Area Forecast Discussion
Issued by NWS Chicago, IL

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FXUS63 KLOT 271055

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
555 AM CDT Sun May 27 2018

324 AM CDT

Through Tonight...

Early season heat remains the focus of this forecast as a
pattern resembling mid-summer continues. Have inched up today`s
high temperatures to solidly mid 90s apart from near the lake, and
cannot rule out 97+ readings in a few spots. Heat index readings
in some locations are likely to reach upper 90s this afternoon.
With the early nature of this heat and multi-day duration
overlapping the holiday weekend, we continue to message awareness
to try to avoid the elements sneaking up on anyone, especially
those at work or strenuous activity outside.

The center of the stout mid to upper level ridge is building
gradually eastward from the Plains early this morning. The 00Z
raobs last evening sampled height rises of 30-40 dm at 500mb at
DVN and ILX, and this should once again keep any afternoon
convection very widely scattered. There is a subtle lower level
trough and associated moist axis around 700-850mb that will inch
from Iowa into northwest and north central Illinois this
afternoon. Surface dew points in the upper 60s are less likely to
mix much in this area, and its possible a couple thunderstorms
could trigger mid-late afternoon as convective temperatures are
reached. Also cannot rule out the lake breeze boundary leading to
some towering cumulus and possibly a stray storm. Overall, drier
air entrainment indicated further east in guidance has warranted
not mentioning anything in the forecast for the Chicago area at
this time.

The low-level thermal ridge of 20C+ at 850mb and 27C+ at 925mb was
sampled at DVN last evening (these values are at the 99th
percentile for late May). With the aforementioned upper ridging,
this thermal ridge will spread further across the CWA today. Based
on temperatures upstream yesterday, starting point values this
morning, and local area climatology, highs in the mid 90s today
have high confidence. The lake breeze will push inland maybe at
just a tad more speed than yesterday, but overall similar timing
to the temperature trend of yesterday for lake adjacent counties.
The earlier mentioned weak low-level trough has allowed upper 60s
to around 70 dew points to pool near the Mississippi River into
parts of the western CWA early this morning. Mixing should allow
for some afternoon drying, and maybe even to around 60 further
east including into the metro. But western areas (north central
Illinois) could hold onto those upper 60s dew points, yielding
heat index readings that may near 100.

Temperatures will not cool much tonight with lows around 70.



324 AM CDT

Monday through Saturday...

The northern stream jet remains north of the area through the
week, meaning a pattern that favors above normal temperatures,
including heat Memorial Day and Tuesday. There will be frequent
onshore flow, but with the jet north the tropospheric heights and
temperatures will generally support temperatures at least
slightly above normal. The one kink in this is Subtropical Storm
Alberto, which continues to trend that it will have effects on the
local area on Wednesday in the form of clouds and showers,
possibly with some thunderstorms.

The thermal ridge will remain over the area on Memorial Day so
synoptically temperatures similar to today are supported. A
building surface high over Lower Michigan will produce a more
easterly wind, albeit light, which should allow an earlier lake
breeze. So slightly cooler temperatures are forecast in lake
adjacent counties. Inland, mid 90s should again not be a problem.
Dew points may be a couple degrees lower areawide thanks in part
to the light easterly flow.

The thermal ridge does begin to ease Tuesday, but only slightly.
More of a factor will be an increasing east wind as then inland
Subtropical Storm Alberto lifts northward from the Deep South.
The east wind should keep highs a little cooler, again mainly near
the lake, while inland lower 90s are forecast.

The National Hurricane Center forecast of Alberto, as well as for
the most part the guidance consensus, favors the center of
circulation to lift north-northeast into Indiana by Wednesday.
The GFS and EC both indicate this will become more of a open,
negatively tilted 500 mb wave by the Wednesday p.m. time frame.
On its western and northern side of circulation, there may be
quite a cutoff in rain showers, but the close proximity certainly
supports likely showers Wednesday for northeast Illinois and
northwest Indiana. Rainfall amounts of over an inch locally are
possible if the moisture and widespread rain activity is
maintained. As for thunder potential, with the system likely
maintaining some subtropical/tropical characteristics, it may be
limited. This is especially true given the cloudy conditions
expected Wednesday.

Beyond, the pattern remains generally favorable for re-
amplification of the upper ridge, with upstream troughing
supported in the west.



324 AM CDT

The following are daily record highs for Chicago and Rockford over
the next two days.

               Sunday       Monday
Chicago       97 (2012)    95 (2012)
Rockford      99 (2012)    93 (2006)



For the 12Z TAFs...

Light flow is expected through much of the day. Another lake
breeze is expected to push through the eastern terminals this
afternoon. Latest guidance suggests the lake breeze may arrive an
hour or so earlier than currently forecast, but I decided to keep
the previous timing going. Isolated thunderstorms may form along
the lake breeze and across north central IL. Due to low confidence
in occurrence and coverage, I kept the TAFs dry. Mainly clear
skies and light and variable winds continue through Monday



330 AM CDT

Little in the way of concern over Lake Michigan for the remainder
of the holiday weekend and possibly much of this week. Last
night`s scattered storms over the northern half of the lake have
left some residual variable winds over that area, but those will
become southerly by mid-morning. Afternoon lake breezes are once
again favored into the nearshore areas the next two days, before a
more prevailing northeast to east wind is established for midweek.

The remnants of Subtropical Storm Alberto will lift northward
into the Indiana region by Wednesday, and this will increase the
pressure gradient and winds across the southern part of the lake.
It is possible 3-5 ft waves are seen in the Illinois and Indiana
nearshore on Wednesday, but there is quite a bit of uncertainty
right now.






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