Preliminary Forecasts
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FXUS02 KWNH 180620
PREEPD

PRELIMINARY EXTENDED FORECAST DISCUSSION
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
219 AM EDT SAT APR 18 2015

VALID 12Z TUE APR 21 2015 - 12Z SAT APR 25 2015

...OVERVIEW...
THIS MEDIUM RANGE PERIOD...THE JET STREAM PATTERN DIPS SOUTHWARD
ACROSS THE EAST CENTRAL PACIFIC---TAKING A PATH ACROSS
CALIFORNIA...AND SOUTHERN GREAT BASIN AND SOUTHERN ROCKIES. THIS
ANTICIPATED CHANGE IN THE FLOW PATTERN SHOULD PRODUCE AN ACTIVE
PERIOD OF APRIL WEATHER FOR THE SOUTHWEST QUARTER OF THE
NATION...WITH ACTIVE WEATHER PROGRESSING ACROSS THE SOUTHERN
ROCKIES AND SOUTHERN PLAINS LATE IN THE FORECAST PERIOD.

ACTIVE AND AT TIMES `CHILLY` APRIL WEATHER PATTERN WILL FOLLOW A
SLOW-MOVING CLOSED LOW MIGRATION FROM THE MIDWEST TO THE
NORTHEAST. BELOW NORMAL TEMPERATURES AND ABOVE NORMAL
PRECIPITATION ARE EXPECTED ACROSS THE NORTHEAST QUARTER OF THE
NATION THIS MEDIUM RANGE PERIOD.

...UNCERTAINTY ASSESSMENT/MODEL PREFERENCES...
OVERALL---THE 17/12Z ENSEMBLE GUIDANCE LOOKED MUCH BETTER THROUGH
THE PERIOD THAN THE DETERMINISTIC RUNS---FROM WEST TO EAST. AND
AFTER DAY 5---RECOMMEND SOMETHING CLOSER TO THE 17/12Z ECENS/NAEFS
MEANS AT 500 MILLIBARS AND THE SURFACE. I HAD NO ISSUE USING
EITHER THE 17/18Z GFS OR 17/12Z ECMWF ALONG THE NORTHEASTERN US
COAST THROUGH THE DAY 4/5 PERIOD.

CONCERNING THE EVOLUTION OF A SLOW-MOVING CUTOFF 500MB LOW
MIGRATION ACROSS SOUTHEAST CANADA AND SURFACE WAVE ENERGY
SHORTWAVE ENERGY ROTATING THROUGH ITS BASE---THE ENSEMBLES ARE
AFFORDED A CONSISTENT JET-LEVEL WIND FORECAST ACROSS THE LOWER 48
(WITH MINIMAL AMPLIFICATION EAST OF THE DIVIDE). COMPRISED OF A
SINGLE JET AXIS---RATHER OUR PERSISTENT `SPLIT-FLOW` REGIME---THIS
SINGLE JET AXIS REMAINS RATHER `FAST-ZONAL` THROUGH THIS MEDIUM
RANGE PERIOD---EXTENDING FROM WEST TEXAS TO THE DELMARVA. THE
UPPER LOW MIGRATING FROM THE WEST CENTRAL PLAINS TO THE ST
LAWRENCE VALLEY WILL UNDERGO A TRANSFORMATION---FROM MORE OF A
COMPACT...CLOSED CIRCULATION OVER LAKE SUPERIOR (DAY 3) TO A MORE
SHEARED OR ELONGATED TROUGH AXIS (BY DAY 7) OVER MAINE. THIS
TRANSFORMATION---ESSENTIALLY RESULTING FROM THE `UNDERCUTTING`
NATURE OF THIS FAST WESTERLY JET AXIS ALONG THE BASE OF THE
TROUGH.  GIVEN THE COMPLEXITY OF THE TRANSFORMATION---THE MEANS
ARE NOT FAR OFF THE MARK OVER NORTHERN NEW YORK AND NEW ENGLAND AT
DAY 7 WITH A CONSISTENT APPROACH---FORECASTING THE 540 DM AND 546
DM HEIGHT CONTOURS TO ANCHOR INVOF THE GULF OF MAINE AT DAY 7. AND
THROUGH THURSDAY NIGHT---THE MEANS ARE VERY CONSISTENT WITH THEIR
SURFACE PRESSURE FORECASTS ACROSS THE NORTHEAST. SO LONG AS THE
JET AXIS DOES NOT GAIN AMPLITUDE---THE TRANSFORMATION PROCESS
ACROSS SOUTHEAST CANADA SHOULD COME TO FRUITION `WITHOUT A HITCH`
BY THE DAY 7 PERIOD.

IN THE WEST...THE MEANS ARE CONSISTENT THROUGH DAY 6---DEPICTING
AN UPPER-LEVEL TROUGH AXIS ALONG 120W---AND PACIFIC JET AXIS
DIGGING INTO THE BASE OF THIS TROUGH ---INVOF SOUTHERN CALIFORNIA.
WHAT IS YET TO BE DETERMINED IS---TO WHAT EXTENT THE DIGGING
PACIFIC FLOW WILL ALLOW FOR THE 500MB TROUGH AXIS TO BECOME
NEGATIVELY-TILTED THROUGH TIME. AS FAR AS THE 17/12Z DETERMINISTIC
GUIDANCE WENT---ONLY THE CANADIAN AND NAVGEM LIKE A CLOSED LOW
SOLUTION OFF THE OREGON COAST AT DAY 6 (24/12Z).

...SENSIBLE WEATHER HIGHLIGHTS...
A WET PATTERN CONTINUES TO BE THE PRIMARY FOCUS DAY 3 ALONG AND
EAST OF THE MISSISSIPPI RIVER VALLEY AND GREAT LAKES. AS THE
CUTOFF 500MB LOW AND ASSOCIATED SURFACE CYCLONE BEGINS TO STALL
AND SHEAR OUT ACROSS WESTERN ONTARIO---PRECIPITATION WILL SHIFT
NORTHEASTWARD ACROSS THE APPALACHIANS AND INTO NEW ENGLAND AND THE
MID-ATLANTIC STATES. APPRECIABLE PERIODS OF COLD ADVECTION SHOULD
BE PRODUCED OVER THE GREAT LAKES AND NORTHEAST---GIVEN THE VERY
GOOD ENSEMBLE GUIDANCE AGREEMENT FOR THIS PATTERN`S EVOLUTION.

IN THE WEST... ISOLATED TO SCATTERED PRECIPITATION STILL APPEARS
TO BE `PRE-FRONTAL` AND CONVECTIVE IN NATURE FROM NORTHERN
CALIFORNIA TO EASTERN WASHINGTON WITH A SOUTHERLY MID-LEVEL FLOW
PRECEDING A SLOW-MOVING...LEAD PACIFIC COLD FRONT.

THE RESULT OF A DIGGING TROUGH AXIS ALONG THE WEST COAST---PULLING
MARITIME PACIFIC AIR SOUTHWARD RATHER THAN EASTWARD SHOULD PRODUCE
AN ACTIVE SPRING WEATHER PATTERN TO THE SIERRA...CENTRAL AND
SOUTHERN CALIFORNIA AND DOWNSTREAM ACROSS THE COLORADO BASIN AND
SOUTHERN ROCKIES.

A PERSISTENT WESTERLY FLOW ALOFT OVER NEARLY ALL OF THE THE RIO
GRANDE VALLEY AND DIGGING TROUGH IN THE FOUR CORNERS NORMALLY
PROMOTES BREEZY TO WINDY CONDITIONS DOWNWIND ACROSS THE SOUTHERN
PLAINS---BUT THE APPARENT LACK OF LOW-LEVEL GULF MOISTURE---DOES
NOT BODE WELL FOR WIDESPREAD DIURNAL CONVECTION IN THE SOUTHERN
PLAINS AND RIO GRANDE VALLEY.

VOJTESAK

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