Area Forecast Discussion
Issued by NWS Chicago, IL

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FXUS63 KLOT 261807
AFDLOT

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

.UPDATE...1003 AM CDT

Earlier this morning a couple clusters of showers and
thunderstorms were moving through Northern Illinois. One had
lifted North across Winnebago county...meanwhile the stronger area
of thunderstorms were positioned along the southern forecast area
and have since pushed East of the area into Central Indiana.
Visible satellite imagery continues to show considerable cloud
cover...however expect a steady erosion to the cloud cover will
take place over the next several hours with mostly sunny skies
developing by this afternoon. The sunshine with temperatures
nearing the lower to middle 80s coupled with dew points in the
lower to middle 60s will result in humid conditions this
afternoon.

For this afternoon the lack of vorticity and well defined forcing
feature does not appear to support much convective growth or
development. PWAT values will continue to nudge up above 1.5
inches...and some guidance indicates that we will approach 2
inches of moisture within the atmosphere later this evening. The
atmosphere will become unstable and there may be an isolated
thunderstorm that could develop...however given the lack of a
sheared environment the greater concern from any thunderstorms
will be brief heavy downpours and possibly a brief wind gusts to
40-50mph.

Beachler

&&

.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 18Z TAFS...

Earlier this morning a couple areas of showers and thunderstorms
were approaching the TAF sites...however one area of thunderstorms
lifted Northwest and dissipated and the other went South and has
since developed into a line of strong thunderstorms stretching
across Southeast Indiana stretching West through Southeast
Missouri. In the next several hours across Northern Il/Northwest
IN quiet weather looks to persist...with visible imagery
indicating a steady erosion to the lower clouds with continued VFR
conds. Most forecast guidance continues to lean towards a dry
afternoon and evening timeframe...with the next potential for
showers and thunderstorms nearing the TAF sites around 9Z Fri at
the earliest...and perhaps around daybreak across the ORD/MDW
sites. This continues to be a low confidence scenario though...and
may end up remaining dry through this period as well. Winds
throughout the TAF time will be generally from a 220 degree
direction trending to 190 with speeds between 7-12kt.

Thunderstorms will increase by late Friday morning through the
afternoon hours...and could easily see the VCSH trending towards
either a PROB30 or likely a TEMPO for thunderstorms Fri aftn at
ORD. This has increased confidence in impacting the TAF sites.

Beachler

&&

.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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