were to occur, there would be little if any
wintry precipitation in these areas. However, farther north
towards the I-80 corridor, it appears that sleet and freezing rain
is a real possibility. This is certainly not the time of year we
would anticipate significant freezing rain over the area, and this
may end up being the case with this event as well. However, with
surface temperatures progged to be just below freezing around and
north of the I-80 corridor, it does appear probable that at least
some minor freezing rain and snow/sleet accumulation could occur
Friday night. However, any accumulations could end up being
limited to elevated and grassy surfaces due to the possibility of
warmer road temperatures, but uncertainty is high with regards to
the extent of accumulations at this early stage. The threat for
wintry precipitation should end during the morning on Saturday.

After this wintry mix, it appears that the rest of Saturday and
Sunday will be dry. Temperatures also look to moderate into the
lower 50s by Sunday. However, forecast guidance suggests that the
weather pattern could become cool and active again by early next
week. In particular is the potential for a rather stout storm
system to impact the region later Monday into Tuesday. This system
could result in rain possibly changing to snow by Monday night
locally, but this will ultimately depend on the actual track that
system takes. Some of the latest deterministic forecast guidance
has shifted most of the active weather with this storm system to
our south, which would keep much of the area dry during this
period. However, at this early stage I am not convinced that we
are out of the woods yet. I would like to see the trends in the
ensembles and deterministic models continue to point in this
direction for the next day or two before pulling all mention of



For the 18Z TAFs...

Main near term concern is wind shift from northwest to northeast
at Chicago terminals early this afternoon as lake breeze moves

Weak surface high pressure ridge was over northwest IL at midday,
with temps warming through the mid-40`s away from the Lake
Michigan shore. This was allowing a lake breeze to develop and
push inland, with northeast winds already noted in surface obs
east of I-294. Expect this lake breeze boundary to push inland and
through ORD/MDW by around 19Z/2 pm CST.

Winds then are expected to remain northeast or east-northeast
through Friday, as surface ridge moves east tonight and low
pressure develops to the west across the Plains. Some patchy
2500-3000 ft cu/stratocu over the lake may try to push inland
later tonight, with some indication of a high-MVFR layer for a
time early Friday, though confidence is low on any persistent
ceiling at this point.



215 PM CDT

High pressure over Manitoba and western Ontario will weaken while
drifting southeast across the northern Lakes tonight, and across
Lake Michigan later Friday and Friday night. This will allow north
winds to gradually diminish tonight, becoming somewhat variable
in direction over central Lake Michigan Friday near the ridge
axis. East winds will increase somewhat on southern parts of the
Lake Friday night, as an area of low pressure moves from the
Plains to downstate Illinois by early Saturday, though the low is
expected to fill as it reaches the lower Ohio Valley. Another
weaker low will progress across the northern Lakes Saturday night
into Sunday, with a weak, baggy gradient across much of the Lake
as another area of weak high pressure spreads in from the west. A
cold front will sag across the lake Sunday, with a period of east
to northeast winds lingering early next week.

In the near term, weaker gradient within cold frontal trough
axis/lake breeze circulation has resulted in relatively light
winds (15 kts) along the IL/IN shores this afternoon. As north
to northeast winds 15-20 kts become better established for a
time across southern Lake Michigan this evening, marginal small
craft advisory conditions are possible. Have inherited a
headline into early Friday, and am inclined to hold onto it
for now, as 20-25 kt wind gusts noted along eastern shore.



LM...Small Craft Advisory...IN nearshore waters until 10 AM Friday.

     Small Craft Advisory...IL nearshore waters until 10 AM Friday.




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