Scientific Forecaster Discussion
Area forecast discussion
National Weather Service Chicago/Romeoville Illinois
630 PM CDT Friday Oct 9 2015
207 PM CDT
Tonight through Sunday...
The main forecast concerns in the near term are cloud cover
trends tonight...then increasingly gusty winds and warming temperatures
Cool air continues to spill into the region this afternoon...in
the strong surface pressure gradient on the eastern periphery of
high pressure spreading east from the plains and upper Midwest.
Extensive stratus/stratocu cloud cover remains across the
area...with steep low level lapse rates and moisture trapped
beneath strengthening subsidence inversion. Dry advection
continues with breezy winds from the north...and this low level
drying and loss of diurnal stratocu production. Overall have
trended a bit slower than guidance with clearing this evening
however...with considerable closed-cell stratocu and likely some
continued Shallow Lake-enhanced stratocu production...but skies
should eventually become partly cloudy overnight as high pressure
moves in. With the approach of the high winds will also fall off
fairly quickly as the gradient relaxes. The ridge axis will be
over the County Warning Area by sunrise...allowing temperatures to sink to near 40 in
outlying areas and into the middle-upper 40s in the city.
A nice warm-up is in store as we head into the weekend
however...as the surface ridge drifts southeast of the area on
Saturday and winds pick up from the southwest. Heights rise aloft
as a broad upper ridge nudges into the Midwest...with subsidence
clearing skies and allowing plenty of sunshine. Low pressure will
move well north of the area tightening the gradient between it and
the high to our southeast...with winds becoming a little breezy in
the 12-20 miles per hour range Saturday. Further strengthening of the
gradient into Sunday looks to produce fairly gusty southwest winds
of 15-25 miles per hour...with gusts around 30 miles per hour likely during the
afternoon hours. Strong and persistent warm advection boosts low
level thermal fields...with 925/825 mb temperatures supporting highs from
65-70 Saturday...and upper 70s/around 80 in many spots on Sunday.
While low level moisture does begin to return through the
Mississippi Valley...strong mixing should cause surface dew points
to lower into the 40s in many areas during the midday/afternoon
hours both days...making for windy... warm and dry conditions to
round out the weekend.
235 PM CDT
Sunday night through Thursday...
The warmer than normal conditions will continue into Sunday night
and Monday morning as strong southwesterly flow continue in advance of the
approaching cold front. The timing of the frontal passage will have
a major impact on maximum temperatures for Monday. Much of the model guidance
is in relatively good agreement on the front pushing through the
region Monday morning. A wind shift to northwesterly will be the first
impact of the frontal passage...but that the strongest cold advection will not
occur until later in the day...so temperatures on Monday will still have an
opportunity to rise into the middle 60s over the northern portions of the
County Warning Area to around 70f over the far southeastern locations. As winds shift to
westerly and northwesterly through the afternoon and become strong and gusty with
forecast soundings suggesting gusts to around 30kt through the day.
As the colder air filters over the region temperatures will likely
drop off through the afternoon and evening...with lows back in the
middle 40s over much of the area away from the Chicago-area urban
heat Island. A Pacific shortwave dropping out of the northern
plains will help turn winds back to more southwesterly...bringing a warming
trend which will see temperatures in the middle 60s to lower 70s for
the rest of the week. There will be little available with the
initial frontal passage Monday morning or the passage of the Pacific
shortwave Tuesday night...so...no precipitation is likely with either of
these system. The next chance for any precipitation for the region until
Thursday or Thursday night as a northern stream shortwave drops out
of Canada. However...the longer range guidance is suggesting that
the track of the main upper level impulse should be to the
north...across central WI into Southern Lake Michigan...so have limited
probability of precipitation to slight chance levels.
//Ord and mdw concerns...updated 00z...
* wind speeds diminishing under 10 knots prior to 01z.
High pressure over the upper Mississippi Valley will center itself
over the local area tonight bringing light and variable winds. A
few stratocumulus clouds may hang on near the lake and namely gyy
during this evening. Otherwise only cirrus is expected tonight with
some light speckling of cumulus possible on Saturday. As the high
moves east on Saturday afternoon it will allow for southwest winds
to increase in speed with a few gusts to around 15 knots forecast.
//Ord and mdw confidence...updated 00z...
* high in all elements.
//Outlook for Ord/mdw for 12z Sunday-12z Friday...updated 00z...
Sunday...VFR. Gusty southwest winds.
Monday...VFR. Gusty west winds.
Tuesday...VFR. West winds.
Wednesday...VFR. Northwest winds.
Thursday...slight chance of showers. Northwest winds.
235 PM CDT
Conditions across Lake Michigan will start out the period relatively
quiet as high pressure builds across the region tonight and
Saturday. By Saturday night...the high will settle to the southeast
as a trough of low pressure develops over the plains and a deepening
low pressure system drops out of the Canadian rockies and into the
prairies. This low will continue to deepen as it tracks east across
Canada...dragging a cold front across the plains. Strong south to
southwest winds will strengthen in advance of the cold front...with
winds reaching close to gale force levels. The front is expected to
push across the lake Monday morning as the deepening low drops into
southern Ontario. The low will quickly lift northeastward Monday evening as winds
over the lake veer to westerly and then northwesterly by Monday night. A short
period of strong northwesterly winds will persist through Monday night...but
a ridge of high pressure is expected to quickly build in behind the
cold front...allowing winds to taper off. A quickly progressive
pattern is expected for the remainder of the week...with a series of
weak systems crossing the lake...though none is expected to be
Lm...Small Craft Advisory...nearshore waters
until 4 am Saturday.
Visit US at http://weather.Gov/Chicago (all lowercase)
Follow US on facebook...twitter...and youtube at: