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Risk of Snow Showers tomorrow, cold for Friday

By Lee Carlaw On Wednesday, January 23, 2008 At 5:19 PM
Current Weather:
Temperatures currently in the lower 40s should drop into the mid 20s overnight under mainly clear skies and light winds in and around the Beltway. Farther to the north and west, temperatures are presently in the mid to upper 30s, and should fall into the upper teens overnight.

Forecasts For the Next Few Days:

Thursday:

By 8 or 9 AM tomorrow, clouds should begin streaming in from the west, as energy associated with a tiny pocket of mid and upper level energy slides east from the Midwest. High temperatures tomorrow will hover in the lower to mid 30s in and around DC, which will support a few snow showers, which may develop during the late morning and into the late afternoon hours. Some minor snow accumulation is possible on grassy and elevated surfaces, but most roadways should remain snow-free due to above-freezing road temperatures.

Probability of Snow: 40%

Image at right: NAM forecast precipitation for Thursday afternoon showing the possibility of some light snow showers around the metro region. Image courtesy of the Penn State E-Wall

Friday:
Cold, Canadian air is expected to pour into the region overnight Thursday as a cold front sweeps through the Mid Atlantic. Temperatures early Friday morning will range from 15 around Hagerstown and surrounding areas, to near 20 near Washington, D.C.

Clouds should scour out by mid morning, leaving us with mainly sunny to skies. High temperatures will be in the upper 20s and lower 30s.

Saturday:
Clouds will increase late Friday night and into Saturday morning as a very weak upper level disturbance rotates through the region. At this point, there does not appear to be enough lift to get any precipitation over the mountains. Temperatures will hover around 40.

Paying Attention to What the Forecaster Actually Says:
With the recent forecasts of snow in the metro region, it has gotten to me when people ask me, "hey, the weather channel said there was supposed to be snow last night. How come there was none/How come it didn't stick?"

It seems incredible to me that, with all the precision and time forecasters put into their work, the vast majority of the public really doesn't pay attention to some of the crucial details. Here's a portion of the text forecast for Montgomery county for Thursday from the National Weather Service:

.THURSDAY...MOSTLY CLOUDY WITH SCATTERED SNOW SHOWERS. HIGHS IN THE UPPER 30S. NORTHWEST WINDS 10 TO 15 MPH. GUSTS TO 30 MPH IN THE AFTERNOON. CHANCE OF SNOW 40 PERCENT.
Seems simple enough, right? Now, take a quick look at the image to the right. It's The Weather Channel's forecast for Thursday. If it DIDN'T snow at all tomorrow, I would bet that most people would say, "the meteorologists got it wrong again. Where was the snow??"

Very few public viewers actually take the time to read the forecast text. Notice that little percentage at the bottom? In the NWS text forecast, this percentage is 40%, and in The Weather Channel's it's 30%. These are known as Probabilities of Precipitation.

Most people think of POP as the percentage of getting precipitation on that day. That, unfortunately, is only one part of the formula. POP is based on two things:

1) The probability that any precipitation will fall in the time period and
2) The predicted areal coverage if precipitation actually develops

So take this hypothetical situation (it would more than likely never happen, but what the heck):
Washington, D.C. records a POP of 20% for 100 consecutive days. If this POP were accurate over the long term, then Washington should have experienced some type of precipitation for 20 of those 100 days (not a great number, right?) The likelihood of precipitation being recorded when the POP is 20% is extremely low.

So, the next time you read or hear a forecast, it would be wise to also look or listen for the Probability of Precipitation. Weather isn't exactly and exact science.

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