Friday, October 25, 2013

Persistent ridges deliver abundant fog and stratus

Impacts of a persistent ridge

A strong ridge of high pressure continues to sit over the Pacific Northwest as of this afternoon (10/24). The ridge has essentially remained stationary all week and consequently we have been subject to rather stagnant weather conditions.  

500 mb map and IR satellite 10 am 10/24

One of the weather conditions associated with ridges this time of year is widespread fog. The fog has typically been forming late at night and eroding over most locations by midday. Here's a look at what the fog looked like at 10 am on Tuesday (10/22).
Visible satellite 10/22 10 am
And here's how it looked 48 hours later. Although the images look similar the coverage has expanded westward into the Okanogan  Valley. 
Visible satellite 10/24 10 am

This is a typical occurrence with persistent ridges. Each day the fog has been burning off a little slower as well. While most of the Inland Northwest has seen the fog erode by midday. One persistent are of low clouds and fog has remained fixed over the Kootenai River Valley from Bonner Ferry (north of Sandpoint) and north into British Columbia. 

Below is a loop of how the fog eroded on 10/24. This erosion process has been consistent each day with most locations seeing widespread sunshine by early afternoon. Again note the clouds remain fixed over the Bonners Ferry and extreme north Idaho through the entire day. This erosion process takes place as solar heating works its way through the low clouds and fog until it can effectively break up the inversion. 

Visible satellite loop courtesy of CIMMS University of Wisconsin

Another process that can break up fog in the valleys has to do with a typical valley and valley slope heating. On a typical day the sides of a valley will heat up before the valleys below. This leads to a phenomena termed upslope flow. As the winds begin to blow up both sides of the valley it creates a void in the center of the of the valley. Consequently a circulation forms with a sinking motion developing over the center of the valley. This often times will lead to a clearing in the center of a cloud/fog choked valley. The image below shows a couple good examples of that.

polar orbiting visible satellite imagery 10/24

The fog pattern will stick with us through Saturday or Saturday night. After that a big change in the weather is expected care of a strong Canadian cold front.  

Monday, October 21, 2013

Is the ridge going away yet?

The ridge that won't go away

The last blog entry discussed the strong upper level ridge over the West Coast and how long it would stick around and this post will continue on that note. Since last week, there has been little change in the strength of the ridge although it shifted a little farther east, as seen on the satellite image below (green lines= 500 mb heights superimposed on the infrared satellite.  

So how will this pattern change through the week? The answer is not much. The model solutions are in strong agreement that the ridge will persist at least through Friday. Below is the 500 mb map with moisture (green infers a moist atmosphere with high level clouds). Looks somewhat similar to the image above, doesn't it. 
500 mb height pattern for 5pm 10/28
This pattern will continue to bring generally clear skies to the region with areas of fog reappearing over parts of northeast Washington and the northern Idaho Panhandle. Similar to what's seen in the visible satellite picture below (notice the fog extending from Spokane north and east to Sandpoint, Bonners Ferry, and Colville). The fog has been shallow enough over our area that its burned off over most if not all locations around noon or sooner. That's not the case west of the Cascades, where the fog has persisted through most of the day.

Visible Satellite 1200pm 10/21

So is there is there any sign that this stagnant pattern will ease or breakdown? Actually there are changes in the future, its just a question of to what extent will the changes occur. On the 500 mb height chart below, note the ridge axis retrogrades off the coast (moves west) and the flow over our area turns north-northwest and allows a disturbance to track south through southern Saskatchewan. This is likely too far east to deliver much in the way of precipitation (except for some light precipitation over the Idaho Panhandle) , but it would likely bring cooler air down the Purcell Trench, from Bonners Ferry to Coeur d'Alene, which should lessen the chances for fog. 

Taking a look at the same ensemble plots as the last blog entry it supports this change...with decent model agreement (note the line clustering and trough signature just to our east).

Spaghetti Charts of 500 mbs 10/27

Here's a look at what sort of temperatures we can expect with the transition to northerly flow. Notice we go from well above average temperatures in Spokane (the red land blue lines are forecast and the dotted lines are the averages) for most of the week to below normal temps by early next week. 
7-day temperature outlook for Spokane 10/22-10/28

This cooler weather will could persist into early next week, but look what the GFS is forecasting to appear by the middle of next week. The return of the ridge. There is some model disagreement with this notion, but its possible much of the region will remain dry through the end of the month. If no more rain falls in Spokane through the end of the month it will be the 4th driest October on record. 

500 mb height pattern for Wednesday 10/30

Wednesday, October 16, 2013

Dry weather to continue...but for how long?

Dry weather to continue...but for how long?

A strong ridge of high pressure currently sits directly off the coast of Washington as seen on the 500mb height (around 17-18k ft above ground) and satellite image below. Ridges of this strength tend to persist for extended periods of time and this one will be no exception. This time of year, we can be fairly certain the ridge will keep dry weather in place, however the longer they persist, the more prone the area  is to the formation of low clouds and fog due to growing temperature inversions. If low clouds and fog form, they can have a dramatic impact on the temperatures especially if the fog persists through most of the day. Typically that does not occur as readily in October as it does later in the year, but that's for a later discussion. For now lets focus on how long this ridge could persist. 

Here's what the pattern looks like for next Wednesday off the GFS model. Notice the ridge is still in place; in fact it moves very little from its current position. (the shading shows moisture levels at 500 mbs, green=moist, white=drier). While the Inland Northwest will likely stay dry and relatively mild, the same can't be said for the Great Lakes region.  Notice the very deep low that forms over Upper Michigan. If this were to verify, it's likely that area (and especially just east of there) could see a major snow event as temperatures will certainly be cold enough. Not for our area...yet.

The latest GFS run finally breaks the ridge down by late next Friday or Saturday...and that's when we might take our turn at dealing with late fall or early winter like weather. Temperatures just off the ground will be well below freezing...while moisture associated with the low will be plentiful. So the big question is will this model verify and will our bout of dry, mild weather come to a sudden end?

If you play the odds game and give some merit to the images below, you might not break out the snow shovels ready quite yet. The upper left image simply is another representation of the image above. The lower right and left panels show what happens when we take the initial model run and introduce small perturbations or changes. We call this ensemble forecasting. This is a useful tool for determining how much confidence to place in any one model run. Generally speaking the farther out we are in time the more uncertainty there is in a forecast. This shows up quite well in the lower right panel. Each of the lines corresponds with a different model run or perturbation while the red line is the GFS. When the lines are closely clustered (such as over Mexico and the Gulf States) the forecast confidence is generally high. Over our latitude though, there is huge amount of variation or uncertainty. That doesn't help with the forecast process significantly, however we can then take all the perturbations and average them resulting in a mean forecast. Theoretically there is a better chance of the mean verifying than any of the individual perturbations or the lone GFS (that's not to say that they can't verify though!). That is what is shown in the lower left portion of the image below. In this case, the mean is suggesting the ridge will remain strong through at least next Saturday, however it does hint at a slight retrogression (westward shift). If that verifies it could introduce some cooler air from the north, but certainly not to the extreme that's being advertised from the lone GFS run.