Saturday, March 23, 2013

Why don't we get rain and wind at the same time?

An interesting aspect of Northwest storms is that generally we don't get heavy rain and strong winds at the same time.    This week's early spring storm was a good example:  first came the rain and THEN came the wind.  Why don't they occur at the same time?  It has to do with the nature of the structure of Pacific storms.

Let's start with a plot of wind, precipitation, and other weather parameters over a 72-hr period this week at the University of Washington.  The top panel shows the winds and the fifth panel presents precipitation.  Time is in Greenwich Mean Time (GMT or UTC).  Lots of rain from 0000 to 1500 UTC on the 20th, but the big winds waited until 1800 UTC on the 20th.  Classic.  Remember the Chanukah Eve Storm in 2006?  Pouring rain during the day, but the big winds came during the evening and early morning hours after nearly all the rain was over.  I could give you a hundred examples of this behavior.

So why the separation? Consider the storm on the 20th.  Here is the surface weather map for 1200 UTC on the 20th during the rainy period.   The low center and the big pressure differences (gradient) was still offshore, but moist, warm southwesterly flow was coming into the area and the storm's front was approaching the coast.   Wet, but not particularly windy.

Here is the radar for that time.  Lots of rain....yellow is very heavy stuff.


Now, fast forward to 2100 UTC (2 PM)--here is the map.  Lots of pressure gradient and strong winds as the low moves past us to the north.  But the front is inland and the flow has switchws to the west and northwest....much cooler and drier are moving in.  Yes, we can get a few showers, but not much more.

Here is the radar at this time---pretty wimpy precipitation--except for the convergence zone over the northern Sound.  Major rain shadowing fro Seattle to Olympia.


A schematic of an oceanic cyclone and attendant fronts summarizes the situation. Shading shows the main clouds.  The big precipitation is generally with the fronts, which lead the low center.  The big winds are south and west of the low.


 Yes, there are some situations where we can get driving rain and wind, but generally and with most of our windstorms, there is a substantial separation between heavy rain and wind.

4 comments:

Rod said...

Hi Cliff,

Thank you for the explanation. I have noticed this, as well. We are fortunate here in the Puget Sound area that the wind and rain are usually separate...less water damage to battered structures.

seattle said...

It's also interesting how fast the wind can dry things out after it rains.

And in other news, I finished reading your book. It took me about two and a half years to finish (with some breaks), but I finally finished it--one chapter at a time. Nice to have such a comprehensive guide to PNW weather--I would recommend it to people. I appreciate the thoughtful work that goes into your blog as well, and look forward to a year of interesting weather.

Mark Olwick said...

Perfect combination to blow down my fence. First saturate the ground and turn it to mush. Then it's easy for the wind to knock it over. Great.

Kelly Vander Linda said...

As a one of those windsurfing nuts who goes out on the puget sound during storms, I always look more forward to the post frontal (clearing) winds. Usually has a westerly cant to it. But it also means more sun (warmth for my wetsuit). Generally speaking if there is a squall, winds will pick up before, then die down during and next to nothing after. Then will pick back up. I've always seen situations where a lot of wind is predicted, but if I know the rain was going to be there along with it, it was very sketchy at best for high winds. Of course there are those storms that are windy with rain, just not your big time winds. Thanks for the post.