Areal Flood Warning 1, NWS Anchorage, AK

SOUTHCENTRAL AND SOUTHWEST ALASKA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE ANCHORAGE AK
500 PM AKDT SUN MAY 6 2012

.ANALYSIS AND UPPER LEVELS...
HIGH AMPLITUDE LONGWAVE TROUGH STILL SITS OVER MUCH OF THE GREATLAND
TODAY. TWO MAIN FEATURES ARE EVIDENT ON THE WATER VAPOR LOOP
ASSOCIATED WITH THE PLANETARY WAVE...A POTENT SHORTWAVE TROUGH MOVING
OVER SOUTHCENTRAL ALASKA BY WAY OF THE GULF...AND THE SECOND...THE
MAIN UPPER LOW PRESSURE CENTER PROPAGATING EASTWARD IN THE NORTHERN
PACIFIC JUST SOUTH OF THE ALASKAN PENINSULA. THE GULF OF ALASKA
SYSTEM HAS SPAWNED WIDESPREAD CLOUD COVER OVER MUCH OF THE SOUTHERN
MAINLAND AS WELL AS A SOLID PRECIPITATION SHIELD...IN THE FORM OF
RAIN...OVER THE NORTHERN GULF AND PRINCE WILLIAM SOUND. WHILE MOST
AREAS AROUND WESTERN PRINCE WILLIAM SOUND AND EASTERN KENAI MOUNTAINS
ARE RECEIVING CONSTANT RAIN...THE KENAI PENINSULA...ANCHORAGE
BOWL...AND MATANUSKA VALLEYS ARE REMAINING PRECIPITATION FREE THANKS
TO A STOUT EASTERLY/SOUTHEASTERLY DOWNSLOPING FLOW OVER THE
CHUGACH/KENAI RANGES.

.MODEL DISCUSSION...
GUIDANCE HAVING A HARD TIME GETTING THE EXACT POSITION OF THE
MULTIPLE SURFACE CIRCULATION SOUTH OF THE AKPEN. NOT SURPRISINGLY ALL
STRUGGLE WITH THE POSITION OF THE SAME SURFACE FEATURE AS IT MOVES
INTO THE GULF. THE UPPER LEVEL HEIGHT/VORTICITY PATTERN IS AT LEAST
AGREED UPON BY MOST SOLUTIONS. GENERALLY THE UPPER LOW CENTER WILL
SLOWLY MIGRATE EASTWARD AND A WEAK SURFACE LOW WILL DEVELOP AS THE
UPPER TROUGH AXIS SWINGS INTO THE GULF. WITH LITTLE ACCESS TO WARM
SURFACE AIR OR STRONG JET A WEAK SURFACE FEATURE MAKES SENSE...ALSO
WHY THERE IS DISCREPANCY BETWEEN GUIDANCE.

.SHORT TERM FORECAST...
SOUTHCENTRAL ALASKA...MAIN FORECAST CHALLENGES FOR TODAY REVOLVE
AROUND PRECIPITATION AND WIND CHANCES FOR THE COOK INLET REGION. AS
THE SURFACE LOW CROSSES THE MOUNTAINS THIS EVENING A COUPLE SOLUTIONS
ARE POSSIBLE...A CONSOLIDATED LOW LEVEL CIRCULATION OVER THE KENAI
PENINSULA WHICH WILL ACT TO KEEP THE MOUNTAIN NORMAL FLOW
STRONGER...OR A WEAKER DISORGANIZED MID-LEVEL CIRCULATION BRINGING
LIGHTER MOUNTAIN TOP WINDS ALLOWING FOR PRECIPITATION TO SPREAD OVER
MOST LOCATIONS. LEANING TOWARD THE DRIER SOLUTION AT THIS POINT WITH
THE STRENGTH OF THE UPPER LEVEL WAVE. THE BEST CHANCE FOR
PRECIPITATION WILL LIE OVER WINDWARD COASTAL AREAS AND THE KENAI
PENINSULA AS THE SURFACE LOW MIGRATES ACROSS THE MOUNTAINS. AS FAR AS
WINDS GO...TURNAGAIN ARM WINDS WILL INCREASE TONIGHT WITH THE SURFACE
LOW TO THE SOUTH. AS THE LOW MOVES WESTWARD THE PANC/PAHO PRESSURE
GRADIENT WILL EASE AND ALLOW FOR THE WINDS TO TURN UP INTO WEST
ANCHORAGE DURING THE EVENING/OVERNIGHT HOURS. ON MONDAY A FEW
LINGERING SHOWERS WILL REMAIN OVER THE GULF WHILE THE AREA REMAINS
UNDER CYCLONIC FLOW. SUBSIDENCE BEHIND THE INCOMING SHORTWAVE TROUGH
WILL KEEP MOST AREAS AROUND THE SOUTH CENTRAL DRY.

SOUTHWEST ALASKA...SHOWERS WILL INCREASE THIS EVENING AND TONIGHT AS
AN UPPER LEVEL WAVE CROSSES THE ALASKA RANGE. ACTIVITY WILL BE ON THE
WANE OVER THE SOUTHERN PORTION OF BRISTOL BAY TOMORROW AS SUBSIDENCE
BEHIND THE UPPER WAVE TAKES HOLD. THE UPPER TROUGH WILL PIVOT AROUND
ON TUESDAY KEEPING ISOLATED SHOWERS IN THE FORECAST.

BERING SEA AND ALEUTIANS...STACKED LOW PRESSURE SYSTEM OF THE PAST
TWO DAYS IS DEPARTING...LEAVING A COLD NORTHERLY FLOW IN ITS WAKE.
SHOWERS WILL BE POSSIBLE ALONG THE ALEUTIAN CHAIN...MAINLY FOR THE
EASTERN ALEUTIANS AND AKPEN. A QUICKER MOVING LOW WILL RACE ACROSS
THE CHAIN ON TUESDAY BRINGING ANOTHER SHOT OR RAIN ON THE FRONTSIDE
AND SNOW ON THE BACKSIDE.

.LONG TERM FORECAST...
THE AMPLIFIED PATTERN WILL CONTINUE INTO THE EXTENDED PERIOD WITH A
QUICK MOVING SYSTEM MOVING OVER THE ALEUTIANS INTO THE GULF AND A
DEEPER UPPER LOW PRESSURE DROPPING DOWN FROM THE ARCTIC REINFORCING
THE TROUGH OVER MOST OF THE MAINLAND. IN THIS PARTICULAR PATTERN MOST
OF THE ACTIVE WEATHER WILL BE MOVING NORTHEASTWARD INTO THE GULF WITH
SCATTERED DAYTIME ACTIVITY OVER THE MAINLAND MOUNTAIN RANGES.

.AER/ALU...WATCH/WARNING SUMMARY
PUBLIC...FLOOD WATCH 152
MARINE...STORM 120
         GALE 125 170 172
FIRE WEATHER...NONE

MTL MAY 12