Scientific Forecaster Discussion
Area forecast discussion
National Weather Service Chicago/Romeoville Illinois
1147 PM CST Friday Nov 27 2015
745 PM CST
Going forecast tonight largely in decent shape. Main tweak was to
lower hourly probability of precipitation in southern County Warning Area as observation have only been showing
drizzle and light rain with no visibility restrictions so its probable
not all areas are seeing measurable precipitation. Kept drizzle wording
going into the overnight in this area with low dewpoint spreads
and continued broad lift from upper waves moving across the
region. Solid stratus deck stretches from southern Minnesota across
central WI...therefore locked in full cloud cover until about middle
day Saturday. Temperatures will only slowly drop a few degrees
from current levels by daybreak. Tweaked a few locations up a
degree or so considering clouds and winds not going calm. Will be
able to let the Lakeshore Flood Advisory go as planned at
9pm...with waves at south buoy down to 8 feet as of this writing
and winds gradually diminishing at marine ob sites.
Only other item of note is lake effect plume into far northeast
Illinois and southeast Wisconsin on last several hourly runs of
hi-res guidance such as hrrr and hrrr-x. With unimpressive lake to
850 mb Delta T/S of 9 or 10 celsius yielding low inversion heights
and not much instability...not expecting any measurable precipitation out
of lake plume. Kept "silent" probability of precipitation around 10% in grids.
212 PM CST
Lingering light rain and drizzle this afternoon is slowly sliding
southeast together with an axis of pronounced low level fgen aligned
along a cold front. Farther north...a ridge axis associated with
high pressure centered over South Dakota is dropping across Lake
Superior toward Lake Michigan. Drier air ahead of this ridge is
starting to work into the local area as evidenced by falling
dewpoints north of the departing cold front. After the area dries
out this evening...the remainder of the weekend should remain dry
and near seasonally normal highs under the continued influence of
the ridge and the corresponding height rises aloft.
212 PM CST
Sunday night through Friday...
A very large upper low spinning over the southwest Continental U.S. Will begin
to make its presence felt locally by late in the weekend and early
next week. The variety of model solutions generally agree in
lifting this upper low across The Rockies during the weekend and
into the plains by Monday morning. It is a little hard to buy
into such a progressive pattern from a considerably wrapped up
feature...but the kick appears to come from a strong shortwave
moving onshore Saturday night and dropping into the base of the
upper trough. This feature is apparent on the WV imagery and both
the GFS and European model (ecmwf) seem to have it analyzed well...so will plan to
follow the ideas of these two solutions for the remainder of the
Most of Monday...or at least through early afternoon...the local
area should remain generally dry as a developing surface low lifting
out of the Gulf ahead of the large upper trough stays far enough
south and west of the area. Precipitation chances increase quickly later
Monday afternoon into Monday night as a warm advection rain shield
lifts across the area. Even into Tuesday morning this would all
appear to remain in the form of rain under the continued influence
of warm advection ahead of the low. In fact...temperatures likely
will climb overnight Monday into Tuesday morning.
On Tuesday and Tuesday night into Wednesday the surface and upper
low centers both lift north of the area and wrap colder air around
to their south. Low levels dry out considerably during this
period...but strong upper shortwaves and midlevel moisture likely
will support continued periods of showers and perhaps some light
By Thursday and Friday the area again should have a break in the
weather...and clearing skies would appear to support daytime highs
near seasonable norms.
//Ord and mdw concerns...updated 06z...
* brief light rain or drizzle possible prior to 08z.
* Northeast winds at 10 knots with occasional higher gusts possible
* MVFR ceilings through the morning and possibly into afternoon.
Northeast winds will continue over the region through the taf
though the atmosphere will remain slow to dry out today. MVFR
ceilings are expected to continue at Chicago area taf sites aided by a
Stout inversion and unstable air and added moisture off of Lake
Michigan. There is some weak forcing aloft causing patchy light
drizzle/rain northern Illinois. This is translating east so dpa
and Ord...and possibly mdw and gyy...may experience this prior to
The MVFR ceilings should slowly start to erode late this morning and
into the afternoon...though northeast Illinois will likely see at
least scattered low clouds through the entire afternoon...so
broken will be possible. Further to the northwest...rfd is
expected to remain near the edge of the MVFR ceilings through much of
North-northeast winds should lose gusts into this afternoon and
then drop down further in speed this evening.
//Ord and mdw confidence...updated 06z...
* high that any precipitation will be light and temporary and done
* Low-medium in timing of MVFR ceilings scattering.
* High in all other elements.
//Outlook for Ord/mdw for 12z Sunday-12z Friday...updated 00z...
Sunday and Monday...VFR.
Monday night and Tuesday...periodic -shra. Chance of -dz/fg. MVFR
ceilings likely with IFR or lower possible.
Wednesday...MVFR ceilings possible early.
212 PM CST
Between a ridge of high pressure building over the lakes from the
west...and a cold front departing to the southeast...strong north
winds over the lake today will gradually diminish tonight into
Saturday morning. As the ridge axis moves overhead Saturday...winds
across the north half of the lake will back westerly while winds
across the south half will veer northeasterly. The center of the
ridge slides north of the lake on Sunday and turns all winds to the
east. The easterly flow persists into Monday and Monday night as a
center of low pressure lifting out of the Gulf approaches the area.
By Tuesday the low is expected to be lifting through Wisconsin and
then across the north end of the lake Tuesday night. With most of
the lake remaining east of this low...winds will gradually veer from
east to south to southwest and all the way around to northwest by
Lm...Small Craft Advisory...nearshore waters
until 3 PM Saturday.
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