Tuesday, October 27, 2015

Weekend Soaking in Perspective...and Another Round Ahead?

By Steve Caparotta

Were you among the plethora of south Louisiana residents begging for some rainfall through the first few weeks of October? I was certainly in that crowd, growing tired of running my sprinkler to keep my lawn and garden from wilting in the dry and warm weather. But as they say, be careful what you wish for.

In a remarkable turn of events, we went from 'extreme drought' in some locations to dealing with flooding in the course of just a couple of days. In fact, the most recent Drought Monitor that was released last Thursday, October 20th, had more than 50% of Louisiana classified as being in an 'extreme drought', with over 13% of North Louisiana under the worst classification of 'exceptional drought'. It's fair to say the map below will see significant changes when a new update is released this Thursday.

As it turns out, a stretch of 24 days without measurable rain in Baton Rouge and much of the surrounding area and an overall dry stretch since the beginning of September turned out to be a blessing this past weekend. While we certainly dealt with some localized flooding in urban areas and continue to monitor rising rivers, it could have been much worse had there been any rain of consequence in the prior days and weeks.

How big of a rain event did we see this past weekend? For most of the area, it was the most significant multi-day rain since Tropical Storm Allison produced flooding in 2001. Sunday's rainfall ranks as the 5th largest single-day total in Baton Rouge in records that date back to 1892!

And the 2-day rain total for Saturday and Sunday in Baton Rouge ranks as the 6th largest 2-day event since 1892.

It's also remarkable to consider that through October 23rd, Baton Rouge Metro had only recorded a trace of rainfall. Three days later, 2015 stood as the third wettest October on record!

One other way to measure a rain event is by using a tool that climatologists call 'recurrence intervals'. Without getting into the dirty (and boring to many) details, recurrence intervals are a statistical measure of how frequently a given location can expect certain rain amounts over a given duration. Using a NOAA tool available online, we find that the 2-day rain total of just under 11 inches in Baton Rouge is a roughly 1-in-25-year event. In other words, people in Baton Rouge can expect to see rains like this past weekend roughly once every 25 years when averaged over a LONG period of time. Another way of looking at it is that there's a 4% chance in any given year of seeing a 2-day rain like this past weekend.

Recurrence intervals for rainfall in Baton Rouge. Source: http://hdsc.nws.noaa.gov/hdsc/pfds/pfds_map_cont.html?bkmrk=la
While most of our neighborhoods are beginning to dry out, rivers are still rising and we'll see additional problems through the week for those who live along or near rivers. And the bad news? It looks like we've got more rain on the way for the weekend.

The rains will be somewhat of a double-whammy, not only arriving over the weekend, but potentially turning into a real issue for Trick-or-Treaters and Halloween parties. There's still some uncertainty in the timing of the rains over the weekend, but it looks like the wet weather will begin to arrive by Saturday afternoon and continue into Sunday.

The map below shows that NOAA's Weather Prediction Center is already forecasting an additional 2" to 4" of rain for much of our area over the weekend.

One more thing to note that we've already mentioned at times in our weathercasts. One of the strongest El Niño events on record is ongoing in the Pacific. History tells us that during El Niño events, fall and winter tend to see above-normal rainfall along the Gulf Coast. With that in mind, it's quite possible that these rain events will only become more frequent in the coming months. The currently 3-month outlook for Nov-Jan from NOAA's Climate Prediction Center is in line with that thinking, calling for above-normal chances for above-normal precipitation in our region.

Stay dry, everyone, and count on the WAFB First Alert Storm Team on air, online and on your mobile device for the latest on any storm systems headed our way.

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