Sunday, August 10, 2014

Thunderstorm Potential for Monday and Tuesday

After 2 damaging severe thunderstorm episodes this summer, is it possible to have another one?  At this point, the answer is "yes, it is possible".  That said, it is too early to tell the exact timing, intensity, and location of any strong thunderstorm.  So at this point, there is no reason to panic.  Just pay attention to the forecasts today and Monday as they become more detailed.  There is still a good chance that all of the ingredients don't come together just right, and the event winds up being rather benign or localized.  So let's take a look at some of the ingredients.

First we want to see instability in the atmosphere.  There's lots of things that go into this, more than we can cover here.  But essentially, we want to see an atmosphere that is very hot at the surface or cold aloft, or the combination of the two.  Also, moisture in the low levels is needed.  We measure these parameters by calculating the Convective Available Potential Energy of the atmosphere, or CAPE.  Values of 1000 for CAPE are fairly large for our area.  Below is the probability of CAPE > 1000 on Tuesday.  As you can see, the shaded areas have a 70% chance or greater of having CAPE > 1000.  So we will have instability.

15Z SREF Probability of CAPE > 1000 J/kg on Tuesday 12 August

Moisture is another key ingredient.  A dry atmosphere can be unstable, but it won't form any clouds.  Again, there's several ways to assess this.  One way is what we call "precipitable water".  Despite the name, it is not a measure of how much rain you'll get.  Below is the forecast precipitable water.  A "normal" value for this time of year is about 0.70".  So a forecast of 1.25" is nearly twice what we normally see in August.

15Z SREF Precipitable Water on Tuesday 12 August

Another ingredient is what we refer to as "upper level dynamics".  That is, a larger scale feature in the atmosphere that will support and enhance thunderstorms.  The satellite image below shows a large swirl of low pressure off the northern California coast.  That low is forecast to move over our area on Tuesday.

Water Vapor image at 10 August 2014 at 2pm

There are other ingredients that are more subtle, but can still have a big impact on the event.  For example, if there are too many clouds on Tuesday, we won't heat up as much as forecast.  That decreases the instability and would lessen the event.  On the other hand, a bright sunny day could result in hotter-than-expected temperatures and more instability.

So what do we expect to happen?  Here's a basic breakdown of what to look for:

Thunderstorms will fire over central Oregon on Monday.  The remnants of these storms will drift northward into eastern Washington Monday night.  These storms could create what we call a "haboob", which is a strong wind storm, often with blowing dust and very little if any rain.  This would be most likely in southeast Washington in the evening, moving into the Spokane metro area late in the evening.  Here's a computer forecast of what the radar might look like.  
NAM reflectivity forecast for Monday evening

This activity will gradually weaken overnight.  Tuesday will see more thunderstorms.  The difference is that the low we previously talked about will be moving into our area on Tuesday, helping to support the storms.   Storms will fire over the Washington and Oregon mountains.  The Oregon storms will move into Washington from the south.  The low will keep these storms going, not letting them die.  Here's the computer forecast of what the radar might look like for Tuesday evening.

NAM reflectivity forecast for Tuesday evening

The one missing ingredient in all this is something we like to call "vertical wind shear".  There will be some, but not a lot, and we don't expect that to change.  That could hinder storms from becoming very strong.

So here's a breakdown of what the potential impacts might be from these storms::
  • Flash Flooding: Precipitable water values will be up to 200% of normal for August. Storms will not be moving fast, probably about 15 mph. Debris flows are possible for recent burn scar areas. 
  • Wind: The main threat will be strong gusty winds. The previous two events this summer have shown the potential problems this poses. It's impossible at this point to say where this will occur, so stay tuned. 
  • Fire starts: Given our very dry fuels, any lightning will likely start fires. Even wet storms can start fires. 
  • Hail: Instability over the mountains would certainly support large hail. The SREF has 70% probability of CAPE > 1000 J/kg. However, we are lacking strong atmospheric wind shear, so that could limit hail production.
How does this event compare to our previous 2 events this summer?  This event is similar to 23 July in that there is a strong low aloft to provide forcing.  The August 2nd event was lacking this.  In that case, the thunderstorms provided all of the lift themselves.  This event is also similar to 23 July in the track of the storms, moving from south-to-north.  The August 2nd event saw storms that tracked from west-to-east.  But July 23rd had a stronger low and more instability.

Similar to the August 2nd event, this one will feature a drier lower atmosphere initially.  This favors more wind than hail.  Also, the lack of vertical wind shear is similar to August 2nd.

All of these ingredients have to come together just right for a large convective event.  If the low is a little slower and arrives at night, we won't see as much thunderstorm activity.  Or the moisture may be over-forecast. So please, keep up to date on the forecast.  

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