A cold front is on the way. Are your weekend plans in jeopardy?

5:41 PM, Sep 20, 2013   |    comments
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Weekend forecasts can often be tough. A lot of people have outdoor plans-- maybe you're one of them-- and you want to know if the weather is going to cooperate. As I'm about to show you, when a cold front is on the way, sometimes it's tricky to predict exactly when the wet weather will arrive!

If you've been following our forecast, then you've known all week that rain was likely on Saturday. Our forecast model data has been consistent on that. But now that the weekend is upon us, we would like to have a more precise idea of when the rain will fall, rather than generalizing with a blanket statement such as "rain and storms are likely on Saturday". 

So, how can we get more precise? I like to look at different forecast model sources and compare them to each other. I do this every day for the 7-day forecast, but in trying to fine tune the timing of precipitation, I need to look at the model data more in-depth. 

First, the European model. This one seems to be going for a slightly later arrival of the precipitation, and also a longer-lasting rain event in the DMV. At 2pm on Saturday, the model still shows the rain lingering to the west for the most part: 

With the bulk of the moisture moving through from dinnertime through the wee hours of the morning. This is the European model's depiction of the rainfall from 8pm Saturday until 2am Sunday:

Notice in the legend that the blue shading indicates ares of precipitation of up to 1" during that 6-hour period. Typically, when you see rainfall amounts that high with a frontal passage in late September, you can expect some thunderstorms along with rain showers. (Other atmospheric indicators are pointing to the possibility of non-severe storms on Saturday, too).

Continuing to follow the European model's progression of the front, we are dry again in the DC Metro by 8am on Sunday, with the only lingering showers in southern Maryland. The Euro's forecast for precip at 2pm Sunday shows this clearing trend: 

So, if we just trusted the European model and didn't take any others into account, then we would expect Saturday to be completely dry until at least 2pm, but the rain lingers through the overnight hours into Sunday morning. The GFS model, which is the main American forecast model, paints a slightly different picture. At 2pm on Saturday, the rain is already falling in parts of the DC Metro area, according to the GFS. 

The most intense rain, again, with thunderstorms possible, is during the afternoon and early evening on the GFS model. 

You can also see how the GFS model's forecast keeps most of the heavier rain (depicted in oranges and yellows) to the north of the Beltway. 

Just as the rain moves in earlier on GFS, it also moves out earlier. So by late Saturday night, we are dry again. Here's a look at 2am Sunday morning's 6-hour precip forecast:

Compare this map to Map #2. Notice how much further east the front is on the GFS than on the Euro by 2am Sunday! So, the American and European models are approximately 12 hours apart on their forecast for the front's passage. Even just one day before the front is expected to move through, we still don't know exactly when the rain will arrive! 

Model discrepancies are the main reason why our forecasts are not always 100% accurate, and it's also the main reason why forecasts can differ from one source to another (different TV stations, National Weather Service website, etc.) But at least, in this case, we can say with confidence that Sunday will be the better day of the weekend for anything you want to do outside. Enjoy!

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