Area Forecast Discussion
Issued by NWS Chicago, IL

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FXUS63 KLOT 301056
AFDLOT

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
556 AM CDT MON MAY 30 2016

.SHORT TERM...
337 AM CDT

Today through tonight...

The main forecast concern on this Memorial Day is with the potential
for a few isolated showers or thunderstorms. Above normal warmth
will continue through the holiday weekend.

The modest cold front which pushed through the area Sunday has
settled across the Ohio Valley early this morning...and was
already lifting back north as a warm front across the Central
Plains. Well north of the surface front...a few isolated showers
had developed over northern IL...in a small region of moist ascent
associated with a short wave trough propagating eastward across
MN/WI. High resolution short range guidance (RAP...HRRR and 4km
NAM) forecasts indicate forcing will weaken through sunrise...
resulting in these showers dissipating. Recent radar trends
support this with a decrease in both coverage and intensity
noted. Large scale subsidence is then expected to strengthen in
the wake of the early morning short wave passage...as indicated by
rising model mid-level heights of building short wave ridge over
the region. This induces weak surface high pressure to spread
across the area through early afternoon...with light synoptic west
winds expected to allow lake breeze development. 950-925 mb
temperatures fields support highs reaching the low-mid 80s across
the forecast area...with mid-upper 70s in onshore flow along the
lake. While a mix of high cirrus and scattered daytime cu is
expected...partly to mostly sunny skies are anticipated.

Turning attention to our west...a strong short wave noted over
southern Alberta in early morning water vapor imagery is progged
to dig southeast into the Northern Plains through tonight. Surface
low pressure will deepen across the Dakotas later today in
response...strengthening the southerly return flow across the
Central Plains. In response...the surface warm front will lift
north across the mid-Missouri Valley this afternoon and tonight...
with the potential for isolated to scattered thunderstorm
development occurring as far east as parts of
western/northwestern IL as indicated by several convection allowing
models. While the aforementioned synoptic pattern across the cwa
today in not particularly conducive for widespread or organized
convective development...forecast soundings from the WRF and GFS
do depict a conditionally unstable weakly capped thermodynamic
profile across portions of the forecast area with 500-600 J/kg of
MLCAPE this afternoon. Thus while initiation is not expected in
the forecast area...there is a low chance potential for isolated
storms over eastern IA or western IL to propagate east into our
western cwa counties late this afternoon or early this evening.
Even into tonight however...the focus for additional thunderstorm
development should remain largely west/northwest of the forecast
area where the low level jet axis develops into northern IA/MN and
WI.

Ratzer

&&

.LONG TERM...
355 AM CDT

Tuesday through Sunday...

Primary concerns in the extended forecast focus on a period of
unsettled weather mid-week. The strong short wave over the Dakotas
will deepen and drift to the east-northeast Tuesday through early
Thursday...with a slow moving cold front trailing through the
Plains. A somewhat narrow south-north oriented axis of deep
moisture and warm/unstable air will exist along and east of the
front...with the late Tuesday through Wednesday night period
looking active with the potential for thunderstorms with slow
front-parallel movement. This slow movement and high precipitable
water values progged at around 1.5 inches would be supportive of
a locally heavy rainfall threat. The potential for severe weather
will increase by Wednesday as well...as the upper level jet streak
associated with the upper trough spreads across the western and
northern Great Lakes region. SPC has included the forecast area
in a marginal severe thunderstorm risk in their day 3 outlook.
Though clouds and precipitation will likely have some effect on
temperatures...above normal warmth will continue through mid-week
with afternoon highs in the upper 70s to mid 80s.

The cold front moves through the area by early Thursday...with
drier and cooler air moving in as surface high pressure develops
across the Midwest. While cooler...temperatures are progged to
remain close to average for early June with highs in the 70s and
lows in the 50s/low 60s. A significant shift in the upper pattern
develops late in the week as an upper ridge amplifies over the
western CONUS...with a corresponding deep east coast upper trough.
Northwest flow will persist across the area through next
weekend...with a series of disturbances bringing periodic chances
of showers.

Ratzer

&&

.AVIATION...
For the 12Z TAFs...

No major changes from the 06Z TAFS.

The only aviation weather concern today will be the timing of a lake
breeze boundary this afternoon...otherwise VFR conditions are
expected. Light westerly winds will start out the day. However...a
lake breeze is likely to develop by midday...then shift inland over
the area through the afternoon. Currently it appears that it could
reach ORD and MDW as early as 20-21 UTC...with an associated
easterly wind shift expected. Wind speeds could be up around 10KT
for a period with the passage of the boundary...but should
gradually abate into the evening.

KJB

&&

.MARINE...
116 AM CDT

South to southwest winds will continue over the northern half of
the lake today. However...a surface ridge of high pressure setting
up over the southern half of the lake today will allow for lake
breeze development...and onshore flow across the southern end of
the lake this afternoon. Expect the winds to become east-
southeast and begin to increase on Tuesday as a storm system
begins to approach upper Midwest. It appears that winds up around
20 KT will occur late Tuesday into early Wednesday in response to
this storm system...which is forecast to shift eastward into
southern Ontario by late Wednesday. This will push a cold front
over the lake Wednesday night. A period of thunderstorms looks
likely Wednesday into Wednesday evening as this front approaches.
Following the frontal passage expect lighter westerly flow to set
up for Thursday.

Late in the period...eighter late Friday or Saturday another storm
system could impact the western Great Lakes region with another
possible period of storms and higher winds. However...confidence
with the timing and track of this system is low at this time.

KJB

&&

.LOT WATCHES/WARNINGS/ADVISORIES...
IL...NONE.
IN...NONE.
LM...NONE.
&&

$$

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