Area Forecast Discussion
Issued by NWS Chicago, IL

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FXUS63 KLOT 260846

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
346 AM CDT THU MAY 26 2016

328 AM CDT

Through Tonight...

Concerns for today and tonight are once again on thunderstorm
timing and coverage with a more unstable and moist air mass than
yesterday...however less of a forcing mechanism. Have boosted
high temperatures with fairly widespread upper 80s forecast.

Another morning with a couple MCSs to the west over IA and
northern MO. A low amplitude short wave trough has been helping
to sustain these on the nose of a 40 kt low-level jet /LLJ/ and
this feature will continue east-northeast. Recent IR trends have
indicated sustained cloud top expecting at least
a broken line of showers and probably some storms making it into
north central Illinois around 8 a.m. or so. These should be in a
fading state with the LLJ gradually weakening. These very well
could completely fade by Chicago/northwest Indiana later this
morning as they outrun their environmental support.

Convective debris clouds this morning...especially over parts of
northern Illinois...may initially slow the rate of warming. But
the past two days any clouds have done little to prevent readings
from reaching several degrees above guidance. The 925mb
temperatures observed last eve on soundings were 22C-23C. In late
May over the past 30 years readings of 22C or warmer correlate to
a mean high of 88 at Chicago ORD. So feel comfortable with the
upper 80s in most areas with dew points inching into the upper
60s. This will certainly give a muggy feel with apparent
temperatures in the lower 90s.

The high theta-e air warm sector spreading over the region this
afternoon will support large amounts of instability characterized
by MLCAPES in excess of 2500 J/KG...assuming T/Td forecasts are
on track. A cap...albeit expected to be in place along
with limited low-level convergence/focus or a forcing mechanism.
The exception to this may be if there is any lingering weak
forcing or elevated boundary from the morning activity...which
probably would be over the southeast forecast area this afternoon.
The experimental HRRR and a couple other convective allowing
models try to indicate some afternoon development there. In this
high CAPE/PWAT air mass feel comfortable keeping a chance for
isolated storms areawide this afternoon into the evening. Any
that do trigger will need to be monitored for brief large hail
and downbursts...but again that is a conditional threat.

In response to the upper low ejecting from the Four Corners
region tonight...strong moisture transport in the low to mid
levels will likely foster convection across IA and potentially
western IL this evening. Convection should want to grow northeast
within improving upper diffluence and some scattered mainly
elevated storms may develop into the northern IL portion of the
CWA. Confidence on this is low at this time and may depend on just
how much is developing in the Ozarks/MO that could rob our
moisture replenishment this way. Severe potential with any of
these late night storms is also marginal...and would be mainly
brief hail.



345 AM CDT

Friday through Wednesday...

The ejecting upper low as mentioned above will shift a high PWAT
air mass /AOA 1.5 inches/ over the Mississippi River valley by
midday Friday. The challenge is how quickly and how much rain will
spread east as similar upper lows over the past few weeks have
struggled to expand rain into the area within a similar semi-
blocked pattern. The difference with this is a wide open Gulf of
Mexico with a strong fetch of moisture transport. Almost all
guidance provides the western CWA with more rainfall on
Friday/Friday Night than the east and have tightened the gradient
on PoPs and rainfall amounts but feel it will probably need to be
done more so during the day Friday and possibly Friday evening.
Still expect eastern areas to get showers...just not sure how
quickly and it may not be until Friday night or even Saturday
morning. The synoptic setup gets less favorable for severe weather
and more so for heavy rainfall with PWATs approaching the 99th
percentile. The model ensemble spread for rainfall is quite large
over the area...with higher members in the two to three inch will need to monitor hydrology concerns...especially
with some localized amounts over 1.50 inches last night. The
chances for rain and some storms will continue into
Saturday...with more of an unstable air mass trying to push back
into the area...although forcing could be diminishing.

Upper ridging atop a weak surface high is expected to set up by
Sunday. As this draws near and our confidence has increased...we
are able to keep more of the area dry in the forecast for Sunday
and Memorial Day. Highs are forecast in the lower to mid 80s.




1235 am...Ifr/lifr cigs mainly north of ord to pwk are expected to
scatter out as southeasterly winds affected by earlier convection
settle back to southerly over the next few hours so not expecting
prevailing lower cigs at the terminals...though these lower cigs
may persist near the lake. A period of mostly clear skies early
this morning combined with the light winds and higher moisture
should allow some fog to develop. As mid/high clouds spread back
across the area by sunrise...this may prevent any widespread lower
vis/fog from developing. Possible for mvfr cigs toward morning as
clouds around 2kft over east central IL continue lifting north.

General trend from guidance is for dry conditions through the
period. Convection over northern MO appears to track south of the
terminals and weakens later this morning. Convection over
northwest IA will lift northeast and the trailing edge of this
decaying activity may reach rfd after sunrise...perhaps centered
around 14z. Will continue dry forecast there and monitor trends
for any precip mention with the 09z update. Isolated showers or
maybe a thunderstorms can/t be completely ruled out this afternoon
into this evening but expected coverage too low and timing too
uncertain for any mention in this forecast. Next best chance of
any organized thunderstorms appears to be late tonight into Friday
morning but confidence here also low.

Southeasterly winds will turn more southerly but remain light
through sunrise. Winds will then turn more south/southwesterly
during the morning with speeds increasing into the 10-15kt range.
Winds have trended stronger then guidance the past few days and
gusts into the mid/upper teens are possible. Wind direction may
remain southerly enough for a lake breeze right along the IL shore
but not expecting it to reach ord/mdw. cms



142 am...A warm front will continue lifting north across northern
portions of Lake Michigan this morning shifting winds southerly. A
large trough of low pressure is expected to remain across the
plains into Saturday maintaining moist southerly flow across the
lake. This will lead to continued fog development and likely dense
fog today through Friday and a dense fog advisory may be needed.

A weak cold front is expected to move across the lake Saturday
night shifting winds westerly with weak high pressure settling
over the lakes region early next week resulting in winds turning
northeast Monday night or Tuesday. cms


LM...Dense Fog Advisory...LMZ777-LMZ779 UNTIL 9 AM Friday.




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