Scientific Forecaster Discussion
Area forecast discussion
National Weather Service Chicago/Romeoville Illinois
709 PM CDT Monday Mar 30 2015
339 PM CDT
Tonight through Wednesday night...
Main focus in the near term is on the clippery-like system
currently dropping southeast across the upper Midwest. Surface low
pressure analyzed over southwest Minnesota is prognosticated to continue
to move southeast over northeast Illinois late tonight...then move
quickly off into Ohio by Tuesday morning. Ahead of the
low...breezy westerly winds have warmed temperatures into the 60s and
even around 70 (sterling il) this afternoon. As the low moves
southeast toward the area however...winds will likely back to the
east/southeast this evening and will likely cause some temperature
fluctuatons as flow shift from offshore to onshore along the
Illinois Lake Michigan shore. Thus our current mild weather will
likely turn chillier at least along the lake later this evening.
In addition...area of light to moderate rain across WI will also
shift southeast early this evening... clipping northeast Illinois and
Northwest Indiana into tonight. While much of the rain will be
light...steep middle-level lapse rates noted in rap soundings and a
period of decent f-general forcing around 850 mb may produce some
heavier banded showers this evening. Showers will gradually pull
off to the east after midnight as the best forcing and warm/moist
advection shifts away from the County Warning Area with the wave.
Cooler north-northeast winds are in store for the area
Tuesday...as weak high pressure ridging spreads across the region.
While highs in the 55-60 range are expected well west/southwest of
the lake... readings closer to the shore will likely not make it
out of the middle 40s. The good news however is that this cool down
will be brief... as upper ridge develops across the central Continental U.S.
Tuesday night into Wednesday...as a deep trough works out of the
northern rockies and out across the Canadian prairies. Surface
pressure falls across the northern plains and upper Midwest will
allow winds to shift south Wednesday...increasing through
Wednesday night as the surface low tracks north of the border.
Warmer temperatures...in the upper 60s to lower 70s...will make for a
breezy more Spring-like day. A cold front trailing from the
Canadian low will approach the area late Wednesday night...with
mild and more humid conditions ahead of the boundary.
Precipitation chances will be best immediately along/ahead of the
front and mainly west of our County Warning Area through the evening...before
increasing potential for showers/thunderstorms spreads across
northern Illinois late Wednesday night.
339 PM CDT
Thursday through Monday...
Surface cold front continues to push southeast across the
forecast area on Thursday...with best chance of showers and
thunderstorms for most of the County Warning Area. Potential for some strong
thunderstorms may evolve south of the front (mainly our south
counties) depending upon cloud cover and destabilization by
afternoon...as upper jet streak moves overhead and supports strong
deep layer shear in excess of 40 kts. With the front moving
slowly...temperatures should remain fairly mild...in the 60s except for
areas near the lake where winds may shift onshore behind the front
later in the day.
The cold front continues to sag south Thursday night into
Friday...though guidance does support a continued chance of precipitation
into Friday as another wave is depicted moving along the elevated
baroclinic zone north of the surface front. Cooler and drier
weather does look to develop later Friday and Friday night as weak
high pressure moves in. The high pressure ridge then moves south
into Ohio Valley Saturday...with winds shifting west and allowing
another warm up. Long range solutions indicate a short wave moving
across the northern lakes and trailing a cold front into area by
Saturday evening however...with another weak high moving across
the area Saturday night as cold front continues to sag south.
Models differ a bit in details by days 6-7...but generally
indicate front lifting back north as a warm front Sunday as
surface low pressure develops across the plains. Precipitation chances
increase during the period in this scenario...with milder
temperatures possibly much warmer if more aggressive GFS pans out
with warm front north of the area.
//Ord and mdw concerns...updated 00z...
* northeast winds around 10kt early to middle evening
* chance of a lake breeze/northeast wind Tuesday late afternoon
Winds have flipped northeast this evening due to developing front
with some added influence from Lake Michigan. Northeast winds
around 10kt likely to continue early this evening until the low
pressure moves across northern Illinois late this evening at which
time winds will likely become lighter and variable. Once the low
passes overnight looks for northerly winds to develop later
tonight into early Tuesday morning. Winds could be close to 10kt
and at this distance it is tough to say which side of due north
they will start the day on at ohare. Confidence in wind direction
is fairly low Tuesday as synoptic gradient would tend to favor
winds backing to northwest and then west during the
afternoon...but with speeds mostly dropping below 10kt a lake
breeze is likely to develop. How far inland the lake breeze makes
it is tough to pin down...but for now have kept lake breeze/wind
shift to the east out of the tafs with thinking being about a 40
percent chance of it making it through Ord/mdw late Tuesday
Generally expect VFR conditions through the period...though there
is an outside chance that there could be some lake stratus early
Tuesday morning. Forecast soundings are hitting this threat pretty
hard...but low levels are dry and the push of marine air is not
particularly cold so have kept any ceilings out of the tafs for now
and will just monitor satellite trends tonight.
//Ord and mdw confidence...updated 00z...
* medium confidence in winds through middle evening
* low confidence in winds late this evening/early overnight
* medium confidence in wind direction late tonight through Tuesday
* medium-high confidence in no lake effect ceilings late tonight/early
* high confidence in remaining forecast elements
//outlook for Ord/mdw for 12z Wednesday-12z Monday...updated 00z...
Thursday...shra/some thunderstorms and rain likely. Periodic IFR possible.
Friday...chance of MVFR ceilings. NE winds 10kt or greater possible.
Saturday and Sunday...primarily VFR.
134 PM CDT
Stronger westerly winds over the lake have since diminished this
afternoon with a weak ridge of high pressure overhead. Along with
the diminishing trend this afternoon...winds have become highly
variable across the entire the lake. Expect either an onshore
component or a period of variable winds over the nearshore this
afternoon into the evening...with a south to east wind over the
open waters. This will occur until late in the evening...when a
low pressure system quickly moves southeast to the southern end of
the lake. Increasing speeds over the entire lake but still
variable over the south half...will then transition to a northerly
wind over all of the lake by early Tuesday morning. Speeds may
still be in the 15 to 25 knots range initially with this
transition...but will likely observe a quick diminishing trend
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