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
000
FXUS02 KWNH 290613
PREEPD

PRELIMINARY EXTENDED FORECAST DISCUSSION
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
112 AM EST SAT NOV 29 2014

VALID 12Z TUE DEC 02 2014 - 12Z SAT DEC 06 2014

...OVERVIEW...
AN ACTIVE WEATHER PATTERN WILL PREVAIL ACROSS THE NORTHERN HALF OF
THE NATION THIS FORECAST PERIOD. DURING THE DAY 3-4 PERIOD...THE
NORTHEAST QUARTER OF THE COUNTRY WILL BE UNDER THE INFLUENCE OF
COLD CANADIAN HIGH PRESSURE WHILE A BROAD CYCLONE---ENTERS THE
WEST COAST AND GREAT BASIN---BRINGING MUCH-NEEDED RAINFALL AND
SNOWFALL TO THE NORTHERN ROCKIES AND CASCADES.

ON DAY 5...THE MODIFIED ARCTIC AIRMASS AND SURFACE RIDGE MIGRATES
NORTHEASTWARD INTO THE CANADIAN MARITIMES...BUT A WEDGE OF HIGH
PRESSURE REMAINS IN PLACE---TO THE LEE OF THE APPALACHIANS FROM
NOVA SCOTIA TO THE PIEDMONT. MEANWHILE...THE LEADING EDGE OF THE
BROAD PACIFIC SYSTEM EJECTS A MID-LEVEL SHORTWAVE INTO THE THE
NORTHERN PLAINS AND MID-LEVEL WARM ADVECTION RACES NORTHEASTWARD
FROM THE SOUTHERN PLAINS TOWARDS THE GREAT LAKES AND ST LAWRENCE
VALLEY. THIS LEAD SYSTEM ADVECTS SUFFICIENT MOISTURE ATOP THE
RETREATING COLD AIRMASS AND SURFACE RIDGE---TO GENERATE POCKETS OF
SNOW AND A WINTRY MIX ACROSS THE UPPER GREAT LAKES...NORTHERN
APPALACHIANS AND NORTHERN NEW ENGLAND. THE UPPER OHIO VALLEY AND
WEST CENTRAL NEW ENGLAND ARE WHERE SOME TIMING DIFFERENCES COULD
BECOME A TRICKY P-TYPE FORECAST.

...MODEL PREFERENCES/UNCERTAINTY ASSESSMENT...
THE PREVAILING FAST-ZONAL WESTERLY FLOW IS GENERALLY HANDLED BY
THE ECENS...GEFS AND NAEFS AS A SERIES OF LOW-AMPLITUDE WAVES FROM
THE NORTH CENTRAL CONTINENTAL DIVIDE TO THE GREAT LAKES THIS
MEDIUM RANGE PERIOD. THE KEY FEATURE OF THE FLOW IS THE RATHER
EXPANSIVE VERTICAL STRUCTURE OF THE TROUGH MIGRATING ACROSS THE
WEST COAST---DAY 3-4. THE SERIES OF SUBTLE 700MB WAVES THAT EJECT
FROM THE BASE OF THIS TROUGH---AS IT RACES ACROSS THE GREAT
BASIN...NORTHERN ROCKIES---AND TRANSLATE THEIR MOMENTUM DOWNSTREAM
INTO THE NORTHERN PLAINS AND NORTHEAST QUARTER OF THE NATION WILL
BE THE FORECAST CHALLENGE. TIMING THE SUBTLE WAVE ENERGY MIGRATION
AND THEIR ABILITY TO PULL CANADIAN AIR INTO THE LOWER 48---IN
THEIR WAKE---WILL BE ANOTHER FORECAST CHALLENGE FOR THE
NORTHERN/CENTRAL PLAINS AND THESE MID-LEVEL WAVES ARE BEST
CAPTURED IN THE 850MB-700MB LAYER WIND FIELDS. WITH A 1040MB
SURFACE RIDGE IN PLACE DOWNSTREAM OF THE PACIFIC TROUGH...THE
SURFACE REFLECTION(S) ASSOCIATED WITH THESE MID-LEVEL WAVES WILL
BE DIFFICULT TO DISCERN AND TRACK EAST OF THE ROCKIES.

THE WPC GRAPHICS THIS MORNING---FOLLOWED THE PREVIOUS SHIFT
GUIDANCE AND USED THE 28/12Z ECENS MEANS TO COVER THE TRANSITORY
NATURE OF THE FLOW. THIS APPROACH MAINTAINED CONTINUITY WITH THE
PREVIOUS MASS FIELD FORECASTS...MINIMIZES ANY TIMING ERRORS AT
THAT TIME RANGE---BUT GENERALLY LACKS THE DETAIL THE PACIFIC
TROUGH WILL EMBODY AS IT MIGRATES ACROSS THE SOUTHERN CASCADES...
SISKIYOUS... SIERRA (ON DAYS 3-4)...AND THE WASATCH/NORTHERN
ROCKIES AND DIVIDE ON DAY 5. THE ECENS MEAN SOLUTIONS IMPLIES A
FASTER NORTHEASTWARD MIGRATION OF THE SURFACE RIDGE ACROSS THE
SOUTH CENTRAL PLAINS AND OHIO VALLEY---AND SLIGHT AMPLITUDE TO THE
UPPER-LEVEL FLOW IN THE EAST BY DAY 6-7 VERSUS THE GEFS/NAEFS.
COULD NOT ARGUE FOR OR AGAINST THE PREVIOUS  PREFERENCE---WITH THE
BROAD WESTERLIES GENERATING SEVERAL ADDITIONAL ORGANIZED
SHORTWAVES IN THE EASTERN PACIFIC/GULF OF ALASKA IN THE DAY 6-7
FORECAST PERIOD.

...SENSIBLE WEATHER HIGHLIGHTS...
BIG TEMPERATURE SWINGS FOR THE UPPER MISSISSIPPI VALLEY...UPPER
GREAT LAKES AND NORTHERN NEW ENGLAND...BUT WITH A MILDER PACIFIC
AIRMASS GRADUALLY WORKING ACROSS THE LOWER 48...A BROAD-BRUSH
DESCRIPTION IS FOR AT/SLIGHTLY ABOVE NORMAL HIGHS---AWAY FROM THE
INFLUENCE OF THE SURFACE RIDGE IN THE EAST.

PERIODS OF LOCALLY HEAVY PRECIPITATION ACROSS CALIFORNIA...AND THE
CASCADES/NORTHERN ROCKIES AND NORTHERN HIGH PLAINS THIS PERIOD.

VOJTESAK

$$





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