Europe Model: Crazy Rain Offshore

Tuesday, June 9, 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.