Friday, May 1, 2015

April Verification and May Forecast 2015

First, let's recap the April tornado activity forecast:

The forecast called for 20% chance of being below average, 40% chance of being average, and 40% chance of being above average. 

April averages 131 tornado reports (1950-2014).  This average is slightly higher (163) if considering only the period 1985-2014. This year, the U.S. tallied 181 preliminary tornado reports during April (calculated using the filtered preliminary tornado reports from the Storm Prediction Center), thus putting us at 138% or 111% of normal tornado reports this month, depending on the averaging period.  For ERTAF, I have been using <75%, 75<=x<=125%, and >125% of normal climatology (based on the 85-14 reports) to classify BA, A, and AA activity respectively.  This is obviously subjective and unimportant, as the atmosphere could really care less how we try to split these categories.  I am going to run with the lower, 111% of normal for April 2015, thus putting the month at average tornado reporting activity.  This aligns nicely with the April forecast for a 40/40 split between A and AA conditions. 

Environmentally, one can see that the atmosphere was much more favorable for severe convective storms this April relative to the previous March post! 

Red pixels indicate negative LI anomalies, which are indicative of an anomalously unstable atmosphere supportive of thunderstorms.  The greatest LI anomalies were found in the southern Great Plains and Gulf Coast areas, coincident with the greatest density of tornado and other severe weather reports. 

So, what's up for May?

Based on the current large scale pattern, dynamical models, analog approaches, and ongoing research, here is my forecast for May 2015:

10% Chance Below Average |  50% Chance Average | 40% Chance Above Average

For those interested, sub-monthly forecasts (ERTAF) are being made weekly (issued on Sunday evenings) and can be found here.

Here is a map of reported tornadoes in the month of May:

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