Prognostic Meteorological Discussion
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FXUS02 KWBC 181535
PMDEPD

EXTENDED FORECAST DISCUSSION
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
1135 AM EDT SAT APR 18 2015

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

...OVERVIEW...

AMPLIFIED/BLOCKY PATTERN OVER MUCH OF THE NORTHERN HEMISPHERE --
IN TYPICAL SPRINGTIME FASHION -- WILL BE THE THEME OF THE MEDIUM
RANGE. RIDGING IS FORECAST TO SURGE TOWARD THE VERY HIGH LATITUDES
FROM NW CANADA INTO THE CANADIAN ARCHIPELAGO... WHICH FAVORS AN
UPPER LOW TO ITS SOUTH NEAR THE GREAT LAKES AND SPLIT FLOW IN THE
NE PACIFIC. SUBTROPICAL RIDGING WILL CONTINUE TO SLOWLY BUILD
ALONG 20N AS WE HEAD TOWARD THE END OF APRIL.


...UNCERTAINTY ASSESSMENT/MODEL PREFERENCES...

SHIFTS IN THE GUIDANCE... RATHER THAN A SUBSTANTIAL CHANGE... HAVE
BEEN BOTH INCREMENTAL -- AS IN THE GFS/GEFS -- AND IN UNISON -- AS
IN FROM CYCLE TO CYCLE. AGREEMENT AMONG THE GUIDANCE IS GOOD TO
START AND THEN SLOWLY DRIFTS APART AS SMALLER DETAILS GROW WITH
TIME... ESPECIALLY IN THE NORTHEAST PACIFIC BUT ALSO IN THE EAST.
MODELS HAVE YET TO IRON OUT HOW TO HANDLE INCOMING SHORTWAVES INTO
WESTERN NORTH AMERICA TUESDAY AND HOW MUCH WILL PUSH INTO CANADA
VS DROP SOUTHWARD TOWARD SOUTHERN CALIFORNIA. THEN... NEXT
UPSTREAM SHORTWAVE OR CLOSED LOW WILL AFFECT HOW QUICKLY THE LEAD
LOW EXITS THROUGH THE SOUTHWEST ONTO THE SOUTHERN PLAINS... AND
THE ENSEMBLES HAVE TRENDED TOWARD EMPHASIZED THE TRAILING ONE OVER
THE LEAD ONE IN THE 00Z RUNS. IN THE EAST... FORECAST IS AND HAS
BEEN MORE STEADY WITH A WRAPPED UP SFC SYSTEM AROUND AN
INCREASINGLY STACKED AND FILLING UPPER LOW IN SOUTHERN ONTARIO
MIDWEEK. THIS WILL SPIN OFF A TRIPLE POINT SYSTEM INTO ATLANTIC
CANADA AROUND WEDNESDAY AND THEN WEAKER SHORTWAVES WILL LIKELY
ROTATE UNDER THE MAIN LOW AND AMPLIFY ENOUGH TO SHOW A SFC
REFLECTION -- COLD FRONT -- THROUGH THE NORTHEAST THU.

BY NEXT FRI/SAT... ENSEMBLES SUGGEST MAINTAINING TROUGHING IN THE
WEST AND ALLOWING SOUTHERN ENERGY TO SHEAR EASTWARD THROUGH THE
SOUTHEASTERN STATES AS THE UPPER LOW IN QUEBEC BEGRUDGINGLY MOVES
EASTWARD. GOOD AGREEMENT EXISTS BETWEEN THE 00Z GEFS AND ECENS
MEANS... AND DETAILS FROM THE 00Z ECMWF AND SOME OF THE GFS WERE
USED FOR MUCH OF THE PERIOD. HOWEVER... THESE DETAILS HAVE BEEN
SHIFTING WITH EVERY CYCLE.


...SENSIBLE WEATHER HIGHLIGHTS...

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

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 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 BIG
BEND/RIO GRANDE VALLEY AND SOUTHERN NM. FOCUS SHOULD BE FARTHER
NORTH AND EAST... ALBEIT ONLY MODESTLY AT BEST... ALONG THE
QUASI-STATIONARY BOUNDARY EAST OF THE NORTHERN/CENTRAL ROCKIES
WHERE MOISTURE WILL HAVE A BIT LONGER TO ADVECT WITH TIME.


FRACASSO/VOJTESAK

$$




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