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Area forecast discussion 
National Weather Service Chicago/Romeoville Illinois
1004 PM CDT Thursday Oct 30 2014

Update...
921 PM

Middle-evening update...

Have made some additional minor refinements to probability of precipitation for showers with
approaching cold front tonight. More importantly...have bumped quantitative precipitation forecast
and snowfall amounts associated with lake effect precipitation
expected Friday afternoon and evening in Northwest Indiana.

Cold front continues to develop southeastward across Iowa...
Minnesota and southwest Wisconsin this evening...as cold Canadian
high pressure expands southeast toward the region. Gusty winds still
remain a few hours away for the local forecast area with
strengthening gusts more sharply falling temperatures after
midnight. Generally weak radar echoes noted across the region as
cold advection and low-level frontogenesis strengthens upstream of
the County Warning Area. High-res guidance continues to depict development of
scattered light showers/sprinkles after midnight as the colder air
spreads in...and have largely based hourly pop trends along these
trends.

All headlines remain unchanged through the period...with no changes
in expectations of the very gusty winds...and the resulting large
waves and Lakeshore flood potential along Lake Michigan. Becoming a
bit more concerned about potential for significant lake effect snow
over Northwest Indiana Friday afternoon/evening...and even portions
of the Illinois Lake shore areas Friday night. Really impressive
thermodynamic set-up with cold air over Warm Lake waters and strong
unidirectional northerly fetch over the lake. Allowing for lake
surface temperatures in the low 50s f...lake-850 hpa Delta T approaches 20
degree c during the period and yields lake-induced forecast convective available potential energy up to
1200 j/kg and inversion levels above 15 kft. The only potential
limiting factor...and item of great uncertainty...is the warmth of
the water/surface wet bulb temperatures supplied by the long fetch over
warm waters. Given strong instability...with good likelihood of
thunder...thermodynamic instability may well be able to overcome
warm low levels and produce accumulating wet snowfall. With much
uncertainty remaining...have increased quantitative precipitation forecast/snowfall amounts slightly
across Northwest Indiana Friday afternoon/night to the 1-3 inch
range...and have indicated the potential for locally higher amounts
in the advisory west-southwest text. If column is able to cool sufficiently for
long duration +sn/tssn...could be several inches more in some spots.
In addition...though instability weakens later Friday night as winds
shift northeast...expect less intense rain/snow or all snow showers
to impinge upon parts of the Illinois shore and Stateline areas as
well.

Ratzer

&&

Short term...(tonight through saturday)
332 PM CDT...

The main forecast focus this afternoon continues to center around
the storm system expected impact the region late tonight through
Friday night. Strong winds...and the potential for some periods of
heavy lake effect snow are the primary concerns.

We are issuing a Wind Advisory for most of the eastern half of the
area...with the exception of Lake...Porter...Jasper and Newton
counties in Indiana...where a Winter Weather Advisory is being
issued. See below for more details.

Water vapor imagery early this afternoon shows a rather vigorous
upper level impulse now moving just east of Lake Winnipeg and over
far southwestern Ontario. This wave is expected to continue digging
south-southeastward tonight along the eastern periphery of an upper
ridge...current centered across the inter mountain west. As this
occurs...an area of surface low pressure will shift southward across
lower Michigan into Friday morning. Farther northwest...a rather
Stout 1036 mb high is also expected to rapidly build southeastward
across the Dakotas and upper Midwest into Friday. This still looks
to set the stage for strong northern winds across the area during
the day Friday as strong pressure rises and strong cold air
advection lead to a quick ramp up of northerly winds in the wake of
a cold frontal passage late tonight. Model soundings all suggest
mean mixing layer wind speeds could top 40 knots across mainly the
eastern half of the area inland from the lake...meaning wind gusts
could easily reach this magnitude in most areas. However...areas
directly adjacent to Lake Michigan in northeastern Illinois and
northwestern Indiana could experience even stronger gusts up around
50 knots with the strong northerly onshore component to the winds.
Winds look to remain into the evening on Halloween...though wind
speeds should gradually weaken through the night.

The Lake Shore Flood Advisory will continue for areas near the lake
as large battering waves in excess of 20 feet appear likely...and as
the previous discussion mentioned...lake levels are about 2 feet
higher than past years...which could make the impacts of these waves
higher then similar wave events in the past couple of years.

Due to the likelihood of these strong winds...we have issued a Wind
Advisory for much of the eastern half of my County Warning Area. This
includes areas basically along and east of a line from Harvard
southward through Kankakee and Iroquois counties. The only exception
to this Wind Advisory is for Lake County Indiana...as well as
Porter...Jasper and Newton counties...where a Winter Weather
Advisory is being issued in place of a Wind Advisory for the
combination of these strong winds and also the increasing potential
for some accumulating lake effect snow on Friday into Friday
evening.

Precipitation with the approaching cold front and upper level
impulse
will likely begin to enter far northern Illinois after 10 PM this
evening...and be in the form of rain showers. This initial round of
showers should move southward across the area through the night...in
close proximity the increasing frontogenesis in the vicinity of the
cold frontal boundary. This should be a fairly progressive area of
showers...with most areas across north central Illinois drying out
by early Friday morning.

The second stage of precipitation will be more of a true lake effect
setup. A cold airmass featuring -7 degree c temperatures at 850 mb
will move over the lake...which is currently reporting a 53f (11.7c)
degree water temperature at the south buoy. Therefore...Delta t's
should end up very favorable around 19 degrees c. In addition...Deep
Lake induced instability is still expected...with equilibrium levels
likely to reach or even exceed 16,000 feet above ground level. Therefore...the threat
for thunder and lightning has not changed...with convective
processes
likely to be more than deep enough through the -10 to -20 layer to
result in good charge separation.

It looks as though this lake effect band could begin to set up
across northwestern Indiana and perhaps portions of southeastern
Cook County by late Friday morning. Hi res guidance suggests
uncertainty on the exact areas that will get this lake effect
banding. Some have hinted in a farther west band...which would
impact more of the Chicago area. However...I think at this point
areas in Northwest Indiana will be more favored...with the
likelihood of a nearly due north boundary layer flow setting up over
Lake Michigan. This would tend to favor better boundary layer
convergence along the southern tip of Lake Michigan Friday
afternoon...which suggests that Porter and lake Indiana will be more
in line with the potential heavier precipitation. Due to the strong
winds...it is also likely that this lake effect precipitation could be
driven well inland across northwestern Indiana...possibly impacting
much of Jasper and Porter counties as well.

Uncertainty in precipitation type is still somewhat high...as
some warmer air moving off the lake should make boundary layer
temperatures marginal for snow to reach the surface. However...with
the aforementioned convective instability and the good potential for
some heavier precipitation rates...it is very much in the realm of
possibilities that enhanced dynamic cooling could result in a cold
enough boundary layer supportive of heavy snow. It appears that
areas inland from the immediate Lake Shore would have the best
potential for snow...though even areas along the lake could change
over/mix with snow. Overall...with the potential for some heavier
bands of snow with some accumulations possible (possibly a few
inches in some areas) along with the very strong northerly
winds...we have chosen to issue the Winter Weather Advisory for the
4 northwestern Indiana counties. If...however...the transition to
snow does not pan out as current thinking suggests...winds would be
the main hazard on Friday...and the Winter Weather Advisory could
then be replaced with a Wind Advisory. Therefore...in spite of the
uncertainties on when change overs to snow will occur and how much
snow will actually fall...we feel at this point...to error on the
side of caution...as there is a potential for some very heavy snow
in addition to the strong winds.

The winds are expected to gradually veer more northeasterly Friday
night...and this could allow some of the lake effect precipitation
to get into northeastern Illinois for a brief period Friday evening.
However...model guidance continues to suggest that inversion heights
will crash in a hurry by late Friday evening as heights and
temperatures begin to rise aloft. Therefore...any snow showers that
get into northeastern Illinois would likely be light and come to an
end late Friday night/early Sat morning.

High temperatures area-wide look to remain in the lower 40s at best
on Friday and Saturday

Kjb

&&

Long term...

High temperatures will moderate into early next week as the
western Continental U.S. Upper ridge builds eastward over the region. Another
cold front looks to take aim on the area around next
Tuesday...which will result in our next chance of showers.
Otherwise temperatures look to remain near normal during the later
half of the week.

Kjb

&&

Aviation...

//Ord and mdw concerns...updated 03z...

* rain showers overnight.
* Snow showers Friday morning and possibly late Friday afternoon.
* MVFR ceilings overnight through Friday morning.
* Strong/gusty northwest winds overnight.
* Very strong/gusty northerly winds to 40 knots Friday. Cms

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

Multiple forecast concerns this period. Low pressure over the
eastern u.P. Will move to Lake Huron by morning. A trailing cold
front will move across the area late this evening into the early
morning hours...shifting westerly winds to the northwest with
speeds slowly increasing. As colder air spreads across the area
late this evening and overnight...rain showers are expected to
develop...mostly likely right over the terminals. Thermal profiles
suggest precipitation should remain mainly rain through much of the night
and perhaps only last a few hours though confidence on timing is
low enough to maintain vicinity mention.

Winds continue to increase Friday morning as colder air continues
to pour into the region. As winds shift to the north middle/late
morning...there will be another chance for precipitation...by this
time...thermal profiles are cold enough for at least a mix of rain
and snow but maintained snow showers from previous forecast.
Confidence regarding specifics during this time period are low...
including duration and intensity. As this time period approaches
and trends emerge on radar...a short duration tempo for lower
conditions may be needed.

Ceilings should remain VFR this evening and lower to MVFR overnight
with the potential for a short period of low MVFR possible. Once
precipitation ends in the morning...ceilings should lift to either high MVFR
or low VFR.

Winds will steadily increase from the early morning through the late
morning and turn northerly. Prevailing speeds in the middle 20 knots
range look on track with prevailing gusts into the middle/upper 30kt
range with the potential for gusts to 40kts in the late morning
and early afternoon. Speeds/gusts will slowly diminish late in the
afternoon...especially at rfd...but windy conditions will continue
into Friday evening.

As for gyy and lake effect...a band of lake effect rain and snow
is expected to develop by middle morning and continue into Friday
evening...affecting Northwest Indiana for much of the day. This
precipitation is expected to be quiet intense with thunder likely.
If this intense precipitation is sufficient enough to cool the lower
layers...the precipitation may switch over to all or predominantly snow.
Confidence in how long this may persist is low and maintained a
rain/snow mix at gyy with a period of reduced visible/ceilings in the
early/middle afternoon. Specifics are difficult from this distance
and changes to the gyy forecast should be expected.

Some of the guidance shows this lake effect precipitation moving
back west toward Ord/mdw late Friday afternoon into Friday
evening. Confidence regarding this trend is also low. However...
the band will be close enough to mdw to add mention late in the
afternoon with this forecast. If precipitation is snow and does make it
to Ord/mdw...IFR ceilings/visible are certainly possible. Cms

//Ord and mdw confidence...updated 03z...

* high for rain showers overnight.
* Medium for snow showers Friday morning...low for duration/
intensity.
* Low for snow showers late Friday afternoon.
* Medium for MVFR ceilings overnight/Friday morning.
* High for wind speeds/gusts/direction through the period...medium
for timing/changes. Cms

//outlook for Ord/mdw for 12z Saturday-12z Thursday...updated 00z...

* Saturday...dry/VFR. Northeast winds 10-15 knots.
* Sunday...dry/VFR. South winds 10-15 knots.
* Monday...slight chance of rain showers. Gusty southwest winds.
* Tuesday...shra/MVFR likely. Southwest winds.
* Wednesday...dry/VFR. Southwest winds.

&&

Marine...
250 PM CDT

A very active period over Lake Michigan beginning later tonight. The
area will find itself in a favorable and anomalous synoptic weather
pattern for strong northerly winds...and this will be further aided
by the warm waters/extreme instability to produce regular higher end
storm force gusts persisting into Friday. A strong cold front will
move down the lake rapidly from around midnight through 5 am.
Immediately behind this a very tight pressure gradient in sync with
cold air advection will produce a ramp up in northerly wind speeds.
The highest wind speeds of storm force to 50 and possibly even 55 knots
are expected late tonight through middle Friday afternoon for the
central and southern part of the lake...and after daybreak through
the day on Friday for the nearshore areas of Illinois and Indiana.
The wave maximum of over 20 footers is expected to develop over the
central part of the lake early Friday morning and evolve southward
reaching the Illinois and Indiana nearshore Friday afternoon and
early evening. Waves will be very slow to subside through late
Friday night and early Saturday. High pressure will move quickly
over the lake Saturday night before southerly flow returns to the
lake.

Mtf

&&

Lot watches/warnings/advisories...
Illinois...Wind Advisory...ilz014...7 am Friday to 10 PM Friday.

Lakeshore Flood Advisory...ilz014...11 am Friday to 4 am
Saturday.

Wind Advisory...ilz005-ilz006-ilz012-ilz013-ilz020-ilz022...7 am
Friday to 7 PM Friday.

Wind Advisory...ilz023-ilz033...10 am Friday to 7 PM Friday.

In...Lakeshore Flood Advisory...inz001-inz002...11 am Friday to 4 am
Saturday.

Winter Weather Advisory...inz001-inz002-inz010-inz011...10 am
Friday to 10 PM Friday.

Wind Advisory...inz019...10 am Friday to 7 PM Friday.

Lm...Gale Warning...lmz080-lmz366-lmz565-lmz567-lmz669-lmz671-lmz673-
lmz675-lmz741-lmz742-lmz743-lmz744-lmz745-lmz777-lmz779-
lmz868-lmz870-lmz872-lmz874-lmz876-lmz878...7 PM Friday to
4 am Saturday.

Storm Warning...lmz080-lmz366-lmz565-lmz567-lmz669-lmz671-lmz673-
lmz675-lmz777-lmz779-lmz868-lmz870-lmz872-lmz874-lmz876-
lmz878...3 am Friday to 7 PM Friday.

Gale Warning...lmz261-lmz362-lmz364-lmz563 until 10 PM Friday.

Gale Warning...lmz740...4 am Friday to 4 am Saturday.

Storm Warning...lmz741-lmz742-lmz743-lmz744-lmz745...4 am Friday
to 7 PM Friday.

&&

$$

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