Area Forecast Discussion
Issued by NWS Chicago, IL

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FXUS63 KLOT 270719

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
219 AM CDT TUE SEP 27 2016

824 PM CDT

Going forecast for tonight looks good and mainly made some minor
tweaks to hourly temps. Biggest change to the forecast tonight and
into tomorrow was to lower dewpoints somewhat significantly.
Dewpoints mixed out into the low to mid 30s Monday afternoon and
forecast sounding suggest a similar set-up for efficient mixing
will be in place Tuesday afternoon. This could send afternoon RH
values to near critical levels again again Tuesday afternoon.
Winds will likely be quite strong and gusty again creating a
heightened fire danger, particularly southern CWA.



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 06Z TAFs...

1233 am...Forecast concerns include wind speeds/gusts today and
showers/mvfr cigs tonight into Wednesday morning.

West/southwest winds at/just under 10kts early this morning will
turn more westerly after sunrise with speeds/gusts steadily
increasing through the mid/late morning. Gusts into the mid 20kt
range look on track but its possible speeds/gusts may be a little
higher than currently expected. Speeds/gusts will diminish with
sunset this evening and then turn more northwest tonight.

Low pressure over Ontario will move south down Lake Michigan
tonight reaching Northern Indiana Wednesday. Cloud cover will
begin to increase from north to south late this afternoon into
this evening and should be vfr at first...but is expected to lower
into mvfr by late this evening. Possible cigs may lower into ifr
late tonight into Wednesday morning. Scattered showers are also
expected especially from late evening through the predawn hours of
Wednesday morning. cms



219 am...Low pressure north of Lake Superior this morning will
move south across Lake Michigan tonight and then to the mid
Atlantic region Wednesday evening. Strong westerly winds will
slowly diminish from north to south as this low approaches. There
could still be some gale gusts early this morning...but appears
prevailing gales will be ending so will be letting the gale
warning for the north half of Lake Michigan expire.

Winds will diminish tonight as they shift to the northwest but as
the low moves south...winds will shift northeasterly overnight
into Wednesday morning. High pressure building across the upper
midwest and Ontario will tighten the gradient with northeast winds
increasing to 30 kts Wednesday and possibly continuing into
Thursday. The low pressure is expected to move back west toward
the southern Great Lakes region Friday into Saturday and if this
does occur...there will be a prolonged period of northeast flow
into this weekend.

As this low moves across the lake combined with showers and cold
air aloft...there will be a chance of waterspouts later today and
continuing through Wednesday. cms


     UNTIL 7 PM Tuesday.

     UNTIL 10 PM Wednesday.




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