June 09, 2015

Crazy Rain Offshore

Looking at a recent radar image from the Langley Hill weather radar near Hoquiam, you would be running for an umbrella and your rain gear.  Here is the image around 8:51 PM Monday.   Wow.  This is the radar reflectivity (the amount of radar signal scattered back to the radar) from the lowest radar beam (.5 degrees above the horizon).   VERY heavy precipitation is indicated offshore.   Such dark reds are either torrential rain or hail.    But none of the forecasts are calling for rain.  What gives?

Next, lets check the visible satellite image a few hours earlier (6:30 PM).  NOTHING IS OUT THERE!

Our computer model forecasts indicate no chance of precipitation over the eastern Pacific.

So that is going on?    A profound case of anomalous propagation produced by a low-level inversion.

Radar beams emitted by weather radars normally don't go straight:  they are bent slightly (refracted) downward by the change of atmospheric density with height (air is denser near the surface and less dense aloft).  Such refraction is good, because it gives our radar extra range, partially compensating for the curvature of the earth (see figure).

An inversion, when warm (less dense) air is above cooler (more dense) air, can really rev up the refraction by causing a greater decrease in density with height.    And with very warm air aloft and cool air near the Pacific Ocean surface, the last day or two has had strong inversions.

To show this, here is the vertical temperature sounding at Quillayate, on the WA coast on Monday morning.  The dew point temperature is also shown (left line).    The Y-axis is height (700 hPa is about 10,000 ft) Very strong inversion above a layer of cool air near the surface.

So down worry too much about rain along the Washington Coast for the next few days.

There were some isolated thunderstorms over eastern Oregon yesterday (see radar image for 7:45 PM Monday).  A few isolated lightning strokes.

But big changes are a'coming this weekend....with far cooler temperatures and light showers on tap.  Enjoy the warmth...it won't last.


  1. This was a fascinating post. Thank you--I enjoyed it very much.

  2. Thank you Cliff. Very interesting how our weather radar can be fooled. During the Vietnam war, I maintained Electronic Warfare Equipment which intentionally scrambled enemy SAM radars.

    May 2015: All-Time Wettest Month on Record for the U.S.

  3. Hi Cliff,

    Wasn't sure how to email you. I love your blog. Can you make some suggestions on what brand of weather station (rain gauge, temp, etc) a amateur weather guy could ask for for Father's day. There are a lot of ones out there and hard to know what makes sense. Could be a blog post for ya! Thanks,


  4. Hey Cliff,

    any interest in talking about how the Weather is affecting the Race To Alaska? I thought I heard it was really much more windy than normal?


    Tim Koontz

  5. Jay...if you search my blog you will find a few blogs on purchasing amateur equipment. If you have a substantial budget, the Davis Vantage Pro weather stations seem to be of high quality. If your budget is modest, get a good rain gauge and digital thermometer...cliff

  6. My friends are in Sacramento. Yesterday there was strange rain where the clouds seemed to be forming over northern Nevada and coming from the east. It was really hot and the ground was dry afterwards because it evaporated so quickly.

  7. Hi Jay,
    I ran an Oregon scientific system atop my roof for years. You can find them on-line and at some hardware stores.

    For the last 8 years, I've been running a Davis Vantage Pro 2 on south Vashon Island. When SeaTac recorded a record high of 103, I recorded a max temp of 102 but I live in a deep woods. It's almost problem free. The lithium 123 battery lasts about 6 to 8 months. Last winter, I had to replace the transmitter board as the capacitor leaked. This spring, I cleaned the radiation shield from a buildup of moss and dirt.

    I have a poor location for wind speed and direction due to the proximity of 150 foot tall Doug firs (Pseudotsuga menziesii) to the north and west. I apply a surface friction factor to compensate for wind speed. I've considered developing a wind direction algorithm by comparing my wind direction to SeaTac but I haven't done the analysis. It might be a work of fiction as surface wind direction through the dense forest tends to oscillate.

    My dream machine would be an R. M. Young and a 10 meter tower.

    You can view my weather system at:

  8. Thanks! It is always tough to know what works best. I love idea of having it live on the web.

    Cliff, can you tell us when rain is coming? My veggies are really stressed and things are drying out!



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