SPC Day 3 Moderates In Context

Posted by pmarshwx on 8 Apr 2011 | 2 comments

This morning, weather enthusiasts woke to a Storm Prediction Center (SPC) Day 3 Moderate Risk outlook for severe thunderstorms. Most weather enthusiasts already know a Day 3 Moderate Risk outlook is a pretty rare occurrence, but just how rare is it? The simple and misleading answer is that since 2000, and including today’s, only 10 Day 3 Moderate Risk outlooks have been issued. The more precise answer is a bit more complex.

The SPC is continually refining their products based on the state of the science and user feedback. As such the criteria for a Day 3 Moderate Risk outlook has changed over time. Currently, it takes an “Any Severe” probability of 45% and a “Significant Severe” probability of 10% to reach Day 3 Moderate Risk category. Previously it was possible to ascertain the Day 3 Moderate Risk with as little as an “Any Severe” probability as low as 30%, which has happened several more times since the change in probability criteria. Therefore, it is difficult to compare old Day 3 Moderate Risks to current Day 3 Moderate Risks. However, I’ve attempted to break them down below.

Below is a table of the date of issuance for Day 3 Moderate Risk outlooks:

Broken down by year:

For additional information regarding SPC Outlooks, this time focusing on Day 1, please check out the following posts from earlier this year:

• Average Location and Date of First SPC Day 1 Moderate and High Risks
• SPC Day 1 Moderate and High Risk Climatologies
• SPC Day 1 Slight Risks Climatology

All SPC Moderate and High Risk Climatologies

Posted by pmarshwx on 3 Feb 2011 | 0 comments

After posting the climatology of where the first moderate and high risks occur, I’ve received a couple of requests for additional graphics. One that was extremely easy to produce, and also one most frequently requested, is a climatology of moderate and high risks. Using the same Kernel Density Estimation technique described in the original post, I’ve calculated the number of moderate and high risk outlooks an area might expect during a given year, based on data from 1990 through 2008.

Please note that each day consists of multiple outlooks and so it is possible for a grid point to receive multiple “hits” for being located in a Moderate or High risk outlook on the same day. In other words, if your location is located within 2 high risk outlooks, this does not guarantee that you will have two days of high risks. The location might simply be contained within a high risk from two separate outlooks issued for the same day.

For moderate risks, central Oklahoma appears to be the clear winner with nearly 30 moderate risks averaged per year. Increase probabilities extend both north and east from here.

As expected, the average number of high risks per years is considerably less than the average number of moderate risks. (In fact, I had to change the color scale!) Northeast Kansas, northern Missouri, and west-central Illinois are the most likely areas to experience a high risk in a given year with slightly more than 2 expected. A minor axis of increased probability extends southward from the eastern edges of this highest probability band, reaching portions of eastern Arkansas and far northern Mississippi.

Notice how in both of these climatologies, the maximum probabilities are centered in the central United States — east of the Rocky Mountains and west of the Appalachian Mountains. We’ll leave discussion as to why this is for another blog post.

AOTW: Storm Prediction Center Moderate & High Risks

Posted by pmarshwx on 3 Feb 2011 | 11 comments

Although most of the country is still digging out and cleaning up from a historic winter storm, commonly referred to as the “Groundhog’s Day Storm”, I decided to go ahead and answer the Question of the Week regarding Storm Prediction Center (SPC) Moderate and High Risks.

Before I answer the questions, first let me explain how I arrived at the answers…

I took the polygon outlining the first Moderate [High] risk during a given year, regardless of time of issuance. This means I treated a 12 UTC issuance the same as a 1630 UTC issuance and 2000 UTC issuance. I took the risk polygon, placed it on a 4km grid (specifically grid number 240), and activate all grid points that fell inside the risk polygon. This left me with a grid of 1′s (inside outlook) and 0′s (outside outlook). I created a grid for each year and then summed all the grids together. This gave me a grid containing the number of times each grid point was within the first Moderate [High] risk of the year. I then divided each grid point by the number of years I was examining. This left me with the probability that a grid point would be contained in the first Moderate [High] risk of the year.

The resulting plots were extremely blocky because of the straight edges of the outlook polygons and the relatively small size of the resulting sample size (19 years for Moderate risks and 18 years* for High risks). In an attempt to better retrieve the underlying probability distribution, I employed a technique called Kernel Density Estimation. I used a gaussian kernel with a bandwidth of 120 kilometers (per Brooks et al. 1998), and extended the resulting distributions out to 5 standard deviations.

To determine when the first Moderate [High] risk is typically issued, I converted the day and month of the first Moderate [High] risk into the corresponding day of year (e.g., 1 July is the 182 day of the year). I then took the mean day of the year and converted it back to day and month. This means that the average first issuance of a Moderate [High] risk may not fall on a day that actually had a Moderate [High] risk issued.

For this analysis I only evaluated the years between 1990 and 2008. This is because in the early years of the National Severe Storms Forecast Center (NSSFC) / Storm Prediction Center categorical risks had a different meaning than they do now. Thus, I tried to limit the analysis to relatively current definitions.

So, here are this week’s answers:

In the graphics below, the colored contour intervals are every 1% and the thin black contours are every 5%.

• Climatologically, where is the most likely location to experience the first SPC Moderate Risk of a given year?
There are actually two areas when using the raw probabilities: southwest Arkansas and southern Alabama. However, when applying the kernel density estimator, the area in southern Alabama has a slightly higher probability. As for when this first Moderate risk is issued, the average first issuance is 28 January.
• What about High Risk?
There is actually more variability (both spatially and temporally) in the issuance of the first High risk than the first Moderate risk. This is most likely the result of the greater variability in time of year, and thus greater variability of the large scale pattern. High risks have been issued as early as 21 January (1999) and as late as 1 July (1997). [Technically it has been issued as late as never, because 2000 did not have a single High risk issued.] For early High risks, the favored location is in the southeast United States where the ingredients that favor severe thunderstorms is more often confined to the Gulf Coast areas. Later in the year, more northern locations are favored as the cyclone track, and ingredients for severe thunderstorms, lifts northward. With this said, the first High Risk is typically issued across portions of southeast Arkansas and northern Mississippi between 27-28 March.

QOTW: Storm Prediction Center Moderate & High Risks

Posted by pmarshwx on 24 Jan 2011 | 4 comments

This week’s “Question of the Week” comes from Trevor Gramling and Ryan Vaughan.

• Climatologically, where is the most likely location to experience the first Storm Prediction Center Moderate Risk of a given year?
• What about High Risk?

Since I do not know the answer ahead of time, this may end up being a two week “Question of the Week” as I write the code to create the answers…

This ought to be fun!