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Area forecast discussion 
National Weather Service Chicago/Romeoville Illinois
642 PM CDT Friday Oct 31 2014

Update...
556 PM CDT

Minor update to near term grids in particular increasing probability of precipitation
over northeast Illinois and heightening frozen precipitation type...as
well as extending the High Wind Warning through this evening.

Latest radar imagery showing unorganized bands of lake effect
precipitation continuing to spill into portions of northeast Illinois and
Northwest Indiana at this hour. Current observational data as well
as many spotter/public/social media reports depicting a variety of
precipitation types currently ongoing ranging from snow...sleet...and
rain. While overall column is sufficiently cold enough for frozen
precipitation...latest amdar and short term forecast soundings still
showing a highly variable thermal profile within the boundary
layer. Warmer lake air spilling into areas immediately near the
lake still helping for a rain snow mix...while areas inland
observing more of a snow and sleet mix. These varying precipitation types
across those locations will likely continue over the next several
hours...before boundary layer sufficiently cools for all snow over
all locations. This will occur while low level flow and strongest
observed convergence over Northwest Indiana veers...with the focus
then shifting more into Illinois. Although during this
time...better synoptic support will exit...inversions will
lower...saturation for snow growth diminishes...and convergence
also weakens.

Winds still rather strong at this hour and although they will
diminish in the near term...strongest winds should still remain
near High Wind Warning criteria...especially for areas near the
lake. Did extend the current High Wind Warning to 10 PM...but will
possibly observe a further diminishing trend prior to this hour.

Rodriguez

&&

Short term...
307 PM CDT

Through tonight...

No changes planned with headlines.

Several bands of lake effect snow have developed across Southern
Lake Michigan early this afternoon...with some of these snow showers
impacting portions of the northeastern Illinois shores.
However...the most substantial area of lake effect rain and snow has
been occurring along the southern tip of Lake Michigan and across the
advisory area of northwestern Indiana. Based on numerous phone
calls across this area it appears areas near the lake are slightly
warmer...resulting in a rain snow mix. However...areas roughly 20 to
30 miles inland (basically along and south of a Crown Point to
Valparaiso Indiana line) the precipitation is mainly in the form of
snow. Several reports of accumulating snow have been received south
of this line...with low visibilities under a mile.

Based on current radar trends and short range hires guidance...it
appears this activity will continue to be focused mainly across
northwestern Indiana for the next few hours...through late
afternoon/early this evening. However...some of the less organized lake
effect snow showers will likely impact portions of Cook and lake
counties in Illinois as well during this time. Finally...as the
surface low shifts southeast over the Ohio Valley into this evening
the boundary layer flow will veer and weaken a bit. This should
allow the better focus for lake effect showers to shift farther west
across portions of eastern Illinois...including the Chicago metropolitan
area near the lake. While there could be a short period of some
moderate...or even briefly heavy snow showers into northeastern
Illinois into this evening...I have chosen not to extend the winter
advisories farther west into Illinois. This is mainly due to the
fact that it appears the threat for better more organized lake
effect snow will be of short duration...limiting overall
accumulations. Height rises and warming temperatures in the middle
levels should allow inversion heights to drop below the dendritic
growth zone by later this evening/overnight. The models have been
insistent on this for the past few days and see no reason why this
would not occur as the upper ridge to our west begins to build
eastward. Therefore...any left over lake effect snow showers will
likely not be efficient snowfall producers and likely be loosely
organized...and hence the threat for accumulating snowfall should
come to an end by later this evening.

The winds with this system have been impressive. Kgyy has been
experiencing gusts to 58 knots for several hours...with similar
magnitudes noted along the Lake Shore. It appears these strong winds
will slowly ease tonight as the surface high overspreads the area on
Saturday. Therefore...no changes are planned with the current wind
advisories or warnings.

Lake Shore flooding continues to be an issue...especially in
Chicago...where waves have been getting water onto Lake Shore drive.
We considered to upgrade to a Lake Shore Flood Warning...but decided
to just beef up the wording in the current advisory as a warning
would not due much good at this stage of the game.

Kjb

&&

Long term...
307 PM CDT

Tomorrow through Thursday...

Saturday and Sunday...
the upper level trough pushes east allowing a large upper level
ridge to move overhead through the weekend. The corresponding
surface high moves over the region tomorrow evening.

Guidance continues to feature a narrow convergence band along the
southwest end of Lake Michigan early Saturday morning. Kept a
slight chance of precipitation in for far northeast Illinois with the band
dissipating by late morning/early afternoon as forcing and moisture
weaken. Not expecting any additional significant accumulation and
the precipitation type will be a snow/rain mix as temperatures will be
above freezing over the lake. Have medium confidence in the band
persisting until morning...medium high confidence in the band mainly
impacting the Illinois side of the lake...and high confidence in little to
no additional accumulation.

Winds decrease Saturday as the high moves overhead. Warmer air
begins to move in as well with highs in the low 40s tomorrow. Winds
turn south behind the high Sunday and usher in even warmer air with
highs in the upper 40s to around 50. Saturday afternoon through
Sunday night will be dry.

Monday through Thursday...
the ridge axis moves overhead Monday as the surface low slides over
the southeast U.S. The warm-up continues with highs in the upper 50s to
around 60 Monday...and middle 50s expected through middle week. Overnight
lows vary from the middle 40s Monday night to the middle to lower 30s
Thursday night.

A surface low moves into Ontario Tuesday...and it drags a cold front
across the region Tuesday. Expecting widespread showers Tuesday
morning with rain ending from west to east as the front slides east
Tuesday evening/night. Guidance differs on how much precipitation will
fall...but its looking like a good wetting rain is expected.

A weaker low passes just to our north middle week and have medium-low
confidence in precipitation making it into northern Illinois Wednesday. Thinking
precipitation will remain closely tied to the low which right now will be
over southern WI. If the low/S track were to dip a bit more
south...there would be a better chance for precipitation across far
northern Illinois.

Winds turn northwest behind the low and another lake effect precipitation
situation is possible for Northwest Indiana Wednesday night. Only have rain
for now as temperatures will be above freezing. Winds remain off of the
lake through Thursday night as another high pressure system
approaches from the west.

Jee

&&

Aviation...

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

* north winds gusting around 30 knots at 00z...slowly diminishing
this evening.

* Lake effect snow showers affecting mdw...and expected to drift
west across Ord later this evening. MVFR ceilings/brief IFR visible in
-shsn especially at mdw. Snow showers continue overnight but
weaken after midnight.

* MVFR ceilings linger Saturday...though rise with time and scatter
late.

Ratzer

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

Very gusty north winds continue to slowly diminish early this
evening as region of tight surface pressure gradient and strong
pressure rises ahead of high pressure move southeast of the
terminals. Gust should decrease below 30 knots 00-02z for Chicago
terminals with the exception of gyy...where wind right off Lake
Michigan will produce slowly diminishing gusts of 40+ kts. Winds
continue to shift slowly to the north-northeast Saturday morning
with speeds and gusts continuing to steadily diminish during the
Day.

Lake effect rain/snow showers occurring in multi-cellular north-
south band across the lake were affecting gyy and mdw at taf
issuance...though with gradual north-northeastward shift of
boundary layer flow snow showers will likely spread west into Ord
by middle evening. Within area of snow showers...MVFR ceilings and
occasional IFR visibilities can be expected...with a gradual
decrease in intensity after midnight as high pressure builds
toward the region and inversion levels Lower. North-northeast
winds will likely maintain middle-high range MVFR ceilings into
Saturday...though with rising bases and eventual scattering later
in the day.

Ratzer



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

* high confidence in wind trends.

* Medium confidence in -shsn trends...and ceiling/visible impacts.

* Medium confidence in MVFR ceiling duration Saturday.

Ratzer

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

Sunday...dry/VFR. South winds 10-15 knots.

Monday...dry/VFR during the day. Rain showers likely Monday night. Gusty
southwest winds 15-20 knots.

Tuesday...shra/MVFR likely. Southwest winds.

Wednesday...dry/VFR. Southwest winds.

Thursday...dry/VFR. West-northwest winds.

Ratzer

&&

Marine...
212 PM CDT

Storm force winds will slowly ease to gales tonight. No changes
planned to current headlines.

For the nearshore zones...will need a prolonged Small Craft Advisory
through at least Saturday as very large waves subside. May flirt
with Small Craft Advisory criteria winds and waves Sunday and
Monday. Have medium confidence as conditions may be marginal.

North to northeast winds to 30 knots slowly diminish Saturday and winds
turn southwest behind a high pressure system Sunday morning. South
to southwest winds increase to 30 knots Sunday afternoon as the
gradient tightens between the departing high and an approaching
low. South to southwest winds finally diminish to under 30 knots
Monday night...and then winds turn west 10-20 knots behind the lows
cold front Tuesday. West winds increase to 20-25 knots and become
northwest Wednesday night...and may see 30 knots winds over the
northern half of the lake Thursday. Winds diminish Thursday night
as high pressure moves over the area.

Kjb/jee

&&

Lot watches/warnings/advisories...
Illinois...High Wind Warning...ilz014 until 10 PM Friday.

Lakeshore Flood Advisory...ilz014 until 4 am Saturday.

Wind Advisory...ilz005-ilz006-ilz012-ilz013-ilz020-ilz022-ilz023-
ilz033 until 7 PM Friday.

In...High Wind Warning...inz001-inz002 until 10 PM Friday.

Lake effect Snow Advisory...inz001-inz002-inz010-inz011 until 10
PM Friday.

Lakeshore Flood Advisory...inz001-inz002 until 4 am Saturday.

Wind Advisory...inz010-inz011-inz019 until 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 until 4 am
Saturday.

Storm Warning...lmz080-lmz366-lmz565-lmz567-lmz669-lmz671-lmz673-
lmz675-lmz741-lmz742-lmz743-lmz744-lmz745-lmz777-lmz779-
lmz868-lmz870-lmz872-lmz874-lmz876-lmz878 until 7 PM Friday.

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

Gale Warning...nearshore Winthrop to Wilmette harbors until 4 am Saturday.

&&

$$

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