12:58 Mon May 13, 2019
Bring on the Snow!
Once again there is snow in the forecast for the summit of Mount Washington. Snow storms like this one are always exciting up here, no matter the season, at least for me anyways. This storm has been fairly complicated to forecast for due to weak dynamics aloft. This is really to be expected at this time of the year as the jet stream weakens and there is not as big of a temperature gradient between the north pole and the equator to help increase baroclinicity (One of the major sources of energy for mid latitude cyclones).
Below is a map of the 250 millibar (roughly 35000 feet above sea level) winds and the strongest parts of the jet stream is well to our south and the wind velocities are barely over 100 mph. Being north of the jet stream means we are on the cold side of the storm and that is why we are expecting snow. The area of low pressure, which is right where the red L is on the map, is sitting just north of the jet max, which is giving it some upper level support. Due to the weak flow aloft, this area of low pressure will be sitting just off of the coast for a few days, with clouds and precipitation moving in off shore.
Near the coast, with on shore flow, temperatures will be warm enough for rain but near the foothills of the White Mountains, there will be enough cold air aloft to possibly see some snow flakes all the way down to valley level. Luckily at this time of the year, the ground is warm, which will prevent any snow accumulations below 2000 feet or so. We will need to see high snow fall rates to overcome the melting of the snow on the warm ground in the valleys.
Up here on the summit is a different story. Temperatures have been well below freezing most days so the ground is still frozen. We do not typically see the ground really start to warm up until later in May or early June so snow will accumulate up here, even in lower snow fall rates. With topographical enhancement, snowfall rates up here will be approaching an inch an hour for most of the night, so the possibility of high accumulation is there. Though a small change in the intensity of the storm or placement of the low will have a huge impact on the amount of snow we will see. The weak flow could mean the forecasted placement of the low could be 50 miles off, even if it is only 12 hours into the future.
Right now we are expecting right around a foot of snow up here by the time the snow ends early Wednesday morning. I personally am hoping we get a similar event to the May 14th 2017 storm that dropped 33 inches of snow!
Adam Gill, Weather Observer/IT Specialist