Halloween Atmospheric River Forecast
Good year to be a pineapple for Halloween
Weekend rainfall totals reached a half-inch over the City to just over an inch on the Central Cascades. That’s more rain than was predicted by medium-range models most of last week, but it was exactly what was expected 24-hours prior. It was all welcome in SPU’s watersheds, of course, and very few drainage problems were reported by customers in-town.
About that much rainfall is expected again on Wednesday, as yet another front splits before coming ashore. The main forecast event, though, is lining up for this weekend.
An atmospheric river (AR) will almost certainly hit the Northwest. At the moment [Monday morning] it’s poised to hit us directly, which may not be good news for urban drainage but it’s exactly what our water supply managers want to see. But as we’ve seen in the past AR’s tend to wave about and there’s plenty of time for the forecast to change.
Seattle looks to be shadowed for most of the event with forecast 24-hour totals remaining under an inch and a half, but the mountains could see 5-10 inches over the course of this week, depending on where the AR lands.
To monitor changes, I will continually update this blog with snippets from National Weather Service Area Forecast Discussions along with screenshots of UW WRF-GFS water vapor and precipitation accumulation forecasts. Oldest images are at the bottom. I’ve chosen Saturday evening at 5PM for its trick-or-treat implications; the most intense period will likely be earlier. Also keep in mind that the model imagery is one of many tools used by forecasters, but it should provide insight into forecast uncertainty.
Thursday evening: MOIST WESTERLY FLOW WILL DEVELOP FRIDAY AND CONTINUE INTO SATURDAY WITH A FRONTAL SYSTEM DRAPED ACROSS THE REGION FOR WET AND BLUSTERY WEATHER. THE HEAVIER PRECIPITATION LOOKS LIKE IT WILL COME IN TWO WAVES…THE FIRST FRIDAY MORNING. THE SECOND HEAVIER SHOT OF RAIN WILL DEVELOP FRIDAY NIGHT AND CONTINUE INTO SATURDAY. THE HEAVY RAIN WILL END FROM NORTH TO SOUTH SATURDAY AFTERNOON AND EVENING AS THE FRONT SAGS SOUTH OF THE AREA. MINOR RIVER FLOODING IS POSSIBLE BY SATURDAY OR SATURDAY NIGHT.
Thursday morning: MOIST ZONAL FLOW WILL DEVELOP FRIDAY AND CONTINUE INTO SATURDAY WITH A FRONTAL SYSTEM DRAPED ACROSS THE REGION FOR WET AND BLUSTERY WEATHER. THE HEAVIER PRECIPITATION LOOKS LIKE IT WILL COME IN TWO WAVES. THE FIRST WAVE WILL BE FRIDAY MORNING. THE SECOND HEAVIER SHOT OF RAIN WILL DEVELOP LATER FRIDAY NIGHT AND CONTINUE INTO SATURDAY. THE HEAVY RAINS WILL END FROM NORTH TO SOUTH SATURDAY AFTERNOON AND EVENING AS THE FRONTAL FEATURE SAGS SOUTH OF THE AREA. MINOR RIVER FLOODING IS POSSIBLE BY SATURDAY OR SATURDAY NIGHT.
Wednesday evening: STRONG WARM ADVECTION AND LOW LEVEL W FLOW WILL BRING HEAVY RAIN TO THE MOUNTAINS. W FLOW ACROSS THE OLYMPICS WILL ALSO PUT THE GREATER PUGET SOUND REGION IN THE OLYMPIC RAIN SHADOW…WITH ONLY LIGHT AMOUNTS OF RAIN LIKELY OVER THE GREATER PUGET SOUND REGION THURSDAY NIGHT. THE GFS20 BRINGS THE WARM FRONT ACROSS W WA EARLY FRIDAY MORNING. THERE WILL BE A LULL IN THE RAIN BETWEEN THE FRIDAY MORNING WARM FRONT AND THE NEXT WARM FRONT FRIDAY NIGHT. THE GFS20 TAKES THE SECOND WARM FRONT NE BY EARLY SATURDAY MORNING…BUT THE TRAILING COLD FRONT WILL KEEP THE RAIN GOING THROUGH SATURDAY AFTERNOON. BREEZY TO WINDY CONDITIONS CAN BE EXPECTED WITH THE FIRST WARM FRONT AND AHEAD OF SATURDAYS COLD FRONT.
Wednesday morning: THE FRIDAY THROUGH SATURDAY PERIOD LOOKS WET WITH STRONG MOIST WESTERLY FLOW ALOFT POINTED AT WESTERN WASHINGTON. THE PERIODS WITH THE HEAVIEST PRECIPITATION SHOULD BE FRIDAY AND THE FIRST HALF OF SATURDAY. HEAVY PRECIPITATION WILL SHIFT SOUTH OF THE FORECAST AREA DURING THE DAY SATURDAY. MODELS CONTINUE TO BACK OFF ON THE TOTAL QPF. FLOODING CANNOT BE ENTIRELY RULED OUT FOR THIS PERIOD BUT THE THREAT APPEARS TO BE LESSENING.
Tuesday evening: THE FRIDAY THROUGH SATURDAY PERIOD LOOKS WET WITH STRONG MOIST WESTERLY FLOW ALOFT POINTED AT WESTERN WASHINGTON. THE PERIOD WITH THE HEAVIEST PRECIPITATION SHOULD BE FRIDAY NIGHT AND THE FIRST HALF OF SATURDAY. HEAVIER PRECIPITATION WILL SHIFT SOUTH OF THE FORECAST AREA DURING THE DAY SATURDAY. MODELS CONTINUE TO BACK OFF ON THE TOTAL QPF. FLOODING CANNOT BE ENTIRELY RULED OUT FOR THIS PERIOD BUT THE THREAT APPEARS TO BE LESSENING.
Tuesday morning: WET AND BLUSTERY WEATHER WILL CONTINUE ACROSS WESTERN WA FRIDAY NIGHT AND SATURDAY AS A STRONG AND MOIST PAC SYSTEM SLOWLY CROSSES THE AREA. BOTH THE GFS AND ECMWF SHOW HEAVY RAIN ON THE COAST AND MOUNTAINS AS THE COLD FRONT DRAPES OVER THE AREA. AN UPPER LEVEL LOW MAY SHIFT THE WAVE /AND HEAVY PRECIP/ TO THE SOUTH INTO OREGON SATURDAY EVENING.
Monday evening: A WET PATTERN WILL PREVAIL FRIDAY THROUGH SUNDAY WITH HEAVY RAINS AT TIMES AND HIGH SNOW LEVELS. IF THE GFS IS TO BE BELIEVED…THE HEAVIER RAINS WILL COME IN A COUPLE OF WAVES…ONE ROUGHLY FRIDAY AFTERNOON AND EVENING…AND ANOTHER SATURDAY AFTERNOON AND NIGHT. OTHER MODELS ARE ALSO WET FOR THIS PERIOD…BUT GENERALLY NOT AS WET AS THE GFS AND THE DETAILS ARE DIFFERENT. THE CURRENT FORECAST LEANS TOWARDS THE GFS WHICH IS THE WETTEST OF THE MODELS. THE GFS SHOWS 5 TO 10 INCHES OVER THE MOUNTAINS FOR THE 72 HOUR PERIOD FRIDAY THROUGH SUNDAY PERIOD WITH 1 TO 3 INCHES FOR THE LOWLANDS. IF THESE AMOUNTS VERIFY…FLOODING MAY OCCUR ON THE EXCEPTIONALLY FLOOD-PRONE SKOKOMISH RIVER AND POSSIBLY ON OTHER AREA RIVERS AS WELL.
Monday morning: THE FRONT THAT PASSES SLOWLY SE ACROSS THE AREA FRI INTO SAT APPEARS THAT IT WILL BE QUITE WET WITH HIGH PRECIPITABLE WATER AMOUNTS GETTING ENTRAINED INTO THE FRONT FROM THE WESTERN TROPICAL PACIFIC. 850-925 MB WINDS ARE EXPECTED TO BE WSW 35-45 KT AHEAD OF THIS FRONT…AND SNOW LEVELS AROUND 9000 FEET. THE GFS AND A MAJORITY OF ITS ENSEMBLE MEMBERS SHOW IMPRESSIVE RAINFALL AMOUNTS. THE OPERATIONAL GFS SHOWS 24 HR RAINFALL AMOUNTS OF 5-7 INCHES IN THE OLYMPICS AND IN THE CASCADES. THE UW WRF 12KM EXTENSION SHOWS 24 HR AMOUNTS 5-10 INCHES FRI AFTERNOON THROUGH SAT AFTERNOON. THE ECMWF HAS LOWER QPF VALUES WITH THE FRONT THAN THE WETTER GFS. IT WILL BE LOCALLY BREEZY TO WINDY AS FLOW ALOFT INTERACTS WITH THE TERRAIN…AND TEMPERATURES WILL BE QUITE MILD FOR THE END OF OCTOBER.
Sunday evening: A WARM FRONT WILL MOVE INTO WESTERN WASHINGTON AROUND FRIDAY NIGHT…AND IT WILL BE AT THE LEADING EDGE OF A LONG FETCH OF MOISTURE EXTENDING WEST-SOUTHWEST OVER THE PACIFIC OCEAN. THIS FRONTAL BAND WILL SINK SLOWLY SOUTHWARD ACROSS THE AREA OVER THE WEEKEND…KEEPING THE WEATHER WET WITH SNOW LEVELS AROUND 8000 TO 9000 FT. THE 12Z SUN MODEL RUNS SHOW CONSIDERABLY LESS RAINFALL FROM THIS SYSTEM THAN THE 00Z SUN MODEL RUNS DID…BUT THERE IS STILL CONSIDERABLE SPREAD IN THE ENSEMBLES.
Sunday morning: MODELS ARE HINTING AT A VERY WET FRONTAL SYSTEM MOVING SLOWLY SE ACROSS THE AREA LATER FRIDAY INTO THE WEEKEND. WHILE MODELS VARY ON TIMING…SOME MODEL SOLUTIONS SHOW THE POTENTIAL FOR AN 18-24 HOUR PERIOD OF 4 INCH RAINFALL IN THE OLYMPICS…ALONG THE COAST…AND OVER PORTIONS OF THE CASCADES.
Saturday evening: ANOTHER FRONTAL SYSTEM WILL SAG INTO THE AREA FOR MORE PRECIPITATION FRIDAY THROUGH SATURDAY. THE MODELS ARE IN FAIRLY GOOD AGREEMENT THROUGH THE LONG TERM.
Saturday morning: CLOUDS AND A CHANCE OF RAIN WEDNESDAY INTO THE END OF THE UPCOMING WEEK.
– JRH