Friday, May 29, 2015

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.

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