Seattle.gov Home Page
Seattle Public Utilities Home page

Search Climate Change

December 2014
M T W T F S S
1234567
891011121314
15161718192021
22232425262728
293031  

Busy Week

Posted: December 10th, 2014

 

It’s been a busy week, and after all the watches, warnings and advisories expire it may end up being remembered not for flooding or wind, but for today’s high temperature. At 11 AM, Seatac hit 66°F, which is the warmest it’s ever been not only on this day, but during any month of December on record. But the warmth isn’t going to cause many problems, unlike what remains forecast.

 

The second of three storms this week is moving through at the moment. It has dropped just under an inch of rain across most of Seattle so far, and the its cold front is still a few hours away. Another half-inch is expected through this evening, perhaps briefly intense when the front passes. The latest High Resolution Rapid Refresh model run has that occurring at about 5 PM, followed by partial clearing and scattered showers.

 

The main event of the week, however, remains tomorrow’s windstorm, which now looks like it will hit tomorrow evening instead of during the day. Forecast models and those of us who interpret them have struggled to agree on just how strong the event will be, but damaging winds (falling trees, power outages) across a large area are certain to occur. That’s especially true given how much is has rained, a fact underscored by the increasing risk of landslides—the next few hours of rain will push us beyond the USGS threshold.

 

From the NWS High Wind Watch

* TIMING…HIGH WINDS ARE NOW POSSIBLE FROM LATE THURSDAY
AFTERNOON THROUGH LATE THURSDAY NIGHT.

* WIND…SOUTH TO SOUTHWEST WINDS OF 30 TO 40 MPH WITH GUSTS TO 65
MPH COULD DEVELOP…INCLUDING IN THE PUGET SOUND I-5 URBAN
CORRIDOR. IN THE MOUNTAINS…GUSTS TO OVER 75 MPH ARE
POSSIBLE…EXCLUDING THE MOUNTAIN PASSES WHICH MAY REMAIN
SOMEWHAT SHELTERED.

* IMPACTS…WINDS THIS STRONG CAN TOPPLE LARGE TREES..
ESPECIALLY GIVEN THE SATURATED SOILS FROM RECENT RAINFALL.
POWER OUTAGES ARE LIKELY TO OCCUR AND MAY BE WIDESPREAD.

 

Fortunately, the windstorm is not expected to be accompanied by much rainfall. However instability associated with deep low pressure systems often breeds high-intensity, short-duration squalls, particularly on the backside, so the radar should not be ignored late tomorrow.

 

Whenever we have windstorms, we must not forget to monitor tides for coastal flooding. Tomorrow’s 12.02’MLLW/9.67’NAVD88 high-but-not-King tide is predicted to occur at 8:32 AM. That’s on the lower side of recent tides that featured waves dramatically crashing into Alki. The windstorm is currently expected to pass well after that 12.02′ mark, however timing is among the most uncertain aspects of the forecast.

 

Sea-level pressure forecasts for the storm as it passes Seattle have ranged from 994mb to as low as 979mb, the latter of which would add over a foot the water level due to the inverted barometer effect. Timing and direction of wind, especially over 10-15 knots sustained, could add another 1-2 feet to water levels. It should also be noted that, save for a few isolated locations near drainage inlets and outfalls, predicted rainfall amounts would contribute neither to water levels nor flooding.

 

Again, tomorrow’s predicted water level is a good place to start, but if a major wind storm verifies and it roughly coincides with lower-high tide at 7:30PM, or even with Friday morning’s 9 AM tide, property flooding and street ponding is likely to occur, especially along Beach Drive SW and South Alki Point.

 

From the NWS Coastal Flood Watch

* IMPACTS…TIDAL OVERFLOW MAY PRODUCE MINOR FLOODING IN LOW LYING
AREAS AROUND THE INLAND WATERS AROUND HIGH TIDE THURSDAY
MORNING.

* TIME OF HIGH TIDE…9 TO 10 AM THURSDAY MORNING.

* PREDICTED TIDE…AT SEATTLE ABOUT 13.5 FT…WHICH IS WHERE MINOR
TIDAL OVERFLOW NORMALLY BEGINS. OTHER LOCATIONS ALONG THE SHORES
OF THE INLAND WATERS WILL SEE TIDAL ANOMALIES RANGING FROM 10 TO
11 FT…VERY CLOSE TO WHERE MINOR TIDAL OVERFLOW BEGINS FOR MOST
OF THOSE LOCATIONS.

 

Light, scattered showers are expected to linger on Friday. Saturday and most of Sunday look nice and quiet. We need it.

 

– JRH

 

NWS Area Forecast Discussion
NWS Forecast Table Interface—Seattle
NWS Forecast Table Interface—Tolt Reservoir
NWS Forecast Table Interface—Chester Morse Lake