Area Forecast Discussion
Issued by NWS Chicago, IL

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FXUS63 KLOT 300821

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
321 AM CDT Tue May 30 2017

319 AM CDT

Through tonight...

Relatively benign though somewhat cool conditions remain in place
early this late meteorological spring morning as deep and
expansive upper level low pressure continues to spin across
Ontario. The low has been far enough away to prevent widespread
precipitation, but close enough to need to watch the radar scope.
Today remains no exception as several notable
shortwaves/circulations remain evident in GOES-R layered water
vapor imagery on the back side/southern flank of the low. This
first weaker wave encroaching the Mississippi River will pass
through dry this morning, but the stronger circulations, one
across northern MN and one over far northeast ND, with a broader
cloud field ahead of them will move south and east today. The
leading one over MN looks to slide by to our north, the upstream
wave will interact with a deeply mixed lower atmosphere again
today to trigger scattered showers and maybe a t-storm this
afternoon and early evening.

RAP/HRRR forecast soundings due to the deep mixing do not depict
much instability this afternoon, and even show a bit of capping
until the wave arrives late today, which may limit coverage for a
while in spite of a thickening cloud field. Therefore have kept
thunder mention as isolated in spite of a slightly higher chance
for showers today then on Memorial Day, especially along the
northern tier of counties. It will again be somewhat breezy with
the surface low holding its presence northeast of the area. With a
bit more cloudiness and some weak cold air advection from the
northwest temps will be held down a touch as well.

The thunder threat should diminish this evening but there could
still be some showers with the plethora of shortwaves in nw flow
shifting southeast and with the weak surface trough/cold front.
With some late clearing it will be another seasonally chilly night
with lows even dipping down into the upper 40s across north
central IL and into outlying areas west of Chicago.



320 AM CDT

Wednesday through Monday...

Quiet weather is anticipated Wednesday as the main large scale
trough axis shifts off to the east of the region, and generally
subsident conditions develop as indicated by 50-60 meter H5 height
rises. While the surface pressure gradient slowly relaxes, it
appears that enough west-northwest flow (15-18 kts at 950 mb) will
persist to prevent any significant lake breeze development,
except for the Indiana shore east of Gary where the northwest
component to the surface winds will produce onshore flow. Low
level thermal fields from models support highs from around 70
north to the lower 70`s south. Surface high pressure ridge drifts
just south of the area Wednesday night into early Thursday,
allowing winds to back to the southwest overnight.

Energy emerging from a long wave trough along the west coast is
progged to cross the northern Rockies Thursday into Friday, with
weaker short wave energy ejecting from the desert southwest
region. This induces a broad area of southerly return flow to set
up across the Plains and into the Mississippi Valley and western
Great Lakes region. Guidance displays some minor detail
differences during this day 3-4 period, though generally depicts
the north-northeastward development of a surface warm front into
the area Thursday night into early Friday. Forcing aloft is fairly
subtle as this feature lifts north, though guidance does indicate
some precipitation development within the region of broad ascent
associated with warm/moist advection on the warm frontal slope.
Modest mid-level lapse rates and increasing theta-E associated
with a 30-40 kt southwesterly low level jet does result in
elevated MUCAPE which should increase thunder potential late
Thursday night into early Friday. The front lifts largely north of
the area Friday, with a warmer and more humid air mass in place.
High temps Friday should reach the lower 80`s across much of the
cwa, with the possible exception of the far northeast where highs
may hold in the 70`s as a slight southeast wind component is
possible off the lake. Forcing aloft remains somewhat weak, though
the presence of stronger northwest flow aloft just north of the
forecast area does suggest the potential for weak disturbances to
move through. This, combined with the warm/humid surface air mass,
suggests the potential for additional scattered thunderstorms and
worth a chance pop during the day. GFS forecast soundings depict
MLCAPE values in excess of 2000 J/kg across the cwa by Friday
afternoon along the southern periphery of stronger 25-35 kt 0-6 km
shear axis to our north, which suggests some strong/organized
storm potential could exist.

Guidance really begins to show some detail differences heading
into next weekend, particularly with a cold front which sags south
toward/into the area. GFS is somewhat of an outlier with the
front pushing south across the area and a more southward track of
an area of surface low pressure which develops and propagates
along the boundary into Sunday. ECMWF (00Z and previous 12Z) and
Canadian don`t push the front as far south, keeping it over the
cwa and tracking the low across the area late Saturday and
Saturday night. This obviously has implication with respect to
temps Saturday, and also rain/thunderstorm trends. Have generally
gone with blend of guidance solutions at this distance, though
have upped thunder potential a bit during the period. All guidance
trends toward developing some sort of closed upper low feature
across the region Sunday-monday, with the surface low deepening
off to our east. This would suggest a blustery and much cooler
weather regime to end the weekend and begin the following week.



For the 06Z TAFs...

Quiet overnight conditions with light westerly winds will give way
to another breezy west wind today as deep mixing is again
expected. Gusts appear to be a tad lower today than on Memorial
Day, though still expected to reach the mid 20s.

No big change in this set of TAFs as guidance still depicts
scattered afternoon showers. Instability on RAP/HRRR forecast
soundings is fairly low as the lower atmosphere is fairly dry and
with deep mixing in place. Therefore expect t-storm coverage to be
too low to include in the TAFs.



216 AM CDT

Low pressure will continue to drift slowly northeastward across
northern Ontario through tonight, reaching James/Hudson Bay on
Wednesday. Meanwhile, high pressure will spread southeast across
the Plains, and will reach the lower Missouri and lower Ohio
Valleys by Wednesday evening. This will continue to produce modest
west-southwest winds across the lake. The northern periphery of
the surface ridge will cross Lake Michigan early Thursday, with
winds becoming southerly Thursday night. Winds will not remain
south for long on the northern half of the lake, as a cold front
is forecast to sag south on Friday. This front will likely push to
the south end of the lake or south of the lake entirely by early
Saturday. Various model forecasts then show some spread in their
representation of an area of surface low pressure that will track
along the front late Saturday into Sunday. Some guidance deepens
this low significantly, leading to a period of strengthening
easterly and then northerly winds late Saturday and Sunday. Will
obviously get a better handle on the details with successive model
runs, though at this point it appears that 25 kt winds be
expected during the latter half of next weekend, with a long
northerly fetch building waves Sunday.



LM...Small Craft Advisory...LMZ740-LMZ741-LMZ742-LMZ743...11 AM
     Tuesday to 7 PM Tuesday.




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