Flash Flood and Heavy Rain in Delaware, March 21, 2001

AREA FORECAST DISCUSSION...CORRECTED
NATIONAL WEATHER SERVICE MOUNT HOLLY NJ
330 AM EST WED MAR 21 2001

WELCOME TO THE FIRST FULL DAY OF SPRING?

CUR FCST APRS ON TRACK. FLOOD WATCH WILL REMAIN IN EFFECT FOR TDY
AND TNGT...EVENT OCRG SLGTLY FSTR...BUT THE HVYST PCPN S/B OCRG FROM
LATE MRNG DOWN S TO AROUND MIDN UP N. MDL QPFS HAVE BEEN CONSISTENT
WITH ABT 1.5 TO 2.0 INCHES ACRS OUR CWA AND THIS UNFORTUNATELY WOULD
CAUSE FLDG FOR SOME OF OUR MORE RESPONSIVE STREAMS. LATEST MDL RUNS
BRING THE SFC LOW MORE TO THE W AND PASS IT JUST E OF THE NJ SHORE
ON WED. THIS BRINGS MOST OF OUR CSTL AREAS INTO LGTR WINDS BY THE
THU MRNG HI TIDE. THUS W/O A STG ANCHORING HI IT APRS TIDAL FLDG
SHLD REMAIN IN THE MINOR CAT.

MDLS HAVE BEEN PRETTY CONSIST AND SHOWING PRETTY GOOD RUN TO RUN
CONTINUITY. ALL THE MDLS CONT A SLO GRADUAL DEEPENING THRUT THE FCST
PD. IF ANYTHING THERE HAS BEEN A WARMING/WWD DRIFT WITH EACH MDL
RUN. ON THE ETA E.G. ITS EXPECTED TO PASS ABT 30 MI CLSR TO CAPE MAY
CITY THAN LAST NGT/S RUN. IN TERMS OF SENSIBLE WX FOR MOST OF OUR
CTYS...THAT ISN/T MUCH OF A DIF...BUT FOR THE THREE NW CTYS WHICH
ARE GOING TO BE CLOSE TO AN ELEV DRIVEN HVY SNOW EVENT THIS WWD
SHIFT IS CRITICAL.

FIRST SPOKE OF PCPN MOVG ACRS OUR SRN ZNS ATTM. THIS SHLD REACH OUR
NRN ZNS CLS TO ISSUANCE TIME AND PLANS ARE TO EITHER JUST SAY RAIN
OR PERIODS OF RAIN. UP N...ESP PRIOR TO 18Z ENUF COLD AIR RMNS IN
PLACE FOR PCPN TO START AS OR MIX WITH SNOW AT THE START. MDLS EVEN
SHOW SOME EVAP COOLG EFFECTS AS LO LVL TEMPS DIP A BIT. FACT PCPN IS
OCRG DURG THE DAYLIGHT HRS AND INTENSITY WILL BE ON THE LIGHT SIDE
SHLD PRECLUDE MUCH SNOW FROM ACCUM. WE WILL LIMIT THE MNTN TO HIER
TRRN OF OUR THREE NRNMST CTYS. DON/T BUY MDL MAX TEMPS. OCEAN TEMPS
ARE IN THE LO-MID 40S...WILL USE THEM IN MOST ZONES.

THE REAL HVY PCPN IS TIED TO THE CONFLUENCE OF A PVA SPOKE...STG
THETA E ADVT AND SCREAMING LLJ. ONE COULD FLW THE THETA E AXIS WHERE
IT IS NOW IN NRN NC AND SEE WHERE THE ACTION WILL BE LTR.
ANOMALOUSLY STG 50KT ERLY LLJ WILL CROSS THE SRN HALF OF OUR CWA
BTWN 18Z AND 00Z AND THE NRN HALF BTWN 00Z AND 06Z. SO WILL GO THE
HVYST OF THE HVY PCPN. IN THE DELMARVA IT WILL PBLY BE OVR CLS TO
00Z...IN THE PHILLY ARE DURG THE EVE AND OUR NRN ZNS ABT MIDN. IT
WILL PBLY BE DURG THIS 6 HR PERIOD THAT THE BULK OF THE HVYR RAIN
WILL FALL.

OTHER PBLM IS UP IN OUR FAR NWRN ZNS WHERE ELEV AND DYNAMIC COOLG
DURG THE HVYR PCPN BURSTS MIGHT CHG THE PCPN OVR TO SNOW. MDL RUNS
HAVE TRENDED WRMR AND USING THE AVN LO LVL FCST FIELDS PCPN WOULD
ALL RAIN WELL NW OF OUR CWA. WE ARE NOT TOTALLY SOLD ON THIS YET AND
CONFID ABT PTYP STILL IS NOT HI. PCPN INTENSITY AND THE FACT THE
FCST SOUNDG IS BARELY ABV FRZG FOR ABT THE 2000 LOWEST FEET WILL
COME INTO PLAY. WE WILL INCLD SOME HIER TRRN ACCUM PSBLTY DURG
HVYR PCPN BURSTS. SINCE FCST CONFID IS NOT HI...WE WILL NOT ISSUE A
2ND PERIOD ADVY ATTM. WOULD EXPECT PCPN INTENSITY TO DIMINISH DURG
THE 2ND HALF OF THE NGT AND MIGHT EVEN END FOR A WHL OVER OUR SERN
ZNS. PRFR COOLER NGM MOS FOR MINS.

MARCH IS NATIONAL WRAPARND MONTH AND THU WON/T BE ANY DIFFERENT AS
LOW BEGINS TO INTENSIFY FSTR AS IT NRS CAPE COD. ALL THE MDLS PUSH
WRAPARND PCPN BACK THRU OUR CWA. QPF/S S/B UNDER A QUARTER OF AN
INCH AND SEE NO NEED TO EXTEND FFA ATTM. PTYP UP N WILL PBLY BATTLE
BACK AND FORTH BTWN WET SNOW AND RAIN. WHL IT GETS COOLER ALF...LACK
OF A COLD AIR SOURCE NEAR THE SFC KEEPS THE BNDRY TEMPS REL WARM.
COUPLED WITH LGTR PCPN INTENSITY AND THE STG (BUT HIDDEN) MARCH
SUN...WE/LL FCST A MIXTURE OF RAIN/WET SNOW IN NRNMST ZNS AND LEAVE
REST PRETTY MUCH LIQUID. COOLER NGM MOS MAX TEMPS PRFRD AGAIN.

MARINE: WILL KEEP GALES GOING AND MINOR TIDAL FLDG HEADLINE INTO
THU. ON THU AM CHCS ARE GREATER UP N WITH STILL A BIT OF AN ONSHR
FLOW. BY THU MRNG S CSTL AREAS COULD BE IN THE COLISH FLOW NR THE
SFC LOW.

TU FOR EXTENSIVE COORD BGM/CTP/OKX AND HPC.

.PHI...CORRECTED FLOOD WATCH TDY AND TNGT ALL AREAS.
       GALE WARNING ALONG THE COAST AND DE BAY.

TFG