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April 2015

Spring Instability

Posted: April 13th, 2015


2015-04-13 FROPA


Another strong cold front is working its way across the area and associated rainfall is expected to be briefly intense, thunderous and and even partly frozen (lowland soft hail and mountain snow).


The front showed up nicely on the coastal radar (note the ribbon-like structure in the above image) and should pass through Seattle between 3-4PM and then wash out over the Cascades an hour later.


This event, in which a cold low pressure system interacts with a strengthening sun, wouldn’t seem so jarring to us if our late winter hadn’t been so warm and sunny. But we’ve seen three or four similar systems this season, the most significant of which occurred on March 31st and April 1st just north of the Ship Canal.


Post-frontal convergence also looks like it will be strong and persistent, this time over Central and North Seattle, according to the latest UW GFS-WRF run. NOAA’s HRRR model kicks off an ESE-oriented Puget Sound Convergence Zone over the common King-Snohomish border spot at about 8PM. The WRF keeps it going over N. Seattle through early in the morning, then revives it tomorrow afternoon between S. Seattle and Tacoma. The duration of this PSCZ event could prove problematic for urban drainage depending on where it settles. At the same time, the event should also enhance mountain snowfall amounts, which at this point is … somewhat strange, but great for forest health and helpful for an already good water supply.


Once the atmosphere settles down tomorrow afternoon, an extended period of dry and eventually warmer weather looks likely. We shall see whether or not our string of (13) consecutive warmer-than-normal months will continue. At the moment we are 0.7º below normal for the month.



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