Area Forecast Discussion
Issued by NWS Chicago, IL

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FXUS63 KLOT 300837

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

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 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 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



246 PM CDT

Monday night through Sunday...

Monday night through Wednesday night:
Mid-lev diffluence will begin to shift East slightly Monday evening;
however, some guidance members continue to indicate enough
ridging/diffluence will hold through Monday evening to hold precip
to the West of the area. Have adjusted POPs to account for better
chance for POPs not arriving until closer to daybreak Tue. Upper
level trough will remain well West through Mon ngt, meanwhile a
displaced surface wave will be trying to develop over the Central
Plains late Monday. This may be strong enough to send a lobe of
vorticity Northeast towards the western Illinois area. Low level
feed of moisture will steadily increase Monday night, helping to
bring dew points back into the lower 60s across Northern Illinois by
Tuesday. With a continued feed of moisture Tuesday, and steady
growth of instability, the periodic chances for
showers/thunderstorms will likely continue Tuesday. Temps Tue will
return to the lower to perhaps middle 80s across much of Northern
Illinois and Northwest Indiana.

Heading into tuesday night, the 500mb trough axis will begin to
pivot east of the Rockies and increasing the forcing along the lee-
side through the Plains. This should likely increase POPs chances
through the mid-sections of the CONUS towards the Western Great
Lakes late Tue into Wed. With the trough axis being slow to arrive
over the forecast area Wed eve, a persistent chance of
showers/thunderstorms is possible. Then the frontal boundary
continues to be progged to arrive late Wed, which will usher in
drier air and bring an end to the precip chances for later in the

Thursday through Saturday: Ensembles continue to point towards a
ridge developing along the western CONUS late in the week into next
weekend. Thursday through Friday look to remain dry with weak
surface ridging arriving, then with the upstream ridge progged to
develop this will likely usher in a quasi-northwest flow for the
upcoming weekend and a return to periodic showers/thunderstorms.
Temps for late week through next weekend will return to the middle
to upper 70s, with overnight lows in the 50s.



For 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.



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.






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