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February 2015
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Major AR

Posted: February 4th, 2015

 

3-day rainfall forecast through Saturday afternoon.

 

 

A major atmospheric river (AR) is about to hit the west coast. It’s expected to hit Northern California most directly, but waves will periodically affect the Northwest tonight all the way through Tuesday, according to the latest model forecasts. Seattle’s National Weather Service office has thus far only issued a Hydrologic Outlook for the City and the Cascades, but with forecast 5-day, 12-inch precipitation bullseyes nearby, flood watches and warnings are likely to follow.

 

ARs are often viewed by those of us who manage water resources as double-edged swords. They can if not bust, then dent droughts, and they can also lead to flooding, both river and urban. Hopes are high that the forecast storm will fill some of California’s dangerously empty reservoirs; high also are flooding fears there. Here in the Northwest, despite scant snow, we’re holding on to plenty of water and now must prepare for too much. That said, local forecasts have been fairly consistent and amounts have remained below intense levels.

 

The incoming storm may also be historical with respect to research. Weather Underground’s Bob Henson pointed out this will likely be the most intensely watched AR in weather history, mostly due to an ambitious study known as CalWater. SPU will take a close look to see if any of the study’s results can applied to our environment. We’re eager to improve forecasting, of course, but we’re also interested in learning more about the nature of ARs, for example, exactly how they behave when they interact with our mountains. And then there’s the question of what they’ll do as the planet warms.

 

SPU has participated in and led a series of climate change studies over the last decade, some of which have shed a little bit of light on the future of ARs in the Northwest. Certainty with respect to local precipitation change has long been notoriously difficult to pin down, especially relative to other climate impacts, but advances have been made recently. Improved modeling efforts (for example, by UW’s Mike Warner) have shown that water vapor, the key AR ingredient, will increase and eventually spread northward toward our region allowing us to more confidently state that extreme rainfall is likely to increase. Again, though, how intense events will be, if they’ll be more frequent, where shadowing will tend to occur, among other questions will continue to be asked by SPU, for the double-edged sword is indeed sharp.

 

Atmospheric Rivers
WU/Bob Henson
CalWater
Mike Warner

 


 

Latest waver vapor imagery, pineapples lining up over Hawaii.

 

The latest NWS precipitation forecast for Seattle shows just over 3 inches falling between tonight and Monday. While only moderate-to-heavy for at most 12-hours at a time, the most intense periods look to occur overnight tonight, Friday morning, Saturday morning and Sunday night. Given the instability, high-intensity, short-duration showers will remain a concern, perhaps on Saturday and again on Monday.

 

In the Cascades around 5 inches is expected to fall also between tonight and Monday. The current heaviest 24-hour forecast total (1-2″) looks to occur through Friday morning. Winds look to be brisk throughout the period, mostly out of the south, with the exception of Sunday when 20-30mph southeasterlies are forecast. Freezing levels are expected to stay between about 5000-7000 feet, frustratingly.

 

The NWS Hydrologic Outlook is worth quoting in full…

…FIVE DAYS OF INTERMITTENT HEAVY RAIN COULD CAUSE FLOODING LATE THIS WEEK…

THERE WILL BE HEAVY RAIN AT TIMES THIS WEEK THURSDAY THROUGH MONDAY MORNING. THE FIRST TWO STRONG WAVES…MAINLY THURSDAY AND THURSDAY NIGHT…COULD BRING ENOUGH RAIN TO THE OLYMPICS TO CAUSE MINOR FLOODING ON THE SKOKOMISH RIVER IN MASON COUNTY. A FLOOD WATCH FOR THAT RIVER IS IN EFFECT FROM THURSDAY MORNING THROUGH FRIDAY AFTERNOON.

THE RAIN KEEPS COMING IN A SERIES OF WAVES…WHICH ALSO IMPLIES THERE ARE BREAKS IN THE HEAVY RAIN. THAT MAY BE THE SAVING GRACE OF WHAT WILL CERTAINLY BE A WET WEEK. THE 120-HOUR TOTAL…5 DAYS OF RAIN…SHOWS 10-13 INCH BULLS EYES OF HEAVY RAIN IN THE MOUNTAINS AND 1-4 INCHES IN THE LOWLANDS. THE SNOW LEVEL WILL BE ABOVE 5000 FEET THE ENTIRE TIME…OCCASIONALLY RISING TO 7000 FEET OR HIGHER.

IF THAT AMOUNT OF RAIN FELL IN 48 HOURS OR LESS THERE WOULD ALMOST CERTAINLY BE WIDESPREAD MAJOR FLOODING ON MANY RIVERS AS WELL AS SOME URBAN AND SMALL STREAM FLOODING. SINCE THE RAIN IS SPREAD OUT OVER FIVE DAYS RIVERS WILL RISE…POSSIBLY TO FLOOD STAGE…BUT MAJOR FLOODING DOES NOT SEEM LIKELY.

RIVERS FLOWING OFF BOTH THE CASCADES AND OLYMPICS COULD FLOOD. THERE COULD ALSO BE FLOODING ON RIVERS THAT DO NOT FLOW OUT OF THE MOUNTAINS SUCH AS THE CHEHALIS AND ITS TRIBUTARIES. LOWLAND RAIN COULD BE HEAVY ENOUGH AT TIMES TO CAUSE MINOR FLOODING PROBLEMS NEAR SMALL STREAMS.

 

After this series of low pressure waves moves through, it looks like high pressure will return to power. In some ways, it’s as if this outbreak is the opposite of that which occurred around Thanksgiving. Recall that a southward-migrating jet stream brought with it a wavy AR and significant rainfall on November 23rd and 28th. Perhaps the jet is returning and Spring begins in earnest later next week? That’s a different double-edged sword.

 

NWS Area Forecast Discussion
NWS Forecast Table Interface—Seattle
NWS Forecast Table Interface—Tolt Reservoir
NWS Forecast Table Interface—Chester Morse Lake
SPU Water Supply Outlook

 

JRH