The Radar Shows Heavy Rain Offshore Without Clouds: How Can That Be?

The radar imagery from the National Weather Service Langley Hill radar near Hoquiam showed heavy showers over the coastal waters last night.   To illustrate, here is the radar imagery over the Northwest for 1:14 AM this morning.   The reds indicate heavy precipitation!  Was it pouring offshore?



A look at the infrared satellite image at the same time shows no weather disturbances out there (see below).  Just some clouds to the north (associated with system that will bring us rain on Saturday).


A high resolution visible satellite image taken just before sunset on Thursday confirms the lack of weather offshore.   But note the low stratus and fog right along the coast....that will be important later!


So is the expensive Langley Hill radar broken?     Is rain falling out of clear skies?   Is there an explanation for this mystery?

There is an answer:   super bending of the Langley Hill radar beam by an unusually strong and low inversion, where temperature increases with height.

Let us take a look at the vertical sounding of temperature, dew point, and winds with height from the balloon-borne radiosonde released at Forks, on the Washington coast, around 5 AM this morning.  

Note how temperature (the right-hand line) increases with height in the lower atmosphere.  That represents an inversion.  The left line is dew point temperature; when dew point and temperature are the same, the air is saturated, which was true near the surface.

Yesterday afternoon, cool, but shallow, marine air moved into the coast, while warm air remained aloft.  The result was a fairly strong inversion in the lower atmosphere.  The visible satellite image showing the low clouds on the coast were a hint of what was going on.

So why do we care about inversions?   Because they can bend the radar beams (which are in the microwave portion of the electromagnetic spectrum--around 10 cm in wavelength).    

A strong inversion can bend the radar beam down to the earth's surface in what is known as superrefraction (see image).  The earth's surface (in this case the ocean) is a pretty decent reflector of the radar signal and a good amount propagates back to the radar, giving a false signal of heavy precipitation.  That is why the radar showed the arc of supposedly heavy rain offshore.   Totally bogus.


Now in the typical atmosphere, with denser air at low levels and less dense air above, there is normally some bending of the radar beam, which is a good thing because it gives the radar greater range (see below)


There was simply too much of a good thing last night!  And don't worry, real rain will get to Washington this weekend, with our weekend weather curse continuing.




Comments

Post a Comment

Popular posts from this blog

Why so little lightning in the Pacific Northwest? And a very nice weekend ahead.

Image
During the past 24-hours, there have been a few lightning strokes over the region (see below) but nothing to write home about (see below). It turns out that the Pacific Northwest and the remainder of the West Coast have some of the lowest frequencies of thunderstorms in the entire country (see below). But why?    My podcast provides the answer. And I also give the weekend forecast....and I think most will like what I have to say. To listen to my podcast, use the link below or access it through your favorite podcast service. Some major podcast servers:     Like the podcast? Support on Patreon 
Read more

The Time of Year You Can See the Air Move

Image
  Most of the time, you can not see the air move.   You can feel it, but the complex, turbulent motions are invisible to you, except when it interacts with leaves and branches on trees and plants. But for a few weeks of the year, the sky is full of small, cotton-like tuffs that like a natural MRI machine reveals the complex three-dimensional motions of the atmosphere.   Cottonwood season.  And we right in the middle of it.   Being cottonwood aware is of particular value this year.  It reveals the active dispersion and movement of air that makes the outdoors essentially COVID safe, and thus should be of some comfort for those nervous about transmission outside. Cottonwood seeds are embedded in small cotton-like fibers.   A million seeds weigh about 3 pounds.  Yes, each seed and tuff weights about .000003 pounds.   And that fact, plus the cottonwood tuff, means that these seeds fall VERY, VERY, VERY slowly.   So slowly that they are excellent markers for the three-dimensional flow of the
Read more

A Much Colder Than Normal April: But How Unusual?

Image
  April was a chilly month over the Northwest, but how chilly?     It turns out the answer depends on where you are located. The difference between this April's temperatures from normal across Washington State (below) shows that the entire state was well below normal.  The eastern side of the state was particularly frigid:  some areas were more than 6 degrees below normal.   Considering the west side of the state, the April mean temperatures at SeaTac Airport are plotted below.   This April was the coolest since 2011. April 2020 was in 15th place for the coolest April since the mid-1940s and was a bit more than 4F below normal.  So if you felt that you were cheated out of normal spring warmth...you are right. But if you really wanted to experience extraordinary April cold you had to be east of the Cascade crest. Consider Yakima.  Wow.  It had the third coldest April temperature since the mid-1940s.   And the all-time record in 1955 was only slightly colder. If you want a longer-te
Read more