Prognostic Meteorological Discussion
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FXAK02 KWNH 181931
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ALASKA EXTENDED FORECAST DISCUSSION
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
330 PM EDT FRI AUG 18 2017

VALID 12Z TUE AUG 22 2017 - 12Z SAT AUG 26 2017

THE UPCOMING FORECAST PERIOD WILL FEATURE A PAIR OF DOMINANT UPPER
LOW CENTERS AFFECTING THE NORTHEASTERN PACIFIC AND WESTERN
ALEUTIANS...RESPECTIVELY. THE FORMER SYSTEM IS EXPECTED TO TRACK
EAST-SOUTHEASTWARD REACHING THE COAST OF BRITISH COLUMBIA BY
ROUGHLY MID-WEEK. MODELS SHOW A DECENT FEED OF LOW/MID-LEVEL
MOISTURE AHEAD OF THE ATTENDANT COLD FRONT WITH PRECIPITABLE WATER
VALUES IN THE 95TH TO POSSIBLY 99TH PERCENTILE BASED ON THE 12Z
GFS. THE 00Z CMC APPEARED TO BE A QUICK OUTLIER WITH THIS SYSTEM
AND WAS POSITIONED AHEAD OF THE ENSEMBLE CONSENSUS. IN ITS
WAKE...A BRIEF PERIOD OF RIDGING WILL EXTEND UP THROUGH MAINLAND
AK IN ADVANCE OF AN EVEN STRONGER UPPER LOW TRACKING TOWARD THE
WESTERN ALEUTIANS. THE PARENT CIRCULATION SHOULD BE MORE ROBUST IN
NATURE WHILE HELPING SUPPRESS THE DOWNSTREAM RIDGE. THERE IS
FAIRLY GOOD DETERMINISTIC/ENSEMBLE AGREEMENT WITH THIS
SYSTEM...ALTHOUGH SOME SURFACE PRESSURE DIFFERENCES ARE EVIDENT.
THE PAST SEVERAL ECMWF ENSEMBLE MEANS HAVE SHOWED A NOTABLE
INCREASE IN CYCLONE STRENGTH...AT TIMES BEING SEVERAL MB DEEPER
THAN THE NEXT ENSEMBLE SOLUTION. THE 00Z ECMWF ENSEMBLE MEAN
BECOMES A TAD SLOW TOWARD THE END OF THE PERIOD BUT REMAINS WELL
WITHIN THE ENSEMBLE ENVELOPE. LOOKING FARTHER TO THE NORTH...A
NEGATIVE HEIGHT ANOMALY SEEMS TO REMAIN A QUASI-STATIONARY ENTITY
ON THE MAP. MOST FORECAST SOLUTIONS FAVOR PINCHING OFF A LOBE OF
VORTICITY AND DRAGGING IT SOUTHWARD WHERE IT WILL BECOME INGESTED
INTO THE LARGER CIRCULATION AFFECTING THE SOUTH-CENTRAL AK COAST
LATE NEXT WEEK.

GIVEN THE 06Z GFS AND 00Z ECMWF/UKMET WERE ALL IN DECENT AGREEMENT
WITH THE PARENT FEATURES ON THE MAP THROUGH DAY
5/WEDNESDAY...FAVORED A COMBINATION OF THESE WITH MINOR
CONTRIBUTIONS FROM THE 06Z GEFS MEAN AND 00Z ECMWF/NAEFS ENSEMBLE
MEANS. GRADUALLY INCREASED THE ENSEMBLE INFLUENCES DEEPER INTO THE
FORECAST GIVEN THE TIMING DIFFERENCES AMONG THE RESPECTIVE
ENSEMBLE MEANS. HOWEVER...CONFIDENCE IS SLIGHTLY ABOVE AVERAGE
THROUGHOUT THE PERIOD GIVEN THE LARGE-SCALE PATTERN IS
CHARACTERIZED BY SUFFICIENT MODEL AGREEMENT.

AS THE PATTERN IS HEAVILY INFLUENCED BY A PAIR OF POTENT UPPER
LOWS AND GENERALLY BELOW AVERAGE MID-LEVEL HEIGHTS...THE FORECAST
WILL FEATURE ABUNDANT CLOUD COVER ALONG WITH NUMEROUS CHANCES FOR
PRECIPITATION. THIS IS PARTICULARLY THE CASE ACROSS THE SOUTHERN
HALF OF AK WHERE THE BETTER MOISTURE...FORCING...AND LOW-LEVEL
ONSHORE FLOW WILL COLLOCATE. ON TUESDAY...RECENT MODELS SHOW 2 TO
3 INCHES OF RAINFALL POSSIBLE ACROSS SOUTHEASTERN AK WITH THE
THREAT EVENTUALLY TURNING UPSTREAM WITH THE NEXT SYSTEM. ON
THURSDAY...THE 12Z GFS IS REMARKABLY WETTER DEPICTING 2 TO 3
INCHES ACROSS SOUTH-CENTRAL AK RELATIVE TO THE TAMER 00Z ECMWF
WHICH IS LITERALLY ONE-FOURTH OF THAT. OVERALL...HIGH TEMPERATURES
IN THE UPPER 50S SHOULD BE COMMONPLACE FROM THE ALEUTIANS EASTWARD
ALONG THE COAST TOWARD THE AK PANHANDLE GIVEN THE UNSETTLED
CONDITIONS.


RUBIN-OSTER


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