072 FXUS66 KSEW 191036 CCA AFDSEW AREA FORECAST DISCUSSION...CORRECTED NATIONAL WEATHER SERVICE SEATTLE WA 336 AM PDT TUE MAR 19 2013 CORRECTED TYPO IN THE KSEA PART OF THE AVIATION SEGMENT. .SYNOPSIS...RAIN WILL SPREAD INTO THE AREA FROM THE SOUTHWEST LATER TODAY AS A FRONT ASSOCIATED WITH A DEVELOPING LOW PRESSURE SYSTEM APPROACHES THE AREA. THIS SYSTEM WILL BRING RAIN...WIND...AND HEAVY MOUNTAIN SNOW TO THE AREA TUESDAY NIGHT THROUGH WEDNESDAY NIGHT. COLDER AIR WILL ARRIVE WEDNESDAY NIGHT AND THURSDAY WITH SHOWERY CONDITIONS. WEAK HIGH PRESSURE COULD BRING A DRYING TREND TOWARD THE WEEKEND. && .SHORT TERM...HIGH CLOUDS ARE SPREADING INTO WESTERN WASHINGTON EARLY THIS MORNING AHEAD OF A DEVELOPING SYSTEM OVER THE OFFSHORE WATERS. A SURFACE NEAR 40N 142W AT 06Z IS EXPECTED TO DEEPEN AT IT TRACKS NORTHEASTWARD TOWARD VANCOUVER ISLAND OVER THE NEXT 24 HOURS. PRECIPITATION WILL BEGIN TO SPREAD INTO THE REGION DURING THE AFTERNOON AHEAD OF A WARM FRONT THAT WILL LIFT NORTHWARD THROUGH THE AREA TONIGHT. THIS WILL PUSH SNOW LEVELS UP TO NEAR 4000 FEET FOR A PERIOD TONIGHT...ESPECIALLY IN THE OLYMPICS AND CENTRAL CASCADES. THE SURFACE LOW OFFSHORE WILL DEEPEN TO NEAR 985 MILLIBARS AS IT MOVES ONSHORE OVER NORTH VANCOUVER ISLAND AROUND 12Z WEDNESDAY. AN ASSOCIATED COLD FRONT WILL MOVE ONSHORE DURING THE MORNING HOURS WEDNESDAY...USHERING IN COLDER AIR ALOFT AND LOWERING SNOW LEVELS. I WENT AHEAD AND CONVERTED THE WINTER STORM WATCH TO A WARNING FOR THE CASCADES AND OLYMPICS STARTING LATE TODAY AND CONTINUING THROUGH WEDNESDAY. THE BRIEF RISE IN SNOW LEVELS TODAY COULD MAKE SNOWFALL TOTALS DIFFICULT TO PREDICT...ESPECIALLY IN THE LOWER PASSES LIKE SNOQUALMIE. HOWEVER...STRONG POST-FRONTAL WESTERLY FLOW ALOFT COMBINED WITH THE COLDER AIR FLOODING IN ON WEDNESDAY SHOULD BE SUFFICIENT TO PRODUCE SUBSTANTIAL SNOWFALL MEASURED IN FEET WEDNESDAY AFTERNOON AND NIGHT AT PLACES LIKE MOUNT BAKER AND PARADISE. IN ADDITION...THE NAM12 AND WRFGFS-4KM SHOW A DISTINCT CONVERGENCE ZONE DEVELOPING WEDNESDAY AFTERNOON AND THAT COULD NAIL STEVENS PASS WITH A LOT OF SNOW IN A SHORT PERIOD TIME...MUCH LIKE WHAT OCCURRED A COUPLE DAYS AGO. STRONG SURFACE GRADIENTS AHEAD OF AND BEHIND THE SURFACE LOW WILL MAKE FOR WINDY CONDITIONS LATE TONIGHT THROUGH WEDNESDAY. AT THIS POINT...IT APPEARS THAT WIND ADVISORIES MAY EVENTUALLY BE NEEDED FOR PLACES WITH EXPOSURE TO EASTERLY GRADIENTS LATE TONIGHT...I.E. THE COAST...STRAIT...AND NORTH INTERIOR. THEN STRONG POST-FRONTAL PRESSURE RISES ON WEDNESDAY WILL LEAD TO WINDY CONDITIONS FOR MUCH OF THE AREA. THE 00Z RUN OF THE GFS INDICATED A POSSIBLE PDX-BLI GRADIENT OF +12 MILLIBARS 00Z THURSDAY. TEMPERATURES ALOFT DIP TO -30C AT 500 MILLIBARS WITH THE UPPER LEVEL TROUGH AXIS SHIFTING ONSHORE WEDNESDAY AFTERNOON. THIS ALONG WITH SOME EARLY SPRING SUN OUGHT TO BE ENOUGH TO DESTABILIZE THE ATMOSPHERE A BIT AND WARRANTS THE MENTION OF A CHANCE OF A THUNDERSTORM. RESIDUAL SHOWERS CONTINUE INTO WEDNESDAY EVENING BUT SHOULD BE DIMINISHING WITH THE NOTABLE EXCEPTIONS OF THE MOUNTAINS AND ANY LINGERING CONVERGENCE ZONE. BY LATE WEDNESDAY NIGHT...SNOW LEVELS COULD BE AS LOW AS 500 FEET...THOUGH ANY ACCUMULATION BELOW 1000 FEET IS RATHER UNLIKELY. A BROAD TROUGH REMAINS OVER THE REGION THROUGH THURSDAY KEEPING A CHANCE OF SHOWERS IN THE FORECAST. .LONG TERM...CONFIDENCE IN THE EXTENDED FORECAST DETAILS REMAINS LOW AS THE MODELS CONTINUE TO HAVE DIFFICULTY RESOLVING UPSTREAM DEVELOPMENTS OVER THE NORTHEAST PACIFIC. BOTH MODELS ARE EDGING TOWARD SOME RIDGING LATE FRIDAY INTO SATURDAY...BUT THE EURO QUICKLY BREAKS THE RIDGE DOWN LATE IN THE WEEKEND WITH ANOTHER SYSTEM WHILE THE GFS KEEPS UPPER LEVEL RIDGING JUST OFFSHORE THROUGH THE WEEKEND. THE CANADIAN IS SIMILAR TO THE EURO. THE EXISTING PARTLY TO MOSTLY CLOUDY WITH A CHANCE OF SHOWERS FOR THE EXTENDED PERIOD LOOKS LIKE THE WAY TO GO FOR NOW. 27 && .HYDROLOGY...FLOODING IS NOT EXPECTED IN THE NEXT 7 DAYS. && .AVIATION...AN UPPER RIDGE WILL PREVAIL OVER THE REGION TODAY. INCREASING SW FLOW ALOFT AND LOW LEVEL OFFSHORE FLOW. THERE IS A POSSIBILITY OF LLWS LATE TODAY BUT MORE LIKELY THIS EVENING DUE TO INCREASING SLY WINDS JUST ABOVE THE SURFACE. ANTICIPATE VFR CONDITIONS TODAY...ALTHOUGH CIGS WILL GRADUALLY LOWER DURING THE DAY. THERE MAY BE SOME AREAS OF CIGS 2-3K FT LATE THIS AFTERNOON OVER THE COAST. KSEA...VFR CONDITIONS WILL PREVAIL OVER THE TERMINAL TODAY. WINDS BECMG ELY 8-12 KNOTS TODAY. && .MARINE... STRONG PRES FALLS OVER THE NE PACIFIC /ASSOCIATED WITH A STRONG FRONTAL SYSTEM/ AND HIGHER PRES E OF THE CASCADES WILL RESULT IN STRENGTHENING OFFSHORE FLOW TODAY. THIS FRONTAL SYSTEM WILL IMPACT THE AREA TONIGHT THROUGH WED FOR GALE FORCE WINDS AND BUILDING SEAS. THERE IS A POSSIBILITY THAT GALE FORCE WESTERLIES THROUGH THE STRAIT OF JUAN DE FUCA ON WED COULD BRIEFLY REACH STORM FORCE. GALE FORCE WINDS ARE NOT EXPECTED OVER THE PUGET SOUND/HOOD CANAL AT THIS TIME BUT THERE IS A POSSIBILITY THAT SOUTHERLY WINDS HERE MAY BRIEFLY REACH GALE FORCE ON WED. THE FRONTAL SYSTEM WILL EXTEND FROM A LOW THAT WILL DEEPEN TO NEAR 985 MB AS IT RACES NE TOWARD VANCOUVER ISLAND EARLY WED. && .SEW WATCHES/WARNINGS/ADVISORIES... WA...WINTER STORM WARNING OLYMPICS AND NORTH AND CENTRAL CASCADES TONIGHT THROUGH WEDNESDAY NIGHT. PZ...GALE WARNING IN EFFECT FOR THE COASTAL WATERS. SMALL CRAFT ADVISORIES ARE IN EFFECT FOR THE WESTERN AND CENTRAL STRAIT OF JUAN DE FUCA. GALE WATCH IN EFFECT FOR THE STRAIT OF JUAN DE FUCA... NORTHERN INLAND WATERS...AND ADMIRALTY INLET THIS EVENING THROUGH WEDNESDAY EVENING. && $$ WWW.WEATHER.GOV/SEATTLE YOU CAN SEE AN ILLUSTRATED VERSION OF THE FORECAST DISCUSSION AT WWW.WEATHER.GOV/SEATTLE/GAFD/LATEST_WEBAFD.HTML