Scientific Forecaster Discussion

Return to Local Conditions & Forecast

Without Abbreviations
With Abbreviations

FXUS63 KLOT 251759

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
1159 AM CST Sat Feb 25 2017

1159 AM CST

Through Sunday...

Main forecast concerns are ending of light snow showers/flurries
early this afternoon, then very windy conditions Sunday. Could be
approaching wind advisory criteria Sunday afternoon, with gusts 40
mph or higher possible.

Surface low pressure which moved across our area Friday continues
to pull away to the northeast of the Great Lakes early this
afternoon. Mid-level trough/vorticity axis was shifting east of
the area as well per water vapor imagery, which will bring light
snow showers to an end early this afternoon across the eastern 
counties of the cwa. A few flurries may linger within cold 
advection stratocu deck, though gradual advection of drier air
will tend to thin the cloud deck and allow flurries to gradually
end as well. With thinning of cloud deck, some sunshine will break
through especially across the western cwa, though persistent cold
advection should limit temp rise with highs generally in the mid-
upper 20's this afternoon with wind chills in the single digits.

Skies clear briefly this evening as weak surface high pressure
ridge approaches, but mid clouds are already pushing east across
western Iowa ahead of a quick-moving mid-level wave which will
move across the area late tonight into Sunday morning. Temps fall
off into the teens/lower 20's tonight with diminishing winds and
brief clearing, then become more steady overnight as surface winds
turn southwest and cloud deck moves in. Gradient then tightens up
Sunday, with gusty southwest winds developing and spreading warmer
air back into the area. Forecast soundings indicate decreasing
cloudiness in the afternoon especially across the northern half of
the cwa, resulting in deep mixing to nearly 750 mb and tapping
45-50 kt winds. While not all of this momentum will mix to the
surface, the potential for very windy conditions with gusts to
around 40 mph appear likely during the mid-afternoon hours,
perhaps a little lower in the southeast where clouds linger a bit
longer. Could be close to wind advisory criteria (45 mph gusts) so
will have to monitor further guidance trends for clouds/mixing 
depths. While guidance highs are generally mid-40's for Sunday, 
progged 925 mb/850 mb temps would support upper 40's to lower 
50's in some spots under ideal conditions.



331 AM CST

Sunday through Friday...

The rest of the weekend into early next week does appear to 
remain on the dry side. Mid level trough expected to push further 
east through the region on Sunday, but it does appear that the 
stronger forcing stays just to the south of the CWA. Some 
moderating of the air mass will occur, and should see temps 
rebound back into the 40s on Sunday. Continued dry conditions 
likely to occur on Monday, with slightly warmer temps in place. 
Westerlies usher in additional shortwave energy across the central
CONUS Monday night into Tuesday. Surface reflection expected to 
lift northeast across the area during this time, with additional 
warming likely. Still some model variability with the next system,
but precip chances do increase by Monday night. Precip type 
should be all liquid Monday night into Tuesday and as additional 
forcing and moisture increase Tuesday, could see some scattered 
thunderstorm development. Guidance begins to really vary beyond 
this point, Tuesday night into Wednesday. Additional precip 
chances are still warranted though, but confidence is low with 
trends/timing. Will need to keep an eye on this system, as it is 
possible for much colder air to filter in across the area while 
the stronger forcing is still in place. This would then support 
more wintry type precip over much of the CWA.



For the 18Z TAFs...

Minor concerns in near term are persistent flurries and gradual
lifting/thinning of mvfr cigs this afternoon. Very windy
conditions develop Sunday, with gusts 35-40 kt from the southwest
expected during the afternoon.

Surface low pressure continues to pull away to the northeast of
the Great Lakes at midday, with strong pressure gradient lingering
across the region driving blustery west-northwest winds. Mid-level
trough axis was shifting east of the area currently, and lingering
light snow showers should be ending shortly with only a few
flurries left into the early/mid afternoon hours before those too
dissipate. MVFR deck gradually rising into the 2500-3000 ft range
and will thin over time, but will likely not completely scatter
out until sunset. Winds, from 280-300 degrees, will also slowly
diminish through this evening as weak surface high pressure ridge
moves in from the west.

Winds, which back southwest overnight, will increase significantly
again Sunday morning. Forecast soundings indicate deep mixing by
afternoon, coincident with loss of mid-level cloud deck which
develops late tonight. Guidance mixes as deep as 7000-8000 ft by
mid/late afternoon, tapping momentum of 40-50 kt winds aloft.
Fairly high confidence in gusting to the mid-upper 30 kt range at
this point, and potentially a bit higher. Direction looks to start
out around 200 degrees, veering slightly to more 230-ish later in
the day.



143 AM CST

The pattern remains active over the Great Lakes this weekend.
Departing low pressure from the central lakes this morning will
continue cold advection and at least occasional northwest gales 
are likely across the north and central, while more marginal 
south. Weak ridging tonight will ease the winds only temporarily,
as a quick moving low over the northern lakes on Sunday will
result in stout southwest winds. The air mass looks inherently
cool enough going into Sunday to prevent an inversion and likely 
support southwest gales during the day Sunday, especially central 
and south. Some near gale force gusts are possible in the Illinois
and Indiana nearshore on Sunday.

The next chance for higher wind speeds comes Wednesday behind a
northeastward moving low pressure which passes during or near
Tuesday night/early Wednesday. Still a decent amount of spread in
our computer model guidance on this system, but this could 
support some near gales, as well as more of a due north wind



LM...Gale Warning...LMZ777-LMZ779 UNTIL 3 PM Saturday.


     Small Craft Advisory...NEARSHORE GARY TO MICHIGAN CITY IN UNTIL 3 AM Sunday.




National Weather Service Glossary of Abbreviations