THE WEATHER PREDICTION CENTER

COLLEGE PARK, MD


ALASKA EXTENDED FORECAST DISCUSSION




ALASKA EXTENDED FORECAST DISCUSSION
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
215 PM EST MON DEC 11 2017

VALID 12Z FRI DEC 15 2017 - 12Z TUE DEC 19 2017

MODELS SHOW A COUPLE WEATHER SYSTEMS OF NOTE AFFECTING ALASKA
DURING THE DAY 4-8 (FRI-TUE) TIME FRAME. DETERMINISTIC MODELS SHOW
GENERAL AGREEMENT ON A SHORTWAVE/SURFACE CYCLONE CROSSING THE
ALEUTIAN CHAIN/SOUTHERN BERING SEA FRI BEFORE EMERGING INTO THE
GULF ON SAT. SOLUTIONS DIFFER, HOWEVER, WITH RESPECT TO WHETHER A
TRIPLE POINT LOW DEVELOPS ACROSS THE GULF FRI NIGHT/SAT AND
QUICKLY MOVES ONSHORE IN SOUTHEASTERN AK. ECMWF AND GFS RUNS FROM
LAST NIGHT HAD BOTH SHOWN SUCH A LOW DEVELOPING BUT THE 12Z RUNS
OF BOTH MODELS BACKED OFF ON THIS IDEA SUBSTANTIALLY. NONETHELESS,
PRIMARY SURFACE LOW IS EXPECTED TO DRIFT EAST ACROSS THE NORTHERN
GULF THROUGH SUN AS THE SURFACE FRONT MOVES INLAND. A MAJORITY
DETERMINISTIC BLEND OF THE 00Z ECMWF/12Z GFS ALONG WITH THE 00Z
ECENS/NAEFS MEANS SERVED AS A STARTING POINT FOR THE FORECAST
DURING DAYS 4-5. BY LATE IN THE WEEKEND INTO EARLY NEXT WEEK
ATTENTION TURNS TOWARD SYSTEM CROSSING THE ALEUTIANS OR SOUTHERN
BERING SEA. MODELS AND ENSEMBLES ARE IN GENERAL AGREEMENT THAT AN
UPPER SHORTWAVE WILL RAPIDLY AMPLIFY AS IT EXITS EASTERN ASIA
RESULTING IN SIGNIFICANT DEEPENING OF THE ASSOCIATED SURFACE
CYCLONE. DETERMINISTIC SOLUTIONS ALL DEEPEN THE CYCLONE INTO THE
940-955 HPA RANGE, BUT DIFFERENCES EXIST ON THE EXACT TRACK AND
TIMING, WITH THE ECMWF TAKING THE CYCLONE ON A MORE NORTHERLY
TRACK TOWARD EASTERN RUSSIA WHILE THE GFS AND CMC KEEP THE LOW
FARTHER SOUTH OVER THE CENTRAL OR EVEN SOUTHERN BERING SEA THROUGH
MON NIGHT. THE FORECAST WAS TRENDED HEAVILY TOWARD THE 00Z
ECENS/NAEFS MEANS FROM DAY 6 ONWARD, WHICH RESULTS IN A LOW
POSITION FOR THIS CYCLONE A BIT CLOSER TO THE GFS THAN THE 12Z
ECMWF.

IN TERMS OF SENSIBLE WEATHER IMPACTS, EXPECT WIDESPREAD
PRECIPITATION AS WELL AS GUSTY WINDS TO AFFECT PORTIONS OF THE
SOUTHERN BERING, ALEUTIANS, AND NORTHERN GULF IN ASSOCIATION WITH
THE SURFACE CYCLONE FRI-SAT, WITH SOME POTENTIAL FOR AN ENHANCED
AREA OF WINDS ACROSS THE NORTHERN GULF IF TRIPLE POINT LOW
DEVELOPMENT OCCURS. PERSISTENT ONSHORE FLOW IN THE WAKE OF THE
FIRST SYSTEM WILL KEEP PRECIP IN THE PICTURE ESPECIALLY ACROSS
SOUTHEAST ALASKA SUN-MON. EXPECT WELL DEVELOPED PLUME OF MOISTURE
IN ASSOCIATION WITH NEXT SYSTEM CROSSING THE ALEUTIANS MON-TUE
ALONG WITH THE POTENTIAL FOR STRONG WINDS IF THE CYCLONE IS AS
INTENSE AS DETERMINISTIC SOLUTIONS ARE INDICATING. TEMPERATURES
SHOULD GRADUALLY DECREASE TO NEAR OR EVEN A BIT BELOW NORMAL
ACROSS MUCH OF THE STATE FRI-SUN AS UPPER RIDGING WEAKENS,
ALLOWING ARCTIC AIR TO PRESS FARTHER SOUTH. BY EARLY NEXT WEEK A
SOMEWHAT WARMER MARITIME AIR MASS COULD SPREAD INTO PORTIONS OF
SOUTHWESTERN MAINLAND AK ASSOCIATED WITH SOUTHERLY FLOW AHEAD OF
THE INTENSE INCOMING LOW PRESSURE SYSTEM.

RYAN





Last Updated: 215 PM EST MON DEC 11 2017