The Human METAR

Plymouth overall saw 10-15 inches of snow (depending on where you were). Down south there was much less, but they also had to deal with some ice issues. The timing of my snowfall forecast was good. I woke up Wednesday morning to only a dusting. Winds varied between 5 and 10 mph throughout the day, but were primarily from the SSE. By the evening we saw about 9 inches on the ground. There was another 3-4 inches overnight, and the winds shifted from SSE to WNW, due to the frontal passage. Today we saw a clear day with “warming” temperatures. Winds were gusty from the W all day.

Let’s see how I did with the high and low temps. I’ll be verifying against Boyd Hall, and another forecast Norman Shippee did.

The only one that appears to be busted is the Wednesday Night forecast, I was off by 10 degrees. I believe it was because the clouds stuck around longer than expected, keeping the infrared radiation in and limiting the temperature from falling.

As for the future, we are going to see a lot of warming…by that I mean above freezing kind of warming. The GFS is showing a big storm coming up the coast next Tuesday. And if the temperatures are still warm, we may even see a rain event. More on this later!

We are over halfway through the storm, and we have about 8-10 inches of snow on the ground in Plymouth. The storm was not enough to cancel classes all day, but they did cancel classes 2pm and beyond.  The storm is getting stronger, but it will stop snowing in Plymouth in about 6-7 hours. There will probably be another 5-6 inches on the ground before the night is done.

I will do my verification tomorrow, but not before a lot of shoveling!

Another snow storm is expected to hit the New England area on Wednesday. The National Weather Service has issued a Winter Storm Watch in effect all day Wednesday. Luckily it isn’t strong as the past storms we have had, and its not going to last long, but it will still dump a few inches. Here is my forecast:

Tuesday Night – Clear skies becoming cloudy overnight. Winds light and from the north. Slight chance of snow showers late into the night with an overnight low of 7°F. Chance of precipitation is 30%. Little or snow accumulation possible.

Wednesday – Cloudy and snowy. High of 26°F with snow accumulations between 6-8 inches. Winds from the NE between 5 and 10 mph . Chance of precipitation 90%.

Wednesday Night – Cloudy with snow tapering off after midnight. Overnight low of 10°F. Northerly wind between 10 and 15 mph.  Nighttime snow accumulations of 2-3 inches. Chance of precipitation is 60%.

Thursday – Partly Cloudy skies. High of 29°F. Winds light and variable.

Looking at this morning’s skew t diagram, it is clear that radiational cooling is present, indicated by the green oval above. However the temperature got a lot colder than forecasted:

So why did the method not work? Well I can think of 2 main reasons. First, we excluded the arctic air mass in the forecasting steps, if we added another 5 degrees, that would have brought us down to -7, making it a closer forecast for Boyd Hall. Also, the 850mb temperatures are not 100% reliable, as warm air advection at that level could mess up the results at the surface. So while this technique is somewhat decent, it should be used carefully.

Forecasting overnight lows can be tricky in the winter months, because the presence of snow, calm winds, and clear skies can induce radiational cooling, where all of the infrared radiation leaves the earth and causes the temperature to drop drastically within the first few hundred meters. This inversion can be seen on morning soundings, such as the following sounding taken at KGYX on the morning of 1/2/09:

Near the surface, the temperature got as low as -11°C, but at just 500m up, the temperature was about 9 degrees warmer. This is a case of radiational cooling.

Well how can you forecast for this? Well there is a simple, semi-accurate way of doing it:

1. Find the coldest 850mb temp overnight (from Models)
2. Using a skew-t chart, start at the temperature at 850mb and come down to the surface moist adiabatically
3. Pick off the temperature and convert to Fahrenheit.
4. IF there are clear skies, subtract 10°F
5. IF there are clear skies AND calm winds, subtract another 5°F
6. IF there are clear skies, AND calm winds, AND fresh snow on the ground, subtract another 5°F
7. IF there are clear skies, calm winds, fresh snow, AND an arctic air mass, subtract another 5°F

As you can see, if there are clear skies, calm winds, fresh snow, and an arctic air mass (like we had last week), the temperature can easily drop 25°F! Man that would be really cold.

Example, using data for tonight’s overnight low

Using the GFS MOS, the coldest 850 temp we will see tonight is -19°C. Moving that down to the surface, that will give us a surface temp of -8°C, or 17.6°F.

Will there be clear skies?: YES! 700mb RH is low, and there is no strong troughs or fronts heading our way, so it looks like there will be clear skies. 17.6 – 10 = 7.6°F

Will the winds be calm?: This one is tricky, becuase some areas of Plymouth may see some winds (downtown, on campus) but some areas could be calm (Baker River Valley). I am going to say however that any winds will die down late into the night, helping to lower the temperature. 7.6 – 5 = 2.6°F

Is there fresh snow?: Maybe, what is the exact definition of fresh snow. The last time we had significant snow fall was Monday. Few days ago, but Im going to say it’s still fresh. 2.6 – 5 = -2.4°F

Arctic Air Mass?: NO! The 850mb temps are above -30°C. I don’t consider that arctic

So after all that im going to round up and say that tonight’s low will be -2°F. Let’s see how this verifies tomorrow