Area Forecast Discussion
Issued by NWS Chicago, IL

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FXUS63 KLOT 270124

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
824 PM CDT MON SEP 26 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 00Z TAFs...

Only potential weather related aviation concern will be the gusty
west winds which will subside early this evening, then increase
and become gusty again Tuesday mid morning through the afternoon.
Peak gusts Tuesday afternoon will likely top out generally in the
25-30kt range. Cloudiness and some showers will spread south into
northern Illinois Tuesday evening, mainly just beyond the current
TAF cycle except for the 30 hour ORD TAF.



305 PM CDT

Low pressure currently over Ontario, will gradually shift
southward over the western lakes region through early Wednesday,
before shifting over the Ohio valley by late week. This system
will continue to drive the wind pattern across Lake Michigan for
most of the week. Westerly winds to 30 kt (aside from a few gale
gusts through this evening) will continue tonight and on Tuesday,
then the area of low pressure will move southward down Lake
Michigan Tuesday night. As this occurs, the winds will shift east-
northeasterly from north to south over the lake following the
passage of the low. It then appears that a prolonged period of
northeasterly winds up around 25 KT will occur later Wednesday
through Friday. This would likely result in small craft advisory
conditions again for a few days as waves over southern Lake
Michigan would likely top 6 feet. As a result, either a new small
craft advisory, or an extension of the current small craft
advisory may be needed in future forecast updates.

The other concern over the lake will be water spout potential,
especially Tuesday night through Wednesday evening. A very cool
airmass, associated with an upper level low will shift over the
very warm Lake Michigan waters during this period. This should set
up favorable thermodyamics over the lake to support lake induced
showers, and possibly thunderstorms as well. This will also be a
setup favorable for water spouts, especially in and around in
any lower level convergence zones over Lake Michigan.




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




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