Scientific Forecaster Discussion
Area forecast discussion
National Weather Service Chicago/Romeoville Illinois
631 PM CDT Wednesday Jul 30 2014
254 PM CDT
A high amplitude longwave pattern that has been in place for the
past few days will persist for the next few days with deep long wave
troughing over the eastern Continental U.S. And an upper low center over James
Bay...with ridging over the western Continental U.S.. the local area will remain
under generally northwesterly flow aloft under the southwestern periphery of the
upper low. This will allow for a series of weak short waves to
ripple through the northwesterly flow aloft over the local area.
For the remainder of this afternoon and this evening...the main
concern will be the lake breeze boundary quickly pushing inland
through northestern Illinois and northwestern in. Temperatures will drop off several
degrees with the passage of the boundary and Lakefront locations
have already dropped into the middle 60s. The environment ahead of
the boundary is modestly moist and unstable. With cyclonic flow
aloft...diurnal cumulus has been developing over the region per the latest
satellite imagery. As of early this afternoon...have not seen any
rain showers/thunderstorms and rain activity over northern Illinois/northwestern in...but the region remains
modestly unstable with dewpoints in the middle to upper 50s. There
will still be a chance for some rain showers/thunderstorms and rain focused along the lake
breeze boundary as well as some isolated rain showers or an even more isolated ts
over the area through sunset. During the evening and into the
overnight hours...any diurnal precipitation will dissipate and and residual
precipitation will be well north of the area in vicinity of on of the seemingly
endless series of impulses dropping through the northwest flow.
Little change is expected for tomorrow through Saturday as the
synoptic scale pattern should change little and a weak gradient
pattern will persist at the surface. Under persistent northwesterly flow
aloft...the series of weak shortwaves will continue to bring
periodic chances for scattered rain showers and isolated thunderstorms and rain...with the better
chances for ts during the afternoon hours. The weak gradient pattern
at the surface should allow for daily lake breeze development...with the
main question being how far inland a lake breeze will penetrate.
Certainly...it is likely that Lakefront locations will turn cooler
in the afternoon...but it is less certain how far inland the lake
cooling will penetrate. So...will keep the maximum temperature forecast a bit
on the conservative side as far as lake influence is concerned and
limit the cooling to the immediate Lakefront and allow maximum temperatures to
be more representative of the large scale pattern. Under the cool
northwesterly flow aloft...highs will remain a few degrees below normal into
the weekend with highs around 80f or so. Timing of periods of precipitation
will be less certain than the temperature trends for the next few
days and will generally be dependent on the timing of shortwave
energy dropping through the northwesterly flow aloft. So...while no day in
the short term forecast period will be a total washout...there
should be periods of scattered rain showers and isolated thunderstorms and rain.
The longer range guidance is in generally good agreement on the
upper level pattern becoming more progressive through the weekend.
This will initiate a warming trend as the upper level ridging moves
out over the Western Plains on Sunday while the upper low over James
Bay kicks out to the northeast...leaving weaker longwave troughing
over the eastern Continental U.S.. Sunday should be a transitional day...with
inland temperatures a little higher...but a weak gradient persisting at the
surface will allow for another lake breeze day and cooler conditions
near the lake. Rising heights over the Midwest should allow for maximum
temperatures to climb into the middle 80s by Monday.
More uncertainty will creep into the forecast during the extended
forecast period as flow becomes more zonal over the Midwest. In the
increasingly zonal flow...short wave strength and timing will become
more uncertain...so...confidence in the sensible weather forecast
will increase with time through the extended forecast period.
However...the latest long range guidance suggests that the warming
trend could be brief as a weak upper low hangs back over the lower
Mississippi Valley through the period and the upper ridging will not
build any farther east than the Central Plains. Also...the surface
pattern should remain weak...so daily lake breeze development will
be likely as well. The trend for the extedned forecast period is
shapping up for more below normal temperatures with periodic chances
for at least some light precipitation.
//Ord and mdw concerns...updated 00z...
* isolated rain showers/thunderstorms and rain Thursday afternoon. Cms
A lake breeze has moved well west of Ord/mdw late this afternoon
and will continue west until it dissipates this evening. The
gradient will be quite weak tonight and winds will eventually
become light and variable and likely calm in many locations. Winds
will become light west/southwest after sunrise and remain westerly
Thursday with speeds increasing to 10-12kts with a few higher
gusts possible in the afternoon. Wind speeds appear strong enough
to prevent a lake breeze from reaching Ord/mdw...but a lake breeze
may still form near the lake.
Cumulus field is quickly dissipating this evening and not expecting any
additional showers. Showers and thunderstorms over northern WI
will move south this evening but are expected to dissipate before
reaching northern Illinois with just few/scattered middle clouds overnight.
Isolated showers/thunderstorms are possible Thursday afternoon
into early evening.
With the light winds...temperatures will likely cool toward the dewpoints
overnight and toward morning with some light/shallow ground fog
possible in the usual spots and added fog mention to dpa. Cms
//Ord and mdw confidence...updated 00z...
* low for isolated rain showers/thunderstorms and rain Thursday afternoon. Cms
//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.
200 PM CDT
A weak pressure gradient is over the lake this afternoon...though a
weak surface low/trough has drifted to the southern tip of the lake.
This generally resulted in a shift to northerly winds down the lake.
Meanwhile an upper level low continues to spin near James Bay with
with surface lows/troughs emanating from this feature that progress
south and east at times while the stronger parent surface low
remains closer to James Bay. Meanwhile high pressure remains in
control of the central and northern plains...resulting in southerly
winds across the northern half of the lake this afternoon.
After onshore flow in the nearshore waters this evening due to the
passing surface trough...expect winds to become more uniform this
evening with southwest winds across the lake by early Thursday
morning. A couple more weak troughs/cold fronts will pass through
the lake at times through weeks end...with one Thursday night and
with a bit more pronounced front Friday night into early Saturday.
Each of these will turn southwest winds to the northwest/north but
speeds primarily remaining below 15 knots as the pressure gradient
remains weak for the near future.
The Great Lakes region will likely experience afternoon and early
evening showers with occasional thunderstorms at times. While these
are not expected to be widespread at any given time...any storms
that form could create gusty outflow winds that would create
hazardous conditions to boaters...so caution is urged during the
late afternoon and evening periods the next few days.
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