Temperatue and weather at LSC weather station

Weather summary for Wed. March 4, 2009 at LSC indicates the following high and low
Hi temp: 22.2F (-5.4C) at 13:55
Low temp: -11.7 F (-24.3C) at 06:10

Overnight temperatures were falling continuously (blue) before the low at 6:10 AM shortly before sunrise. This indicates a relatively clear night in which outgoing longwave radiation (OLR) was able to radiate into outer space without clouds interfering.

The sharp 30 degree rise in temperatures during the morning hours (red) indicates strong solar warming associated with clear skies and higher noontime solar angles.

Finally, temperatures in the early afternoon (green) are relatively constant in the lower 20's after about noon . Indeed, a slight dip in temperatures can be observed at around noon. The pressure trace shows a very slight dip (about 2 or 3 mb) in the pressure trace.

Comparison to temperature and pressure data shows a clear morning with light winds and a sudden appearance of snow over Burke Mountain around noon. This corresponds to pressure fall and constant afternoon temperature. Click here to retrieve larger image at webcam archive.

Local satellite images for Wednesday afternoon

The image at left shows the visible satellite at 21 UTC Wednesday, February 18 2009. Clicking on the image will link you to a a visible satellite loop of the entire afternoon after 17Z.

The loop shows thin, scattered cloud moving in early from the west. Later in the afternoon, a thicker, more extensive wave of cloud moves in, also from the west. This wave of cloud is the one that brought with it light snow.

The second image shows an IR image for the same time. It links to a loop of IR imagery. It confirms the double wave of cloud, and that the second wave appears to be more extensive. In addition, comparing the cloud shading to the temperature scale at the bottom scale at the bottom shows that the cloud tops have a temperature of about -40 C.

Current and archived imagery can be found at the UCAR's satellite page.

High pressure system and eastern cold surge

The image at right is for 0Z Thursday 05 Feb
2009 (7PM EST Wednesday 04 Feb 2009). A line of high pressure systems over the eastern U.S. dominates the eastern half of the continent. A line of high pressure centers is called a ridge. The circulation around this ridge is clockwise and outward. This means that to the east of the high the winds blow from northwest and push in cold air (blue arrows). To the west of the highs, winds blow from the southeast (red arrows). Note that the cold arctic air is transported all the way to southern Florida behind a cold front. The temperature at Savannah Georgia is approaching freezing (36 F).

The second image shows a forecast valid one day later at 0Z Friday 06 Feb 2009 (7PM EST Thursday 05 Feb 2008). The high pressure centers have moved southeastward to the Florida panhandle. The associated ridge to the north shifts eastward, extending across New York State into Canada, indicating that pressure over Vermont and New England will continue to rise. Cold northwesterly winds will continue through all of Thursday. A low pressure system forms over southern Alberta (an Alberta clipper!).


The third image shows a forecast valid two days later at 0Z Saturday 07 Feb 2009 (7PM EST Friday 06 Feb 2008). The high pressure system has moves souteastward to the coast of northern Florida. As a result, a south or southwesterly wind now dominates much of the eastern part of the U.S., bringing with it warmer air. It appears that this will warm up temperatures over the weekend starting Saturday morning.

The dark black arrow indicates the path of the Alberta Clipper moving into the Dakotas from southern Alberta. The counter clockwise circulation around the low enhances the southerly winds to the east of the system. This low pressure system will bring it with cloud and light precipitation as it moves rapidly across the continent this weekend.

Cold air and high pressure

The trace of weather data from LSC for Wed. Feb 5, 2008 indicates rising pressures and falling temperatures. As of 10:30 PM, the temperature is already -3 F, colder than the overnight low of 2 F the previous morning. The pressure trace shows a rise in pressure to about 1015 mb currently from about 10 mb from 1005 mb at midnite the previous evening. (Note however that the pressure values are low compared to other stations, indicating possible problems with the barometer). In the bottom panel, sustained winds (blue) and gusts (purple) become calmer and calmer as the high pressure moves in.

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.

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!