INCREASED POTENTIAL FOR DEBRIS FLOWS IN CASCADES, COAST RANGE

Posted: June 1st, 2010 10:19 PM

THE NATIONAL WEATHER SERVICE HAS ISSUED A SPECIAL WEATHER STATEMENT ADVISING RESIDENTS AND TRAVELERS IN THE COAST RANGE AND THE CASCADES OF HEAVY RAIN FOR THE NEXT 2 DAYS. THE HEAVY RAIN ON SNOW WILL CAUSE THE INCREASED POTENTIAL FOR DEBRIS FLOWS FROM THE LARGE AMOUNTS OF RAIN.

DEBRIS FLOWS ARE DANGEROUS, RAPIDLY MOVING LANDSLIDES. STEEP SLOPES, CANYONS, GORGES AND THE MOUTHS OF MOUNTAIN STREAMS ARE THE LOCATIONS AT GREATEST RISK. PERSONS THAT LIVE OR MAY TRAVEL THROUGH THESE LOCATIONS SHOULD BE ALERT TO THE POSSIBILITY OF DEBRIS FLOWS DURING OR SHORTLY AFTER PERIODS OF INTENSE RAINFALL.

Care should be taken when traveling over the mountains during this time. The most dangerous places include:
• Canyon bottoms, stream channels, and areas of rock and soil accumulation at the outlets of canyons;
• Bases of steep hillsides;
• Road cuts or other areas where slopes of hills have been excavated or over steepened;
• Places where slides or debris flows have occurred in the past.

"Debris flows are rapidly moving landslides that can destroy everything in their paths," said James Roddey, Earth Science Information Officer for the Oregon Department of Geology and Mineral Industries (DOGAMI). "They can easily travel a mile or more, depending on the terrain. They'll contain boulders and logs and transport those in a fast-moving soil and water slurry."

"People want to know what they should do when they hear about the potential for debris flows and landslides," said Roddey. "Some areas are more hazardous than others when the danger of landslides is high. People know that if there's a flood warning, they should stay away from the river. We also want them to start thinking about staying away from steep slopes during intense rainstorms. Knowing ahead of time where the danger areas around your home for potential landslides might be is the first step in being prepared," explains Roddey.

Roddey recommends several steps:
• Stay alert. Listen to the radio, TV, or a weather radio for flood watches, which include the potential for debris flows and if told to evacuate, do so immediately;

• Listen for unusual sounds that might indicate moving debris, such as trees cracking or boulders knocking together. A trickle of falling mud or debris may precede larger landslides;

• If you think there is danger of a landslide, leave immediately;

• If water in a river or stream suddenly turns muddy or the amount of water flowing suddenly decreases or increases, this is a warning that the flow has been affected upstream. You should immediately leave the area because a debris flow may soon be coming downstream;

• Assume highways are not safe. Be alert when driving, especially at night. Don't overdrive your headlights. Embankments along roadsides may fail, sending rock and debris onto the road;

• Landowners and road managers should check road drainage systems and conduct needed maintenance in case the predicted heavy precipitation does occur.

Cleaning up after landslides can also be hazardous. "When it's this wet outside, people need to be careful when they're cleaning up the mess. A small mudslide can actually be part of a larger landslide," explains Roddey. "Cleanup should not be done until after the storm."

The official statement from the National Weather Service can be found at: http://www.weather.gov/alerts/or.html

Updated weather

This afternoon/evening, Dec 11, Friday, it will , begin with snow and transition to freezing rain and rain by late Saturday and Sunday as it moves north. The main impacted areas are the Willamette Valley areas and the Gorge. The coast from south to north from today thru Sunday will experience freezing rain to snow. Coast range could get up to 8 inches on the east slopes. Storm should start to hit the Central Willamette Valley around 10 pm today, transition from snow to freezing rain and to rain by Sunday. The gorge could get 3-10 inches for snow with a transition to freezing rain by Saturday afternoon. Next week should have higher temperatures and rain. There is still some uncertainty with the amounts and types of precipitation.

Winter Weather Advisory

..WINTER WEATHER ADVISORY FOR SNOW IN EFFECT FROM 10 PM THIS
EVENING TO 4 AM PST SUNDAY FOR THE CASCADES OF SOUTH
WASHINGTON...NORTH OREGON AND LANE COUNTY...

A STRONG PACIFIC FRONTAL SYSTEM ALONG THE COAST WILL TRACK INLAND THIS
EVENING BRINGING WET AND WINDY CONDITIONS TO THE CASCADES FOLLOWED
BY RAPIDLY FALLING SNOW LEVELS. AS PRECIPITATION RATES INCREASE
THIS EVENING SNOW LEVELS INITIALLY RANGING BETWEEN 6000 AND 7000 FT
WILL FALL TO 3500 FT NORTH AND 4500 FT SOUTH THROUGH FRIDAY
MORNING. 6 TO 10 INCHES OF NEW SNOW ARE EXPECTED TO FALL OVERNIGHT.

STRONG ONSHORE FLOW COMBINED WITH COLD UNSTABLE AIR ALOFT WILL
ENHANCE POST FRONTAL SNOW SHOWERS FRIDAY THROUGH SATURDAY NIGHT
WITH SNOW LEVELS BETWEEN 3000 AND 4000 FT. SEVERAL SHORTWAVE
TROUGHS ROTATING INLAND OVER THE NEXT COUPLE OF DAYS WILL INCREASE
SNOW FALL RATES AT TIMES BUT ON AVERAGE SNOW FALL RATES ARE
EXPECTED TO RANGE BETWEEN 4 TO 8 INCHES OF SNOW PER 12 HOUR PERIOD
BRINGING STORM TOTALS OF 2 TO 3 FEET OF NEW SNOW THROUGH SATURDAY
NIGHT.

..HIGH WIND WARNING REMAINS IN EFFECT UNTIL 9 PM PST THIS EVENING FOR THE
NORTH AND CENTRAL OREGON COAST...

A STRONG COLD FRONT IS MOVING TOWARDS THE COAST THIS AFTERNOON AND WILL
MOVE ONSHORE AROUND 9 PM THIS EVENING. AS THE FRONT APPROACHES...SOUTH
WINDS OF 30 TO 40 MPH WITH GUSTS UP TO 65 MPH WILL OCCUR.

AFTER THE FRONT MOVES ONSHORE TONIGHT...WINDS WILL DECREASE FROM THE NORTH
TO SOUTH.

A COASTAL FLOOD ADVISORY REMAINS IN EFFECT UNTIL 4 PM PST THIS AFTERNOON.

A HIGH SURF ADVISORY REMAINS IN EFFECT FROM 10 PM THIS EVENING TO 4 PM PST
SATURDAY.

LARGE SWELL GENERATED BY A STORM IN THE GULF OF ALASKA WILL BUILD TONIGHT
THROUGH FRIDAY AND CONTINUE THROUGH SATURDAY...BRINGING HIGH SURF
CONDITIONS TO THE COAST. THE LARGEST SEAS ARE EXPECTED TO ARRIVE FRIDAY
AFTERNOON. HAZARDS FROM HIGH SURF CONDITIONS ALONG THE SHORELINE WILL BE
ENHANCED DURING THE HIGH TIDES AROUND 3 TO 4 PM IN THE AFTERNOON FRIDAY
AND SATURDAY.

HIGH TIDE OF 10.3 FT AND AN ANOMALY OF AROUND 1 FOOT WILL BRING THE HIGH
TIDE LEVEL TO AROUND 11.1 FT THIS AFTERNOON. AT THIS LEVEL...MINOR
FLOODING IS EXPECTED IN AND NEAR THE TOWN OF RAYMOND.

TIDES WILL BE LOWER AT SUBSEQUENT HIGH TIDES. CONSULT LOCAL TIDE TABLES
FOR TIMES AND HEIGHTS OF TIDES IN YOUR AREA.

A HIGH SURF ADVISORY MEANS THAT HIGH SURF WILL AFFECT BEACHES IN THE
ADVISORY AREA...PRODUCING RIP CURRENTS AND LOCALIZED BEACH EROSION.
EXERCISE EXTREME CAUTION ALONG BEACHES. UNPREDICTABLE AND DESTRUCTIVE
WAVES MAY WASH OVER BEACHES...JETTIES AND OTHER STRUCTURES WITH NO
WARNING. BEACH EROSION IS POSSIBLE. THIS IS NOT A TIME TO BE NEAR THE
WATER.

SPECIAL WEATHER STATEMENT

NATIONAL WEATHER SERVICE PORTLAND OR
342 AM PST WED NOV 4 2009
ORZ001>004-WAZ020-021-050500-
NORTH OREGON COAST-CENTRAL OREGON COAST-
COAST RANGE OF NORTHWEST OREGON-
CENTRAL COAST RANGE OF WESTERN OREGON-WILLAPA HILLS-
SOUTH WASHINGTON COAST-
INCLUDING THE CITIES OF...ASTORIA...CANNON BEACH...TILLAMOOK...
LINCOLN CITY...NEWPORT...FLORENCE...VERNONIA...JEWELL...TRASK...
GRANDE RONDE...TIDEWATER...SWISSHOME...FRANCES...RYDERWOOD...
RAYMOND...LONG BEACH...CATHLAMET
342 AM PST WED NOV 4 2009


...WIND...RAIN...AND HEAVY SEAS TO ARRIVE LATER THURSDAY...
A STRONG LOW PRESSURE SYSTEM WILL RAPIDLY STRENGTHEN INTO A
POWERFUL STORM TODAY AND THURSDAY. WHILE THE CENTER OF THIS LOW
PRESSURE SYSTEM IS EXPECTED TO REMAIN IN THE GULF OF ALASKA...IT
WILL PUSH A STRONG COLD FRONT ONSHORE INTO THE PACIFIC NORTHWEST
THURSDAY.

WINDS ASSOCIATED WITH THE FRONT MAY GUST AS HIGH AS 50 TO 60 MPH
NEAR THE BEACHES AND COASTAL HEADLANDS LATER THURSDAY...AS WELL AS
ACROSS THE HIGHER TERRAIN OF THE COAST RANGE. THE FRONT WILL
LIKELY BE ACCOMPANIED BY A PERIOD OF HEAVY RAIN...WHERE WHERE 1 TO
3 INCHES OR RAIN ARE POSSIBLE THURSDAY AND THURSDAY NIGHT. RAIN
CONTINUES AT TIMES INTO SATURDAY...WITH AN ADDITIONAL 1 TO 2
INCHES OF RAIN EXPECTED.

ANOTHER SIGNIFICANT ASPECT OF THE GULF OF ALASKA STORM SYSTEM WILL
BE THE VERY LARGE OCEAN SWELL EXPECTED TO DEVELOP AS THE STORM
STRENGTHENS. THIS SWELL MAY BEGIN IMPACTING THE SOUTH WASHINGTON
AND NORTH OREGON COAST WITH HIGH SURF AS EARLY AS THURSDAY
MORNING...AS SWELL OF 25 TO 30 FEET WILL CREATE CHAOTIC SURF ZONE
THURSDAY NIGHT THROUGH SATURDAY. THE HEAVY SURF MAY PROVIDE THE
POSSIBILITY FOR MINOR COASTAL FLOODING DURING HIGH TIDE...AND THE
LARGE BREAKERS MAY PRODUCE HAZARDOUS CONDITIONS ALONG THE BEACHES.