Area Forecast Discussion
Issued by NWS Chicago, IL
FXUS63 KLOT 211953
Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
253 PM CDT SUN AUG 21 2016
252 PM CDT
For the remainder of the afternoon and, indeed, through the entire
short term forecast period, conditions will remain cool and quiet.
Following the passage of the strong cold front yesterday, persistent
deep layer cold advection under nwly flow aloft has brought much
cooler, drier air across the region. Main issue for the afternoon
will be a very small chance for some isold showers over far nern IL,
but latest radar imagery indicates that any shra activity is well
north of the WI border. Temperatures this afternoon should top out
in the mid 70s, which is almost 10 degrees below seasonal normal
levels. Any remaining diurnal stratocu will dissipate at sunset and
skies should be clear for the overnight hours. The sfc ridge axis
currently over the upper Missouri Valley will move across the local
area overnight tonight, bringing lgt/vrbl winds and clear skies. The
light winds, clear skies and relatively low dewpoints should allow
for temperatures to drop into the lower to middle 50s across the
region. The exception will be the urban heat island of the Chicago
metro area, which should only drop into the middle 60s. For Monday,
expect dry conditions to continue. As the sfc ridge moves off to
the east tomorrow, a trough of low pressure will develop over the
wrn plains, setting up swly flow across the region. Do not expect
much in the way of a return of moisture, but temperatures should be
a few degrees higher with max temps tomorrow in the upper 70s to
around 80 F.
230 PM CDT
Monday night through Sunday...
The long term features some ups and downs, with a transition back
to a period of warm and muggy conditions and chances for storms
mid week, another brief cool down, and another chance for showers
and storms on the weekend.
Surface high pressure will continue to retreat to the east Monday
night through Tuesday as deep upper level low pressure moves from
the Canadian Rockies to the Canadian Prairies and Upper Midwest.
With southerly low level flow in place, a warming trend will
continue with readings getting back to near or possibly slightly
above seasonal norms which now hover in the low 80s, and while not
oppressive dewpoints will recover back into the 60s.
With deep southwest flow in place ahead of the approaching upper
level trough moving through the northern Plains, storm chances will
increase late Tuesday night and more so Wednesday and Thursday.
The main upper trough will not get here due to strong upper level
ridging across the southeastern U.S, but several smaller scale
disturbances in the increasingly moist and unstable airmass will
promote off and on chances for showers and storms until a cold
front moves through Thursday evening. Pwats again approaching 2
inches suggests heavy rainfall would be the most significant
concern during this period.
High pressure builds in for Friday, but provides a brief cool and
dry period as the main upper low/trough remains in place across
the remains in place across the Northern Plains and southwest flow
aloft dominates. Surface flow will be northerly which should keep
moisture away Friday, but the frontal boundary and low level
baroclinic zone looks to remain draped across the area. Increased
moist isentropic lift over this boundary will increase chances for
showers and thunderstorms this weekend, but model uncertainties on
timing preclude more then chance pops from the model blend at this
point, with the more favored and agreed upon period being late
Saturday into next Sunday.
For the 18Z TAFs...
A ridge of high pressure is approaching the middle and upper MS
Valley as strong low pressure lifts newd through nrn Quebec. Cold
advection persists over the region with gusty wnwly winds and
sct-bkn stratocu. The trend has been for cigs to improve to vfr,
so will start out all sites, except GYY at vfr. RFD/DPA have
already becm sct and ORD/MDW should scatter soon as drier air
advects over the region and the sfc ridge continues to build to
the east. GYY will remain mvfr a little longer due to nwly flow
off of Lake Michigan. Expect that sky cover should continue to
diminish through the afternoon and go skc in the evening. Winds
should remain gusty through the afternoon, but diminish as the
ridge continues to build east. There is a very remote chance for
an isolated shower this afternoon, close to Lake Michigan, but the
general trend should be for dry conditions through the period.
Winds should becm lgt/vrbl overnight as the ridge axis moves
overhead. By tomorrow morning, the ridge should push east of the
terminals while low pressure develops over the plains. Winds
should increase and becm swly through the morning hours.
230 PM CDT
Deep low pressure east of James Bay will continue northeast
tonight as high pressure across the plains shifts to southern Lake
Michigan, Gusty northwest winds this afternoon will become light
westerly tonight, then will shift southerly and increase through
midweek as the high moves to the Atlantic seaboard. Low pressure
will pass north of the lake Wednesday evening, with the trough
axis crossing the lake Thursday. High pressure over the plains
Thursday evening will shift over the lake Thursday night allowing
a cold front to pass across Lake Michigan. The high shifts east on
later Saturday and southerly flow will return.
IN...Beach Hazards Statement...INZ001-INZ002 UNTIL 10 PM Sunday.
LM...Small Craft Advisory...NEARSHORE BURNS HARBOR TO MICHIGAN CITY IN UNTIL 10 PM Sunday.
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