Area Forecast Discussion
Issued by NWS Chicago, IL

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FXUS63 KLOT 261140
AFDLOT

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

.SHORT TERM...
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 temperatures...so 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 weak...is 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.

MTF

&&

.LONG TERM...
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
range...so 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.

MTF

&&

.AVIATION...
FOR THE 12Z TAFS...

640 am...Broken line of thunderstorms extends from southwest WI
through the Quad Cities with more widespread thunderstorms across
central IL. Short term/Hi res guidance has been showing this
activity weakening as it moves east across north central IL over
the next few hours. However...this trend has not yet emerged and
some of the activity near the Quad Cities has strengthened. With
uncertainty regarding how long this activity will persist before
weakening...opted to include a short tempo for thunder at all the
terminals this morning. Wind shift across far northeast IA and
southern WI will likely make it into rfd with a period of stronger
westerly winds.

Low confidence regarding thunderstorm chances after this morning.
Convection over the central plains later today and this evening
will likely move east tonight and possibly into northwest or
northern IL overnight or toward daybreak Friday. Despite the low
confidence opted to include prob mention at rfd toward sunrise
Friday morning.

South/southwest winds around 10kt will likely increase into the
12-15kt range this afternoon as skies are expected to clear and
low levels mix. Its possible wind speeds may be a bit higher and
gusts into the mid/upper teen range will be possible. Winds will
settle back to light southerly this evening. Some light fog will
also be possible tonight especially if skies are generally clear.
cms

&&

.MARINE...

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

&&

.LOT WATCHES/WARNINGS/ADVISORIES...
IL...NONE.
IN...NONE.
LM...Dense Fog Advisory...LMZ777-LMZ779 UNTIL 9 AM Friday.

&&

$$

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