Area Forecast Discussion
Issued by NWS Chicago, IL

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297
FXUS63 KLOT 280544
AFDLOT

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
1244 AM CDT SAT MAY 28 2016

.UPDATE...
851 PM CDT

Scattered showers and thunderstorms continue to lift north across
portions of northeast and east central Illinois...and northwest
Indiana this evening. Only anticipate an occasional rumble of
thunder as the showers have been mainly lightning free through
much of the evening...though have slightly intensified over the
Chicago metro area this past hour. Outflow associated with the
showers will also result in a period of stronger southerly winds
gusting 30 to 40 mph. No significant changes were made to the
going forecast.

BMD

&&

.SHORT TERM...
300 PM CDT

Through Tonight...

MCV from earlier morning activity in central and southern Illinois
is moving northeast through the area this afternoon. This activity
is moving into a weakly unstable environment, but is struggling to
grow into deeper convection given this weaker instability, deep
layer shear under 30 kt, and mid level lapse rates that are poor
as well. Expect shower coverage to hold at its current levels or
increase some in the coming hours, with some isolated to scattered
thunder activity as well. The severe threat today is low and
reflected in the marginal risk for severe storms focused farther
west. The concern for scattered storms this afternoon/evening will
be localized heavy downpours in the moisture laden and weak storm
motion environment...along with some gusty winds to 40 mph.
Slightly better instability is found in east central Illinois and
northwest Indiana.

There is another wave moving through the bootheel of Missouri and
southern Illinois that will rotate northeastward tonight ahead of
the negatively tilted trough in the plains, that will keep the
chance for showers and isolated thunderstorms through the evening
and we may see another uptick in activity during this time, but
likely less so overnight as the wave appears to weaken some and
the focus shifts towards the stronger forcing associated with the
upper trough itself. Confidence on coverage during this time is
medium.

KMD

&&

.LONG TERM...
300 PM CDT

Tomorrow through Friday...

The pattern remains active through late next week. Confidence in day
to day convection is not overly high because forcing is weak in most
cases. The atmosphere looks relatively primed for convection,
especially in the near term, but have less confidence in forcing.
Confidence decreases rapidly Thursday night through next Friday due
to model differences.  Highs will remain in the 80s through most of
next week. Lake breezes are possible next week leading to highs in
the upper 60s along the lake.

Saturday is looking very similar to today as far as precip is
concerned. A weak vort streamer lifts north through the day and
expecting convection to increase in coverage to scattered in the
afternoon. Kept PoPs in the generic chance range since the pop up
convection will be hard to catch deterministically in the grids.
Main threat will be heavy rain and perhaps a few will produce gusty
winds. CAPE values look to be less than 1000 J/kg with weak shear.
Pwat values remain high though around 1.75 inches. Storm motion
looks a little better with southwest to northeast movement at around
30-40 kt.

Convection dies off diurnally Saturday night...but left over
convection from the west shifts east through the night. Expecting
the storms to diminish as they do so. Could see some gusty winds as
the storms first breach the western CWA, as depicted by SPC`s day 2
marginal risk.  Areas along and west of I-39 are included in the
marginal risk.

Guidance differs more than I would like with the main upper level
trough Sunday. The GFS is further south than the other models and
thinking precip will be closely tied to the trough. The better storm
coverage will be over Wisconsin...but storms are possible mainly
north and along I-88. Kept a slight chance along and north of I-80
in case the GFS is right. Lifted Pops even further north Sunday
afternoon as the trough lifts northeast with a slight chance along
the IL/WI state line and east of I-55.  CAPE values are forecast to
be below 1000 J/kg again with even weaker shear so not expecting any
severe storms.  Forecast Pwat values are around an inch...but
thinking storms still may produce heavy rain.

We get a break in convection Sunday night through Monday morning!
The GFS and ECMWF bring convection associated with a shortwave into
north central IL Monday afternoon. Have medium-low confidence in the
convection making it into the CWA so capped PoPs in the low chance
range. The GFS is more aggressive than the ECMWF so also limited how
far east the precip gets.  Monday should be mainly dry for most if
not all locations.

Tuesday may be dry, but the GFS features some upper level vort
streamers that may help convection get going. Kept a chance of
thunderstorms in the forecast but have low confidence in PoPs
through the afternoon. Guidance is consistent with additional
convection proceeding an approaching cold front passing through
Wednesday night and Thursday. The GFS is still the most aggressive
with the precip since it has a stronger second upper level low
pressure trough over the plains.  Have low confidence in PoPs
Thursday night into Friday given the drastic differences in
guidance`s upper level patterns. The GFS has a cutoff low over
Kansas while the ECMWF has a closed low over Ontario and a trough
axis moving through the region late it in the week.  Moral of the
story is the pattern remains busy but exact details mid to late next
week are very fuzzy.  Temps in the 80s look solid...with a psbl cool
down into the 70s late next week.

JEE

&&

.AVIATION...
For the 06Z TAFs...

The main concern through the TAF period is with timing the
potential for thunderstorms.

Deep south-southwest flow will persist across the area through
Saturday evening...with warm...moist air mass remaining in place
across the terminals. Forecast soundings indicate this air mass
will become conditionally unstable by this afternoon...with a low
potential for isolated shra/tsra hard to rule out at any particular
time given the potential for subtle minor amplitude mid-level
disturbances to ripple northward ahead of a deeper upper trough
over the central Plains. It appears that Saturday evening will see
the greatest potential for more organized shra/tsra however...as a
stronger mid-level disturbance wraps around the base of the trough
to the west of the area and rotates northeastward into the upper
Midwest. With late afternoon and early evening peak in diurnal
instability and aforementioned forcing aloft...model guidance has
been relatively consistent in depicting thunderstorm development
to the west of the terminals across eastern IA..southwest WI and
western IL. KRFD...in closer proximity to convective development
late in the afternoon likely has a bit higher threat of tsra by
early evening. Threat for Chicago area terminals appears to be
more mid-late evening with model guidance indicating an decaying
line of tsra approaching by 03-04z. While signal for decaying
convection farther to the east is consistent with several models
and over successive runs...forecast soundings indicate sufficient
instability for tsra to continue to move east into Chicago area
should an organized outflow or MCV provide forcing for continued
eastward development. Brief MVFR cig/vis conditions would be
likely with thunderstorms.

Otherwise...south winds will likely gust 20-25 kts later this
morning through this afternoon.

Ratzer

&&

.MARINE...
304 PM CDT

We have expanded the dense fog advisory for the northern two
thirds of the lake into Saturday as warm moist air runs over the
colder lake. It is not until midday before guidance begins to
erode the fog over the lake. Have kept a single advisory through
mid afternoon for the area for now, though it could erode a bit
sooner in the south and make take a bit longer for the north
portion of the lake. Otherwise, expect some showers and
thunderstorms through this evening, with lower coverage in the
coming days.

Otherwise low pressure will continue to slowly move toward Lake
Superior through the weekend before a weak cold front shifts winds
to light northerly with high pressure.

KMD

&&

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

$$

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