Area Forecast Discussion
Issued by NWS Chicago, IL
FXUS63 KLOT 212246
Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
546 PM CDT SAT MAY 21 2016
215 PM CDT
Rather benign pattern will continue this evening through Sunday.
Broad surface high pressure centered over the upper midwest
continues to allow a light East to Northeast wind. Mixing
continues to be good this afternoon with some occasional gusts
being pulled down to the surface to 20 mph. Expect around sunset
mixing will decouple and result in light to at times variable
winds overnight. Clouds will continue to be minimal
overnight...however a thin cirrus shield will likely help to keep
temps elevated overnight in the lower 50s to upper 40s across the
forecast area. With dew points this afternoon falling into the
lower to middle 40s and a few upper 30s...temperatures overnight
may be able to quickly fall and could allow lows to be a few
degrees cooler than currently forecasting.
A high amplitude mid-level ridge extends North through Central
Canada early Sunday and will drift east over the Western Great
Lakes. Surface high pressure will become centered over Northern
Illinois Sunday midday...with a light East/Northeast wind around
5 to 10 mph. Skies will remain mostly clear through much of Sunday
again...with temperatures expected to warm further into the 70s
and a few sites away from Lake Michigan could warm to around 80
degrees Sunday afternoon. Areas adjacent to Lake Michigan will
likely be several degrees cooler with a light onshore flow in the
afternoon...with highs in the lower to middle 60s.
241 PM CDT
Sunday night through Saturday...
Quiet weather will continue Sunday night through Monday night
across northern Illinois and northwest Indiana as high pressure
moves from the Great Lakes into the upper Ohio Valley. Subsidence
from upper ridge building overhead and a very dry airmass will be
in place allowing for lots of sunshine on Monday and temperatures
warming to around 80.
Upper ridge begins to weaken on Tuesday while surface high settles
to our southeast allowing moisture to begin to return to portions
of the midwest. Guidance continues to suggest dew points
building into the upper 50s and low 60s during the day with modest
instability developing. Guidance has slowed the arrival of the
EML however so instability is no longer advertised to be as robust
Tuesday afternoon as earlier guidance would have suggested. In
addition...weak forcing from mid-level wave embedded in the
larger scale ridge may be unable to overcome capping inversion.
Trimmed pops back kept them focused during the afternoon hours in
the western portions of the CWA Tuesday. Maintained chance PoPs
for Tuesday night as low level jet ramps up to our west with
nocturnal convection possible. Chances do appear better to our
west though and if trends hold may be able to lower PoPs for this
time-frame in the coming forecast updates.
Upper level troughing is progged to inch eastward over the plains
Wednesday into Thursday while at the surface broad low pressure
develops. GFS soundings show very steep 8-9 C/km mid-level lapse
rates advecting overhead with 2000+ J/kg MLCAPE Wednesday and 3000+
J/kg Thursday afternoon. In addition...mid/upper flow begins to
markedly increase from midweek on allowing for deep layer shear
more supportive of severe weather. Will have to wait and see if
and how convective debris and upstream conditions impact us and if
this prevents us from realizing the advertised instability...but
certainly looking at the potential for strong instability and at
least marginally sufficient deep layer shear to support severe
thunderstorms Wednesday and especially Thursday. Warm and moist
conditions continue Friday and Saturday and eventually main upper
trough axis is progged to lift across the Great Lakes supporting
additional chances for showers and thunderstorms.
For the 00Z TAFs...
VFR conditions with no wind concerns through period.
241 PM CDT
High pressure will bring quiet weather conditions across Lake
Michigan through the remainder of the weekend. The light flow will
allow lake breezes to form this afternoon and again Sunday
afternoon. The ridge axis will shift east of Lake Michigan by
Monday allowing fresh southerly winds to overspread the lake. The
overall pattern will be slow to change next week with broad low
pressure in place over the plains and high pressure settling
across the southeast providing persistent southerly flow across
the Great Lakes. Several rounds of showers and thunderstorms are
expected...and moisture moving north from the Gulf of Mexico will
increase the threat for fog developing...especially beyond Tuesday
when dew points into the 60s advect across the region.
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