Flood Watch 4, NWS Anchorage, AK

AREA FORECAST DISCUSSION...CORRECTED
SOUTHCENTRAL AND SOUTHWEST ALASKA
300 PM AKDT MON SEP 8 2008

CORRECTED HEADLINES.

.MODEL DISCUSSION...
ALL MODELS INITIALIZE WELL AND ARE IN GREAT AGREEMENT ON MAJOR
FEATURES THROUGH 48 HOURS. SOME DIFFERENCES FROM PREVIOUS RUNS
INCLUDE THE HANDLING OF THE LOW APPROACHING THE ALASKA PENINSULA
TONIGHT. PREVIOUS RUNS WERE CONSISTENTLY BRINGING THE LOW TOWARD
KODIAK ISLAND WHILE A SECONDARY LOW FORMS UNDER THE UPPER LEVEL
CENTER BACK TOWARD BRISTOL BAY. THE GFS WAS THE FIRST AND NOW THE NAM
AND ECMWF AGREE ON A MORE NORTHERLY TRACK OF THE LOW ACROSS THE
ALASKA PENINSULA AND INTO BRISTOL BAY. WHILE IT MAKES SENSE TO TAKE
THE LOW NORTHWARD UNDER THE MAIN UPPER TROUGH...STILL THINKING THAT
SOME REMNANT LOW MAY REMAIN NEAR KODIAK ISLAND WHICH WILL MAKE SUBTLE
DIFFERENCES IN THE COASTAL WATERS FORECAST FROM COOK INLET TO
SHELIKOF STRAIT.

ANOTHER MORE SIGNIFICANT DIFFERENCE AND WHERE THE MODELS ARE
DIVERGING AT 48 HOURS IS THE HANDLING OF A DEVELOPING LOW SOUTH OF
THE KAMCHATKA PENINSULA. THE LATEST RUN OF THE GFS IS MUCH MORE
AGGRESSIVE THAN OTHER MODELS INCLUDING EVEN THE GFS MEAN ENSEMBLE.
AFTER COORDINATION WITH HPC...AGREED TO STICK WITH THE PREVIOUS RUN
OF THE GFS WHICH IS MUCH MORE CONSISTENT WITH THE EUROPEAN AND EVEN
THE NAM AT THIS POINT. IT IS LIKELY THE MODELS WILL CONTINUE TO HAVE
A DIFFICULT TIME TRANSITIONING THE UPPER LOW FROM SIBERIA ACROSS THE
BERING SEA TOWARD THE LONG WAVE TROUGH LOCATED OVER THE WESTERN
MAINLAND LATER THIS WEEK.

.ANALYSIS AND UPPER LEVELS...
A 996 MB LOW IS DEVELOPING SOUTH OF THE ALASKA PENINSULA EARLY THIS
AFTERNOON. THIS IS THE LATEST OF MANY DEVELOPING SURFACE LOWS UNDER
THE LARGE LONG WAVE UPPER TROUGH LOCATED OVER THE WESTERN MAINLAND.
SURFACE HIGH PRESSURE IS CONTROL TO THE WEST OF THIS FEATURE OVER THE
CENTRAL AND WESTERN BERING SEA.

.SHORT TERM FORECAST...
SOUTH CENTRAL...THE APPROACHING SYSTEM WILL BRING SEVERAL CHALLENGES
TO THIS AFTERNOON FORECAST PACKAGE. THE FIRST WILL BE THE STRENGTH
AND TIMING OF THE CHANNELED WINDS THROUGH PORTAGE VALLEY AND
TURNAGAIN ARM. THE INITIAL WARM FRONT WILL INCREASE THE PRESSURE
GRADIENT ACROSS THE CHUGACH MOUNTAINS AND INCREASE THE TURNAGAIN ARM
WIND TONIGHT. THE PRESSURE GRADIENT WILL BE STRONG ENOUGH FOR GUSTS
TO REACH JUST BELOW WARNING CRITERIA FOR A SHORT PERIOD OVERNIGHT
UNTIL THE INITIAL FRONT PASSES. THE WARM FRONT WILL ALSO PROVIDE
ISENTROPIC LIFT THAT WILL BRING CATEGORICAL RAINS FROM THE WESTERN
KENAI UP TO THE MATANUSKA VALLEY LATE TONIGHT. AFTER THE FRONT
PASSES...THE GRADIENT WILL SHIFT ITS ORIENTATION ENOUGH TO FAVOR A
PORTAGE VALLEY WIND EVENT.

ANOTHER CONCERN WITH THIS SYSTEM WILL BE EXCESSIVE RAINFALL OVER
PRINCE WILLIAM SOUND WHERE THE GROUND IS ALREADY SATURATED. THE BREAK
BETWEEN SYSTEMS HAVE ALLOWED RIVER AND CREEK LEVELS TO SUBSIDE BUT
MODERATE TO HEAVY RAIN FORECASTED THROUGH MID WEEK WILL LIKELY BRING
BACK FLOODING CONCERNS. WILL CONTINUE TO COORDINATE WITH THE RFC AND
MONITOR RIVER GAUGES AROUND THE AREA.

RAIN WILL BE LIKELY ACROSS SOUTHCENTRAL AS SOUTHWEST FLOW ALOFT
PERSISTS AND ABUNDANT MOISTURE IS FED NORTHWARD OUT OF THE NORTH
PACIFIC.

WEST OF THE ALASKA RANGE...
CONCERN OUT WEST WILL BE THE EXACT TRACK OF THE WEAKENING LOW AS IT
CROSSES THE ALASKA PENINSULA AND THE AFFECT IT HAS ON SOUTHWEST
MAINLAND. STEADY RAIN AND GUSTY WINDS WILL GIVE WAY TO SHOWERS
TUESDAY. A BRIEF BREAK IS POSSIBLE BETWEEN SYSTEMS MID TO LATER IN
THE WEEK.

BERING SEA AND ALEUTIANS...
WIDESPREAD FOG PERSISTS UNDER HIGH PRESSURE IN THE CENTRAL AND
WESTERN ALEUTIANS. AS MENTIONED IN THE MODEL DISCUSSION...THERE IS
MUCH DISAGREEMENT WITH THE PATH OF THE LOW DEVELOPING WEST OF THE
ALEUTIANS. WHILE THE PATTERN CHANGE SEEMS INEVITABLE WITH A LOW
DEVELOPING WEST OF THE ALEUTIANS AND MOVING TOWARD THE ALASKA
PENINSULA...THE TIMING OF THIS CHANGE REMAINS UNCERTAIN.

.AER/ALU...WATCH/WARNING SUMMARY
PUBLIC...FLOOD WATCH 125.
MARINE...GALE WARNING ZONES 120 125 130 132 175 176 351 352.
FIRE WEATHER...NONE.

AB SEP 08