Scientific Forecaster Discussion
Area forecast discussion
National Weather Service Chicago/Romeoville Illinois
248 PM CDT Thursday Jul 31 2014
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 features...in 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 suspicious...so 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 20z...
* chance for some isolate -shra late this afternoon...with a minimal
chance of thunderstorms and rain.
* Slightly better shot at some showers and thunderstorms on
* Lake breeze possible Friday afternoon with a east-northeasterly
wind shift by middle to late afternoon.
The main concern this afternoon will be if any showers or storms
develop across the area. The atmosphere remains nearly unchanged
from the past couple days. Therefore...cumulus clouds will continue
to increase over the next hour or so across the area...possibly
resulting in a VFR ceiling by early afternoon. Some isolated to widely
scattered showers and perhaps a thunderstorm could then develop
across portions of northern Illinois/northwestern Indiana by middle
afternoon...with the threat continuing through late afternoon/early
evening. However...due to the low areal coverage...and hence low
confidence in a given terminal being hit by a shower/storm...we will
continue remain optimistic in the 18 UTC taf.
The other weather concern during the taf period will be on Friday.
It appears that there will be a somewhat better chance of some
scattered showers and storms during the day as a more defined
weather disturbance moves across the area. Confidence on timing this
far out is a bit low at this time...but a late morning through middle
to late afternoon time frame would certainly appear to be capture
this threat. At this time...I have expanded the probability 30 group for
thunder at Ord to go through 21 UTC Friday.
A very light wind field is expected during the day Friday...and this
will likely promote a lake breeze to develop and move inland across
northeastern Illinois and northwestern Indiana. Therefore...it is
possible that an east-northeasterly wind will set up by late
afternoon Friday across the eastern terminals. This lake breeze
could also be another focus for showers and storms during the
//Ord and mdw confidence...updated 20z...
* low-medium in rain showers coverage and location...low confidence in
showers or thunder impacting the terminals this afternoon.
* Low-medium with rain showers and thunderstorms and rain chances Friday.
* Medium with lake breeze Friday afternoon.
* High confidence in all other forecast elements.
//outlook for Ord/mdw for 00z Saturday-12z Thursday...updated 12z...
Friday night...chance rain showers.
Saturday...chance rain showers then chance of afternoon thunderstorms and rain.
Monday...schc thunderstorms and rain during the day and chance rain showers overnight.
Tuesday and Wednesday...chance thunderstorms and rain.
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
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