The images here represent the weather conditions at LSC as of 11:53 AM Tuesday 22 May, 2007. They were retrieved from the course weatherpage under the 'Current Conditions' section. You may click on the images below for larger versions of these images.

The image at left was retrieved from the 'Summary' link, and indicates that the maximum and minimum temperature normals are 68 and 39 F, respectively. The record high and low temperatures are 88 and 24 F respectively. Record temperatures give some perspective on how extreme temperatures can get at a given time of the year. You may have noticed that both Monday and Tuesday seemed cooler than the normal. The webcam also shows that it was cloudless outside.

The data to the left were downloaded from the 'Details' link. The temperature at 11:53 AM was 61.3 F. At this time, the day's maximum and minimum temperatures were 61.3 F (at 11:45 AM) and 30.9 F (at 5:20 AM), respectively. This means that it is still getting warmer outside at this time; also, it went below freezing (32 F) overnite, giving as a late spring frost. This low is well below the normal overnight minimum temperature, indicating that we are in a cold air mass. Still, it is nowhere near the record cold temperatures of 24 F set for this day.

The previous day's high temperature was 58.0 F at 4:10 PM and the low temperature early Monday morning (just after midnight) was 39.1 F. While the minimum temperature was average for this time of year, the afternoon maximum temperature was much cooler than normal. It was a cold day on Sunday. It was windy too. The highest wind reached 14 knots which is close to 25 mph. This made it seem even cooler.




To understand why it was so cold Monday and Tuesday through despite the bright sunshine, you need to remember what the flow is like around high and low pressure systems. The image at left illustrates this circulation. In particular, notice that the winds east of a High pressure system generally come from the North. This usually brings in cold air into a region.

Going back to the course weatherpage, you can click a current surface pressure map at the link called HPC(archive). The surface pressure map for Monday evening at 8 PM is shown at left (click on the image to enlarge). Vermont is filled in green at the top right of the page. To the west of Vermont lie two High pressure centers. Because the flow around high pressure is clockwise, we expect the winds to be from the north (i.e. northerly) over all of New England. You will notice that the wind barbs do indeed indicate northerly winds east of the high pressure system, with the wind bringing in cold polar air from the north. Hence from the map we would expect cooler temperatures and clear skies as the high moves in from the west.

The image at left shows the weather map for the U.S. valid 5 PM Thu Apr 26 2007. New England is under the influence of a high pressure system just off the Atlantic coast. The weather maker for the next three days is a slow-moving low pressure system that is over Illinois on the map.

Discuss the evolution of this low pressure system and its associated mesoscale features (i.e. fronts, thunderstorms, etc.). Discuss also one of the following: 1) it's influence on New England weather, particularly that of Northern Vermont, in any one of the next three days; 2) it's past effect on weather over the southern U.S. (it was associated with severe weather outbreaks in the past three days).

Here are a few links to help you discuss this case. You may upload imagery and data found here to the discussion board:

Course weather page: Describes local conditions at LSC

HPC's Surface analysis archive: Archive of surface maps, satellite, and radar imagery back to 7 days

HPC Forecasts: Current weather map and forecast out to 5 days

Storm Prediction Center (SPC): Current national radar map with weather warning/watch boxes for severe weather

SPC Storm reports: Current storm reports; you can navigate this page for daily storm reports back several years; the past three days of storm reports

New England Surface weather loop: Station reports and fronts for New England

Solutions in red.

The image at left is another depiction of the relationship between the polar jet stream (remember, the jet is high in the troposphere, not at the surface) and warm and cold air masses in the lower troposphere near the surface of the Earth. Cold air masses generally lie north of the jet, and warm air masses to the south. Using this principle, it's possible to make predictions about temperature trends using 300 mb maps that show the position of the polar jet.

All three images at left represent forecasts for 12 UTC (8 AM) Saturday Morning April 14. The first was issued Monday at 12 UTC and represents a 5-day forecast, and is the same image posted below in Monday's . The second was issued Thursday at 12 UTC (8 AM local time) and represents a 2-day forecast.

The Monday forecast shows a pronounced ridge over New England. The jet stream lies to the north of Vermont. It indicates that there is a warm air mass under that ridge over New England. The more recent Thursday forecast shows a less pronounced ridge over New England. The jet stream in this case lies right over Vermont. This means that the warming below the ridge is not as strong as in the 5-day forecast.

The final image shows the actual jet stream on Saturday morning. The jet is not as intense as predicted. You will also note that the core of the jet over New England is further south than predicted. This indicates that the warm air associated with the ridge did not make it as far north as predicted. This should

How is this reflected in the NWS forecast? On Monday, the NWS called for a forecast high temperature of 42 F at Lyndon Center at the bottom of Vail hill. On Thursday, they were still calling for a high of 41 F. Personally, given the look of maps, I would have adjusted my prediction to about 36 F. It is possible that with the clear skies that usually accompany a ridge will allow the sun to warm things up. However, the nor' easter that brought us today's storm will likely bring in low level moisture that will keep things cool and damp.

Sorry, people it looks like more winter.

Final assignment for Saturday/Sunday.

A) Download a map of the actual 300 mb heights at 12Z Saturday morning and compare them to the forecasts. (Click here and click on 000 under 300 mb sometime after 11 AM on Saturday). Pay particular attention to the ridge and jet stream over New England.

B) Check the Saturday high temperature at the course weatherpage. Is it higher or lower than the predicted temperatures? Explain whether these results consistent with your observations of the jet and ridge mentioned above.

The high temperature was 41.7 degrees F recorded at 2:30 PM. As it turns out, errors in predicting the jet did not affect a very accurate 5-day forecast of the high temperature for Saturday. However, this does illustrate that there was a warming (albeit slight) associated with a northward displacement of the jet.

Chapter 9 introduced how planetary-scale ridges (associated with high pressure aloft) and troughs (low pressure) are associated with warm and cold air masses respectively (see Figure 1). Ridges are also associated with a poleward shift in the jet stream, and troughs with an equatorward shift in the jet. Because large planetary waves in the jet stream tend to remain stationary for several days, they can be used to make predictions about long term temperature trends and and the track of storm systems.

The Geophysical Fluid (GFS) model is a numerical weather prediction (NWP) model (i.e. a huge computer program) used by the National Weather Service (NWS) to predict large-scale weather patterns out to 14 days (usually, forecast accuracy goes down after 2 or 3 days and nosedives after 5 to 7 days). Figure 2 shows the 300 mb GFS model analysis for 12Z (8 AM EDT) Monday, April 9, 2007. Note that the blue shading indicates wind speed in knots as you plotted in Investigation 9A, NOT temperature. This is the data used to initiate the model.

The map shows a large trough over all of eastern North America. You will note that this is so pronounced that the trough has cutoff into closed cyclones in places. Meteorologists call this a cutoff-low (clever, aren't they?) The jet stream is well to the south of the northeastern U.S., looping around the Great Lakes and out over the Atlantic.

This trough has been with us for about a week and is associated with the cold weather we are having. Persistent troughs like this are also associated with damp, humid weather. At this time of year, it makes it feel like winter is never going to leave. Going to the course weatherpage and clicking on the Summary link near the top of the page gives climate data for Monday and shows a high temperature of 36.3 F compared to a normal high if 48.0 F, 12 degrees colder than normal. So if you think it's cold for April, you're right.

Will winter leave us soon? Let's check the 5-day forecast that the model produced Monday morning (Figure 3). This forecast is a 120-hour (exactly five days) valid 12Z (8 AM EDT) Saturday morning Apr. 14. This is a very different pattern. A wavy jetstream snakes its way across the continent. Over the northeastern U.S., there is a ridge that pushes the jetstream to the north.

Weatherblog Assignment 3:
1) Make a prediction for the high temperature for Saturday based on Figure 3. Discuss your reasoning. You may include references to web or broadcast forecasts to support your prediction. The NWS forecast for Lyndon Center can be found here.
2) Update the temperature and 300 mb wind forecast before Wednesday evening. New forecasts for Saturdays are issued every 24 hours at the GFS model Upper air website. Here, new forecasts run at 12Z (8 AM local time) are posted everyday at about 11 AM.
3) Plot the verified 300 mb map for 12Z Saturday and comment on the quality of Monday's original forecast. Discuss the position of the ridge relative to previous forecasts and whether temperature forecasts were accurate. I will post the verification on this blog.

The 300 mb Wind forecast is best for monitoring the polar jet stream. The forecast maps are given every 6 hours. You can loop them, or look at them individually. The numbers represent forecast times: 24 represents a 24 hr (1 day) forecast, 048 a 48 hour (2 day) forecast, and so on. You can check the valid forecast date at the bottom of each chart.

Remember that the Saturday forecast hour will change every day. On Tuesday it will 096 hr, Wednesday 072 hr, Thursday 048 hr, and Friday 24 hr. It will verify on Saturday as the 000 hr forecast.

No, this is not a flying saucer!

This is a lovely lenticular cloud that formed over Burke mountain and was visible from the Vail building late Monday afternoon. It is called a lenticular cloud because it is in the shape of a lens. It is an orographic cloud because it is induced by a mountain (in this case, Burke Mountain).

Click here to see the complete time lapse animation for clouds on Monday. These clouds did not form until late in the day. Notice how these orographic clouds are stationary compared to other clouds which zip along in the jetstream.

Lenticular clouds like these are often wave clouds that form as the flow above mountains becomes wavy (see imiage), much the way fast-flowing rivers do as they cross large rocks in the river bed. Wherever the air flow is upward, clouds begin to condense. You can see several clouds form downwind of Burke in the animation above.

The deep freeze continues.

Monday morning's temperature map of North America shows that the cold pool located over Ontario Saturday morning has migrated over Quebec and extends south of the Great Lakes and into New England. Note that the Great Lakes are not yet frozen and are 32 degrees F. The cold air is warmed and moistened as it crosses the Great Lakes, leading to huge snowstorms in towns downwind of the Lakes (like Grand Rapids, Michigan and Buffalo and Syracuse, New York). A second cold pool centered over Northern Saskatchewan moves in from the Arctic. It looks like this cold pool will keep us in the freezer for the rest of the week.

Can anyone use a pressure and wind map and describe the circulation of wind around high and low systems and explain how these winds push these Arctic cold pools towards Vermont?

Tropopause in Arctic air mass

The image at left shows the temperature at Albany, NY Monday morning. The stratosphere begins at the tropopause, where the temperature becomes constant (about -40 C) with height. In Arctic air masses, the tropopause is usually very low.

Deep freeze will return this weekend.

It will once again get colder for the beginning of the week. Note the pool of cold air over northwestern Ontario in the image at left (Saturday Morning). The circulation around high and low pressure systems will slowly blow this cold air towards us over the course of the next 5 days. You should be able to follow this progession from the links on the course weatherpage.