Preliminary Forecasts
Issued by NWS

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FXUS02 KWNH 200622
PREEPD

PRELIMINARY EXTENDED FORECAST DISCUSSION
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
211 AM EDT MON APR 20 2015

VALID 12Z THU APR 23 2015 - 12Z MON APR 27 2015

...OVERVIEW...
AN UPPER-LEVEL CIRCULATION ANCHORS ACROSS SOUTHEAST CANADA THIS
MEDIUM RANGE PERIOD---WITH A COOL APRIL WEATHER PATTERN FOR THE
GREAT LAKES AND NORTHEAST. A PERSISTENT WESTERLY JET STREAM
PRESENCE ACROSS THE SOUTHERN STATES---FROM COAST TO COAST--- WILL
PROMOTE MID-LEVEL ENERGY MIGRATION AND SCATTERED PRECIPITATION
EVENTS FROM CALIFORNIA TO THE CAROLINAS.

...MODEL PREFERENCES...
THOUGHT THE 19/12Z ECENS AND NAEFS MEANS (A 60/40 BLEND
RESPECTIVELY) ADEQUATELY HANDLE THE FLOW PATTERN---FROM WEST TO
EAST---THIS MEDIUM RANGE PERIOD. AND WILL COVER THE DIFFERENCES IN
THE SHORTWAVE DETAILS POSED BY THEIR INDIVIDUAL DETERMINISTIC
RUNS. THE 19/12Z DETERMINISTIC GFS/ECMWF WAS LESS THAN DESIRABLE
GUIDANCE AFTER THE MID-POINT OF DAY 4---FOR THE LOWER 48---AS A
WHOLE. VARIED RESULTS---AT REGIONAL SCALE ARE NOTED BELOW.

...UNCERTAINTY ASSESSMENT...
CONCERNING THE UPPER-LEVEL CIRCULATION OVER QUEBEC---THE 19/12Z
DETERMINISTIC GFS/ECMWF AND CANADIAN APPEARED TO BE `USE-ABLE`
PIECES OF GUIDANCE UNTIL LATE DAY 4---WHEN THE ECMWF APPEARED TO
BREAK CONTINUITY OVER SOUTHEAST CANADA AND BEGINS TO GENERATE A
SLIGHTLY MORE-AMPLIFIED TROUGH ALONG THE EASTERN SEABOARD.

CONCERNING THE ORGANIZED PACIFIC SHORTWAVE EXITING THE COLORADO
FRONT RANGE ON FRIDAY AFTERNOON---THE 19/12Z DETERMINISTIC
GFS/ECMWF AND CANADIAN WERE IN MUCH BETTER AGREEMENT---AND FRIDAY
NIGHT IS WHEN THE ECMWF BEGINS TO DEVELOP MORE SHORTWAVE RIDGING
OVER THE MISSISSIPPI VALLEY AND TAKES A MORE NORTHEASTWARD TRACK
VERSUS THE GFS AND CANADIAN.

CONCERNING THE EVOLVING UPPER-LEVEL TROUGH PATTERN IN THE WEST AND
PROJECTED DEEP CYCLONE MIGRATING THROUGH THE SOUTHEAST GULF OF
ALASKA TOWARDS THE PACIFIC NORTHWEST (DAYS 6-7)---

THE DETERMINISTIC GUIDANCE STILL LIKES THE IDEA OF A SECOND
MID-LEVEL WAVE TO MOVE INTO SOUTHERN CALIFORNIA SATURDAY
MORNING---BUT THERE REMAINS SOME QUESTION AS TO--IF AND
WHERE---THE WAVE TRIES TO PINCH OFF INTO A CLOSED 500MB
CIRCULATION. THE ECMWF/CANADIAN HAVE COMPLETELY BACKED OFF THE
IDEA OF A CLOSED CIRCULATION AND THE 12Z GFS CONTINUES TO FAVOR
THE FORMATION OF A CLOSED CIRCULATION BETWEEN POINT CONCEPTION AND
THE MEXICAN BORDER ON FRIDAY NIGHT/EARLY SATURDAY MORNING.

THE DEEP CYCLONE EDGING SOUTHEASTWARD TOWARDS THE OLYMPIC
PENINSULA DOES HAVE A GENERAL CONSENSUS WITH THE `UPPER` FRONT
PROGRESSION ACROSS THE CASCADES---ITS STRENGTH---BEING THE
CHALLENGE RIGHT NOW. THE 19/12Z ECMWF/GFS TRENDED A TAD FASTER AND
THE CANADIAN---SLOWER WITH THE HEIGHT FALLS (THE 564 DM HEIGHT
CONTOUR) MOVING THROUGH NORTHERN CALIFORNIA AROUND THE 27/12Z
(MONDAY) TIME FRAME. HOWEVER---THE 19/12Z ECMWF NOTED A MUCH
STRONGER LEAD SHORTWAVE MOVING THROUGH THE PACIFIC NORTHWEST FOR
SATURDAY/SUNDAY---VERSUS THE GFS/CANADIAN.

...SENSIBLE WEATHER HIGHLIGHTS...
`CHILLY` APRIL WEATHER PATTERN FOR THE GREAT LAKES AND NORTHEAST
WITH BROAD CYCLONIC FLOW FROM EASTERN CANADA `CYCLING`
COUNTERCLOCKWISE FROM NORTHWEST ONTARIO TO SOUTHERN NEW
ENGLAND...THE GULF OF MAINE AND NOVA SCOTIA. MORE OF A WESTERLY
LOW-LEVEL WIND COMPONENT FOR THE CENTRAL AND SOUTHERN APPALACHIANS
AND MID-ATLANTIC STATES---AFFORDING THE LOWER ELEVATIONS AND
COASTAL AREAS WITH SOME MODIFICATION OF THIS COLD ADVECTION
`CYCLING`.

IN THE WEST...AN ACTIVE SPRING WEATHER PATTERN COMES TO THE GREAT
BASIN... SIERRA... CENTRAL AND SOUTHERN CALIFORNIA... AND
DOWNSTREAM ACROSS THE COLORADO RIVER BASIN AND SOUTHERN ROCKIES
WITH A COUPLE OF ORGANIZED MID-LEVEL SYSTEMS MIGRATING INTO THE
SOUTHERN HIGH PLAINS DURING THE LATTER HALF OF THE PERIOD.

MEDIUM RANGE PRECIPITATION FOCUS---EAST OF THE CONTINENTAL DIVIDE
SHOULD BE ALONG/BENEATH THE WEST-SOUTHWESTERLY ORIENTED JET STREAM
AXIS FROM WEST TEXAS TO THE CAROLINAS. ACROSS THE SOUTH CENTRAL
PLAINS---WITH AN ORGANIZED SYSTEM FOR FRIDAY/SATURDAY APPEARS ON
TRACK.

VOJTESAK

$$




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