Preliminary Forecasts
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FXUS02 KWNH 170518
PREEPD

PRELIMINARY EXTENDED FORECAST DISCUSSION
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
1217 AM EST WED DEC 17 2014

VALID 12Z SAT DEC 20 2014 - 12Z WED DEC 24 2014

...SOAKING RAIN FOR PORTIONS OF WASHINGTON/OREGON STARTING LATE
THIS WEEKEND...


..OVERVIEW AND GUIDANCE PREFERENCES...

ACTIVE SPLIT-FLOW PATTERN OVER THE CONUS WILL TRANSITION TO A MORE
AMPLIFIED RIDGE/TROUGH WITH A POSSIBLE NEGATIVELY TILT TROUGH
SETTING UP OVER THE GREAT LAKES BY CHRISTMAS EVE. A BREAK/BUCKLING
IN THE STRONG WESTERN/CENTRAL PACIFIC JET BY SUNDAY WILL BE THE
IMPETUS FOR AMPLIFYING THE PATTERN DOWNSTREAM OVER THE CONUS. THE
MODELS/ENSEMBLES HAVE WAVERED IN DETAILS BUT HAVE MOSTLY KEPT ON
TOP OF THE PATTERN SHIFTS... WITH SOME OVER/UNDER AMPLIFICATION
EVIDENT AT BOTH LARGER AND SMALLER SCALES. THAT BEING SAID... A
COMPROMISE/BLENDED APPROACH SEEMED PRUDENT... BANKED MOSTLY ON THE
18Z PARALLEL GFS AND 12Z ECMWF/ECENS MEAN THROUGH MONDAY BEFORE
TRENDING TOWARD A GEFS/ECMWF ENSEMBLE MEAN SOLUTION.


...SENSIBLE WEATHER HIGHLIGHTS...

WITH AN INCREASING JET INTO THE PAC NW THIS WEEKEND INTO MONDAY...
150KTS PLUS... EXPECT A PRETTY WET PERIOD FOR ESPECIALLY COASTAL
WA/OR AND INTO THE CASCADES. NEAR RIGHT ANGLE FLOW INTO THE
TERRAIN COUPLED WITH PW ANOMALIES IN THE +2 TO +3 RANGE SHOULD
HELP MAXIMIZE PRECIP POTENTIAL... WITH SOME HIGHER RES GUIDANCE
SHOWING SEVERAL INCHES OF QPF SAT-MON -- UP TO ABOUT TEN INCHES AT
THE HIGH SIDE OF THE SPREAD. SNOW LEVELS WILL BE FAIRLY HIGH IN
THE PACIFIC AIRMASS BUT WILL ACCUMULATE AT THE HIGHEST PEAKS.

IN THE EAST... THE PESKY SYSTEM FORECAST TO SLIDE OFF THE COAST
WILL LIKELY DEVELOP FAR ENOUGH OFFSHORE TO HAVE LOW IMPACTS ALONG
THE COAST... BUT COULD THROW SOME LIGHT RAIN/SNOW BACK INTO THE
MID-ATLANTIC/NORTHEAST AS IT EXITS INTO THE ATLANTIC.

MID-PERIOD... QUESTIONS FROM THE LAST FEW NIGHTS ABOUT HOW MUCH
ENERGY WOULD DIG/SPLIT INTO N MEXICO AS THE RIDGE BUILDS OFF
CALIFORNIA AND THE TROUGHING DIGS INTO AZ/NM LATE THIS WEEKEND.
TREND HAS BEEN AWAY FROM THE DIGGY SOLUTIONS AND MORE TOWARD WHAT
THE GEFS MEMBERS HAD 48 HRS AGO -- BUT IT APPEARS THAT THE GEFS
AND ECMWF ENSEMBLES CONVERGED SOMEWHAT MIDWAY BETWEEN EACH OF
THEIR FORMER SOLUTIONS. CURRENT 12Z ECMWF ENSEMBLES STILL LAG JUST
BEHIND THE GEFS MEMBERS AND SHOW A BIT MORE AMPLITUDE BUT EVEN A
BLEND BETWEEN THE TWO SIGNALS A NEGATIVELY TILTED TROUGH
DEVELOPING BY NEXT WEDNESDAY... 24 DECEMBER. SLOWER SOLUTIONS
WOULD ALLOW MUCH MORE RAIN TO DEVELOP IN THE SOUTHEAST AHEAD OF
THE TROUGH AXIS. GIVEN ITS TIMING... IMPACTS WILL BE GREATER THAN
AVERAGE. STAY TUNED.


FRACASSO

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