Heavy Snow in Washington, November 15 to 16, 2013

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE SEATTLE WA
355 AM PST FRI NOV 15 2013

.SYNOPSIS...A STRONG WEATHER SYSTEM MOVING DOWN THE BC COAST THIS
MORNING WILL MOVE THROUGH THE AREA THIS EVENING. GOOD OROGRAPHIC
FLOW BEHIND THE FRONT WILL KEEP SNOW GOING IN THE MOUNTAINS TONIGHT.
SATURDAY...NORTHWEST FLOW ALOFT AND AN UNSTABLE AIR MASS WILL
SUPPORT LINGERING SHOWERS. A WEAK WARM FRONT ON SUNDAY WILL BRING
CLOUDS AND LIGHT PRECIPITATION. A FRONTAL ZONE WILL SETTLE OVER
WESTERN WASHINGTON ON MONDAY AND TUESDAY.

&&

.SHORT TERM...SATELLITE IMAGERY SHOWS A VIGOROUS COLD FRONT ALONG
THE CENTRAL BRITISH COLUMBIA COAST THIS MORNING. PLENTY OF CLOUDS
WELL AHEAD OF THE FRONT BUT THE PRECIPITATION IS STILL WELL TO THE
NORTH WITH RAIN JUST BEGINNING AT PORT HARDY AT 11Z. TEMPERATURES
UNDER CLOUDY SKIES WERE IN THE UPPER 30S TO MID 40S AT 11Z.

LITTLE CHANGE IN THE FORECAST THIS MORNING WITH THE FRONT MOVING
SOUTHEAST TODAY. WINDS PICKING UP AHEAD OF THE FRONT AS
SURFACE PRESSURES LOWER TO THE NORTH CREATING A SIGNIFICANT SOUTH TO
NORTH GRADIENT THIS AFTERNOON. BOTH THE NAM AND THE GFS HAVE THE
KOLM-KBLI GRADIENT PLUS 6 MB OR GREATER BY 00Z THIS AFTERNOON.
GRADIENT THROUGH THE STRAIT WILL ALSO BE PICKING UP THIS AFTERNOON
WITH THE KUIL-KBLI GRADIENT IN THE PLUS 2 TO 3 MB RANGE BY 00Z AS
WELL. THIS WILL MINIMIZE THE SOUTHERLY WINDS OVER THE INTERIOR. FOR
NOW WILL GO WITH WINDS JUST BELOW ADVISORY CRITERIA...IN GENERAL 15
TO 30 MPH WITH LOCAL GUSTS TO 40 MPH. RAIN BEGINNING LATER THIS
MORNING OVER THE LOWLANDS WITH SNOW IN THE MOUNTAINS. SNOW LEVEL IS
ALREADY DOWN TO AROUND 3500 FEET AND WILL DROP BELOW 3000 FEET THIS
MORNING. OROGRAPHIC FLOW PICKING UP THIS AFTERNOON WITH THE MODEL
850 MB WINDS INCREASING TO WESTERLY 25 TO 40 KNOTS WITH THE
STRONGEST WINDS AIMED AT STEVENS PASS. SNOWFALL RATES WILL PICK UP
SIGNIFICANTLY THIS AFTERNOON WITH UP TO 10 INCHES OF NEW SNOW
POSSIBLE BY LATE AFTERNOON.

FRONT MOVING THROUGH THE AREA THIS EVENING WITH THE PRECIPITATION
OVER THE LOWLANDS CHANGING TO SHOWERS. STRONG OROGRAPHIC FLOW
CONTINUING IN THE CASCADES INTO THE EARLY MORNING HOURS WITH 850 MB
WINDS REMAINING IN THE 25 TO 40 KNOT RANGE OUT OF THE WEST KEEPING
THE WINTER STORM WARNING GOING FOR THE NORTH AND CENTRAL CASCADES
INTO THE EARLY MORNING HOURS ON SATURDAY. UP TO ANOTHER FOOT OF NEW
SNOW IS POSSIBLE. EXPECT THE HIGHEST SNOWFALL AMOUNTS NEAR STEVENS
PASS WITH A CONVERGENCE ZONE SETTING UP TO ENHANCE THE SNOW AMOUNTS.
SURFACE PRESSURE GRADIENTS EASING OVER THE AREA AS THE SURFACE LOW
MOVES INTO EASTERN WASHINGTON.

NORTHWEST FLOW ALOFT OVER WESTERN WASHINGTON ON SATURDAY WITH THE
AIR MASS SOMEWHAT UNSTABLE. THIS WILL KEEP A CHANCE OF SHOWERS IN
THE FORECAST FOR THE LOWLANDS BUT A GOOD PORTION OF THE AREA WILL BE
DRY FOR MOST OF THE DAY. FLOW ALOFT WEAKENS AS WELL WITH 850 MB
WINDS DOWN TO THE 10 TO 25 KNOT RANGE BY SATURDAY AFTERNOON. THE
COMBINATION OF THE DRYING OF THE AIR MASS AND THE WEAKENING OF THE
FLOW ALOFT WILL RESULT IN MUCH LESS SNOW FOR THE CASCADES ON
SATURDAY. THE WINTER STORM WARNING ENDS AT 6 AM SATURDAY. THIS STILL
LOOKS GOOD WITH ONLY AN ADDITION 2 TO 4 INCHES OF SNOW EXPECTED IN
THE CASCADES DURING THE DAY ON SATURDAY. COOLER AIR MASS MOVING INTO
THE AREA WITH THE SNOW LEVEL DOWN TO NEAR 1500 FEET ON SATURDAY.
HIGHS ONLY IN THE MID 40S WILL BE COMMON IN THE LOWLANDS.

A BIT OF A TRANSITION ON SATURDAY NIGHT AS A WEAK WARM FRONT
DEVELOPS OFF THE COAST WITH THE WARM FRONT MOVING INTO WESTERN
WASHINGTON ON SUNDAY. 500 MB HEIGHTS RISING FROM THE 530S DM 00Z
SUNDAY TO THE MID 540S TO LOWER 550S DM RANGE BY 00Z MONDAY. THE
PARENT LOW FOR THE WARM FRONT IS WELL TO THE NORTH...NEAR HAIDA
GWAII AND NOT VERY DEEP. NOT A LOT OF DYNAMICS WITH THIS FEATURE
WITH THE JET WELL TO THE NORTH. FOR NOW WILL STAY WITH THE CHANCE
POPS SATURDAY NIGHT INTO SUNDAY.

.LONG TERM...EXTENDED MODELS HAVING TROUBLE COMING TOGETHER ON A
SOLUTION FOR MONDAY WITH THE ECMWF BRINGING ANOTHER SYSTEM INTO THE
AREA WHILE THE SLOWER GFS HOLDS THE FRONT OFFSHORE WITH A DRY DAY ON
MONDAY. BOTH THE GFS AND ECMWF BRING THE FRONT THROUGH THE AREA ON
TUESDAY BUT THERE ARE BIG DIFFERENCES IN THE SURFACE PATTERN WITH
THE GFS AROUND 10 MBS DEEPER WITH THE PARENT LOW. THERE IS A
LITTLE BETTER AGREEMENT ON DAY 6 AND 7 WITH BOTH OF THE MODELS
DRYING THINGS OUT FOR A BIT. THE LACK OF AGREEMENT EARLY ON IN THE
EXTENDED DOESN'T MAKE FOR A LOT OF CONFIDENCE IN THE FURTHER OUT
SOLUTION SO FOR NOW WILL NOT MAKE ANY CHANGES TO THE EXTENDED AND
STAY WITH THE CHANCE POPS FOR WEDNESDAY AND THURSDAY AND KEEP THE
LIKELY POPS FOR MONDAY THROUGH TUESDAY NIGHT. FELTON

&&

.HYDROLOGY...SYSTEM MOVING THROUGH THE AREA TODAY INTO TONIGHT WILL
NOT PROVIDE HYDROLOGICALLY SIGNIFICANT PRECIPITATION TO THE AREA
WITH SNOW IN THE MOUNTAINS. IF THE ECMWF SOLUTION OF A MORE
PROLONGED PERIOD OF RAIN MONDAY THROUGH TUESDAY ENDS UP VERIFYING
THEN THERE IS A CHANCE THAT THE MORE FLOOD PRONE RIVERS COULD BECOME
A CONCERN BY TUESDAY. THE GFS SOLUTION IS MUCH LESS THREATENING.
CONFIDENCE IN THE EXTENDED MODELS IS NOT VERY HIGH AT THIS POINT.
FELTON

&&

.AVIATION...A STRONG DISTURBANCE WILL CROSS WRN WA TODAY IN STRONG
NW FLOW ALOFT. A MOIST AIR MASS AND INCREASING LARGE-SCALE LIFT WILL
CAUSE RAIN AND HEAVY MTN SNOW TO BLOSSOM OVER WRN WA THIS MORNING.
HOWEVER...W/NW FLOW ALOFT WILL SUPPORT RAIN SHADOWING DOWNWIND OF
THE OLYMPIC MTNS IN PLACES LIKE THE KITSAP PENINSULA AND SEATTLE.
STEADY RAIN WILL END EARLY THIS EVNG...BUT THE AIR MASS WILL BECOME
MORE UNSTABLE...AND ONSHORE FLOW WILL SUPPORT A MORE SHOWERY PATTERN
FOR MOST OF TONIGHT. IFR CIGS WILL BE COMMON TODAY WITH MODERATE TO
OCCASIONALLY HEAVY RAIN AND LARGE-SCALE LIFT. CIGS LIFTING TO MVFR
OR BETTER THIS EVNG AS WE TRANSITION TO THE SHOWERY PATTERN
FOLLOWING A COLD FROPA.

KSEA...IFR CIGS IN THE 010-020 RANGE WILL BE COMMON TODAY...
OCCASIONALLY LOWER IF RAIN GETS HEAVY ENOUGH. HOWEVER...THE TERMINAL
WILL BE IN OR NEAR A DOWNWIND RAIN SHADOW FROM THE OLYMPIC MTNS IN
W/NW FLOW ALOFT. RELATIVELY LIGHT RAIN AT KSEA WILL HELP KEEP VSBYS
AOB 6SM FOR MOST OF THE DAY. OTHERWISE...SW SFC WIND INCREASING
TODAY. PEAK WIND WILL COME WITH A COLD FROPA ARND 03Z SAT...WITH
GUST POTENTIAL OF 30-35 KT. BEHIND THE FRONT...CIGS WILL LIFT INTO
THE 025-040 RANGE WITH A FEW CONVECTIVE SHOWERS TONIGHT. A DRIER AIR
MASS ON SAT WILL CAUSE SHOWER ACTIVITY TO DECREASE WITH MORE BREAKS
DEVELOPING IN THE CLOUD COVER.     HANER

&&

.MARINE...A 998 MB SFC LOW WAS CENTERED JUST SOUTH OF JUNEAU ALASKA
EARLY THIS MORNING. IT WILL MOVE SE INTO THE INTERIOR OF B.C. AND
STRENGTHEN TO 989 MB TODAY. THE 989 MB LOW WILL CLIP ACROSS NE WA
EARLY SAT MRNG ON ITS WAY TO THE NRN U.S. ROCKIES. WITH SUCH LOW
PRES EAST OF THE CASCADES...ONSHORE FLOW WILL STRENGTHEN FROM NOW
THROUGH THIS EVNG. THE 06Z NAM MODEL PEAKS THE UIL-BLI GRADIENT NEAR
+5 MB THIS EVNG...AND THE 06Z GFS PEAKS THE SAME GRADIENT AROUND +4
MB. W-NW 850 MB WINDS WILL ALSO LINE UP NICELY WITH THE ORIENTATION
OF THE STRAIT AS COLD ADVECTION GETS UNDERWAY THIS EVNG. THE 00Z UW
MM5-ETA SHOWS WINDS AS HIGH AS 36 KT AT THE EAST END OF THE STRAIT
AT 7 PM THIS EVNG...AND THE UW WRF-GFS SHOWS WINDS REACHING 32 KT.
GIVEN THESE FACTORS...HAVE INCREASED WEST WIND THROUGH THE STRAIT
ENOUGH TO HOIST GALE WARNINGS. STRONGEST WINDS WILL ARRIVE EARLY
THIS EVNG. SHOULD ALSO BE GALE-FORCE NORTHWESTERLIES DOWN THE STRAIT
OF GEORGIA WHICH WILL AFFECT THE AREA FROM POINT ROBERTS TO CHERRY
POINT TONIGHT.

ON SAT...A SFC TROUGH WILL DEVELOP ALONG THE SW SIDE OF VANCOUVER
ISLAND...EXTENDING THROUGH THE STRAIT TO WHIDBEY ISLAND. SOUTH OF
THE TROUGH AXIS...MODERATE ONSHORE FLOW WILL CONTINUE. NORTH OF THE
TROUGH AXIS...LIGHT NLY OR NELY WINDS WILL PREVAIL.

OTHERWISE...FRESH SWELL WILL KICK UP TO NEAR 20 FT OVER THE COASTAL
WATERS ON SAT MRNG IN RESPONSE TO TONIGHTS NWLY GALES.      HANER

&&

.SEW WATCHES/WARNINGS/ADVISORIES...
WA...WINTER STORM WARNING FOR THE CASCADES FROM 4 AM FRI THROUGH
     10 AM SAT.
PZ...GALE WARNING ALL WATERS EXCEPT PUGET SOUND AND HOOD CANAL.
     SMALL CRAFT ADVISORY FOR PUGET SOUND AND HOOD CANAL.

&&

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