Preliminary Forecasts
Issued by NWS

Home | Current Version | Previous Version | Text Only | Print | Product List | Glossary On
Versions: 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17
000
FXUS02 KWNH 210623
PREEPD

PRELIMINARY EXTENDED FORECAST DISCUSSION
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
223 AM EDT TUE APR 21 2015

VALID 12Z FRI APR 24 2015 - 12Z TUE APR 28 2015

...OVERVIEW...
THE CENTER OF A DEEP AND EXPANSIVE CYCLONIC FLOW OVER SOUTHEAST
CANADA WILL GRADUALLY SHIFT FROM QUEBEC TO LABRADOR THIS WEEKEND
(DAYS 3-5)...AND A DEEP LOW PRESSURE VORTEX IS EXPECTED TO ANCHOR
IN THE GULF OF ALASKA BY EARLY NEXT WEEK (DAYS 6-7).

ACROSS THE LOWER 48...MULTIPLE MID-LEVEL SYSTEMS WITH PACIFIC
ORIGINS WILL MIGRATE DOWNSTREAM---CARRIED INITIALLY BY A BROAD
UPPER-LEVEL JET AXIS---ESTABLISHED IN THE SHORT TERM AND EARLY
MEDIUM RANGE---ACROSS THE SOUTHERN US. BY THE END OF THE FORECAST
PERIOD---A `NEW` PACIFIC JET AXIS EMERGES IN THE PACIFIC NORTHWEST
AND NORTHERN ROCKIES.

...MODEL PREFERENCES...
THE 20/12Z ECENS...NAEFS AND GEFS MEANS OFFERED THE BEST FIT WITH
THE FLOW PATTERN ACROSS NORTH AMERICA DURING THIS MEDIUM RANGE
PERIOD. IN PARTICULAR---AFTER DAY4---THE RATIO WAS A 50-30-20
BLEND RESPECTIVELY. THE RATIO SELECTED IS MORE OF A BLEND TO
`PROJECT` THE AMPLITUDE OF AN UPPER-LEVEL RIDGE AXIS IN CENTRAL
CANADA AND THE NORTHERN HIGH PLAINS HEADING INTO THE DAY 7-8
PERIOD.

THOUGHT THE 20/12Z DETERMINISTIC ECMWF AND GFS WERE USEFUL FOR THE
LOWER 48---BUT ONLY UNTIL 26/00Z. BOTH DEPICTED THE GRADUAL
MIGRATION OF THE TWO JET AXES DESCRIBED IN THE OVERVIEW---BUT
THEIR HANDLING OF THE MID-LEVEL ENERGY ENTERING THE WEST COAST OF
NORTH AMERICA IS LESS THAN DESIRABLE.

...UNCERTAINTY ASSESSMENT...
ONE OF THE KEY SENSIBLE WEATHER FEATURES ALLOWING A TRANSITION
ALOFT TO TAKE PLACE ACROSS THE LOWER 48---IS THE LEAD PACIFIC
SHORTWAVE EJECTING OUT OF THE COLORADO FRONT RANGE ON FRIDAY
(25/00Z TO 25/06Z). ITS OUTCOME AND EVOLUTION HAVE VERY DIFFERENT
CONSEQUENCES DOWNSTREAM---EAST OF THE MISSISSIPPI WITH RESPECT TO
PRECIPITATION. ANOTHER FEATURE OF THE PATTERN...IS THE OUTCOME OF
THE EXITING CIRCULATION IN SOUTHEAST CANADA AND THE ORIENTATION OF
ITS `ELONGATED` TROUGH AXIS WITH RESPECT TO---THE INTRODUCTION OF
PACIFIC ENERGY ENTERING WESTERN AND CENTRAL CANADA.

CONCERNING THE BROAD CYCLONIC FLOW IN SOUTHEAST CANADA AND PACIFIC
ENERGY ENTRAINMENT INTO ITS SOUTHWESTERN QUADRANT---

THE MEDIUM RANGE FORECAST ACROSS THE SOUTH CENTRAL CANADIAN
PRAIRIE PROVINCES WILL BE WHERE THE MODEL CONTENTION AND SPREAD
WILL DISPLAY THE MOST VOLATILITY THROUGH TIME. AND THE PRIMARY
REASON FOR THIS---IS THE PACIFIC ENERGY AND LEAD JET STREAK MOVING
FROM THE GULF OF ALASKA TO THE CANADIAN ROCKIES ON DAY 4. ITS
EMBEDDED MID-LEVEL ENERGY MIGRATING DOWNSTREAM INTO THE
MID-MISSOURI VALLEY AND/OR HUDSON BAY BY DAY 6 HAS BEEN `QUITE
PERPLEXING` FOR SEVERAL DAYS NOW. ALOFT...THE DETERMINISTIC
GUIDANCE CONTINUES TO FLIP-FLOP WITH SHORTWAVE ENERGY
CONSOLIDATION OVER THE DAKOTAS (INVOF 100W) AROUND 27/00Z---AND
ITS ABILITY TO CARVE OUT AN ADDITIONAL TROUGH AXIS ACROSS THE
GREAT LAKES AND NORTHEAST HAS TAKEN SEVERAL FORMS---NONE OF
WHICH---HAS MAINTAINED MUCH RUN-TO-RUN OR MODEL-TO-MODEL
CONTINUITY.
THE CONSEQUENCES OF THE 20/12Z CYCLE ARE MOST SENSITIVE OVER
MAINE--- AND WILL BE---A WARMUP IN NEW ENGLAND FOR DAY 7-8? (THE
DETERMINISTIC ECMWF)...OR NO WARMUP DAY 7-8? (THE GFS/CANADIAN
SOLUTIONS).

CONCERNING THE LEAD PACIFIC SHORTWAVE ORIGINATING IN SOUTHERN
CALIFORNIA---
AFTER 26/00Z...ONLY THE ECWMF TAKES THE ORGANIZED SYSTEM IN THE
CENTRAL PLAINS---AND LIFTS IT EAST-NORTHEASTWARD INTO THE OHIO
VALLEY---WITH ENOUGH AMPLIFICATION (THANKS TO ITS SECONDARY 250MB
JET STREAK SOLUTION) TO MAINTAIN A SLOWER---MORE EXPANSIVE
DEFORMATION PRECIPITATION SHIELD FROM THE MID-MISSISSIPPI VALLEY
TO THE CAROLINAS AND DELMARVA. THE 20/12Z GFS AND CANADIAN WEAKEN
THE WAVE AS IT SPREADS ACROSS THE APPALACHIANS AND MAINTAIN A
FLATTER---MORE-SHEARED SOUTHWARD...AND FASTER MIGRATION OF THE
PRECIPITATION ACROSS THE SOUTHEAST AND CAROLINAS.

THE DEEP VORTEX IN THE GULF OF ALASKA---
THE 20/12Z DETERMINISTIC ECMWF OFFERED YET ANOTHER `LOOK` (ITS
THIRD DIFFERENT IDEA IN THE LAST 3 RUNS) WITH THE INTENSE
SHORTWAVE ENERGY ROTATING NORTHEASTWARD INTO THE PACIFIC NORTHWEST
AND ALASKA PANHANDLE SATURDAY NIGHT (26/06Z). THE 20/12Z GFS
LOOKED THE MOST CONSISTENT WITH ITS PREVIOUS RUNS---BUT ITS
CENTRAL PRESSURE FORECAST SEEMS TOO DEEP (BETWEEN 974MB TO 981MB)
TO BE GIVEN MUCH CREDENCE AT THIS POINT---AND GOING FORWARD.

...SENSIBLE WEATHER HIGHLIGHTS...
BELOW NORMAL TEMPERATURES CONTINUE TO BE A MAINSTAY FOR THE GREAT
LAKES AND NORTHEAST---WITH THE EXPANSIVE AND DEEP VORTEX OVER
SOUTHEAST CANADA SLOW TO GIVE UP ITS POSITION AND MIGRATE INTO THE
CANADIAN MARITIMES.

OUT WEST...AN ACTIVE SPRING WEATHER PATTERN WILL ACCOMPANY THE
CUTOFF LOW AND ITS MIGRATORY TRACK ACROSS THE SOUTHERN GREAT
BASIN/FOUR CORNERS REGION LATE THIS WEEK...AND EXIT INTO THE WEST
CENTRAL HIGH PLAINS AND LOWER MISSOURI VALLEY THIS WEEKEND.

LATE IN THE PERIOD...HEIGHT FALLS ASSOCIATED WITH THE DEEP VORTEX
SETTING UP IN THE GULF OF ALASKA SHOULD BRING A STEADY INCREASE IN
PRECIPITATION FOR THE PACIFIC NORTHWEST AND NORTHERN DIVIDE EARLY
NEXT WEEK.

VOJTESAK


$$




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