Avalanche in Washington, January 15, 2013

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE SEATTLE WA
350 PM PST TUE JAN 15 2013

.SYNOPSIS...HIGH PRESSURE ALOFT WILL PREVAIL UNTIL ABOUT NEXT
TUESDAY...WITH TEMPORARY FLATTENING OF THE RIDGE TAKING PLACE ON
FRIDAY. HIGH PRESSURE WILL LEAD TO DRY WEATHER AND EXTREMELY STABLE
CONDITIONS. AIR STAGNATION WILL LEAD TO THE POSSIBILITY OF
DETERIORATING AIR QUALITY.

&&

.SHORT TERM...A STRONG UPPER RIDGE AXIS IS LOCATED NEAR 130W THIS
MORNING. IT WILL CONTINUE TO EXPAND INTO WRN WA THROUGH WED MORNING.
A SHORTWAVE WILL MOVE THROUGH NRN B.C. AND THE YUKON ON THU NGT AND
FRI MORNING...LEADING TO A MERE FLATTENING OF THE STRONG RIDGE.
CONFIDENCE IS HIGH IN A DRY FORECAST THROUGH EARLY NEXT WEEK.

THE UPPER RIDGE HAS ALREADY BROUGHT WARMING TEMPS ALOFT. THE 12Z
KUIL SOUNDING SHOWED A FREEZING LEVEL OF 11000 FT MSL...AND
AVALANCHE SITES SUCH AS PARADISE AND MOUNT BAKER HAVE SEEN DRAMATIC
WARMING AND DRYING SINCE LAST NIGHT. PARADISE (ELEV 5400') IS NOW UP
TO 55 DEGREES... WHILE MOST LOWLAND SITES HAVE ONLY REACHED THE LOW
TO MID 40S THIS AFTN...EVIDENCE OF A VERY STRONG INVERSION. THE
STRONGEST PART OF THE SUBSIDENCE INVERSION IS NOW BTWN 3000 AND 4000
FT MSL. THIS WILL CONTINUE TO DROP TONIGHT AND WED...GETTING BELOW
1000 FT MSL DURING THE DAY ON WED. THE SUBSIDENCE INVERSION WILL
THEN CONTINUE AT OR BELOW 1000 FT MSL UNTIL FRI MORNING. IN
ADDITION...LOW-LEVEL WINDS WILL BE LIGHT (5 MPH OR LESS) THROUGH THU
EVNG. THIS LEAVES POLLUTANTS WITH NOWHERE TO GO...SO AIR QUALITY
ISSUES WILL TAKE THE SPOTLIGHT OVER THE NEXT FEW DAYS.

TEMPORARY FLATTENING OF THE UPPER RIDGE WILL SIGNIFICANTLY WEAKEN
INVERSIONS BY FRI AFTN...AND A LITTLE BREEZE (5-10 MPH) WILL DEVELOP
NEAR THE SURFACE. 925 MB WINDS WILL INCRS TO ABT 20 KTS...SO
IMPROVEMENT IN STAGNANT CONDS EXPECTED FRI.

TOUGHEST PART OF FORECAST WILL RELATE TO FOG AND LOW STRATUS
COVERAGE. ONCE THE STRONGEST INVERSION IS NEAR THE SURFACE OVER THE
LOWLANDS...LOW CLOUDS AND FOG WILL BE CONFINED TO ELEVATIONS BELOW
1000 FEET. WITHIN THAT REALM...THE COVERAGE OF STRATUS CAN MORPH AND
OOZE IN SOME INTERESTING BUT UNPREDICTABLE WAYS. WITH SUCH A STRONG
LOW-LEVEL INVERSION...FOG AND LOW STRATUS WILL NOT RELENT TO DAYTIME
HEATING ON WED AND THU. HAVE INCLUDED FOG AND FREEZING FOG FOR THE
LOWLAND FORECAST...BUT EXACT COVERAGE IS IN QUESTION...AND WONDERING
IF FOG MIGHT SIMPLY FROST OUT.

OTHERWISE...TEMPERATURES ARE PRONE TO LARGE FORECAST ERRORS IN THE
DAYS TO COME...DEPENDING LARGELY ON LOWLAND STRATUS COVERAGE AND THE
EXACT LEVEL OF THE WARM TONGUE ON FUTURE SOUNDINGS. IN THE MTNS...
UNSEASONABLY WARM TEMPS (ABOVE THE PASSES)...CLEAR SKIES...AND VERY
DRY AIR ARE OCCURRING AND EXPECTED TO CONTINUE.       HANER

.LONG TERM...AFTER FLATTENING ON FRI...THE UPPER RIDGE WILL REBOUND
AND PERSIST FROM SATURDAY THROUGH MONDAY...RENEWING STRONG LOW-LEVEL
SUBSIDENCE INVERSIONS AND POTENTIALLY STAGNANT CONDS...BUT OTHERWISE
KEEPING THE WEATHER DRY AND BENIGN. GFS AND ECMWF FINALLY SHOW A
MORE SUBSTANTIAL FLATTENING OF THE UPPER RIDGE AROUND TUE OR WED OF
NEXT WEEK WITH AN EVENTUAL RETURN TO MORE ZONAL FLOW BEYOND DAY 7.
HANER

&&

.HYDROLOGY...FLOODING IS NOT EXPECTED FOR THE NEXT 7 DAYS.  HANER

&&

.AVIATION...HIGH PRESSURE AT THE SURFACE AND ALOFT CENTERED OFFSHORE
WILL SHIFT INLAND ACROSS WESTERN WASHINGTON TONIGHT THROUGH
WEDNESDAY. EXPECT A VERY STABLE AND DRY AIR MASS WITH LOW LEVEL
MOISTURE INCREASINGLY SQUASHED INTO A 1000 FT THICK -OR LESS- LAYER
ABOVE THE SURFACE STARTING THIS EVENING.

MODELS CONTINUE TO SHOW STRONG SUBSIDENCE WARMING AND DRYING OF THE
AIR MASS ABOVE 030. MOUNTAIN OBSERVATIONS CONFIRM THE WARMING ALOFT
WITH TEMPERATURES AT 042 AT MT BAKER SPIKING AT 49F. CLOUDS WRAPPING
AROUND THE TERRAIN WILL GRADUALLY LOWER AND OTHER LOWER STRATUS
CLOUDS WILL DEVELOP AT THE BASE OF THE INVERSION THIS EVENING. ANY
PERSISTENT HOLES OVER THE INTERIOR WILL TEND TO ALLOW STRONG
RADIATIONAL COOLING AND THE FORMATION OF A DENSE FOG. FOG AND VERY
LOW STRATUS WILL TEND TO PERSIST WELL INTO THE DAY ON WED WITH ANY
TERRAIN ABOVE ABOUT 005 POPPING ABOVE THE CLOUDS LATE IN THE DAY
INTO THE EVENING AS THE INVERSION BECOMES VERY STRONG AND LOW. WINDS
ALOFT WILL BECOME QUITE LIGHT LATER TONIGHT INTO WEDNESDAY...AND THE
LIGHT WINDS AND VERY STABLE AIR WILL PREVENT MIXING.

AREAS TO THE SOUTH OF SEATTLE DOWN INTO THE SOUTH INTERIOR WILL BE
LEAST LIKELY TO BREAK ON WED. AREAS NORTH OF SEATTLE WILL TEND TO
BREAK DURING THE AFTERNOON HOURS. CONFIDENCE IN THE SEATTLE AREA IS
LOWER.
ALBRECHT

KSEA...THIN CLOUDS WITH BASES 025-030 WILL CONTINUE TO FADE AND
RETURN EARLY THIS EVENING...THEN WILL FORM AROUND THE LOWERING
INVERSION OVERNIGHT. BY ABOUT 08Z EXPECT FOG WITH VIS M1/4SM LIKELY
AT THE TERMINAL AT TIMES LATER TONIGHT THROUGH THE DAY WED. LIGHT NE
WIND LESS THAN 5 KT. ALBRECHT

&&

.MARINE...HIGH PRESSURE AND LIGHT GRADIENTS WITH AREAS OF FOG
INCREASING TONIGHT AND WED. EXPECT SEAS 6 FT OR LESS OVER THE
COASTAL WATERS THRU THU NIGHT.

SEAS WILL INCREASE TOWARD THE END OF THE WEEK AS SOME LONG PERIOD
SWELL ARRIVES AFTER A LONG TRIP ACROSS THE PACIFIC. THIS SWELL
DEVELOPED AROUND A DEEP LOW THAT WAS IN THE FAR WESTERN PACIFIC
EARLY IN THE WEEK. ALBRECHT

&&

.SEW WATCHES/WARNINGS/ADVISORIES...
WA...AIR STAGNATION ADVISORY UNTIL NOON ON FRIDAY FOR THE PUGET SOUND
      LOWLANDS FROM EVERETT SOUTH AND FOR LAND AREAS ADJACENT TO THE
      EASTERN STRAIT OF JUAN DE FUCA.
PZ...NONE.

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