Area Forecast Discussion
Issued by NWS Chicago, IL

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FXUS63 KLOT 261528
AFDLOT

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE CHICAGO/ROMEOVILLE IL
1028 AM CDT TUE MAY 26 2015

.UPDATE...
1007 AM CDT

A SFC LOW IS OVER IOWA AND ITS WARM/STATIONARY FRONT IS WELL TO
OUR NORTH OVER CENTRAL WI. ITS COLD FRONT STRETCHES FROM THE LOW
THROUGH CENTRAL KANSAS. AN AREA OF MAINLY RAIN AND A FEW
THUNDERSTORMS IS SLOWLY SPREADING NNE ACROSS NORTH CENTRAL IL. SO
FAR THE SHOWERS HAVE BEEN BEHAVING ACROSS MY CWA...BUT A FEW CELLS
TO THE WEST HAVE SHOWN SIGNS OF ROTATION.

LIKING THE LATEST RAP GUIDANCE WHICH KEEPS CAPE VALUES ON THE LOW
SIDE WITH ONLY A FEW HUNDREDS J/KG OF CAPE TO WORK WITH. THIS IS
ASSUMING SKIES STAY OVERCAST...AND THINKING WE WILL BE OVERCAST
TODAY. THAT BEING SAID...ANY LOCATIONS THAT SEE SUNSHINE COULD
DEVELOP ENOUGH CAPE TO SUPPORT TALLER CELLS AND POSSIBLY SEVERE
WEATHER. IN GENERAL THINKING DAMAGING WINDS AND AN ISOLATED
TORNADO ARE THE MAIN THREATS...BUT STRONGER STORMS WITH HIGHER
CAPE VALUES COULD SUPPORT HAIL. RADAR IMAGERY IS SHOWING GRAVITY
WAVE TAGS MOVING ALONG THE LINE OF STORMS AND WHEN THE TAGS
INTERSECT THE LINE...STORMS BEGIN TO SPIN. THEREFORE THE TORNADO
THREAT IS NOT ZERO.

THIS FIRST BATCH OF PRECIP WILL PUSH EAST THROUGH THE EARLY AFTN
HOURS AND EXPECTING ADDITIONAL SCATTERED TO NUMEROUS THUNDERSTORMS
DEVELOPMENT BEHIND THE LINE OVER NORTH CENTRAL IL. THE SECOND
WAVE SHIFTS EAST THROUGH THE EARLY EVENING HOURS. PWAT VALUES ARE
1.2-1.5 INCHES SO HEAVY DOWNPOURS ARE PSBL.

JEE

&&

.SHORT TERM...
319 AM CDT

THROUGH TUESDAY NIGHT...

FOR THE REMAINDER OF THE OVERNIGHT HOURS...CONDITIONS WILL REMAIN
QUIET ACROSS THE CWA AS A SERIES OF SHORT WAVES CONTINUE TO TRACK
NORTH AROUND THE TOP OF A PERSISTENT UPPER LEVEL RIDGE WHICH EXTENDS
FROM THE SERN CONUS THROUGH THE UPPER MISSISSIPPI VALLEY.  THIS
UPPER RIDGE HAS BEEN THE FEATURE WHICH HAS CONTROLLED THE PATH OF
THE SERIES OF SHORTWAVES LIFTING OUT OF THE SOUTHERN PLAINS...AND
WILL CONTINUE TO DO SO THROUGH THE DAY TODAY. SLY...MODERATELY
STRONG...WINDS WILL PERSIST THROUGH THE NIGHT...AND MUCH OF THE DAY
ALLOWING FOR SFC DEWPOINTS TO REMAIN CONSISTENT OR POSSIBLE INCH A
LITTLE HIGHER.

MAIN CONCERN FOR THE MORNING HOURS WILL BE THE TRACK OF A STRONG
SHORT WAVE AND ASSOCIATED SFC LOW LIFTING NORTHWARD THROUGH THE
MIDDLE MISSISSIPPI VALLEY.  THIS FEATURE WILL PREDOMINANTLY IMPACT
ERN IOWA AND WRN IL BY ARND DAYBREAK.  THROUGH THE MORNING...THE
COMBINATION OF ISENTROPIC LIFT IN THE WARM SECTOR OF THE
SYSTEM...ALL OF IL AND INTO NWRN IN...WILL LEAD TO SOME
DESTABILIZATION ACROSS THE REGION. A SFC TROUGH...AS WELL INDICATED
BY THE LATEST HIGH RES GUIDANCE...WITH LIFT THROUGH NRN IL/IN
THROUGH THE DAY...COINCIDENT WITH A STRONG SSWLY LOW LEVEL JET OF 45-
50 KT...WITH PROVIDE THE POTENTIAL FOR SHOWERS AND STRONG TO SEVERE
THUNDERSTORMS ACROSS THE CWA.  BASED ON WIND PROFILES AND MID LEVEL
DRYING...THE PRIMARY CONCERN WITH REGARD TO SEVERE WEATHER WILL BE
STRONG WIND GUSTS.  HOWEVER...SOME OF THE HIGH RES SFC WIND GUIDANCE
SUGGEST SOME SLIGHT BACKING OF SFC WINDS FROM 180-190 DEGREES TO 160-
170 DEGREES...WHILE WINDS ALOFT REMAIN SWLY.  THIS SLIGHT BACKING OF
THE SFC WINDS COULD PROVIDE ENOUGH DIRECTIONAL SHEAR THROUGH THE
LOWER LEVELS TO BRING AN ISOLD TORNADO THREAT. LOCALLY HEAVY
RAINFALL WILL ALSO BE CONCERNED WITH PWATS REACHING 1.75 INCHES
LATER THIS MORNING.  HOWEVER...FLOODING WILL LIKELY NOT BE A
SIGNIFICANT CONCERN AS THE SWLY WINDS RIGHT OFF THE DECK WILL RANGE
FROM ARND 45 KT ABOVE THE BOUNDARY LAYER TO ARND 90KT AT UPPER
LEVELS WHICH SHOULD ALLOW ANY OF THE STRONGER STORMS THAT DO DEVELOP
TO MOVE QUICKLY.

BY EARLY AFTERNOON...THE MAIN SHORT WAVE WILL LIFT INTO WISCONSIN
WHILE A SFC TROUGH TRACKS EAST ACROSS THE AREA.  THIS WILL LEAD TO
WINDS AT THE SFC VEERING TO SWLY...SETTING UP A UNIDIRECTIONAL WIND
PROFILE...EFFECTIVELY MINIMIZING THE SEVERE WEATHER THREAT TO THE
REGION.  BY LATE AFTERNOON AND INTO THE EVENING THE MODELS ARE
ADVERTISING THAT A DRY SLOW SHOULD NOSE INTO NRN IL...EFFECTIVELY
CUTTING OFF THE PCPN AREA INTO 2 SEPARATE AREA...ONE OVER WISCONSIN
AND ERN IOWA AND ANOTHER MORE FOCUSED OVER INDIANA.  THE NWRN
INDIANA COUNTIES WILL STILL HAVE A CHANCE FOR SOME SHOWERS AND
THUNDERSTORMS INTO THE EVENING...WHILE PCPN CHANCES OVER NRN IL
BEGIN TO WANE.  MUCH OF THE MODEL GUIDANCE ALSO INDICATED THAT THE
SHORTWAVE SHOULD SLOW IT`S PROGRESSION THROUGH WISCONSIN AS IT
APPROACHES THE APEX OF THE UPPER RIDGE.  AN ASSOCIATED SFC
TROUGH/COLD FRONT EXTENDING SOUTHWEST FROM THE SFC LOW WILL LINGER
OVER ERN IOWA IN THE EARLY EVENING BEFORE STEADILY PUSHING ACROSS
NRN IL AND INTO NWRN IN OVERNIGHT.  THROUGH THE NIGHT...THE
STRONGEST UPPER LEVEL FORCING WILL BE WELL NORTH OF THE CWA...SO ANY
LINGERING CHANCES FOR PCPN OVERNIGHT WILL BE LOW AND LIKELY CONFINED
TO THE NRN TIER COUNTIES OF IL...ALONG THE WI BORDER.

KREIN

&&

.LONG TERM...
229 AM CDT

WEDNESDAY THROUGH MONDAY...

WEDNESDAY THROUGH THURSDAY...
OPEN 500MB WAVE WILL BE LIFTING NORTHEAST ACROSS THE UPPER MIDWEST
INTO ONTARIO AND EVENTUALLY QUEBEC...WITH HEIGHTS SLOWLY INCREASING
WED AFTN. SFC FEATURES REMAIN DISORGANIZED WED...WITH A BOUNDARY
SLOWLY SHIFTING EAST. OPER GUIDANCE DIFFERS ON HOW QUICKLY PRECIP
WILL DEPART THE REGION. ENSEMBLE DATA SUGGESTS HOLDING ONTO CHC TO
SLIGHT CHC POPS MAINLY FOR THE CENTRAL/EASTERN CWFA WED/WED EVE.
ALTHOUGH WITH SOME HINT OF SFC HEIGHTS NUDGING UP...IT IS POSSIBLE
THAT SOME DRY AIR COULD FILTER IN AND KICK PRECIP EAST BY MIDDAY AND
ALLOW FOR A MAINLY DRY WED. SFC WINDS WILL BE TURNING SOUTHWEST TO
WEST...KEEPING AN OFFSHORE FLOW FOR LOCATIONS CLOSE TO LAKE
MICHIGAN. TEMPS SHOULD YET AGAIN TOUCH UPR 70S TO ARND 80 WED...IF
CLOUDS AND PRECIP CAN PUSH EAST BY MIDDAY...SOME CLEARING WOULD
ALLOW SFC TEMPS TO PUSH A COUPLE DEGREES WARMER INTO THE LOW 80S IN
SOME LOCATIONS. LATE IN THE EVENING THE GRADIENT SLACKENS
FURTHER...AND COULD ALLOW A LATE DAY ONSHORE FLOW TO DEVELOP FOR
AREAS ADJACENT TO LAKE MICHIGAN.

MID-LVL FLOW BEGINS TO FLATTEN AS A WEAK SFC RIDGE BECOMES ORIENTED
FROM WESTERN QUEBEC SOUTH THROUGH THE EASTERN OHIO RIVER VALLEY.
THIS WILL GENERALLY ALLOW A SOUTHERLY FLOW...ALTHOUGH THE GRADIENT
IS NOT VERY STRONG. THIS SHOULD SET THE STAGE FOR A LAKE
BREEZE/ONSHORE FLOW TO DEVELOP THUR. TIMING COULD END UP BEING
SLIGHTLY EARLIER GIVEN THE LIGHT SFC GRADIENT. GUIDANCE HAS BEEN
CONSISTENT IN HOLDING PRECIP WEST/SOUTH OF THE FORECAST AREA
THUR...AS A RESULT OF THE WEAK SFC RIDGING. SO ONE MORE MILD DAY
WITH HIGHS WARMING BACK INTO THE UPR 70S/LOW 80S...HOWEVER AS
PREVIOUSLY NOTED...A LAKE BREEZE IS LIKELY TO DEVELOP. THIS WILL
BLEED THE MUCH COOLER MARINE AIR WEST ACROSS NORTHEAST IL AND
DOWNTOWN CHICAGO. IF THE TIMING IS EARLIER IN THE DAY THUR...TEMPS
MAY NOT WARM BEYOND THE LOW/MID 60S ALONG THE CHICAGO LAKEFRONT.

FLOW THEN TURNS SOUTHERLY THUR NIGHT WITH THE MORE PROGRESSIVE FLOW
IN THE MID-LVLS PUSHING THE RIDGING EAST. A RETURN FLOW MOISTURE
WILL CONTINUE TO BE FOCUSED ACROSS THE CENTRAL CONUS THUR EVE...WITH
A SLOW MOVING FRONTAL BOUNDARY NUDGING EAST THUR NIGHT TOWARDS
WESTERN IL. THIS FRONTAL BOUNDARY WILL BECOME ENERGIZED AS A ROBUST
500MB VORT MAX BREAKS SOUTH FROM CENTRAL CANADA LATE THUR NIGHT.

THIS SHOULD ALLOW FOR AN ATMOSPHERIC RIVER TO DEVELOP FROM THE
SOUTHERN PLAINS STRETCHING NORTH THROUGH THE MID-MISSOURI VALLEY
INTO THE WESTERN GREAT LAKES EARLY FRI MORNING.

FRIDAY THROUGH EARLY NEXT WEEK...
ENSEMBLES CONTINUE TO PROG A ROBUST TROUGH SLIDING SOUTH FROM
CENTRAL CANADA FRI. THIS COUPLED WITH DOWNSTREAM MID-LVL RIDGING
ACROSS THE SOUTHEAST CONUS WILL STRENGTHEN THE FRONTAL BOUNDARY
THAT WILL BE ORIENTED FROM THE SOUTHERN PLAINS STRETCHING
NORTHEAST THROUGH THE GREAT LAKES REGION INTO WESTERN QUEBEC.

THIS SETUP IS IDEAL FOR THE DEVELOPMENT OF AN ATMOSPHERIC RIVER WITH
ABUNDANT LOW LEVEL MOISTURE AND EFFICIENT PRECIP PRODUCING
CONVECTION.

OPER GUIDANCE HAS BEEN RELATIVELY CONSISTENT WITH BRINGING A SERIES
OF WAVES ALONG THE SLOW MOVING FRONTAL BOUNDARY FRI AFTN/EVE...THEN
THE BOUNDARY SHOULD SLIDE EAST WITH WINDS QUICKLY VEERING
NORTH/NORTHEAST FOR SAT AND USHERING IN MUCH COOLER AIR TO THE
REGION.

CURRENT INDICATIONS ARE THAT HIGHS SAT MAY STRUGGLE TO WARM BEYOND
THE UPR 50S ACROSS NORTHEAST IL/NORTHWEST IN...WITH ONLY LOW 60S
ELSEWHERE. STRONG SFC RIDGE WILL REMAIN NORTH OF THE REGION...SO THE
PRIOR FRONTAL BOUNDARY MAY BECOME QUASI-STATIONARY FOR THE WEEKEND
ALONG THE OHIO VALLEY AND CONTINUE TO FEED PRECIP CHANCES TO THE
SOUTHERN CWFA.

BEACHLER

&&

.AVIATION...

//ORD AND MDW CONCERNS...UPDATED 14Z...

* MVFR CEILINGS CONTINUE THIS MORNING.

* PERIODS OF -SHRA WITH PERIODIC TSRA THIS AFTERNOON.

* SOUTH TO SOUTH-SOUTHWEST WINDS INCREASING THROUGH THE MORNING TO
  GUST OVER 20KT

KREIN/MM

//DISCUSSION...UPDATED 12Z...

A MID LEVEL DISTURBANCE MOVING FROM THE SOUTHERN PLAINS TO NRN IL
BY THIS AFTERNOON MAY BRING ANOTHER ROUND OF SHOWERS WITH SCT
THUNDERSTORMS TO THE REGION. UPPER LEVEL FORCING FROM THE MID
LEVEL DISTURBANCE AND AN UPPER LEVEL JET STREAM WILL ENHANCE THE
LIKELIHOOD FOR PCPN AND THE CHANCE FOR TSRA...WITH THE MOST
LIKELY CHANCE FOR ACTIVITY FROM LATE MORNING THROUGH EARLY
EVENING. SOME UNCERTAINTY REMAINS IN PINNING DOWN THE EXACT
PERIOD FOR THE TSRA...SO WILL MAINTAIN THE GOING TEMPO FOR LATE
MORNING THROUGH EARLY AFTERNOON. HOWEVER...THE PCPN TIMING COULD
BE SHIFTED OR EXPANDED IN TIME.

LOW LEVEL MOISTURE STREAMING NORTHWARD AHEAD OF LOW PRESSURE OVER
THE MID MISSISSIPPI VALLEY HAS BROUGHT A LOW MVFR STRATUS DECK
ACROSS THE REGION. SOME SITES ARE REPORTING CIGS DROPPING INTO IFR
RANGE...SO WILL INCLUDE THE POSSIBILITY IN A TEMPO GROUP THROUGH
LATE MORNING. DAYTIME WARMING SHOULD ALLOW FOR INCREASE DEEP LAYER
MIXING WHICH SHOULD SCATTER OUT THE STRATUS DECK.

WINDS WILL BE GENERALLY SOUTHERLY TODAY...THOUGH DIRECTION MAY BE
A BIT VARIABLE AROUND CONVECTIVE ACTIVITY. SOME GUSTINESS IS
LIKELY TO BEGIN BY LATE MORNING...WITH THE LATEST GUIDANCE
SUGGESTING THAT WINDS COULD STRENGTHEN AND BECOME EVEN MORE GUSTY
TONIGHT WITH WINDS VEERING SWLY AS SURFACE LOW PRESSURE MOVES
NORTHEAST ACROSS SOUTHERN WISCONSIN.

KREIN

//ORD AND MDW CONFIDENCE...UPDATED 14Z...

* HIGH CONFIDENCE IN WIND TRENDS TODAY. MEDIUM IN WIND TRENDS
  TONIGHT.

* HIGH CONFIDENCE IN MVFR CIGS THIS MORNING.

* HIGH CONFIDENCE IN SHOWERS AND MEDIUM CONFIDENCE OF A PERIOD OF
  TSRA. LOW CONFIDENCE IN SPECIFIC TSRA TIMING AND IMPACT ON THE
  TERMINALS WITH MULTIPLE ROUNDS POSSIBLE BETWEEN 18Z AND 02Z.

KREIN/MM

//OUTLOOK FOR ORD/MDW FOR 00Z THURSDAY-12Z TUESDAY...UPDATED 12Z...

WEDNESDAY NIGHT...WX NIL/VFR. SOUTHEAST WINDS.

THURSDAY...SLIGHT CHANCE OF TSRA AT NIGHT. SOUTHEAST WINDS.

FRIDAY...TSRA/SHRA LIKELY THROUGH THE DAY AND NIGHT. MVFR
POSSIBLE. SOUTHWEST WINDS.

SATURDAY...SHRA LIKELY WITH A CHANCE OF TSRA DURING THE DAY.
NORTHEAST WINDS.

SUNDAY...CHANCE OF SHRA. EAST WINDS. MVFR POSSIBLE.

MONDAY...CHANCE OF SHRA. SOUTHEAST WINDS. MVFR POSSIBLE.

KREIN

&&

.MARINE...
229 AM CDT

WARM MOIST AIR CONTINUES TO FEED NORTH ACROSS THE LAKE
TODAY...WHICH MAY HELP TO FURTHER ENHANCE ANY AREAS OF FOG THAT
HAVE ALREADY DEVELOPED. IT IS POSSIBLE PATCHY DENSE FOG MAY
DEVELOP...ALTHOUGH WINDS MAY BE JUST STRONG ENOUGH TO LIMIT THE
COVERAGE AND INTENSITY OF THE FOG. A SURFACE LOW PRESSURE SYSTEM
WILL BE LIFTING NORTH ACROSS EASTERN IOWA INTO CENTRAL WISCONSIN
THIS EVENING. THE GRADIENT WILL TIGHTEN ACROSS THE
CENTRAL/SOUTHERN PORTIONS OF THE LAKE...WHICH WITH WARMER
NEARSHORE WATERS EXPECT SOME SLIGHTLY BETTER MIXING TO OCCUR.
FURTHER OUT ACROSS THE OPEN WATERS THE WATER REMAINS COLD AND
EXPECT THE STABILITY TO MAKE IT DIFFICULT TO MIX TO THE SFC THE
HIGHER GUSTS. HIGH PRESSURE THEN ARRIVES LATE WEDNESDAY WITH LIGHT
WINDS DEVELOPING. THEN HIGH PRESSURE SHIFTS EAST THURSDAY
NIGHT...WITH SOUTHERLY WINDS DEVELOPING AGAIN FRIDAY. A FRONTAL
BOUNDARY WILL APPROACH LAKE MICHIGAN LATE FRIDAY NIGHT THEN
STEADILY SLIDE SOUTH ALONG THE LAKE. THIS WILL ALLOW WINDS TO
QUICKLY VEER FROM SOUTH TO NORTH/NORTHEAST SATURDAY. ALONG WITH
THIS QUICK WIND SHIFT WILL BE COOLER AIR SLIDING SOUTH OVER THE
LAKE. WAVES SHOULD EASILY BUILD SATURDAY FOR THE CENTRAL/SOUTHERN
PORTIONS OF THE LAKE.

BEACHLER

&&

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

$$

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