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Area forecast discussion 
National Weather Service Chicago/Romeoville Illinois
611 PM CST sun Feb 1 2015

Short term...
220 PM CST

Through tonight...

Only changes to the going headlines was to add Porter County Indiana
to the Blizzard Warning. This was done as it appears they will be
getting a period of stronger winds off the lake this evening...which
will produce significant visible restrictions.

The 19 UTC objective surface analysis places the center of a 1003 mb
low about 50 miles northeast of kstl...with a surface boundary
draped eastward over the southern fringe of my central Illinois and
Indiana counties. To the north of this front...winds have become
gusty out of the northeast...and this is beginning to lead to
significant blowing snow as we are now transitioning to a dryer
snowfall as colder air filters southward. This trend will continue
this evening as strong wind gusts of 40 to 45 miles per hour set up over
northern Illinois/northwestern Indiana. The combination of falling
snow and blowing snow will result in white out conditions at
times...especially in open areas.

The snow is expected to finally begin to abate from west to east
later this evening and into the overnight hours. The threat of
significant lake effect snow showers looks low on the back side of
this the thermodynamic setup over the lake is not looking
as good. With this in mind...any lingering lake induced snow showers
should remain any additional accumulations across
northeastern Illinois/northwestern Indiana after midnight tonight
should generally remain under an inch. An additional 3 to 5 inches
is still expected before the snow ends later this evening. This
should result in around 12 to 16 inches across much of northern
Illinois...with lighter amounts far south closer to the track of
this system.

Wind speeds look to remain elevated overnight tonight and this will
likely continue to result in blowing snow over the area after the
current 06 UTC ending time of the blizzard and Winter Storm Warning.
I though about extending the headlines some...but once the snow
stops widespread whiteout conditions will be unlikely.
Therefore...if conditions still warrant a headline overnight...a
Winter Weather Advisory may be the better Route to go.



Long term...
118 PM CST

Monday through Monday night...
low pressure will continue to depart the region to the east...with
anti-cyclonic flow replacing it over the forecast area for Monday.
Longwave flow pattern continues to be in a somewhat northwest
orientation. This coupled with the surface ridge center positioned
just south of the County warning forecast area will keep winds generally from the
west/northwest. Enough dry air should mix down to the surface and erode
the bulk of the clouds...however not too confident that clear
skies will linger very long. The flow will remain progressive and
help to push the surface ridge east/southeast of the area by late Monday

This will setup a weak middle-level wave sliding southeast across the
northern plains to arrive late Monday night in the far western County warning forecast area. Low level
flow will turn southwesterly and could keep some dry air locked in
long enough to keep any precipitation west of the forecast area until late
Monday night.

Highs Monday will struggle to warm despite the thinning solar
shielding...with temperatures generally in the low/middle teens. Then Monday night
temperatures aloft will easily radiate with light winds and thin cloud cover.
Fresh snow pack will likely help temperatures to cool into the single
digits above zero. With clouds returning late Monday night from a weak
middle-level system...temperatures could rise a degree or two after
midnight/early Tuesday.

Current expectation is that any accumulations Monday night would be less
than one half inch.

Tuesday through Wednesday...
with a middle-level wave pivoting east overhead Tuesday morning...low level flow
will remain southwesterly with some weak warm air advecting into the
forecast area. Temperatures should warm into the 20s Tuesday...and with a good
push of warm air advection in the southern tier of counties...temperatures
could tag 30 degrees Tuesday afternoon.

500mb trough axis is prognosticated to send yet another middle-level wave diving
southeast across the northern plains Tuesday morning. Guidance has
continued to forecast the arrival of this feature into the County warning forecast area Tuesday night.
Forcing appears to be rather weak...however enough moisture and lift
could easily produce some light snow once again late Tuesday night. At this
time it would appear around 1 inch of snow.

Considerable cloudiness Tuesday night will help to hold temperatures up...with
overnight lows in the upper teens to middle 20s.

Some lingering light snow Wednesday morning...with flow starting to flatten
by midday Wednesday. As the low lifts northeast Wednesday morning across lower
Michigan...a trailing boundary will slowly sag south across the western
Great Lakes and middle-Missouri Valley. A weak wave will try to develop
over the Central Plains early Wednesday...and could bring addtl light precipitation
back into the far west/southwest County warning forecast area late Wednesday afternoon.

Wednesday night through Saturday...
ensembles continue the theme of an active flow...with troughing
generally over the Great Lakes region and ridging remaining over
western Canada. There is some weak flattening in the longwave
pattern Thursday...but this appears to be only for one period.

Then ensembles are beginning to suggest broad ridging will develop
across the southwest Continental U.S....which could shift the northwest flow
orientation to the northeast. Robust thermal trough swings over the
region Thursday...bringing much cooler air back with highs in the single
digits to low teens. Thursday night temperatures could then drop below zero.
Then for the start of the upcoming weekend temperatures could warm back
into the low 30s.




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

* snow through the evening with the heaviest prior to around 02z
with lighter lake effect snow through late evening Ord and early
tonight mdw.

* Visibility mainly 1/4-1/2sm until 02-03z with some improvement to near
3/4sm after that.

* Northeasterly winds gusting to 30-35 knots through late this evening while
turning north-northeast...then easing overnight.

* Blowing and drifting snow through tonight.

* Mainly IFR ceilings through late evening.


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

Snow continues to move across the terminals this evening with
radar imagery showing that lake enhancement is occurring
underneath the larger scale system snow /brighter southwestward
moving echoes underneath weaker higher level echoes/. The back
edge of the heaviest larger scale snow is working eastward and
should be east of rfd soon and then through the Chicago metropolitan
around 02z. Lighter snow will continue for a time behind this more
intense area but blowing snow will likely become the main issue as
snow lets up. Lake effect/enhancement will continue at Ord/mdw/gyy
and to some extent dpa as winds turn from northeast to north into
the overnight. Currently...areas of more persistent 1/4sm or less
visibility is being driven by the combination of large scale snow and
lake enhancement so it is tough to gage how low visibility will be the
lake processes take over. Given parameters overall visibility should
improve somewhat but embedded heavier snow showers may persist.
Expect a few hours of lake effect snow showers at Ord into late
evening with a bit longer duration at mdw as the band sets up
along the Illinois shoreline and focuses toward gyy. Basically
expect slowly improving conditions late this evening into the
early overnight.

Winds will still be an issue causing blowing snow and limiting
visibility. Once snow intensity lets up expect visibility in blowing snow will
still be IFR this evening. Wind speeds will ease up after midnight
allowing for improved visibility in blowing snow.

For Monday...Lake snow showers will probably linger at or near gyy
during the morning. MVFR ceilings should clear toward midday leaving
only some high clouds. Winds will gust to around 20 knots early but
high pressure will be moving overhead allowing winds to diminish
and turn west then southwest late in the day.


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

* medium to high confidence in weather/visible trends.

* Medium confidence in ceiling trends.

* Medium confidence in timing of improving cig/vis.

* Medium to high confidence in wind trends.


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

Tuesday...slight chance snow early with MVFR cigs/vis. VFR/weather none
afternoon. Snow likely with IFR ceiling/visible overnight.

Wednesday...slight chance snow with MVFR ceiling/visible early. VFR/weather none

Thursday...VFR/wx none.

Friday...VFR/wx none.

Saturday...slight chance snow with MVFR ceiling/visible possible.



135 PM CST

Low pressure has been slow to lift northeast across the Ohio
Valley...reaching east central Illinois by early this afternoon. This
low continues to deepen and will allow for gales to increase from
35 knots to likely closer to 40 knots. The far northern portions of Lake
Michigan may struggle to see the stronger gradient develop and
stronger gales...but the expectation is that gales will eventually
reach the northern portions later this evening. With much colder
air sliding across the lake with gale conditions...heavy freezing
spray will be likely. Low pressure will then slide northeast late
tonight...and this will allow the gradient to steadily relax
overnight. This will gales/freezing spray to come to an end late
tonight. Broad high pressure will build in early Monday...bringing
much lighter winds to Lake Michigan.

The flow remains relatively active...with the high pressure
quickly sliding east Monday evening and flow turning
southwesterly. The gradient begins to increase late Monday night
into Tuesday...but at this time it appears winds will remain
between 10-20kt possibly up to 25kt.

A series of low pressure systems will continue to feed from west
to east across Lake Michigan through mid-week...but at this time
the systems are not expected to be strong.



Lot watches/warnings/advisories...
Illinois...Lakeshore Flood Advisory...ilz006-ilz014 until 10 am Monday.

Blizzard Warning...ilz003-ilz004-ilz005-ilz006-ilz008-ilz010-
until midnight Monday.

Winter Storm Warning...ilz023-ilz032-ilz033-ilz039 until
midnight Monday.

In...Blizzard Warning...inz001-inz002 until midnight Monday.

Winter Storm Warning...inz010-inz011-inz019 until midnight

Lm...Gale Warning...lmz080-lmz261-lmz362-lmz364-lmz366-lmz563-lmz565-
lmz874-lmz876-lmz878 until 3 am Monday.

Heavy freezing spray warning...lmz080-lmz261-lmz362-lmz364-
lmz868-lmz870-lmz872-lmz874-lmz876-lmz878 until 3 am Monday.



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