Prognostic Meteorological Discussion
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FXAK02 KWNH 201902
PMDAK

ALASKA EXTENDED FORECAST DISCUSSION
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
301 PM EDT THU JUL 20 2017

VALID 12Z MON JUL 24 2017 - 12Z FRI JUL 28 2017

THE LARGE SCALE FLOW PATTERN ACROSS ALASKA SHOWS A RELATIVELY HIGH
DEGREE OF PREDICTABILITY FOR DAYS 4-8, WITH MODELS AND ESPECIALLY
ENSEMBLES SHOWING A RELATIVELY LOW AMOUNT OF SPREAD. A PERSISTENT
UPPER LEVEL RIDGE OVER THE NORTH PACIFIC WILL WORK ITS WAY
EASTWARD INTO THE ALEUTIANS BY THE MIDDLE TO END OF NEXT WEEK.
MEANWHILE, A TROUGH WILL BECOME REESTABLISHED ACROSS THE NORTHEAST
PACIFIC OFF THE SOUTHEAST ALASKA/BRITISH COLUMBIA COAST. FARTHER
NORTH, AN AREA OF NEGATIVE HEIGHT ANOMALIES WILL PERSIST ACROSS
THE ARCTIC OCEAN. THE COMBINATION OF THESE THREE LARGE SCALE
FEATURES WILL KEEP MUCH OF MAINLAND ALASKA IN FAIRLY PROGRESSIVE
WEST-NORTHWESTERLY FLOW AT THE MID/UPPER LEVELS, WITH A SERIES OF
SHORTWAVES TRAVERSING THE STATE THROUGH THE FORECAST PERIOD. THESE
FEATURES ARE WHERE A MODERATE DEGREE OF SPREAD/UNCERTAINTY IS
INTRODUCED INTO THE FORECAST, PRIMARILY DUE TO THE SMALLER SCALE
OF THE SHORTWAVES AND THEIR RELATIVELY LOW LEVEL OF PREDICTABILITY
IN THE MEDIUM RANGE. ONE SUCH SHORTWAVE IS FORECAST BY THE
GFS/ECMWF TO CROSS THE STATE ON DAY 4 (MON). THE GEM GLOBAL IS A
BIT SLOWER WITH THIS WAVE, BUT GIVEN THE RELATIVELY GOOD AGREEMENT
BETWEEN THE GFS/ECMWF, THESE SOLUTIONS WERE WEIGHTED FAIRLY
HEAVILY INITIALLY IN THE FORECAST PERIOD. A SERIES OF WEAKER
SHORTWAVES WILL FOLLOW THE SAME PATH IN THE WAKE OF THE ONE ON DAY
4. GRADUALLY RISING HEIGHTS ACROSS THE ALEUTIANS AND SOUTHERN
ALASKA AS THE NORTH PACIFIC RIDGE BUILDS EAST SHOULD SERVE TO
SHIFT THE MOST ACTIVE MID/UPPER FLOW SLOWLY NORTH THROUGH THE
PERIOD. BY DAYS 7-8 (THU-FRI) ANOTHER MORE SIGNIFICANT SHORTWAVE
IS FORECAST BY MOST GUIDANCE TO CROSS THE KAMCHATKA PENINSULA AND
PERHAPS REACH WESTERN AK. SPREAD IS A BIT HIGHER BY THIS TIME,
WITH THE ECMWF SHOWING A STRONGER AND SOMEWHAT FASTER WAVE THAN
THE GFS/GEM. GIVEN THE INCREASING SPREAD AND UNCERTAINTY WITH
RESPECT TO THE SMALLER SCALE ASPECTS OF THE FLOW, THE FORECAST WAS
TRENDED GRADUALLY TOWARD HEAVIER ENSEMBLE WEIGHTING DURING THE DAY
6-8 TIME PERIOD.


RYAN

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