For some reason, it seems appropriate that a record for the highest winds measured in the US takes place in New Hampshire. On April 12, 1934, a gust of 231 mph was measured at Mount Washington Observatory as high pressure built in from the east as a low strengthened over the Great Lakes to the west. When surface pressure changes greatly over short distances, winds tend to be strongest. Friction between the Earth’s surface and the atmosphere usually serves to lower wind speeds...this beautiful, clear, unobstructed view of Mount Washington shows it open to winds from the west.
The diagram to the right is surface data from the area around the Nor’easter off Cape Cod, as initialized by the GFS model at 1 pm Eastern on February 8. Cape Cod is expecting blizzard conditions, which is no surprise as the isobars are very close together.
The blue and red dashed lines measure the thickness of the 1000-500 mb layer. Where the lines are blue, the layer is thin (under 5400 m) and any precipitation falling is usually in the form of snow.
Behind the big nor’easter is a second, weaker system centered just east of Detroit. Note that the surface pressure of the Great Lakes storm is around 1004 mb while that of the nor’easter is 980 mb.
Now, here’s a snapshot of the NAM model’s prediction for 7 am on February 9. This model shows surface pressure, 1000-500 thickness and 6-hour QPF, that is, the water equivalent of any precipitation that may fall over a 6-hour period. All of New Hampshire is in the area of 0.10 to 0.25 inch QPF. The amount of snow this corresponds to depends on multiple factors: temperature being one of them. Under optimal conditions, 1 inch of water makes 20 inches of snow. A more common shorthand is 10 inches of snow per inch of water.
Most of the day, most of New Hampshire will be in the light-green area for snow: in practice that means uncertain, scattered flurries and snow showers that do not really accumulate, but may make driving less safe.
What will temperatures be like?
weather.unisys.com 1200 GMT 850 hPa
These are 850 mb (850 hPa) temperatures and winds from radiosonde data taken this morning around 7 pm Eastern. The flags point in the direction where winds are blowing FROM. The -14 C air over Caribou, ME and the -8 C air over Portland are both moving southwest. It is reasonable to expect that 850 mb temps will be near -12 C in northern New Hampshire and -8 C in the south, and that’s just what NAM (not pictured) predicts.
What to do with these temperatures? One rule (that I saw on weather.unisys,com) says to add 10 C to the reading, then take 10 F back if there is a snowpack. On the other hand, it will be cloudy and windy amidst mountainous terrain, so we can expect the atmosphere to be well-mixed. A first estimate for lowland temperatures would be highs around 15 north and 26 south, with lows Tuesday night around 5 north and 16 south. NWS grid forecasts indicate low 20s north and high 20s south, with lows in the single digits north and 15-20 south. Mountain peaks will be colder.
