Flood Watch 1, NWS Anchorage, AK

AREA FORECAST DISCUSSION
SOUTHCENTRAL AND SOUTHWEST ALASKA
600 AM AKDT SUN MAY 3 2009

.ANALYSIS AND UPPER LEVELS...
BIGGEST FEATURE CONTINUES TO BE THE UPPER LEVEL RIDGE THAT EXTENDS
FROM THE INTERIOR THROUGH WESTERN CANADA AND INTO THE HIGH PLAINS OF
THE LOWER 48. A FAST MOVING SHORTWAVE SOUTH OF SHEMYA IS MOVING
SOUTHEAST. AN EASTERLY SHORTWAVE THAT CROSSED THE AKPEN YESTERDAY
MORNING HAS SLOWED OVER THE PRIBS THIS MORNING AS IT BEGINS MOVE
SOUTH ALONG THE PERIPHERY OF ITS PARENT LOW IN THE NORTHERN PACIFIC.

.MODEL DISCUSSION...
MODELS CONTINUE REMAIN IN VERY GOOD AGREEMENT THROUGH THE FIRST FOUR
DAYS OF THE FORECAST...LEADING TO CONTINUED AND ABOVE AVERAGE
CONFIDENCE IN THE FORECAST IN THE SHORT TO MEDIUM RANGE...IN THE LONG
TERM THE ECMWF AND GFS CONTINUE TO BATTLE OUT THE PROGRESSION OF THE
RIDGE TRAVERSING THE ALEUTIANS. THE MODELS ARE IN GOOD AGREEMENT
THROUGH LATE WEDNESDAY ... BEYOND THAT THE GFS CONTINUES THE TYPICAL
TREND OF BREAKING DOWN THE RIDGE TOO QUICKLY WITH A LOW THAT
SLINGSHOTS ACROSS THE WESTERN AND CENTRAL ALEUTIANS BY FRIDAY. THE
MORNING FORECAST PACKAGE WAS BASED OFF OF THE GFS IN THE SHORT TO MID
TERM...WHILE THE ECMWF WAS PREFERRED IN THE LONG TERM.

.SHORT TERM FORECAST...

SOUTHCENTRAL...AN END TO THE PROLONGED STRETCH OF ABOVE NORMAL AND
UNSEASONAL WEATHER IS UNFORTUNATELY WITHIN SIGHT...HIGH PRESSURE OVER
MUCH OF THE STATE WILL CONTINUE TO WEAKEN THROUGH EARLY IN THE WEEK
AS AN ARCTIC TROUGH SWEEPS DOWN FROM NORTON SOUND. A THERMALLY
INDUCED TROUGH OF LOW PRESSURE (SOMETIMES CALLED A HEAT LOW) WILL
FORM OVER THE COPPER AND SUSITNA RIVER VALLEYS TODAY. UPPER LEVEL
MOISTURE WILL BEGIN TO FILTER BACK INTO THE AREA. SHOWERS WILL
DEVELOP MAINLY OVER THE MOUNTAINS TODAY AND SPREAD OVER THE VALLEYS
DURING THE EVENING. THE POTENTIAL FOR OUR FIRST LIGHTNING STRIKES OF
THE SEASON IS INCREASING ALONG THE ALASKA RANGE IN ZONES 141 AND 145.
WINDS WILL ALSO INCREASE TODAY IN RESPONSE PRESSURE GRADIENT
INCREASES FROM THE THERMAL TROUGH. RH VALUES ACROSS FOR ZONES
101...111 AND 141 RANGED ANYWHERE FROM THE 20S TO 30S YESTERDAY AND
WILL LIKELY BE AT THE SAME LEVELS...IF NOT LOWER TODAY. THIS COUPLED
WITH WINDS THAT WILL INCREASE THIS AFTERNOON WILL CREATE A POTENTIAL
FIRE HAZARD...THUS A RED FLAG WARNING WAS ISSUED FOR ALL THREE OF
THOSE ZONES FOR LATE THIS AFTERNOON INTO THE EVENING FOR RH BELOW 30%
AND WINDS OF 25 MPH OR GREATER. THE ISSUES IS NOT ON THE LOW RELATIVE
HUMIDITIES...IT IS ON IF THE WINDS WILL INCREASE ENOUGH TO SATISFY
CRITERIA.

SOUTHWEST ALASKA...PERSISTENCE IS KEY AND SOUTHWEST WILL AGAIN SEE
FAIR AND TRANQUIL CONDITIONS THROUGH THE EARLY WEEK. LATE TUESDAY IN
TO WEDNESDAY...WINDS AND CLOUDS WILL INCREASE ALONG THE ALEUTIAN AND
ALASKA RANGES AROUND TUESDAY AND SPILL OVER INTO EASTERN PORTIONS OF
ZONES 151 AND 161.

BERING SEA/ALEUTIANS...THE MAIN WEATHER FOCUS FOR THE STATE HAS BEEN
OVER THE ALEUTIANS THE LAST COUPLE OF DAYS BUT THE NICE WEATHER IS
EVEN EXTENDING BEYOND THE EASTERN ALEUTIANS NOW. HIGH PRESSURE WILL
BUILD OVER THE BERING SEA THROUGH THE REMAINDER OF THE WEEKEND AND
PERSIST INTO MID WEEK.

.LONG TERM FORECAST...
EARLY ON TUESDAY AN ARCTIC TROUGH WILL BEGIN TO MOVE OVER THE WESTERN
COAST AND WORK ITS WAY EAST. THIS TROUGH IS LACKING ANY REAL KICK AND
WILL ONLY USHER IN COOLER TEMPERATURES AND CLOUDY SKIES. THIS TROUGH
WILL BE THE MAIN IMPETUS BEHIND THE RETURN TO MORE SEASONAL
TEMPERATURES FOR MOST OF SOUTHWEST AND SOUTHCENTRAL.

.AER/ALU...WATCH/WARNING SUMMARY
PUBLIC...NONE
MARINE...NONE
FIRE WEATHER...RED FLAG WARNING FOR ZONES 101,111, AND 141

SHEA MAY 09