Forecast: 15 Inches of Rain at Our Water Cooler

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What do you overhear at your office water cooler? Seinfeld‘s George Costanza prefers to get his gossip at the office coffee machine (“Nobody drinks from a water cooler any more — they use bottles”), but trading gossip in the office — whether at the water cooler or the coffee machine — is as old as the office itself.  Location aside, you’ll probably agree there’s more going on most times than mere gossip. That’s why my ears perked up yesterday when I heard one of our StormWatch forecasters mention an upcoming rain event with a possible 15 inches of rainfall. You don’t need a weatherman to tell you 15 inches of rainfall is a significant event, however how and why a forecast for 15 inches came to be definitely requires a weatherman to deconstruct and understand.

Taking shape: It doesn't look like a rainmaker for the Gulf Coast just yet, but the low complex over the Great Basin isn't even warmed up yet. Click for larger image. Image: ImpactWeather StormWatch

Let’s take a closer look at our water cooler gossip: 15 inches is a crazy amount of rainfall, but is it really worth considering? Maybe. First, this forecast is for an event still four and five days, perhaps even six days away. A lot can — and usually does — change in a 120+-hour weather forecast. Second, it’s not a one-day rainfall total, but an amount based on total accumulations during the course of a multi-day event. Third, could it (the 15-inch prediction) be an attention-grabbing device? Kind of like an attempt to shut out the background noise of other meteorological chatter by dropping the Mother Load? Fourth, 15 inches is a crazy amount of rainfall.

By Thursday the low has moved to the Plains and is getting acquainted with the humidity of the Gulf of Mexico. Severe weather — including tornadoes and flooding — in Oklahoma, Eastern Texas and the Mississippi River Valley is likely. Click for larger image. Image: ImpactWeather StormWatch

Here’s what I think: 75% of me feels that 15 inches of rainfall is an attention-grabbing attempt to say, “Hey, this upcoming rainfall amount is beyond your everyday big rainfall amount — this one is going to be huge.” However, I’ll be the first to admit that the forecaster who dropped that bomb is not one to play such attention-grabbing stunts. The other 25% of me feels that too much can change in a 4- and 5-day forecast to bank on 15 inches of rainfall. What’s the answer? Bank on a significant rainfall event, but pay attention to the daily forecast leading up to the event because it will change.

1983's Hurricane Alicia brought nearly 10 inches of rainfall to areas of the upper Texas Coast, while 1989's Tropical Storm Allison brought more than 20 inches to Southeast Texas. 1979's Tropical Storm Claudette produced the maximum 24-hour rainfall in U.S. history: 43 inches over Alvin, Texas. Image: Wikipedia

Meteorologically, what exactly is going on? There’s a low pressure complex and frontal system over the Great Basin of the American West that is strengthening today. It’s expected to move southeastward to the Plains by Thursday. Here’s where it gets interesting: as it reaches the Plains, it taps the vast moisture of the Gulf of Mexico — much of it already in place over the central and southern Plains and the Mississippi River Valley, but of course a bottomless supply lies just south of the coast in the form of the Gulf of Mexico. But what is really the source of the “15 inches of rainfall” comment is what happens — or what is expected to happen — to the low once it reaches Louisiana: it slows and/or stalls. Once stalled, a low pressure system such as this becomes, essentially, a rainfall machine. It will feed off the Gulf of Mexico while remaining powerful due to the expected structure of the atmosphere supporting it. 15 inches is beginning to not sound so crazy.

Five days from now — an eternity in the world of weather forecasting — the low is showing signs of moving off to the east while the frontal system becomes stationary in the Gulf. Whether the low stays or goes, the instability remaining in the Gulf is a sure sign of heavy rainfall to come. Tornadoes and flooding will certainly be a concern. Click for larger image. Image: ImpactWeather StormWatch

At this time there are many variables left to understand and fine-tune. Where exactly will the low track? How fast (or slow) will it move? How likely is it to stall? If not 15 inches, then how much — and where will it fall? To further complicate the matter, computer model guidance is not being very helpful: one rushes the system east (with significantly less accumulated rainfall), while two lag the system across the Plains but differ on their placement of the low and of the rainfall. An ensemble group of computer models suggests a similar “lingering” pattern, yet it too is different from the other computer models.

Time, as always, will tell, so I’ll continue to buzz the water cooler — and coffee machine — as needed. I’ll report back here in a couple of days.

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