Scientific Forecaster Discussion
Area forecast discussion
National Weather Service Chicago/Romeoville Illinois
544 am CST Friday Mar 6 2015
305 am CST
Cloudiness for a time later this morning through early-middle afternoon
before skies clear out late. Some guidance...particularly the
NAM...is aggressive in saturating the boundary layer this evening
and depicting stratus overcast developing. This is known bias in the NAM
over snowpack...the GFS and especially the European model (ecmwf) are much less
bullish with very low level moisture/stratus tonight and have based
sky grids more on their solutions. Despite the lack of stratus a
moderate southwest wind and continued warm advection should result
in steady/slowly rising temperatures through the night tonight.
Skies should become mostly sunny Saturday and a continuation of the
warm up is expected...though likely modulated fairly significantly
by the existing Glacier Aka deep snowpack. Recent sunny days have
shown a distinct difference in albedo on visible satellite imagery
over urbanized/highly suburbanized areas vs majority of the County Warning Area
which is more rural. Interestingly...Thursday afternoon infrared imagery
showed a distinct temperature difference between these two regions which
obviously showed up in the maximum temperatures. Have attempted to depict this
island of relative warmth in the grids Saturday and beyond. Will try
a Post a satellite pic demonstrating this on our office facebook and
twitter this morning showing this difference.
305 am CST
Saturday night through Thursday...
Fairly benign period expected Sunday through pretty much all of next
week. Weak shortwave passing by Sunday could pose a threat for some
light rain or snow showers...but guidance has been trending drier as
a whole and have only maintained slight chance probability of precipitation for Sunday over
northern County Warning Area. Should transition to an increasingly zonal flow next
week with rising heights and warming temperatures. Snow cover will
undoubtedly play a role in slowing the warm up...particularly in
more rural areas where snow pack is often slower to melt. A few days
of sunshine and temperatures above freezing could largely wipe out the
majority of the snow in the more urbanized corridor allowing those
areas to realize more of the warmth as we get into the middle of
next week. GFS continues to advertise a frontal passage and Flipping winds off
the lake Tuesday night into Wednesday which would have a dramatic impact on
forecast high temperatures Wednesday. Majority of the guidance and
ensembles do not support this earlier frontal passage and have maintained
temperatures into the 50s Wednesday...though by then snow cover could be a
non-factor for immediate Chicago area so if outlier GFS is wrong the
majority of models pan out then we could make a run at 60 even. Late
in the week a weak backdoor front could result in a slightly cooler
temperatures except sharply colder temperatures near the Frigid Lake...though
details like that could easily change that far into the future.
//Ord and mdw concerns...updated 12z...
* gusty south-southwest winds with maximum gusts up to 25 knots in the afternoon.
* MVFR ceilings are possible this evening.
High pressure has shifted over the Appalachians and still
expecting south-southwest winds gusting to around 20 knots by 14z. A few bands of
middle to upper level clouds are over Illinois and Iowa and they will
pass overhead through this afternoon while winds gust to around 25
knots. Winds subside this evening...and guidance still features MVFR
ceilings. Given the low moisture content of the upstream
air...thinking MVFR ceilings are not likely so only went few015. SW
winds begin gusting to around 20 knots by middle morning Saturday.
//Ord and mdw confidence...updated 12z...
* low confidence in MVFR or lower ceilings developing Friday evening.
* High confidence in all other forecast elements.
//outlook for Ord/mdw for 00z Sunday-12z Friday...updated 12z...
Sunday...schc rain/snow during the day.
Monday through Thursday...dry and VFR.
247 am CST
Expecting winds to pick up on the back side of the high this
afternoon as the high shifts southeast. Will keep the Small Craft
Advisory as is. Still expecting a few hours of gale force gusts over
the north half of the lake this afternoon...but the window should be
narrow enough where a gale headline will not be needed. Southwest
winds diminish as the gradient weakens and a weak low passes over
the northern Great Lakes. Winds become west behind the low Saturday
afternoon and remain west through the weekend. Southwest winds
15-25 knots set up ahead of the next low Monday night and Tuesday...and
then become north behind the low Wednesday night.
Lm...Small Craft Advisory...lmz740-lmz741-lmz742-lmz743-lmz744-
lmz745...9 am Friday to 4 am Saturday.
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