A Wet Weekend During a Month When Drying is the Norm

The last four weeks have been quite dry, with Sea Tac Airport receiving about 2.5 inches less than normal over that period (see plot, cyan color is normal).

But everything changes this weekend, with substantial precipitation heading our way.  A good weekend to finish you taxes (as I will be doing myself).  The first, weaker, system is now coming in, and the next 24 hours will bring light rain to the lowlands and modest rain to the coastal mountains and the Oregon Cascades.

On Sunday, a potent atmospheric river of moisture will hit British Columbia, with only light stuff over Puget Sound and Oregon during the 24 h ending 5 PM Saturday.   Saturday will be the day to take that walk and enjoy the fading cherry blossoms on the UW campus.

But Saturday night, the atmospheric river will push into our region.  This image shows the transport of water vapor by the atmospheric river (it shows something called integrated water vapor transport).   A plume of water vapor is directed from the subtropics into our region.

As this current of water vapor is forced to rise by our terrain, it cools and converts much of the water vapor into precipitation.  This is shown by the 24-hr precipitation total ending 5 PM Sunday.  Very wet, with some areas in the mountains getting over 2.5 inches (black colors).  But most of that precipitation will fall on Sunday morning and by lunchtime you can head outside again.  Do your taxes in the morning.

Climatologically, April is a month of substantial drying in western Washington. For example, the climatological probability of getting a trace of precipitation in Seattle goes from around 50% in the beginning of the month to about 32% at the end.   Big drop.  Then we plateau in May at around 30% before the big decline in June and early July.

And the average monthly (30 day) rainfall drops from about 4 inches to 2.2 inches this month.

Thus, if the 16-day National Weather Service total precipitation forecast is correct (see below), we probably are in store for at least a normal month in terms of precipitation.  Perhaps you can work on your taxes next weekend as well.

from Cliff Mass Weather and Climate Blog http://bit.ly/2Ia62f4