Gale Warning 207, NWS Anchorage, AK

AREA FORECAST DISCUSSION
SOUTHCENTRAL AND SOUTHWEST ALASKA
500 AM AKST FRI FEB 20 2009

.ANALYSIS AND UPPER LEVELS...

ALOFT...A HIGHLY AMPLIFIED TROUGH OVER THE ALCAN IS BEING SQUEEZED
BETWEEN UPPER RIDGING OVER WESTERN CANADA AND OVER THE EASTERN BERING
SEA.  THIS TROUGH IS MOVING VERY SLOWLY EASTWARD AS RIDGING HOLDS
TOUGH DOWNSTREAM.  BETWEEN THE BUILDING UPSTREAM BERING RIDGE
ADVANCING EASTWARD AND THE UPPER TROUGH...STRONG NORTHERLY FLOW IS
CONTRIBUTING TO STRONG WINDS NEAR THE SURFACE OVER SOUTHCENTRAL AND
THE COPPER RIVER DELTA. AN UPPER SHORTWAVE WITH ACCOMPANYING STRONG
COLD FRONT IS SPLITTING AS IT APPROACHES THE RIDGE OVER THE BERING
SEA AND CENTRAL ALEUTIANS...WITH MOST OF THE ENERGY ASSOCIATED BEING
PUSHED NORTHWARD AROUND THE RIDGE AXIS.  TRANSITORY RIDGING EXISTS
OVER THE WESTERN ALEUTIANS JUST DOWNSTREAM OF A 494 DM LOW CURRENTLY
SITUATED JUST SOUTH OF THE SOUTHERN TIP OF THE KAMCHATKA PENINSULA.

AT THE SURFACE...A 1046 MB HIGH WITH VERY DRY AIR ASSOCIATED WITH IT
HAS ESTABLISHED ITSELF OVER MOST OF THE STATE...WHILE A 1016 MB LOW
IS LOCATED NEAR THE BASE OF THE UPPER TROUGH NEAR 58N 141W IN THE
GULF OF ALASKA.  THE PRESSURE GRADIENT BETWEEN THESE TWO FEATURES IS
CONTRIBUTING TO VERY STRONG NORTHERLY OUTFLOW WINDS SUFFICIENT TO
MEET HIGH WIND CRITERIA THROUGH THE COPPER RIVER DELTA AND THOMPSON
PASS...AND ALSO PRODUCING GALE FORCE WINDS THROUGH CHANNELED AREAS IN
COASTAL WATERS.  A VERY STRONG COLD FRONT WITH STORM FORCE WINDS IS
MOVING EASTWARD ACROSS THE BERING SEA AND THE CENTRAL ALEUTIANS.

.MODEL DISCUSSION...

THE GFS AND NAM WERE USED IN THE SHORT TERM THROUGH 84 HOURS ON THE
EAST SIDE...WHILE THE GFS WAS ADEQUATE FOR USE THROUGH THE SAME TIME
FOR THE WEST SIDE.  AFTER 84 HOURS...THE ECMWF WAS AGAIN THE
PREFERRED MODEL...PARTICULARLY OUT WEST...WHERE THE ECMWF
TRADITIONALLY OUTPERFORMS THE GFS IN HANDLING SPLIT-FLOW REGIMES IN
THE MEDIUM RANGE.  NUDGED EXISTING WIND GRIDS IN THE EXTENDED TOWARDS
THE ECMWF ON THE EAST SIDE.

.SHORT TERM FORECAST...

SOUTH AND EAST OF THE ALASKA RANGE...WINDS SHOULD GRADUALLY DIMINISH
IN INTENSITY THROUGH CHANNELED TERRAIN OVER THE NEXT 24 HOURS AS THE
SURFACE LOW WEAKENS INTO A TROUGH ALONG THE NORTH GULF
COAST...DIMINISHING THE CURRENTLY LARGE PRESSURE GRADIENT.  AS
SURFACE RIDGING CONTINUES TO SETTLE OVER THE COOK INLET AND COPPER
RIVER BASIN...COLD AIR SHOULD BE ABLE TO DRAIN DOWN FROM THE
NORTH...BRINGING A RETURN TO NORMAL TO SLIGHTLY BELOW NORMAL
TEMPERATURES THROUGH THE WEEKEND.  WINDS SHOULD ALSO CONSIDERABLY
WEAKEN AS SURFACE RIDGING PREDOMINATES AND UPPER RIDGING BUILDS OVER
THE ALASKA RANGE THROUGH THE WEEKEND.

WEST OF THE ALASKA RANGE...STRONG COLD FRONT CURRENTLY OVER THE
CENTRAL BERING SEA AND ALEUTIAN CHAIN SHOULD SLOWLY PUSH EASTWARD AS
UPPER RIDGING BUILDING OVER THE MAINLAND SHUNTS MOST OF THE UPPER
ENERGY NORTHWARD THROUGH THE BERING STRAIT.  WITH THE LOSS OF THE
UPPER ENERGY...THE FRONT WILL WEAKEN CONSIDERABLY AND STALL BY SUNDAY
WEST OF THE ALASKA RANGE...BUT NOT BEFORE PRODUCING BLIZZARD
CONDITIONS ALONG THE COASTLINE OF THE KUSKOKWIM DELTA DURING THE DAY
TODAY.  ANOTHER STRONG DISTURBANCE WITH ACCOMPANYING SURFACE LOW WILL
WORK ITS WAY ACROSS THE WESTERN ALEUTIANS SUNDAY NIGHT AND
NORTHEASTWARD THROUGH THE BERING SEA MONDAY...BRINGING PRECIPITATION
TO THE WESTERN AND CENTRAL ALEUTIANS AND THE RETURN OF AT LEAST GALE
FORCE WINDS TO MOST OF THE OFFSHORE WATERS.

.LONG TERM FORECAST...

HIGHLY PROGRESSIVE PATTERN WILL MEAN RAPIDLY CHANGING WEATHER
CONDITIONS OVER A VAST PART OF THE STATE THROUGH THE EXTENDED...WITH
THE TIMING OF DETAILS VERY DIFFICULT TO PIN DOWN WITH SUCH A PATTERN
IN PLACE.  IT WILL BE HARD TO KEEP COLD AIR FROM BECOMING ESTABLISHED
FOR ANY AMOUNT OF TIME IN SUCH A PATTERN...SO SHOULD SEE HIGHLY
VARIABLE WEATHER CONDITIONS THROUGH THE EXTENDED PERIOD WITH MOSTLY
NORMAL TO ABOVE NORMAL TEMPERATURES.

.AER/ALU...WATCH/WARNING SUMMARY
PUBLIC...HIGH WIND WARNING...131 135
         BLIZZARD WARNING...155
MARINE...GALE WARNING...120 121 129 130 155 160 165 172 175 176 351
                        352 411
         STORM WARNING...170 171 179 180 185 412 413 414
         HEAVY FREEZING SPRAY WARNING...121 129 130 140
FIRE WEATHER...NONE

CC/DS FEB 09