So lets start out with today and move forward from there. Our current pattern has a ridge of high pressure directly over the Inland Northwest keeping conditions calm and pleasant. The ridge also allows for warm southwesterly flow to advect into our area bringing above normal temperatures today. We will take a look at the current set up in the following image.
From the image we can see the area of low pressure to our west and the current ridge over us. The ridge is also diverting much of the cloud cover to our north. As the low slowly moves onshore the ridge will continue to be shifted to the east allowing clouds to move east of the Cascades. Not only will this bring increased cloud cover for tomorrow afternoon, but also increased moisture through the atmospheric column allowing better instability. So lets take a look at the increased moisture moving in. This can be accomplished in a couple of manners, but we will look at Precipitable Water or PWATs in the atmospheric column.
|11am PDT Infared satellite map with 500mb heights|
|Precipitable Water values from 11am today (left image) and 5pm Thursday (right image)|
|NAM forecast soundings from 2pm today (left image) and 2pm Thursday (right image)|
For thunderstorms to occur, many things need to be present including lift (forcing), moisture and instability (atmospheric stability). We will first look at the lift for the area and the one way to do this is to compare from today's conditions to tomorrow. Below is an image looking at the Q vector convergence in the upper atmosphere or Div-Q. Div-Q is a generalized way to assess the lifting potential in a portion of the atmosphere.
|Upper level Div Q from 11am today (left image) and 5pm Thursday (right image)|
From comparing the images we can see the major differences concerning forcing. Tomorrow the low will move onshore bringing a good amount of forcing ultimately aiding in thunderstorm development. With the right image being for 5pm tomorrow, we will most likely be looking at a case that will unfold more in the late afternoon and evening hours rather than early in the afternoon. Would this be a good or a bad thing with it unfolding in the evening? For now it looks good. We have been hitting our high temperatures in the late afternoon/early evening which would be the best time for the stronger storms, so this would also would aid in thunderstorm development. We have already looked at the moisture profiles for the area and have concluded that higher amounts of moisture will be in place. Finally we will look at one of the convective parameters that are normally consulted to address thunderstorm potentials which is CAPE or the Convective Available Potential Energy. First we will once again look at the NAM model. It has been the most aggressive when it comes to CAPE values, but paints a similar picture to the others as to the areas with the best chances for seeing thunderstorms.
|NAM CAPE values for 5pm Thursday|
From the NAM we can see a broad area with values surpassing 1000J/kg and localized areas greater than 1500J/kg (blue and green shading). If you remember around a week ago on the afternoon of June 3rd we had thunderstorms for much of the area. These storms were also working with around 1000J/kg of CAPE. With the NAM being on the upper end of model perspective lets take a look at another one. Next we will examine the model often used by the Storm Prediction Center (SPC) in Norman, OK which is the SREF or the Short Range Ensemble Forecast.
|SREF CAPE values for 5pm tomorrow|
Although not as colorful as the NAM, the SREF is also showing higher values for eastern Washington and into the Idaho Panhandle. It also has peak values in the 1000J/kg to 1250J/kg range. So we do have some consistency among short range models of the potential for higher amounts of CAPE. So with all of this in mind, now comes the million dollar? will we see thunderstorms and if so, how strong will they be? Concerning the chance for thunderstorms, it is a given. We will see thunderstorms tomorrow. The next question is where? As we can see, the best forcing will be in the eastern third of Washington and the Idaho Panhandle so these are the locations with the best chances. We do not want to omit the chance for the east slopes of the Cascades or the Basin, but the threat will not be as great as the other locations. From the SPC, they create a calibrated outlook of the thunderstorm chances for a location so lets take a look at what they think.
While the chances for severe storms do not look very high, they still highlight the potential for portions of the area. This product usually only has slight chances for us when severe events do occur, so the potential is there. The region will want to keep an eye on the skies tomorrow as active weather looks to be a given.
|SPC calibrated thunderstorm probabilities between 5am-5pm Thursday|
So from the SPC, they highlight a 40% chance for much of northeast Washington into the Idaho Panhandle. These are some of the higher values I have seen from this for our area so it would lead me to think the chance for thunderstorms is essentially a slam dunk. Finally the big one.....will any of the storms be severe? Here at the office we seem to think the potential is definitely there for strong storms. Comparing to last week, we had similar CAPE values, but tomorrow we actually have better dynamics to support storms. The SPC also does a product for the chances of severe storms and here it is.
|SPC severe thunderstorm probabilities for 5pm Thursday|