Areal Flood Advisory 9, NWS Anchorage, AK

SOUTHCENTRAL AND SOUTHWEST ALASKA FORECAST DISCUSSION...CORRECTED
NATIONAL WEATHER SERVICE ANCHORAGE AK
500 AM AKDT SAT SEP 15 2012

CORRECTED WATCH/WARNING SECTION

.ANALYSIS AND UPPER LEVELS...
AN EXCITING METEOROLOGICAL SITUATION SHAPING UP OVER THE GREAT LAND
THIS MORNING. A NORTHERLY 150 KT JET OVER THE WESTERN ALEUTIANS AND
ATTENDANT POTENT POTENTIAL VORTICITY ANOMALY ARE DIGGING SOUTHWARD
INTO THE NORTH CENTRAL PACIFIC. THEY ARE WELL INTO THE PROCESS OF
PHASING WITH A SHORTWAVE TROUGH ALONG A LOW LEVEL BAROCLINIC ZONE
WHICH IS COMPLETING A TROPICAL CONNECTION TO THE MID-LATITUDES. THE
DEVELOPING HIGH AMPLITUDE PATTERN IS EXHIBITING SATELLITE DERIVED
PRECIPITABLE WATER VALUES IN EXCESS OF 160 PERCENT OF NORMAL AHEAD OF
THE CYCLONE AND 30 PERCENT BEHIND WHICH GIVE AN INDICATION OF THE
RARITY OF THE EVENT. THE HIGH MOISTURE VALUES INDICATE THE DEVELOPING
THREAT OF PROLONGED MODERATE TO HEAVY RAINFALL FOR MANY LOCATIONS
OVER MAINLAND ALASKA IN ADDITION TO THE HIGH WIND THREAT ALREADY IN
PLACE. THE PRESENTATION OF INFRARED SATELLITE AS WELL AS WATER VAPOR
IMAGERY WITH BAROCLINIC LEAF STRUCTURE WITH DIGGING TROUGH BEHIND
SIGNAL THAT RAPID CYCLOGENISIS IS IMMINENT.

CURRENTLY OVER THE SOUTH CENTRAL...ONSHORE FLOW BEHIND A DECAYING
FRONTAL BOUNDARY IS BRINGING STEADY RAIN TO THE NORTH GULF COAST.
DEEP LAYER SOUTHWESTERLY FLOW ALOFT CONTINUES THE MOISTURE
CONNECTION. LOWER FREEZING LEVELS AND COLDER TEMPERATURES ALOFT ARE
KEEPING INSTABILITY OVER THE COOK INLET REGION...KEEPING NUMEROUS
SHOWERS GOING INTO THE OVERNIGHT HOURS ALONG WITH THE BEGINNING OF
WARM ADVECION/ISENTROPIC LIFT ASSOCIATED WITH THE DEVELOPING TROUGH
OUT WEST. THE WEAKENING BRISTOL BAY LOW COMBINED WITH BUILDING
NORTHERN GULF COAST SURFACE RIDGE IS BEGINNING TO RAMP UP THE WINDS
OVER TURNAGAIN ARM AND HIGHER ELEVATIONS. GLEN ALPS AND UPPER
HILLSIDE LOCATIONS ARE ALREADY GUSTING NEAR 35 MPH EARLY THIS MORNING
AND SHOULD CONTINUE TO INCREASE.

.MODEL DISCUSSION...
GUIDANCE IS IN MUCH BETTER AGREEMENT WITH THE DEVELOPING LOW PRESSURE
SYSTEM. BETTER CONSENSUS EXISTS WITH THE TWO DIFFERENT MODEL CAMPS
CONVERGING ON A SOLUTION. THE EC/GEM HAVE MATCHED THE AMERICAN MODELS
AS FAR AS TROUGH ORIENTATION BECOMING MORE NEGATIVELY TILTED WITH
TIME...AS WELL AS A DEEPER FURTHER WESTWARD SURFACE LOW CENTER WITH
BENT BACK OCCLUSION. THE GFS/NAM HAVE COME INTO AGREEMENT WITH THE
EC/GEM WITH QUICKLY RACING THE SYSTEM INLAND ONCE IT HAS FULLY
MATURED. AS WITH MANY RAPID CYCLOGENESIS EVENTS...GUIDANCE WILL NEVER
FULLY CATCH UP TO OBSERVATIONS...ESPECIALLY IN OBSERVATION DEVOID
AREAS SUCH AS THE BERING SEA AND NORTHERN PACIFIC. WITH SUCH A
DYNAMIC SYNOPTIC SITUATION THE DEEPEST AND FURTHEST WESTWARD GUIDANCE
SHOULD BE PREFERRED...12Z FRIDAY/00Z SATURDAY ECMWF IN THIS
PARTICULAR SITUATION.

.SHORT TERM FORECAST...
SOUTHCENTRAL ALASKA...AN ACTIVE WEEKEND IS IN STORE WITH THE
DEVELOPING STRONG LOW PRESSURE OUT WEST. RAIN WILL CONTINUE TO BECOME
MORE WIDESPREAD AS THE SOUTHERLY FLOW BECOMES MORE ESTABLISHED WITH
EMBEDDED DISTURBANCES AHEAD OF THE PUSH OF RELATIVELY WARMER/MOIST
AIR AHEAD OF THE LOW PRESSURE. MORE CONCERNING THAN THE PRECIPITATION
IS THE THREAT FOR VERY STRONG WINDS THROUGH TURNAGAIN ARM AS WELL AS
HIGHER ELEVATIONS OF THE ANCHORAGE BOWL. THE CURRENT SITUATION VARIES
FROM THE PREVIOUS WIND EVENT IN THAT THERE WILL BE A STRONG
DOWN-INLET COMPONENT TO THE FLOW AS THE LOW DEEPENS IN BRISTOL BAY.
THAT WOULD NORMALLY ACT TO KEEP THE HIGHEST WINDS FROM BENDING INTO
SOUTH AND WEST ANCHORAGE. THE CAVEAT TO THIS IS THE STRONG DOWNWARD
VERTICAL MOTION FROM BOTH PRECIPITATION DRAG AND DOWNSLOPING AIR OFF
OF THE CHUGACH...BOTH WILL AID IN POTENTIALLY MIXING HIGHER MOMENTUM
AIR FROM ALOFT TO THE SURFACE. THE WINDS WILL REALLY RAMP UP LATE
THIS AFTERNOON AND EVENING FOR THE HILLSIDE AND TURNAGAIN ARM WITH
STRONGER WINDS SPREADING TO EAST ANCHORAGE AND LOWER HILLSIDE
OVERNIGHT INTO SUNDAY. AS THE SURFACE LOW MOVES INTO THE CENTRAL
INTERIOR AND FRONT PASSES OVERHEAD THE STRONGEST WINDS WILL BEND INTO
WEST ANCHORAGE ON SUNDAY AFTERNOON.

WINDS WILL CONTINUE TO INCREASE FOR THE COPPER RIVER BASIN AS SURFACE
RIDGING INCREASES AND NORTHEAST GULF COAST WITH THE APPROACHING
BARRIER JET TONIGHT INTO SUNDAY. THE OTHER THREAT WITH THE DEVELOPING
SYSTEM IS HEAVY RAINFALL ALONG COASTAL PRINCE WILLIAM SOUND AS RAIN
LOOKS TO CONTINUE FOR THE DURATION OF THE EVENT. FLOOD WATCHES ARE
POSTED FOR THESE AREAS ALREADY.

SOUTHWEST ALASKA...LOOK FOR OCCASIONAL SHOWERS TO GIVE WAY TO
WIDESPREAD STEADY RAIN TODAY AS A STRONG LOW PRESSURE DEVELOPS SOUTH
OF THE ALASKAN PENINSULA. RAIN WILL CONTINUE THROUGHOUT THE
WEEKEND...ACCOMPANIED BY GUSTY SOUTHERLY WINDS AS THE SURFACE LOW
MOVES ASHORE SUNDAY AFTERNOON.

BERING SEA AND ALEUTIANS...WHILE THE STRONG LOW PRESSURE WILL MAINLY
AFFECT THE FAR EASTERN PORTION OF THE CHAIN AND THE ALASKAN PENINSULA
THE EFFECTS WILL STILL BE FELT OVER THE CENTRAL ALEUTIANS IN THE FORM
OF GUSTY NORTHERLY WINDS AS WELL AS COOLER TEMPERATURES. A FRONT WILL
MOVE FROM WEST TO EAST OVER THE WESTERN ALEUTIANS SATURDAY AFTERNOON
INTO THE EVENING BRINGING PERIODS OF RAIN AND GUSTY SOUTHERLY WINDS.

.LONG TERM FORECAST...
THE EXTENDED PATTERN DOES NOT LOOK TOO FAR OFF FROM WHAT IS CURRENTLY
BEING EXPERIENCED. AN AMPLIFIED PATTERN WITH A LONGWAVE TROUGH OVER
THE BERING AND STRONG RIDGE OVER THE NORTHEASTERN PACIFIC LEAVE THE
MAINLAND UNDER PROLONGED SOUTHERLY FLOW VERY CONDUCIVE TO SEASONABLY
WET FORECAST.

.AER/ALU...WATCH/WARNING SUMMARY
PUBLIC...HIGH WIND WARNING 101 125 135
         FLOOD WATCH 125 131 135
MARINE...HURRICANE FORCE WIND WARNING 120
         STORM 125 130 138 140
         GALE 121 129 132 141 150 155 160
              165 170 171 172 176 179 180 185

FIRE WEATHER...NONE

MTL SEP 12