Area Forecast Discussion
Issued by NWS Chicago, IL

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FXUS63 KLOT 160336

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
936 PM CST Sun Jan 15 2017

935 PM CST

For Evening Update...

Elected to add DuPage, Cook and Lake counties to the winter
weather advisory for Monday morning based on the expectation for a
period of light freezing rain and some glazing potential.

Surface high pressure ridge lingers across northern IL and
northwest IN this evening, with temperatures ranging from around
the freezing mark across the southern half of the cwa to the 20`s
across the north. While a surface warm front was located well
south of the Missouri and Ohio river valleys, broad warm advection
was already developing aloft as depicted by south-southeast winds
in the 925-850 mb layer in 00Z raob data and modest veering wind
profile from sfc-850 mb in both DVN/ILX soundings. Indeed, an area
of light precipitation has been brushing the southern tier of cwa
counties during the evening in association with a weak mid-level
short wave which has enhanced ascent and moist-advection across
central IL/IN. Had spread some slight chance pops eastward along
this area earlier, though with dry low levels and now weakening
forcing it appears that precip at the surface will be spotty.
Surface temps generally 32-34F in these areas as well, which is
likely limiting any icing potential.

Precipitation is expected to blossom northeastward during the
pre-dawn hours of early Monday morning however, as large scale
ascent increases in response to the strong negative-tilt mid-
level wave lifting out of the Southern Plains. Increasing south
flow above the warm-frontal inversion and associated moisture-flux
will saturate mid-levels, with resulting precipitation expected to
then saturate the lower levels of the column during the morning.
Very dry low levels as noted by surface dew point temps in the
teens-20`s and large temp-dew point spreads in nearby 00Z
soundings suggest some potential for a brief period of snow/sleet
before persistent warm/moist advection yields mainly liquid
hydrometeors. Of course the concern is temperatures currently near
or below freezing and the potential for icing/glazing especially
on untreated surfaces (or thinner paved surfaces such as
sidewalks and driveways). Areas south of I-80 are less likely to
fall appreciably below freezing overnight with thicker cloud cover
and a slow rise in temps expected toward morning. Areas to the
north however, especially along/north of the I-88 corridor are
likely to have temps below freezing and are more at risk for ice
accumulation. With this in mind, have elected to add DuPage, Lake
and Cook counties to the winter weather advisory for Monday
morning, with the greatest impact expected from the I-88 corridor
northward. Warmer temps in the heart of Chicago and along the lake
will likely limit icing there, while the western parts of DuPage
and Cook counties and much of Lake county likely to see more
potential for impacts. Surface temps/dew points will continue to
warm slowly across the entire area through the morning, rising
above freezing from south to north by mid-late morning and ending
the threat of freezing precip.

Updated grids/headlines/zfp/lfp already available.



244 PM CST

Through Monday...

Quiet weather conditions will prevail through the remainder of the
day and into the evening with lgt/vrbl winds and increasing mid and
high cloud cover. The main forecast concern will be centered around
the time on onset of precipitation and precipitation moving into
the area tonight.

This afternoon, strong high pressure extends from southern Quebec
through the Great Lakes to the middle Mississippi valley.
Temperatures this afternoon have been able to climb into the lower
to middle 30s under cloudy skies with calm to lgt/vrbl winds.

Weather concerns will quickly shift tonight to the developing
winter weather system developing over the plains. For the
afternoon forecast update, have made a few changes to the going
forecast with regard to timing of onset of pcpn as well as pcpn
types. As is usually the case with deep closed lows dropping into
the desert southwest, the system has trended slower with every
model run, up to and including this morning`s runs. Latest thermal
and moisture profiles suggest that the predominant p-type, at
least initially, should be freezing rain, with the chance for some
snow mixing in on the nrn fringe of the pcpn shield and some
sleet possibly mixing in farther south. Temperature profiles
already indicate a warm, over +1to +2 C layer over areas where the
sfc temp remains at or below 32 F. Going into the evening, any
areas that had been above 32 F will quickly cool of and sfc temps
will be below 32 F before the onset of pcpn overnight tonight.
have introduced slight chance to low chance PoPs into the far swrn
portions of the CWA shortly before midnight, but any pcpn that
manages to reach the ground before midnight should be light,
perhaps even only trace amounts. As the system gradually lifts
northeast overnight, pcpn will begin to spread from southwest to
northeast across the CWA, reaching the Rockford area by arnd 08-
09z and then the Chicago metro area by arnd 12Z. The window of
opportunity for the potential for the greatest ice accumulation
from freezing rain will a relatively short period durg the early
morning hours. Will hoist a Winter Weather Advisory, which will
cover the morning rush hours. Again, while there could be some
snow or sleet mixing with the freezing rain for short periods, the
main wither weather concern will be the freezing rain. As for
duration, the latest guidance continues to bring the warmer sfc
air into the area relatively quickly, with the changeover from
freezing to liquid pcpn occurring durg the late morning hours.
However, given the strength of the warm, moist advection in
advance of the sfc low, there will be the potential for the pcpn
to change over to all liquid a little earlier than currently
outlooked in the Winter Weather Advisory. Currently anticipate
that temps will reach 34-35 F by arnd 14z over the far south
portions of the CWA, with the entire CWA reaching the middle 40s
to around 40 F by 18z. Highs should reach onto the lower 40s over
the far south this afternoon. Since the strong high will be slow
to move out, light esely flow and sfc ridging will still poke into
nrn IL/srn WI into the afternoon, so locations toward the WI
border will only hit the middle to upper 30s.


237 PM CST

Monday night through Sunday...

The surface low of the mature mid-latitude cyclone is agreed upon
within guidance to pass directly over the CWA overnight Monday
night as the negatively tilted upper wave lifts northeast. With
still a closed upper feature and the continuous slowing trend in
guidance with this system, it could still further slow, but the
main theme for Monday night should remain consistent...and that is
wet. Precipitable water of around an inch, which is in the top
percentile for mid-January, is forecast to nose into the area
just ahead of the low in a zone of strong upper ascent Monday
evening. An area of elevated instability is predicted to encroach
ahead of the quasi dry slot and low-level warm front, so continue
the mention a chance of thunder for most of the area. Rainfall
amounts from Monday through Monday night are forecast to be one
half to one inch across the area. As the low center moves
overhead, fog and drizzle become more likely overnight and very
early Tuesday morning. There is potential for this to be dense.

Temperatures will warm overnight Monday night to around 50
degrees in the far south within the system warm sector, while
hanging in the mid-upper 30s in the far north. As winds turn west
behind the low passage, temperatures will drop some, before
recovering slightly or holding steady on Tuesday. There still
should be plenty of cloud cover and some spotty rain showers
remain possible. Guidance is struggling a bit with handling a
northern stream wave Tuesday afternoon and night, and how it
interacts with the preceding system, but global guidance indicates
there could be at least scattered shower coverage in northern
areas during Tuesday evening. With the column cooling, can`t rule
out some brief wet snow if enough saturation and lift exists in
the mid-levels.

The pattern remains advertised to become quite amplified and
blocked going forward, with the CWA under anomalously high
heights and 850mb temperatures for the time of year by late week.
A closed low is forecast to lift across the Plains states Thursday
and have introduced some precip chances mainly Thursday night and
Friday, but again a slowing, more westward trend, may be noted
with that system too. Temperatures look to moderate to well above
normal values whether clouds or not given the projected 925mb
temperatures and lack of snow cover here and to our south.
Nonetheless, clouds still look to be key on just how far above
normal. Continue introducing some 50s in the south on Friday and
areawide on Saturday, which looking back at local climatology and
CIPS analogs is certainly within reason.



For the 00Z TAFs...

558 pm...Multiple forecast concerns including a period of
freezing rain Monday morning and then cigs/vis lowering to lifr by
Monday afternoon and likely to vlifr by Monday evening/night.

Current area of precipitation across mainly central IL is expected
to stay south of the terminals this evening with additional precip
developing and spreading north across the terminals during the
predawn hours Monday morning. There will be a layer of dry air
that this precip will need to saturate before reaching the surface
and this should occur quickly but a brief period of sleet/snow is
possible before precip changes to freezing rain. As temps slowly
rise into the mid 30s...the freezing rain will change to all
liquid rain and rain will then continue through the rest of the
period and may become moderate/heavy at times into Monday

Cigs will quickly lower through mvfr into ifr as precip arrives
Monday morning and then should lower to lifr by late Monday
morning. Confidence on cig trends is high and cigs may end up
being lower sooner than currently advertised. Visibilities will
also lower to mvfr with precip arrival and as the low levels
continue to moisten...vis will likely lower into ifr. However...
confidence regarding vis trends is only medium. Its possible that
visibilities remain ifr through sunset Monday evening. But as low
pressure moves north across the area Monday night...its possible
that visibilities will drop under 1sm and there is some potential
for dense fog especially later Monday night into Tuesday morning.

As the surface low noted above approaches the area Monday night...
there is also a chance of thunderstorms across the terminals.
Confidence regarding how widespread these will become is low...
but prob30 mention may be needed with later forecasts. Best timing
of thunder chances would likely be mid/late Monday evening.

Light southerly winds will become southeasterly this evening and
more easterly by morning...speeds well under 10kts. Easterly winds
will gradually increase to 10-12kts Monday afternoon and there is
some potential for gusts up to 20kts Monday afternoon/evening.
Winds become light and variable Monday night with the low moving
over the terminals early Monday morning. cms


237 PM CST

High pressure over the general lakes region will depart east
Monday as low pressure approaches from the Plains states. The
pressure gradient should sharpen quite a bit over the
southern/central parts of the lake Monday evening, and there
could be 30 kt easterly gusts in response. Waves should build to
potentially small craft advisory criteria along the northern
Illinois shore. As the low moves east of the lake on Tuesday
afternoon, winds will become west-northwest, although there is not
a particularly strong push of cold advection, so speeds should
remain in check. A prolonged period of southerly winds are then
expected, with some southeast component at times. No period stands
out for real strong wind speeds Wednesday through Saturday.



IL...Winter Weather Advisory...ILZ003-ILZ004-ILZ005-ILZ006-ILZ008-
     AM Monday TO NOON Monday.




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