Saturday Morning "Deluge" Shows the Limitations of Modern Weather Prediction

This morning was very wet around Puget Sound, with some locations getting 3/4 of an inch or more.  And eastern Washington is being hit quite hard today with vigorous thunderstorms.

To illustrate, here is the total precipitation over the past 24 hr, ending 4 PM Saturday (click on figure to expand).  About 2/3rds of an inch in Seattle...close to the usual amount for the entire month of August.   Several locations in the southern Cascades and the eastern slopes of the mountains got over an inch.

Over western Washington, precipitation was associated with a deformation band north of a land-falling low center to our south.  What is a deformation band?  It occurs then two streams of air converge together, often north of a low center, resulting in upward motion--and thus clouds/precipitation.  Here is the forecast winds for 8 AM this morning at 700 hPa (about 10,000 ft)...you can see the deformation pattern.

The result was a band of precipitation that hung over Seattle for several hours, thoroughly wetting the area.  A radar image around 7 AM  shows the band very well.

Unfortunately, although the models had shown that a band would set up somewhere in western Washington, they universally got the timing and location wrong.  For example, the 24h precipitation forecast from the NOAA/NWS HRRR model, their best at the short-range, kept the band too far south, with less than .25 inches over Seattle. 

This afternoon the action moved to eastern Washington, where surface heating released a great deal of instability, resulting in strong thunderstorms, lightning, and lots of rain.   Take a look at the radar for 5 PM.  Wow.  The red areas are very heavy rain or hail. Lots of places over the eastern Cascade slopes are getting hit hard.

These storms are really impressive in the visible satellite imagery at the same time (see below).   You see the high high ice clouds extending northwestward from the storms?  These are anvils--the high cloud blow off from the thunderstorms.     Some of this anvil cloudiness is spreading into western Washington!

The big issue, of course, is the potential for lightning from these storms causing more fires.  Will have to be watchful.

Sunday should be a bit drier, but eastern Washington will get more thunderstorms, and a few showers might well drift over the west.

from Cliff Mass Weather and Climate Blog https://cliffmass.blogspot.com/2019/08/saturday-morning-deluge-shows.html