Area Forecast Discussion
Issued by NWS Chicago, IL

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000
FXUS63 KLOT 012205
AFDLOT

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE CHICAGO/ROMEOVILLE IL
505 PM CDT WED APR 1 2015

.SHORT TERM...
322 PM CDT

THROUGH FRIDAY NIGHT...

MAIN FORECAST CONCERNS ARE WITH SHOWER/THUNDERSTORM POTENTIAL
LATE TONIGHT INTO THURSDAY MORNING...AND THEN WITH A SLIGHT
POSSIBILITY FOR A PERIOD OF WINTRY WEATHER FRIDAY.

GENERALLY QUIET CONDITIONS ONGOING THIS AFTERNOON...WITH ABOVE
NORMAL TEMPS AND SUNNY SKIES IN PLACE. SURFACE PRESSURE FALLS WITH
A TIGHTENING SURFACE GRADIENT IS ALLOWING FOR INCREASING SOUTHERLY
FLOW THIS AFTERNOON AND WITH WARM FRONT LIFTING NORTH THROUGH THE
CWA...TEMPS HAVE RISEN TO THE LOW TO MID 70S. COOLER CONDITIONS
ARE STILL IN PLACE ACROSS FAR NORTHEAST ILLINOIS AT THIS TIME BUT
STILL EXPECT A PERIOD OF WARMER CONDITIONS IN THE 60S TO FINALLY
REACH THIS LOCATION LATE THIS AFTERNOON AND EARLY EVENING...AS
WINDS TURN MORE SOUTHERLY AND USHER THE INLAND WARM AIR. DRY
CONDITIONS WITH MAINLY CLEAR SKIES WILL BE IN PLACE THROUGH MUCH
OF THE EVENING WITH EVEN A PERIOD OF STRENGTHENING WINDS
EARLY...AS SYSTEM TO THE WEST APPROACHES.

DONT ANTICIPATE PRECIP TO REACH THE CWA UNTIL AFTER THE MIDNIGHT
TIME FRAME AS FOCUS FOR DEVELOPMENT WILL REMAIN WELL TO THE WEST
AND SOUTHWEST DURING THIS TIME. INITIAL WAVE OF PRECIP WILL
ACTUALLY STEM FROM NORTHWARD LIFTING SHORTWAVE ENERGY CURRENTLY
OVER THE SOUTHERN PLAINS. AS THIS ENERGY LIFTS NORTH TOWARDS THE
CWA...A VEERING AND STRENGTHENING LLJ WILL BE FOCUSED TOWARDS
NORTHERN ILLINOIS. BOTH OF THESE FEATURES WILL AID IN THE
DEVELOPMENT OF SHOWERS ACROSS NORTHERN ILLINOIS TOWARDS THE 7-9Z
TIME FRAME. HAVE MAINTAINED MENTION OF THUNDER TONIGHT AS GUIDANCE
IS INDICATING THAT THE INSTABILITY AXIS WEST OF THE CWA WILL SHIFT
TOWARDS THE CWA LATE TONIGHT. HOWEVER...AS IT SHIFTS EAST...IT
DOES APPEAR AS IT WILL BE SHUNTED MORE TOWARDS THE SOUTH. EVEN IF
THIS WERE TO OCCUR STILL EXPECT AT LEAST WEAK INSTABILITY TO IN
PLACE...ALONG WITH POTENTIAL FOR STEEPER MID LEVEL LAPSE RATES. SO
THIS SHOULD ALL COME TOGETHER FOR THE POTENTIAL OF RUMBLES OF
THUNDER LATE TONIGHT INTO EARLY THURSDAY MORNING.

COVERAGE OF SHOWERS WITH EMBEDDED THUNDERSTORMS SHOULD INCREASE
THROUGH MID TO LATE MORNING THURSDAY AS ANOTHER WAVE OF ENERGY
SHIFTS EAST ACROSS THE UPPER MISSISSIPPI VALLEY...AND WITH SURFACE
TROUGH/FRONTAL BOUNDARY APPROACH THE CWA. AS SURFACE DEWPOINTS
INCREASE OUT OF THIS FRONT...DO FEEL THAT THUNDERSTORMS WILL STILL
BE POSSIBLE ALONG THE FRONT. HOWEVER...ONCE AGAIN...INSTABILITY
REMAINS TO BE WEAK AT BEST AND SKINNY CAPE AS WELL. NOT OVERLY
CONCERNED WITH WIDESPREAD STRONG TO SEVERE THUNDERSTORMS THURSDAY
MORNING ACROSS THE CWA WITH THIS WEAK CAPE IN PLACE. GUIDANCE DOES
INDICATE THAT MOISTURE AND INSTABILITY WILL CONTINUE TO TRY AND
INCREASE THROUGH MID AFTERNOON PRIMARILY ACROSS THE FAR SOUTHEAST
CWA. AS THE FRONT FINALLY PUSHES THROUGH THIS AREA DURING THIS
TIME...CANNOT RULE OUT AN ISOLATED STRONGER STORM. HOWEVER...THIS
BETTER DEVELOPMENT WILL LIKELY END UP OCCURRING JUST SOUTH OF THE
CWA.

GUIDANCE VARIES TO THE SOUTHWARD PUSH OF THIS PRECIP THURSDAY
NIGHT AND HAVE MADE LITTLE CHANGES TO THE GOING FORECAST FOR THIS
PERIOD...WITH HIGH CHANCE POPS FOR ONLY RAIN ACROSS THE SOUTHERN
CWA. ACTIVE MID LEVEL FLOW WILL USHER IN ADDITIONAL SHORTWAVE
ENERGY OUT AHEAD OF DIGGING LONG WAVE TROUGH ON FRIDAY. THIS
SHOULD ALLOW PRECIP TO BLOSSOM OVER THE SOUTHERN HALF OF THE CWA
THROUGHOUT THE DAY. HAVE MAINTAINED RAIN DURING THIS TIME BUT HAVE
SOME CONCERNS THAT SOME SNOW FLAKES COULD BE OBSERVED AS WELL.
COLD AIR FILTERING ON THE BACKSIDE OF THIS SYSTEM ON FRIDAY WILL
ALLOW THE COLUMN TO COOL WITH CRYSTALS PRESENT BY THE AFTERNOON.
BOUNDARY LAYER TEMPS APPEAR TO BE THE LIMITING FACTOR BUT IF
ENOUGH FORCING AND COOLING WERE TO OCCUR...A FEW SNOWFLAKES COULD
BE POSSIBLE LATE IN THE DAY FRIDAY.

RODRIGUEZ

&&

.LONG TERM...
322 PM CDT

SUNDAY THROUGH WEDNESDAY...

UNSETTLED WEATHER TO START THE NEW WEEK WITH SERIES OF WAVES TO
MOVE ALONG A WEST-EAST ORIENTED FRONTAL BOUNDARY TO BE IN THE
VICINITY THROUGH THE PERIOD. INITIALLY SPRAWLING HIGH PRESSURE
ACROSS THE OHIO VALLEY AND CONTINUED RISING HEIGHTS ALOFT AS
RIDGING BUILDS EAST OUT OF THE PLAINS STATES WILL MAINTAIN A
MILDER DAY SUNDAY.

AS A FRONTAL BOUNDARY SAGS SOUTH ACROSS WISCONSIN SUNDAY AND LOW
PRESSURE APPROACHES THE AREA FROM THE WEST... EXPECT AN AREA OF AT
LEAST SCATTERED RAIN TO BREAK OUT ACROSS AREAS MAINLY NORTH OF
I-80. LIMITED MOISTURE MAY CURTAIL PRECIP COVERAGE UNTIL BETTER
FORCING ALONG WITH INCREASE GULF MOISTURE ARRIVES MONDAY.

BEST PERIOD FOR RAIN AND EVEN THUNDERSTORMS AT THIS POINT APPEARS
TO COME MONDAY NIGHT AND TUESDAY AS MODELS CONTINUE TO DEVELOP
STRONG LOW PRESSURE ACROSS THE CENTRAL PLAINS MONDAY ASSOCIATED
WITH A SHARP UPPER LEVEL TROUGH. THIS LOW THEN RIDES EAST ALONG
AFOREMENTIONED FRONTAL BOUNDARY NEAR THE IL-WI BORDER EARLY
TUESDAY AND DRAGS A TRAILING COLD FRONT ACROSS THE AREA LATER
TUESDAY.

WITH SECONDARY UPPER TROUGH COMBINED WITH COLD AIR ALOFT DROPPING
SOUTH INTO THE AREA WEDNESDAY BEHIND COLD FRONT... WILL MAINTAIN
SHOWER CHANCES THROUGH EVENING.

ED F

&&

.AVIATION...

//ORD AND MDW CONCERNS...UPDATED 22Z...

* GUSTY SOUTH WINDS THROUGH MID EVENING.

* SMALL CHANCE FOR TSRA LATE TONIGHT INTO THURSDAY MORNING.

* LLWS POTENTIAL TONIGHT.

KJB/BMD

//DISCUSSION...UPDATED 18Z...

AN AREA OF LOW PRESSURE MOVING EASTWARD ACROSS SOUTHERN CANADA WILL
CONTINUE TO DEEPEN AS IT MOVES OVER SOUTHWESTERN ONTARIO THIS
EVENING. OVERALL THIS WILL CONTINUE TO LEAD TO AN INCREASE IN
SOUTHERLY WINDS ACROSS THE AREA THROUGH THIS EVENING. HOWEVER...IN
SPITE OF THIS SURFACE WIND INCREASE...IT APPEARS THAT A LOW LEVEL
JET FEATURING 50+ KT SOUTH-SOUTHWESTERLY WINDS AT AROUND 2,000 FEET
AGL WILL DEVELOP THIS EVENING...LIKELY SETTING THE STAGE FOR LLWS
OVERNIGHT. A COLD FRONT...ASSOCIATED WITH THE SOUTHERN CANADA LOW...
WILL SHIFT ACROSS THE AREA BY EARLY THURSDAY AFTERNOON...WITH WEST-
NORTHWESTERLY WINDS LIKELY IN ITS WAKE.

THE OTHER CONCERN DURING THE PERIOD IS THE POSSIBILITY FOR A PERIOD
OR TWO OF SHOWERS WITH SOME EMBEDDED THUNDERSTORMS...PARTICULARLY
VERY LATE TONIGHT AND AGAIN BY LATE THURSDAY MORNING. UNCERTAINTY
STILL EXISTS ON THE EXTENT OF THE THUNDER WITH THIS ACTIVITY...BUT
IT DOES APPEAR THAT AN UPPER LEVEL DISTURBANCE...CURRENTLY MOVING
NORTHEASTWARD OVER SOUTHWESTERN MISSOURI...WILL SHIFT OVER PORTIONS
OF NORTHERN ILLINOIS BY DAYBREAK THURSDAY. THIS COULD RESULT IN SOME
SCATTERED SHOWER AND THUNDERSTORM DEVELOPMENT OVER THE TERMINALS
AFTER 09 UTC. FINALLY...ANOTHER AREA OF SHOWERS AND PERHAPS SOME
ADDITIONAL STORMS LOOK TO MOVE OVER THE AREA BY LATE MORNING INTO
THE EARLY AFTERNOON AHEAD OF THE APPROACHING COLD FRONT.

KJB

//ORD AND MDW CONFIDENCE...UPDATED 22Z...

* MEDIUM-HIGH IN WIND TRENDS.

* LOW-MEDIUM IN EXTENT OF TSRA LATE TONIGHT INTO THURSDAY MORNING.

* HIGH IN REMAINING ELEMENTS.

KJB/BMD

//OUTLOOK FOR ORD/MDW FOR 00Z FRIDAY-12Z WEDNESDAY...UPDATED 12Z...

THURSDAY NIGHT...VFR LIKELY.

FRIDAY...CHANCE LIGHT RAIN AND MVFR. GUSTY NORTH TO NORTHEAST
WINDS.

SATURDAY...VFR. CHANCE -RA AT NIGHT.

SUNDAY...CHANCE -RA. SOUTH WINDS SHIFTING TO EAST.

MONDAY...CHANCE -SHRA AND IFR/MVFR. SLIGHT CHANCE TSRA AT NIGHT.

TUESDAY...CHANCE TSRA AND MVFR.

RC

&&

.MARINE...
317 PM CDT

TIGHTENING GRADIENT BEHIND DEPARTING HIGH OVER THE EASTERN GREAT
LAKES AND AHEAD OF APPROACHING DEEP LOW PRESSURE OVER THE
NORTHERN PLAINS/SOUTHERN MANITOBA WILL PRODUCE A PERIOD OF BRISK
SOUTH WINDS ACROSS THE LAKE TONIGHT. GIVEN STABLE THERMAL
PROFILE... EXPECT A MUCH LARGER THAN TYPICAL VARIATION BETWEEN THE
10 M WIND AND WINDS AT A 30 M PILOT HOUSE LEVEL. WINDS WILL THEN
SLOWLY SUBSIDE AS SOMEWHAT MORE RELAXED GRADIENT IN VICINITY OF
FRONTAL BOUNDARY MOVES EAST THURSDAY.

NORTHWEST WINDS COMBINED WITH A MORE NEUTRAL TO SLIGHTLY UNSTABLE
THERMAL PROFILE BEHIND A SECONDARY COLD FRONT THEN ADVECT ACROSS
THE LAKE LATER THURSDAY NIGHT INTO FRIDAY. TIGHTENING GRADIENT
COMBINED WITH COLD AIR ADVECTION MEANS THIS PERIOD WILL HAVE TO BE
MONITORED CLOSELY FOR A PERIOD OF BRISK NORTHWEST FLOW BEFORE
GRADIENT RELAXES AS HIGH PRESSURE APPROACHES FROM THE WEST.

A SPRAWLING HIGH PRESSURE RIDGE INFLUENCES THE LAKE SATURDAY
BEFORE LOW PRESSURE THEN APPROACHES FROM THE WEST LATER SUNDAY.

ED F

&&

.LOT WATCHES/WARNINGS/ADVISORIES...
IL...NONE.
IN...NONE.
LM...SMALL CRAFT ADVISORY...INDIANA NEARSHORE WATERS UNTIL 1 PM THURSDAY.

     SMALL CRAFT ADVISORY...ILLINOIS NEARSHORE WATERS UNTIL 10 AM THURSDAY.

&&

$$

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