Does Monday still look like a severe day?

A few days have come and gone since our last post about the potential severe weather event for the first day of June. So how are things looking now that the event is nearing? Well according to the water vapor imagery below, it's looking pretty good. See the swirl off the northern California Coast? That's the deep low-pressure system which will deliver the active weather as it moves toward the Inland Northwest. Why is this low important? Well for a good thunderstorm day we need three key ingredients: lift, instability, and moisture. The low will provide us with the first ingredient.

Animated water vapor imagery. Note the swirl off the Northern California coast. 

The models are very consistent in taking this low from its current position off the California Coast toward the Washington/Oregon border by afternoon and then toward the Washington/BC border by evening. This pattern resembles the typical negative tilted pattern we associate with severe weather in the Inland Northwest. The track of the low will ensure that the region will be subject to strong lifting potential beginning in southern Washington and central Idaho in the morning and spreading northward through the day. 

500 mb forecast with precipitable water forecast

Notice the shading in the 500 mb map above? That represents the precipitable water forecast. The greens, blues, and purples depict where the juiciest air will be located, Initially the pool of moisture, currently over far northern Oregon, will shift northward tonight and then become enhanced with even more moisture from the incoming low. How much moisture will we see? We forecast that using  a parameter termed precipitable water. Precipitable water is a figure used to represent how much water the atmosphere is holding. The precipitable water forecasts are expected to reach  values are forecast to exceed an inch over portions of the Inland Northwest by afternoon. How unusual is that? According to the graphic below, it would be placed in the 97.5-99th percentile for this time of year. So it will be far from typical. 

Precipitable Water anomaly

So with the two key pieces in place, what about the third, instability? Well it looks like that piece of the puzzle will be realized as well. If you recall our previous blog post, one of the ways to measure potential instability is looking at the lifted index values. If we see negative lifted index values, that indicates potential instability. So as early as 5 am, we begin to see some of this instability. Notice the yellow and orange shading over southeast Washington? This is enough instability to support thunderstorms even without the benefit of daytime heating. 

5am Monday Lifted Index Forecast

How about later in the day? Suffice it to say the instability will not be lacking. The entire region is expected to see negative lifted index values, with the best instability represented by the -6 to -8 values over extreme eastern Washington and north Idaho. That's about as good of a lifted index as you can expect to see over the region. 

5 pm Monday Lifted Index Forecast

So with all the pieces in place what would we expect to happen? We think there will be a band of showers and thunderstorms early in the morning spreading across the Washington/Oregon border, which should slowly work their way northward through the morning. How extensive this band will be remains rather uncertain. One of the weather models is forecasting the radar to look like the image below. Notice the nice cluster of showers and thunderstorms near the Blue Mountains in SE Washington. This coincides nicely with the good pocket of instability forecast by another model.  

7 am simulated radar image

By midday, this band is expected to drift farther north, but could lose some of its definition and strength.  

11 am simulated radar

However, the first band which moves through will further moisten and destabilize the atmosphere before the strong upper-level disturbance tracks toward our region. So what will the afternoon look like? This is far from certain, and there are as many answers as there are high-resolution models. Here's just one of the forecasts for 5 pm. 

5 pm simulated radar

That is a pretty impressive simulation with strong thunderstorms located across the northern portions of Washington as well as in the southern Idaho Panhandle. However recall that the best instability (or lifted index values) were located over the Idaho Panhandle. So we have better faith in the eastern portions of this radar simulation verifying. Another thing we can look at is an ensemble of simulated radar returns. The image below shows where the greatest risk will be for the biggest thunderstorms (chances of having a 40 dbz or stronger radar echo). The regions shaded in purple have the best chances (over 90%) followed closely by the reds (over 70%). This would highlight two areas. The Cascade crest and over the southeast corner of Washington and the southern Idaho Panhandle.

Ensemble chances for 40dbz or higher echoes

So what will the main risk of severe weather involve tomorrow? Based on the instability, the biggest risk looks to be large hail. If we look at the model soundings they are likely a little too moist to support widespread wind damage, however, we still expect to see some strong wind potential with a few of the storms. 

So what is the typical weather we experience with this type of upper-level pattern? We can utilize a fascinating tool produced by the Cooperative Institute for Precipitation Systems (CIPS) at Saint Louis University. They created a tool that makes an analog of the 15 closest weather patterns (since 1980) to what the forecast weather pattern is supposed to be. So below we see the forecast for tomorrow in red compared to the mean of the top 15 analogs (or pattern matches). This looks like a pretty good fit.

500 mb forecast for tomorrow (red) plotted against the 15 top weather analogs since 1980

So what weather was experienced on those days? Interestingly enough, quite a bit. Of those 15 days that matched Mondays expected weather, there were widespread severe reports. Most of them were related to hail, but a good sample of the reports were related to wind as well. Also, notice there were 3 tornado reports in the Inland Northwest. We do not expect to see tornadoes on Monday, as the wind patterns are not quite right to support violently rotating storms, however, they have occurred in this weather pattern before. 

Severe reports from the top 15 analog days to Monday's weather pattern

The other risk we see tomorrow will be for flash flooding. The storms which develop tomorrow will obviously contain copious amounts of moisture. And more importantly they could be slow movers. 

The risk of heavy rains will likely continue through Monday night before tapering off. Here's a look at the 24-hour precipitation forecast from 4 different weather models.

Precipitation forecast from 11am Mon-11am Tue

That's a lot of precipitation (purple amounts are above  0.75") and there is a fairly consistent message that the bulk of it will occur across the northern quarter of Washington, the Cascade Crest, and over the Idaho Panhandle. Since much of this will be attributed to thunderstorms, there is likely to be a high variability over short distances, with some areas likely to receive much heavier amounts. 

Stay tuned for updates to the forecast as this will likely be a very active weather pattern. 

Severe Weather Monday?

You may have heard about the potential for severe thunderstorms on Monday, June 1st.  We've been talking about it in our forecasts for the past couple of days.  So how likely is this?  Let's take a look at a few things.

First, thunderstorms rely on instability.  There's several ways that meteorologists assess the instability of the atmosphere.  One of the more straightforward methods is called the Lifted Index, or LI.  There are more sophisticated methods, but we'll stick with the LI for simplicity sake.  Essentially, if the LI is less than 0, then the atmosphere could be unstable, leading to thunderstorms.  The more negative the LI value is, the stronger the thunderstorms could be.

So here's 4 LI forecasts for Monday afternoon.

Lifted Index forecasts from four models valid Monday afternoon, 1 June 2015

The yellow, orange, red, and pink colors show areas of LI less than 0, with pink showing the areas with LI less than -4.  So as you can see, these 4 models all show instability on Monday afternoon, with some areas of rather strong instability (pink shading).  The exact location varies a bit, but in general they agree on the southern Idaho panhandle.  So from an instability standpoint, Monday certainly looks favorable for strong thunderstorms.

Another parameter we look at is wind shear.  We look at this over a layer of the atmosphere, typically from the surface up to about 4 miles.  Shear measures the change in wind speed as you go up.  For strong storms, we want lots of shear.  This helps the thunderstorm develop.  Weak shear means the storms will be more vertical, which isn't as good for strong storms.

Here's the shear forecast from the GFS model (the others are similar to it)

Forecast 0-6km Wind Shear for Monday afternoon, 1 June 2015

The colors of purple indicate shear of less than 30 knots.  This isn't very strong.  The cyan color shows shear of 30-40 knots, which is moderate, but still not strong.  So the shear forecast doesn't look very promising for strong thunderstorms.  

We also need a "kicker", something to get the ball rolling so to speak.  Often times this can just be the sun warming the ground.  So let's see what the forecast looks like for clouds.

Relative Humidity Forecast for Monday morning, 1 June 2015

This is the Relative Humidity forecast from the GFS model for Monday morning.  The green shading shows the atmosphere nearly saturated, which usually means clouds.  As you can see, there's the potential for a lot of clouds Monday, which would limit surface heating from the sun, and thus diminish the thunderstorm chances.

But there are other ways to make thunderstorms without sunshine.  A strong low pressure system can do the trick.  And that's what is causing the spiral of green off the Oregon coast in the above image.  This low could bring enough "dynamic" lift to kick off thunderstorms.  In fact, for  strong/severe storms, we usually want both (sunshine and strong low).

Here's the model precipitation forecast from 4 models.  Again, there's fairly good agreement.  These would seem to indicate that the best activity could fire off in the Panhandle and quickly move into western Montana.

Forecast precipitation from four computer models, valid Monday evening 1 June 2015

This is still a few days away, so there's still time for the models to refine their forecast.  Timing will be critical.  If the low comes in too fast, there will be too much clouds; too slow, and the sun will be setting before things can get going.  So as always, stay tuned to the forecast.

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.

Severe thunderstorms tomorrow? It's possible.

After a seemingly endless barrage of weather systems, yet another is already beginning to show its stripes. Take a look at the satellite picture below. This is a water vapor image.Notice the green area covering the western portions of Oregon and Washington. That's associated with a plume of sub-tropical moisture which should deliver rain (and high mountain snow) to most of the Inland Northwest between tonight and Thursday morning.  Also notice the dark area over the eastern Pacific and the kink in the blue lines located along 130w. This is associated with a fairly vigorous shortwave trough. This trough will become a crucial player for our weather across the Inland Northwest by tomorrow afternoon.

10pm PDT Water Vapor image and 500 mb heights

So as mentioned before we expect widespread rains (and mountain snow) tonight into early Thursday. This will likely prime the atmosphere full of moisture and could set the stage for some active weather tomorrow afternoon across portions of the Inland Northwest. Lets look at some model data for details. First we will start with a course resolution model, the 90km GFS. Here's a look at what that models is showing as far as the shortwave trough moving through the region. The first image shows the previously displayed 500 mb heights combined with what we term Q-Vector convergence. This is a fancy term that essentially shows upper level lifting. In this case the purple shading shows where the lifting in the upper levels is strongest. The top image shows the trough extending from the central Washington Cascades to the SE corner of Washington around 11 am with the best upper level lifting focused over most of the region. The second image shows the same thing only at 5 pm. Notice by this time the trough is focused along a line from Sandpoint to Missoula, however the strongest lifting by that time has shifted into eastern Montana and SE British Columbia. That's all well and good but what does that translate to weatherwise?

11am PDT Thursday 500 mb heights and Q-vector convergence

5pm PDT Thursday 500 mb heights and Q-vector convergence

Lets take a look at one more 90km GFS image before moving on. This time we will look at CAPE values. CAPE or Convective Available Potential Energy is simply the amount of potential energy that can be released should we be able to lift an air parcel. You can read more about it here if you desire. Suffice it to say, the higher the CAPE values there more energy or explosiveness there is in the atmosphere. So what is explosiveness tomorrow afternoon? Its actually fairly impressive at least for this region and this time of year. Notice the axis of the highest CAPE values extend from the NE corner of Washington southeast toward the Clearwater Mountains southeast of Lewiston. Keep in mind this area of instability coincides with the passage of the shortwave trough and upper level lifting...a good thing for producing thunderstorms.

CAPE forecast for 5pm Thursday

The parameters shown above are what we meteorologist have looked at for many years, however we are now also getting more refined and specific data from finer resolution models and simulated radar data. So what is this newer model data showing? Its actually quite interesting. Here a look at simulated radar for 5 pm Thursday that was run 42 hours previously (at 11 pm Tue). Notice over NE Washington and the SE corner of Washington there are a pair of bright orange and yellow cells. This model was suggesting that there could be thunderstorms over this area. We see this a lot from the finer resolution models but often can't put a lot of faith in them unless there is some consistency.

Simulated 42hr Composite reflectivity (radar) from 11pm Tue Model run

 So has there been consistency? Lets see the next model run. This one was run 6 hours later and sure enough there are the two yellow and orange areas over NE Washington and a slightly larger one over SE Washington.

Simulated 36hr Composite reflectivity (radar) from 5am Wed  Model run

So that gives us some confidence. Now how about the latest model run? If you look at the image below you can see that the information isn't  quite as convincing as the previous two but it still showed two yellow and orange areas in the same general vicinity. Interesting. So is this enough information to convince us that we will see severe thunderstorms tomorrow afternoon. Probably not, but it does at least hint at a possibility of it. Our confidence is much higher that we will see some thunderstorms...a few of which will produce hail and possible some gusty winds. If the latest models were showing what we see depicted from northern Louisiana toward northern Illinois our confidence would be much higher!

Simulated 24hr Composite reflectivity (radar) from 5pm Wed  Model run