Scientific Forecaster Discussion

Return to Local Conditions & Forecast

Without Abbreviations
With Abbreviations

Area forecast discussion 
National Weather Service Chicago/Romeoville Illinois
852 PM CDT Thursday Jul 31 2014

845 PM CDT

As the sun has set...clouds have thinned. Temperatures have been
slow to fall this evening...however expect a general trend to
around 60 by daybreak. The exception will be locations adjacent to
Lake Michigan...where temperatures will likely hold in the upper 60s to
perhaps around 70.

Precipitation this evening has been very minimal. Infrared/water vapor imagery
indicates a wave over Iowa/southern Minnesota sliding southeast this
evening. Local arw8km indicates this wave will arrive
overnight...and could help to touch off an isolated shower. At
this time it does not appear any convection will develop over the
forecast area...however a few lightning strikes have been noted
well upstream of the County warning forecast area.



Previous discussion...
218 PM CDT

Very little change to the going forecast for the next few days as
the pattern...both observed and forecast remains consistent. The
biggest changes to the going forecast will be in the longer term
which looks like it could be a little wetter.

For the remainder of the afternoon and evening...there will be a
continued small chance for diurnal isolated thunderstorms and rain and scattered rain showers as the
region remains under cyclonic flow aloft...under the southwestern periphery
of the deep upper low sitting over James Bay. The main difference
for today over the past couple days is that low level winds are a
bit stronger and more westerly which has kept any lake breeze
development at Bay. So...the warmer air will be able to penetrate
up to the Lakefront. As could cover this afternoon is predominantly
diurnally induced...expect that skies should become mostly clear
overnight...with low temperatures around 60f expected for much of the
area...except the heavily urbanized areas of the Chicago metropolitan
area...which should remain in the middle to upper 60s.

The remainder of the short term forecast period will also see little
change from the going forecast as the latest model runs remain
consistent with previous runs and with each other. The very slowly
progressive high amplitude pattern will persist into the weekend
with a series of weak shortwaves dropping through the northwesterly flow
aloft. This will keep the chances for isolated/scattered showers over the
area...with the chance for isolated afternoon thunderstorm. Given the
slight timing/intensity differences between the models with these
weak shortwaves...and the inherent uncertainty in the specific
details of the sensible weather particular precipitation...will
keep probability of precipitation generally low and follow diurnal trends. Still anticipate
a slight warming trend into the weekend as upper ridging builds out
over the Western Plains...bringing rising heights to the
Midwest...but the models continue to advertise broad troughing
lingering into the lower Mississippi Valley as the seemingly endless
series of weak short waves dropping out of south central Canada
continues to maintain a long wave trough over the eastern Continental U.S.. at the
surface...expect a return to a weaker surface gradient as weak high pressure
builds over the region. So...after only one day of warmer air
reaching the Lakefront...anticipate a return to daily lake breeze
development Friday through Sunday...maximum inland temperatures topping out
around 80f and Lakefront temperatures remaining several degrees lower.

For the longer term forecast period...from Monday through
Thursday...the the longer range models remain consistent on lifting
out the deep upper low from the James Bay area. It still looks like
the only real opportunity for temperatures to hit seasonal normal
levels...highs in the middle 80s...will be Monday as the axis of the
upper ridge extends into the upper Mississippi Valley and surface low
development over the northern plains sets up a short period of modest
southerly-southwesterly flow at the surface. By Tuesday...short wave
energy will reinforce the broad upper trough over the eastern Continental U.S..
the European model (ecmwf) wants to generate a closed low over the lower Mississippi
Valley by Tuesday morning...which then cuts off from the westerlies
and slowly retrogrades into the Texarkana area. The GFS starts out
early Tuesday with a similar upper trough...but then becomes much
more progressive with the upper trough. The net effect on the
pattern is that the European model (ecmwf) is trending toward maintaining a more
highly amplified pattern while the GFS becomes more zonal aloft over
the upper Midwest and allows for higher chances for precipitation through the
extended forecast period from a series of shortwaves traversing the
region. At this point...the idea of a cut-off low developing over
the lower Mississippi Valley looks a bit have
trended the longer term forecast closer to the GFS...which would
imply better chances for precipitation over the local area than the
European model (ecmwf)...which favors the upper Mississippi Valley through the
eastern Great Lakes zone for higher precipitation chances.




//Ord and mdw concerns...updated 00z...

* chance of rain showers/thunderstorms and rain Friday.
* Lake breeze middle/late Friday afternoon. Cms

//discussion...updated 00z...

Isolated showers have been confined to areas generally south of
I-80 and then thunderstorms and rain east into northern Indiana. Cumulus field over
northern Illinois has been gradually dissipating and not expecting any
showers this evening at the terminals. A bit stronger upper wave
will move across the area from late Friday morning through
middle/late Friday afternoon but expected coverage will probably
still only be isolated to scattered and maintained current

Westerly winds will slowly diminish through sunset and then
continue to diminish to 5kts or less overnight...with some areas
likely becoming light and variable or calm. The gradient will be
rather weak Friday thus southwesterly winds should be under 10kts.
Though wind directions could be problematic if showers and
thunderstorms do form and produce outflow boundaries near the
terminals. In addition...a lake breeze is also expected to form
and should move through the terminals during the middle/late

Light winds and mainly clear skies will allow temperatures to cool to
the dewpoints overnight and some light/shallow fog is expected
again in the usual spots and included mention at dpa/gyy. Cms

//Ord and mdw confidence...updated 00z...

* low/medium for rain showers/thunderstorms and rain Friday.
* Medium for lake breeze middle/late Friday afternoon. Cms

//outlook for Ord/mdw for 12z Saturday-12z Thursday...updated 00z...

* Saturday...chance of shra/tsra. Afternoon lake breeze.
* Sunday...dry/VFR.
* Monday through Wednesday...chance of shra/tsra.


211 PM CDT

High pressure in the plains with low pressure in eastern Ontario is
keeping the lake breeze at Bay today with westerly winds in the
nearshore waters and south winds on the Open Lake. There are even
some patches of fog with visibilities reported as low as 2
nautical miles. The low in Ontario will continue shifting north
as high pressure expands over the Great Lakes the next few days.
This will slacken the gradient over Lake Michigan and keep
variable winds under 10-15 knots...while lake breezes return Friday.
With the weak gradient also expect fog at times. A weak cold
front will push in Saturday as the high builds shifting winds back
to northerly for a time. Expect lake breezes into early next week
with a return to southerly flow in the open waters as stronger low
pressure organizes in the Central Plains. The low will track
eastward through the Great Lakes region middle to late week...but
still some uncertainty as to whether it will pass over the lake or
dip to the south. Kmd


Lot watches/warnings/advisories...


Visit US at http://weather.Gov/Chicago (all lowercase)

Follow US on facebook...twitter...and youtube at:

National Weather Service Glossary of Abbreviations