Prognostic Meteorological Discussion
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FXAK02 KWNH 211915
PMDAK

ALASKA EXTENDED FORECAST DISCUSSION
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
314 PM EDT THU SEP 21 2017

VALID 12Z MON SEP 25 2017 - 12Z FRI SEP 29 2017

ENSEMBLE MEAN GUIDANCE IS SHOWING REASONABLE AGREEMENT WITH THE
LARGE SCALE FLOW PATTERN SETTING UP OVER ALASKA NEXT WEEK, WHICH
SHOULD FEATURE AN UPPER LOW GRADUALLY SINKING
SOUTHWARD/SOUTHEASTWARD OUT OF THE BERING SEA (CENTERING ITSELF
NEAR BRISTOL BAY ON MONDAY AND THEN ONLY JUST SOUTH OF KODIAK BY
FRIDAY).  ENSEMBLE MEANS WERE ALSO IN AGREEMENT WITH LOWERING
HEIGHTS OVER THE NORTHERN/NORTHEASTERN PORTION OF THE STATE WHILE
A DEEP UPPER VORTEX DROPS SOUTH TOWARDS THE REGION EARLY NEXT
WEEK.   DESPITE THE SIMILAR ENSEMBLE MEAN GUIDANCE, LARGE SPREAD
BETWEEN INDIVIDUAL ENSEMBLE MEMBERS AND RUN-TO-RUN FLUCTUATIONS IN
DETERMINISTIC MODELS AFTER DAY 4/MONDAY SUGGESTS THERE IS STILL
QUITE A BIT OF UNCERTAINTY WITH THE SMALLER SCALE DETAILS OF THE
FORECAST, ESPECIALLY WITH THE TIMING AND EVOLUTION OF MULTIPLE
SURFACE LOWS POSSIBLY LIFTING NORTH/NORTHEASTWARD THROUGH THE GULF
OF ALASKA NEXT WEEK.

THE WPC FORECAST STARTED WITH A BLEND BETWEEN A SIMILAR 00Z
ECMWF/CMC/ENSEMBLE MEANS IN ORDER TO CAPTURE SOME OF THE SMALLER
SCALE FEATURES EARLY IN THE PERIOD, BUT THEN THE FORECAST QUICKLY
TRANSITIONED TO A BLEND OF ENSEMBLE MEANS SINCE THE SPREAD AND
UNCERTAINTY RAPIDLY INCREASES BEYOND DAY 5 (TUESDAY).  THE BIGGEST
SENSIBLE WEATHER ISSUE SHOULD BE A CONSTANT STREAM OF MOISTURE
IMPACTING THE SOUTHERN PORTION OF THE STATE, FROM THE UPPER
ALEUTIANS THROUGH THE SOUTHERN PENINSULA TO THE PANHANDLE.


GERHARDT


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