Monday, April 18, 2016

April heat, mountain snow melt, and thunderstorms

April is sure off to a warm start by Inland Northwest standards, with some locations off to the warmest start on record.   Will the warm weather continue?  The answer is a definite yes!   The warm weather is going to lead to significant snow melt in the mountains this week which could cause some river flooding, as well as the potential for thunderstorms for the end of the week.  Lots to discuss, so let's get into it.

First off, the warm start to April.  Here is a map showing the temperature anomalies for the first 16 days of April across the Continental US.

The pattern here is clear, much above normal temperatures over the west (about 6 degrees F Northwest), and colder than normal over the east.  The following COOP sites are reporting the warmest start to April on record:  Lind, Quincy, Plain, Stehekin, Mazama, Priest River, and Colville.  

The warm weather is going to continue this week, with temperatures 15 to 25 degrees above normal through Friday. This is partly in response to high pressure centered right over the area for multiple days allowing continued warming of the air mass over the region.   This is what Meteorologists call an omega block.  Take a look at the GFS model forecast for Tuesday, do you see it?

00z/18th GFS model run forecast of the 500mb heights 18z Tuesday (11 AM PDT Tuesday)
The table below shows our current forecasts as of Monday morning for Tuesday and Wednesday for selected cities, as well as the current records for the date.

The pattern by the end of the week begins to shift.  The strong ridge moves east allowing increased moisture to be brought into the region from the south.  The increased moisture combined with the warm temperatures will increase our chances for thunderstorms.  Below is a graphic from the GFS model portraying the expected pattern for Thursday.

00z/18th GFS model run forecast of the 500mb heights 06z Friday (11 PM PDT Thursday)

It's still too early to narrow down which days and where thunderstorms will occur, and to what intensity.  However the pattern looks favorable and chances will be gradually increasing for late in the week.

The warm weather will also cause snow melt in the mountains with this water making it into area rivers.  This may lead to minor flooding on rivers along the East Slopes of the Cascades and north Idaho Panhandle.  

Here is the current situation with the snow pack across the region.

NOHRSC SWE analysis for April 18th, 2016 09z

As you can see, lots of snow pack remains in the Cascades, Rockies, and north Idaho Panhandle.  The warm weather and snow melt means rising rivers.

The Okanogan River at Tonasket is currently forecast to reach flood stage by the end of the week with several other rivers in the Cascades and north Idaho Panhandle likely to reach at least action stage. Those with interest near area rivers should monitor the latest forecasts by clicking on the link below:

So how long will this warm weather last?  Will this be the warmest April on record?  It's still a bit too early to make this call, but the near record warmth for much of this week combined with the warm start to April will sure help the cause.  There is still next week to get through, and here is the latest outlook from CPC (Climate Prediction Center)

The outlook for April 25th - May 1st calls for slightly elevated odds for warmer than normal temperatures.

In summary, expect near record warmth this week resulting in increased snow melt and rising rivers, as well as an increasing chance for thunderstorms by Thursday or Friday,