Prognostic Meteorological Discussion
Issued by NWS

Home | Current Version | Previous Version | Text Only | Print | Product List | Glossary On
Versions: 1 2
000
FXAK02 KWNH 221843
PMDAK

ALASKA EXTENDED FORECAST DISCUSSION
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
242 PM EDT SAT APR 22 2017

VALID 12Z WED APR 26 2017 - 12Z SUN APR 30 2017

GUIDANCE HAS STABILIZED PRETTY WELL WITH THE IDEA OF A MEAN RIDGE
EXTENDING OVER THE MAINLAND FROM WRN CANADA WHILE ALEUTIANS LOW
PRESSURE AND UPR LOW/TROUGH GRADUALLY WEAKEN AS THE SYSTEM
APPROACHES THE SWRN MAINLAND, LIKELY DISSIPATING BY EARLY SAT.
THE MEAN RIDGE ALOFT SHOULD PERSIST IN SOME FASHION THROUGH DAY 8
SUN WHILE SFC HIGH PRESSURE OVER THE ARCTIC EXTENDS INTO NRN
CANADA.  THERE ARE STILL TYPICAL UNCERTAINTIES FOR STRENGTH/SHAPE
OF THE RIDGE ALOFT WITH A MODEL/ENSEMBLE MEAN CONSENSUS ACCOUNTING
FOR THESE ISSUES WELL.  THIS SOLN REFLECTS A SLIGHTLY STRONGER
TREND VS PRIOR FCST AT SOME VALID TIMES.

THE NERN PACIFIC CONTINUES TO BE THE MOST TROUBLESOME AREA OF THE
FCST WITH INDIVIDUAL MODELS AND ENSEMBLE MEMBERS STILL AT ODDS
OVER WHETHER ONE OR MORE MID-LATITUDE WAVES COULD TRACK FAR ENOUGH
N TO HAVE SOME INFLUENCE ON THE PANHANDLE/GULF OF ALASKA.  EARLY
IN THE PERIOD WED-THU 00Z CANADIAN ENSEMBLES ON A PERCENTAGE BASIS
SHOWED A HIGHER POTENTIAL FOR A FARTHER N SFC LOW TRACK WITH THE
12Z CMC AND ESPECIALLY 12Z UKMET PART OF THIS CAMP.  IT STILL
LOOKS LIKE A FINE LINE BTWN SUPPORTING ENERGY ALOFT BEING
DEFLECTED AROUND THE WRN SIDE OF THE NRN BC PART OF THE RIDGE
ALOFT OR UNDERCUTTING TO THE S.  THEN THE 00Z GFS AND ALONG WITH
SOME GEFS MEMBERS SHOWED SUCH POTENTIAL BY FRI ONWARD.  THIS GFS
FEATURE (FARTHER SE IN THE 06Z RUN) ORIGINATES FROM ENERGY OVER
THE WRN BERING SEA AS OF 12Z TODAY AND FLOWS ALL THE WAY AROUND
THE MEAN CYCLONIC CIRCULATION.  THE 00Z UKMET/NAVGEM AGREED WITH
THE 00Z ECMWF/CMC THAT ANY DEVELOPMENT FOR THIS SPECIFIC FEATURE
SHOULD BE WEAKER/FARTHER SE.  THE 12Z GFS/UKMET/CMC FURTHER AGREE
IN LEANING AWAY FROM THE 00Z/06Z GFS FOR THIS FEATURE.
INCONSISTENT SIGNALS EARLY AND MAJORITY TRENDS LATER FAVOR A
DETERMINISTIC FCST THAT KEEPS ANY WELL DEFINED SFC WAVES TO THE S
OF THE GULF OF ALASKA/PANHANDLE FOR THE TIME BEING.

OVERALL THE 00Z ECMWF/ECMWF MEAN/NAEFS MEAN PROVIDED THE BEST
COMBINATION OF CLUSTERING AND CONTINUITY FOR THE NWRN PAC SYSTEM
FCST TO TRACK NEAR THE ALEUTIANS DURING THE LATTER HALF OF THE
PERIOD.  THREE OF THE PAST FOUR OPERATIONAL ECMWF RUNS (00Z/21 RUN
BEING THE EXCEPTION) HAVE BEEN FAIRLY CLOSE TO THIS CONSENSUS.
THE LAST COUPLE GEFS MEAN RUNS SLOWED DOWN VERSUS EARLIER RUNS
WHILE THE LATEST ECMWF MEAN HAS ACCELERATED A BIT.  INCLUDING THE
SLIGHTLY SLOWER NAEFS MEAN WOULD HELP TO NUDGE THE ECMWF-BASED
SOLNS BACK A BIT TO ACCOUNT FOR CONTINUITY.  THE FULL
MODEL/ENSEMBLE SPREAD IS STILL FAIRLY BROAD, INCLUDING SOME GFS
RUNS THAT HAVE BEEN HOLDING THE PARENT LOW WELL WWD WHILE THE
LEADING WAVY FRONT CONTINUES ALONG AT A PACE MORE SIMILAR TO THE
FAVORED SFC LOW CLUSTER.

BASED ON CONSIDERATIONS FOR THE ABOVE AREAS/SYSTEMS, TODAY`S FCST
STARTED WITH A BLEND OF MOSTLY 12Z GFS/00Z ECMWF DAYS 4-5 WED-THU
TRENDING TO A NEARLY EVEN WEIGHT OF THE MODELS AND 00Z NAEFS/ECMWF
MEANS DAY 6 WED.  THE LAST TWO DAYS OF THE FCST CONSISTED
PRIMARILY OF THE ENSEMBLE MEANS WHILE RETAINING 25 PCT OF THE 00Z
ECMWF.

RAUSCH

$$




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