Area Forecast Discussion
Issued by NWS Chicago, IL

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FXUS63 KLOT 280806

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

306 AM CDT

Through tonight...

The main weather concerns are for the potential of some
strong thunderstorms over portions of the area...especially this

a few widely scattered showers and perhaps a thunderstorm will be
possible today. This may end up especially being the case east of I-
57 as some weak mid level disturbances shift northward over the
area. However...most areas look to be dry for much of the day...and
as a result...I have lowered POPS to just slight chances in most
areas. However...the chances of more storms will be gradually
increasing over north central and northwestern Illinois late this
afternoon...and especially this evening.

An upper level trough...currently over the Plains...will gradually
shift northeastward to the upper Mississippi Valley by this evening.
This system will likely spawn additional thunderstorms to our west
over eastern Iowa this close proximity to the
entrance region of an 80 KT upper level jet...and as diurnal
destabilization maximizes. This activity then looks to progress
eastward into northern Illinois into the early evening. The main
question with this activity is how far eastward will it progress
before weakening. Many of the Convective Allowing Models
(CAMs)suggest this activity will weaken quickly through the evening
as the storms shift towards northeastern Illinois. While this
certainly seems plausible given the poor diurnal timing...I still
have mentioned higher end chance POPS into the Chicago area later
this evening as I think some storms could still persist this far

Some stronger storms will be possible early this
evening...especially over north central and northwestern Illinois.
Farther east...the expected weakening trend with the convection
should limit the severe threat. These storms could present a hail
risk...along with some locally strong wind gusts given the higher
DCAPE values. This activity looks to wane during the late evening
and overnight as it shifts eastward.



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.



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 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. 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.



214 AM CDT

Relatively quiet period for marine weather...with the only issues
being some lingering dense fog over northern parts of the lake and
breezy south winds today and tonight. No recent visibility
observations...though model forecasts indicate fog should become
less widespread with time this morning/today. Will maintain
current dense fog advisory into this afternoon... though would
expect it could be cancelled earlier.

A diffuse area of low pressure was over the central Plains early
this morning...with a warm front arcing across the upper Midwest
and about two thirds of the way up the lake. This low will lift
slowly to the northeast over the weekend...reaching the upper
Mississippi Valley by this evening and moving across far western
Lake Superior by early Sunday. As the low...and the warm front...
move north...winds will shift to the south across the lake. Winds
will gust into the 20-25 kt range today...and 15-20 kt later
tonight and Sunday. Winds across WI and IL will shift west-
southwest behind a cold front on Sunday with gusts 20-25 kt also
affecting the immediate western shore of the lake. Cool water and
warm air will make for a stable marine boundary layer over the
open waters resulting in lighter winds away from shore especially
on the southern part of the lake. The low will continue to pull
slowly away to the northeast of the Great Lakes early next week
with a weaker surface pressure gradient across the lakes bringing
lighter winds. Another low may lift across the upper Midwest mid-
week...with another frontal passage Wednesday or Wednesday night.






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