Scientific Forecaster Discussion
Area forecast discussion
National Weather Service Chicago/Romeoville Illinois
708 PM CDT Thursday Sep 18 2014
203 PM CDT
Forecast concerns on our warm up for Friday and Saturday...followed
by thunderstorm chances for Saturday and Saturday evening.
Surface high pressure is in control to our north and is gradually
shifting to the east this afternoon. Cold front has stalled out over
the area this afternoon and onshore flow is in place. Stratus has been
eroding from the south as lower level flow begins to turn more
southerly...and we are left with some lingering stratocumulus
clouds that look to dissipate after sunset. Expect another cool
night under clear skies.
Friday will be a transition day as the high drifts to the east.
Middle/upper level heights increase with zonal flow with any shortwave
energy passing by to the north. Low level southerly flow will boost
temperatures well into the 70s with an increase in high clouds from the
west and north later in the day...though it should be mostly sunny.
Expect an increase in southerly winds through the day. Friday night
temperatures will be up some 5-10 degrees with warm advection still
in place and cloud cover drifting overhead.
Our next feature of interest is an upper trough axis along the
Pacific coast. The closed off portion of this low will continue to
sink south and cut off from the main middle level westerly flow.
Meanwhile...a strong upper level jet off the British Columbia coast
will continue to push inland across the Canadian rockies Friday. Lee
surface troughing will develop east of The Rockies. As the upper jet
nears the area Saturday...a sharp neutrally tilted upper trough will
develop over the northern plains with the main surface low expected
to pass along Lake Superior on Saturday closer to the main upper
On Saturday morning...there may be some lingering warm advection
induced showers and storms out ahead of the upper trough...but the
low level jet Friday night into Saturday morning is largely focused
west and north of our area before veering over our area...but
guidance still suggests some of this activity may sneak into our
northern zones then drift south before dissipating through the
morning. Saturday looks to be warmer than normal...even in spite of
lingering morning cloud cover. Am reluctant to cut temperatures given that
that low level air mass is certainly supportive of low 80s...but
will need to watch cloud cover as there is some bust potential here. The
better chance for showers and thunderstorms comes as cold front
extending southwest from the surface low approaches in the
afternoon/evening time frame. Expecting a pretty quick moving line
of storms along the front as it moves through. Bulk shear values get
to 40-45 knots...but questions around available instability and
marginal middle level lapse rates at this time. Still...dewpoints
should be in the middle to upper 60s producing mixed layer cape in the
700-1200 j/kg range. This suggests that there may be some severe
wind gusts or hail with any stronger storms. Precipitable water values are 2 S.D.
Above normal...thus some heavier downpours are possible but training
is not anticipated. With the main surface low and upper dynamics
passing by to the north...still some concerns on coverage of
storms...but there is a pretty strong signal from models and
ensembles with a period of storms Saturday afternoon into Saturday
evening...some which may be strong to severe.
The front is quickly through Saturday night into early Sunday.The
core of the upper trough passes by to our north during this time...thus
any Post-frontal showers look to remain out of the area. Surface
high pressure gradually builds in behind the front bringing a return
to generally dry and cooler conditions. The upper flow looks highly
amplified as the ridge in the west continues to build...and a
another stronger ridge remains in place over the Atlantic. This will
keep the upper trough axis in place over the eastern third of the
country. The result for the early half of the week is afternoon
cumulus in the northwest flow on the back side of the upper
trough. By middle week the ridge in the west builds overhead for dry
and seasonably warm conditions. Kmd
//Ord and mdw concerns...updated 00z...
* northeast winds diminishing this evening...becoming southeast
tonight and then more southerly on Friday.
* Fog overnight.
VFR conditions prevail while a steady northeast wind persists
early this evening. Speeds will diminish over the next 1-2 hours
before shifting more southeast tonight and then south on
Friday...with gusts likely by midday. Overall quiet forecast
period outside of fog development tonight...and with a low chance
for redeveloping MVFR/IFR ceilings...mainly for dpa and rfd. Low
levels across these areas remain fairly moist and am concerned
that ceilings could develop once again with cooling later tonight.
The bulk of any low clouds will likely stay to the north but with
rfd possibly observing some of these lower clouds.
//Ord and mdw confidence...updated 00z...
* high confidence with wind speed/direction.
* Medium high confidence with fog overnight...low medium
confidence with exact timing and lowest visible.
//outlook for Ord/mdw for 12z Saturday-12z Thursday...updated 00z...
148 PM CDT
The cold front has pushed south of the lake with high pressure
extending along the West Shore of the lake and over Wisconsin and
into northern Illinois...while an inverted trough has developed over the east
side of the lake and western lower Michigan. This has set up a
mesolow over the south end of the lake with a line of convergence
cloud over the southern portion of the lake. There could be some isolated
rain showers along the boundary...but otherwise...conditions should remain
quiet through the evening and overnight. As high pressure pushes
off to the east and a trough of low pressure and an associated
cold front develop over the Central Plains...winds will become
southerly by Friday morning and strengthen as the parent low of
the cold front deepens over Hudson Bay. Since the southerly winds
will bring much warmer air across Lake Michigan...there remains a
question of how soon the airmass will modify over the lake to the
point where the stronger winds aloft will be able to mix down to
the surface. The forecast models are advertising winds in excess
of 45kt at around 1500 feet above the surface...but the marine layer
should prohibit the strongest winds from mixing all the way down
to the surface...but at this point...feel that gale force winds
are possible for the northern 2/3 of the lake...while the surface
layer of the southern portions of the lake should remain too
stable for gale force winds to mix down to the surface. So...will
hoist a gale watch for the period from Friday afternoon through
late Saturday morning as the period most likely to see gales over
the lake. While gales are currently not anticipated for the
southern 1/3 of the lake...winds will likely reach Small Craft
Advisory levels. With southerly flow...waves should remain under
criteria...but a Small Craft Advisory may be needed for both the
Illinois and Indiana nearshore waters. Following the frontal
passage Saturday...winds should turn northwesterly for Saturday afternoon
and into Sunday...but there is not a strong push of cold air
behind the front...so only expect a short period of brisk northwesterly
winds. Anticipate that conditions could be close to Small Craft
Advisory levels for a short period...especially for the Indiana
nearshore waters Saturday night. High pressure will then
overspread the Midwest and lower Great Lakes region while low
pressure drops out of northern Canada and south of James Bay by
Monday night and then high pressure will quickly build in behind
this system...so...winds across the lake will remain relatively
light from Sunday through midweek.
Lm...Small Craft Advisory...lmz740-lmz741-lmz742-lmz743-lmz744-
lmz745...1 PM Friday to 4 PM Saturday.
lmz669-lmz868-lmz870...4 PM Friday to 10 am Saturday.
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