Rain Off the Coast in Clear Skies?

The weather radar Friday evening at 8 PM looked really wet, with moderate to heavy rain offshore.

But the strange thing is that the visible satellite imagery showed clear skies.  Rain without clouds?  Very strange.

It turns out that the origin of this wacky situation was due to anomalous propagation of the radar signal from the Langley Hill radar near Hoquiam.  The radar beam was doing strange things because there was a stable layer aloft that bent (or refracted) the radar beam down to the ocean surface, where it was reflected back to the radar.  Thus, we were seeing the ocean surface, not precipitation.

Here is the vertical sounding from the radiosonde at Quillayute at 5 PM Friday.  There was a region where the temperature (red line) did not decline much with altitude between roughly 900 and 800 hPa (roughly between 3000 and 6000 ft).  In the lower layer beneath, temperature decreased rapidly with height.  Such an elevated stable layer caused the radar beam to be bent downward.

This situation is called ducting and is illustrated in the figure below.

This ducting configuration was associated with easterly flow aloft (causing warming) surmounting a cooler northerly flow near the surface.

Alberta Fires

The Alberta fires, which has resulted in the evacuation of Fort McMurray,  are extremely unusual this early in the warm season.  On Monday, the fires were quite evident on the NASA MODIS high resolution imagery (see below).  You will notice there are several smoke plumes from multiple fires.

A close up from the Fort McMurray fires today is quite impressive, with dense smoke moving to the southeast.

The fires were set up by very dry conditions, as illustrated by the percent of normal precipitation during April (see below).  Some locations in the fire are had less than 25% normal precipitation.

And temperatures in the same area were 3-5C  (5-9F) above normal.

The proximate cause of all this warmth and dryness?   Here is the pressure (height) anomaly for the last 3 months in the midtroposphere (500hPa). You will notice very strong, small-scale, persistent area of high pressure (red color).   The same ridge that gave us a wet winter and warmth.

Why was the ridge there?  At this point it appears to be natural variability, but perhaps someone may find a connection with human forcing.  But that connection does not exist today.  

El Nino (which is a natural cycle) could have contributed as well.  This year was one of the strongest El Ninos on record, and such events are highly correlated with warmer and drier conditions across the Canadian prairies.

The slow warming of the planet due to global warming from anthropogenically increased greenhouse gases surely made a modest contribution, but clearly natural processes did the heavy lifting setting up this event.  How do we know this?   Here is the springtime  temperature trend over the past 50-years from the excellent Goddard Institute of Space Studies surface temperature web site.  The Alberta area has had only a modest warming, roughly -.2-.5C.  This is much smaller that the observed anomaly of the past few months.   Much larger warming was observed over the Arctic, much of that associated with human forcing.

Finally, there is one other important element.   Suppression of fires over the years that has led to a very mature forest ready to explode.  So human mismanagement of the boreal forest is a major element in this situation.