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Area forecast discussion 
National Weather Service Chicago/Romeoville Illinois
752 PM CST Friday Nov 21 2014

Update...
746 PM CST

Minor changes to the going forecast this evening...mainly slowing
timing of precipitation into the region. Dewpoints remain pretty dry
across the region with single digits still in place north of I-80
and low/middle teens to the south. Regional radar mosaic shows
scattered showers moving across NE MO/southeast Iowa into west central Illinois
this evening and should start to overspread our southwestern
counties by around 04z or so...and into the Chicago area closer to
the 06-07z hour. Lowered probability of precipitation a little mainly due to limited
coverage noted upstream. While temperatures have some downward
potential from wet bulbing this evening...clouds have started to
move overhead with a few sites showing temperatures rebounding a degree
or two...with middle/upper 30s not too far away. Latest rap guidance
actually has air temperatures rising above freezing around the
onset of precipitation...so it appears the window of potential freezing
rain will remain fairly short. Will maintain current advisory
timing as even a light glaze of ice can be hazardous...but if
anything we may be able to let it go a little early.

Bmd

&&

Short term...
352 PM CST

Through tonight...

Conditions will remain quiet for the remainder of the afternoon
under mostly clear skies. Latest satellite imagery shows a dense
mass of cloud cover associated with a surge of moisture crossing the
Missouri Valley and approaching western Illinois. At the surface...latest observation
indicate dewpoints rapidly increasing into the 40s and 50s with the
moisture surge...while dewpoints over northern Illinois/northwestern in under the
mostly clear skies remain in the single digits to the lower teens.
The precipitation shield associated with the approaching system will take a
little time to overcome the very dry air in place...but upstream
trends and the short term model guidance align well with the trend
to bring increasing probability of precipitation into the far southwestern portions of the County Warning Area after
sunset and then quickly spreading to the north and east through the
evening and overnight. There will be a slow increase in surface temperatures
through the night under strong warm advection driven by a southwesterly low
level jet to 45kt. Available deep layer moisture with system is
limited...so actual quantitative precipitation forecast amounts expected should be relatively
light. Will maintain and make no changes to the going Freezing Rain
Advisory. With light quantitative precipitation forecast amounts expected...any ice accumulation
should range from a light glazing to a little over a tenth of an
inch. There is also a chance for some glazing as early as 23z-00z
over the Pontiac area...with some light drizzle or light rain coming
in contact with Road surfaces and walkways that are currently below
freezing due to the recent unseasonably cold weather. Through the
late evening and into the overnight hours...the freezing precipitation will
gradually change over to a mix of freezing or liquid to all liquid
precipitation as the warmer air surges into the region late this evening and
overnight tonight. The freezing precipitation potential should diminish from
southwest to northeast as the warmer air overspread the
region...with the freezing precipitation potential ending over the far northestern
portions of the County Warning Area...including the far northern portions of the
Chicago area by daybreak Saturday morning.

Krein

&&

Long term...
352 PM CST

Saturday through Friday...

For the remainder of the day on Saturday...with low pressure
deepening over the northern plains while high pressure moves off the
south east Atlantic coast...the persistent fetch of warm...moist air
will continue to stream over the area...with light rain persisting
through the day. Temperatures will continue to increase through the
day on Saturday with highs ranging from around 50f over the Pontiac
area to the middle 40s over the remainder of the County Warning Area. Dewpoints are
also expected to continue to rise through Saturday...reaching into
the middle 40s...which will be a Stark contrast to the unseasonably
cold...dry air that has been the prevailing condition for the past
several days. With the main focusing mechanism to maintain the precipitation
being widespread...but gradual...isentropic lift...rainfall amounts
will continue to be relatively light through Saturday night...with
widespread amounts of a couple tenths of an inch.

More significant rainfall is setting up for Sunday and into Sunday
night as a strong southern stream shortwave lifts out of the south High
Plains phases with another shortwave lifting from the southwest Gulf
Coast region. These system will result in a deepening surface low over
southern Illinois by Sunday evening as a northern stream shortwave tracks across the
northern plains to the western Great Lake region. The southern stream system will
be the primary focus for heavier rain. The models continue to
indicate almost unprecedented levels of deep layer moisture for late
November...with precipitable waters increasing to in excess of 1 inch through the
day on Sunday. While any thunder remains unlikely due to a lack of
insolation under persistent cloud cover...the unusually high
moisture content should bring areas of fog through Sunday as well as
some locally heavy rainfall. Widespread rainfall of 1 to 1.5 inches
is possible for Sunday through Sunday night with localized areas of
even heavier rain possible is some convective elements can develop.
Conditions will remain much warmer/moister than recent days through
Sunday with highs once again expected to be in the upper 40s to
around 50f.

The models have settled into a relatively consistent solution of
phasing the northern and southern stream shortwaves over the eastern Great Lakes
by Monday morning with a surface low around 980mb developing. As the low
deepens further on Monday...strong west winds will develop over the
area with winds increasing to around 25 miles per hour with 35mph gusts.
Considering a possibility that the models could even be under
forecasting the potential deepening of this low...which is a
possibility given the late fall contrast of warm...moist Gulf air
associated with the southern stream system and much colder air with the
northern stream system...there is the chance that Wind Advisory criteria
winds of sustained 30 miles per hour winds with 45 miles per hour gusts may occur on
Monday.

The deep surface low is expected to lift out to the northeast Monday
night and Tuesday...weak high pressure will build across the plains
as the surface while broad upper troughing develops over the central
Continental U.S.. the upper level pattern is expected to become more
amplified...but slowly progressive by midweek through the end of next
week...the area remaining under persistent northwesterly flow and cold
advection aloft. So the weekend warmup should come to an abrupt end
by Monday as a strong cold front passes through the region and
temperatures drop back into the upper 30s to around 40f on Monday with
highs only in the upper 20s to lower 30s for Tuesday. Through the
remainder of next week...the temperature trend will go back to well
below normal...with highs on Thanksgiving only in the lower to
middle 20s. A weak clipper-type system dropping out of the northern
rockies on Tuesday is expected to dive into the lower Mississippi
Valley by Wednesday as another weak shortwave drops through the
upper Great Lakes. This could bring some light snow for
Wednesday...but then strong high pressure should build across the
middle Mississippi Valley through the Great Lakes. This could set
up a weak pressure gradient pattern over the region...bringing the
possibility for a lake effect plume to develop over Lake Michigan.
While it is still way too early to consider the finer details of
lake effect snow associated with mesoscale banding of a lake
plume...the pattern would be conducive for such type of development.

Krein

&&

Aviation...

//Ord and mdw concerns...updated 00z...

* coverage of freezing rain showers increases after around 06z.
Dry air may limit activity to freezing sprinkles initially.

* Temperatures expected to rise above freezing by 12z.

* Ceilings lower to MVFR into the overnight with a good chance for IFR
Saturday morning.

* Low level wind shear possible tonight with winds increasing to
45 knots around 2000 feet above ground level.

* Liquid rain showers linger Saturday morning with drizzle
possible through the day.

Mdb

//discussion...updated 00z...

Freezing rain showers are expected to arrive toward 06z but very
dry air in the low levels may limit how much actually reaches the
ground initially. Much of the early activity may fall as
sprinkles. Do expect that coverage will increase overnight with
moistening allowing freezing rain to occur at the surface at the
terminals. Surface air temperatures will be warming overnight and latest
trends support a rise above freezing by 12z or so. Showers will
likely linger beyond 12z and be possible through midday or even
much of the afternoon...with drizzle possible as well.

Ceilings will lower through the night with MVFR expected early
tonight. Further lowering to IFR is expected Saturday morning and
there are mixed signals on whether it will improve to MVFR going
into the afternoon or not. If drizzle can be more persistent then
IFR may remain through the afternoon. Conditions may lower with
fog developing Saturday evening before widespread rain arrives
late Sunday night. South winds will increase slightly into the
overnight while winds above the surface near 2000 feet increase to
45 knots leading to decent low level wind shear potential through
daybreak.

Mdb

//Ord and mdw confidence...updated 00z...

* high confidence in freezing rain showers increasing in coverage
after about 06z.

* Medium-high confidence in temperatures warming above freezing by 12z.

* High confidence in ceilings lowering to MVFR tonight. Medium
confidence in a period of IFR Saturday morning...low confidence
in duration.

* High confidence in 45 knots winds at 2000 feet above ground level tonight.

* Medium confidence in intermittent rain showers/drizzle Saturday.

Mdb

//outlook for Ord/mdw for 12z Sunday-12z Friday...updated 00z...

Sunday...rain. Heavy at times. IFR likely. Gusty S to southeast winds.

Monday...chance -rasn early and -sn later. MVFR ceilings possible.
Gusty west to SW winds.

Tuesday...chance of flurries. MVFR ceilings possible. Breezy west winds.

Wednesday...light snow possible. MVFR ceilings/visible possible. West to
southwest winds 10 knots or less.

Thanksgiving day...VFR likely.

Mdb

&&

Marine...

242 PM...high pressure over the southern lakes and Ohio Valley
will move southeast tonight reaching the Carolina coast on
Saturday as low pressure moves across northern Ontario and James
Bay tonight. The gradient between these two systems will steadily
increase over Lake Michigan tonight with gale force winds
expected...possibly gusting as high as 45kts across the northern
portion of the lake. The gradient will begin to weaken later
Saturday afternoon into Saturday evening...but another low moving
across the northern plains Saturday night into Sunday will tighten
the southerly gradient again. This low will begin to weaken Sunday
evening and then be absorbed by another stronger low which will
develop over the Southern Plains Saturday night into Sunday...then
move north across Lake Michigan Sunday night into Monday morning.
This low has been forecast to deepen to as low as 975mb near Lake
Superior for several days. If this occurs...westerly high end
gales to 45 kts would likely develop Monday with the potential for
some storm force winds or gusts. Cms

&&

Lot watches/warnings/advisories...
Illinois...Freezing Rain Advisory...ilz019-ilz021-ilz023-ilz032-ilz033-
ilz039 until 6 am Saturday.

Freezing Rain Advisory...ilz003-ilz004-ilz005-ilz006-ilz008-
ilz010-ilz011-ilz012-ilz013-ilz014-ilz020-ilz022...11 PM
Friday to 9 am Saturday.

In...Freezing Rain Advisory...inz001-inz002...11 PM Friday to 9 am
Saturday.

Freezing Rain Advisory...inz010-inz011-inz019 until 6 am
Saturday.

Lm...Gale Warning...lmz080-lmz671-lmz673-lmz675-lmz777-lmz779-lmz872-
lmz874-lmz876-lmz878 until 9 am Saturday.

Gale Warning...lmz261-lmz362-lmz364-lmz366-lmz563-lmz565-lmz567-
lmz669-lmz868-lmz870 until 9 am Saturday.

Small Craft Advisory...nearshore waters
until 3 PM Saturday.

&&

$$

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