Scientific Forecaster Discussion
Area forecast discussion
National Weather Service Chicago/Romeoville Illinois
632 am CDT Wednesday Aug 27 2014
321 am CDT
Convective trends remain the main focus of the forecast through the
weekend. Water vapor imagery shows a split upper flow with a
northern stream of progressive flow running across southern Canada
into the upper Great Lakes. A low amplitude trough axis is
traversing the flow and moving into Wisconsin at the current time.
The southern stream features a cutoff upper low which is over Utah
and slowly moving eastward. Upper ridging is starting to build just
downstream of the upper low with several embedded waves moving
northeastward ahead of the low. At the surface...the frontal
boundary which has been discussed over the past several days is now
south of the County Warning Area and runs east-northeast across the lower Great
Lakes and St. Lawrence River Valley. High pressure has spread across
the upper Midwest with its southern flank into the forecast area.
Today...the northern stream upper trough axis will
cross the local area this morning. The greatest ascent associated
with it is to the north and is generating bands of showers across
Wisconsin...with additional returns working across central Iowa into
northern Illinois. This activity seems to be in better conjunction
with the right entrance region of an upper jet maximum to the north in
addition to a broad area of height falls induced by the upper
trough. A cluster of thunderstorms has also developed across SW Iowa
in response to better focused middle level warm advection. Most of this
upper support will be passing by this morning resulting in a
decrease in activity. With the surface front to the south the
instability gradient is also to the south with a westerly low level
jet above it suggesting that the convection across southwest Iowa
would not be a factor too far north of its current latitude. Once
the current shower activity exits it appears that most areas will
remain dry for the rest of the day but far southern areas do have a
chance to see some showers and storms this morning given the
westerly low level jet along the boundary and middle level moisture
gradient but warm/moist advection is pretty weak. High temperatures
will range from the upper 70s north to middle 80s south with a
northeast wind holding Lakefront areas to the lower to middle 70s.
Tonight and Thursday...as the upper trough shifts east the ridging
downstream of the cutoff low to the west will pivot across the
Central Plains. Surface low pressure will be developing across
eastern Colorado and western Kansas which will drive the frontal
boundary northeastward across the Central Plains with the nearby
portion of the front remaining south of the area into Thursday
evening. With warm advection increasing aloft with the ridge
building back in tonight showers and thunderstorms are expected to
be moving east-northeast across Iowa/Missouri. Some of this activity
may reach the western County Warning Area very late tonight and may end up seeing a
similar situation to recent days with remnant convection festering
and/or spawning new development across the County Warning Area Thursday
morning/afternoon. The main surface low and upper wave is expected
to be moving across Iowa/Minnesota/Wisconsin so the bulk of the
convection may stay to the northwest of the local area. Will focus
highest probability of precipitation across the western County Warning Area and slow down the arrival until
later tonight. Thursday looks to be slightly warmer...especially
with convection looking to be more scattered in coverage.
Friday through early next week...the upper trough will start to make
quicker east-northeast progress thanks to another large scale upper
trough pushing into the western U.S./Canada. This will take another
surface low northeastward with it which will likely track northwest
of the area Saturday. Friday should see a noticeable warm up with
another lull in widespread convection with it focusing to the north
and west. Will have higher probability of precipitation Saturday with the low passing but am
seeing mixed signals in the track which will affect precipitation coverage.
Chances will diminish Saturday night with question Marks surrounding
Sunday into early next week with the GFS showing a more active flow
while the European model (ecmwf) is quieter though it does bring a front through the
area Tuesday. Do not see much of a cool down overall and a dry
forecast Sunday/Monday would coincide with a warming trend if it
comes to pass. Will continue with middle to upper 80s Friday with
readings several degrees cooler Saturday and Sunday.
//Ord and mdw concerns...updated 12z...
* northeast winds through tonight...possibly around 10kt this
* East winds Thursday morning. Cms
High pressure will continue moving across the upper Midwest and
northern lakes today and tonight. This will maintain northeast
winds today turning more easterly overnight in Thursday morning.
The gradient may tighten just a bit this afternoon allowing speeds
in the 10kt range...perhaps a bit higher in the middle/late afternoon.
Lower VFR clouds...3-5kft...have been scattering out over the past
few hours. Will continue to see middle clouds this morning with high
clouds expected this afternoon and tonight. More middle clouds
possible overnight into Thursday morning. Cms
//Ord and mdw confidence...updated 12z...
* medium for winds through the period. Cms
//outlook for Ord/mdw for 00z Friday-12z Wednesday...updated 12z...
* Thursday night and Friday...chance of thunderstorms and rain.
* Saturday...thunderstorms and rain likely.
* Sunday through Tuesday...chance of thunderstorms and rain.
307 am...high pressure centered over Minnesota early this morning
will move across the lakes region today and tonight. As it does
the gradient will slowly weaken over Northern Lake Michigan while
it slowly tightens over the southern portion of the lake. Speeds
will increase into the 10-20kt range. As the high shifts east...
winds will turn more easterly by Thursday morning then shift
southeast to southerly Thursday night as a warm front lifts north
across the lake. The gradient could tighten enough for a period of
15-25 kts Thursday night into Friday. A frontal boundary is then
expected to move south across the lake Saturday afternoon into
Saturday evening. But at the same time...low pressure is expected
to develop over the western lakes and then move across Northern
Lake Michigan by Sunday morning. This makes for a challenging wind
forecast. If the low begins to deepen...northerly winds may
increase into the 15-25kt range. Changes can be expected during
this time period as trends emerge. Winds then quickly turn back
southerly Sunday night into Monday. Cms
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