Area Forecast Discussion
Issued by NWS Chicago, IL

Home | Current Version | Previous Version | Graphics & Text | Print | Product List | Glossary Off
Versions: 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50

FXUS63 KLOT 160835

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
335 AM CDT Fri Mar 16 2018

330 AM CDT

Through Saturday...

A challenging precipitation type forecast for later tonight. A
period of freezing rain is expected and as noted in recent AFDs,
the uncertainty continues on the northeastward extent and what
impacts to roads will be across the area. In the zone that does
receive freezing rain, elevated surfaces/objects are likely to see
some ice accumulation, and given a few hour duration the roads
may see patchy ice as well. A collaborated Winter Weather
Advisory has been issued for the most confident area of freezing
rain for several hours. The impact time looks to be centered on
overnight, but could start in some areas late this evening and
extend in some locations to just after sunrise Saturday.

Satellite imagery early this morning shows the well-developed
closed upper low across the Kansas/Nebraska border region. Warm
advection-driven echoes on regional radar over Iowa and eastern
Missouri have shown a struggling trend to advance northeast and
that should be the theme through today. Nearly full sun this
morning will become more filtered as high clouds start to spread
in this afternoon, but not before allowing the drier low-level
air mass to warm into the lower to mid 40s in most areas. The
heart of Chicago with the onshore flow looks to stay shy of 40.

The upper low will start to feel the effects of the strong upper
ridging currently in place over the Great Lakes this evening,
which should lead to 1.) a gradual system weakening into the
overnight and 2.) a sharp northeast gradient in precipitation.
However, during the mid-evening, the leading axis of
precipitation could be modest and even convective as it spreads
into the southwest CWA, assisted by differential instability
wrapping northeastward ahead of the low. Precipitation type should
be rain at first in the southwest CWA, and cannot rule out some
thunder. As the system slows and gradually weakens late this
evening into overnight, an axis of deeper moisture convergence
should temporarily set up draped east-southeast over the
area...most favored on the suite of guidance where the current
Winter Weather Advisory is. Given the northeast dry low-level
flow, a fairly sharp gradient on the northeast side should be

The warm layer aloft in this system is not particularly warm, with
the maximum wet-bulb aloft mainly below 1.5C in the area of
interest. However, the depth of the isothermal layer is several
thousand feet, and sufficient for melting of hydrometeors using
the Bourgouin method. This combined with the wet-bulb cooling in
the inherent drier air mass presents the rare mid-March freezing
rain threat. Some precipitation will go toward wet-bulb cooling
and thus not the ice, and it is unlikely all QPF will be ice
overnight, so ice amounts in may locations that see freezing rain
may only be a few hundredths, and if so roads will likely be
alright. However, at this time the QPF forecast is for an axis of
0.10-0.15 to fall as ice during that precipitation type window for
freezing rain, most of that favored 11 pm - 4 am on high-
resolution guidance.

Air temperatures at the surface early this morning are running
well below freezing and northeast flow will only inch down 925mb
temperatures over the next 24 hours, so after dark tonight would
think central and northern areas would not have a problem dropping
to or below freezing, though it may not be more than a couple
degrees below. Again this time of year boundary layer temperatures
can be quite challenging to get much below freezing in a
precipitation event. As for the pavement temperatures, they were
noticeably warm yesterday after a sunny day, but dropped into the
30s into the evening (32 presently at Pontiac on an IDOT sensor).
So confidence on impacts, especially road impacts, is low at this

Precipitation should be weakening in the pre-dawn hours on
Saturday into the morning as the low dampens and slides east-
southeast. Anything falling Saturday morning would likely be
liquid due to lack of ice aloft, but should be quite light.
Depending on cloud trends, temperatures on Saturday look to warm
into the 40s for most non-lake adjacent areas.



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 (frontogentically
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 8 AM Saturday.

IN...Winter Weather Advisory...INZ010-INZ011...11 PM Friday to 8 AM

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




WWW.YOUTUBE.COM/NWSCHICAGO is the U.S. government's official web portal to all federal, state and local government web resources and services.