Winter Weather in Alaska, September 25, 2015

NORTHERN ALASKA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE FAIRBANKS AK
533 AM AKDT FRI SEP 25 2015

.DISCUSSION...

UPPER AIR...
AN UPPER LOW CENTERED NEAR CAPE LISBURNE WILL MOVE EASTWARD
TO NEAR OLD CROW BY 4 AM SATURDAY...THEN PROCEED EASTWARD
INTO NORTHWEST CANADA DURING THE DAYTIME ON SATURDAY. A
MODERATE WESTERLY FLOW PATTERN WILL PERSIST OVER THE
SOUTHERN HALF OF NORTHERN ALASKA THROUGH SATURDAY NIGHT.
AN UPPER LEVEL RIDGE WILL BEGIN TO BUILD OVER THE WESTERN
ALASKA INTERIOR SUNDAY MORNING...AND INCREASE IN AMPLITUDE
AS IT MOVES EASTWARD TO THE ALCAN BORDER BY MIDNIGHT
SUNDAY NIGHT. THE RIDGE WILL CONTINUE ON EASTWARD INTO
NORTHWEST CANADA LATE SUNDAY NIGHT AND MONDAY. SOUTHWEST
FLOW ALOFT ON THE BACK SIDE OF THE RIDGE WILL BRING WARMER
AIR ALOFT INTO THE NORTHERN ALASKA MAINLAND SUNDAY NIGHT.
THE WARMER AIR ALOFT WILL SHIFT SLOWLY EASTWARD MONDAY
AND MONDAY NIGHT AND BE REPLACED BY COLDER AIR ALOFT.

OVER CENTRAL SIBERIA...A SHORT WAVE TROUGH CURRENTLY EXTENDS
FROM AN UPPER LOW NEAR 74N/160E SOUTHWESTWARD INTO THE SIBERIAN
INTERIOR. THIS SHORT WAVE WILL SWING EASTWARD AND DEEPEN INTO
AN UPPER LOW JUST WEST OF THE GULF OF ANADYR BY SUNDAY
AFTERNOON. THIS UPPER LOW WILL DEEPEN SUBSTANTIALLY SUNDAY
NIGHT THROUGH MONDAY EVENING AS IT MOVES EASTWARD TO THE
SEWARD PENINSULA OR NORTON SOUND...WITH A TROUGH EXTENDING
SOUTHWARD. THE UPPER LOW IS THEN EXPECTED TO BECOME A TROUGH
WHICH WILL EXTEND FROM BARROW SOUTHEASTWARD ACROSS NORTHERN
ALASKA BY WEDNESDAY. OUT THROUGH WEDNESDAY...THERE ARE ONLY
MINOR DIFFERENCES BETWEEN THE OPERATIONAL GFS AND ECMWF
500 MB HEIGHT/850 MB TEMPERATURE PROGS.

SURFACE...
A 988 MB LOW CENTERED ABOUT 50 MILES NORTHWEST OF CAPE LISBURNE
WILL WEAKEN AND DISSIPATE OVER THE NORTHWEST ALASKA INTERIOR BY
THIS AFTERNOON AS A WEAK ELONGATED 992 MB LOW EXTENDING FROM
BARROW TO BARTER ISLAND DEVELOPS. THIS ELONGATED LOW WILL WEAKEN
INTO A TROUGH OVER THE CENTRAL AND EASTERN BEAUFORT SEA LATE
TONIGHT.

A WEAK 995 MB LOW IS EXPECTED TO DEVELOP NEAR 54N/177E...ABOUT
100 MILES NORTHEAST OF SHEYMA...SATURDAY MORNING. THE 25/00Z
OPERATIONAL RUNS OF THE GFS/NAM/ECMWF MODELS RAPIDLY DEEPEN THIS
LOW AS IT MOVES NORTHEASTWARD THROUGH SUNDAY NIGHT. BY 4AM MONDAY
THE GFS AND ECMWF PUT THE LOW CENTER OVER OR NEAR NORTON SOUND
WITH A CENTRAL PRESSURE IN THE 980-985 MB RANGE. THE NAM MOVES THE
TAKES THE LOW FARTHER EASTWARD...TO NEAR GALENA...WITH A CENTRAL
PRESSURE OF ABOUT 990 MB. THE CANADIAN NH MODEL PUTS A 980 MB
CENTER OVER EASTERN NORTON SOUND. MOST OF THE 12 GEFS ENSEMBLE
MEMBERS DISPLAYED IN AWIPS ARE CLUSTERED OVER OR NEAR NORTON SOUND
AT 4 AM MONDAY...THE OPERATIONAL FORECAST POSITION APPEARS TO BE
NEAR THE CENTER OF THE CLUSTER AND AGREES WELL WITH THE GEFS
ENSEMBLE MEAN. BEYOND 4 AM MONDAY...FORECAST UNCERTAINTY
INCREASES AS THE OPERATIONAL SOLUTIONS BEGIN TO DIVERGE. BY 4AM
TUESDAY...THE GFS PUTS THE LOW CENTER NEAR SHISHMAREF...ECMWF LOW
POSITION IS ON THE EASTERN COAST OF NORTON SOUND...AND THE
CANADIAN MODEL TAKES THE LOW CENTER NORTHEASTWARD TO NEAR ICY
CAPE. ALL THREE MODELS DEEPEN THE LOW DOWN INTO THE 970S. THERE IS
POTENTIAL FOR COASTAL FLOODING IN NORTON SOUND MONDAY IF THE LOW
CENTER EXITS TO THE NORTH OR NORTHEAST OF NORTON SOUND. IN SUCH A
SCENARIO...SOUTHWESTERLY GALE FORCE WINDS WOULD INITIALLY DEVELOP
OVER SOUND...GRADUALLY SHIFTING TO WEST AND THEN TO NORTHWEST.
THIS WIND PATTERN WOULD PUSH WATER INTO NORTON SOUND LIKELY
CAUSING HIGH SEAS AND SIGNIFICANTLY ELEVATED WATER LEVELS. IT IS
TOO EARLY AT THIS TIME TO ISSUE ANY WATCHES...BUT SUBSEQUENT MODEL
RUNS NEED TO BE CLOSELY EVALUATED FOR THE EVOLUTION OF THIS EVENT
AND POTENTIAL FOR COASTAL FLOODING. THIS SYSTEM WILL LIKELY BRING
SIGNIFICANT RAINFALL TO THE WEST COAST...WESTERN AND CENTRAL
INTERIOR BY MONDAY. RAIN WOULD LIKELY BEGIN SUNDAY OVER THE
SOUTHERN PARTS OF THE WEST COAST AND WESTERN INTERIOR...THEN
SPREAD NORTHWARD AND EASTWARD THROUGH MONDAY. RAIN COULD SPREAD TO
MUCH OF THE EASTERN INTERIOR BY TUESDAY.

QUICK LOOK AT THE 25/06Z GFS AND ECMWF MODEL RUNS SHOWS
NO SIGNIFICANT CHANGE IN THE 4 AM MONDAY FORECAST POSITIONS OF
THE INCOMING LOW CENTER...WITH BOTH MODELS TRACKING THE LOW
NORTHWARD TO NEAR KOTZEBUE BY MONDAY EVENING.

IN THE SHORTER TERM...AN EASTWARD MOVING UPPER LOW AND
WEATHER FRONT WILL SPREAD PRECIPITATION ACROSS THE INTERIOR TODAY
AND TONIGHT. PRECIPITATION IS EXPECTED TO BE MAINLY SNOW OR MIXED
RAIN AND SNOW.

IN ZONES 221 AND 222...SNOWFALL AMOUNTS OF 2 TO 5 INCHES ARE
EXPECTED TODAY...SOME SMALL ADDITIONAL AMOUNTS POSSIBLE TONIGHT.
THE HEAVIEST SNOW AMOUNTS ARE EXPECTED TO BE AT HIGHER
ELEVATIONS. SNOW COULD BE MIXED WITH RAIN AT AT OR NEAR VALLEY
LEVEL.

IN ZONE 225...5 TO 10 INCHES OF SNOW EXPECTED TODAY. HEAVIEST
AMOUNTS ABOVE 3000 FT MSL.

IN ZONE 227...5 TO 10 INCHES OF SNOW EXPECTED TODAY IN AREAS
NEAR THE ALASKA RANGE...HEAVIEST AMOUNTS ABOVE 3000 FT MSL.

OTHER MAJOR WEATHER...
ZONE 205...UP TO 5 INCHES OF SNOW TODAY THROUGH SATURDAY.
ZONE 206...UP TO 6 INCHES OF SNOW TODAY THROUGH SATURDAY.
ZONE 207...STRONG NORTHWEST WINDS TODAY.
ZONE 208...UP TO 2 INCHES OF SNOW TODAY.
ZONE 209...STRONG WEST TO NORTHWEST WINDS TODAY.
ZONE 210...WEST TO NORTHWEST WINDS TO 35 MPH TODAY.
ZONE 213...NORTH WINDS 25 TO 35 MPH ON THE BERING STRAIT COAST
 TODAY.
ZONE 217...2 TO 4 INCHES OF SNOW THROUGH TONIGHT.
ZONE 218...UP TO 3 INCHES OF SNOW TODAY.
ZONE 219...ABOUT 4 INCHES OF SNOW THROUGH TONIGHT.

COASTAL HAZARD POTENTIAL...
IN THE SHORT TERM...HIGH SURF CONDITIONS CONTINUING IN ZONES
201-207-209-210 THROUGH 10 PM AKDT THIS EVENING.

IN THE LONGER TERM...POTENTIAL FOR COASTAL FLOODING IN NORTON
SOUND MONDAY. SEE MAIN DISCUSSION FOR FURTHER DETAILS.

&&

.HYDROLOGY...
NONE.

&&

.AFG WATCHES/WARNINGS/ADVISORIES...
HIGH SURF ADVISORY FOR AKZ201-AKZ207-AKZ209-AKZ210.

WINTER WEATHER ADVISORY FOR AKZ221-AKZ222-AKZ225-AKZ227.

SMALL CRAFT ADVISORY FOR PKZ200-PKZ210-PKZ230.

GALE WARNING FOR PKZ215-PKZ220-PKZ225.
&&

RF SEP 15