Area Forecast Discussion
Issued by NWS Chicago, IL

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FXUS63 KLOT 261922

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
222 PM CDT MON SEP 26 2016

158 PM CDT

Through Tuesday...

Following the passage of a strong cold front last night, much
cooler, drier air has filtered across the region.  Also, sfc high
pressure is building across the middle Mississippi Valley, helping
keep mostly clear skies across the region.  Under persistent deep
layer cold advection, modest pressure rises and a strong pressure
gradient between a deep low over the upper Great Lakes and the high
pressure building across the middle Mississippi Valley, strong,
gusty west winds will persist through the day.  Conditions should
remain rather breezy into the night tonight.  There should be a
brief lull in the winds as a weak sfc trough rotates around the swrn
periphery of the low, but expect brisk, gusty winds to return for
tomorrow.  Latest satellite imagery shows a widespread area of
backwash cloudiness over the the upper midwest, but latest low-mid
level RH progs suggest that the moisture should remain north of the
WI border and not not expect any sgfnt cloud cover to work south of
the border in the short term forecast period, so expect mostly clear
skies to persist through the day tomorrow.  Temperatures across the
CWA will top out at 15 to 20 degrees lower than yesterday, with
highs in the upper 60s to lower 70s.  Under clear skies tonight,
conditions should be rather chilly, with lows in the mid to upper
40s away from downtown Chicago where lows will only be in the low
50s.  Max temps should be a couple degrees lower tomorrow due to a
combination of persistent cold advection and lowering heights aloft
as the deep upper low begins to move south to near lake Superior by
tomorrow evening.


221 PM CDT

Tuesday night through Monday...

The main weather story during the extend period will be the
continued impacts from the Great Lakes upper low. This upper low
is expected to only gradually drop southward over the lower Great
Lakes and the Ohio Valley by mid to late week. As a result, it
appears that there will be a few days of cool weather, along with
a few periods of rain showers.

Late Tuesday and Tuesday night a significant PV anomaly is
expected to dig southward along the westerly periphery of the
upper low. This will effectively begin to drive the upper low and
the associated surface low and cold front nearly due south right
over the area by Wednesday morning. A period of showers rain
showers will also likely accompany the approach of this feature
Tuesday night into Wednesday. Breezy northerly winds, cloudy skies
along with periods of showers will likely make Wednesday the
coolest day of the week, with highs only in the low to mid 60s.

Once the vertically stacked low shifts towards the Ohio Valley
late Wednesday into Wednesday night, expect the gusty winds to
turn north-northeastward over the area into Thursday. The colder
airmass (850 MB temps in the low to mid single digits above 0 C)
moving over Lake Michgian should set up favorable thermodyamics
for some thunderstorms, and possibly some water spouts as well
over the lake during this period. Therefore, as the surface winds
shift northeasterly by late Wednesday, a lake induced
thunderstorm or two could try to move onshore over Northeast IL
and northwest IN.

The vertically stacked system should fill and continue to
gradually become ingested back into the main belt of westerly
either later this weekend or by early next week. Model and
ensemble guidance continues to show uncertainty on exact timing
of this, however. Until this system moves out of the Great Lakes,
expect northerly flow and cool, but pleasant, temperatures for
the later portion for forecast period. There will also continue to
be very low chances for some lake enhanced rain showers for
northeast Illinois and northwestern Indiana.



For the 18Z TAFs...

Only concern this period remains winds. West winds have been
gusting to 25-30 kt with a few higher gusts. Expect that
prevailing gusts to 30 kt through arnd sunset should hold though
an isold higher gust is not out of the question, with direction
remaining prevailing wly through the late evening hours. A brief
lull in winds should occur overnight as a weak trough rotates
around the swrn periphery of a deep sfc low north of Lake
Superior. Expect wind speeds/gusts will increase again after
sunrise tomorrow morning, though gusts should only be to arnd 25kt.
Otherwise, dry/vfr through the period.



220 am...Low pressure over western Ontario will slowly move
southeast to Lake Superior Tuesday afternoon and then move south
across western lower Michigan Tuesday it slowly
weakens. Westerly winds will persist across the lake through
Tuesday with speeds to 30 kts. There remains the potential for
some gale gusts this afternoon into tonight but confidence on how
widespread these will become remains low and will maintain gale
gust mention. Winds will turn northerly Tuesday night as the low
moves south with speeds diminishing. As the low moves to the mid
Atlantic Wednesday...high pressure will build across Ontario and
the upper midwest...tightening the gradient again across the
western Great Lakes. North to northeast winds may increase back to
30 kt Wednesday night before diminishing on Thursday.

With this low slowly moving across the lakes region and associated
cold air aloft...there will be a chance of waterspouts Tuesday
into Wednesday. cms



     Small Craft Advisory...NEARSHORE GARY TO MICHIGAN CITY IN UNTIL 10 PM Wednesday.




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