Area Forecast Discussion
Issued by NWS Chicago, IL

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FXUS63 KLOT 160750

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
250 AM CDT Fri Mar 16 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.



310 PM CDT

Friday night through Thursday...

Friday Night Wintry Mix Potential:

The main concern continues to be the threat for a wintry mix of
freezing rain and sleet and possibly snow from Friday evening
into early Saturday. Overall thinking and uncertainty with this
event haven`t changed substantially from the previous forecast
issuance. Any travel impacts may be limited to elevated/colder
surfaces due to well above freezing air and pavement temps on

The initially stout closed mid-upper level system will
dampen/weaken as it tracks east Friday night. Nonetheless, there
will be sufficient large scale and mesoscale (frontogenetically
driven) forcing to spread precipitation (possibly somewhat banded
in nature) northward into the region during the evening. The
northward advance of precip will be fighting dry air due to east-
northeasterly boundary layer flow north of the weakening surface
low that will track well to our south, which could delay precip
start time toward midnight or later in parts of NE IL.

Main questions regarding trends into the overnight include liquid
equivalent precipitation amounts given the weakening of the
system and spread in the guidance, temperature response to low
level wet bulbing as well as onshore flow particularly into IL
shore including Chicago, and specific p-types. Regarding liquid
equivalent precipitation amounts, while there is fairly
substantial spread in the ensembles especially with northeastward
extent, among the operational guidance the 12z 12km NAM appeared
to be a wet outlier. Kept total QPF amounts at roughly 0.25 to 0.4
inch in the southwest half of the CWA. There could also be a
sharp cut- off where little or no precip occurs over far northeast
Illinois due to the dry air and weakening of the system.

With east-northeasterly surface winds and sub-freezing web bulbs
near the surface beneath a melting layer aloft with partial to
full melting likely, it appears the area could be in for a mix of
freezing rain and sleet, with freezing rain favored as winter
p-type south of I-90 anywhere surface temps are supportive (~32F
or less). Snow is also possible over far northern IL late Friday
night. There continues to be questions as to the extent of the
freezing rain potential across 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.

Farther north towards the I-80 corridor, it appears that freezing rain
and sleet continues to be a real possibility despite it being late
in the season. The surface temperatures are progged to be at or
just below freezing around and north of the I-80 corridor, so it
does appear probable that at least some minor freezing rain and
sleet accumulation could occur Friday night. A likely exception to
this will be in the city of Chicago with onshore winds from Lake
Michigan with water temps in mid to upper 30s along with urban
heat island effect. However, even where air temps are cold enough,
any accumulations could end up being limited to colder elevated
and grassy surfaces due to the possibility or even likelihood of
warmer road temperatures after starting with partly cloudy skies
on Friday. For reference, ORD City OPS data indicated runway temps
at ORD around mid day today in the mid 70s.

Uncertainty is therefore high with regards to the extent of
accumulations at this stage, particularly as they relate to any
road/travel impacts. It could very well be the case that impacts
are exclusive to elevated/colder surfaces, but since we do not
forecast pavement temps, this is not something that can be said
with high confidence. The current forecast indicates very light
sleet/snow accums 1/2" or less generally I-80 and north and ice
accretion amounts up to one tenth of an inch or so (slightly
higher up to ~0.15 in spots) in a corridor from near Sterling and
Rochelle southeastward to Kankakee, north of Rensselaer and up to
VPZ. Lighter amounts under 1/10" surround this corridor, with 0
ice accumulation indicated in most of the city of Chicago. The
threat for any wintry precipitation should end during the early to
mid morning on Saturday.

Saturday PM through Thursday:

The rest of Saturday/St. Patrick`s Day will feature decreasing
clouds and temps rising into the 40s except likely upper 30s near
Lake Michigan due to persistent onshore winds. After a chilly
start Sunday will be a pleasant day under surface high pressure,
with highs reaching the lower to mid 50s. A late day lake breeze
shift will quickly knock down temps lakeside and a bit inland of
the lake.

Monday through Thursday will be marked by generally seasonably
chilly temps, especially near the lake as stout onshore flow
persists much of the time. As a blocking pattern (-NAO)
reinvigorates over eastern North America, the latest operational
and ensemble guidance consensus favors the early week system
initially tracking safely mainly south of most or all of the CWA
under the block. However, there is some potential for the mid and
upper system to close off and get trapped east of us, possibly
even retrograding WNW a bit due to downstream blocking. If this
occurs, the threat for some precipitation spreading back into the
eastern half or third of the CWA Tuesday through Wednesday.



For the 06Z TAFs...

MVFR cigs along the lake have mainly dissipated, and remain near
GYY. SCT/BKN high level, VFR clouds will continue to filter in,
and MVFR clouds return to the eastern terminals this morning. The
MVFR clouds scatter in the late morning/early afternoon. East
winds will be around 10 kt.

The first wave of wintry precip associated with the next system
arrives after midnight Friday night. Guidance still differs on
timing, so I kept precip in a PROB30. Precip type will likely be a
wintry mix including freezing rain. Forecast soundings strongly
suggest freezing rain will be possible, and ice accumulation of a
few hundredths of an inch is possible. MVFR or lower cigs may
accompany the wintry mix, but I did not have enough confidence to
include lower cigs in the forecast at this time.



247 AM CDT

I kept the Small Craft Advisory as is as waves should quickly
subside and no longer pose a threat to small craft by mid-morning.

North winds to 30 kt will weaken this morning as high pressure
spreads over the lake. Wind direction will vary across the lake this
afternoon with west winds over the northern end of the lake and
northeast to east winds over the southern end of the lake. The
high dissipates tonight while low pressure over Kansas weakens as
it reaches the Mid Mississippi Valley. The low continues to weaken
as it passes over the Ohio Valley Saturday. North winds set up
Saturday as the low passes to the south. A weak low passes over
the northern Great Lakes Saturday night/Sunday followed by high
pressure early next week. The next low forms over the Plains
Sunday night, and the low passes over the Ohio Valley Tuesday.



IL...Winter Weather Advisory...ILZ008-ILZ010-ILZ011-ILZ012-ILZ019-
     ILZ020-ILZ021-ILZ022-ILZ023...11 PM Friday to 6 AM Saturday.

IN...Winter Weather Advisory...INZ010-INZ011...11 PM Friday to 6 AM

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




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