Prognostic Meteorological Discussion
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FXUS02 KWBC 140649
PMDEPD

EXTENDED FORECAST DISCUSSION
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
246 AM EDT MON JUL 14 2014

VALID 12Z THU JUL 17 2014 - 12Z MON JUL 21 2014

...VERY HEAVY RAINFALL THREATENS THE SOUTHERN PLAINS...
...COOL AND DRY CANADIAN AIRMASS TO INVADE THE LOWER 48...

...OVERVIEW/MODEL PREFERENCES...
THOUGHT THE NAEFS AND ECENS MEANS CONTINUE TO BE THE STEADIER
INDICATORS OF THE SUBTLE PATTERN CHANGE ALONG THE WESTERN COAST
AND MOUNTAIN CHAINS OF NORTH AMERICA. THE END RESULT CONTINUES TO
BE A GRADUAL MEAN LONGWAVE OVER WEST CENTRAL CANADA INVOF 110W
LONGITUDE AND THE DISPERSAL OF THE TROUGH IN EASTERN CANADA.

THE GRADUAL WEST-NORTHWESTWARD MIGRATION AND TEMPORARY
DISPLACEMENT OF THE  ANTICYCLONE IN THE GREAT BASIN AND SOUTHWEST
CANADA TO MORE OF AN OFFSHORE POSITION ALLOWS HIGH-LATITUDE FLOW
TO RE-ENTER THE NORTH AMERICAN CONTINENT ACROSS WESTERN CANADA AND
CARVE OUT A BROAD-SCALE WEST-NORTHWESTERLY FLOW OVER ALBERTA...THE
NORTHERN DIVIDE OF MONTANA/IDAHO AND DOWNSTREAM INTO THE CENTRAL
CANADA.

...UNCERTAINTY ASSESSMENT...
FOR DAYS 3 AND 4 (THROUGH 18/12Z)...THE 13/12Z GUIDANCE AND THE
14/00Z DETERMINISTIC GFS...ECMWF AND CANADIAN WERE NOT DRASTICALLY
DIFFERENT WITH THE MID-LEVEL TROUGH POSITION IN SOUTHWEST CANADA
AND THE NORTHERN ROCKIES. LIKEWISE...THEY ALL GENERALLY CARRY A
SIGNIFICANT---AND UPSTREAM--- CLOSED SYSTEM IN THE GULF OF ALASKA.

WHAT UNCERTAINTY REMAINS---BEGINS AROUND DAY 5 IN THE EASTERN GULF
OF ALASKA AS A BURST OF WESTERLY FLOW IN THE NORTH PACIFIC INVADES
BRITISH COLUMBIA...THE ALASKA PANHANDLE AND PACIFIC NORTHWEST.
THIS BRIEF PERIOD OF MID-LEVEL WIND ACCELERATION SHEARS OUT A
SERIES THE MID-LEVEL DISTURBANCES ACROSS EASTERN ALBERTA WITH
VARIED DEGREES OF SUCCESS IN GENERATING SIGNIFICANT DOWNSTREAM
CYCLOGENESIS AND SOUTHWARD/SOUTHEASTWARD PROGRESSION OF MORE
CANADIAN AIR INTO THE LOWER 48. FOR TIMING THE SURFACE WAVE
FORMATION(S) AND MIGRATION OF THE PRIMARY 500MB TROUGH ACROSS
WESTERN CANADA...USED THE DETERMINISTIC 13/12Z ECMWF APPROACH TO
HANDLE THE SURFACE FRONT SCENARIO AND A SUCCESSION OF 500MB WAVE
MIGRATIONS THROUGH CENTRAL CANADA. THE LAST 4-5 RUNS OF THE
DETERMINISTIC GFS LOOKED TOO DRASTICALLY DIFFERENT TO GLEAN ANY
SORT OF TREND FROM THEM.

OVERALL...THE DETERMINISTIC GUIDANCE CONTINUES TO STRUGGLE WITH
THE AMPLITUDE OF THE FLOW OVER THE CANADIAN ROCKIES AROUND DAY 7.
AND CONSEQUENTLY---A NEED TO BLEND THE ECENS AND NAEFS FROM DAY 5
TO DAY 7...TO HANDLE THE TRANSITION FROM OUR CURRENT PATTERN TO
THE `NEW PATTERN` ACROSS CANADA. THOUGHT IT MADE MORE SENSE TO GO
WITH A LOWER-AMPLITUDE FLOW REPLACING THE CURRENT `BLOCKY`
HIGH-AMPLITUDE RIDGE OVER WESTERN CANADA. UNFORTUNATELY...THE
14/00Z DETERMINISTIC GUIDANCE REMAINS AT ODDS WITH THE PLACEMENT
OF THE MASS FIELDS---LOWEST HEIGHTS OVER SOUTHWEST CANADA AND
HIGHEST HEIGHTS ACROSS THE SOUTH CENTRAL CONUS. SO WHAT TRANSPIRES
BEYOND DAY 7 APPEARS OUT OF REACH.

...SENSIBLE WEATHER HIGHLIGHTS...
HEAVY RAINFALL POTENTIAL FOR THE SOUTHERN PLAINS STATES...INVOF
THE RED RIVER AND SOUTHERN OZARKS THURSDAY AND FRIDAY...WITH
OVER-RUNNING ENVIRONMENT ALONG THE REMNANT COLD FRONT. OF
NOTE...THE ENSEMBLE MEANS ARE PRODUCING 2-3 INCH TOTALS FOR
PORTIONS OF SOUTHERN OKLAHOMA AND NORTHERN TEXAS DURING THIS 48
HOUR PERIOD. SOME INDIVIDUAL DETERMINISTIC RUNS ARE GENERATING
48-HOUR TOTALS IN THE 4-7 INCH RANGE.

IN THE WEST...THE NORTHWESTWARD-MIGRATION OF THE BROAD ANTICYCLONE
AND EMBEDDED MID-LEVEL DISTURBANCES ALONG THE WESTERN PERIPHERY OF
THIS CIRCULATION WILL SPREAD PERIODS OF MONSOON MOISTURE NORTHWARD
ACROSS CALIFORNIA...INCLUDING THE SIERRA AND SISKIYOUS THURSDAY
AND FRIDAY.

FRIDAY...SATURDAY AND SUNDAY...THE CANADIAN COLD FRONT WILL BECOME
AN ACTIVE PRECIPITATION ZONE FROM THE LOWER MS VALLEY
NORTHEASTWARD INTO THE APPALACHIANS...UPPER OHIO VALLEY AND EAST
COAST.

COOL AND DRY CANADIAN HIGH PRESSURE WILL DESCEND UPON THE CENTRAL
PLAINS...MIDWEST AND SPREAD EASTWARD INTO THE OHIO VALLEY. THE
BEST PROSPECTS FOR RECORD TEMPERATURES TO BE BROKEN WITHIN THIS
HIGH PRESSURE SYSTEM APPEAR TO BE ON THURSDAY AND FRIDAY...IE
OVERNIGHT LOWS AND LOW DAILY MAXES.

VOJTESAK

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