Monday, May 26, 2014

Memorial Day Nowcast and an Amazing Record

Yesterday's forecast was not a great success;  rain came in on Sunday far earlier than predicted.  This illustrates a weakness in the weather prediction toolbox:  weak, small scale systems are difficult to get right.    A major issue is the lack of high-resolution weather data over the ocean, something that should improve with advancing satellite sensors and better data assimilation (the use of data to describe the 3D state of the atmosphere).

So let me switch into nowcasting...short term prediction....mode.   Today looks quite a bit better than expected, particularly if you are in the right places.

Here is the 10 AM radar.  Showers are occurring on the windward slopes of the Cascades and coastal mountains as moist, unstable flow is forced to rise.  Dry in the Columbia Basin.  Some light bands of showers coming in off the ocean and if you look closely there are some showers around Everett...associated with a Puget Sound convergence zone.


A recent visible satellite image shows the shows offshore, enhanced clouds on the windward (western) side of the mountains and a clear indication of the Puget Sound convergence zone north of Seattle.  Do you see the enhanced clearing over Seattle and the south Sound?  A classic CZ signature.


So you want to decide on your outdoor activity this afternoon?  Western slopes of the WA Cascades will be damp and cloudy.  Sorry.  The convergence zone may strengthen and move southward to Seattle.  Here are the 3-h UW WRF forecasts ending 2 PM, 5 PM, and 8 PM today (Monday).  Outside of the convergence zone the lowlands look dry...and don't take this forecast as gospel...the phasing and timing of the CZ could be shifted by hours.     Watch the radar




A completely independent system is the High Resolution Rapid Refresh run by NOAA.  It's one-hour precipitation forecast ending 2 PM and 5 PM are similar to WRF, but a bit drier.  Still some convergence zone activity but less near Puget Sound.



So if you are south of Seattle...fire up the grill.  If you are in Seattle, watch the weather radar!  The convergence zone precipitation is relatively narrow (perhaps 5 miles) and you can time you way to a dry meal.

And the amazing record?   Here is a notice released by the Seattle National Weather Service office:

.CLIMATE...THE RAINFALL TOTAL AT SEATTLE-TACOMA AIRPORT WAS 0.22 
INCHES SUNDAY. THIS MAKES THE RAINFALL TOTAL SINCE FEBRUARY 1ST 
22.87 INCHES. THIS BREAKS THE RECORD FOR THE WETTEST FEBRUARY 
THROUGH JULY IN SEATTLE. THE OLD RECORD WAS 22.81 INCHES SET IN 
1972.   FELTON/MCDONNAL

You knew this was a wet late winter/spring, particularly mid-February through mid-March.   But to beat the Feb-July record in MAY is really notable.


4 comments:

Ryan Williams said...

I spent Memorial Day Weekend in Gold Bar, right in the heart of an moderately active convergence zone. For those of us who appreciated the rain, the convergence zone was truly a wind-wind situation. Ba-dum-bum.

Anonymous said...

it appears the forecast for seattle has been wrong for about the last week. virtually every day has been forecast for rain, but what have we had? sun with some clouds.

Ansel said...

Went sailing Memorial Day weekend in the North sound. I did not expect sun, but there was no wind either. And the "curse" continues: I was led to believe it would rain today. So far a beautiful sunny day- a Wednesday.

I hardly dare trust this weekend's good forecast!

Michael Gersh said...

This just shows the limits we operate under. Chaos has a greater part to play than most people want to admit. Even a 24 hour forecast that the best minds and models had confidence in is subject to failure. Just as a Jumbo Jet can disappear. All records are always broken over time. Time for some humility, not just for weather, but in many areas where we don't know as much as we like to think. Thanks Cliff, for not shying away from admitting a blown forecast as many others would, and are.