Scientific Forecaster Discussion
Area forecast discussion
National Weather Service Chicago/Romeoville Illinois
317 PM CDT Wednesday Jul 23 2014
315 PM CDT
Summer struggles to maintain a grip on the region. Cool weather
tomorrow and Friday will only briefly yield to typical temperatures
and increased thunderstorms and rain chances for the weekend before moving right back
into the area to start next week.
Behind the passage of a cold front last night...cool and dry
northeasterly flow today has been keeping highs about 10 to 15
degrees below average for this time of the year. This flow ahead of
a surface ridge centered west of the area today will gradually
transition to southerly warm and moist flow by late Friday as the
ridge slides overhead tomorrow and then moves off to the east.
Shower and thunderstorm chances increase by late Friday and remain
elevated through the weekend as the warm and moist airmass settles
into place. By Sunday night the cooler and drier air returns with
the passage of yet another cold front. So the first half of next
week is setting up much like the last half of this one. The main
questions for the next several days involve the timing and extent of
rain showers/thunderstorms and rain through the weekend.
Near term...tonight through Saturday
Tonight and Thursday will remain dry and cool underneath a broad
area of high pressure. Thursday night an upper shortwave and
pronounced moisture flux supported by the low level jet across Iowa
increase thunderstorms and rain chances in our western counties early Friday. Often
this Summer the surface ridge has been too dominant locally and the
low level moisture over the area has been too limited to maintain
thunderstorms and rain activity as it moves out of Iowa into northwest Illinois.
Given these trends...will elect not to increase probability of precipitation above the
chance category Thursday night and Friday...with the highest values
focused west of I-39 during the Friday morning period.
By Friday night the Gulf moisture has returned and the surface high
has moved well off to the east. Models depict an upper level
shortwave embedded in fairly strong zonal flow across the northern
Continental U.S....with its associated surface low moving across Iowa into the
local area along with a frontal zone. While the diurnal timing may
not be great...the combination of low level convergence...moderate
middle level lapse rates...and a shortwave aloft all would suggest
increased thunderstorms and rain chances again Friday night into early Saturday.
Models show the front pushing through Saturday morning and stalling
out south of the area for the remainder of the weekend. This would
keep the warmer and more humid air over the area but focus the
better thunderstorms and rain chances to our southern counties toward central Illinois.
Extended period...Sunday into Wednesday
The zonal flow which develops briefly on Saturday begins to amplify
again on Sunday. At 500 mb...the GFS and Euro both point to a
vorticity maximum moving from south-central Canada into the Great
Lakes region by Sunday morning...deepening an upper trough over the
eastern Continental U.S.. at the surface...southerly to southwesterly flow
Saturday into early Sunday helps maintain moderate dewpoints in
the middle to upper 60s during the day on Sunday. North of the
frontal boundary...with winds from the west or northwest during
the day on Sunday...warm air advection is limited. The upper level disturbance
combined with moderate moisture and warmth leads to a continued
chance of storms on Sunday in the middle morning to afternoon
timeframe...however...due to a lack of clear forcing due to the
position of the front...not expecting coverage to be widespread.
Meanwhile...Saturday night into Sunday morning...a pronounced ridge
begins building over the western US. As mentioned above...the
downstream response to this ridge is a trough deepening over the
eastern half of the Continental U.S. Sunday night into Monday. This amplified
trough over the eastern half of the US remains in place through the
end of the forecast period. Thus...after the front south of our area
finally pushes out of the region Sunday night...below average
temperatures are likely for the first half of next week.
//Ord and mdw concerns...updated 20z...
* gusty north-northeast/NE winds through this evening.
* Lake breeze with east-northeasterly winds around 10kt tomorrow afternoon.
A cold front has pushed across the region with brisk northerly to northeasterly
winds covering the region. The strongest winds will be near the
Lakefront with gusts approaching 30kt through into the early
evening. Winds will taper off farther inland...but Ord/mdw/dpa
should still see at least occasional northeasterly gusts to 25kt through
sunset. With high pressure building across the region tonight and
tomorrow...winds will diminish to 5kt or less overnight. A very
weak pressure gradient in place across the region tomorrow should
allow lake breeze development...with winds becoming east-northeasterly to around
10kt by early afternoon. Otherwise...no sig weather expected through
//Ord and mdw confidence...updated 20z...
* high in wind direction and speed...medium in timing of gusts
diminishing this evening.
* Medium confidence in timing/speed/direction of lake breeze
* High confidence in all other forecast elements.
//outlook for Ord/mdw for 00z Friday-12z Wednesday...updated 12z...
Friday through Sunday...periodic chances for thunderstorms.
Monday through Tuesday...slight chances for showers.
220 PM CDT
Winds remain in the 20 to 25 knots across much of Lake Michigan except
in the far north behind a strong cold front which is now well south
of the Great Lakes. Significant wave heights peaked earlier today
with 10 feet observed near Michigan City...and we are still seeing 6
to 8 feet waves along the southern half of the lake where rip current
activity has been reported along the Indiana shoreline. A Small
Craft Advisory remains in effect for the Indiana and Illinois
nearshore waters due to the elevated winds and waves.
High pressure is nosing into the upper Midwest...and it will
continue sliding south and east across Lake Michigan tonight allowing
winds to subside overnight into Thursday. Winds will generally
remain northerly over the southern half of the lake with onshore
winds on the Indiana and Illinois shores while winds begin to turn
southerly along the north half of the lake. The high will depart to
the east as low pressure moves out of the northern rockies into the
plains on Friday allowing winds to have a southerly component across
The next organized cold front will pass through on Sunday. The
surface low associated this front will deepen in the eastern lakes
on Monday while high pressure again builds south into the upper
Midwest. This will create a period of gusty northwest to north winds
which reach 25 to 30 knots.
Illinois...beach hazards statement...ilz006-ilz014 until 10 am Thursday.
In...beach hazards statement...inz001-inz002 until 10 am Thursday.
Lm...Small Craft Advisory...nearshore waters
until 10 am Thursday.
Visit US at http://weather.Gov/Chicago (all lowercase)
Follow US on facebook...twitter...and youtube at: