AREA FORECAST DISCUSSION
NWS SEATTLE WA
AFDSEW 215 AM PDT SUN APR 15 2001
.OVERVIEW...
A HIGH AMPLITUDE PATTERN IS IN PLACE OVER THE PACIFIC AND WESTERN
NORTH AMERICA WITH LONG WAVE RIDING ALONG 170W...A TROUGH ALONG
140W...AND AN OMEGA BLOCK CENTERED ALONG 115W. ON THE SYNOPTIC SCALE
THE MODELS ARE ALL SHOWING GOOD CONSISTENCY AND CONTINUITY THROUGH
LATER IN THE WEEK BY SLOWLY PROGRESSING THE LONG WAVE PATTERN. THE
FORECAST PROBLEM LOCALLY WILL BE IN THE SUNDAY NIGHT THROUGH TUESDAY
TIME PERIOD AS MESOSCALE FEATURES AND JET STREAKS ROTATE AROUND THE
OFFSHORE TROUGH AND MOVE ACROSS THE AREA. THE MODELS HANDLE THESE
FEATURES DIFFERENTLY...AND BASED ON SATELLITE IMAGERY...ACARS
DATA...SATELLITE IMAGERY...AND SHORT TERM FORECAST PERFORMANCE I HAVE
NO BASIS FOR PREFERENCES. MAYBE THE BEST BET IS FOR A MODEL BLEND
BETWEEN THE FASTER ETA AND THE SLOWER AVN. IN THE LONGER RANGE ALL
MODELS ARE SHOWING UNSETTLED CONDITIONS THROUGH NEXT SATURDAY AS THE
LONG WAVE TROUGH MOVES INTO OUR VICINITY...THEN GETS RECHARGED BY
ANOTHER CUTTING OFF LOW NEARBY.
.SHORT TERM...
WHILE THERE IS NO CURRENT EVIDENCE OF MOUNTAIN WAVE CONDITIONS WILL
MAINTAIN GUSTY EAST WINDS IN THE FORECAST FOR THE FOOTHILLS TODAY.
OFFSHORE GRADIENTS ARE INCREASING AND SHORT TERM MODELS...
SPECIFICALLY THE UW MM5...CONTINUE TO INDICATE THE DEVELOPMENT OF
MOUNTAIN WAVE CONDITIONS AS EASTERLY PRESSURE GRADIENTS INCREASE...
THE FLOW REVERSES NEAR THE CASCADE CREST LEVEL...AND A FRONTAL
INVERSION MOVES IN FROM THE SW. THIS SHOULD HELP WARMING IN WRN
WASHINGTON TO AROUND THE 60 DEGREE LEVEL THIS AFTERNOON AS CURRENTLY
FORECAST. OFFSHORE FLOW AND ASSOCIATED LOW LEVEL SUBSIDENCE SHOULD
KEEP POPS BELOW MOS GUIDANCE ALL AREAS THIS AFTERNOON AND IN THE
INTERIOR TONIGHT...BUT WILL RAISE POPS IN THE DIRECTION OF GUIDANCE A
BIT TONIGHT BASED ON TIMING UNCERTAINTIES. THE FRONTAL BAND MOVES E
INTO THE INTERIOR MON AFTN THEN GETS ENERGIZED MON AFTERNOON AS A
STRONG JET MAX ROTATES AROUND THE OFFSHORE LOW. GIVEN FORECAST OF
HEALTHY UPPER LEVEL DIFFLUENCE IN THE LEFT EXIT REGION OF A STRONG
JET STREAK...LIFTED INDICES M1-M2...AND CAPES OF 400-600J/KG...WILL
INCLUDE POSSIBLE EMBEDDED THUNDERSTORMS MONDAY AFTERNOON FOR ALL
AREAS --CURRENT FORECASTS ONLY MENTION THE CASCADES.
.EXTENDED...
NEW EXTENDED GUIDANCE KEEPS POPS HIGH THROUGH WEDNESDAY. THEN MOSTLY
CLOUDY AND COOL WITH A CHANCE OF SHOWERS LOOKS GOOD FOR THE REMAINDER
OF THE EXTENDED IN THE PRESENCE OF THE LONG WAVE TROUGH.
UIL 279 SEA 147 OLM 147 ALBRECHT
.SEW...SMALL CRAFT ADVISORY WEST ENTRANCE TO THE STRAIT OF JUAN DE
FUCA.
.KATX...VCP31/CSR31...OPERATIONAL