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!

Observations of Jetstream: Surface Maps

With a little knowledge of geography, you can observe the jet stream on a surface map too! The circled station (click on the map to see details) is from the Mount Washington observatory high atop the mountain about 1.5 miles above sea-level. Whereas winds are calm almost everywhere in New Hampshire, a steady westerly breeze of 25 knots (a little more than 25 mph) is blowing here. You can see a current surface plot of this map here.

Observations of jet stream: Satellite Imagery

This high resolution visible satellite imagery of VT and HM shows the sun going down at the end of today. It confirms oue obervations of clouds being moved from west to east by the mid-latitude westerlies. A stratocumulus cloud mass (meteorologists sometimes call it a "blob of cloud" for obvious reasons) passes over the Northeast Kingdom of stratocumulus late in the day.

You can see current high resolution visible satellite imagery of New England at the Meteorology Department's Satellite page.

Observations of jet stream: WebCams

LSC Webcam imagery for Monday, 8 Sept 2008. Click here for current animation.

The LSC webcam is in the staff room on the 4th floor of the Vail Building looking southeastward over the campus in summer and eastward towards Burke Mountain in the winter. It's a good way of watching clouds develop, storms pass, and the day go by. You can guess at which direction the jet stream is moving too.

The clouds seen in this video are generally stratocumulus clouds: sometimes they look like puffy cottonballs (cumulus clouds), and sometimes they flatten out into a flat cover (a stratus deck). It looks like they are low, maybe a mile or two above the surface.

Note that even though the wind was relatively calm all day on the ground, they are moving at a good clip due to a strong westerly jet stream, pushing them towards the east. This is the norm in the middle latitudes. Winds usually increase with height in the lower 10 miles of the atmosphere due to the jet stream.

Severe Storm Reports

Severe storm reports of damage associated with thunderstorms (mostly) is compiled by the National Weather Service at the Storm Prediction Center (SPC) in Norman, Oklahoma. You can access their website on the course weatherpage. You can view todays reports by clicking on the "Reports" links on the menu at the top of the SPC site. Click here to view the more detailed updated reports for Sept. 3, 2008 (9 PM version shown above). You can click to previous days' reports at the top of the page. Note that what's left of tropical depression Gustav is still triggering some tornado activity and flooding along the southern Mississippi River.

Closer to home, a line of thundershowers seems to be triggering hailstorms in the Berkshires just to the south of Vermont. The radar below for this Wednesday evening shows storms moving southward from Vermont and into the Berkshires (click on image for animation). You may have seen the towering clouds move past us to our west during sunset today. The red colors show the storms that were probably generating the hail.

The direction of the storms is somewhat unusual. Usually storms will move in from the south. It explains why the SPC missed issuing a watch for these. To be fair, they were also probably watching the tornadoes in Louisiana more closely, because they are much more dangerous.

The southward path of the New England storms can be explained by the conterclockwise circulation around the low pressure system situated off the shores of New England. As discussed in class, this results in northerly winds winds FROM the north over New England (see Sea-level Map below). They move the thunderstorms southward into Massachusetts.

The image at left shows the frequency of Atlantic Hurricanes over the last 100 years as described by the climatology at the NHC Homepage. It shows that while the hurricane is well underway in June and July (the season officially begins June 1), the season really spins up in August and peaks in mid-September. So we can expect an increase in tropical storm activity and intensity throughout August. This peak is due to the fact that ocean waters actually reach their peak temperatures in September (dry land reaches peak temperatures in late July).

This is also accompanied by a slight change in the track of the storms. The July climatology shows tropical storms developing over the West Indies and tracking either to the Gulf of Mexico or along the coast of the Southeastern States. Note that hurricanes Dolly and Cristobal both followed these paths, respectively, in the past week.The August climatology shows an over all intensification of tropical cyclone activity, but also a new track towards Florida not visible in July. As well, storms appear to be more likely to veer northward in the Gulf of Mexico, unlike July storms like Dolly which hook westward towards the coast of Southern Texas and Mexico.

Vapor channel from tropics

Warm front advances
The northeastern frontal map for 12Z Thu. 24 July, 2008 (left) shows that the stationary front that was over southern New England on Monday is being pushed northward by southerly winds circulating in a counterclockwise direction around a low pressure system over Lake Ontario. Cloudy conditions. Dewpoints in the high 60's almost everywhere in New England indicate humid conditions. Overcast conditions keep temperatures int he 70's all day.

Transport of humidity
The main weather story today (and all of this week!) is the rain and humidity. This is largely because of a channel of water vapor (i.e. humidity) that has opened up between New England and the Gulf of Mexico. This occurs along the narrow band of clouds ahead of a cold front along the Atlantic coast in the image at left (valid 12Z Thu. 24 July, 2008). A cold front extends southward from the low pressure system over southeastern Ontario all the way to the Gulf of Mexico. Red arrows in the image at left show the direction of the warm, humid flow along the Atlantic coast of the U.S. ahead of the cold front. This flow is pushing the warm humid air northward. The cold front is pushed eastward by a surge of cooler, drier westerly winds behind the front (blue arrows).

Warm conveyer belt
This is pretty typical of a mature phase of mid-latitude cyclone development shown at left (see Jetstream section of cyclone model for details). The image shows a warm, humid conveyer belt (red arrows) overrunning cooler air to the north where the warm flow meets the warm front. This rising motion leads to large amounts of precipitation ahead of the warm front. Note that the occluded front (purple at left) does not appear on todays map. Occluded fronts are rare in the summertime because weather systems evolve more slowly, with fronts moving much more slowly. Your text discusses this in more detail in Chapter 10.

Vapor channel in satellite imagery
The vapor channel is easy to see in satellite animations. Click here to see an animated loop of the warm conveyor belt (or vapor channel) discussed above. The cold front can be discerned along the border between clear and cloudy skies in the eastern half of the U.S. The thunderstorms and clouds along the eastern seaboard ahead of the front all moves northward in the fast moving southerly flow ahead of the front. Note also Tropical Storm Dolly along the Gulf coast of Texas/Mexico.

Hurricane/Tropical Storm Dolly

The image at left shows the current Atlantic tropical storm outlook. Tropical Storm Dolly made landfall along the south Texas coast as a Category 1 Hurricane but has since been downgraded to a tropical storm. The clouds in the yellow circle represent a tropical depression (weaker than a tropical storm) that is slowly moving westward across the Atlantic. This could develop into a tropical storm or hurricane over the weekend, but it is too early to tell. You can find updates to this graphic along with a brief forecast discussion at the National Hurricane Center's Webpage.


The second image shows the swath of tropical storm and hurricane force winds (winds greater than 38 mph and 64 mph, respectively) associated with Dolly. The National Hurricane Center has introduced this map because it shows the real effects of the storm. Dolly picks up energy shortly before it hits the coast and attains hurricane status. Dolly then loses much of its "punch" as it hits land. This is typical of tropical storms. Increased friction over land puts a break on the winds, and the storm gets cut off from the warm waters that give them their energy. This image is also updated at the NHC webpage.
The final image is the station map of the south-central U.S. taken 18Z Wed 23 July, 2008 as Dolly made landfall. This map illustrates how small the storm actually is. There is no station plot actually showing hurricane force winds. This is because the actual region of hurricane force winds is very small, but also because there is little data in the eyewall of a hurricane (hurricane force winds tend to topple wind towers over land, and ocean going ships avoid hurricane centers). Most data from hurricane centers is taken by dropsondes every six hours (dropsondes are like rawinsondes except that they are dropped from reconnaissance aircraft flying out of the NHC headquarters in Miami). You can find regional maps and loops like this one at the HPC North American Surface Analysis webpage.

Steering flow

Clicking on the image at left will show a 24-hr animation of the stationary front over New England on Monday 21 July, 2008. It is being distorted by small mesoscale disturbances (or low pressure systems) that advect parts of the front northward and southward as they rapidly move eastward. They also trigger local showers as they pass through a given area. These mesoscale systems are in fact smaller than the large synoptic-scale systems we usually track in the other seasons. Recent animations of New England station plots and fronts can be found and the HPC surface analysis page.

These disturbances are steered over New England by the upper flow. Clicking on the image of left for 8 AM EDT Sunday 20 January 2008 shows the jet stream on a 500 mb map dipping south of the Great Lakes and across New England roughly above the stationary front described above. It is this westerly flow that guides system eastward across New England.

The 24 hr precipitation field for 8 AM Sunday through 8 AM Monday reflects the path of small scale distirbances in the steering flow. Precipitation fall in the northern U.S. follows the same curved pattern around the Great Lakes and into New England as the systems guided by the jet stream. These maps (and more) can by found at the Daily Weather Maps webpage of the HPC.

Stationary Front over New England

Fronts are usually analyzed at the leading edge of cold air. The past few days have seen a stationary front stalled over New England leading, with a moist polar air mass lying to the north and a moist tropical air mass lying to the south . High humidity in these air masses combined with the front itself has given us lots of rain over the past few days.
The image at left shows temperatures taken at 3PM Monday, July 21 2008. Northern New England and Southern Quebec are in a uniform polar air mass where most temperatures (with a few exceptions in the Burlington VT area) lie in the 70's. South of the front, temperatures are more tropical (generally in the 80's).

The station analysis (click on image below for better view) for 18Z (2 PM EDT) on the same date shows dewpoints (green numbers) in the 60's almost everywhere in the northeast, indicating an abundant supply of water vapor. These high humidity conditions make for sticky, uncomfortable weather everywhere. The nightime temperatures are not likely to sink below the dewpoint, meaning warm, clammy nights where you constantly have to flip your pillow to get the cold side (grrrr ... ).


A stationary front extends along the temperature gradient (the area of contrasting temperatures) across New York State and Connecticut. The strong temperature gradient is actually north of the front. Very weak lows associated with cloud and thundershowers are shown over Massachusetts, southern Ontario, and southern Michigan. Weak winds around these systems blow across the stationary front, leading to warm and cold advection. These weak systems are typical of the summer, when extratropical cyclone development is not as well defined as in the winter, making these systems difficult to track. Tropical cyclones, as we will see this week, are much more frequent and obvious at this time of year!

Flooding in Iowa

You may have been hearing news stories of the disastrous flooding in Cedar Rapids, Iowa.

This is largely due to a stationary front that persisted over the Iowa region for more than a week. Stationary fronts are a lot like like warm fronts in that they result in moderate precipitation over a broad area

The visible satellite image at left shows the tops of thunderstorms mushrooming along the trough extending southward from the low pressure system over Quebec described in the previous post below. These storms flipped a truck on the Champlain Bridge in the middle of rush hour in Montreal, leading to a 7 truck pile up. Amazingly, no one was killed. Winds also knocked out power at LSC and at my home in Frelighsburg, QC.

A long line of thunderstorms like this is called a squall line and accounts for most of the severe thunderstorms in our area.

The animated visible satellite image shows how quickly these thunderstorms grow in the hour between 18Z and 19Z. It seems that these storms form as the trough hits the Lake Champlain Valley. This rapid formation is why thunderstorms are so difficult to predict. The trough and cold front which triggered the storms could be seen days before, but if these storms erupted an hour later the Champlain Valley and Montreal would have been missed entirely!

High resolution visible satellite satellite imagery for Vermont can be accessed at the LSC Meteorology Department website.

Air mass advection and fronts

The image at left from 21 Z (5 PM EDT) Tuesday 10 June 2008 will show how air mass advection and fronts around midlatitude cyclones can change the weather in New England.

A low pressure center can be seen in Quebec to the northwest of Vermont. Wind barbs indicate a counterclockwise and inward circulation around the low that is usually seen around a cyclone. A cold front extends soutwestward and a stationary front extends southeastward from the system center.

Three airmasses can be identified in the map. Over Vermont and most of New England, a marine tropical air mass predominates. It's this air mass that brought us near-record high temperatures between the weekend and Tuesday. It is bordered by the cold front to the west and a stationary front to the east, forming a wedge south of the low pressure center. Temperatures are in the 80's and 90's F and dewpoints are in the high 60's. Southerly winds in this air mass advect warm moist air northward in this region all the way from the Gulf of Mexico.

West and northwest of Vermont lies a continental polar air mass, with both temperatures and dewpoints in the 50's and 70's, the cooler temperatures residing to the north. It's this air mass that has moved in today, giving us cooler and drier conditions outside. Winds all have a westerly, with those over Canada having a more northerly component. They advect this airmass eastward. You probably noticed that it cooled off considerably yesterday evening as the front moved through with the storms yesterday.

Finally, over Maine and the Atlantic Ocean, a marine polar air mass had been destroying the hopes of anyone looking for a summer day at the beach. Temperatures and dewpoints along the coast of Maine and the adjacent ocean are all in the 40's and 50's, indicating the strong influence of cool Atlantic waters. It's these air masses that typically lead to ocean fog. Easterly and south easterly winds advect this cooler air onshore. These winds are enhanced by the afternoon seabreeze circulation.

The final feature to notice on the map is the dashed brown line, representing a trough. You can see this also in the isobar looping southward from the center of the low. This was associated with the line of thunderstorms that swept across northern Vermont and southern Quebec yesterday evening and knocked out our power.

You can access a 24 hr loop of New England fronts and weather station data at the NWS-HPC surface analysis website.

Climate data for yesterday

Clicking the image at left will give you an idea of the high temperatures reached yesterday. It was in the 90's pretty much everywhere in Vermont, with the hot spots being near the south end of the Connecticut Valley and in the southern Champlain Valley region. Mountain stations were cooler, but the 79 F maximum temperature on Mount Mansfield tied a record set for this date in 1974.

The second image shows the total precipitation that fell yesterday afternoon and evening. The effect of the Green Mountains is strong here. The largest precipitation amounts were in excess of one inch in the area of Mount Mansfield (3.67'' at Underhill, 2.52'' at Jeffersonville) and Jay (1.80'' at Jay Peak, 1.33'' at East Berkshire). In the Northeast Kingdom and Connecticut Valley, less that 1'' fell.

Precipitation is a difficult quantity to get a feeling for. Most areas of the northeastern U.S. get on average about 3 inches of rain in a month. So 3'' is about what you would normally expect to get in month. If that amount falls in a day, you're getting a lot of rain!

Both of these examples illustrate climate data that record weather extremes and totals for a given day. Local climate maps can be retrieved at the NWS-Burlington Office website.

Hot Sunday

Data from the LSC weather station around noon Sunday reads:

Time: 12:35 EST
Temp: 84.3 °F
Dew Point:67.2 °F
RH: 55.8 %

Summer is not a time of year that starts at the June 21 summer solstice and ends September 21 at the fall equinox. Summer in New England happens when southerly winds advect in a marine Tropical (mT) air mass from the Gulf of Mexico.

Clicking on the the mesoscale analysis of temperature at left (see NWS website for updated map) shows the effect of elevation on these temperatures. Temperatures on top of Jay Peak and Mount Mansfield are 79 F, whereas low elevation stations in the southern Connecticut and Champlain valleys are already in the 90's.

Everyone knows that 84 degrees F is hot in the noon day sun. What about a dewpoint of 67 F? What does that feel like? Well, you might note that it makes working outside a sticky experience. The humidity index chart shows that this humidity (corresponding to 57% relative humidity) makes 85 F feel more like 90 F and 90 F feel more like 100 F.

My personal weather gauge, Spooky the Cat, has climbed down from her current warm spot on the sofa and is sprawled out on the floor wondering when this will all end; she wants to go back to flushing out the small burrowing mammals attacking my cabbage patch. I think this means that it's uncomfortable for all species. More on why this is happening on Mondays blog.

Warm, humid air mass in Vermont

Data from the LSC weather station around noon Sunday reads:

Time: 12:35 EST
Temp: 84.3 °F
Dew Point:67.2 °F
RH: 55.8 %

Summer is not a time of year that starts at the June 21 summer solstice ends September 21 at the fall equinox. Summer in New England happens when southerly winds advect in a marine Tropical air mass is advected in from the Gulf of Mexico. Those of us inVermont is a warm, humid (mT) air mass.

Clicking on the the mesoscale analysis of temperature at left (see NWS website for updated map) shows the effect of elevation on these temperatures. Temperatures on top of Jay Peak and mount Mansfield are 79 F, whereas low elevation stations in the southern Connecticut and Champlain valleys are already in the 90's.

Everyone knows that 84 degrees F is hot in the noon day sun. What about a dewpoint of 67 F? What does that feel like? Well, you might note that it makes working outside a sticky experience. The humidity index chart shows that at this humidity (57% relative humidity), it makes 85 F feel more like 80 F and 90 F feel more like 100 F, My personal weather gauge, Spooky the Cat, has climbed down from her current warm spot on the sofa and is sprawled out on the floor wondering when this will all end and go back to flushing out the small burrowing mammals attacking my cabbage patch. I think that means that it's uncomfortable for all species.






Temperatures at 15Z (11 AM EDT) are already in the low 80's and dewpoints in the and high 60's in Vermont.

The situation is serious.

reads: The map at left shows three large distinct air masses clashing over the northeastern U.S.
between a warm front to the east and a cold front to the west.




By

RH:	55.8 %

80 F is obviously hot.

Cool, humid Sunday

Another great day for the indoors. Yesterday's low pressure system (see below) has moved northeastward and is now situated over Atlantic Canada. If you remember that the flow is counterclockwise and inward around the low, you'll see that this brings a flow of maritime polar air from the North Atlantic ocean across Quebec and into New England (see red arrows in figure at left). This keeps things cool, cloudy and humid, with a few showers.

Rainy Saturday I (warm front, and low pressure system

A rainy Saturday. A good day to stay inside and work on your weather studies.

The map at left for 15Z (11 AM) shows us why. We have a low pressure system currently situated just north of the Great Lakes and moving in from the west. A warm front extends eastward and across New England. This is bringing in steady rain of moderate intensity extending over a broad area from New York State to Atlantic Canada. This type of rain is called stratiform precipitation because it falls from a low-level, flat layer of stratus cloud. Rain like this usually lasts about a day because it is so widespread. It takes time for a system this large to move through an area.

As studies in Chapter 1, the wind circulation around this low (red arrows) is counterclockwise and inward. East of the low (i.e. New England and eastern Canada), this circulation causes southerly winds (FROM the south) that bring in a moist tropical air mass from the south behind the warm front. You'll notice that even though it is cloudy outside, it is still pretty warm.

You can find more maps like this at the NWS-HPC website.

Temperature Inversions

In Chapter 2, we learn that the temperature in the troposphere generally decreases with height at a rate of about 6.5 degrees C/km. These are average conditions, and in fact the temperature profile of the atmosphere varies considerably. In the case of cool, polar airmasses like the one currently over Vermont, there are small layers of the troposphere where the temperature actually increases with height. These are called temperature inversions.
The temperature profile chart at
right (the red line measures temperature) is taken this morning at 12Z (8AM) over Maniwaki, Quebec, and represents the sounding station closest to Vermont from the the North. You will note here that the temperature between 925 and 850 mb (about 2500 to 4500 ft) actually rises from about 6 to 10 degrees C (about 43 to 50 degrees F). On days like this, you'll find that the temperature is actually warmer at the top of a mountain than at its foot!

Charts like this that plot temperature against heigh are called STUVE diagrams. You can accesscurrent charts like this one at the LSC-MET webpage for upper-air data.

High pressure system over North America II

The image at left provides some additional information about the high pressure system discussed below, adding station plots and radar echoes to the analysis. Wind barbs confirm the circulation of wind around the high pressure center over the Great Lakes. In particular, the wind barb over Portland, Maine confirms a northwesterly flow (i.e. wind coming FROM the northwest) over New England. Remember that wind barbs point in the direction the wind is coming from. They point INTO the wind, whereas the arrows in the post below point in the opposite direction (WITH the wind).

It should be no surprise that it is so cool and clear today in Vermont. It's common knowledge that northerly winds bring in cool, dry air (i.e. continental polar air) from Canada.

The image above is updated hourly the Unisys website. It too can be accessed on the course website.

High pressure system over North America

The image at left represents a depiction of pressure contours, fronts, high and low pressure systems, and IR satellite imagery from the National Weather Service. It is valid at 8AM (12 UTC) this morning. The continent is dominated by a massive high pressure system that extends into New England. Note the large area of clear skies associated with this system. A band of cloud associated with a long cold front that extends from the Labrador sea to the southeastern U.S. and is moving out to sea. Arrows indicate the direction of winds circulating around the high pressure system. Note how over New England, northwesterly winds (winds blowing FROM the northwest) push the band of cloud out to sea.

Note that this image is updated and archived every six hours at the NWS/HPC Surface analysis archive and can be accessed on the course weatherpage.

Welcome to ProfWW's weatherblog for Spring 2008

You have managed to get to ProfWW's weatherblog for Lyndon State College's online course in Elementary Meteorology Online. Please watch this space for weather discussion and other discussion topics.