Dense Fog Advisory 2, NWS Boston/Norton, MA

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE TAUNTON MA
732 AM EDT TUE APR 5 2011

.SYNOPSIS...
A WARM FRONT WILL LIFT THROUGH SOUTHERN NEW ENGLAND THIS MORNING...
BRINGING A BRIEF PERIOD OF WARMTH BEFORE A COLD FRONT CROSSES THE
REGION THIS AFTERNOON. A WEAK COLD FRONT MAY BRING A FEW SHOWERS
WEDNESDAY AFTERNOON. THE FRONT WILL THEN STALL SOUTH OF NEW
ENGLAND. PERIODS OF WET WEATHER MAY AFFECT THE REGION LATER THIS
WEEK AND INTO THE WEEKEND...BUT CONFIDENCE IS LOW.

&&

.NEAR TERM /UNTIL 6 PM THIS EVENING/...
SHOWERS APPROACHING THROUGH THE HUDSON VALLEY...A FEW LIGHTNING
STRIKES REPORTED THERE. BASED ON MOTION...EXPECT THE BEST CHANCE
OF ANY LIGHTNING TO BE ALONG/SOUTHEAST OF A HARTFORD-WORCESTER-
LAWRENCE LINE.

DENSE FOG CONTINUES IN THE GENERAL VICINITY OF THE MERRIMACK
VALLEY. WARM FRONT MADE IT TO JAFFREY AND BOSTON BY 7 AM BUT STILL
DEAD AIR TO THE NORTH/EAST. EXPECT CONDITIONS TO IMPROVE DURING
MID MORNING AS THE LINE OF SHOWERS APPROACHES.

RELIED ON BLEND OF NAM/GFS TIMING FOR POPS. COLD FRONT SHOULD CROSS
CT VALLEY 15Z-16Z...RI AND E MA BY 18Z AND CAPE COD BY 20Z.

HIGH TEMPERATURES WILL OCCUR THIS MORNING PRIOR TO PASSAGE OF COLD
FRONT. ALL GUIDANCE SOURCES APPEAR TO BE TOO COOL BASED UPON NAM
2M TEMPERATURES...SO BLENDED LATEST BOX FORECAST WITH NAM MOS FOR
HIGHS TODAY. SHOULD BE WELL INTO 50S ESPECIALLY IN SOUTHERN ZONES.

WIND ADVISORY CONTINUES FOR CAPE COD AND ISLANDS TODAY AND HAS
BEEN EXPANDED TO INCLUDE BLOCK ISLAND. THE ADVISORY IS MAINLY FOR
SUSTAINED 25 TO 35 MPH WINDS. SOUNDINGS SHOW STRONG INVERSION
WHICH WILL LIMIT GUST POTENTIAL...BUT MOMENTUM TRANSFER SHOULD
EASILY PRODUCE GUSTS UP TO 45 MPH. WINDS ALOFT BEGIN TO DIMINISH
THIS AFTERNOON AND IT DOES NOT APPEAR LIKELY THAT STRONG WINDS
WILL PERSIST INTO TONIGHT SO WE CUT BACK ON ENDING TIME OF WIND
ADVISORY.

&&

.SHORT TERM /6 PM THIS EVENING THROUGH 6 PM WEDNESDAY/...
2 AM...FRONT HEADS OFFSHORE EARLY THIS EVENING FOLLOWED BY INCREASING
SUBSIDENCE AND DRYING AS HIGH PRESSURE BRIEFLY BUILDS INTO REGION.
LOOKS TO BE A RATHER COOL NIGHT AS WINDS DROP OFF ACROSS INTERIOR
AND NAM MOS LOOKED MOST REASONABLE FOR LOWS TONIGHT.

MODELS BRING NEXT SHORT WAVE RAPIDLY ACROSS REGION WED UNDER ZONAL
FLOW ALOFT. LOW LEVELS REMAIN FAIRLY DRY WHICH SHOULD LIMIT EXTENT
OF SHOWER DEVELOPMENT...BUT WITH STEEP LAPSE RATES WE THINK
SHOWERS WILL BE MORE SCATTERED IN NATURE DURING AFTERNOON. CUT
BACK ON HIGHS A FEW DEGREES WED GIVEN EXTENT OF CLOUD COVER.

&&

.LONG TERM /WEDNESDAY NIGHT THROUGH MONDAY/...
00Z MODELS SHOW A SIMILAR LARGE SCALE UPPER FLOW OVER NORTH
AMERICA THROUGH LATE WEEK WITH A DIGGING WEST COAST/ROCKIES TROUGH
AND FLAT EASTERN RIDGE. PLENTY OF JET ENERGY FLOWING THROUGH THIS
FLAT RIDGE...THE GFS-GGEM-ECMWF SHOW 120 KNOTS MOVING THROUGH THE
NORTHEAST THURSDAY-FRIDAY. THE MAIN WESTERN TROUGH THEN SHIFTS
EAST BY THE END OF THE WEEK...WITH THE MODELS SUGGESTING
ISENTROPIC LIFT MOVING THROUGH NEW ENGLAND DURING LATER SUNDAY AND
MONDAY.

THIS WOULD SUGGEST TWO PERIODS OF WET WEATHER...WITH SURFACE HIGH
PRESSURE PROVIDING PUNCTUATION ON THURSDAY AND SATURDAY. THE GFS
HAS A FAST BIAS...WHICH LOWERS CONFIDENCE IN ITS SOLUTION. WE USED
A BLEND OF HPC AND GMOS DATA...BUT SHADED TOWARD THE HPC VALUES.
BEYOND THE GENERAL DETAILS SKETCHED ABOVE...LOW FORECAST CONFIDENCE
CONTINUES.

THURSDAY...HIGH PRESSURE BUILDS SOUTH FROM QUEBEC. THIS WILL BRING
DRY WEATHER. TEMPS AT 850 MB SUGGEST MAX SURFACE TEMPS IN THE
50S...BUT WARM ADVECTION SHOULD KEEP MIXING BELOW THAT LEVEL AND
THE WEAK SURFACE FLOW WILL PROMOTE ONSHORE FLOW. WE USED A RANGE
OF 45 TO 50.

FRIDAY...ALTHOUGH CLOUDS MAY INCREASE THROUGH THE DAY...WE FOLLOWED THE
SLOWER MODEL SOLUTIONS WHICH BRING PRECIP TO NEW ENGLAND LATE IN
THE DAY AND THROUGH FRIDAY NIGHT.

SATURDAY-MONDAY...VERY LOW CONFIDENCE FORECAST...AS MENTIONED
ABOVE. TIMING ISSUES ABOUND AMONG THE DIFFERENT MODEL SOLUTIONS.
FOR NOW...LEANED TOWARD A DRY FORECAST FOR SATURDAY...THEN CHANCE
OF PRECIP ON SUNDAY AND MONDAY AS MORE WEAK LOWS MOVE ALONG THE
FRONT TO THE S.

&&

.AVIATION /12Z TUESDAY THROUGH SATURDAY/...
FORECASTER CONFIDENCE LEVELS...
  LOW...LESS THAN 30 PERCENT
  MEDIUM...30 PERCENT TO 60 PERCENT
  HIGH...GREATER THAN 60 PERCENT

12Z...MEDIUM CONFIDENCE IN TAFS.

WARM FRONT FROM BDL-ORH-BOS EARLY THIS MORNING WILL LIFT N AND
BRING INCREASING S/SW WINDS...BUT IFR/LIFR EXPECTED TO PERSIST
THROUGH MUCH OF MORNING AS SHOWERS AFFECT TERMINALS 10Z-18Z. TSTM
POTENTIAL LIMITED AND NOT MENTIONED IN TAFS BUT COULD SEE ISOLD
TSTMS THIS MORNING ACROSS CT/RI/E MA.

MOST CONFIDENT IN TIMING OF WIND SHIFT WITH COLD FRONT AND
IMPROVING CONDITIONS THIS AFTERNOON. ALSO CONFIDENT ON LLWS AS WE
ARE ALREADY SEEING 60+KT WINDS AT 020 AROUND JFK WHICH WILL SHIFT
INTO SOUTHERN NEW ENGLAND EARLY THIS MORNING.

RAPID IMPROVEMENT TO VFR TONIGHT AS SKIES CLEAR. VFR CIGS AND SCT
SHOWERS EXPECTED WED AFTERNOON AS WEAK FRONT CROSSES REGION.

KBOS TAF...MEDIUM CONFIDENCE. LINE OF SHOWERS/ISOLATED THUNDER IN
NY EXTRAPOLATES THROUGH ABOUT 14Z...VSBYS SHOULD LOWER AS THE
SHOWERS GO THROUGH AND THEN IMPROVE AGAIN THE FOLLOWING 1-2 HOURS.
CONDITIONS IMPROVING TO VFR MID-LATE AFTN AND THEN VFR AT NIGHT
AND ON WEDNESDAY.

OUTLOOK...WEDNESDAY NIGHT THROUGH SATURDAY...

WEDNESDAY NIGHT...WEAK COLD FRONT MOVES SOUTH OF NEW ENGLAND.
WINDS THEN SHIFT TO NORTHWEST AND DIMINISH. WITH DRIER AIR MOVING
IN...CEILINGS AND VSBYS SHOULD IMPROVE TO VFR ESPECIALLY AFTER
06Z.

THURSDAY...VFR. MEDIUM TO HIGH CONFIDENCE.

FRIDAY-SATURDAY...LOW TO VERY LOW CONFIDENCE. TIMING AND IMPACT
OF LOW PRES SYSTEMS PASSING S OF THE REGION REMAINS UNCERTAIN. MAY
SEE MVFR-IFR CIGS/VSBYS IN AREAS OF RAIN. CURRENT TIMING SUGGESTS
LOWEST CONDITIONS DURING FRI NIGHT WITH IMPROVEMENT ON SAT EXCEPT
FOR POSSIBLE MVFR CIGS/VSBYS ACROSS W MA/SW NH/N CENTRAL CT.

&&

.MARINE...
GALES REMAIN POSTED FOR MOST OF WATERS AS STRONGEST WINDS AFFECT
S COASTAL WATERS AND WELL OFF E MA COAST. DECIDED TO KEEP SCA FOR
BOS HARBOR...ADJACENT E MA WATERS AND NARRAGANSETT BAY SINCE CORE
OF STRONGER WINDS PASSES A BIT FARTHER OFFSHORE. WINDS SHOULD
DIMINISH AS FRONT CROSSES THIS AFTERNOON...BUT SHOULD PICK UP
AGAIN THIS EVENING.

MAIN TIME FRAME FOR SHOWERS/ISOLATED TSTMS IS FROM LATE MORNING
INTO MID AFTERNOON. AREAS OF FOG EXPECTED THROUGH MUCH OF TODAY.

INCREASING S FLOW WILL BRING HIGHEST SEAS /10-14 FT/ TO WATERS S
OF CAPE COD. THESE HIGHER SEAS WILL SHIFT OFFSHORE LATER TODAY AS
FLOW BECOMES MORE W/NW.

SCA CONDITIONS PROBABLE TONIGHT AND WED...MAINLY FOR SEAS. WEAK
FRONT EXPECTED TO BRING SCATTERED SHOWERS WED AFTERNOON.

OUTLOOK...WEDNESDAY NIGHT THROUGH SATURDAY...

WEDNESDAY NIGHT...DIMINISHING SEAS AND NORTHWEST WINDS WITH VALUES
BELOW SMALL CRAFT CRITERIA LATE AT NIGHT.

THURSDAY...HIGH PRES WILL BUILD N OF THE WATERS AS A FRONT STALLS
S OF THE WATERS. EXPECT LIGHT E WINDS AND SEAS...HIGHEST VALUES
WILL BE ON THE SOUTHERN OUTER WATERS.

FRIDAY-SATURDAY...LOW PRES WILL PASS S OF THE WATERS FRI NIGHT INTO
EARLY SAT...SO EXPECT E-NE WINDS. WINDS AND SEAS CURRENTLY LOOK TO
REMAIN BELOW SMALL CRAFT THRESHOLDS...BUT THIS IS A LOW CONFIDENCE
FORECAST.

&&

.HYDROLOGY...
315 AM...INCORPORATED LATEST NERFC GUIDANCE WHICH BRINGS 0.25 TO
0.50 INCH OF RAINFALL TODAY ON AVERAGE WITH HIGHER AMOUNTS NEAR
CT RIVER VALLEY.

FLOODING IS NOT EXPECTED AT THIS TIME. RISES ARE FORECAST ALONG
MAINSTEM CT AND MERRIMACK RIVERS OVER NEXT FEW DAYS...WITH BRIEF
RISES ON SMALLER TRIBUTARIES.

&&

.BOX WATCHES/WARNINGS/ADVISORIES...
CT...NONE.
MA...DENSE FOG ADVISORY UNTIL 9 AM EDT THIS MORNING FOR MAZ003>007-
     012-026.
     WIND ADVISORY UNTIL 6 PM EDT THIS EVENING FOR MAZ022>024.
NH...DENSE FOG ADVISORY UNTIL 9 AM EDT THIS MORNING FOR NHZ011-012-
     015.
RI...WIND ADVISORY UNTIL 6 PM EDT THIS EVENING FOR RIZ008.
MARINE...GALE WARNING UNTIL 2 PM EDT THIS AFTERNOON FOR ANZ232>235-237.
     SMALL CRAFT ADVISORY UNTIL 1 AM EDT WEDNESDAY FOR ANZ230-236-
     251.
     GALE WARNING UNTIL 4 PM EDT THIS AFTERNOON FOR ANZ231.
     GALE WARNING UNTIL 4 PM EDT THIS AFTERNOON FOR ANZ250-254>256.

&&

SYNOPSIS...WTB/JWD
NEAR TERM...JWD
SHORT TERM...JWD
LONG TERM...WTB
AVIATION...WTB/JWD
MARINE...WTB/JWD
HYDROLOGY...

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