Prognostic Meteorological Discussion
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FXUS02 KWBC 020625
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EXTENDED FORECAST DISCUSSION
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
224 AM EDT THU JUL 02 2015

VALID 12Z SUN JUL 05 2015 - 12Z THU JUL 09 2015

...OVERVIEW...
CURRENT MEDIUM RANGE FORECAST FROM WEST TO EAST LOOKS TO BE ON
TRACK --- AND FOR EARLY JULY... THE AMPLIFIED PATTERN LOOKS TO BE
A FAIRLY PROGRESSIVE ONE ALONG THE US/CANADIAN BORDER.

...MODEL PREFERENCES/UNCERTAINTY...
THE 1 JUL/12Z ECENS...GEFS AND NAEFS ENSEMBLE GUIDANCE APPEARED TO
HAVE REACHED A CONSENSUS OVER CENTRAL CANADA --- ESTABLISHING A
MEAN UPPER-LEVEL CYCLONE NEAR 60N 90W ON DAY 6. THE ENSEMBLE MEANS
SUPPORT A WAVELENGTH THAT TRANSPORTS A SERIES OF CANADIAN FRONTAL
PROGRESSIONS FROM SOUTHEAST ALBERTA TO THE GREAT LAKES --- AND
MAINTAINS A BROAD (AND CONVECTIVELY-ACTIVE) MID-LEVEL TROUGH ---
ALONG AND NORTH OF THE MISSOURI AND OHIO RIVERS THROUGH THIS
PARTICULAR MEDIUM RANGE PERIOD. PERIODS OF SUSTAINED CONVECTION
(AND ITS SUBSTANTIAL OUTFLOW) EMBEDDED WITHIN THIS MID-LEVEL
TROUGH WILL OCCASIONALLY SPREAD SOUTHWARD BENEATH THE BASE OF THE
MEAN MID-LEVEL TROUGH BASE. I SAW MERITS WITH BOTH THE
DETERMINISTIC ECMWF AND GFS THROUGH DAY 5 AND DAY 6. TEMPERATURES
SHOULDN`T BE A MAJOR CONCERN FOR DAYS 3-4... THOUGH THE DEGREE OF
SIGNIFICANT CONVECTION (MCS PROPAGATION) IN THE HIGH PLAINS MAY
THROW SOME GFS-BASED TEMPERATURE FORECASTS OFF ALONG 40N EAST OF
THE DIVIDE (MAINLY FOR DAY 5).

THIS RIDGE-TROUGH CONFIGURATION ACROSS THE LOWER 48 STARTS WITH A
DAY 3 --- WESTERN CANADA/ALASKA PANHANDLE SHORTWAVE THAT MIGRATES
ACROSS THE MONTANA/IDAHO BORDER ---  AND FLATTENS THE NORTHERN
FLANKS OF THE ANTICYCLONE OVER THE CENTRAL GREAT BASIN. AN ONSHORE
FLOW ERODING/ERASING THE THERMAL TROUGH FROM NORTH TO SOUTH ALONG
THE I-5 CORRIDOR. THE DETERMINISTIC GUIDANCE SEEMED REASONABLE
WITH THE PROJECTED HEIGHT FALLS WEST OF THE DIVIDE AND ---THOUGH
NOT PERFECT --- THE 1/12Z CYCLE DETERMINISTIC GUIDANCE
(CANADIAN...GFS AND ECMWF) ARE AT LEAST ON THE SAME PAGE INVOF 40N
125W BY DAY 6 VERSUS 24 HOURS AGO --- CONSOLIDATING MID-LEVEL
ENERGY ROUNDING THE PERIPHERY OF THE UPPER-LEVEL RIDGE AND SENDING
THIS ENERGY NORTHEASTWARD ALONG THE NORTHERN FLANKS OF THE
ANTICYCLONE (UPPER-LEVEL RIDGE).

THE BROAD ANTICYCLONIC FLOW ALOFT OVER THE WEST CENTRAL MOUNTAINS
OF NORTH AMERICA WILL SPAWN INTERVALS OF TERRAIN-BASED ELEVATED
CONVECTION --- ENCOMPASSING A LARGE PORTION OF SOUTHERN AND
WESTERN FLANKS OF THE RIDGE. ACROSS THE NORTHERN AND EASTERN
PERIPHERY OF THE ANTICYCLONE (GENERALLY DOWNWIND OF THE CENTRAL
DIVIDE) --- THE CONVECTIVE CYCLE AND STORM CELL TRAJECTORIES WILL
BE A LITTLE MORE COMPLEX. THE COMPLEXITY RESULTING FROM
SOUTHWARD-AND SOUTHEASTWARD-PROPAGATING CELL MOTIONS AND THEIR
SUSTAINED LOW-LEVEL OUTFLOW --- OPPOSING A MEAN NORTHWESTERLY
MID-LEVEL STEERING CURRENT THAT FLOWS DOWNSTREAM INTO THE BASE OF
THE MEAN TROUGH.

...SENSIBLE WEATHER HIGHLIGHTS...
THE CHALLENGING PART OF THE MEDIUM RANGE FORECAST --- TRACKING THE
MORE ORGANIZED CONVECTIVE CLUSTERS DOWNWIND AND INTO THE BASE OF
THE LONGWAVE TROUGH (AND EASTWARD FROM 90W LONGITUDE FROM THE
LOWER MISSOURI VALLEY/OZARKS TO THE CAROLINAS/DELMARVA AND INTO
THE WESTERN ATLANTIC OCEAN BASIN.

THE MEANS DEPICT THE PERSISTENCE OF THE PATTERN WITH A ZONE OF
LOW-LEVEL CONFLUENCE AND CONVERGENCE DOWNWIND OF THE SOUTHERN
APPALACHIANS. APPEARS TO BE MORE LIKE THE GRAVEYARD OF DECAYING
FRONTAL PROGRESSIONS COUPLED WITH CONVECTIVE DEBRIS FROM UPSTREAM
CONVECTION. IN EITHER CASE --- THERE IS A SUBSTANTIAL WEAKENING OF
THE UPPER-LEVEL FLOW IN THIS PORTION OF THE COUNTRY AND A
COMBINATION OF FACTORS THAT SUGGEST THE SUBTROPICAL RIDGE IN THE
SOUTHEAST WILL WEAKEN AND LOW-LEVEL INSTABILITY WILL BE ON THE
INCREASE DURING THE LATTER HALF OF THE FORECAST PERIOD.

A SECOND AREA OF SENSIBLE WEATHER CONCERN WILL BE ACROSS THE
SOUTHERN HIGH PLAINS --- WITH THE PERSISTENCE OF
SOUTHWARD-PROPAGATING CONVECTION CYCLING A RAIN-COOLED OUTFLOW
BOUNDARY SOUTH AND SOUTHWEST INTO EASTERN NEW MEXICO AND THE TEXAS
PANHANDLE. THE MODELS ARE FOCUSING ON A BOUNDARY THAT PARALLELS
THE I-70/I-40 CORRIDOR WITH LOCALLY HEAVY RAINFALL IN THE DAY 5-6
TIME FRAME. HERE I USED MORE OF A BLENDED MEANS APPROACH (50/30/20
OF THE EC/NAEFS AND GEFS) TO GET A GENERAL VIEW OF WHAT THE
UPPER-LEVEL RIDGE WILL LOOK LIKE AND WHERE ITS STABLE CORE WILL BE
LOCATED. A STRONG MID-LEVEL CAP IS ANTICIPATED TO COINCIDE WITH
THE DESCENT OF RAIN-COOLED AIR FROM CENTRAL TEXAS EASTWARD TO THE
LOWER MS VALLEY AND ENCOMPASS MUCH OF THE WESTERN GULF COAST.

VOJTESAK











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