The Ridge Begins to Shift

This has been the winter of the ridge, an area of high pressure in the in the lower to mid atmosphere that has brought us consistently warm temperatures.   Downstream of our persistent ridge there has been a trough over the eastern U.S. that given them unending cold and snow.

But the ridge is shifting, with substantial implications for our weather in the Northwest.

Let me remind you of Ridge 101.  The plot below shows a situation with a ridge over the western U.S.  The color shades are the heights of a pressure surface (in this case 500 hPa).  Where the higher heights push northward, there is a ridge, pushed southward--a trough.   When the ridge is over us (like in this figure) we are generally dry.  During most of the year, we are also warmer than normal when the ridge is overhead (but during the middle of winter sometimes we can be engulfed by fog at low levels).  East of the ridge there is northwesterly flow, bringing cold air from the north.   If a disturbance is embedded in this NW flow, we can have precipitation--yes, even snow.  On the western side of the ridge, there is southwesterly flow:  warmer than normal temps and usually wet.  This is when we get the atmospheric rivers, with heavy precipitation.

This winter we have gone between having the ridge over us (dry, and often a bit warmer than normal) to brief situations when the ridge slipped a bit eastward, give us warm, wet conditions.   As a result, we have had an overall warm winter, with normal precipitation.   Rarely, has the ridge slipped west of us, putting us in the cooler flow.   

But that is going to change.

Let's see how things have evolved and will evolve this week by looking a series of upper level (500 hPa maps).

First, last Monday at 10 PM PST.  Big ridge over the coast, but most of the northerly flow is to the east of the region. (winds are parallel to the lines, where lines are closer together, the winds are stronger, higher heights to right of the winds).  The jet stream is associated with the large gradients in the heights, with the jet stream being the conduit for weather disturbances.

 Saturday at 4 PM.  Looks familiar?  But there has been a subtle westward shift of portions of the ridge and northerly flow is working its way into eastern Washington.   Temperatures have cooled a bit...and there was even some frost in my neighborhood this morning.

 Fast forward to the prediction for Wednesday morning.  The ridge has broadened and shifted to the west.  Disturbances (troughs) are getting around the ridge on its northern flank.

 And by Friday AM, the ridge has strengthened offshore and we are now on the cold side of the jet stream

So we are not only moving into cooler air, but are now vulnerable to feeling the effects of weather disturbances embedded in the northwesterly flow.    This is good for mountain snow lovers.

On Thursday/Friday a disturbance WILL reach our region.   Here is an upper level map for Friday at  one AM;  you can see the upper level trough over us and the big ridge offshore.

Thw 48 hr total precipitation ending 4 PM on Friday is shown below.  Modest amounts, particularly over the western side of the Cascades (1-2 inches)

And there will be snow, as shown by the 48h snowfall totals over the same period (6-8 inches in the Cascades) and even eastern Oregon gets whitened up.  Not a game changer, but helpful.

It is not unusual for a late winter weather regime change, particularly for years with dry/warm mid-winters.  It doubt we will make up for the abysmal snowpack, but we may be able to mitigate the situation at the end.

Rider Oasis Questionnaire

A group of bicycle enthusiasts are developing the idea to place convenient kiosks around the city with items of interest to cyclists.  They have a questionnaire available on their website.  Please take a look and give them feedback if you have a minute...thanks.

The Pacific Northwest Weather Workshop

Interested in attending the big local weather workshop of the region?  The Pacific Northwest Weather Workshop will be held in Seattle at the NOAA facility on February 27-28th.   Everyone is invited and the majority of talks are accessible to laypeople.  To attend you have to register or they won't let you in the gate.  There will be a major session on the Oso landslide.  There is a registration fee that covers refreshments and food, and special student pricing.  If interested, check out this website.