Storm Warning 157, NWS Anchorage, AK

AREA FORECAST DISCUSSION
SOUTHCENTRAL AND SOUTHWEST ALASKA
200 PM AKDT THU SEP 30 2010

.ANALYSIS AND UPPER LEVELS...
HIGHLY AMPLIFIED PATTERN IN PLACE OVER MUCH OF THE NORTHERN
HEMISPHERE. A DEEP UPPER LEVEL TROF IS NEUTRALLY TILTED WITH AXIS
ABOUT 150W. THE DEEP TROF IS TAPPING SUBTROPICAL MOISTURE AS
EVIDENCED BY THE WATER VAPOR IMAGERY DEPICTING A 1800 MILE FETCH OF
MOISTURE STREAMING NORTH TOWARD SOUTHCENTRAL ALASKA.

AT THE SURFACE...A VERTICALLY STACKED LOW IS WEAKENING OVER BRISTOL
BAY WHILE ALL THE ENERGY IS TRANSFERRED TO THE DEVELOPING SYSTEM AT
THE BASE OF THE TROF. MOISTURE OVERRUNNING THE WEAKENING LOW IS
PRODUCING WIDESPREAD LIGHT SNOW ACROSS THE KUSKOKWIM VALLEY.
PRECIPITATION IS FALLING IN THE FORM OF RAIN OVER SOUTHCENTRAL ALASKA
WHERE FREEZING LEVELS INCREASED ABOVE 4000 FT THIS MORNING.

.MODEL DISCUSSION...
THE FOCUS IS ON THE SHORT TERM AS WE DETERMINE THE CORRECT TRACK OF
THE DEVELOPING LOW IN THE GULF. THE 12Z NAM CAME INTO AGREEMENT WITH
PREVIOUS RUNS OF THE GFS AND ECMWF WHICH TAKES THE LOW ON A MORE
NORTHEASTERLY TRACK TOWARD KACHEMAK BAY. THIS MORNINGS GFS TOOK THE
TRACK A BIT FURTHER WEST TOWARD KODIAK CITY BEFORE WEAKENING IT OVER
THE SOUTHERN INLET. THE EC WAS TO THE EAST...SO THE NAM WAS A NICE
COMPROMISE. THIS FORECAST PACKAGE WAS BASED ON THE 12Z NAM AND DETAIL
AND FOCUS WAS GENERALLY IN THE FIRST 48 HOURS. THE MEDIUM AND LONG
RANGE FORECAST WAS NOT CHANGED.

.SHORT TERM FORECAST...
SOUTHCENTRAL ALASKA...CHALLENGING FORECAST TODAY WITH MANY LOCAL
EFFECTS NEEDING TO BE FINE TUNED. FIRST CONCERN IS THE THREAT FOR
HIGH WIND WARNING CRITERIA AS THE LOW DEEPENS AND A BARRIER JET FORMS
ALONG THE NORTH GULF COAST. HURRICANE FORCE WIND WARNING HAS BEEN
POSTED FOR PARTS OF THE GULF AS WELL. THE TRACK OF THE LOW STILL A
BIT OF A CONCERN SINCE THE SOUTHERN AND WESTERN TRACK WOULD KEEP
WINDS BELOW HIGH WIND WARNING CRITERIA. CONFIDENT ENOUGH AT THIS TIME
TO ISSUE A HIGH WIND WATCH FOR SEVERAL ZONES THROUGH THE NORMAL
CHANNELED TERRAIN AND HIGHER ELEVATIONS. WE WILL HAVE TO FINE TUNE
THESE AS THE 0Z RUNS COME IN THIS AFTERNOON/EVENING.

HEAVY RAIN WILL BE THE OTHER CONCERN WITH THIS SYSTEM AS A WARM FRONT
MOVES ONSHORE THIS EVENING ACROSS PRINCE WILLIAM SOUND. SEVERAL
INCHES OF RAIN HAS ALREADY BEEN REPORTED FROM CORDOVA TO THE EASTERN
KENAI PENINSULA SO WE ISSUED A FLOOD ADVISORY AS MORE HEAVY RAIN IS
EXPECTED THROUGH TOMORROW. DEPENDING ON HOW THE RIVERS RESPOND...THE
ADVISORY MAY NEED TO BE EXTENDED INTO SATURDAY.

THE LOW PRESSURE SYSTEM BOTTOMS OUT AT APPROX 958 MB JUST EAST OF
KODIAK CITY FRIDAY EVENING AND THEN BEGINS TO WEAKEN AS IT MOVES
INLAND NEAR THE SOUTHERN INLET. THE TROF AXIS REMAINS NEARLY
STATIONARY OVER THROUGH THE WEEKEND SO UNSETTLED WEATHER WILL
CONTINUE INTO EARLY NEXT WEEK.

SOUTHWEST ALASKA...
SNOW AND BLOWING SNOW THE MAIN STORY ACROSS THE KUSKOKWIM VALLEY. THE
WARM AIR ALOFT OVERRUNNING THE COLD AIR AT THE SURFACE IS CAUSING THE
SNOW TO FALL ACROSS THE REGION. STRONG NORTHEASTERLY WINDS ARE
BLOWING THE SNOW AROUND REDUCING VISIBILITY TO A HALF MILE AT TIMES
WITH SNOW ACCUMULATING 2 INCHES OR LESS. SNOW WILL CONTINUE TO TAPER
OFF THIS EVENING. ANOTHER ROUND OF PRECIPITATION WILL BEGIN AGAIN
EARLY FRIDAY MORNING ASSOCIATED WITH THE STRONG GULF LOW. THIS
PRECIPITATION WILL FALL IN THE FORM OF SNOW AS ALL THE COLD AIR IS
PUSHED NORTHWARD. THE EXCEPTION MAY BE ALONG THE KUSKOKWIM DELTA
WHERE NORTHEASTERN WINDS WILL HOLD THE LONGEST...ALLOWING THE COLD
AIR TO STICK AROUND THROUGH POSSIBLY FRIDAY AFTERNOON.

SATURDAY...THE LOW WEAKENS OVER THE INTERIOR AS THE SYSTEM BECOMES
VERTICALLY STACKED. THIS WILL BE A GOOD SETUP FOR SHOWERY WEATHER
THROUGH THE REST OF THE WEEKEND.

OUT WEST...BROAD NORTHWESTERLY FLOW WILL PREVAIL ACROSS MOST OF THE
CENTRAL AND EASTERN BERING SEA. THE GRADIENT WILL INCREASE ACROSS THE
ALASKA PENINSULA TONIGHT AS THE GULF LOW DEEPENS. WITH SOME UPPER
LEVEL SUPPORT...EXPECTING STRONG WIND TO AFFECT CHANNELED TERRAIN AND
POSSIBLY REACH HIGH WIND WARNING CRITERIA IN PLACES LIKE KING COVE.
RAIN AND SNOW SHOWERS WILL PREVAIL ELSEWHERE.

.AER/ALU...WATCH/WARNING SUMMARY
PUBLIC...HIGH WIND WARNING 181
         HIGH WIND WATCH 101 125 135
         FLOOD ADVISORY 125 131 135
MARINE...HURRICANE FORCE...120 351
         STORM 125 129 130 140 352
         GALE 121 132 138 141 150 155 160 165 180
FIRE WEATHER...NONE.

AB SEP 10