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Area forecast discussion
National Weather Service Chicago/Romeoville Illinois
146 PM CDT Friday Apr 18 2014
352 am CDT
Period of quiet/dry weather through Easter Sunday puts focus of
forecast on temperature trends through the weekend...warming into the 70s
after cool northerly winds today. Precipitation potential appears to be
mainly later Sunday night-Monday. Progressive pattern continues next
week with another potential warm-up middle-late in the week.
Isolated showers were occurring across the far southeastern part of
the forecast area early this morning...south of about a
Pontiac-Kankakee-valpo line. These showers were occurring in a
region of low level f-general and weak moisture transport associated
with the elevated baroclinic zone behind a departing cold front...
and just ahead of a sheared middle-level short wave trough propagating
northeast into the central Great Lakes region. Radar trends
supported by short-range guidance which indicates a quick
diminishing of this precipitation as it moves off to the east/northeast
early this morning...and it should not be a factor beyond sunrise if
even that long. Clouds also quickly decreasing from the west per GOES
infrared imagery this morning...setting up mostly sunny conditions for
today. Surface high pressure building southeast from Ontario and the
upper Mississippi Valley in response to subsidence in the wake of
the upper trough will result in cooler north-northeast winds today
especially immediately downwind of Lake Michigan where highs in the
middle 40s are anticipated...while areas well inland to the west and
south of The Heart of the metropolitan area should see upper 50s/low 60s
for highs. Cool/dry weather will continue through tonight as the
surface high continues to drift east across the area as broad upper
ridging develops aloft.
South-southeast winds return Saturday as a fairly strong middle-level
short wave currently over the Pacific northwest tracks along the
Canadian border...and eventually occludes across far northwest
Ontario. Low-level moisture return will be impeded by southern
periphery of our surface ridge which will block flow off the western
Gulf of Mexico...keeping precipitation potential north/west of the forecast
area along cold front which trails southwest across WI-MN-IA through
Saturday night. Dry/breezy south winds will support a nice warm-up
Saturday with low level model thermal forecasts supporting lower 70s
across our western County Warning Area...and middle-60s farther east. It will be cooler
along the lake from the city northward however...due to a southeast
component to the wind off of the Lake. Lake-modified air should
support temperatures in the low-middle 50s near the shore. Modest south winds
continue Saturday night...which along with slowly increasing surface
dew points will make for overnight lows in the 40s/near 50
Another upper trough will slowly emerge eastward from the Desert
Southwest on Easter Sunday...with models depicting a series of much
weaker short-wave disturbances rippling northeast into the Midwest.
Guidance remains largely dry through the day Sunday however...with
relatively weak forcing and moisture transport continued to be
focused along cold front to our northwest. Despite potential for
some increased middle-high cloud cover especially north...south winds
and warm advection look to support temperatures in the middle-70s most areas
and even along the lake as flow becomes more southwest. Rain chances
then increase slowly from the northwest Sunday night into Monday as
the front eventually moves into the forecast area and upper trough
axis approaches late Sunday night and passes across the County Warning Area Monday.
Current model forecasts indicate the cold front clears the eastern
counties of the County Warning Area around midnight...with showers ending from the
west behind the front. Model soundings are not very impressive with
lapse rates...with no widespread thunder expected.
Meanwhile...guidance remains fairly consistent in digging a northern
stream short wave across the northern lakes Monday-Monday night
phasing into a deeper eastern Continental U.S. Trough by Tuesday with high
amplitude upper ridge across the west. Surface high pressure once
again builds across the upper Midwest and lakes Tuesday and
Wednesday with dry weather...but attendant north/northeast winds and
cooler temperatures. Extended period continues to feature persistent high
amplitude but still progressive upper pattern...leading to warmer
temperatures again by Thursday...but with a better potential for showers
and thunderstorms as western Gulf opens up.
//Ord and mdw concerns...updated 18z...
* northeast winds between 040-070 degree with speeds around 11-14kt
through this afternoon.
* Wind speeds decrease slightly to around 8kt and turn 080-100 degree
Surface high pressure centered to the north of the region is allowing
near cloudless skies this afternoon with VFR conds at area
airfields. Winds have turned northeasterly to between 040-070 degree
this afternoon...and speeds have nudged up slightly to around 11-14kt.
Expect the speeds to remain through early this evening...then diminish
after 03z to around 08kt. Then winds turn more easterly to 080-100
degree. High pressure will then slide east...with winds beginning to
turn southeasterly. It does appear that wind directions may hold
linger at 110-130 through much of Sat morning...then by late Sat afternoon
clouds begin to thicken and winds begin to turn to a 130-150 degree
//Ord and mdw confidence...updated 18z...
* high confidence in all forecast elements.
//outlook for Ord/mdw for 00z Sunday-12z Friday...updated 12z...
Sunday...chance of rain during the night.
Monday...chance of rain.
Wednesday...east flow. Chance of rain.
Thursday...chance of T-storms.
144 PM CDT
Surface high pressure is currently building across Ontario and the
upper Great Lakes. This is resulting in mainly light northerly flow
across the lake. However...some enhanced northerly winds...around
20kt...are ongoing across the far southern sections of the lake.
These winds look to continue into this evening before abating and
gradually shifting easterly. This north-northeasterly fetch will
will likely continue to result in elevated wave heights up around
the 4 foot mark across Southern Lake Michigan...including the near
shore waters into this evening. This is right on the boarder line of
Small Craft Advisory conditions. However...in spite of this...I have
opted to continue without a headline and mention 3-5 footers into
this evening for the near shore forecast.
The surface flow will come around out of the south-southeast during
the day Saturday...then increase up around 25 knots Saturday evening in
response to an area of low tracking eastward across southern Canada.
It appears the strongest flow will remain out over the open waters
of the Lake. A frontal boundary is expected to slowly shift
southward down the lake early next week...as another area of low
pressure is expected to develop eastward along the boundary. This
will set up northerly flow again lake-wide by Tuesday.
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