Area Forecast Discussion
Issued by NWS Chicago, IL

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789
FXUS63 KLOT 270901
AFDLOT

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
401 AM CDT FRI MAY 27 2016

.SHORT TERM...
401 AM CDT

Today through Sunday...

Forecast challenges focus primarily on attempting to identify
timingof shower and thunderstorm clusters through the Memorial Day
holidayweekend. High perceptible water air mass and slow storm
movementwill present the potential for locally heavy rainfall with
thesestorm clusters.

Upper air analysis and GOES water vapor imagery depict a broad
uppertrough across much of the central and western CONUS this
morning...with a closed upper low over eastern Colorado.
Downstream to theeast...upper ridging was noted over the Atlantic
and across the mid-Atlantic and southeastern states. Model
guidance is in goodagreement in depicting amplification of the
upper ridge across thenortheast through the weekend...while the
Plains upper trough takeson a negative-tilt orientation and slowly
lifts northeast toward theupper Mississippi Valley Saturday and
eventually across westernOntario and the western Great Lakes
through Sunday. Anelongated area of surface low pressure from the
Texas Panhandle intosouth-central Kansas is progged to move slowly
northeast across theupper Mississippi Valley and northern Lakes
region in associationwith the upper low. East of the track of the
upper trough/surfacelow a very warm and moist will remain in place
across the region...with embedded smaller-scale short wave troughs
and MCV featuresmodulating areas of convection. Identifying and
timing of these moretransitory features...while difficult...will
be key to attempting toadd some degree of detail to timing the
most likely periods ofconvection to affect the forecast area
through the period.

In the near-term...one such feature is noted over
northeasternMissouri and western Illinois early this morning. SPC
RAP-basedmesoanalysis data depicts this activity occurring within
an elevatedwarm frontal zone...featuring 25-30 kt south-southwest
flow andstrong moisture advection and frontogenesis within region
of 500-1000 J/kg of MUCAPE. High-res guidance including the HRRR
and 4kmNAM indicate this activity will continue to lift to the
north-northeast early this morning...though with weakening trend
noted inforecast reflectivity fields. Current radar trends suggest
greatestpotential for convection early this morning will likely be
forwestern portions of the cwa...where weakening but
persistentmoisture transport is depicted. With persistent
warm/moist advectionit is hard to rule out additional scattered
convection at just aboutany time today...though it appears that
this afternoon and eveningwill be the next period with a slightly
greater potential forconvection given diurnal instability max and
guidance indications ofenergy from another mid-level wave lifting
out of Arkansas. Afterthis evening...have generally maintained low
chance pops intoSaturday before more organized forcing develops
late Saturday andSaturday evening...as more significant height
falls develop acrossthe upper Midwest with the approach of the
negatively tilted uppertrough...and a cold front which trails the
surface low and pushesacross the forecast area overnight. For this
period...have carriedhigh chance/likely pops particularly across
the western portions ofthe cwa where forcing and frontal passage
will be juxtaposed moreclosely with the diurnal instability
maximum during the afternoonand evening hours. Increasing
mid/upper level wind speeds will yieldstronger deep-layer bulk
shear at this time as well...supportingorganized storms and the
potential for strong to severe storms. SPCday 2 outlook does bring
the marginal severe risk into the westerncounties of the forecast
area. Precip threat then looks to diminishsubstantially Sunday and
Sunday night behind the cold front.

Temperatures through the weekend are expected to continue to
beabove average for late May...with low level thermal
fieldssupporting low-mid 80s for highs today and Saturday.
Considerablecloud cover and the prospects for areas of precip will
tend tomoderate the warmth just a bit however...and have carried
lower 80sover much of the area per slightly warmer GFS MAV
guidance. Despiteslightly cooler low level temps Sunday...more sun
with drierwesterly flow behind the front should still yield lower
80s in mostareas.

Ratzer

&&

.LONG TERM...
257 PM CDT

Friday night through Thursday...

The extended continues to look busy with a chance of thunderstorms
every day.  High temperatures will be in the 80s each day...but lake
side cooling develops next week.  Could see even warmer high temps
if we get more sun than forecast.

For thunderstorms...forcing will be the big question each day.
Thinking we will have leftover convection spread along and west of I-
55 Friday night and then dissipate Saturday morning.  A weak vort
streamer swings through the region Saturday while the main short
wave stays well west of the region over SD and NE.  Cannot rule out
a few thunderstorms so kept a chance of thunderstorms going in the
forecast.  Forecasted CAPE looks skinny and we are also lacking
shear. A few storms may become strong...but not expecting any severe
storms.  Pwat values will still be around 1.5 inches and storm
motion will be around 20-30 kt.  Storms will be probably produce
heavy rain but should be moving quick enough to avoid flooding.
However...a few training storms could produce localized flooding
like we saw yesterday.

The story looks similar for next week with no clear forcing but
ample CAPE and moisture.  Lingering convection weakens as it spreads
over the region Saturday night and once again expecting better
thunderstorm coverage along and west of I-55.  The big questions
Sunday will be how quickly we recover from the morning convection
and how much forcing we will get from the shortwave/vort streamer
that rotates through. Forecast soundings feature a decent cap and
little to no shear again.  If we recover...could see better
thunderstorm coverage than what is currently forecast.  Still only
expecting a few stronger storms due to the lack of shear.  Thinking
there will be better thunderstorm coverage over WI closer to the
shortwave.

Thinking Monday will be dry so backed off on precip chances. A
better chance of thunderstorms arrives ahead of a cold front Tuesday
and more likely Tuesday evening/night. Long range guidance does
differ with the exact path of the low next week so have low
confidence in timing and details of storms from Tuesday onward.

JEE

&&

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

The main forecast concern will be with the potential and timing
of any thunderstorms over the area today.

An atmospheric disturbance is currently driving a cluster of showers
and storms to our southwest over northeastern MO and into portions
of west central IL. This activity is expected to shift to the
northeast through the night...but will likely do so in a weakening
fashion. Remnants of this activity could move over the terminals
after daybreak today...but at this time...I only prefer to mention a
VCSH.

The convective picture looks a bit murky through this afternoon and
evening. The only real feature of interest...which could drive a
period of thunderstorms over the area later this afternoon...is an
upper level disturbance currently noted over the ARKLATEX region.
Forecast guidance suggests this disturbance will shift
northeastward over northeastern IL later in the day during peak
heating. As such...this could be enough to result in thunderstorm
development over the area after 20Z this afternoon. Overall
confidence is low to medium at best at this juncture. However...I
felt justified to include a PROB30 mention for thunder at the
terminals.

KJB

&&

.MARINE...
159 AM CDT

The main weather concerns over Lake Michigan will be the continued
dense fog today...especially over the northern half of the
lake...and also the potential for more thunderstorms.

Light winds and high a dew point air mass over Lake Michigan is
resulting in dense fog...especially over the northern half of the
lake. It appears that this fog will persist through the day
today...and possibly well into tonight as well. As a result...we
have extended the dense fog advisory through late tonight for the
north end of the lake. It appears that although some fog could
persist over the south end of the lake...that the fog advisory there
may be allowed to expire later this morning.

An area of low pressure over the plains...is expected to shift
northeastward over the upper Midwest late Saturday. This storm
system will result in the possibility of a few waves of showers and
storms over the next couple of days. Additionally...it appears that
south winds will be on the increase into the 15 to 20 KT range for
Saturday and Sunday as the low lifts into Ontario.

KJB

&&

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

&&

$$

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