Preliminary Forecasts
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FXUS02 KWNH 290629
PREEPD

PRELIMINARY EXTENDED FORECAST DISCUSSION
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
227 AM EDT MON JUN 29 2015

VALID 12Z THU JUL 02 2015 - 12Z MON JUL 06 2015

THE MEDIUM RANGE PERIOD ACROSS THE LOWER 48 REMAINS AMPLIFIED ---
WITH A STRONG ANTICYCLONE LOCATED INVOF THE FOUR CORNER REGION AND
A WEAK UPPER-LEVEL TROUGH POSITIONED OVER THE EASTERN GREAT LAKES
AND NEW ENGLAND.

IN A GENERAL OVERVIEW SENSE...
THE SUBTLE CHANGES EXPECTED TO TAKE PLACE DURING THE LATTER HALF
OF THIS FORECAST PERIOD --- AND THE GREATEST UNCERTAINTY IN THE
DETERMINISTIC FORECASTS --- ARE CONFINED TO THE US-CANADA BORDER
REGION (A CORRIDOR BETWEEN 48N AND 52N LATITUDES) --- AND INCLUDE
THE CANADIAN PRAIRIE PROVINCES...UPPER MISSOURI VALLEY...UPPER
GREAT LAKES AND NORTHERN NEW ENGLAND.

AND VERY LITTLE OF THE CURRENT MEDIUM RANGE FORECAST AND
PHILOSOPHY REQUIRES CHANGES AND/OR ALTERATIONS.

THE SUBTLE CHANGES RESULT FROM THE INTRODUCTION OF A HIGH-LATITUDE
PACIFIC JET STREAK THAT MIGRATES ACROSS CENTRAL CANADA AND
`RE-SHUFFLES`THE UPPER-LEVEL CYCLONE`S POSITION ACROSS ARCTIC
CANADA. A SECONDARY IMPACT FROM THE `RE-SHUFFLE WILL BE THE
INTRODUCTION OF A FRESH CANADIAN AIRMASS ---EAST OF THE
DIVIDE---AND ACROSS THE NORTHERN PLAINS FROM EASTERN MONTANA TO
THE WESTERN SHORES OF LAKE SUPERIOR BETWEEN NEXT SATURDAY NIGHT
AND MONDAY AFTERNOON.

THE 28/12Z ECENS AND GEFS MEANS WERE VERY REASONABLE PIECES OF
GUIDANCE THROUGH THE JULY 6-7 FORECAST PERIOD WITH THE 28/12Z
DETERMINISTIC ECMWF AND 28/18Z GFS BEING THE MORE AGGRESSIVE
SOLUTIONS OVER THE SOUTHERN CANADIAN ROCKIES ON/AFTER 4/12Z. AND
BY NEXT MONDAY NIGHT...THE DETERMINISTIC 28/12Z CANADIAN BECOMES
THE FASTER SOLUTION ACROSS EASTERN NORTH AMERICA --- WHILE THE
MEANS AND DETERMINISTIC ECMWF AND GFS MAINTAIN A PROGRESSIVE ---
BUT SLIGHTLY SLOWER FRONTAL PROGRESSION SOLUTION COMPARED TO THE
DETERMINISTIC CANADIAN.

CANNOT NECESSARILY RULE OUT DETERMINISTIC RUNS --- WHICH ARE
SLIGHTLY STRONGER SOLUTIONS WITH RESPECT TO THE CANADIAN FRONTAL
INTRUSION VERSUS THEIR RESPECTIVE MEANS. AND IN ADDITION --- EACH
DETERMINISTIC SOLUTION ESSENTIALLY FLATTENS THE NORTHERN FLANKS OF
THE STRONG ANTICYCLONE IN THE GREAT BASIN AND ALONG THE WEST COAST
FROM PUGET SOUND (48 NORTH) TO CAPE MENDOCINO (40 NORTH) ALONG
125W.

BUT I DO PREFER--- USING A BLEND OF THE 28/12Z ECENS/GEFS MEANS (A
70/30 RATIO) BY DAY 7. THIS WILL AT LEAST KEEP THE MID-LEVEL WAVE
PATTERN AND WAVELENGTH OVER CENTRAL CANADA `SEMI-STABLE` ALONG 95W
AND PROVIDE VERY LIMITED INTERACTION BETWEEN THE SUBTROPICAL AND
MID-LATITUDE FLOW REGIMES. AM SUGGESTING HERE THAT EVEN THOUGH THE
PATTERN WILL BE AMPLIFIED --- THE LOWER 48`S MID-LEVEL FLOW
PATTERN WILL CARRY A PREVAILING (BUT RELATIVELY BONE DRY)
NORTHWESTERLY FLOW DOWNWIND OFF THE NORTHERN HALF OF THE DIVIDE---
AND MAINTAIN A MID-LEVEL SUBTROPICAL RIDGE (EXTENDING
EAST-TO-WEST) ALONG 30N LATITUDE FROM FLORIDA TO SOUTH CENTRAL
TEXAS.

AND LITTLE TO NO INFLUENCE AND/OR INTERACTION BETWEEN WITH THE
MID-LATITUDE FRONTAL PROGRESSIONS AND MOISTURE FROM THE GULF OF
MEXICO. WHAT GULF MOISTURE CAN MOVE ASHORE --- IS DIRECTED INTO
NORTHEAST MEXICO AND CONFINED TO THE RIO GRANDE VALLEY --- ALONG
AND SOUTH OF THE MID-LEVEL SUBTROPICAL RIDGE AXIS.

VOJTESAK




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