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October 2014
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Weekend Wx Outlook

Posted: October 8th, 2014

 

Atmospheric river (water vapor) modeled by UW WRF-GFS

Atmospheric river (water vapor) modeled by UW WRF-GFS

 
Our little summer reprise is coming to an end. At least through early next week… After one more day of morning fog and afternoon sun, the pattern will shift and set the stage for the region’s second atmospheric river of the season.

 

The prelude will occur on Friday and Saturday when a weak system will provide the area with scattered light rain. Forecast models continue to bicker over the details, but by late on Sunday a warm front should pass through and provide a round of moderate-to-heavy rainfall, along with gusty southerly winds and high humidity. The bulk of the precipitation, heavy at times, will follow early Monday morning. Post-frontal showers look to continue through at least Wednesday.

 

As currently forecast, this storm should not be too problematic with respect to SPU’s systems. Predicted rainfall amounts look similar to those that fell three weeks prior, which Seattle weathered well. One difference between then and now, however, is that those kind-of-pretty-but-actually-annoying leaves are getting ready to drop, which can lead to drainage impacts. We’re still a good 2-3 weeks away from peak leaf fall, but it’ll be windy overnight Sunday and therefore somewhat messy on Monday morning.

 

Citywide, the NWS currently predicts 0.13″ Fri-Sat, followed by 1.61″ Sun-Mon. The most intense rainfall (0.80″) looks to occur during the 12-hour period ending 5 AM Mon. Again, plenty of uncertainty remains (timing, rain shadowing, etc.) but the models have yet to indicate rainfall intensities of greater than 2-year recurrences at any duration. For what it’s worth, the latest ECMWF and UW WRF-GFS runs both indicate less rainfall than the NWS, suggesting that the atmospheric river will hit Oregon and perhaps Northern California more directly.

 

Over SPU’s mountain reservoirs, quite a bit more rainfall is expected. The NWS is forecasting 0.10-0.20″ Fri-Sat, and about 3.50-4.00″ Sun-Mon, again with peak intensities occurring during the 12-hour period ending early Monday morning. Current wind forecasts are only in the 10-20 mph range, and higher gusts are likely, but their southwesterly direction should favor our trees.

 

Excerpts from this morning’s NWS Area Forecast Discussion (emphasis added):

 

UPPER FLOW IS A LITTLE MORE RIDGELIKE THURSDAY SO THERE SHOULD BE MORE AFTERNOON CLEARING. THIS SHOULD ALLOW HIGHS TO BE CLOSER TO 70 DEGREES. ON FRIDAY A WEAK WEATHER SYSTEM WILL REACH THE AREA. SOME RAIN IS LIKELY ON THE COAST IN THE AFTERNOON. BY FRIDAY NIGHT RAIN IS LIKELY EVERYWHERE.

THE GENERAL CONSENSUS WAS THAT THERE WILL BE A CHANGE TO A MORE TYPICAL AUTUMN WEATHER PATTERN FOR WESTERN WA BUT THERE CONTINUED TO BE DIFFERENCES IN THE MEDIUM RANGE SOLUTIONS. THIS CAN BE EXPECTED. THE MODELS TEND TO HAVE DIFFICULTY TRANSITIONING FROM A RELATIVELY STABLE PATTERN TO A STRONG JET ACROSS THE PACIFIC NORTHWEST…ESPECIALLY IF THERE ARE FLIES IN THE OINTMENT [SUCH AS SUPER TYPHOON VONGFONG]. THE ONLY CHANGES IN THE EXTENDED WERE TO BLEND BETTER WITH NEIGHBORS. STILL LEANING TOWARD THE IDEA OF AN ACTIVE AND WET PERIOD. THERE IS THE POTENTIAL FOR HEAVY RAINFALL EARLY NEXT WEEK AND POSSIBLY EVEN HIGH WINDS OVER PARTS OF THE CWA IF THE CANADIAN MODEL VERIFIES.

 

References:

 

NWS Area Forecast Discussion
NWS Forecast Table Interface