Expires:No;;633122 FZUS56 KSEW 220510 CWFSEW AREA FORECAST DISCUSSION NATIONAL WEATHER SERVICE SEATTLE WA 910 PM PST MON FEB 21 2011 .SYNOPSIS...A COLD...LOW PRESSURE SYSTEM WILL SETTLE OVER THE PACIFIC NORTHWEST BY MIDWEEK...RESULTING IN SHOWERY WEATHER AND COLDER TEMPERATURES. SNOW LEVELS WILL BE LOW ENOUGH FOR THE POTENTIAL FOR A FEW INCHES OF SNOW ON THE LOWLANDS. && .SHORT TERM... SCATTERED TO NUMEROUS SHOWERS WERE PRESENT OVER THE CWA THIS EVENING...WITH THE NWS KATX RADAR SHOWING SW TO NE ORIENTED CONVECTIVE BANDS OVER THE CWA. A RAIN/SNOW MIX OR WET SNOW WAS FALLING WITHIN THESE BANDS OVER THE LOWLANDS. THERE WAS ALSO SOME PEA SIZED HAIL WITH THE HEAVIER SHOWERS. ONE OF THE CONVECTIVE BANDS EXTENDED FROM ABOUT WHIDBEY ISLAND NE INTO WHATCOM COUNTY. THE SECOND BAND WAS ALONG THE KING/SNOHOMISH COUNTY BORDER. THIS BAND WAS THE PUGET SOUND CONVERGENCE ZONE /PSCZ/...WHICH WILL LIKELY REMAIN QUASISTATIONARY THROUGHOUT MUCH OF TONIGHT AND POSSIBLY THRU TUE MORNING. WILL NEED TO CLOSELY MONITOR THE PSCZ OVERNIGHT BECAUSE THE POSSIBILITY EXISTS FOR A COUPLE OF INCHES OF SNOW TO ACCUMULATE BY MIDMORNING TUE. MEANWHILE...A FAIRLY STRONG JET WAS NOSING INTO THE AREA AT THIS TIME AND HELPING TO CARVE OUT AN UPPER TROF OVER THE PAC NW AND NRN GREAT BASIN. THIS TROF WAS COLD AS EVIDENCED BY THE OPEN CELL TCU OR CLOUD STREETS OFF THE COAST OF BRITISH COLUMBIA. THIS COLD AIR WILL INVADE THE REGION OVERNIGHT THRU TUE. ANTICIPATE SHOWERY WEATHER ON TUE BUT MUCH OF THE ACTION WILL OCCUR OVER THE MOUNTAINS...THE CASCADES ESPECIALLY...AND WITH SHOWER BANDS. EXPECT A RAIN/SNOW MIX OR SMALL HAIL ON THE LOWLANDS WITH THE HEAVIER SHOWERS DURING THE DAY TUE. EXPECT PRECIP TO INCREASE TUE NIGHT AS AN UPPER LOW DEVELOPS OVER SW BRITISH COLUMBIA. THIS LOW WILL DEEPEN AND SLOWLY SAG SWD WED THRU THU. BY LATE THU...THIS FEATURE IS ANTICIPATED TO BE W OF THE COLUMBIA RIVER BAR. THE LATEST NAM AND GFS DIFFERED IN THE DETAILS CONCERNING THIS SYSTEM. THE GFS MOVED THE ARCTIC FRONT S ACROSS THE CWA LATE WED NIGHT THRU THU WHEREAS THE NAM HUNG IT UP ACROSS THE N INTERIOR. THE NAM INDICATED THAT THE BEST CHANCE OF PRECIP WILL BE OVER ROUGHLY THE SRN THREE QUARTERS OF THE CWA WED THRU THU WHEREAS THE GFS INDICATED THE BEST CHANCE OF PRECIP...BESIDES THE MOUNTAINS...WILL BE WITH THE ARCTIC FRONT ON WED. THIS MODEL ALSO KEPT MUCH OF THE PRECIP OVER THE SRN PORTION OF THE CWA ON THU. IF THE NAM VERIFIES...WED AND THU WILL BE COLDER ACROSS THE CWA BUT NOT TOO MUCH AND IT WILL BE LESS WINDY. IF THE GFS VERIFIES...IT WILL BE COLDER AND WINDIER. IN GENERAL...CONFIDENCE WAS HIGHEST IN REGARDS TO SNOW IN THE CASCADES AND WITHIN THE PSCZ AS WELL AS THE TREND TOWARD COLDER TEMPS DURING THIS PERIOD. LESS CONFIDENCE WAS IN THE DETAILS CONCERNING SNOWFALL AMOUNTS IN ANY PARTICULAR LOWLAND ZONE...HOW QUICKLY THE ARCTIC FRONT SWEEPS ACROSS THE AREA...AND WHETHER THERE WILL BE ANY SIGNIFICANT PRECIP ASSOCIATED WITH THE ARCTIC BOUNDARY. .LONG TERM FROM PREV DISCUSSION... LONG WAVE PATTERN OF A RIDGE AROUND 150W AND TROUGHING IN THE WEST CONTINUED IN THE EXTENDED. COLD CONTINUED ON FRI WITH SLOW MODERATION SATURDAY. LOWS 15-25 AND HIGHS IN THE 30S. IT WILL BE DRY WITH ARCTIC HIGH PRES IN PLACE. FURTHER MODERATION WITH SHOWER CHANCES SUN-MON AND LOW SNOW LEVELS AROUND 1000 FT APPEARED TO BE A GOOD BET. LATEST GFS AND ECMWF SOLUTIONS HINTED AT A RELOAD OF ARCTIC AIR FOR NEXT WEEK...BUT LOW CONFIDENCE AT THIS RANGE. ALBRECHT && .HYDROLOGY... A FLOOD PRODUCING PATTERN IS NOT EXPECTED TO DEVELOP DURING THE NEXT TEN DAYS. THE SNOW LEVELS WILL BE LOW ENOUGH TO PRECLUDE ANY THREAT OF FLOODING ON AREA RIVERS...INCLUDING THE GREEN. && .AVIATION...AN UPPER TROUGH WILL BE OVER THE PACIFIC NORTHWEST TONIGHT AND TUESDAY WITH NORTHWEST FLOW ALOFT AND MODERATE LOW LEVEL ONSHORE FLOW. AIR MOIST AND UNSTABLE. SHOWERY WEATHER NEXT 24 HOURS WITH CONVERGENCE ZONES COMPLICATING MATTERS. AIR COLD ENOUGH FOR SNOW...ESPECIALLY IN HEAVIER SHOWERS AND IN CONVERGENCE ZONE ACTIVITY. KSEA...SOUTH WIND 10-15 WITH GUSTS 20-25 KNOTS. CONVERGENCE ZONE ACTIVITY EXPECTED TO STAY NORTH OF THE TERMINAL. AIR COLD ENOUGH FOR SNOW BUT WITH JUST HIT AN MISS SHOWERS NO SNOW ACCUMULATION EXPECTED AT THE TERMINAL THROUGH TUESDAY. SCHNEIDER && .MARINE...MODERATE/STRONG ONSHORE FLOW TONIGHT. GALES IN THE CENTRAL/EAST STRAIT. HIGH END SMALL CRAFT ADVISORIES ELSEWHERE. ONSHORE FLOW WILL RELAX A BIT TUESDAY...THEN TURN OFFSHORE ON WEDNESDAY AS A MODIFIED ARCTIC BOUNDARY MOVES SOUTH THROUGH THE WATERS. OFFSHORE FLOW WILL BECOME STRONG ON THURSDAY WITH GALES LIKELY SOME WATERS. THE STRONG OFFSHORE FLOW WILL WEAKEN AND BECOME ONSHORE SATURDAY AS THE HIGH PRESSURE OVER SOUTHERN BRITISH COLUMBIA WEAKENS AND SHIFTS SOUTHEAST AND IS REPLACED BY A SURFACE TROUGH. SCHNEIDER && .SEW WATCHES/WARNINGS/ADVISORIES... WA...WINTER STORM WARNING IN EFFECT FOR THE CASCADES TIL 4 AM WED. A WINTER WEATHER ADVISORY IS IN EFFECT FOR THE OLYMPICS TIL 4 AM WED. WINTER STORM WATCH REMAINS IN EFFECT FOR THE N INTERIOR LOWLANDS AND STRAIT OF JUAN DE FUCA WED THRU THU MORNING. PZ...GALE WARNING REMAINS IN EFFECT FOR THE CENTRAL AND EASTERN STRAIT OF JUAN DE FUCA TIL 9 AM TUE. A SMALL CRAFT ADVISORY REMAINS IN EFFECT FOR THE REMAINING WATERS. SMALL CRAFT ADVISORY FOR ROUGH GRAYS HARBOR BAR CONDITIONS REMAINS IN EFFECT TIL 4 PM TUE. $$ WWW.WEATHER.GOV/SEATTLE