Area Forecast Discussion
Issued by NWS Boston, MA

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000
FXUS61 KBOX 220559
AFDBOX

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE TAUNTON MA
159 AM EDT FRI MAY 22 2015

.SYNOPSIS...
FAST-MOVING LOW PRESSURE WILL BRING A PERIOD OF LIGHT RAIN TO
RHODE ISLAND AND SOUTHEAST MASSACHUSETTS THIS MORNING. THEN A
COLD FRONT MAY TRIGGER AN ISOLATED SHOWER LATE THIS AFTERNOON
AND EARLY EVENING. BEHIND THIS FRONT...FROST AND MAYBE EVEN
RECORD- BREAKING COLD...IS POSSIBLE FRIDAY NIGHT INTO SATURDAY
MORNING. HIGH PRESSURE WILL BRING ABOUT DRY WEATHER SUNDAY BUT A
PERIOD OF WET WEATHER MAY RETURN LATER MONDAY/MONDAY NIGHT.
WARMER BUT MORE UNSETTLED WEATHER IS EXPECTED NEXT WEEK.

&&

.NEAR TERM /THROUGH TODAY/...

2 AM UPDATE...

MID LEVEL FGEN RAIN BAND COMING ACROSS LI/LI SOUND...COASTAL CT
AND RI AT 2 AM...LIFTING NE AND WILL TRACK INTO INTERIOR RI AND
SOUTHEAST MA...PROBABLY STAYING SOUTH OF BOSTON. STEADIEST RAINS
WILL BE FOCUSED OVER CAPE COD AND THE ISLANDS THRU 12Z. PREVIOUS
FORECAST CAPTURES THESE DETAILS NICELY SO NO MAJOR CHANGES WITH
THIS UPDATE. EARLIER DISCUSSION BELOW.

================================================================

DESPITE RADAR RETURNS OVER BID/MVY AND ACK LOW LEVEL DRY AIR AS
SEEN ON THE 00Z OKX SOUNDING CONTINUES TO EVAPORATE ANY RAINFALL
BEFORE REACHING THE SURFACE ACROSS THIS AREA. EVENTUALLY BOUNDARY
LAYER WILL SATURATE AND RAIN WILL REACH THE GROUND ALONG THE SOUTH
COAST INCLUDING CAPE COD AND THE ISLANDS THE SECOND HALF OF THE
NIGHT INTO EARLY FRI MORNING.

AS FOR MODEL GUIDANCE...12Z ECMWF/18Z RUNS OF GFS AND NAM/21Z SREF
AND 12Z NMM AND ARW ALL BRING NORTHWEST PORTION OF RAIN SHIELD
INTO THE BOSTON-PROVIDENCE 06Z-12Z...ALBEIT VERY LIGHT RAIN WITH
STEADIER RAIN CONFINED TO IMMEDIATE SOUTH COAST INCLUDING CAPE COD
AND ISLANDS. AT 00Z RADAR AND SATELLITE TRENDS SUPPORT THIS
SCENARIO. LATEST RUNS OF THE HRRR AND RAP ARE FARTHER SOUTHEAST.
NONETHELESS CURRENT FORECAST CAPTURES THIS THEME WITH BULK OF RAIN
ALONG THE SOUTH COAST WITH MUCH LIGHTER RAIN NORTHWARD INTO
PROVIDENCE- BOSTON CORRIDOR. ALL OTHER FORECAST PARAMETERS ON
TRACK AS WELL...THUS LITTLE IF ANY CHANGES WITH THIS FORECAST
UPDATE. EARLIER DISCUSSION BELOW.

==================================================================

TONIGHT...
NOT MUCH CHANGE FROM PREVIOUS THINKING. NORTHERN STREAM SHORTWAVE
ENERGY WILL ARRIVE LATER TONIGHT. THIS FEATURE WILL SPAWN A LATE
SEASON COASTAL LOW WAVE OVER THE SOUTHEAST STATES...THAT WILL
TRACK NEAR OR JUST SOUTHEAST OF THE 40N/70W BENCHMARK BY FRIDAY
MORNING.

21/12Z GUIDANCE CONTINUES ITS EXCELLENT AGREEMENT ON STEADY
RAINFALL WORKING INTO SOUTHEAST MA AND PORTIONS OF RI OVERNIGHT.
THERE IS STILL SOME QUESTION HOW FAR WEST THE BACK EDGE OF THE
PRECIPITATION SHIELD WILL MAKE IT. THE 09Z SREF IS THE FARTHEST
WEST...WITH THE 12Z NAM THE FARTHEST EAST. STAYED WITH A MORE
MODERATE APPROACH...AS EVIDENCED BY THE GFS AND ECMWF SOLUTIONS.
SINCE THIS WILL BE A DEVELOPING COASTAL LOW...TIMING OF THE
DYNAMIC COMPONENTS COMING TOGETHER WILL BE CRUCIAL. WILL NEED TO
WATCH THIS ASPECT OF THIS STORM CLOSELY.

REGARDING CONVECTIVE POTENTIAL...MODEL SOUNDINGS ARE NOT FAVORABLE
FOR ANY SORT OF CONVECTION OVERNIGHT.

&&

.SHORT TERM /TONIGHT THROUGH 6 PM SATURDAY/...
FRIDAY...

A 3-PART DAY WITH MORNING RAINS LIKELY OVER RI AND SOUTHEAST
MA...POSSIBLY NORTHWARD TO BOSTON. DRIER WEATHER...ALONG WITH
MORE SUNSHINE...BRIEFLY DEVELOPS MID TO LATE MORNING AS COASTAL
LOW PRESSURE MOVES FARTHER OFFSHORE.

ANOTHER MID-LEVEL SHORTWAVE IS STILL EXPECTED TO MOVE THROUGH
SOUTHERN NEW ENGLAND FRIDAY AFTERNOON. COMBINED WITH AN
APPROACHING COLD FRONT...THERE SHOULD BE ENOUGH OF A TRIGGER TO
GENERATE SCATTERED SHOWERS ACROSS SOUTHERN NEW ENGLAND...MAINLY
ACROSS EASTERN MA. UNSEASONABLY STRONG COLD POOL ALOFT WILL ALSO
MEAN FAIRLY STEEP LAPSE RATES. THE BIG QUESTION WHICH REMAINS TO
BE ANSWERED WILL BE THE AMOUNT OF AVAILABLE MOISTURE. JUST ABOUT
ALL OF THE GUIDANCE REMAINS VERY DRY ABOVE 10 KFT. THUS CANNOT
COMPLETELY RULE OUT THE POSSIBILITY OF ISOLATED THUNDERSTORMS.
SHOULD ANY THUNDERSTORMS DEVELOP...SMALL HAIL WOULD BE A RISK IN
THE STRONGER CELLS.

FRIDAY NIGHT...
CONFIDENT WITH THE COLD FRONT PUSHING OFFSHORE THAT COLD AIR
ADVECTION WILL BE IN FULL SWING INTO SATURDAY MORNING. PROBABLY
LOOKING AT TEMPERATURES COLD ENOUGH TO WARRANT FROST ADVISORIES
ACROSS PORTIONS OF WESTERN MA...AGAIN. AS PREVIOUSLY MENTIONED...
THERE IS STILL AN ISSUE WITH WINDS. STILL EXPECTING A STEADY
NORTHWEST WIND...WHICH WOULD BE ENOUGH TO KEEP THE SURFACE LAYER
MIXED...AND PRECLUDE WIDESPREAD FROST DEVELOPMENT. WILL NEED TO
KEEP AN EYE ON THIS POTENTIAL. WILL BE CLOSE TO RECORD LOWS FOR A
FEW LOCATIONS /SEE THE CLIMATE SECTION BELOW/.

&&

.LONG TERM /SATURDAY NIGHT THROUGH THURSDAY/...
*/ HIGHLIGHTS...

 - UNSEASONABLY COOL SATURDAY
 - FROST / FREEZE HEADLINES LIKELY FOR SATURDAY MORNING
 - SEASONABLE AND DRY FOR SUNDAY
 - BEGINNING MEMORIAL DAY...A WET-WEATHER PATTERN BEGINS TO EMERGE
 - COULD REMAIN SEASONABLE AND WET THROUGH THE REMAINDER OF THE WEEK

*/ DISCUSSION...

CONFIDENT WITH THE COLD FRONT PUSHING OFFSHORE FRIDAY NIGHT THAT
COLD AIR ADVECTION WILL BE IN FULL SWING INTO SATURDAY MORNING.
BREEZY NW FLOW REARWARD OF THE PIVOTING VORTEX THRU N NEW ENGLAND
WILL USHER AN ANOMALOUSLY COLD AIRMASS S ALOFT. EXPECT WIDESPREAD
FROST/FREEZE HEADLINES ACROSS THE INTERIOR WITH THE BIGGEST IMPACT
OVER N/W PORTIONS OF MA. BUT AGREE WITH PREVIOUS FORECASTER THINKING
THAT THE BREEZY NW-FLOW COULD MITIGATE FROST DEVELOPMENT DESPITE
COLD TEMPERATURES. WILL BE CLOSE TO RECORD LOWS /SEE THE CLIMATE
SECTION BELOW/. COLD AIRMASS LINGERS AS HIGH PRESSURE BUILDS S OF
NEW ENGLAND. COOL AND DRY FOR SATURDAY. MORE SEASONABLE SUNDAY WITH
RETURN S-FLOW.

BEGINNING MEMORIAL DAY AM LESS CONFIDENT WITH THE FORECAST. PACIFIC-
ORIGIN IMPULSES ROTATING THROUGH THE W-HALF OF THE CONUS INVOKE DOWN-
STREAM RIDGING INTO NEW ENGLAND. BERMUDA HIGH REAFFIRMS ROUND THE W-
PERIPHERY WHICH SW-FLOW PREVAILS. THE WARM-FRONTAL LEADING EDGE OF
WARM-MOIST AIR WILL BE A FOCUS FOR WET-WEATHER AS PACIFIC IMPULSES
EJECT E THROUGH RIDGE PATTERN. COMBINATION OF ISENTROPIC ASCENT
BENEATH DIFFLUENCE ALOFT ALLOWING FOR BROADSCALE LIFT...CAN NOT RULE
OUT SHOWERS AROUND MONDAY - TUESDAY. FEEL BETTER CHANCES ARE N/W
AWAY FROM THE ANTICYCLONIC FLOW / DRY AIR AND TOWARDS MORE FAVORABLE
DYNAMICS.

FOR THE REST OF THE WEEK ENSEMBLE AND DETERMINISTIC GUIDANCE SUGGEST
A FLAT-FLOW REGIME ACROSS THE CONUS THROUGH WHICH PACIFIC-ORIGIN
IMPULSES PREVAIL. CONSIDERING THE OFFSHORE BERMUDA HIGH AND A VORTEX
PREVAILING ACROSS CANADA...MAY BE DEALING WITH A MORE ACTIVE PATTERN
AS THE WARM-MOIST FLOW IS STRETCHED ACROSS THE NE CONUS
UNDERGOING LIFT AND ENHANCEMENT AS MID-LEVEL IMPULSES TRANSLATE
THROUGH THE REGION. UNCERTAINTY AS ANY WOBBLE IN THE FLOW PATTERN
BETWEEN THE VORTEX N AND THE BERMUDA HIGH S WOULD MODIFY THE
CONVERGENT FLOW. GFS SUGGESTS A +1 SD OF H925 TEMPERATURES FOR
NEXT WEEK. IF +2 IT WOULD BE MORE LIKELY THAT THE CONVERGENT FLOW
WOULD BE N AND W WITH S NEW ENGLAND WARMER AND DRY BENEATH THE
RIDGE. WITH A CONSENSUS BLEND...WILL KEEP IT SEASONABLE WITH
SLIGHT CHANCE TO CHANCE POPS. ECHOING THE POINT MADE BY THE
PREVIOUS FORECASTER...COULD FIND OURSELVES WITHIN THE STEERING
FLOW OF UPSTREAM MCS/S. CONTINUED SIGNALS OF A CUTOFF LOW WOBBLING
ACROSS THE NW-ATLANTIC. WILL HAVE TO KEEP AN EYE ON THAT.

&&

.AVIATION /06Z FRIDAY THROUGH TUESDAY/...
FORECASTER CONFIDENCE LEVELS...

LOW...LESS THAN 30 PERCENT.
MODERATE...30 TO 60 PERCENT.
HIGH...GREATER THAN 60 PERCENT.

SHORT TERM /THROUGH FRIDAY/...

TODAY...RAIN SHIELD TRACKS OVER SOUTHEAST CT...SRN HALF OF RI AND
INTO SOUTHEAST MA THRU 12Z...THEN OFFSHORE. MVFR WITHIN THIS AREA
ALONG WITH LOW RISK OF BRIEF IFR. RAIN MAINLY LIGHT. VFR ELSEWHERE.
THEN BECOMING VFR AFT 12Z AS RAIN MOVES OFFSHORE. LOW RISK OF A
SPOT SHOWER LATE THIS AFTERNOON INTO EARLY EVENING.

TONIGHT...WNW WINDS INCREASE UP TO 25 KT IN GUSTS. SPOT SHOWER
POSSIBLE UNTIL 03Z OR SO. OTHERWISE VFR AND DRY WEATHER.

SATURDAY...WEST WINDS UP TO 20 KT IN GUSTS. VFR AND DRY WEATHER
PREVAIL.

KBOS TERMINAL...HIGH CONFIDENCE IN TAF.

KBDL TERMINAL...HIGH CONFIDENCE IN TAF.

OUTLOOK...SATURDAY NIGHT THROUGH TUESDAY...

SATURDAY NIGHT INTO SUNDAY NIGHT...HIGH CONFIDENCE.

VFR. BREEZY NW-WINDS THROUGH SATURDAY VEERING SW INTO SUNDAY.

MONDAY INTO MONDAY NIGHT...MODERATE CONFIDENCE.

LOW-END VFR TOWARDS THE LATER-HALF WITH INCREASING -SHRA CHANCES.

&&

.MARINE...
FORECASTER CONFIDENCE LEVELS...

LOW...LESS THAN 30 PERCENT.
MODERATE...30 TO 60 PERCENT.
HIGH...GREATER THAN 60 PERCENT.

SHORT TERM /THROUGH FRIDAY NIGHT/...

RAIN OVERSPREADS THE SOUTHERN WATERS OVERNIGHT AS LOW PRES TRACKS
NEAR OR JUST SOUTH OF 40N/70W BENCHMARK. SOME UNCERTAINTY ON
NORTHERN EXTENT OF RAIN SHIELD. STEADY RAINFALL GIVES WAY TO
SCATTERED SHOWERS FRIDAY AFTERNOON AND EVENING AS A COLD FRONT
APPROACHES FROM THE WEST. GUSTY NORTHWEST WINDS DEVELOP BEHIND
THIS FRONT...WITH THE POTENTIAL FOR GUSTS UP TO 30 KT IN PLACES.
SMALL CRAFT ADVISORIES MAY BE NEEDED SOMETIME FRIDAY NIGHT INTO
SATURDAY...ESPECIALLY ACROSS THE OUTER COASTAL WATERS.

OUTLOOK...SATURDAY THROUGH TUESDAY...

BREEZY NW-WINDS WITH GUSTS OVER 25 KTS THROUGH SATURDAY VEERING SW
INTO SUNDAY AND REMAINING BRISK WITH GUSTS UP TO 20 KTS. LOOKING AT
WAVES BUILDING IN EXCESS OF 5 FEET ON THE OUTER WATERS

MONDAY INTO MONDAY NIGHT...MODERATE CONFIDENCE.

CONTINUED BREEZY SW-WINDS WITH GUSTS UP TO 20 KTS. WAVES IN EXCESS
OF 5-FEET MAY REMAIN ON THE S/SE-WATERS.

&&

.CLIMATE...
RECORD LOWS FOR MAY 23RD...

BOSTON - 40 /1967/
HARTFORD - 37 /1976/
PROVIDENCE - 40 /1950/
WORCESTER - 32 /1929/

&&

.BOX WATCHES/WARNINGS/ADVISORIES...
CT...NONE.
MA...NONE.
RI...NONE.
MARINE...NONE.

&&

$$
SYNOPSIS...BELK/DOODY
NEAR TERM...NOCERA
SHORT TERM...BELK
LONG TERM...SIPPRELL
AVIATION...NOCERA/SIPPRELL
MARINE...BELK/SIPPRELL
CLIMATE...STAFF



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