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000
FXUS63 KGRR 211737
AFDGRR

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE GRAND RAPIDS MI
137 PM EDT TUE MAY 21 2013

LATEST UPDATE...
AVIATION

.SYNOPSIS...
ISSUED AT 323 AM EDT TUE MAY 21 2013

LOW PRESSURE OVER MINNESOTA THIS MORNING WILL SLOWLY MOVE ACROSS THE
GREAT LAKES REGION THROUGH WEDNESDAY NIGHT.  THIS SYSTEM WILL
PRODUCE PERIODS OF SHOWERS AND THUNDERSTORMS.  MUCH COOLER AIR WILL
ARRIVE BY THURSDAY AS THE RAIN GRADUALLY COMES TO AN END.

THE END OF THE WEEK AND INTO THE HOLIDAY WEEKEND LOOKS MAINLY DRY.
TEMPERATURES SHOULD GRADUALLY WARM TO AROUND NORMAL VALUES BY
MEMORIAL DAY.

&&

.UPDATE...
ISSUED AT 1124 AM EDT TUE MAY 21 2013

SOUTHWEST LOWER MICHIGAN IS IN A LULL FOLLOWING LAST NIGHT/S WAVE
OF SHOWERS AND STORMS. A FEW SHOWERS REMAIN AT 1130 AM...DRIFTING
NORTHEAST OUT OF THE FORECAST AREA AT THIS TIME. THE AREA IS BEING
SUPPRESSED AT THE CURRENT TIME BY CLOUD COVER AND SYNOPTIC SCALE
SINK BEHIND THE OVERNIGHT STORMS.

FOCUS NOW SHIFTS TO A SHORTWAVE LIFTING NORTHEAST OUT OF ILLINOIS
THAT SHOULD SLIDE BY THE FORECAST AREA BETWEEN 19Z AND 02Z.
SATELLITE IMAGERY WOULD SUGGEST THAT WE SHOULD AT LEAST SEE BREAKS
TO ALLOW HEATING TO OCCUR. CAPE VALUES ARE PROGD TO PUSH INTO THE
1500-2000 J/KG RANGE WITH LI/S DIPPING TOWARD -6 C. SO ENOUGH
INSTABILITY TO FIRE STORMS AND A TRIGGER VIA THE SHORTWAVE LIFTING
OUR WAY. NAM WIND FIELDS WOULD FAVOR INITIATION ALONG A LAKE
SHADOW FROM NEAR PAW PAW/KALAMAZOO NORTHEAST TOWARD BIG RAPIDS.
CONFIDENCE OVERALL AT THIS POINT IS MEDIUM IN STORMS REFIRING THIS
AFTERNOON. IF STORMS DO INDEED DEVELOP ONCE AGAIN...HAIL AND
STRONG WINDS WOULD BE THE MAIN THREATS. 0-6KM SHEAR VALUES ARE ON
THE ORDER OF 40 KNOTS...SO STORMS WOULD BE ORGANIZED AND HAVE THE
ABILITY TO SUSTAIN. A LULL IN THE ACTIVITY MAY OCCUR IN THE LATE
EVENING BEFORE ANOTHER SURGE OF ACTIVITY LATE TONIGHT.

&&

.SHORT TERM...(TODAY THROUGH THURSDAY)
ISSUED AT 323 AM EDT TUE MAY 21 2013

A BAND OF SHOWERS AND THUNDERSTORMS WILL MOVE ACROSS THE CWA EARLY
THIS MORNING.  THIS AREA WAS DRIVEN BY A WEAKENING LOW LEVEL JET AS
WELL AS UPPER DIVERGENCE FROM THE UPPER JET.  THE UPPER FORCING WILL
ALSO WEAKEN EARLY THIS MORNING.  SO EXPECT THE PCPN TO GRADUALLY
BREAK UP AND ANY STORMS WILL GRADUALLY WEAKEN TOWARD DAYBREAK.

FEEL THE THREAT FOR SEVERE WEATHER HAS DIMINISHED TODAY...ALTHOUGH
IT CAN NOT BE COMPLETELY RULED OUT.  THE SURFACE LOW BECOMES BAGGIER
AS IT MOVES ACROSS WISCONSIN AND NO LONGER SUPPORTS A FOCUSED LOW
LEVEL JET.  MEANWHILE THE RIGHT ENTRANCE REGION OF THE UPPER JET
WILL BE FOCUSED TO OUR SOUTH OVER IL/IN.  SO ANY STRONG DYNAMIC
FORCING IS MUCH WEAKER TODAY AS COMPARED TO YESTERDAY.  HOWEVER WE
ARE PROGGED TO BECOME RATHER UNSTABLE...ESPECIALLY OVER THE EASTERN
CWA.  THE WILD CARD WILL BE HOW LONG THE DEBRIS CLOUDS HOLD IN...AND
WHETHER WE CAN REALIZE THIS INSTABILITY.  IF WE CAN BREAK OUT AND
THE MORNING CONVECTIVE LEAVES A FEW BOUNDARIES...SEVERE POTENTIAL
WILL INCREASE ESPECIALLY EAST OF U.S. 131.

FEEL ANY RISK OF SEVERE STORMS WILL WIND DOWN QUICKLY THIS EVENING
AS THE SURFACE LOW BRINGS IN DEEP MOISTURE...BUT DIMINISHING
INSTABILITY.  STRONG INDICATION THAT A VORT MAX WILL ROTATE AROUND
THE UPPER LOW AND MOVE INTO LOWER MI LATE TONIGHT.  THIS SHOULD
BRING AN EXPANDING AREA OF SHOWERS AND STORMS TO SW MI AFTER
MIDNIGHT...BUT SEVERE STORMS ARE NOT EXPECTED.

THE DEEP MOISTURE LINGERS OVER THE CWA WED AND WED NIGHT.  WE WILL
HAVE TO WATCH HOW THIS UNFOLD AS THE RISK TURNS TO THE POTENTIAL FOR
HEAVY RAIN.  IF THESE RAINS WERE TO FOCUS IN ONE PARTICULAR AREA...
LOCALIZED FLOODING COULD OCCUR.

THE SURFACE LOW AND IT/S COLD FRONT FINALLY EXITS THE CWA ON
THURSDAY.  STILL LOOKS LIKE QUITE A WET DAY OVER THE SOUTHERN CWA.
HOWEVER CLEARING SHOULD ARRIVE FOR NORTHERN AREAS BY LATE IN THE
DAY.  TEMPS WILL BE MUCH COOLER...WITH MID 60S IN AREAS THAT SEE
SOME SUN...BUT ONLY UPPER 50S IN THE WETTER SOUTH.

.LONG TERM...(THURSDAY NIGHT THROUGH MONDAY)
ISSUED AT 323 AM EDT TUE MAY 21 2013

THERE ARE TWO SIGNIFICANT WEATHER ISSUES WE ARE TRACKING. THE FROST
ISSUE UP NORTH SATURDAY MORNING AND THE THREAT OF SHOWERS AND
THUNDERSTORMS SATURDAY INTO SUNDAY.

THE LARGE SCALE PICTURE FEATURES TROUGHS ON BOTH COASTS OVER THE
WEEKEND WITH A LARGE SCALE MEAN RIDGE OVER CENTRAL NORTH AMERICA
FROM TEXAS NORTH INTO CENTRAL CANADA.  THE PACIFIC SYSTEM THAT
CLOSES OFF A LOW OVER THE PACIFIC NORTHWEST SENDS A SHORTWAVE OVER
OVER THE TOP OF THE RIDGE AND CREATES A WAVE ON THE FRONT THAT IF
THE ECMWF FROM THE 12Z RUN WERE CORRECT WOULD BRING RAIN AND THE
THREAT OF THUNDERSTORMS TO THE AREA SATURDAY INTO SUNDAY.  THE GFS
KEEPS THIS SYSTEM SOUTH OF THE AREA.  BASED ON CONTINUITY OVER THE
PAST FEW DAYS I AM FAVORING THE DRY SOLUTION OF THE GFS.

THAT WOULD ALLOW FOR COLDER TEMPERATURES FRIDAY NIGHT /SATURDAY
MORNING AND MAINTAIN THE THREAT FOR FROST. IT WOULD ALSO MEAN PARTLY
SUNNY SKIES AND COOL TEMPERATURE THROUGH THE MEMORIAL DAY WEEKEND.

&&

.AVIATION...(FOR THE 18Z TAFS THROUGH 18Z WEDNESDAY AFTERNOON)
ISSUED AT 137 PM EDT TUE MAY 21 2013

THE MAIN FOCUS OF THE 18Z FCSTS IS ONCE AGAIN CONVECTION TRENDS
OVER THE NEXT 24 HOURS.

THE AREA HAS BEEN SLOW TO HEAT UP SIGNIFICANTLY THIS MORNING DUE
TO THE RAIN EARLY THIS MORNING AND THE MID LEVEL CLOUD COVER THAT
HAS BEEN PLAGUING THE AREA. MORE BREAKS ARE OPENING UP...AND
INSTABILITY IS BUILDING. WE EXPECT JUST SPOTTY STORMS TO DEVELOP THIS
AFTERNOON ALONG AND INLAND OF THE LAKE BREEZE/LAKE SHADOW...MAINLY SE
OF A LINE FROM SOUTH HAVEN TO CADILLAC. SPOTTY MVFR CONDITIONS
SHOULD LIFT TO VFR EXCEPT DIRECTLY UNDER ANY STORMS THAT DEVELOP.

WE SHOULD SEE A BREAK IN CONVECTION THIS EVENING AND DURING THE
EARLY OVERNIGHT HOURS. WE WILL SEE THE CHCS OF RAIN INCREASE AFTER
08-10Z WED AS A WAVE OF LOW PRESSURE APPROACHES. THIS SHOULD BE
MAINLY SHOWERS/RAIN WITH ONLY A FEW EMBEDDED STORMS DOWN SOUTH.
THE STORM CHCS ARE LOW ENOUGH AT ANY ONE TERMINAL TO LEAVE OUT OF
THE FCST FOR NOW. WIDESPREAD MVFR CONDITIONS SHOULD ACCOMPANY THIS
PCPN WITH SOME LOCAL IFR NOT OUT OF THE QUESTION. THIS SHOULD HOLD
IN AT MOST LOCATIONS THROUGH 18Z BEFORE IT MOVES OUT AFTER 18Z WED.

&&

.MARINE...
ISSUED AT 323 AM EDT TUE MAY 21 2013

SOUTHERLY FLOW WILL GRADUALLY WEAKEN OVER THE NEXT COUPLE OF DAYS AS
A SURFACE LOW MOVES TOWARD THE LAKE.  HOWEVER NORTHERLY WINDS WILL
INCREASE ON THE BACK SIDE OF THE LOW.  WE MAY NEED A SCA BY THURSDAY.


&&

.FIRE WEATHER...
ISSUED AT 323 AM EDT TUE MAY 21 2013

THE AREA WILL SEE PERIODS OF RAIN THROUGH THURSDAY.  OVER AN INCH OF
RAIN IS LIKELY KEEPING THE FIRE RISK LOW.

&&

.HYDROLOGY...
ISSUED AT 323 AM EDT TUE MAY 21 2013

PERIODS OF HEAVIER RAIN IS EXPECTED BY LATE TONIGHT AND SHOULD
LINGER UNTIL THURSDAY.  IF THIS SETS UP OVER THE SAME AREAS WE MAY
SEE LOCALIZED FLOODING.  A FLOOD WATCH MAY NEED TO BE CONSIDERED IF
A FOCUSED AREA OF HEAVY RAIN IS EXPECTED.

&&

.GRR WATCHES/WARNINGS/ADVISORIES...
MI...NONE.
LM...NONE.
&&

$$

UPDATE...DUKE
SYNOPSIS...JK
SHORT TERM...JK
LONG TERM...WDM
AVIATION...NJJ
FIRE WEATHER...JK
HYDROLOGY...JK
MARINE...JK