Scientific Forecaster Discussion
Area forecast discussion
National Weather Service Chicago/Romeoville Illinois
1209 PM CDT Wednesday Jul 30 2014
315 am CDT
Synopsis...today will be very similar to yesterday with a lake
breeze pushing inland. Scattered showers and isolated thunderstorms
will form along the boundary. There are chances for afternoon
thunderstorms through Saturday with the better chances Friday and
Saturday. Additional thunderstorms are possible early next week.
Temperatures slowly rise through early next week with middle 80 high
temperatures possible Monday.
Today and tonight...
the upper level low is still parked over James Bay. A short wave
trough/cold front associated with the low is over central WI and it
will slide down the lake through this morning. The front loses steam
this afternoon and stalls over northern Illinois and in.
A lake breeze is also expected today so there should be adequate
forcing for showers and a few storms between the lake breeze and
stalled out cold front. Cape will once again be limited to below
500 j/kg so not expecting a ton of thunderstorm activity. Upper
level temperatures and surface temperatures will be a bit cooler today so thinking
that will also limit instability. Any storms that do form will not
be severe...but may be downdraft driven producing gusty winds
like yesterday. High temperatures will be in the upper 70s...except along
the Lake Shore where the lake breeze will keep high temperatures in the
Showers and storms dissipate this evening with light southwest winds
expected overnight. Skies should clear out nicely so expecting min
temperatures to approach the dewpoints. So looking at lows in the upper 50s
in the outlying areas...but in the low 60s downtown.
Thursday through Saturday...
The area remains on the southwest side of the eastern Canadian
upper long wave trough through the end of the week with little
change in the weather pattern. As is typically the case with such
an established long wave...minor perturbations will traverse
down the back side of the trough over the western Great
Lakes. Each of these look to bring subtle enhancement to low-
level convergence and keep cool air aloft for elongated cape
profiles near peak heating. All in all these continue to present
mainly afternoon and early evening scattered shower/storm chances.
There continues fair model agreement on bringing an Upper/Middle
level speed maximum and an associated wave southward for better
coverage of afternoon showers/storms. Both the sref and gefs
support higher probability of precipitation over a good amount of the area and while the
magnitudes of both may be extreme /70 percent plus/...the idea of
scattered coverage more so than today and Thursday seems likely.
The associated surface front hangs up over the eastern forecast
area. The lake breeze will continue to also be a focus for showers
and storms...so do have a little higher probability of precipitation in the northeast
forecast area. Very slight temperature moderation is expected
through this time.
Early next week...
high pressure looks to prevail Saturday night through the latter
half of the weekend. The upper level pattern looks to flatten
somewhat with a quick moving system north of the Great Lakes
advertised early next week by guidance. This looks to steer a cool
front into the region with large discrepancy on timing and just
how far south this reaches. Have not made any changes to a well-
verifying blend of guidance besides inching up temperatures a couple
degrees on Monday ahead of this cool frontal feature.
//Ord and mdw concerns...updated 18z...
* lake breeze expected with winds shifting east-NE around 10
knots...through mdw around 1930 UTC and Ord between 20 and 21 UTC.
* Widely scattered rain showers this afternoon and early evening. A few thunderstorms and rain also
A light wind field across the area is allowing a lake breeze to
make some good progress inland across northeastern Illinois early
this afternoon. Current timing looks to put the boundary through
kmdw around 1930 UTC and through kord in the 20 to 21 UTC time
frame. Winds do not appear to be overly strong behind the lake
breeze...with generally only around 10 knots of east-northeasterly
flow expected. The winds will gradually shift back out of the
west-southwest tonight into Thursday. Another lake breeze may
develop on Thursday. However...with a slightly stronger west-
southwesterly wind field expected on Thursday...the lake breeze
may struggle to reach the terminals.
The other concern this afternoon will...similar to yesterday...be
the threat for some widely scattered showers and perhaps a
thunderstorm across the area. Cumulus clouds will likely be on an
uptick over the next hour or so as temperatures continue to warm
through the middle and upper 70s. This cloud cover could even lead to a
period of lower end VFR ceiling around 4000 feet above ground level...especially ahead
of the lake breeze boundary. Given the lower areal coverage
expected...I will continue with the vcsh and leave the mention of
thunder out of the taf at this time.
Winds look to continue out of west-northwesterly direction into
Wednesday. However...another lake breeze may develop Wednesday
afternoon...and due to weaker westerly flow around 925 mb...the
boundary could threaten the eastern terminals with an easterly wind
shift later in the afternoon. Confidence is a bit low this far
out...so I have left this mention out of the 30 hour Ord taf.
//Ord and mdw confidence...updated 18z...
* medium-high with lake breeze timing.
* Medium-high in isolated to scattered -shra across the region...low
confidence in thunderstorms and rain coverage and development.
* High confidence in all other forecast elements.
//outlook for Ord/mdw for 00z Friday-12z Wednesday...updated 12z...
Thursday night...schc thunderstorms and rain early in the evening...schc rain showers late
in the evening.
Friday...chance rain showers with a chance of thunderstorms and rain in the afternoon.
Saturday...schc rain showers in the morning...chance of thunderstorms and rain in the afternoon.
Monday...chance thunderstorms and rain overnight.
Tuesday...chance thunderstorms and rain.
255 am CDT
A weak pressure gradient is over the lake with a weak cold
front/upper level wave moving down the lake early this morning.
Winds become north around 10 knots behind the front. High pressure
over the plains attempts to spread east today leading to a
continued very weak pressure gradient. Onshore flow is expected
across nearshore areas leading to variable winds 10 knots or less at
times over the open waters. Winds become more uniform this evening
with southwest winds across the lake by early Thursday morning. A
couple more weak cool fronts are expected by weeks end...at this
time Thursday night and again Friday night into early Saturday.
Each of these will turn southwest winds to the northwest/north but
speeds primarily remaining below 15 knots.
The Great Lakes region will likely experience afternoon and early
evening showers and some storms mainly driven by the heating of
the day over land. Some of these could be able to move into
nearshore areas on the western side of the lake as seen over WI/Illinois
shores last evening...and profiles over land do support some quick
racing outflows/gust fronts from showers the next few days...so
something for nearshore mariners to keep some tabs on despite the
mainly quiet wind/wave regime.
Visit US at http://weather.Gov/Chicago (all lowercase)
Follow US on facebook...twitter...and youtube at: