Severe Weather Tomorrow??? It's a Possibility.

We have all heard of the calm before the storm, today could be just another chapter in that book. This afternoon will feature highs in the 70's and 80's under mostly sunny skies. Come tomorrow the chance for showers and thunderstorms will be on the rise as an area of low pressure approaches the Inland Northwest. Temperatures will continue on the warm side of normal, but increased moisture will push ahead of the low pressure center increasing our chances for convective activity or thunderstorms. In this post we will evaluate the chances for thunder around the region and the potential timing through various weather maps and products.

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.

 11am PDT Infared satellite map with 500mb heights

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.

Precipitable Water values from 11am today (left image) and 5pm Thursday (right image)

The images above show the increase in atmospheric moisture from today to tomorrow from the North American Model (NAM). For much of the Inland NW we will see an increase of nearly a quarter of an inch of precipitable water. Another method to address the amount of moisture would be the change in forecast soundings which show temperature and dewpoints up through the atmosphere. Next we will examine the forecast soundings to see the change in moisture from today to tomorrow.

NAM forecast soundings from 2pm today (left image) and 2pm Thursday (right image)

From the sounding you can see the major differences in the amount of moisture present. The red line would indicate the temperatures and the green indicates the dewpoint which reflects the amount of moisture in the atmosphere. For the left image a dry layer is noted around 500mb (where the green line moves away from the red) whereas for tomorrow (right image) much more moisture is present (green and red lines are closer throughout the image). So from these two examples the change in moisture can easily be noted, but will we have the instability to promote the thunderstorms? We will now at these parameters that promote thunderstorms.

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.

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

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.

Snow, Rain, Wind, (and Thunder?)

Yesterday's weather system brought a nice 1 to 3" of snow to many locations.  You can see an extensive list of snow reports on the National Weather Service website.  Now we have Storm #2 arriving 24 hours later.  This one will be a little different in a couple of ways.  First, this storm is a little wetter and more extensive than the first.  This means more people will see precipitation.  Here's the radar forecast from the HRRR model valid at 5pm this evening:

5pm radar forecast from the HRRR model

This is a forecast from a computer, not an actual radar image.  But it gives you an idea what the radar will look like around 5pm.  And just about everyone in the Inland NW will be seeing some sort of precipitation at this time.  Which leads us to the other difference.  While Storm #1 was all snow, this one will be more of a mixed bag of rain and snow.  The HRRR forecast for precipitation type at 5pm is shown below.

5pm precipitation type forecast from HRRR

This graph is a little hard to read, but the horizontal blue lines represent snow, while the vertical green lines are for rain.  The HRRR expects the precipitation by 5pm to be rain south of a line from Deer Park to Grand Coulee.  This includes Spokane and the Palouse.  The precipitation may initially start in the metro area as wet snow, but it won't be able to accumulate much and will change to rain later this afternoon. Also notice there are some small pockets of red scattered around the region. This represents a chance of freezing rain. If freezing rain really does occur it will be rather isolated and likely quite brief. 

Here's the storm total snow forecast from the National Weather Service.

Storm total snow ending 4pm Thursday

Most of the valleys across the north (Republic, Colville, Sandpoint) will see all snow and pick up 2-6" of snow, while the mountains will receive up to a foot.

Storm #3 will follow on Thursday afternoon/evening this will be a weaker system.  It will once again bring rain or snow to Spokane and the Palouse while the northern valleys will pick up a little more snow.

Storm #4 is still taking shape but it consistently looks like it will be the strongest of the bunch.   Today's GFS model forecast backed off a little from it's previous forecast, while some of the other models came out a little stronger today.  As a result, they're all in very good agreement at this time.  We often take this as a sign to increase our confidence in the forecast.  Even though they all agree, could they all be wrong?  Yes, but the odds are growing increasingly slim.  

Here's a breakdown of how this storm will unfold:

• Precipitation will spread over the area on Friday around or shortly after sunset.
• Spokane and the Palouse will see rain from this storm, with a possible mix of snow at the onset.
• Saturday morning will be rainy and breezy.  Spokane will be in the lower 40s by sunrise and top out in the mid 40s during the day.
• The northern valleys will start as snow and will accumulate a few inches.  They will gradually change to rain overnight.  Colville will probably change to rain before midnight, while Bonners Ferry could hold on to snow until Saturday morning.
• The cold front will move through the area during the middle of the day.  Behind the front winds will really blow.  Below is our forecast peak winds for Saturday.  Areas in white can expect gusts in excess of 50 mph.

Peak wind gusts expected on Satuday

We will likely be issuing Wind Advisories for much of the area and possibly High Wind Warnings.  You'll want to stay tuned to this forecast.

Here's a couple of other things to consider with this storm.  The rain along with the melted snow won't be able to soak into the ground since the ground is solidly frozen (from our early December cold snap).  So expect some water  ponding, even on your lawn.

The other interesting tidbit is the outside chance of a thunderstorm.  Thunder?  In early January?  Yes, it's happened before, and this is pretty much the classic set up for it:  warm moist air ahead of a strong cold front.  Below is the SREF model probability of thunderstorms. 

24 hr thunder chances ending 10pm Saturday. 

You can see a small area of 10% over southeast Washington.  If thunder does occur, we don't expect to see hundreds of lightning strikes. However a few claps of thunder are a possibility especially over southeast Washington and the southern Idaho Panhandle, including Pullman and Lewiston.