Scientific Forecaster Discussion
Area forecast discussion
National Weather Service Chicago/Romeoville Illinois
901 am CDT Tuesday Apr 21 2015
256 am CDT
Main concerns for today will reside on the gusty winds...which could
gust to 45 miles per hour this afternoon. The second concern for today will be on
strong mixing...which will produce considerable subsidence with dew
points being pushed into the low/middle 20s...leading to min relative humidity values
nearing red flag conditions.
The bulk of the clouds early this morning have been confined to the
north in wisc/mich. Winds have become southwesterly with the 500mb
trough axis pivoting overhead...however winds will become westerly
after daybreak. With minimal cloud cover to start the day...expect
mixing to quickly increase and begin producing gusts to 35 miles per hour by
late this morning. There continues to be some model differences with
respect to afternoon/evening clouds rotating southeast across wisc into
northern Illinois/northwest in. Some of the hi-res guidance hold clouds
north until after 00z. If clouds can push in earlier in the
afternoon...this may help to limit wind gusts below 40mph. However
confidence remains relatively high that the frequency of gusts will
remain to warrant hoisting a Wind Advisory from 16z-00z.
Temperatures will slowly warm today...with highs generally in the low/middle
50s. With clouds pivoting southeast late this afternoon...there is the
possibility that light rain showers may develop but should remain confined to
areas north of I-80.
Strong anti-cyclonic flow will continue to slide south from central
Canada to the Central Plains tonight. This coupled with the surface low
over Ontario will result in continued tight gradient across the
western Great Lakes and breezy conds tonight. Cold air will continue
to advect over the forecast area tonight...with lows in the low/middle
30s. A few spots could dip to freezing late tonight across the
northwest County warning forecast area. Winds and low dew points will help to prevent any frost
Dry air will return for Wednesday...with less cloud cover and another
breezy day. Winds will be less robust...however cold air will
continue to advect southeast and limit afternoon highs from warming
beyond 49-53. Wednesday night winds diminish with dry conds and lows dipping
into the low 30s. A portion of the County warning forecast area could see temperatures dip below
freezing Wednesday night.
256 am CDT
Thursday through Monday...
ensembles continue to show minimal spread amongst members in the
medium term...keeping the Great Lakes region in a troughing
pattern with blocking over the North Atlantic slowing the eastern
progression to the trough. This will keep unseasonably cool conds
through at least Thursday/Friday for the region. Oper solutions slide a surface
ridge over the forecast area Thursday night...allowing light winds to
develop. With continued dry weather from the prior couple of
days...lows could easily dip to the low/middle 30s with possible
Oper solutions diverge on how to resolve the southern stream late in
the week into the first portion of the weekend. It appears surface
ridging should linger through much of Friday...then a brief period light
showers Friday night before the southern stream pushes back south from
another surface ridge Sat night. Then the remainder of the weekend looks
to remain dry...with temperatures trying to return to seasonal conds in the
upper 50s to near 60 sun/Mon.
between 40 and 45 miles per hour. Deep mixing from a dry airmass will likely
lead to steady drying and dew points falling into the lower to
middle 20s. Current projections are for afternoon minimum relative humidity
values to fall into the 25 to 35 percent range...however several
models hint that we may trim an additional few degrees off of dew
points resulting in relative humidity values closer to 20 percent.
Min relative humidity values will be nearing red flag warning criteria...however
given the recent rainfall from Sunday night between a half to
three quarters of an inch of rain...10 hour fuel moisture may not
reach red flag criteria. Faster drying fuels however will likely
dry out quickly given the conditions today resulting in an
elevated fire danger.
//Ord and mdw concerns...updated 14z...
* very strong/gusty westerly winds through early evening. Cms
No real change in this set of tafs. Expect winds to rapidly
increase during the day Tuesday...especially middle to late morning.
Current tafs reflect what would generally be the expected steady
state conditions with gusts 35 knots (higher at gyy)...but gusts at
most terminals could reach 40 knots at times. Expect a rapid decline
at sunset with a slow shift from a 270 direction to more of a northwest
wind by daybreak Wednesday. VFR ceilings expected Tuesday afternoon
with maybe a passing and short lived sprinkle or shower. A more
a quick moving and more organized band of rain will pass south of
the main terminal areas Tuesday late evening and overnight.
//Ord and mdw confidence...updated 14z...
* high for wind speeds/gusts/directions through the period. Cms
//outlook for Ord/mdw for 00z Thursday-12z Tuesday...updated 12z...
Wednesday through Sunday...primarily VFR.
228 am CDT
Winds over the lake will be predominantly west-northwest for a few days as low
pressure works its way toward the Atlantic coast...while a high
pressure ridge builds across the Canadian prairies and then
southeast into the plains. Strong mixing today over land will be
realized close to shore along the nearshore waters given the
offshore wind component...and therefore the gale watch will be
upgraded to a Gale Warning. Prime period will be middle morning through
late afternoon. Still feel that the open waters are too stable to
support strong mixing today needed for gale force wind gusts. Gusty
west-northwest winds are expected again in this pattern on Wednesday...though
the wind field aloft is weaker...so not expecting gales Wednesday.
Winds will gradually shift more northerly as high pressure shifts east
and several lows pass by well to the south of the lake.
ilz032-ilz033-ilz039 until 7 PM Tuesday.
In...Wind Advisory...inz001-inz002-inz010-inz011-inz019 until 7 PM
Lm...Gale Warning...nearshore waters until 9 PM Tuesday.
Small Craft Advisory...nearshore waters until 9 am Tuesday.
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