Prognostic Meteorological Discussion
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FXUS02 KWBC 161458
PMDEPD

EXTENDED FORECAST DISCUSSION
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
1058 AM EDT WED APR 16 2014

VALID 12Z SAT APR 19 2014 - 12Z WED APR 23 2014

...OVERVIEW...
THE MEDIUM RANGE PERIOD WILL DISPLAY A BRIEF DE-AMPLIFICATION OF
THE FLOW PATTERN ACROSS THE EASTERN HALF OF THE CONUS--AND MORE
THAN THE USUAL AMOUNT OF UNCERTAINTY--WITH LOW-AMPLITUDE PACIFIC
WAVE ENERGY EJECTING OUT OF THE NORTHERN ROCKIES. BY DAY
6-7...INTENSE ENERGY SWIRLING IN THE GULF OF ALASKA WILL APPROACH
THE WEST COAST AND A MODERATELY-STRONG SPRING CUTOFF SYSTEM WILL
AMPLIFY AND MIGRATE INTO THE SOUTHERN GREAT BASIN AND LOWER HALF
OF THE COLORADO RIVER BASIN. TO ADD CHALLENGE TO THE FLOW PATTERN
FORECAST...INJECTION OF A SUBTLE...BUT ACTIVE SOUTHERN STREAM JET
AHEAD OF THE PACIFIC CUTOFF WILL PROVIDE ADDITIONAL OPPORTUNITIES
FOR CONVECTIVE PRECIPITATION ACROSS THE SOUTHERN PLAINS...LOWER TO
MID-MISSISSIPPI VALLEY AND PORTIONS OF THE OHIO VALLEY AND
CAROLINA COASTAL PLAINS.

...PREFERENCES/UNCERTAINTY ASSESSMENT...
CONSIDERING THE COMPLICATED FLOW PATTERN EXPECTED ACROSS NORTH
AMERICA...AND LESS THAN IDEAL AGREEMENT/CONTINUITY IN RECENT
DAYS...THE 16/00Z GUIDANCE SEEMED TO BE IN `BETTER` AGREEMENT EAST
OF THE ROCKIES THROUGH DAY 5. BUT WPC STILL RECOMMENDS MORE OF A
ECENS-ECMWF DETERMINISTIC APPROACH DAY 3 THROUGH DAY 7 FOR NORTH
AMERICA...THE CENTRAL AND EAST CONUS...ALONG WITH THE NORTHEAST
PACIFIC OCEAN AND ALASKA.

MUCH OF THE `BETTER` AGREEMENT IS THE RESULT OF A MORE `STABLE
WAVE` SOLUTION IN THE GULF OF ALASKA AND ACROSS THE NORTH CENTRAL
PACIFIC. THE `PROJECTED` TIMING AND DEPTH OF A SPRING-LIKE
`CUTOFF` 500MB LOW MIGRATION INTO THE WEST COAST HAS ...MORE OR
LESS...RESOLVED ITSELF WITH THE 16/00Z GEFS AND THE 16/00Z ECENS
MEANS MEETING HALFWAY AT APPROXIMATELY 21/12Z ALONG 130W
LONGITUDE. AT THIS APPROXIMATE TIME FRAME AND LONGITUDE...THE
`MEAN` 500MB TROUGH AXIS WILL HAVE CROSSED 130W AND THE `PINCHING
OFF` OF AN ACTIVE SPRING CUTOFF...WILL GENERALLY BE AN `ON GOING`
EVENT.

BUT THERE IS MORE TO THIS...AND WHERE THE UNCERTAINTY REMAINS
BEYOND DAY 5...IS THE CUTOFF TIMING...AT WHAT LATITUDE IT
OCCURS...AND WHEN THE 500MB TROUGH AXIS MIGRATES THROUGH THE
CASCADES. NEVER MIND...WHEN THE `CUTOFF` ENTERS THE SOUTHWESTERN
US...GREAT BASIN...AND SOUTHERN ROCKIES...FOR THOSE DETAILS ARE
RELATED TO AN `UPSTREAM` TROUGH AND AMPLIFIED RIDGE POSITION THAT
THE GEFS/ECENS CONTINUE TO BE `AT ODDS` WITH TIMING-WISE IN THE
ALEUTIANS AND WESTERN GULF OF ALASKA. BEYOND DAY 5...DID NOT PUT
MUCH STOCK IN THE DAY 6-7 GUIDANCE WITH RESPECT TO THEIR TIMING
AND `PERFECT PROG MIGRATION` OF A DEEP CYCLONE EXITING THE FRONT
RANGE OF THE ROCKIES. THESE SOLUTIONS ARE BASED ON A TIMING
UPSTREAM IN THE ALEUTIANS THAT IS FAR FROM RESOLVED. THE GEFS
CONTINUING TO BE A FASTER SOLUTION VS THE ECMWF. THAT SAID...THERE
WILL BE ROOM FOR AT LEAST 1-2 MID-LEVEL SHORTWAVES TO MIGRATE
THROUGH SOUTHERN CANADA AND THE NORTHERN PLAINS BETWEEN DAYS
3-6...OUT AHEAD OF THE COMPLEX...AND MORE
IMPORTANTLY...UNACCOUNTED FOR IN THE 16/00Z GUIDANCE...THAT ARE
ORIGINATING IN THIS DIGGING PACIFIC TROUGH/SYSTEM ON DAYS 6-7.

ON THIS NOTE...THE 15/00Z ENSEMBLE PACKAGES (GEFS VS ECMWF) WERE
APPROXIMATELY 15 DEGREES OF LONGITUDE APART WITH THE ALEUTIAN LOW
AND `STABLE WAVE` PATTERN IN THE NORTH CENTRAL PACIFIC. THE
DIFFERENCES IN THE 16/00Z CYCLE WERE CLOSER TO 10 DEGREES APART.
SO THE SPREAD IS NARROWING...BUT THESE DIFFERENCES ARE STILL
CAUSING TOO MUCH SPREAD DOWNSTREAM ALONG THE WEST COAST...FOR
22/00Z (DAY 5.5) AND BEYOND.  THE 16/00Z GUIDANCE HAS NARROWED
SOME...BUT WHAT HAS BEEN `STABLE` WITH THIS ALEUTIAN LOW PATTERN
AND ITS DEVELOPMENT...IS THE LATITUDE OF THE DEEP UPPER-LEVEL
LOW...WHICH PROVIDE SOME SUBTLE BUT KEY POINTS DOWNSTREAM. IN
THAT...WHAT SHAKES OUT ALONG THE COAST OF WESTERN CANADA AND THE
PACIFIC NORTHWEST FOR DAYS 6 TO D8+ LOOKS TO BE MORE OF A
NEGATIVE-TILT TROUGH AXIS IN THE EASTERN GULF OF ALASKA AND WEST
CENTRAL NORTH AMERICA. AND WHAT HELPS THIS ALONG IS THE STABILITY
OF THE UPPER-LEVEL RIDGE OVER CENTRAL/NORTHERN ALASKA AND THE
YUKON.

WHAT CAN BE GLEANED AT THIS POINT IS...THAT THE PACIFIC NORTHWEST
SHOULD REMAIN IN MORE OF A WEAK...COLD ADVECTION PATTERN FOR DAYS
6-7 (ALBEIT A MARITIME POLAR AIRMASS) THAT SPREADS ACROSS THE
NORTHERN GREAT BASIN...ROCKIES. WARM ADVECTION ALONG WITH
UPPER-LEVEL RIDGING BEGINS TO BECOME A TREND IN THE FAR EXTENDED
ACROSS THE CENTRAL CONUS AND EVENTUALLY THE EASTERN CONUS. THIS IN
CONTRAST TO THE CONTINENTAL POLAR AIRMASS INTRUSION THAT SPILLED
INTO THE NORTHERN HIGH PLAINS LAST WEEKEND AND PRODUCED SNOWFALL
FROM MONTANA TO COLORADO TO THE OHIO VALLEY. THIS UPCOMING DAY 6-7
PATTERN WILL NOT BE THE SAME WITH A DIGGING/SHEARING UPPER-LEVEL
PACIFIC TROUGH AXIS. IF ANYTHING...MORE SOUTHWESTERLY FLOW ALOFT
AND DIFFLUENT FLOW REGIME... POSSIBLY GENERATING SOME DEEP
CONVECTION IF THE LOWER LEVELS CAN GET INTO MORE OF AN
EAST-SOUTHEAST OR SOUTHEAST UPSLOPE FLOW. WOULD THINK THAT IN THE
NEXT 2-3 12-HR FORECAST CYCLES...THIS `STABLE WAVE` WILL TRANSLATE
DOWNSTREAM INTO THE LOWER 48.

...SENSIBLE WEATHER HIGHLIGHTS...
SYSTEM CROSSING THE NORTHEAST DURING THE WEEKEND WILL LIKELY
GENERATE AREAS OF MOSTLY LGT PCPN PSBLY INCLUDING SOME POCKETS OF
SNOW IN NRN AREAS.  AT THE SAME TIME THE SERN COAST MAY SEE
LINGERING CLOUDS/RNFL DUE TO A SLOW MOVING/CLOSED MID LVL SYSTEM
AND ASSOC SFC WAVE.  UPSTREAM THE COMBINATION OF A NRN STREAM
PLAINS SHRTWV AND ENERGY EJECTING FROM THE SW WILL SUPPORT AN AREA
OF RNFL SPREADING EWD/NEWD FROM THE FOUR CORNERS/SRN HIGH PLAINS
AND NERN PLAINS/UPR MS VLY.  SOME ACTIVITY MAY BE LOCALLY HVY.
EXCEPT FOR NRN AREAS MOST OF THE WEST SHOULD SEE WARM TEMPS
SAT-MON WITH SOME 10-15F ABOVE NORMAL HIGHS.  AS THE AMPLIFYING
ERN PAC TROUGH ARRIVES IN THE WEST NEXT WEEK EXPECT A COOLING
TREND WHILE THE WARMTH HEADS INTO THE PLAINS... WHILE AREAS FROM
THE NRN HALF OF THE WEST COAST INTO THE NRN PLAINS SHOULD SEE
INCREASED COVERAGE OF PCPN.  MSTR COULD EXTEND FARTHER SWD OVER
THE WEST IF THE TROUGH ALOFT ENDS UP BEING DEEPER/MORE CLOSED AND
SLOWER THAN EVEN THE MOST RECENT 16/00Z CYCLE GUIDANCE.

VOJTESAK/RAUSCH

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