Flood in Alaska, September 30, 2004

AREA FORECAST DISCUSSION SOUTHCENTRAL ALASKA
1230 PM ADT THU SEP 30 2004

.DISCUSSION...

.ANALYSIS...WATER VAPOR IMAGERY SHOWS MAJOR UPPER CLOSED LOW CENTERED
AT 47N 167W WITH TROUGH EXTENDING NNE TO ANOTHER LOW AT 64.5N 159W.
HIGH AMPLITUDE LONG-WAVE RIDGE LOCATED OVER THE EAST PACIFIC. LONG
FETCH MOISTURE TRANSPORT CONTINUES FROM BOTTOM OF TROUGH AROUND 40N
TO TOP OF RIDGE NEAR 70N. SURFACE LOWS AND CONNECTING COLD FRONT
LOCATED 100 TO 200 NM EAST OF UPPER REFLECTION. STEADY PRECIPITATION
CONTINUES FROM AKPENN NORTHEASTWARD TO MOST OF SOUTH CENTRAL ALASKA.
PRECIPITATION IS ALL RAIN WITH THE EXCEPTION OF NORTHERN SUSITNA
VALLEY WHERE HEAVY SNOW HAS BEEN FALLING SINCE WEDNESDAY NIGHT.
BETWEEN 10 PM LAST NIGHT TO 9 AM THIS MORNING CHULITNA RECEIVED
APPROXIMATELY ONE FOOT OF SNOW. ISSUED A HEAVY SNOW WARNING THROUGH
THIS AFTERNOON FOR THIS AREA. SURFACE TEMPS CURRENTLY RIGHT AROUND
FREEZING SO SNOW WILL BE WET. MAY EVEN MIX WITH RAIN THIS AFTERNOON.
BEHIND SURFACE FRONT AND AHEAD OF UPPER TROUGH AXIS PRECIP IS
TRANSITIONING BACK TO SNOW. SHORT-WAVE RIDGE OVER WESTERN BERING
RESULTING IN PARTLY CLOUDY SKIES ACROSS MUCH OF THE WESTERN BERING
AND ALEUTIANS. HOWEVER CIRRUS SHIELD FROM AN EASTERN RUSSIA LOW
QUICKLY SPREADING EASTWARD.

.FORECAST...GFS/ETA/UKMET AND YESTERDAYS 00Z RUN OF ECMWF ALL HANDLE
SYNOPTIC FEATURES SIMILARLY OUT TO 36 HOURS BEFORE DIVERGING. OF
COURSE...AS IS USUALLY THE CASE...THE DETAILS ARE A BIT DIFFERENT.
UPPER TROUGH OVER MAINLAND WILL FLATTEN EAST PACIFIC RIDGE TONIGHT
AS IT LIFTS ENE. THIS WILL LEAVE CUT OFF LOW SOUTH OF AKPENN TO
WANDER SLOWLY EASTWARD. SHORT-WAVE WILL THEN MOVE OUT OF EASTERN
RUSSIA AND MAY OR MAY NOT PHASE WITH CUT OFF LOW AND OPEN TO ONE
TROUGH. THIS IS WHERE MODELS DIVERGE. ETA/ECMFW DROP DIG SHORT-WAVE
SOUTHEASTWARD TOWARD CLOSED LOW WHILE UKMET/GFS KEEP SHORT-WAVE OVER
NORTHERN BERING. POSITION OF UPPER JET COMING OUT OF EAST ASIA AND
SOUTHEASTWARD ACROSS WESTERN ALEUTIANS SHOULD PUSH SHORT-WAVE
SOUTHEASTWARD. SO HAVE SIDED WITH THE ETA/ECMFW IN ALL AREAS FOR
THIS FORECAST PACKAGE. SO A SUMMARY OF THE FORECAST THINKING...PRECIP
WILL DIMINISH OVER SOUTH CENTRAL THIS EVENING AS SURFACE FRONT MOVES
EASTWARD. SINCE UPPER TROUGH AXIS WONT PUSH THROUGH UNTIL LATER
TONIGHT WILL BE CHANCE OF RAIN AND SNOW BEHIND THE FRONT. DO NOT
EXPECT ANY SIGNIFICANT ACCUMULATION. THERE WILL BE PARTIAL CLEARING
ONCE UPPER TROUGH MOVES THROUGH. THIS WILL PRODUCE AREAS OF FOG.
SHORT-WAVE RIDGE WILL NUDGE OVER TO SOUTH CENTRAL FRIDAY SO WILL
HAVE A CHANCE TO DRY OUT A BIT. MEANWHILE AS SHORT-WAVE AND
ASSOCIATED SURFACE LOW MOVE INTO WESTERN BERING FRIDAY TIGHT
PRESSURE GRADIENT WILL PRODUCE MAX GALES TO MIN STORMS FOR WESTERN
ALEUTIANS/WESTERN BERING. THIS GRADIENT WILL EASE AS SHORT-WAVE RIDGE
IS SQUASHED TO THE SOUTH. LOW WILL TRACK ACROSS SOUTHERN BERING AND
LOSE SUPPORT AS TROUGH AND UPPER LOW PHASE TO ONE TROUGH. AS HEAD
INTO WEEKEND TROUGH WILL LIFT NORTHWARD WITH RIDGE BEING PUSHED TO
WESTERN CANADA. THIS SHOULD BRING ANOTHER ROUND OF RAIN TO SOUTH
CENTRAL THIS WEEKEND. UNSETTLED WEATHER EXPECTED TO CONTINUE THROUGH
NEXT WEEK ACROSS MOST OF AOR AS UPPER LOW SETS UP OVER MAINLAND
ALASKA.

.AER...WATCH/WARNING SUMMARY
PUBLIC...HEAVY SNOW WARNING 145.
MARINE...NONE.
FIRE WEATHER...NONE.

SEB SEP 04