Scientific Forecaster Discussion
Area forecast discussion
National Weather Service Chicago/Romeoville Illinois
358 am CDT Thursday Jul 31 2014
257 am CDT
Synopsis...rounds of afternoon showers and thunderstorms will
continue through the end of the week. Minimal chances of thunder
today...but higher chances look to arrive tomorrow and Saturday.
Temperatures remain around 80 through the weekend except for along
the lake where lake breezes will keep highs in the 70s tomorrow
through the weekend. A brief warm up into the middle 80s is expected
early next week.
Today and tonight...
prepare for a very familiar song and dance...broad high pressure is
over the region with light flow and patchy fog. The fog will burn
off quickly this morning as mixing gets going. Looking at a breezy
day with west winds at 10-15 knots. A lake breeze will form...but
expecting it to stay just off shore as wind speeds are at 10+ knots
through the column.
An upper level vorticity streamer approaches from the northwest today.
Therefore...weak forcing will be over northern Illinois and far northern
in...so have another round of isolated to scattered showers in the
forecast. Based on yesterdays limited activity...thinking we will
see few if any thunderstorms. The dvn sounding featured a cap
around 650mb yesterday and model guidance has a similar cap in
store for US today. The best chance for showers will be north of
I-80. Not expecting much precipitation from any showers that do form.
High temperatures will be around 80 and overnight lows will be around
60...except for the middle 60s downtown. May see fog again tomorrow
night...but not overly impressed so left fog out of the forecast for
the upper pattern remains the same for one more day into Friday with
the forecast area remaining on the southwest side of the eastern
Canadian long wave trough. One or two short wave impulses are
projected to move around the trough in the northwest flow aloft
during the day and evening on Friday...keeping 500mb and 700mb
temperatures at one to two standardized anomalies below normal.
Combined with a little more low-level heating on Friday...the
slight warm temperature nose/cap near 650mb yesterday and today is
expected to be overcome by this and thus forecast soundings
provide 800 to 1300 j/kg of MLCAPE over southern WI/northern Illinois
despite forecast dew points only in the upper 50s. Overall
coverage of storms is expected to be higher in the region than the
past couple days because of this...maybe similar to Tuesday. A
few stronger updrafts and associated gusty downdrafts are possible
in the region during middle- afternoon through mid-eve.
high pressure...albeit weak...is forecast to build southeastward
across the region early in the weekend in association with height
rises aloft. A developing weak cool front ahead of the high looks
to be a modest focus for scattered afternoon showers/storms in the
lower Great Lakes region. This could be primarily east of the
local area based on some guidance...but given the model spread and
the subtle features that could spark some showers...felt prudent
to keep chance probability of precipitation during the day. Northeast flow is anticipated
to develop and depending on timing could provide a stronger lake
breeze with afternoon cooling in lake adjacent counties. The high
looks to make for a pleasant weather Saturday night through the
end of the weekend.
Monday through Wednesday...
the upper flow will be more dampened and thus a little more
progressive. Long range ensembles and forecast models have modest
agreement on a jet maxima traversing the northern U.S. Sometime
midweek with an associated wave in the middle-levels and a closed
surface low developing in the plains and moving eastward. Given
the distance though see little opportunity to improve much from a
well-verifying blend of models providing chance probability of precipitation area-wide in
that time period. With a closed surface low of 1005 or stronger on
both the 31.00 GFS and ec...cant rule out stronger storms in the
region but far too early to get anywhere near specific.
//Ord and mdw concerns...updated 09z...
* isolate -shra with a minimal chance of thunderstorms and rain.
High pressure is over the region with many sites indicating calm
winds and the early stages of fog development. Expecting fog to be
the worst at gyy with IFR fog possible. Dpa will also see MVFR fog.
Fog will quickly burn off this morning with ample mixing expected.
West winds will be 10-15 knots with occasional gusts possible. The strong flow
will keep the lake breeze along the shoreline. Isolate rain showers are
expected to form along the lake breeze and northern Illinois. Confidence
is medium in location and coverage so backed off on precipitation by
replacing the vcsh with a prob30. Stability will limit thunderstorms and rain
development as well. Showers dissipate this evening and light SW
winds are expected overnight.
//Ord and mdw confidence...updated 09z...
* medium confidence in rain showers coverage and location.
* Medium-high confidence in a very very low chance of thunderstorms and rain
* High confidence in all other forecast elements.
//outlook for Ord/mdw for 12z Friday-12z Wednesday...updated 00z...
* Friday and Saturday...afternoon isolated shra/tsra. Afternoon
* Sunday and Monday...dry/VFR.
* Tuesday...chance of thunderstorms and rain.
257 am CDT
Broad high pressure is over the region with a weak low over James
Bay. Southwest 10-15kt winds will become south this afternoon. May
see patchy fog form over the lake again...but with showers...a few
storms...and decent flow...decided to keep patchy fog out of the
grids for now. If fog does form...it would be in the afternoon
through the evening. A weak cold front will move down the north half
of the lake tonight with winds becoming variable behind it. The
south half will continue to see southwest winds. The front will
dissipate over the lake early Friday.
A continued light pressure gradient over the lake through Monday
will mean speeds less than 15 knots and even being variable at
times...with afternoon lake breezes in nearshore areas. Friday looks
to be a day similar to this past Tuesday...with scattered storms of
over central WI to northern Illinois...some of which could be gusty and
traverse into the western nearshore areas where stability is not
as high. So while generally tranquil conditions the next several
days...that is something for mariners in the nearshore to be aware
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