Prognostic Meteorological Discussion
Issued by NWS

Home | Current Version | Previous Version | Text Only | Print | Product List | Glossary On
Versions: 1 2 3 4 5 6
000
FXUS02 KWBC 120700
PMDEPD

EXTENDED FORECAST DISCUSSION...CORRECTED TYPO PARA 3
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
255 AM EDT SAT JUL 12 2014

VALID 12Z TUE JUL 15 2014 - 12Z SAT JUL 19 2014

...UNSEASONABLE TEMPERATURE READINGS FOR MUCH OF THE LOWER 48...

...OVERVIEW...
THE NORTH AMERICAN CONTINENT REMAINS UNDER A HIGHLY-AMPLIFIED FLOW
PATTERN THROUGH DAY 5...THEN THE PATTERN AT HIGH LATITUDE SLOWLY
RETROGRADES WESTWARD...ALLOWING THE AMPLIFIED RIDGE-TROUGH PATTERN
FROM THE ALEUTIANS TO HUDSON BAY TO SHIFT WESTWARD ABOUT 15
DEGREES OF LONGITUDE BY PERIOD`S END. THIS SUBTLE SHIFT SHOULD
RE-DIRECT COOLER PACIFIC AIR INTO SOUTHWESTERN CANADA AND THE
NORTHERN ROCKIES.

...UNCERTAINTY ASSESSMENT...
MUCH OF THE UNCERTAINTY IN THIS MEDIUM RANGE PERIOD AND PATTERN
REMAINS AT HIGH LATITUDE (ABOUT 55N TO 57N)...AND HAS TIES TO A
`RETROGRADING` UPPER-LEVEL RIDGE ALONG THE WEST COAST OF
CANADA...AND UPPER-LEVEL LOW IN THE NORTHWEST GULF OF ALASKA.
WHERE IT EVENTUALLY TIES INTO THE LOWER 48 WEATHER FORECAST HAS
MORE TO DO WITH THE PROJECTED `RE-POSITIONING` OF THE LONGWAVE
TROUGH OVER CANADA AND A SUBTLE ALTERATION IN THE CONFIGURATION OF
THE ANTICYCLONE IN THE GREAT BASIN. BOTH ARE NOT NECESSARILY
RELATED TO EACH OTHER.

WITH THE UPPER-LEVEL TROUGH OVER CANADA CURRENTLY LODGED INVOF 90W
LONGITUDE... COOLER AIR HAS BEEN ABLE TO DESCEND DIRECTLY DOWNWIND
OF THE RIDGE THROUGH SOUTH CENTRAL AND EAST CENTRAL CANADA...AND
INTO THE NORTHERN PLAINS AND GREAT LAKES. BUT WITH A WESTWARD
SHIFT IN THE UPPER-LEVEL RIDGE TO AN OFFSHORE POSITION IN THE GULF
OF ALASKA...THE CURRENT FLOW PATTERN ENDS---EAST OF THE DIVIDE.
THIS MAY ONLY BE A BRIEF PERIOD OF CHANGE...BUT ENOUGH TO
ESTABLISH THE LONGWAVE TROUGH CLOSER TO 100W-105W OVER CANADA BY
DAY 7. THE AXIS OF MONSOONAL MOISTURE WILL ALSO ATTEMPT TO BRIEFLY
RE-POSITION ITSELF...BUT IN RELATIONSHIP TO THE SUBTLE SHIFT OF
THE ANTICYCLONE IN THE FOUR CORNERS AND FLOW ACROSS NORTHERN
MEXICO.

AND IN ESSENCE...THE THREE MEANS---ECENS...GEFS AND NAEFS ALL SEEM
TO HANDLE THIS GRADUAL TRANSITION IN THE DAY 6-7 PERIOD.

...MODEL PREFERENCES...
THOUGHT THE 11/12Z DETERMINISTIC GFS...ECMWF AND CANADIAN WERE
USEFUL UNTIL ABOUT 16/12Z WITH RESPECT TO THE CLOSED 500MB LOW
MIGRATING THROUGH ONTARIO AND WITH A MID-LEVEL SHORTWAVE MIGRATING
INTO THE PACIFIC NORTHWEST AND NORTHERNMOST US ROCKIES.

THE CANADIAN WOULD BE A SLIGHTLY SLOWER SOLUTION HEADING INTO DAY
5 (17-00Z TO 17-12Z) VS THE ECMWF ACROSS EASTERN CANADA. THE
11/12Z CANADIAN AND 11/18Z GFS ARE STRONGER SOLUTIONS OVER EASTERN
MONTANA AT 16/00Z WITH THE PACIFIC NORTHWEST SHORTWAVE MIGRATING
FROM OREGON EASTWARD FROM THE DIVIDE DOWNWIND INTO WYOMING AND
COLORADO. THE SPREAD GETS TOO LARGE IN THE PLAINS TO GO WITH ANY
DETERMINISTIC RUN BY 17/00Z WITH EMBEDDED NORTHWEST FLOW OVER THE
CENTRAL PLAINS AND THE COLD FRONT WORKING SOUTHWARD ACROSS
OKLAHOMA...TEXAS AND THE SOUTHEAST.

NOT SURE WHAT TO MAKE OF THE 11/12Z AND 11/18Z DETERMINISTIC GFS
RUNS AFTER MID-POINT DAY 4 OVER WESTERN CANADA. THEY LIKED MORE OF
A PHASED SOLUTION OVER THE YUKON...WITH A SOUTHWARD EXTENSION OF
THE 500MB WAVE ACROSS BC AND INTO THE PACIFIC NORTHWEST. THIS
LOOKED TO BE A PREMATURE ATTEMPT TO ESTABLISH THE LONGWAVE ALONG
THE WEST COAST OF NORTH AMERICA. WOULD THINK THIS RETROGRESSION
TAKES ON A `MORE GRADUAL` APPROACH. NOT THAT THE DETERMINISTIC
11/12Z ECMWF AND CANADIAN WERE MUCH BETTER...BUT THESE SOLUTIONS
SEEMED TO BETTER FIT THE ENSEMBLE MEANS (12Z NAEFS...ECENS AND
GEFS) WITH A STEADY PROGRESSION OF THE HIGH-LATITUDE FLOW PATTERN
DOWNWIND OF THE UPPER-RIDGE AXIS.

THE 11/12Z GUIDANCE MAINTAINED NOTEWORTHY ANOMALIES AT VARIOUS
LEVELS BENEATH THE UPPER-LEVEL RIDGE AND UPPER-LEVEL TROUGH INTO
DAY 5...GIVING ADDED CONFIDENCE THAT THE LOWER 48 WILL BE
EXPERIENCING SOME TEMPERATURE DISTRIBUTIONS THAT WILL PRODUCE
"EXTRAORDINARY" VALUES FOR THE THIRD WEEK IN JULY (MAINLY DAYS
3-5).

ALTHOUGH THE DETERMINISTIC RUN-TO-RUN DEPICTION OF THE SURFACE LOW
TRACKING ACROSS SOUTHERN CANADA HAS BEEN LESS THAN STELLAR...THINK
A SUB 996 MB SURFACE LOW IN EASTERN CANADA AROUND DAY
3...WEAKENING BY DAY 5 INVOF NORTHEAST QUEBEC...SHOULD ALLOW THE
CANADIAN COLD FRONT TO PROGRESS THROUGH THE NORTHEASTERN QUARTER
OF THE NATION DURING DAYS 4-5. THE BAROCLINIC CONTRASTS ALONG THE
FRONT OVER THE EAST CENTRAL US WARRANT THE SPC OUTLOOK PROJECTION
FOR SEVERE WEATHER ALONG THE FRONT.

...SENSIBLE WEATHER HIGHLIGHTS...
THE RARITY OF THIS DRY CANADIAN AIRMASS FOR JULY JUST MIGHT
APPROACH TEMPERATURE RECORDS (LOW/MINS AND LOW/MAXES) FOR A VAST
MAJORITY OF THE EASTERN HALF OF THE NATION.

HIGH TEMPERATURE READINGS ARE LIKELY TO BE 15F-25F ABOVE NORMAL IN
THE PACIFIC NORTHWEST (WASHINGTON...OREGON...IDAHO) BENEATH AN
UPPER-LEVEL RIDGE OF HIGH PRESSURE EXTENDING FROM THE CONTINENTAL
DIVIDE WESTWARD TO THE PACIFIC AND GULF OF ALASKA. AS THE
UPPER-LEVEL RIDGE MIGRATES SLOWLY NORTHWESTWARD...THE THERMAL
TROUGH AND MONSOONAL MOISTURE WILL MOVE WITH IT...BRINGING
MID-LEVEL MOISTURE AND THE THREAT OF CONVECTION TO CALIFORNIA AND
THE CASCADES.

THE LESS HUMID---ALMOST AUTUMN-LIKE FEEL OF THE AIRMASS
ORIGINATING IN CANADA--- WILL EVENTUALLY SPREAD ALL THE WAY TO THE
I-10 CORRIDOR FROM WEST TEXAS EASTWARD INTO LOUISIANA---AND TO THE
I-20 CORRIDOR FROM MISSISSIPPI TO GEORGIA AND SOUTH CAROLINA. HERE
THE HIGH TEMPERATURES WILL BE CLOSER TO 10-20F BELOW NORMAL.

DURING THIS MEDIUM RANGE PERIOD...ACTIVE WEATHER---FROM A
PRECIPITATION PERSPECTIVE---WILL BE CONCENTRATED ALONG THE
NORTHERN AND CENTRAL DIVIDE WHERE CONFLUENT FLOW---A MID-LEVEL
WEST TO NORTHWESTERLY WIND---WILL BE CARRYING RESIDUAL MONSOONAL
MOISTURE AND CONVECTIVE DEBRIS FROM THE MOUNTAINS INTO THE HIGH
PLAINS. A SECONDARY FOCUS FOR SHOWERS AND THUNDERSTORMS WILL BE
DIRECTED ALONG AND IMMEDIATELY AHEAD OF THE DAY 3-5 PROGRESSION OF
THE CANADIAN COLD FRONT THROUGH THE EASTERN HALF OF THE NATION.

VOJTESAK

$$




USA.gov is the U.S. government's official web portal to all federal, state and local government web resources and services.