Small Craft Advisory 102, NWS Fairbanks, AK

NORTHERN ALASKA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE FAIRBANKS AK
523 PM AKDT TUE SEP 18 2012

.DISCUSSION...
A SERIES OF STRONG AND FAST MOVING LOW PRESSURE SYSTEMS FROM THEIR
ORIGIN IN THE EASTERN NORTH PACIFIC CONTINUES TODAY. THE FIRST IN
THIS SERIES CROSSED THE ALASKA PENINSULA ON SATURDAY MORNING. IT
THEN GAINED STRENGTH AS IT SLOWLY MOVED OVER THE SOUTHWESTERN
MAINLAND OF ALASKA ON SUNDAY. THE GREAT WIND STORM OVER MUCH OF
THE ALASKA MAINLAND CAME ON SUNDAY NIGHT/MONDAY MORNING.

ON MONDAY...THE LOW PRESSURE SYSTEM SPLIT INTO TWO LOW PRESSURE
SYSTEMS JOINED IN AN OBLONG AREA OVER THE ARCTIC COAST OF ALASKA.

THE SECOND LOW IN THIS SERIES APPROACHED NUNIVAK ISLAND TODAY.
BRISK SOUTH WINDS OVER MUCH OF THE MAINLAND WERE REGAINED STRENGTH
AS THE LOW PASSED BY.

THE THIRD LOW IN THE SERIES WAS FORMING ABOUT 1000 MILES SOUTH OF
ALASKA TODAY...AND WILL REACH THE SOUTHERN MAINLAND LATE TONIGHT
AND EARLY WEDNESDAY.

THE GFS COMPUTER WEATHER FORECASTING MODEL HAS NOT INITIALIZED THE
RAPIDLY INBOUND FRONTAL WAVE NOW NEAR 44 DEGREES NORTH LATITUDE
AND 170 DEGREES WEST LONGITUDE. THE NAM MODEL IS MORE CONSISTENT
WITH PREVIOUS MODEL RUNS AND HAS A CREDIBLE PROJECTION OF ITS
FUTURE DEVELOPMENT AND PATH ACROSS THE PACIFIC TO ALASKA.

THIS IS A DIFFERENCE BETWEEN COMPUTER MODELS THAT IS NOT OFTEN
SEEN. MODELS WITH GLOBAL COVERAGE -- SUCH AS THE GFS MODEL /
GLOBAL FORECAST SYSTEM / GENERALLY PERFORM BETTER IN THE
FORECASTING OF FAST MOVING WEATHER SYSTEMS WHICH TRAVEL A LONG
DISTANCE. MUCH OF THE AREA OVER WHICH SUCH SYSTEMS MOVE IS OFTEN
OUTSIDE OF THE GEOGRAPHIC DOMAIN WITHIN WHICH NESTED MODELS
COMPUTE FORECASTS.

SO IT IS WITH TODAYS SWIFTLY INBOUND NORTH PACIFIC LOW. THIS MAY
BE A CASE IN WHICH A GOOD GLOBAL MODEL RUN PROVIDED GOOD
INITIALIZATION FOR A SUBSEQUENT RUN OF A SMALLER AREA NESTED
MODEL...AND THE FOLLOWING GLOBAL MODEL RUN GOES OFF TRACK.

THE GEOSTATIONARY SATELLITE IMAGERY TODAY HAS SHOWN A CLASSIC
EXAMPLE OF THE DEVELOPMENT OF A FAST TRACK OF NORTHWARD MOVING
CLOUDS ALONG A STRONG WEATHER FRONT WHICH NOW EXTENDS FROM ABOUT
27 DEGREES NORTH LATITUDE AND 170 DEGREES WEST LONGITUDE...NORTH
TO THE ALASKA MAINLAND. THIS FEATURE IS SOMETIMES DUBBED THE
PINEAPPLE EXPRESS BECAUSE ITS ORIGIN IS NOT FAR FROM HAWAII.

THE FRONTAL WAVE ON THIS SYSTEM IS NOW CAUGHT INTO THE FAST
NORTHWARD MOVING WINDS OVER THE FRONT. THE NAM MODEL APTLY
FORECASTS IT TO RACE NORTHWARD...AT A SPEED AROUND 70 / SEVENTY /
MPH. THE NAM DOES NOT FORECAST THIS LOW TO REACH A VERY STRONG
CENTRAL PRESSURE...ONE LESS THAN 970 MILLIBARS.

THIS IS CONSISTENT WITH PAST EXPERIENCE. THE CIRCULATION AROUND A
LOW PRESSURE SYSTEM OF SUCH MODEST DIMENSIONS IS USUALLY NOT
STRONG. ...HOWEVER... THE GREAT ENERGY OF THE FAST MOVEMENT OF THE
WHOLE SYSTEM IS ABSORBED WHEN SUCH A LOW PRESSURE SYSTEM REACHES A
GEOGRAPHIC BOUNDARY...IN THIS CASE...THE MAINLAND AND MOUNTAINS OF
ALASKA.

AT THE TIME OF ARRIVAL...THE ENERGY STORED IN SUCH A LOW PRESSURE
SYSTEM IS EXPENDED AS IT CAUSES STRONG AND TURBULENT WIND FLOW
OVER ITS DESTINATION. THE END RESULT IS THE PRODUCTION OF HEAT IN
THE AIR MASS. THIS IS WHAT IS GOING ON TODAY...AND DUE TO FINISH
THE PROCESS ON WEDNESDAY.

BRISK SOUTH WINDS HAVE BEEN BLOWING OVER THE ALASKA RANGE ALL
DAY...BUT THE ARRIVAL OF THIS INCOMING LOW PRESSURE SYSTEM LATE
TONIGHT AND WEDNESDAY MORNING WILL CAUSE A BIG INCREASE IN THE
WINDS OVER THE INTERIOR...ESPECIALLY IN THE LEE OF THE ALASKA
RANGE MOUNTAINS. THIS IS THE REASONING FOR THE WIND ADVISORIES NOW
IN EFFECT FOR THE SOUTHERN INTERIOR TONIGHT...AND FOR THE HIGH WIND
WARNINGS IN EFFECT WEDNESDAY MORNING AND EVENING.

TO PUT IT IN OTHER WORDS...TODAYS WEATHER CHARTS AND ESPECIALLY
THE SATELLITE IMAGERY LOOPS...ARE GOOD MATERIAL FOR A TEXTBOOK.

THE COURSE OF EVENTS AFTER THE ARRIVAL OF THE FAST MOVING LOW
PRESSURE SYSTEM HAS CONSIDERABLE UNCERTAINTY. THE MODELS DO AGREE
THAT A LARGE AREA OF LOW PRESSURE ALOFT WILL SETTLE OVER THE
WESTERN MAINLAND OF ALASKA BY LATE THIS WEEK. THIS IS A PATTERN
THAT CAN LAST FOR DAYS OR SEVERAL WEEKS. THE WEATHER THAT
ACCOMPANIES THIS IN NORTHERN ALASKA IS USUALLY PARTLY CLOUDY AND
MILD IN THE EAST...AND A LITTLE COOL AND MOSTLY CLOUDY...WITH
SCATTERED SHOWERS OR RAIN OR SNOW...IN THE WEST.

VISIBLE GEOSTATIONARY SATELLITE IMAGERY TODAY HAS SHOWN THE
DRAMATIC INFLUX OF A HUGE AREA OF CLOUDS ACCOMPANYING LOW NUMBER
TWO...NOW SOUTH OF NUNIVAK ISLAND. A CLEAR AREA OVER THE
NORTHEASTERN MAINLAND SHOWED SOLID SNOW COVER OVER THE BROOKS
RANGE ABOVE AN ALTITUDE FROM 1500 TO 2000 FEET.

COASTAL HAZARD POTENTIAL DAYS 3 AND 4...NONE

&&

.HYDROLOGY...
PRECIPITATION NORTH OF THE ALASKA RANGE OVER THE NEXT THREE OR
MORE DAYS WILL BE VERY LIGHT. ABOUT ONE HALF OF AN INCH OF RAIN
FELL OVER WESTERN ALASKA IN THE PAST 24 HOURS. SOME RIVERS IN
NORTHWEST ALASKA ARE RISING...BUT ALL WATERCOURSES IN THE REALM
ARE AT STAGES WELL WITHIN THEIR BANKS.

&&

.AFG WATCHES/WARNINGS/ADVISORIES...
WIND ADVISORY FOR AKZ223-AKZ224-AKZ225-AKZ226-AKZ227.

HIGH WIND WARNING FOR AKZ223-AKZ224-AKZ225-AKZ226.

SMALL CRAFT ADVISORY FOR PKZ210.

SMALL CRAFT ADVISORY FOR HAZARDOUS SEAS FOR PKZ225.
&&

TF SEP 12