Scientific Forecaster Discussion
Area forecast discussion
National Weather Service Chicago/Romeoville Illinois
551 PM CST sun Mar 1 2015
258 PM CST
Through Monday evening...
Quiet weather through Monday evening as high pressure traverses the
region. Skies will clear out this evening and start out sunny Monday
with just some high thin cloudiness arriving during the afternoon.
The sun angle now is as high as it is in early October and given the
expected sunshine Monday along with similar 800 mb/h9 temperatures suspect that
highs will climb well into the 20s...except in the urbanized/highly
suburbanized Chicago metropolitan area where i've gone a bit above guidance
due to lower albedo which should allow sun to do its thing and push
temperatures into the lower 30s.
258 PM CST
Tuesday night through Sunday...
Primary forecast concern is in the later Monday night into Tuesday
time frame when GFS/European model (ecmwf) have remained rather consistent and in
pretty good agreement on ejecting a piece shortwave energy from the
desert SW low northeast into the Midwest...while a northern stream
shortwave drops south and amplifies a trough over the northern
plains. Guidance has continued to indicate that these waves will
remain unphased and thus lower amplitude/quicker moving which should
result in a shorter duration of significant precipitation and lower amounts
given less time for significant return flow. Thermal profiles in
both models continue to show a substantial warm air advection
resulting in freezing line aloft to lift northward across the County Warning Area
overnight Monday night into Tuesday morning. Still time for models
to change their tune and adjust track farther south...but given the
consistency of late thinking that appears less and less likely.
Given the current forecast thermal profiles would expect precipitation to
start as snow or sleet Monday night and then transition to a period
of freezing rain then all rain Tuesday as column continues to warm.
Extensive deep snow pack will likely retard the warming near the
surface but given 850mb/925mb temperature climatology given forecast values
Tuesday would support highs in the 50s...think we should be able to
warm above freezing given the fairly high sun angle. Pavement temperatures
this afternoon have warmed into the 40s (with surface temperatures in 20s)...so
suspect that any snow/sleet/freezing rain that does fall Tuesday
will not be high impact on travel after sunrise unless it were to
fall quite heavily and temperatures be colder than forecast. Ice/snow
accums should be confined more to elevated surface than to pavement
as temperatures warm...so while there looks to be a quick shot of moderate
wintry precipitation centered around rush hour Tuesday morning...not
anticipating much ice/snow accumulate on most roadways. Having said that
even rain causes bad traffic so even if most roads are just wet and
not ice/snow covered weather will likely result in travel delays.
Drying aloft should result in steadier/heavier precipitation tapering off
to mainly drizzle and occasional light rain during the afternoon Tuesday.
Another fairly significant but progressive shot of late season
Arctic air spills into the region midweek as northern stream trough
digs southward in wake of the tuesday's sloppy system. Fairly quiet
pattern then settles in with little significant precipitation threat middle
week through next weekend and alternating bouts of cold/warm air
advection with a couple clipper passages. Given the high sun angle
and less harsh temperatures (other than quit hit middle week) would expect
that we will slowly begin to eat away at the deep snow pack over the
region the next week or so.
//Ord and mdw concerns...updated 00z...
* patchy MVFR ceilings until 03z.
* Southwest winds turning northwest and increasing with gusts to
17kt around 2z through 08z. Then winds decreasing in speeds after 08z.
A few low clouds continue to slide over northern Illinois/northwest in
airfields early this evening. Skies are thinning and are expected
to possibly become clear overnight. Winds will be turning
west/northwest and increase later this evening...with gusts
nearing 17kt. A few gusts could touch 20kt...but should not occur
with any frequency. Then as high pressure builds in from the
northwest...winds will steadily diminish after 6z and possibly
become calm for a few hours Monday morning. Then winds begin to turn
southwest to south in advance of the next weather system Tuesday
morning. Skies will remain VFR conds Monday afternoon through Monday
//Ord and mdw confidence...updated 00z...
* high confidence in all forecast elements.
//outlook for Ord/mdw for 12z Tuesday-12z Sunday...updated 00z...
Tuesday...snow mixing with freezing rain and sleet. IFR likely.
Thursday through Saturday...mainly VFR. Possible MVFR ceilings
312 PM CST
Lake Michigan is almost 80% covered with only a small ice free
corridor from the south central portion of the lake to just
northwest of Manistee. Ice may continue to expand through Monday
night while a cold air mass remains over the Lake. A cold front
will cross the lake this evening...and bring west-northwest winds
to 30 knots on the southern 2/3 of the lake...with a few 30kt gusts
on the far north immediately behind the front. Freezing spray is a
decent bet where open water remains. Winds will quickly diminish
as high pressure moves over the region on Monday then become
southerly and increase Monday night as the high slides southeast
and low pressure approaches from the plains. South winds to 30 knots
are expected ahead of the low on Tuesday...followed by a few
periods of northwest winds to 30 knots in the wake of the low on
Tuesday night and Wednesday. Expansive high pressure will move
just south of the lake on Thursday...resulting in diminished
winds...but then southwest winds could increase to 30 knots on Friday
as the high slides southeast. Small Craft Advisory criteria winds
are likely on Tuesday into Tuesday night...possible on Wednesday
and then possible on Friday.
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