Warm layers and precipitation type near Portland, Maine

The image at left illustrates how a warm layer (above freezing) aloft affects precipitation type. Where temperatures are below freezing everywhere, snow falls. Where the layer is above freezing near the surface, rain falls. In between, the type of frozen precipitation depends on teh depth of warm and cold layer. Warm layers need to be deep enough to melt the snow, and cold surface layers need to be deep enough to freeze the melted precipitation.

The images below show the evolution of the warm layer ahead of the coastal warm front for 24 hours on Friday, 12 December, 2008 near Portland Maine. Note that an extra radiosonde was launched at 6UTC (1 AM EST) to document this exceptional event. The freezing line is marked by a bold blue line at 0C (32 F).



At 0Z (7PM Thursday), a small warm layer above freezing can be seen forming at 850 mb. An inversion can be seen through most of the layer below this level. This would likely produce snow mixed with some ice pellets and freezing rain.

By 6Z (1 AM Friday morning) the warm layer has expanded to include the entire 900-700 mb layer. Southwesterly winds keep this layer warm, whereas the cold layer below 900 mb is sustained by cold northeasterly winds.

By 12Z, the warm layer reaches temperature of 10 C (50 F) while surface temperatures hover around freezing. Saturated conditions can be observed throughout the entire troposphere during this entire period. At rhis point on Friday morning, the warm layer and associated frozen precipitation extended all the way across the Northeast Kingdom, albeit for a very brief period.

By 0Z 13 December (Friday 7PM), cold advection and northwesterly winds are pushing out the warm layer, ending the ice storm. Note that temperatures everywhere are zero, and that conditions are far drier than before.

Ice Storm surface map

A New England surface map for 12Z (7AM Friday Dec 12, 2008 (click image for larger map) at the height of the storm shows a low pressure center tracking along a coastal front. It is a warm front because it is being advected toward the coast by the southerly winds in the warm sector of the low pressure system. This front had been in stationary for the previous two or three days. These winds overrun the front and the White mountains of New Hampshire. Cold air remains wedged northwest of the front in the region of strong precipitation northeast of the front. Freezing rain and sleet at this point extended all the way across Vermont into Canada. The rawinsonde stations of Albany, NY (ALB), Gray, Maine (GYX) near P0rtland), and Cariboo, Maine (CAR) are marked with boxes. The station in northern New Hampshire is Littleton (HIE).

Cold air damming

In addition to a warm front, mountains help exasperate the situation through cold air damming. Cold air, which is denser than the warm air advected toward the mountain gets trapped east of North American mountain ranges such as the White Mountains in New England and the Appalachians in the Carolinas. The warm moist air then overruns this cold air, generating precipitation. If the warm air is above freezing, rain will fall from the warm layer and freeze in the cold layer.

References:
USA Today
WW2010

Warm fronts and frozen precipitation

The structure of warm fronts (stationary fronts are similar) is illustrated at left. Warm, humid air from the warm sector of the system is advected over a wedge of cold air. The lift leads to cloud formation, and a layer of warm, humid air above the cold wedge. However, precipitation falls into the colder air, and will freeze if the air is below freezing. Click here for a more detailed explanation of warm fronts.

Ice Storm hits Maine, New Hampshire, and New York

Friday's storm hit parts of southern New England much harder than the Northeast Kingdom. There are 400, 000 homes and businesses are without power in New Hampshire, and with the an arctic air mass moving this weekend, people are moving into shelters to escape the cold.

Here are a few news stories covering the story:

American Press
Maine News
Nashua Telegraph
Albany Times Union

Stationary and slow moving warm fronts are generally responsible for most severe ice storm events. They are rare, occurring once or twice a decade. The front provides a) lift to drive ascending motion b) cold air to the north or west to freeze the precipitation c) warm air aloft above the cold sector of the storm to melt the falling the precipitation. Because they are slow moving, they concentrate their precipitation fall over one region.

Frontal analysis for 21 Z Thursday 11 Dec shows that the front pasing over New England yesterday has stalled off the coast. A coastal cyclone will track along this front tomorrow, intensifying the precipitation over New England. This indicates that the precipitation pattern over New England will not change much until the cyclone has passed tomorrow evening. This will be a nasty ice storm in coastal Maine, southern New Hampshire, and Western Massachusetts. If you were planning to drive into these regions tomorrow, don't. Roads will be a sheet of ice.

Intellicast provides radar imagery identifying precipitation type. The image at upper left represents radar echoes at 23:15 Z (6:15 PM) Thursday. Blue represents snow, red represents mixed precipitation (wet snow, freezing rain, and sleet), and green represents rain. Click here for a current radar loop.

It is just starting to snow over the NEK. Mixed precipitation extends from along the coast of Maine across southern New Hampshire and Vermont, across the Albany area of New York, and into northern Pennsylvania.

The surface plot for approximately the same time confirms that the mixed precipitation is freezing rain. Subfreezing temperatures extend across this region. Cold air continues to be advected into New England by northerly winds.

Monitoring melting layers aloft

Freezing rain requires surface temperatures below freezing and a warmer layer aloft that is above freezing. For the NEK, the Mount Washington Observatory has weather sensors at various elevations that allow the weather observer to identify a warm layer up to 6000 ft (approximately 800 mb).

As of 6 PM Thursday, the temperature the base of Mount Washington (1600 ft) was 22.5 F and the summit (6288 ft) was 25.7 F, indicating a weak inversion. Precipitation would be falling as snow under these conditions. If th temperature at the summit were above 32 F, we could expect freezing rain or ice pellets.

Weather observations for Mount Washington can be found at this link.

Ice Storm or Snow Storm?

With cold air firmly in place over Vermont and a strong front well to the southeast of Vermont, the forecast question for tomorrow is whether tomorrow's precipitation will fall as freezing rain or snow. Here are a few pointers for predicting precipitation type using a modern forecasting technique known as ensemble forecasting.

Ensemble forecasts are numerical (i.e. computer) forecasts that have slight differences in the the way the forecast parameters such as temperature, precipitation, and humidity are calculated. Nobody knows ahead of time which precise method is best, so several (21 to be precise) computer model runs are made to see how variable these predictions are. They give forecasters a feel for how prone a numerical forecast are to error. Taking the average gives you a "best guess" at what may happen .

The image at top left (click on image for larger image) shows a 27 hour forecast of the WRF model ensemble forecast for the NE U.S. It is initialized 12 Z (7 AM) Thursday, 11 December, 2008; it is valid 3Z Friday, 12 December, 2008(10 PM Thursday night). Shown is the precipitation type (snow (blue); freezing rain (pink); rain (green). An ice storm will occur somewhere along a large swath extending from Maine to Pennsylvania. The question is whether this swath will make it to the NEK in Vermont. About 1/3 of the runs at 10 PM Thursday show no precipitation, another 1/3 show snow, and yet another 1/3 show freezing rain.

As the blue band of snow is very narrow, skiers and snowboarders will be disappointed. Any snow we get at Burke will likely be mixed with all sorts of sleet and freezing rain. Because there will be a strong inversion, it could be that mountaintops will even experience mostly rain. New Hampshire ski areas are almost certain to get freezing rain, and Jay is likely to get only a little snow early in the day, major snowfall occuring late in the day. The best skiing will likely be at Jay on Saturday morning, if you can make it up there in strong winds and ice cold temperatures.

Three hourly forecasts out to four days are available at the Penn State Meteorology Map Wall. Mouse over the forecast time (F03, F06, F09, ... ) for the forecast of the given time. This will be a major event that will intensify throughout the afternoon. Stay tuned for possible class cancellations Friday morning!