NWS Philadelphia/Mount Holly wrote its last area forecast discussion before the first event at Aug 21, 3:03 PM EST, 2 hours 57 minutes earlier.
Area forecast discussion, Aug 21, 3:03 PM EST, 2 hours 57 minutes before
AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE MOUNT HOLLY NJ
403 PM EDT SUN AUG 21 2011
.SYNOPSIS...
A STRONG COLD FRONT WILL MOVE THROUGH OUR AREA TONIGHT, FOLLOWED BY
HIGH PRESSURE BUILDING INTO THE MID ATLANTIC REGION FOR MONDAY AND
TUESDAY. HIGH PRESSURE SLIDES EAST AND OFFSHORE DURING WEDNESDAY. A
COLD FRONT SLOWLY APPROACHES THE MID ATLANTIC ON THURSDAY AND THEN
PASSES SOUTH AND EAST OF OUR AREA BY THE START OF NEXT WEEKEND.
&&
.NEAR TERM /UNTIL 6 AM MONDAY MORNING/...
AN 18Z SURFACE ANALYSIS PLACED A COLD FRONT FROM SOUTHEASTERN CANADA
DOWN THROUGH THE OHIO VALLEY. A TROUGH WAS ACROSS EASTERN
PENNSYLVANIA DOWN INTO VIRGINIA. AN UPPER AIR ANALYSIS PLACED A
LARGE TROUGH ACROSS EASTERN CANADA AND GREAT LAKES INTO THE OHIO
VALLEY. A STRONG 300 MB JET WAS FROM THE MIDWEST TO THE EASTERN
GREAT LAKES REGION.
SEVERE THUNDERSTORM WATCH 794 REMAINS IN EFFECT FOR OUR ENTIRE CWA
UNTIL 01Z.
A STRONG UPPER-LEVEL TROUGH WILL SETTLE ACROSS THE NORTHEAST AND MID
ATLANTIC REGIONS TONIGHT WITH FALLING HEIGHTS. THE COMBINATION OF
INCREASED SHEAR WITH AT LEAST MODEST INSTABILITY /MLCAPE 1500-2000
J/KG/ AND INCREASING LARGE SCALE ASCENT IS ALLOWING FOR CONVECTIVE
DEVELOPMENT. GIVEN THE FLOW NEARLY UNIDIRECTIONAL ABOVE THE SURFACE,
STORM MODE HAS BEEN TENDING TO BE IN LINE SEGMENTS WITH A LOCALLY
INCREASING DAMAGING STRAIGHT LINE WIND THREAT. HOWEVER SPECIAL
16-18Z SOUNDINGS INDICATE THE FLOW BACKING A BIT IN THE LOWEST
LEVELS WHICH WILL ENHANCE THE LOW-LEVEL SHEAR. THE PRESENCE OF A
LOW-LEVEL JET THROUGH THIS EVENING MAY ALSO ENHANCE THE DAMAGING
WIND THREAT ACROSS PARTS OF THE AREA. THE INITIAL CONVECTION LOOKS
TO SHIFT EASTWARD, HOWEVER A SECOND ROUND LOOKS TO ACCOMPANY
STRONGER HEIGHT FALLS AND THE ACTUAL COLD FRONT THIS EVENING. THE
MID LEVEL TEMPERATURES AROUND -10C WILL ALLOW FOR SOME HAIL WITH
SOME POSSIBLY SEVERE IN ASSOCIATION WITH MORE ROBUST UPDRAFTS
ESPECIALLY ANY WITH ROTATION ALOFT. THE MOIST LOW LEVELS WITH SOME
VEERING WITH HEIGHT IN THE LOWEST LEVELS ESPECIALLY IN THE PRESENCE
OF A LEE SIDE TROUGH AND OUTFLOW BOUNDARIES COULD ALLOW FOR LOCALLY
ENHANCED LOW-LEVEL STORM ROTATION AND DISCRETE STORMS. THE MAIN
THREAT IS DAMAGING STRAIGHT LINE SURFACE WINDS WITH ANY LINE
SEGMENTS, ALTHOUGH CANNOT RULE SOME LOCAL LOWER LEVEL ROTATION
GIVEN THE SHEAR IN PLACE. WE WILL CONTINUE THE ENHANCED WORDING OF
STRONG/DAMAGING WINDS IN THE FORECAST THROUGH ABOUT 01Z.
THE COLD FRONT CROSSES THE AREA OVERNIGHT, HOWEVER DESPITE THE
AIRMASS BECOMING MORE STABLE WITH THE LOSS OF HEATING AND ONGOING
CONVECTION, THE WIND FIELD IS FORECAST TO STRENGTH ALOFT WHICH MAY
BE ABLE TO SUSTAIN CONVECTION INTO THE OVERNIGHT HOURS. WE WILL
STEP THE POPS DOWN FROM WEST TO EAST THROUGH THE NIGHT AS THE
COVERAGE AND SEVERITY IS ANTICIPATED TO LESS DEEPER INTO THE NIGHT.
OTHERWISE, THE CLOUDS WILL THIN AND WITH THE LOWER LEVELS OF THE
ATMOSPHERE REMAINING MOIST, THERE COULD BE PATCHY FOG. GIVEN LOWER
CONFIDENCE WITH THIS WE OPTED TO LEAVE IT OUT ATTM. AS FOR
TEMPERATURES WE GENERALLY USED A BLEND OF THE NAM/MOS GUIDANCE.
&&
.SHORT TERM /6 AM MONDAY MORNING THROUGH 6 PM MONDAY/...
A LARGE UPPER-LEVEL TROUGH MOVES INTO THE NORTHEAST AND MID ATLANTIC
REGIONS AS SURFACE HIGH PRESSURE BUILDS INTO THE OHIO VALLEY. SOME
SHORT WAVE ENERGY IN COMBINATION WITH COLDER AIR ALOFT MAY COMBINE
WITH SURFACE HEATING TO GENERATE A FEW SHOWERS ACROSS THE FAR
NORTHWEST ZONES IN THE AFTERNOON. OUR CONFIDENCE IS ON THE LOWER
SIDE REGARDING HOW TALL THE CUMULUS CAN BECOME DURING THE DAY
DESPITE THE CONVECTIVE TEMPERATURES BEING REACHED ESPECIALLY IN THE
CENTRAL AND NORTHERN AREAS. FOR NOW, ADDED A CHC OF SPRINKLES ACROSS
THE FAR NORTHWESTERN ZONES. IF SOME SHOWERS ARE ABLE TO DEVELOP, THE
DRYING LOW LEVELS MAY ALLOW FOR LOCALLY ENHANCED SURFACE WINDS DUE
TO SOME EVAPORATIVE COOLING AND DOWNWARD MOMENTUM. ELSEWHERE, SOME
CLOUDS ARE EXPECTED.
THE PRESSURE GRADIENT WILL TIGHTEN BETWEEN HIGH PRESSURE IN THE
OHIO VALLEY AND LOW PRESSURE NORTH OF NEW ENGLAND. AS A COOLER AND
DRIER AIRMASS MOVES INTO THE AREA, HEATING AT THE SURFACE WILL ALLOW
FOR BETTER VERTICAL MIXING. THEREFORE SURFACE GUSTS MAY REACH THE 20
TO 25 KNOT RANGE FOR A TIME BEFORE SUBSIDING LATE IN THE AFTERNOON
AS VERTICAL MIXING STARTS TO WEAKEN.
AS FOR TEMPERATURES, WE GENERALLY USED A BLEND OF THE MOS GUIDANCE
THEN MADE SOME MAINLY MINOR TWEAKS.
&&
.LONG TERM /MONDAY NIGHT THROUGH SUNDAY/...
AT THE START OF THE LONG TERM PERIOD, AN UPPER LEVEL TROUGH WILL BE
EXITING THE REGION AS SURFACE HIGH PRESSURE CONTINUES TO BUILD INTO
THE MID ATLANTIC. THE GFS AND EURO GIVE A DRY PICTURE FOR MONDAY
NIGHT, WHILE THE NAM STILL HINTS AT A WEAK DISTURBANCE SWINGING
THROUGH OUR SOUTHERN ZONES BRINGING SOME MINOR PRECIP. A DRY
FORECAST HAS BEEN MAINTAINED. PERHAPS THE ONLY OTHER CONCERN WOULD
BE THE POSSIBILITY OF LOCALIZED PATCHY FOG MONDAY NIGHT WITH LIGHT
NORTHWEST WINDS AND MAINLY CLEAR CONDITIONS EXPECTED. FOR NOW,
PATCHY FOG HAS BEEN LEFT OUT OF THE GRIDS, AS THIS SHOULD BE VERY
LIMITED.
AS THE SURFACE HIGH PRESSURE BUILDS OVER THE AREA TUESDAY INTO
WEDNESDAY, THE UPPER LEVEL FLOW BECOMES MORE ZONAL FOR A TIME. DRY
WEATHER CONTINUES FOR TUESDAY AND WEDNESDAY WITH AMPLE SUN AND LIGHT
WINDS. HIGH PRESSURE MOVES EAST AND OFFSHORE BY WEDNESDAY, THOUGH,
AND A RETURN FLOW SETS UP WITH WINDS BECOMING SOUTHERLY. WEDNESDAY
WILL STILL BE A PLEASANT DAY WITH DEWPOINTS MAINLY IN THE 50S, BUT
WITH THE RETURN FLOW AROUND THE BACKSIDE OF HIGH PRESSURE AND
MOISTURE STREAMING INTO THE REGION, DEWPOINTS RISE INTO THE 60S
WEDNESDAY NIGHT INTO THURSDAY.
BY THURSDAY, LOW PRESSURE TRACKING ACROSS SOUTHEASTERN CANADA WILL
BE DRAGGING A COLD FRONT THROUGH THE GREAT LAKES AND APPROACHING THE
NORTHEAST AND NORTHERN MID ATLANTIC. THE TIMING OF THIS FRONT
TRACKING THROUGH OUR AREA THURSDAY AND FRIDAY IS A BIT UNCERTAIN AT
THIS POINT. THERE IS SOME UPPER LEVEL TROUGHING PRESENT IN THE
MODELS, BUT OVERALL MID AND UPPER LEVEL SUPPORT DOES NOT APPEAR TO
BE AS DEFINED WITH THIS FRONTAL PASSAGE. THUS, A SLOWER FRONTAL
PASSAGE IS EXPECTED AT THIS TIME, WITH POPS EXTENDING FROM THURSDAY
MORNING THROUGH FRIDAY AFTERNOON. HOWEVER, THE HIGHEST POPS ARE
FOCUSED THROUGH THE THURSDAY AFTERNOON AND THURSDAY NIGHT TIME FRAME.
BY THE BEGINNING OF THE WEEKEND, THE FRONT IS EXPECTED TO BE SOUTH
AND EAST OF THE FORECAST AREA.
&&
.AVIATION /20Z SUNDAY THROUGH FRIDAY/...
VFR CONDITIONS IN GENERAL, HOWEVER GIVEN THE AMOUNT OF LOW-LEVEL
MOISTURE IN PLACE THERE WILL STILL BE PATCHY MVFR CLOUDS FLOATING
AROUND OUTSIDE OF THE CONVECTION. SHOWERS AND THUNDERSTORMS WILL
INCREASE IN COVERAGE THROUGH THE EVENING AS A COLD FRONT ARRIVES
FROM THE WEST. THIS ACTIVITY SHOULD TEND TO ORGANIZE INTO CLUSTERS
AND LINES WITH LOCALLY VERY HEAVY RAIN AND STRONG WINDS. THIS WILL
LOCALLY REDUCE THE CEILINGS AND VISIBILITIES FOR A TIME. WE
INDICATED A FEW HOURS THIS AFTERNOON WHERE STRONGER STORMS MAY
IMPACT THE TERMINALS WITH PERHAPS A SECOND ROUND THIS EVENING WITH
THE ACTUAL COLD FRONT. BASED ON THE STRENGTH OF THE WIND FIELD AND
ANTICIPATED STORM INTENSITY, WE ADDED SOME ENHANCED WINDS ALONG WITH
THE THUNDER MENTION IN A TEMPO GROUP. WE WILL MONITOR AND CONTINUE
TO ISSUE AMENDMENTS AS THE CONVECTION DEVELOPS AND PROPAGATES.
ONCE THE FRONT CLEARS THE AREA, EXPECT A RETURN TO VFR ALTHOUGH
PERHAPS SOME LOCAL FOG OVERNIGHT WITH ALL THE LOW-LEVEL MOISTURE
IN PLACE. FOR NOW, WE FOCUSED ON THE CONVECTION AND OPTED TO NOT
INCLUDE ANY FOG OVERNIGHT. WINDS GENERALLY IN THE 10 TO 15 KNOT
RANGE WITH GUSTS AS HIGH AS 20 KNOTS INTO THE EVENING HOURS, THEN
DECREASING AND SHIFTING TO THE WEST AND NORTHWEST BEHIND THE FRONT.
FOR MONDAY, HIGH PRESSURE GRADUALLY ARRIVES FROM THE WEST AS THE
COLD FRONT SETTLES TO OUR SOUTH AND EAST. HOWEVER WITH A TIGHTER
PRESSURE GRADIENT AND COOLER/DRIER AIR MOVING IN, THE VERTICAL
MIXING WILL INCREASE ESPECIALLY WITH THE DAYTIME HEATING. AS A
RESULT, SURFACE WINDS SHOULD GUST IN THE 20-25 KNOT RANGE FOR A TIME
BEFORE DROPPING OFF LATE IN THE AFTERNOON AND EARLY EVENING. AS THE
COOLER AIRMASS ARRIVES ALONG WITH SURFACE HEATING, CUMULUS SHOULD
DEVELOP AND COVERAGE SHOULD INCREASE UP TO SCATTERED.
OUTLOOK...
MONDAY NIGHT AND TUESDAY...WITH HIGH PRESSURE OVER THE REGION, A
NORTHERLY TO NORTHWEST FLOW IS EXPECTED. POSSIBLE VISIBILITY
RESTRICTIONS WOULD BE LIMITED TO LOCALIZED EARLY MORNING FOG,
OTHERWISE VFR CONDITIONS ARE EXPECTED. ALSO FOR TUESDAY AFTERNOON, A
SEA BREEZE MAY DEVELOP.
WEDNESDAY...HIGH PRESSURE WILL STILL BE OVER THE REGION AND
GRADUALLY PUSHING OFFSHORE DURING THE DAY. VFR CONDITIONS WILL
PERSIST WITH A SOUTHERLY WIND FLOW.
THURSDAY AND FRIDAY...A COLD FRONT IS FORECAST TO TRACK INTO THE
REGION THURSDAY AFTERNOON AND NIGHT. LOCALLY HEAVY SHOWERS AND
THUNDERSTORMS ARE ANTICIPATED. THE POTENTIAL EXISTS FOR HEAVY RAIN
TO ACCOMPANY THIS ACTIVITY WITH LOCAL MVFR CONDITIONS. SOME SHOWER
OR THUNDERSTORM ACTIVITY MAY EXTEND INTO FRIDAY DEPENDING ON THE
PROGRESSION OF THE FRONT.
&&
.MARINE...
A STRONG COLD FRONT WILL CROSS THE AREA LATE TONIGHT AND EARLY
MONDAY MORNING. THE SOUTHERLY FLOW CONTINUES TO INCREASE IN ADVANCE
OF THIS AND THE MODEL GUIDANCE INDICATES A 30-KNOT 925 MB JET MOVING
THROUGH DURING THE NIGHT. THE FORECAST SOUNDINGS INDICATE THAT WINDS
COULD GET TO 25 KNOTS THEREFORE WILL MAINTAIN THE SMALL CRAFT
ADVISORY ON THE OCEAN UNTIL 15Z MONDAY. BASED ON THE SOUTHERLY FLOW
AND THE ASSOCIATED GRADIENT FOR A TIME, WE WILL GO AHEAD AND ERR ON
THE SIDE OF CAUTION AND HOIST AN ADVISORY FOR DELAWARE BAY UNTIL 10Z
MONDAY. THE SOUTHERLY FLOW WILL TEND TO BUILD THE SEAS AND THEY
COULD GET TO 5 FEET FOR A TIME ON THE OCEAN ESPECIALLY OFFSHORE. ANY
THUNDERSTORMS WILL BE CAPABLE OF PRODUCING LOCALLY STRONG WINDS AND
HIGHER SEAS. LOCALLY HEAVY RAIN AND HAIL WILL ALSO BE POSSIBLE
THROUGH THIS EVENING WITH ANY STRONGER ACTIVITY.
AS THE FRONT SETTLES TO OUR SOUTH AND EAST MONDAY, A TIGHTENING
PRESSURE GRADIENT WILL BE IN PLACE BETWEEN LOW PRESSURE WELL TO OUR
NORTH AND HIGH PRESSURE IN THE OHIO VALLEY. A COOLER AIRMASS
WILL BE SETTLING IN THEREFORE IMPROVED MIXING WILL TAKE PLACE. WHILE
GUSTINESS IS ANTICIPATED, WE GENERALLY HELD WINDS AT 20 KNOTS OR
LESS AT THIS POINT AS CONFIDENCE IS NOT REAL HIGH ON THE TRUE
MIXING. THE WINDS SHIFTING TO THE NORTHWEST WILL ALLOW THE SEAS TO
SETTLE SOME, HOWEVER FUNNELING DOWN DELAWARE BAY COULD KEEP THE
WATER A BIT MORE ROUGH FOR A TIME.
OUTLOOK...
AS HIGH PRESSURE STRENGTHENS OVER THE AREA MONDAY INTO MONDAY NIGHT,
LIGHTER WINDS ARE EXPECTED OVER THE COASTAL WATERS. WINDS AND SEAS
ARE EXPECTED TO BE BELOW SMALL CRAFT LEVELS FROM MONDAY NIGHT
THROUGH WEDNESDAY. BY WEDNESDAY HOWEVER, A RETURN FLOW DEVELOPS AS
HIGH PRESSURE PUSHES OFF TO THE EAST, AND SOUTHERLY WINDS ARE
EXPECTED FOR THE WATERS. THE SOUTHERLY FLOW WILL CONTINUE INTO
THURSDAY AHEAD OF THE NEXT APPROACHING COLD FRONT. THIS FRONT IS
EXPECTED TO NEAR THE COASTAL WATERS BY THURSDAY EVENING AND NIGHT,
BRINGING LOCALLY HEAVY SHOWERS AND THUNDERSTORMS. IN ADDITION, THE
INCREASED GRADIENT BETWEEN THE DEPARTING HIGH PRESSURE AND THE
ADVANCING COLD FRONT MAY BRING SEAS AND WINDS CLOSE TO SMALL CRAFT
ADVISORY CRITERIA FOR LATE THURSDAY. BUT FOR NOW, THIS IS A LOW
CONFIDENCE FORECAST.
&&
.HYDROLOGY...
PW VALUES NEARING 2.0 INCHES THIS AFTERNOON AND EVENING ALONG WITH
DEEP CONVECTION WILL ALLOW LOCALLY VERY HEAVY RAIN TO OCCUR. 1-HOUR
AND 3-HOUR FLASH FLOOD GUIDANCE IS RATHER LOW WITH MANY AREAS DOWN
THE I-95 CORRIDOR AT OR BELOW 1.25 INCHES IN AN HOUR. THE RAINFALL
RATES WITH THE CONVECTION HAVE BEEN RATHER HIGH GIVEN THE MOISTURE
PRESENT, ALTHOUGH THE FORWARD SPEED SHOULD BE FASTER THAN PREVIOUS
EVENTS BUT TRAINING CELLS COULD OCCUR. THE END TIME OF 06Z MAY BE A
BIT LONG, BUT WILL MAINTAIN THIS FOR NOW AS IT CAN ALWAYS BE
CANCELED EARLY.
&&
.PHI WATCHES/WARNINGS/ADVISORIES...
PA...FLASH FLOOD WATCH UNTIL 2 AM EDT MONDAY FOR PAZ067>071.
NJ...FLASH FLOOD WATCH UNTIL 2 AM EDT MONDAY FOR NJZ001-007>010-012-
015>019-021.
DE...FLASH FLOOD WATCH UNTIL 2 AM EDT MONDAY FOR DEZ001.
MD...FLASH FLOOD WATCH UNTIL 2 AM EDT MONDAY FOR MDZ008-012.
MARINE...SMALL CRAFT ADVISORY UNTIL 11 AM EDT MONDAY FOR ANZ450>455.
SMALL CRAFT ADVISORY UNTIL 6 AM EDT MONDAY FOR ANZ430-431.
&&
SYNOPSIS...KLINE
NEAR TERM...GORSE
SHORT TERM...GORSE
LONG TERM...KLINE
AVIATION...GORSE/KLINE
MARINE...GORSE/KLINE
HYDROLOGY...GORSE