Heavy Snow in Alaska, February 17 to 19, 2014

SOUTHCENTRAL AND SOUTHWEST ALASKA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE ANCHORAGE AK
1150 AM AKST MON FEB 17 2014

.ANALYSIS AND UPPER LEVELS...A VERY LARGE AND COLD UPPER LEVEL LOW
IS NEARLY STATIONARY OVER BRISTOL BAY THIS MORNING. CYCLONIC FLOW
AROUND THIS LOW EXTENDS ACROSS ALL OF ALASKA. THE PRIMARY FORECAST
CHALLENGE IS TRACKING THE MANY WAVES EMBEDDED WITHIN THE CYCLONIC
FLOW. A LOOK AT SATELLITE IMAGERY SHOWS THAT THE BULK OF THESE
FEATURES (AND ASSOCIATED UPWARD VERTICAL MOTION) ARE ON THE WEAK
SIDE. HOWEVER...COLD AIR ALOFT HAS PRODUCED SOME STEEP LAPSE
RATES...ESPECIALLY NEAR THE THE LOW CENTER WHERE 500 MB TEMPS ARE
SITTING AT 40 TO 45 BELOW ZERO. THIS INSTABILITY COMBINED WITH
WEAK SYNOPTIC LIFT FROM THE UPPER WAVES IS PRODUCING LOCALLY HEAVY
SHOWERS. THE OTHER PROCESS AT PLAY IS A STEADY SUPPLY OF COLD AIR
STREAMING ACROSS THE RELATIVELY WARMER WATERS OF THE CENTRAL AND
SOUTHERN BERING SEA...RESULTING IN WIDESPREAD SNOW SHOWERS AND
GUSTY WINDS.

.MODEL DISCUSSION...AS WOULD BE EXPECTED IN THIS PATTERN...MODELS
ARE STRUGGLING WITH THE EXACT POSITION OF THE MANY WEAK FEATURES
ROTATING AROUND THE UPPER LOW. MOST NOTABLY...WITH SOME VERY WEAK
SURFACE ROTATIONS SOUTH OF THE ALASKA PENINSULA AND OVER THE GULF
OF ALASKA...ALL MODELS ARE A LITTLE BIT OFF ON LOW CENTER PLACEMENT.

OBSERVATIONS COMBINED WITH SATELLITE AND RADAR IMAGERY ARE THE BEST
BET FOR THE VERY SHORT-TERM FORECAST.

THE PRIMARY FOCUS FOR THE SHORT-TERM FORECAST IS THE PHASING OF
TWO SHORT-WAVES OVER THE SOUTHERN GULF EARLY TUESDAY WHICH LEADS
TO STRENGTHENING OF A SURFACE LOW OVER THE NORTHWEST GULF. THE
GFS IS A CLEAR OUTLIER WITH A MORE SOUTHERLY POSITION OF THE UPPER
TROUGH AS IT BECOMES MORE AND MORE NEGATIVELY TILTED. ALL OF THE
OTHER SOLUTIONS SHOW A SIMILAR POSITION. EXPECT A FAIRLY NARROW
BAND OF HEAVY SNOWFALL OUT AHEAD OF THIS TROUGH...SO THE EXACT
POSITION IS CRITICAL TO DETERMINING WHICH AREAS WILL OBSERVE THE
GREATEST IMPACTS. FORECAST CONFIDENCE IS ABOVE AVERAGE THAT HEAVY
SNOW WILL OCCUR...BUT SINCE MODELS HAVE JUST STARTED TO REALLY
LOCK ON TO THIS EVENT THERE IS STILL SOME UNCERTAINTY IN THE
LOCATION OF HEAVIEST SNOW.

NOTE...THE 18Z NAM LOOKS COMPLETELY DIFFERENT WITH HANDLING OF
TROUGH MOVING ACROSS THE GULF...BUT WILL NOT STRAY FROM THE
ORIGINAL PLAN AT THIS POINT IN TIME.

.SHORT TERM FORECAST...

SOUTHCENTRAL ALASKA...AN UPPER WAVE OVER THE NORTHERN GULF THIS
MORNING WILL WEAKEN AS IT MOVES ONSHORE THIS AFTERNOON AND
EVENING...SO MOST OF THE SNOW WILL REMAIN ALONG THE COASTAL
REGION (PRINCE WILLIAM SOUND AND THE KENAI PENINSULA).

A MORE SIGNIFICANT EVENT IS SHAPING UP FOR TUESDAY/TUESDAY NIGHT
TIME PERIOD. AS INDICATED IN THE MODEL DISCUSSION ABOVE...A FAIRLY
STRONG UPPER TROUGH WILL TAKE SHAPE OVER THE GULF AND LIFT
NORTHWARD TOWARD SOUTHCENTRAL ALASKA. MEANWHILE A STRENGTHENING
SURFACE LOW WILL TRACK NORTHWESTWARD ACROSS THE NORTHWEST GULF
AND KENAI PENINSULA. THIS IS A GOOD SET-UP FOR SNOW FROM THE KENAI
PENINSULA AND ANCHORAGE. HOWEVER...THERE ARE A COUPLE KEY ASPECTS
WHICH WILL LIKELY LEAD TO AN AREA OF INTENSE SNOWFALL. FIRST...AS
THE UPPER TROUGH TAKES ON MORE OF AN EAST-WEST ORIENTATION A FAIRLY
STRONG 500 MB JET WILL BE POINTED TOWARD THE KENAI PENINSULA WITH
AN AREA OF SPEED DIVERGENCE INDICATING ENHANCED UPWARD MOTION.
SECOND...THE ENVIRONMENT WILL REMAIN UNSTABLE...WHICH WILL JUST
ADD TO THE VERTICAL MOTION BEING GENERATED BY THE UPPER TROUGH
AND JET. THE CHALLENGE IS DETERMINING EXACTLY WHERE THE AREA OF
GREATEST LIFT WILL BE. RIGHT NOW...BASED ON AN AVERAGE POSITION OF
THE MODELS (EXCLUDING THE GFS) IT LOOKS LIKE THE HEAVIEST SNOW WILL
FALL IN THE WHITTIER TO GIRDWOOD AND EASTERN TURNAGAIN ARM CORRIDOR
WITH POTENTIAL FOR PERIODS OF HEAVIER SNOW ACROSS THE LARGER AREA
INCLUDING THE NORTHERN HALF OF THE KENAI PENINSULA AND INTO
ANCHORAGE. WILL TREND TOWARD HIGHER SNOWFALL AMOUNTS...RECOGNIZING
THAT THESE WILL NEED TO BE UPPED EVEN FURTHER ONCE THERE IS BETTER
CONFIDENCE IN EXACT POSITION OF THE UPPER TROUGH.

THERE IS ALSO POTENTIAL FOR BLOWING SNOW...DEPENDING ON WHERE THE
SURFACE LOW TRACKS. THE MOST LIKELY AREA FOR WINDS IS WHITTIER TO
EASTERN TURNAGAIN ARM. SINCE THE BIGGER THREAT LOOKS TO BE THE HEAVY
SNOW...WILL LIKELY ISSUE A WINTER STORM WATCH WITH THE AFTERNOON
PACKAGE. THERE IS ALSO SOME POTENTIAL FOR BLOWING SNOW ISSUES IN
THOMPSON PASS...ALTHOUGH SNOWFALL IN THAT AREA WILL BE A LOT LESS
IMPRESSIVE. WILL CONSIDER ISSUING A BLIZZARD WATCH UNTIL CAN NAIL
DOWN THE DETAILS.

THE UPPER LEVEL TROUGH WILL SHEAR APART AS IT TRIES TO ROTATE BACK
AROUND THE UPPER LOW CENTER LATE TUESDAY NIGHT AND THE UPPER LOW
WILL ULTIMATELY BECOME RE-CENTERED IN THE VICINITY OF THE KENAI
PENINSULA. SO THE HEAVIEST SNOW SHOULD BE OVER BY WEDNESDAY
MORNING...BUT SNOW SHOWERS WILL PERSIST UNTIL THE UPPER LOW EXITS
WEDNESDAY NIGHT AND THURSDAY.

SOUTHWEST ALASKA...SOUTHWEST ALASKA WILL REMAIN MOSTLY ON THE DRY
AND COLD SIDE ON THE BACK SIDE OF THE UPPER LOW AS IT TRANSITIONS
TO SOUTHCENTRAL ALASKA. THERE IS POTENTIAL FOR SOME SNOW ASSOCIATED
WITH THE GULF TROUGH TO WORK ITS WAY WESTWARD ACROSS THE ALASKA
RANGE INTO THE SOUTHERN KUSKOKWIM VALLEY AND INTERIOR BRISTOL BAY.
IF IT DOES MAKE IT ACROSS EXPECT MINOR ACCUMULATION.

BERING SEA/ALEUTIANS...A SHORT-WAVE TROUGH OVER THE NORTHERN
BERING/EASTERN SIBERIA WILL DIG SOUTHWESTWARD THE UPSTREAM RIDGE
CUTS-OFF AND RETREATS NORTHWESTWARD. THIS WILL RESULT IN AN
EAST-WEST ORIENTED TROUGH AXIS ACROSS THE BERING SEA WHICH WILL
BRING AN END TO THE DEEP AND COLD NORTHERLY FLOW ACROSS THE EASTERN
HALF OF THE BERING SEA. THUS...EXPECT WINDS AND SNOW SHOWERS TO
DIMINISH TONIGHT THROUGH TUESDAY. WITH THE TROUGH LIFTING NORTHWARD
IN THE GULF AND THE TROUGH DIGGING OVER THE WESTERN BERING...FLOW
WILL BECOME ZONAL SOUTH OF THE ALEUTIAN CHAIN. A FRONTAL SYSTEM
CURRENTLY SOUTH OF THE WESTERN ALEUTIANS WILL ELONGATE SOUTH OF
THE CHAIN...BRINGING SOME PRECIP AS FAR NORTH AS THE CHAIN. AS THE
WESTERN BERING TROUGH CONTINUES TO AMPLIFY MID WEEK FLOW WILL THEN
SHIFT ENOUGH TO BRING PACIFIC SYSTEMS NORTHWARD TOWARD THE ALEUTIANS.

.LONG TERM FORECAST...LOOKS LIKE ANOTHER MAJOR PATTERN SHIFT IS IN
STORE LATE THIS WEEK THROUGH NEXT WEEKEND. THE AMPLIFYING TROUGH OUT
WEST LEADS TO DEVELOPMENT OF A LARGE COMPLEX LOW OVER THE WESTERN
BERING/ALEUTIANS/NORTHWEST PACIFIC WHICH THEN LEADS TO AMPLIFICATION
OF A DOWNSTREAM RIDGE. THIS WILL BRING A RETURN TO MUCH MORE ACTIVE
WEATHER ACROSS THE BERING SEA AND ALEUTIANS AND A WARM-UP TO THE
SOUTHERN MAINLAND OF ALASKA. IT IS A BIT TOO EARLY TO SAY WHAT THE
EXTENT OF THE WARM-UP WILL BE...BUT THIS PATTERN IS FAVORABLE FOR
EXTREME WARM-UPS LIKE WE HAD DURING THE SECOND HALF OF JANUARY.

.AER/ALU...WATCH/WARNING SUMMARY
PUBLIC...NONE.
MARINE...GALE WARNING...185.
         HEAVY FREEZING SPRAY WARNING...150 165 179 185.
FIRE WEATHER...NONE.

SEB FEB 14