Prognostic Meteorological Discussion
Issued by NWS

Home | Current Version | Previous Version | Text Only | Print | Product List | Glossary On
Versions: 1 2 3 4
000
FXUS02 KWBC 221542
PMDEPD

EXTENDED FORECAST DISCUSSION
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
1141 AM EDT MON SEP 22 2014

VALID 12Z THU SEP 25 2014 - 12Z MON SEP 29 2014

...OVERVIEW...

THE MEDIUM RANGE FORECAST WILL BE DOMINATED BY A VERY LARGE UPPER
RIDGE STRUCTURE COVERING MUCH OF THE US WITH SOME WEAK BUT
IMPORTANT EMBEDDED SHORTWAVE SYSTEMS MOVING SLOWLY WITHIN THE
OVERALL STRUCTURE OF THE RIDGE.  AT THE SAME TIME...A WELL
DEVELOPED UPPER TROUGH WILL BE APPROACHING THE WEST COAST BEFORE A
CLOSED LOW MOVES INLAND.

THE MAIN DRIVER OF THE UPPER RIDGE WILL BE THE DOWNSTREAM
AMPLIFICATION OF THE RIDGE OVER THE SOUTHWEST US AS THE UPPER
TROUGH NEARING THE WEST COAST AMPLIFIES. AS THIS SYSTEM
DEEPENS...THE DOWNSTREAM RIDGE COMMENSURATELY AMPLIFIES NORTHWARD
FROM THE SOUTHWEST TO THE NORTHERN PLAINS AND SOUTHERN CANADA
BEFORE A CUTOFF RIDGE DEVELOPS OVER THE NORTHERN PLAINS/GREAT
LAKES BY FRIDAY INTO SATURDAY. THIS RIDGE MOVES SLOWLY EASTWARD
AND WEAKENS TOWARD THE END OF THE FORECAST AS NUMEROUS SMALLER
SCALE SYSTEMS ENCROACH THE RIDGE FROM A VARIETY OF DIRECTIONS.

AT THE SAME TIME...A BROAD YET WEAK UPPER TROUGH OVER THE NORTHERN
PLAINS/MISSISSIPPI VALLEY ON THURSDAY...A BROAD AND WEAK UPPER LOW
OVER THE SOUTHEAST AND ANOTHER RIDGE OFF THE EAST COAST KIND OF
MEANDER AROUND EACH OTHER AS THE LARGE SCALE CHANGES EVOVLE WITH
THE TROUGH/RIDGE AMPLIFICATION OVER THE WESTERN US.

AS A RESULT...THESE SYSTEMS BECOME STRETCHED AND PULLED IN VARIOUS
DIRECTIONS AS THEY ENCOUNTER THESE MORE LARGE SCALE CHANGES.  ON
ONE HAND...THIS CREATES LARGE UNCERTAINTY IN THE EVOLUTION OF
THESE SMALL FEATURES BECAUSE THEIR MOVEMENT/DEVELOPMENT OCCUR
ALONG VARIOUS SHEAR LINES/DEFORMATION ZONES THAT CAN EITHER HAVE
THESE SYSTEMS SQUEEZED IN A NUMBER OF UNUSUAL DIRECTIONS. FOR
EXAMPLE...THE BROAD WEAK UPPER LOW OVER THE EAST COAST ENCOUNTERS
THE DEVELOPING UPPER RIDGE TO ITS NORTH WHILE SOME OF ITS ENERGY
COULD BE SHUNTED EASTWARD WHILE ALSO SOME OF IT GOING WESTWARD.
THIS IS THE SOURCE OF THE UNCERTAINTY.  THE SAME ISSUES HOLD FOR
THE BROAD TROUGH OVER THE PLAINS/MISSISSIPPI VALLEY...WITH ITS
EVOLUTION TO THE SOUTH AND WEST A POSSIBLE RESULT OF THE THIS
LARGER SCALE INTERACTION.

...MODEL GUIDANCE...

DESPITE THESE COMPLEXITIES...THE MODEL ENSEMBLE MEANS PAINT A
FAIRLY UNIFORM PICTURE OF HOW THESE SYSTEMS EVOLVE OVER 4 DAYS
WITH RELATIVELY SMALL DIFFERENCES BETWEEN THE RESPECTIVE ENSEMBLE
MEANS OF THE GEFS/ECMWF/NAEFS AND CANADIAN MODELS.  THE
CORRESPONDING OPERATIONAL RUNS ON THE OTHER HAND SHOW CONSIDERABLE
DIFFERENCES...EVEN WITH THE EVOLUTION OF THE LESS CONFUSING AND
LARGE SCALE WESTERN TROUGH.  FOR MUCH OF THE CENTRAL AND EASTERN
US...AN EXAMINATION OF THE EVOLUTION OF ONE OR MAYBE 2 HEIGHT
CONTOURS DEFINING TROUGH/RIDGE SYSTEMS BECOMES A DIFFICULT MEANS
TO EVALUATE THE FORECAST.

AS A RESULT...THE ENSEMBLE MEANS BECOME KEY TO  DEFINING THE
SURFACE AND UPPER LEVEL FIELDS AND RESULT IN A RATHER
STRAIGHTFORWARD DEPICTION ON THE SURFACE AND 500 MB MAPS.
HOWEVER...THE SENSIBLE WEATHER IS ANOTHER STORY IN THAT SMALL
CHANGES COULD SPELL THE DIFFERENCE BETWEEN SUNNY SKIES OR RAINY
WEATHER...ESPECIALLY IN THE EAST.

...SENSIBLE WEATHER...

THE MODEL GUIDANCE SHOWS THAT THE A LARGE SURFACE HIGH PRESSURE
SYSTEM WILL COVER THE CENTRAL AND EASTERN US FROM THURSDAY THRU
SUNDAY.  ON THURSDAY/DAY 3...THE HIGHEST PROBABILITIES FOR RAIN
SPREADING UP ALONG THE SOUTHEAST AND MIDDLE ATLANTIC COAST ARE
HIGHEST UNDER THE INFLUENCE OF THE WEAK UPPER TROUGH MOVING
NORTHWARD ALONG THE EAST COAST. BY FRIDAY...THIS AREA OF MOISTURE
MAY MOVE EASTWARD...AS THE TROUGH SHEARS UNDER THE INFLUENCE OF
THE DEVELOPING UPPER RIDGE OVER THE UPPER MIDWEST AND THE WESTWARD
DEVELOPMENT OF A SURFACE HIGH PRESSURE CENTER.  HOWEVER...WHILE
SOME OF THE ENERGY MOVES EASTWARD...SOME OF IT MAY REMAIN AND EVEN
MOVE WESTWARD...INDICATING CLEARING SKIES OVER THE MIDWEST INTO
NEW ENGLAND BUT UNCERTAIN CONDITIONS FROM THE MID ATLANTIC INTO
THE SOUTHEAST.  WHILE THE SOUTHEAST COAST LOOKS WET...IT COULD BE
WET OR COULD CLEAR ACROSS THE MID ATLANTIC.  THUS...THE
UNCERTAINTY.

FARTHER WEST...THE BROAD TROUGH OVER THE PLAINS/MISSISSIPPI VALLEY
RESULTS IN SCATTERED SHOWERS ACROSS PARTS OF THE MISSISSIPPI
VALLEY INTO THE SOUTHERN PLAINS ON THURSDAY.  AS THIS SYSTEM GETS
SQUEEZED FROM ALL SIDES...SOME OF THE RESIDUAL MOISTURE COULD
REMAIN OVER THE SOUTHWEST INTO THE SOUTHERN ROCKIES.

MEANWHILE...UNDER THE INFLUENCE OF THE LARGE EXPANSIVE UPPER RIDGE
PATTERN...WARM CONDITIONS WILL BE THE RULE ACROSS THE PLAINS INTO
THE ROCKIES...WITH HOT CONDITIONS APPEARING MOST LIKELY ACROSS
EASTERN MONTANA/ WYOMING INTO THE DAKOTAS WITH NEAR RECORD
TEMPERATURES POSSIBLE.

FINALLY...THE LARGE WESTERN TROUGH COOLS AND MOISTENS THE WEST
COAST THAT MOVES SLOWLY EASTWARD ACROSS THE ROCKIES BY SUNDAY INTO
MONDAY.

KOCIN

$$





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