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November 2014
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Rain Shadows

Posted: November 3rd, 2014

 

Coconut express?

Coconut Express: neither sushi (via Japan) nor pineapple (via Hawaii).

 

A strong storm currently centered over the Gulf of Alaska will provide our region with significant rainfall and wind today and tomorrow. However, due to the orientation of the system, not all locations will be affected equally. The Olympic Mountains are expected to shield the City and lowlands from heavy rainfall while the north-central Cascades get soaked.

 

After the atmospheric river lashes the Northwest early tomorrow morning, it’ll slide southward but leave behind scattered showers and mild temperatures through Wednesday. Some forecast models indicate that the “coconut express” will return and provide moderate-to-heavy showers and wind on Thursday, but we’ll have to watch it develop over the next few days.

 

Seattle remains on the wet side of the USGS landslide threshold. Fortunately, the forecast looks relatively dry compared to the past few weeks, and we may slip below the threshold by the end of the week.

 

Citywide, the NWS currently predicts 0.44″ today, 0.10″ tomorrow, 0.20″ on Wednesday, 0.35″ on Thursday, and 0.00″ on Friday. The latest UW WRF-GFS and ECMWF model runs are each a little drier throughout the week. Breezy 15-20 mph southerly winds are expected across the lowlands overnight tonight, and perhaps briefly on Thursday.

 

Over SPU’s mountain reservoirs, much more rainfall is expected today, particularly over the Tolt River Watershed where 3-5″ is forecast in the 24-hour period ending tomorrow at 4 AM. The Cedar River Watershed is looking at half those amounts due the orientation of the storm and rain shadowing. Winds should not be problematic throughout the forecast period, and snow levels should remain above 6000 through the weekend.

 

Excerpts from this morning’s NWS Area Forecast Discussion (emphasis added):

 

WARM FRONTAL RAIN HAS SPREAD INTO WESTERN WASHINGTON AND RADAR SHOWS IT AFFECTING ALL AREAS. RAIN WILL FALL AT TIMES TODAY THROUGH TUESDAY MORNING. GRADIENTS WILL TIGHTEN THIS AFTERNOON AND EVENING AS A COLD FRONT APPROACHES. THE WEATHER WILL GET BREEZY TONIGHT AND TUESDAY MORNING…WINDY NORTH AND COAST. THE FREEZING LEVEL ROSE ABOVE 7000 FEET…SO UNTIL THE COLD FRONT PASSES TUESDAY MOST OF THE PRECIPITATION IN THE MOUNTAINS WILL BE IN THE FORM OF RAIN…AT LEAST AT THE PASS LEVELS AND SKI AREAS.

A SURFACE RIDGE OF HIGH PRESSURE WILL BUILD TUESDAY AFTERNOON AND EVENING. ANOTHER WARM FRONT FROM A PARENT LOW FAR OUT TO SEA WILL AFFECT WESTERN WASHINGTON ON WEDNESDAY. RAIN WILL PROBABLY DEVELOP ON THE COAST IN THE MORNING AND SPREAD INLAND WEDNESDAY AFTERNOON OR EVENING. HIGH TEMPERATURES ALL THREE DAYS MONDAY THROUGH WEDNESDAY WILL BE 55-60 WITH LOWS WELL ABOVE FREEZING.

THE BIGGEST QUESTION REMAINS HOW THE VIGOROUS SURFACE LOW AFFECTING THE REGION ON THURSDAY WILL PLAY OUT. THE 00Z ECMWF AND CANADIAN AGREE THAT A STRONG SURFACE LOW WILL MOVE NE ACROSS CAPE FLATTERY AROUND MID-DAY THURSDAY. THE ECMWF HAS THE LOW AT 1001 MB AND THE CANADIAN AT 1004 MB. THE MAIN IMPACT WOULD BE STRONG PRESSURE RISES BEHIND THE LOW FOR POTENTIALLY STRONG S-SW WINDS COAST AND INTERIOR THURSDAY AFTERNOON. THE GFS IS JUST BEING KOOKY. THE 00Z RUN TOOK A 1004 MB LOW INLAND PAST CENTRAL VANCOUVER ISLAND BY 18Z THURSDAY…WHILE THE NEW 06Z GFS HAS THE LOW 330 NM SW OF CAPE FLATTERY AT 18Z THURSDAY…ABOUT A 500 NM DIFFERENCE. BUT GIVEN THAT THE 00Z ECMWF DID NOT LOOK AT ALL LIKE THE OLDER 12Z ECMWF… CONFIDENCE REMAINS VERY SHAKY.

BEYOND SHAKY THURSDAY…BOTH THE GFS AND ECMWF BUILD A BROAD LOW AMPLITUDE UPPER LEVEL RIDGE ALONG THE W COAST ON FRIDAY…DRYING THE AREA OUT FRIDAY AFTERNOON THROUGH AT LEAST SATURDAY. COULD EVEN BE THE WHOLE WEEKEND IF THE ECMWF BACKS OFF ON THE UPPER LEVEL TROUGH IT BRINGS OVER THE AREA ON SUNDAY.

 

 

Climate Notes:
 
October went a long way toward making 2014 both our warmest and wettest years on record. With a mean temperature of 58.0°F, October ended up being the warmest on record by an astonishing 5.2°. Most of that warmth was seen at night with abnormally high low temperatures, but average highs were up there, too.
 
October also ended up being the fourth wettest on record with 6.75″. It was our 8th wetter-than-normal-month of 2014, pushing the annual total above average at 38.87″. If November and December are simply average, we’d finish at number two on record.
 

– JRH
 

References:

 

NWS Area Forecast Discussion
NWS Forecast Table Interface
USGS Landslide Information