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October 2014
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Highly Active Pattern

Posted: October 21st, 2014

 

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Total precipitable water (atmopsheric river) satellite imagery via CIMSS

 

It’s been a while since we’ve seen weather systems race toward us like this, especially at this point in the year. We’re currently looking at over 2.5″ (lowlands), 3.5″ (mountains) of rainfall through the weekend and three separate periods of gusty winds.

 

Rainfall is not forecast to be particularly intense during the period, but it will be relatively steady. With respect to impacts, we should lose a lot of leaves this week, as well as a few tree limbs. And then by the weekend we may have to monitor tide predictions and pressure for coastal flooding, and even soil saturation for landslide conditions. It’s Fall!

 

First up is an atmospheric river that is already starting to affect the Olympic coast. The warm, humid system that stretches back to the subtropical Pacific is expected to graze our region and provide periods of moderate-to-heavy rainfall tomorrow. After sliding down to California, it’s then forecast to make a final surge back into the Northwest late Friday/early Saturday.

 

Related to that surge is the first windstorm of the season to threaten the area. At the moment, models take a deep low pressure system into Vancouver Island on Saturday morning. If it verifies, we’ll be on the windy side of the system. But it also means that coastal flooding during high tide is possible (a modest high tide of 11’2″ MLLW/~9’NAVD88 will occur at 6:56 AM on Saturday).

 

Next up after that are the remnants of Hurricane Ana, which recently passed by Hawaii. While there’s lots of uncertainty looking that far into the future, we’ll have to watch sea level pressure, wind speed, and precipitation forecasts closely.

 

Citywide, the NWS currently predicts 0.98″ for tomorrow, 0.40″ Thu, 0.33″ Fri, 0.65″ Sat, and 0.14″ Sun. The UW WRF-GFS has 0.97″ tomorrow and 1.04″ on Thu. The ECMWF is much drier overall. Winds should peak at 15-20mph with 35mph gusts tonight, then again overnight tomorrow.

 

Over SPU’s mountain reservoirs, the NWS currently predicts 0.92″ for tomorrow, 0.62″ Thu, 0.46″ Fri, 0.74″ Sat, and 0.64″ Sun. Winds should peak at 10-15mph with 30mph gusts tonight, then again overnight tomorrow, and then may increase to 20-25mph with gusts to 45mph Fri night (easterly) and Sat morning (southerly).

 

Excerpts from today’s doozy of an NWS Area Forecast Discussion (emphasis added):

 

THE UPCOMING WEEK WILL BE QUITE ACTIVE AS A SERIES OF SYSTEMS MOVE THROUGH THE PACIFIC NORTHWEST. THE JET ACROSS THE NORTH PACIFIC IS BECOMING RATHER STRONG AS COLD AIR DUMPS OUT OF THE BERING SEA INTO THE GULF OF ALASKA AND SUBTROPICAL AND TROPICAL MOISTURE AND ENERGY FROM SYSTEMS IN THE CENTRAL NORTH PACIFIC GETS ENTRAINED INTO THE FLOW.

AN OCCLUDED FRONT SITS FROM CENTRAL VANCOUVER ISLAND SOUTHWESTWARD INTO THE PACIFIC. THE AIR MASS THIS AFTERNOON OVER THE COASTAL WATERS CONTINUES TO BE QUITE UNSTABLE AND THERE IS A GOOD DEAL OF LIGHTNING BEING OBSERVED 40 TO 70 MILES OFF THE NORTH COAST. SOME THUNDERSTORM ACTIVITY MAY CLIP THE NORTH COAST THIS EVENING…BUT THE AIR MASS IS LIKELY TO BECOME A BIT MORE STABLE LATER TONIGHT AS OVERRUNNING DEVELOPS AHEAD OF THE INCOMING DEVELOPING LOW. A DEVELOPING WAVE NOW SEEN NEAR 46N 142W WILL DEVELOP TO A 992 MB LOW AS IT MOVES INTO SOUTH CENTRAL VANCOUVER ISLAND LATE TONIGHT. THIS LOW IS DEVELOPING IN THE LEFT EXIT REGION OF A 150 KT 300 MB JET THAT IS CHARGING EASTWARD ACROSS THE EAST CENTRAL NORTH PACIFIC AND HAS BEEN CHARGED WITH SUBTROPICAL MOISTURE FROM AN UPPER TROUGH NOW SEEN NEAR 37N 165E.

TWO ISSUES ARE LIKELY WITH THIS LOW TONIGHT…WIND AND HEAVY RAINFALL. WIND IS LIKELY TO REACH 20 TO 35 MPH WITH GUSTS TO 50 MPH ALONG THE COAST TONIGHT AND 20 TO 30 MPH WITH GUSTS TO 45 MPH THROUGH THE NORTH INTERIOR FROM ABOUT ADMIRALTY INLET NORTHWARD LATE TONIGHT INTO WEDNESDAY MORNING. A WIND ADVISORY WAS ISSUED LAST NIGHT AND WILL BE MAINTAINED. QPF FROM THE 18Z NAM12 SHOWS 3 TO 6 INCHES OF RAIN OVER THE SOUTHWEST FACING OLYMPICS THROUGH TONIGHT AND ANOTHER INCH OR TWO EARLY WEDNESDAY MORNING. THE 12Z WRFGFS SHOWS UP TO 10 INCHES OF RAIN TONIGHT INTO WEDNESDAY MORNING IN THE
RAIN FOREST AREAS OF THE SW OLYMPIC PENINSULA…QUITE IMPRESSIVE. THIS HEAVY RAIN APPEARS TO BE DUE TO A COMBINATION OF CONVECTION AND STRONG UPSLOPE FLOW WITH SSW WINDS 55 KT AT 850 MB. RAINFALL IS EXPECTED TO BE HEAVY VERY LATE TONIGHT INTO WEDNESDAY EVENING OVER THE NORTH CASCADES…AND THE LATEST WRFGFS SHOWS SEVERAL INCHES OF RAIN ON THE WEST SLOPES OF THE MOUNT BAKER AREA IN WHATCOM COUNTY AND THE MOUNTAINS NW OF DARRINGTON.

A FLOOD WATCH IS IN EFFECT THROUGH WEDNESDAY FOR THE OLYMPIC PENINSULA. THE FLOOD WATCH HAS BEEN EXPANDED AND EXTENDED TO INCLUDE RIVERS FLOWING OFF THE NORTH CASCADES FROM SNOHOMISH COUNTY NORTHWARD MIDDAY WEDNESDAY THROUGH WEDNESDAY NIGHT.

THE FRONT IS PROGRESSIVE AND WILL PUSH EASTWARD REACHING FROM THE N CASCADES TO THE NW OREGON COAST WED AFTERNOON. THEN RAIN BECOMES SHOWERY FROM THE NW WED EVENING AS THE FRONT CONTINUES TO SLOWLY PUSH SOUTHEASTWARD. BREEZY AND SHOWERY WEATHER CAN BE EXPECTED WED NIGHT THROUGH THU IN STRONG SW FLOW ALOFT.

LARGE SCALE MODELS CONTINUE TO SHOW DEVELOPMENT OF A RATHER DEEP LOW OFF THE NORTHERN CALIFORNIA COAST THURSDAY NIGHT INTO FRIDAY AS ENERGY DUMPING SOUTHWARD FROM THE BERING SEA INTO THE NORTH PACIFIC PHASES SOMEWHAT WITH THE REMAINS OF HURRICANE ANA THAT HAS RECENTLY IMPACTED WESTERN HAWAII. THE MODELS VARY SIGNIFICANTLY IN THE EVENTUAL STRUCTURE OF THIS DEVELOPING SYSTEM AND WHERE IT MOVES
INLAND ON SATURDAY. IT IS A GOOD BET THAT SATURDAY WILL BE WET AND WINDY OVER WESTERN WASHINGTON. THIS SYSTEM WILL NEED TO BE CAREFULLY MONITORED.

THE OFFSHORE LOW MOVES INLAND SOMEWHERE OVER THE PACIFIC NW ON SATURDAY BRINGING WET AND WINDY WEATHER TO THE AREA. POST FRONTAL SHOWERS DECREASE ON SUNDAY…THEN ANOTHER WARM FRONT AND LOW APPEAR TO BE ON TRACK TO MOVE ACROSS THE AREA MONDAY AND TUESDAY. MODEL DETAILS DIFFER…BUT IT APPEARS THAT NEXT WEEK WILL BE WET AND BREEZY WITH MOUNTAIN SNOW LEVELS DECREASING TO AT LEAST HIGHER PASS LEVEL MIDWEEK AND TEMPERATURES IN THE LOWLANDS COOLING OFF WHERE HIGHS STRUGGLE TO GET OUT OF THE LOWER TO MID 50S.

 

– JRH

 

References:

 

NWS Area Forecast Discussion
NWS Forecast Table Interface