Area Forecast Discussion
Issued by NWS Portland, ME

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FXUS61 KGYX 211432
AFDGYX

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE GRAY ME
932 AM EST SUN DEC 21 2014

.SYNOPSIS...
HIGH PRESSURE BUILDS OVER NORTHERN MAINE SUNDAY...ALLOWING THE
FLOW TO TURN ONSHORE. A WEAK WEATHER DISTURBANCE MAY TRIGGER SOME
FLURRIES OR LIGHT SNOW OVER SOUTHERN REGIONS RIGHT THROUGH MONDAY.
A SIGNIFICANT LOW PRESSURE SYSTEM SYSTEM WILL GRADUALLY ORGANIZE
WEDNESDAY...BEFORE MOVING OVER THE GREAT LAKES. THIS COULD BRING
WINDY CONDITIONS AND THE POTENTIAL FOR HEAVY RAIN AROUND CHRISTMAS
EVE INTO EARLY CHRISTMAS MORNING.

&&

.NEAR TERM /THROUGH TONIGHT/...
UPDATE...
HAVE UPDATED GRIDS TO RAISE POPS FROM SLIGHT CHANCE/CHANCE TO
CATEGORICAL...AT LEAST FOR THIS MORNING. LOW LEVEL MOISTURE
CONTINUES TO DEPOSIT SNOW ACROSS SOUTHERN NH AND FAR WESTERN
MAINE. WILL FORECAST 1 TO 2 INCHES OF SNOW FOR TODAY AS SOME AREAS
WILL REMAIN NEAR A MILE VSBY IN SNOW. SOME HIGHER REFLECTIVITY
RETURNS IN THE ROCHESTER NH AREA AS WELL WILL PINWHEEL SLOWLY TO
THE NE. THIS IS IN ASSOCIATION WITH AN INVERTED TROUGH OVER
SOUTHERN NEW ENGLAND AS OF 14Z WITH AN UPPER LEVEL LOW SLOWLY
CROSSING THE FORECAST AREA TODAY AS WELL. NUMERICAL MODELS SHIFT
PRECIP SOUTH AND EAST THIS AFTERNOON...WHICH IS IN DISAGREEMENT
WITH THE CURRENT RADAR ANIMATIONS.

OTHERWISE...JUST MINOR MODIFICATIONS TO THE REST OF THE GRIDED
PACKAGE.

PREV DISC...
615 AM UPDATE...JUST MADE SOME MINOR UPDATES TO SKY
AND TEMPERATURES BASED ON CURRENT OBS AND TRENDS...AS STRATUS
REMAINS ENTRENCHED ACROSS THE REGION.

PREVIOUS DISCUSSION...
LOW STRATUS BLANKETS THE CWFA AS A SURFACE HIGH PRESSURE IN
SOUTHERN QUEBEC IS HELPING TO KEEP MOISTURE TRAPPED IN THE LOW
LEVELS. A WEAK SHORTWAVE THAT IS PASSING OVER THE SOUTHERN NEW
ENGLAND THIS MORNING AND THIS MAY HELP KICK OFF SOME LIGHT SNOW
ACROSS THE FAR SOUTHERN AREAS OF NEW HAMPSHIRE AND PARTS OF
SOUTHWEST MAINE...AS ON SHORE FLOW SUPPLIES THE LOW LEVEL MOISTURE
THE LIFT FROM THIS WEAK SHORT MAY BE JUST ENOUGH TO PRODUCE SNOW
SHOWERS HEAVY ENOUGH TO ACCUMULATE IN SOME AREAS. AS THIS WEAK
WAVE MOVES EAST OUR FLOW...ALTHOUGH VERY WEAK...WILL TURN TO
NORTHWESTERLY AND THIS MAY BE ENOUGH TO START CLEARING OUT SOME OF
THE LOW STRATUS SUNDAY NIGHT.

&&

.SHORT TERM /MONDAY/...
FLOW WILL SWITCH BACK TO ON SHORE BY NOON ON MONDAY AND THIS WILL
BRING LIGHT OCEAN EFFECT SNOW OR MAYBE EVEN SOME LIGHT RAIN OR
EVEN FREEZING DRIZZLE INTO THE INTERIOR AS TEMPERATURES WILL HOVER
CLOSE TO FREEZING AS YOU GET AWAY FROM THE COAST.

&&

.LONG TERM /MONDAY NIGHT THROUGH SATURDAY/...
MODELS CONTINUE TO TREND TOWARD UPSTREAM AND DOWNSTREAM
BLOCKING...WITH AN EVENTUAL RETURN TO A +PNA PATTERN AT
500MB...WHICH WOULD SIGNAL COOLING LATE IN THE PERIOD. HOWEVER
ALL THE ACTION WILL BE IN DAYS 3 TO 5 AS THE UPSTREAM DEVELOPS AND
SENDS WAVES INTO TROUGH OVER CENTRAL CONUS...AMPLIFYING THE 500
MB FLOW.

INITIALLY...WILL BE DEALING WITH SMALLER SCALE FEATURE AS
INVERTED TROUGH OVER THE GULF OF ME WORK ITS WAY ONSHORE MON NIGHT
AND WEAKENS. MAY BE ENOUGH DYNAMIC FORCING FOR ICE CRYSTALS
INITIALLY...SO RAIN OR SNOW SHOWERS MOVE ONSHORE AND INLAND DURING
THE EVENING...BUT COULD SEE IT TURN TO DZ/FZDZ AS WE WE JUST
TRANSPORT WATER DROPLETS ONSHORE. WE MAINTAIN AN ONSHORE FLOW INTO
TUESDAY...BUT IT LOOKS LIKE FORCING WILL BE LIMITED TO
UPSLOPE...SO THINKING MORE IN THE WAY OF SHSN/SHRA/VERY LIGHT SNOW
INLAND...WITH JUST CLOUDS ON THE COAST. SHOULD SEE TEMPS ON TUE
WARM INTO THE UPPER 30S TO MID 40S.

THE MAIN EVENT REMAINS THE IMPRESSIVE 500 MB TROUGH THAT DEVELOPS
OVER THE CENTRAL CONUS MID WEEK...NOT SO MUCH IN THAT IT IS VERY
DEEP...BUT MORE ITS AMPLITUDE AS IT EXTENDS FROM N OF THE GREAT
LAKES SWD INTO THE GULF OF MEX....AND THEN TILTS VERY NEG AND
LIFTS POLEWARD WED NIGHT AND THU. NOT SURE HOW MUCH AN EFFECT
CONVECTIVE SCALING ISSUES ARE HAVING ON THE MODELS...BUT IT MAY BE
HAVING SOME EFFECT ON THE FORECAST DETAILS...BUT PROB NOT THE
GENERALITIES OF THE FORECAST. HOWEVER...00Z EURO AND GFS BOTH SHOW
A WEAKER SFC LOW IN THE VCNITY OF THE GRT LAKES AND ST LAWRENCE
VLY /ALTHOUGH IT DOES INTENSIFY FURTHER N AND E THAN PREV MODEL
RUNS. STILL...AS THAT WAVE LIFTS NWD WILL SEE TRIPLE POINT DEVELOP
OVER THE MID ATLANTIC WED EVE...AND LIFT NNE INTO THE CWA BY THU
MORNING. THIS WILL CREATE A DEEP SOUTHERLY FLOW WHICH WILL CONNECT
WITH GULF MOISTURE...ALLOWING FOR SIGNIFICANT RAINFALL...FROM
LATE IN THE DAY WED...INTO EARLY THU MORNING. QPF LOOKS TO RUN
AROUND 2-3 INCHES IN MANY SPOTS...BUT S-SE UPSLOPE AREAS OF THE
FOOTHILLS AND MTNS COULD SEE AN INCH OR TWO MORE...SO THE THREAT
OF BOTH FLASH AND MAINSTEM FLOODING IS A CONCERN...ESPECIALLY
SINCE THIS WILL BE COMBINE WITH SIGNIFICANT SNOWMELT AS HIGHS
RISE WELL INTO THE 40S WED..AND WILL LKLY STAY THERE WED NIGHT
INTO THU.

THERE IS STILL THE POTENTIAL FOR SOME STRONG GUSTS...BUT GIVEN
WEAKER SYSTEM...THE WINDOW FOR OPPORTUNITY WILL BE LIMITED TO A
FE HOURS AHEAD OF THE TRIPLE POINT PASSAGE...WHICH LOOKS TO BE WED
EVE...AT LEAST FOR NOW. WITH ALL THAT SAID...WE ARE STILL DEALING
WITH DAYS 4 AND 5 OF THE FORECAST...WITH THE POTENTIAL FRO SOME
DEEP CONVECTION IN THE BASE OF THE TROUGH...SO FORECAST COULD
CHANGE...BUT BOTH EURO AND GFS PICKED UP THE SIGNALS OF THIS
SYSTEM EARLY...AND HAVE BEEN CONSISTENT ON THE WHOLE SO FAR.

CHRISTMAS DAY WILL SEE RAIN ENDING FROM SW TO NE DURING THE
MORNING...WITH SOME PARTIAL CLEARING POSSIBLE IN THE
AFTERNOON...AND TEMPS IN THE 40S AGAIN. COOLER AIR WILL MOVE IN
FROM THE SW THU NIGHT. FRI AND SAT LOOK MOSTLY DRY /OTHER THAN
POSSIBLE UPSLOPE SHSN ON FRI/. HIGHS WILL CONTINUE TO RUN A FEW
DEGREES ABV NORMAL.

&&

.AVIATION /15Z SUNDAY THROUGH THURSDAY/...
SHORT TERM...MVFR TO IFR CEILINGS WILL REMAIN OVER THE REGION
THROUGH AT LEAST SUNDAY EVENING. CONDITIONS MAY IMPROVE A BIT
SUNDAY NIGHT AS THE ON SHORE FLOW DRIFTING MOISTURE INLAND WILL
GET SHUT OFF. CONDITIONS WILL NOT STAY UP LONG HOWEVER AS THE FLOW
WILL SWITCH BACK TO ON SHORE ON MONDAY BRINGING ANOTHER ROUND OF
MVFR TO IFR CEILINGS.

LONG TERM...POSSIBLE PERIODS OF MVFR TO IFR MONDAY NIGHT AS
INVERTED TROUGH MOVES INLAND. MVFR LIKELY TO HOLD TUESDAY INTO
TUESDAY NIGHT...LOWERING TO IFR LATE TUESDAY NIGHT INTO THURSDAY
MORNING IN RAIN...WITH INCREASING E-SE WINDS WEDNESDAY.

&&

.MARINE...
SHORT TERM...A FEW WIND GUSTS CONTINUE JUST BELOW 25 KTS THIS
AFTERNOON...WITH WAVES 4` OR LOWER...THEREFORE NO FLAGS IN THE
NEAR TERM. WINDS SHOULD CONTINUE TO SLOWLY DECREASE TONIGHT INTO
TOMORROW FROM THE NORTHEAST.

LONG TERM...HIGH PRESSURE WITH LIGHT AND FAIRLY CALM SEAS WILL
PERSIST THROUGH THE PERIOD AND NO FLAGS ARE EXPECTED.

&&

.GYX WATCHES/WARNINGS/ADVISORIES...
ME...NONE.
NH...NONE.
MARINE...NONE.

&&

$$
JC


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