Areal Flood Warning 11, NWS Anchorage, AK

SOUTHCENTRAL AND SOUTHWEST ALASKA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE ANCHORAGE AK
400 PM AKDT WED SEP 19 2012

.ANALYSIS AND UPPER LEVELS...
A LONGWAVE TROUGH IS COVERING THE BERING SEA...WITH MAINLAND ALASKA
CAUGHT BETWEEN THE TROUGH AND A RIDGE OVER WESTERN CANADA AND THE FAR
EASTERN GULF OF ALASKA. TROPICAL MOISTURE CONTINUES TO STREAM INTO
MUCH OF THE MAINLAND...BRINGING ABUNDANT RAINFALL FROM BRISTOL BAY
EASTWARD TO PRINCE WILLIAM SOUND AND NORTH INTO THE INTERIOR. WIND IS
THE OTHER MAIN CONCERN TODAY...AS THE SURFACE PRESSURE GRADIENT
BETWEEN A LOW OVER BRISTOL BAY AND A HIGH OVER THE ALASKA PANHANDLE
INCREASES TODAY.

.MODEL DISCUSSION...
MODELS HAVE A PRETTY GOOD HANDLE ON THIS FIRST SYSTEM...HOWEVER THERE
ARE BIGGER DIFFERENCES WITH THE NEXT LOW MOVING NORTHWARD ACROSS THE
WESTERN MAINLAND. THE MAIN DIFFERENCE WITH THE TRACK OF THAT LOW IS
THAT THE GFS IS LAGGING BEHIND AS IT KEEPS THE MAIN UPPER TROUGH
FARTHER WEST. THE UPPER TROUGH WILL MORE LIKELY CONTINUE TO GRADUALLY
SHIFT EASTWARD...MAKING A NAM/ECMWF SOLUTION MORE PLAUSIBLE.

.SHORT TERM FORECAST...
.SOUTHCENTRAL ALASKA...THE PRESSURE GRADIENT WILL CONTINUE TO
STRENGTHEN ACROSS THE KENAI PENINSULA AND PRINCE WILLIAM SOUND THIS
AFTERNOON...WITH PEAK WINDS EXPECTED BY LATE AFTERNOON. THE WINDS
WILL GRADUALLY DIMINISH OVERNIGHT...ALTHOUGH WINDS THROUGH TURNAGAIN
ARM WILL REMAIN AROUND 40 TO 50 MPH THROUGH THE NIGHT.

AS THE UPPER TROUGH SHIFTS EASTWARD...SO WILL THE MAIN MOISTURE
PLUME. THE HEAVY RAIN WILL THEREFORE DIMINISH ACROSS THE WESTERN
KENAI PENINSULA AND ANCHORAGE BOWL TONIGHT...AND AFFECT AREAS SUCH AS
VALDEZ AND CORDOVA THROUGH THURSDAY. ADDITIONAL RAIN WILL MOVE
THROUGH THE COOK INLET AREA ON THURSDAY...HOWEVER IT WILL NOT BE AS
HEAVY AS THE RAINFALL SEEN TODAY.

.SOUTHWEST ALASKA...THE MAIN RAINFALL OVER THE SOUTHWEST MAINLAND
WILL BE LIMITED TO ALONG THE ALASKA RANGE TONIGHT AS THE UPPER TROUGH
SLOWLY SHIFTS EASTWARD. THE NEXT LOW MOVING NORTHWARD THROUGH THE
AREA FRIDAY NIGHT INTO SATURDAY WILL BRING ANOTHER ROUND OF RAIN.

.ALEUTIANS/BERING SEA...
A RELATIVELY BENIGN PATTERN WILL CONTINUE ACROSS THE BERING SEA AND
ALEUTIAN CHAIN THROUGH FRIDAY. A LARGE LOW WILL AFFECT THE EASTERN
BERING SEA AS IT MOVES NORTHWARD ON SATURDAY...BRINGING GUSTY WINDS
AND PRECIPITATION WITH IT. ANOTHER RELATIVELY WEAK LOW WILL MOVE
EASTWARD ALONG THE ALEUTIANS DURING THE WEEKEND.

.LONG TERM FORECAST...
AN ACTIVE AMPLIFIED PATTERN WILL CONTINUE ACROSS THE REGION OVER THE
NEXT WEEK. A STRONG UPPER LEVEL TROUGH WILL DOMINATE THE BERING AND
SUPPORT A SERIES OF LOW PRESSURE SYSTEMS ACROSS THE BERING AND GULF
COAST. HOWEVER...MODELS ARE BEGINNING TO INDICATE A PATTERN CHANGE BY
EARLY NEXT WEEK.

.HYDROLOGY...
RAINFALL RATES OF ONE TENTH TO ONE QUARTER INCH HAVE BEEN SEEN ACROSS
MUCH OF THE AREA...WHILE SOME LOCATIONS ON THE WINDWARD SIDE OF THE
MOUNTAINS ARE RECEIVING AS MUCH AS 0.6 TO 0.7 INCH PER HOUR. THIS
BAND OF HEAVIEST RAINFALL WILL GRADUALLY SHIFT EASTWARD...BRINGING
THE HEAVIEST RAIN TO LOCATIONS SUCH AS VALDEZ AND CORDOVA TONIGHT AND
THURSDAY. FLOODING CONCERNS CONTINUE ALONG THE ENTIRE GULF COAST AS
WELL AS THROUGH THE ANCHORAGE AREA AND SUSITNA VALLEY AS LONG AS THIS
TROPICAL CONNECTION CONTINUES.

.AER/ALU...WATCH/WARNING SUMMARY
PUBLIC...HIGH WIND WARNING...101 125
         FLOOD WARNING... 125 131 135
         FLOOD WATCH...121 125
MARINE...GALE...120 125 175 176
FIRE WEATHER...NONE

SCHRECK