A 50-50 forecast

Main Threats

Friday night: Heavy rainfall

Summary

We’ll see mostly cloudy skies today but don’t be surprised if the sun tries to sneak in a peak or two. The forecast this week is split about 50/50, with the forecast looking good up until Thanksgiving, but starting to go downhill by Friday. For those of you planning to go shopping on Friday, the first half of the day is looking better than the second half. By Friday night, heavy rainfall will be a possibility as our next storm system really gets its act together.

The first half of Saturday is looking quite wet, while the second half of the day may not be half bad. 

Looking ahead, another strong storm system may move in by Sunday evening, bringing more heavy rainfall, followed by a shot of colder air. 

Discussion

I sometimes have people ask me about rain chances. “What’s the chance of rain today, Mark?” I get that all the time. But do folks understand what I mean when I answer them?

Forecasters have  a complicated job, that is for sure. We’re trying to tell you what to expect from an atmosphere that is made up of millions of billions of equations that are changing every minute. Throw in the earth’s complicated surface (plateau!) and you have yourself a real challenge. It’s made even more complicated by our limited data from the atmosphere. We have all kinds of surface data (temp, rainfall, etc) but our sources of atmospheric data are limited to satellites and weather balloons that are released twice  a day, once in the morning and once at evening at select (and isolated) locations around the globe. It’s a wonder we ever get a forecast right! Thankfully, supercomputers make our jobs a bit easier and on average, help us make a forecast that is right 80-90% of the time.

Forecasting the weather for a location where one grew up makes it a bit less challenging, as well, but it’s still a challenge. 

To make matters even worse, you have to worry that people interpret your forecast correctly. I can’t tell you how many times somebody has gotten a downpour of rain and came to me and said, “So much for that 20% chance of rain, right?” Well, right! Congratulations! You won the 20% chance! lol 

The National Weather Service has a good write-up on this and I’m borrowing from their example.

So, let’s say I forecast a 40% chance of rain. What does this “40 percent” mean? Will it rain 40 percent of of the time? Will it rain over 40 percent of the area?

The “Probability of Precipitation” (PoP) describes the chance of precipitation occurring at any point you select in the area. So, how do we forecasters arrive at this value?

Mathematically, PoP is defined as follows:PoP = C x A where “C” = the confidence that precipitation will occur somewhere in the forecast area, and where “A” = the percent of the area that will receive measureable precipitation, if it occurs at all.

So… in the case of the forecast above, if the forecaster knows precipitation is sure to occur ( confidence is 100% ), he/she is expressing how much of the area will receive measurable rain. ( PoP = “C” x “A” or “1” times “.4” which equals .4 or 40%.)

But, most of the time, the forecaster is expressing a combination of degree of confidence and areal coverage. If the forecaster is only 50% sure that precipitation will occur, and expects that, if it does occur, it will produce measurable rain over about 80 percent of the area, the PoP (chance of rain) is 40%. ( PoP = .5 x .8 which equals .4 or 40%).

In either event, the correct way to interpret the forecast is: there is a 40 percent chance that rain will occur at any given point in the area.

I hope this makes sense! 

Records

On this day in 1900, a violent F-4 tornado cut a swatch of total destruction across Hardin, Wayne, and Lewis Counties in southern Middle Tennessee. The path was 300 yards wide and eight miles long. The tornado reached Columbia and killed two people just west of town. Records indicated that Columbia may have been hit by tornadoes just prior to this bigger one arriving. Cabins in the area were “turned to kindling wood”.  The tornado claimed 27 lives in all and injured 75. This is the fourth deadliest tornado in Middle Tennessee history. Farther north, an F-3 tornado associated with this event (and a separate supercell) went on a 25-mile rampage across Williamson, Davidson, and Rutherford Counties, killing 9 and injuring 40.  

This tornado outbreak of 1900 was a fierce one and there’s no telling how many people and homes would be lost if tornadoes of this intensity took those same paths today. In all, 73 people lost their lives in the South this day to tornadoes and Arkansas, Mississippi, and Tennessee all reported extensive and widespread damage. There is even speculation that some of these tornadoes were more powerful than the ratings given to them. Since the Fugita scale wasn’t invented until the 1970s, we have to look to newspaper accounts, etc to go back in time to rate tornadoes. That is a problematic method, to say the least, but it’s the best we can do.

Almanac

News

Ok, so I’ve been snooping around the extended model data, which is a fool’s game in all honesty. For those of you wanting snow (cough), I think Sunday night/Monday morning may bring us a shot of some snow showers. Nothing major but maybe some snowflakes. And the weather world is all abuzz about the first week of December. I know, I know…that’s a LONG way out but there are strong indications of some arctic air coming in as Gulf moisture may be stirring up some trouble. It’s enough to keep an eye on, for sure. Cross your fingers, snowbirds! You might want to dust off that snow dance, too. 

In other news, arctic air is really invading the Northeast and they may see their coldest Thanksgiving on record this year. 

You all have a great day! 

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