06/29/12 Derecho

There are 921 damaging wind reports from this derecho. And yes, all 921 reports went into this custom radar loop. Each box represents a wind report. Some frames have clusters that show up only to vanish during the next image. This isn't necessarily meaning that there is no wind at the location, rather the reports fell in one frame but not the other. One can see continuing damaging tracks likely from updraft mesovortices along the line edge. These mesovorticies, with the exception of the reports from Raleigh (from a gust front) dominate the northern portion of the line near the end of the loop. There is a great increase of wind reports east of the Appalachian Mountains: this is because descending a leeward slope induces stretching of the vortices along the line. Enjoy the loop!

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***NEW video: better resolution, better for squall line induced vortices***

06/27/12 Forecast

Tropical Storm Debbie

Debbie seems to confirm my suspicion that it would not be making landfall west of New Orleans. The forecast track has changed significantly and is now projected to make landfall along the Florida peninsula. I am actually revising my initial forecast that Debbie will make it to a 70mph tropical storm before landfall. The low level center of circulation is tilted. A tilted circulation displaces the mid-level warm core of the center and can dry out the center of circulation if the shear is strong enough. This gives a tropical cyclone a very asymmetrical appearance.

Structure of a tropical cyclone, from Britannica.

CIMSS animation of water vapor and infrared imagery.

Unisys upper level wind (and low)

 There is wind shear over the center, which keeps displacing the core of convection and tilting the circulation. If you look at the surface, the low level center isn't even underneath the upper level center.

The convection associated with Debbie is far displaced from the center and the upper level circulation is no longer directly over the low level circulation.

And there is another problem with Debbie, which I will show in the next two images.

Twisterdata 500mb relative humidity

Twisterdata 300mb relative humidity

A warm core tropical system derives its strength from the condensation of water vapor, which with condensation, releases heat. The mid-level (below 500mb) is healthy and shows an effective supply of latent heat. But at and above the 500mb level, dry air intrusion is choking off thunderstorm activity. When the rising motion around the center of circulation starts ingesting less buoyant air around it (dry air), it in turn becomes less buoyant and continued rising motion will decrease or cease altogether. We see a healthy environment for tall thunderstorms both along the Florida coast and the Gulf Coast, where the convective activity is. We do not see this over the center, and thus, another reason why we are not seeing convective growth over the center. Debbie is a shallow top system partially because of this.

Verdict - Wind shear will shift to a southerly direction which will not help Debbie strengthen. I have a hard time seeing Debbie get past 70mph, and even that number is optimistic. That would be a scenario if the upper level center were to become more co-located with the low level center. If the core structure does not have a significant change, Debbie will struggle to get past the current strength of 60mph.

Sugar Mountain: Google Earth

And now we get to our assessment of the largest ski resort in North Carolina, Sugar Mountain. What is interesting about Sugar Mountain that makes it stand out from the other resorts is the sheer length of the ski runs. One would have to travel to Snowshoe, WV to find a comparable ski run length. This is from the design of the resort, where instead of 10-12 separate trail, you have 3-5 trails with different sections and ratings. This can be (and will be shown below) a double edge sword for Sugar Mountain.

Advantages:

While Sugar boasts the longest continuous vertical and the longest runs in North Carolina, one inevitably has to go all the way down to the bottom of the mountain to board the lifts. Beginner skiers without question have the most favorable area for them across the Southeast. At no other resort across the Southeast (the closest would be Winterplace, WV) do beginners have a separate area isolated from the rest of the mountain. At all other resorts, intermediate and advance skiers heading down the mountain inevitably have to ski through the majority of the beginner slopes and the beginner terrain is just as much delegated to funnel skiers back to the main lifts as it is to give beginners a comfort zone. The most crowded slopes at any resort not named Snowshoe are at the base where skiers are slowing down and converge, both due to the increase of skier density and due to inevitable choke points from other ski runs merging with the main run. Snowshoe, with it's inverted setup and unusually large number of separate trails, does not see this to the extent of regular ski resorts.

Sugar Mountain has the main run carry the intermediate/advanced skiers to the base, but has a number of separate (and difficult to access from upper areas of the mountain) beginner ski runs, with its own lift. This is very wise. I have noticed a large number of collisions on beginner and lower intermediate trails from a wide gradient of speed between beginners and advanced skiers. Where you have a large gradient in skier speed you have accidents. Strudel and Orchard Run at Appalachian Ski Mountain have both the largest gradient of skier speed, and the largest number of ski accidents.

While I have never gone on the beginner ski lift at Sugar Mountain, it sits very close to the ground. Someone deserves a gold medal for designing a beginner lift that sits low to the ground. Often when I would teach skiers and take them on the chairlift for the first time, they would have a fear of the chair height.

Sugar Mountain also boasts a secondary peak in which intermediate skiers have their own terrain to ski on. The size of Big Red is enormous and eats up the skier traffic.

Disadvantages:

Sugar Mountain's double edge sword is with its layout. With a resort with such a great length but such a narrow horizontal width, you will inevitably have all of your skiers be funneled into a small area. And this is exactly what happens. If traversing from the top, there is no conceivable method of getting around the Flying Mile trail.  The yellow lift has a "loading" station about halfway up the mountain, which is nearly useless on even a semi-crowded day.  Big Red is underused as a trail; since it is difficult to access this trail from the top of the mountain, this trail doesn't quite see the traffic that it can handle. A big drawback are the lifts, mainly the two main lifts. The lifts aren't slow; for fixed grip lifts they are peaking at the skier capacity. Any faster and unprepared beginners will get swept off the lift, leading to it stopping. The length of the lifts, and how these two lifts run from bottom to top are the big drawback. Is it necessary to have THREE unloading points on a single lift?

Current ski traffic setup at Sugar Mountain

If Sugar Mountain required a skier to take two separate lifts instead of one, then intermediate/advance skiers can circulate on the upper half of the mountain and beginners/lower intermediate skiers can focus on the lower part of the mountain. Remember the problem of "long lift times?" Beech Mountain knows about it already; this is why they have a billboard advertising "Fastest lifts in the Southeast" right next to the Sugar Mountain entrance! So if we go for a two-lift design, make the lower lift a fixed grip quad and make the upper lift a....detachable quad! With minor cosmetic changes to some trails (slight widening of the middle part of Flying Mile, a more useful connector to Big Red from Flying Mile, a *possible* advanced intermediate trail next to Tom Terrific, we suddenly have a Sugar Mountain with a very efficient traffic flow.

Two separate lift design

And the final version of the mountain: red lines = fixed grip lifts, pink lines = detachable quad lifts, purple polygons = new terrain.

Meteorological Assessment:

Sitting at a base elevation of 4000' and a peak elevation just under 5200', Sugar Mountain's elevation  passes my GIS test of both being on a peak above 5000' and having a resort base above 3500'. The peak elevation minimum of 5000' ensures that there is sufficient orographic uplift for a Northwest Flow Snowfall. Now, this peak elevation is not an essential. Poga Mountain barely tops out at 4300' yet still racks in heavy upslope snowfall. A big part of this is having nearby peaks that are above 5000' (Snake Mountain, Roan Mountain, Sugar Mountain, Beech Mountain). 

Since orographic snowfall can constitute up to 50% of a season's snow content, every tangible factor which increases this snowfall total is critical. Since Sugar Mountain is *near* the western periphery of the Appalachian Mountains, the moisture content is mostly undiluted and thus maximizes in orographic snowfall. Areas east of here receive far less snowfall, with Deep Gap (just 20 miles away) receives far less snow from an upslope flow. This is because upslope events have little moisture to work with and with this being a stable air mass, vertical uplift is unlikely to occur past the initial windward slopes. For a perspective, there is often a capping inversion between the 725mb and 750mb levels. Above this layer the relative humidity sharply decreases. That is shallow my friends. And that's during peak conditions. I have seen a moisture layer as shallow as the 850mb level (4700') that STILL produced measurable snow. Northwest Flow Snowfall however does not have a very high liquid to water content, so this snow compacts and melts faster than a southerly tracking system. We have had Northwest Flow Snow events range from a 10:1 snow ratio to an outrageous 70:1 ratio (70" of snow to 1" of rain water). I can sweep that off my deck!

With a low density like that, this snow type is likely to have snow drifts associated with it if there is any significant wind. This is a disadvantage for Beech Mountain, as some of the snow is simply blown off the slopes. This disadvantage is somewhat negated at Sugar Mountain due to the slopes facing northeast. Instead, the snow is maintained because of the wind blowing perpendicular to the slopes.
 
Sugar Mountain has does not quite have the weather advantages as Beech Mountain but is still in a very favorable location. Higher elevation = cooler and enhanced Northwest Flow Snowfall = more snow. The enhanced sun exposure on Sugar Mountain (due to the runs either facing northeast or east-northeast) is a downside, even though the runs are sheltered from the wind. These runs will have more sun exposure and will melt more snow on warmer days. In fact, only the Big Red run faces north. Perhaps an advantage to the northeast slopes is that since the sun is hitting these slopes much earlier in the day, then it may reduce the chance of icy slopes.  Unless you are a ski racer, one will appreciate softer snow more than bulletproof ice any day.

Feel the Heat!

From the Miami Heat Website

    You get the idea, but like the team, we will be seeing more of the heat this week. A teleconnection pattern switch will allow for a ridge to engulf the Southeast. At first I thought this was part of the Bermuda high but it is not. Before we focus on our weather, I want to show the Atlantic satellite image. The same cutoff low that I suggested last week that could support a subtropical storm seems to be well, developing into a subtropical storm.

From CIMSS.

With a low/moderate shear environment, marginal water temperatures, the low has a short opportunity to develop into a named (likely subtropical) storm before it races towards the northeast. The storm still has a dependence on the upper level cutoff low but a surface warm core low is beginning to develop underneath it. The system is acquiring tropical characteristics but I believe it will be classified as subtropical, due to the reason stated above.

GIF image of 500mb geopotential height anomalies, from E-Wall (Penn State)

The steady ridge that has been creating a super zonal flow across the eastern Pacific is weakening and the positive height anomaly will subside. This height anomaly along with the negative anomaly to the north will weaken the pressure gradient between the two, and thus, weaken the zonal flow entering into the Pacific Northwest. This isn't necessarily a bad thing though. The super strong zonal flow has pushed right up to the eastern edge of the northern Rockies, and has been the culprit of the lack of precipitation across the Central Rockies. This zonal flow has also kept the polar jet away from the subtropical jet stream, and with the subtropical jet the drought situation across Florida has largely been eradicated.

Zonal flow described above, from Twisterdata.com

Instant Weather Map image of the zonal flow across the eastern Pacific.

This zonal flow across the eastern Pacific will weaken, and instead we get a stronger zonal flow across the northern tier of the United States. With the prior setup, we have had a very strong jet push zonally across the northern Rockies and right up to the eastern crest. With the lack of wave amplitude between the eastern Pacific and the eastern Rockies, the Pacific Northwest has been getting a parade of weaker, but more frequent cyclonic storms. However, with a lack of amplitude across the Rockies, the cyclonic storms encounter the higher terrain and weaken (due to vortex squishing). With the jet exit right at the eastern crest of the mountains, cyclonic storms have been strong coming off the mountains due to vortex stretching. With the peak jet moving farther to the east, the storm track shifts farther to the north. We won't even see frontal activity cross our area until Thursday night.

300mb winds, from instantweathermaps.com

   In the red box is the stronger polar jet flow into the United States and in the yellow box is a building trough associated with an upper level low from the subtropical jet. It is the subtropical jet that has been very beneficial for the Southwest U.S. and will soon provide a huge rain maker for Florida and southern Georgia at the end of this week.

But hang on, are we seeing a ridge building across central Canada in the very last image of the above animation? Yes, and this is in response to higher geopotential heights across eastern Canada and Greenland. If this is starting to become more difficult to understand, don't feel bad because I had to sit and think about this for a while myself.

Surface precipitation (Wednesday) with 1000mb - 500mb height thickness contours (from www.weather.unisys.com)

850mb temperature and pressure contours, from www.weather.unisys.com

The last two images are ugly, with a strong high FIRMLY over the eastern half of the United States. There is enough of a southerly flow in place for isolated convective showers in the afternoon.

500mb height level, Wednesday *very warm*

500mb height level, Friday *still warm*

500mb height level, next week. *Hope?*

 The 3-day and 6-day forecast is bleak, for those like me wishing for a cooler summer. But the end of the model run is encouraging, with a ridge building across both the Midwest and....wait for it....Greenland. The EURO shows this happening a little earlier but is on the same track.

Source: Berlin Stratosphere Group

Verdict - this week = very warm, with highs in the lower/mid 80's. A weak front pushes the temperatures down about 3-5*F on Friday. Another teleconnection shift will favor the reduction of the ridge promoting the hotter weather by next week.

Remember the Bean Collection Theory?

About a year and a half ago, I ventured out to West Jefferson to meet with a man about a theory that "foggy days during the summer correlates to the type of winter we have." Not quick to dismiss this, I looked through five years worth of calenders and photographed the pages, to investigate the days later on. The first question that came up was "What constitutes a foggy day?" The second question was "How much fog constitutes a heavy fog day (and thus a larger bean)? There were two limits I put on the "fog" meeting criteria. One, was it a "cool day" fog or a "warm day fog?" Two, was there precipitation associated with the fog? If condition two was met with a "yes" then the day was immediately discarded. Why? Evaporation fog is very commonly found after precipitating events. Precipitation fog is formed by evaporation of water into water vapor. It can also form if warmer air flows over a cooled rain soaked surface and cools to a saturation point.

Source: Watauga Road Conditions and Weather

There is also radiation fog, which is fog created when land is cooled after a sunset in a clear and stable atmosphere. This is more commonly found during the winter than the summer. However, valley fog associated with this is not unusual and during the clear, calm (has to have little to no wind), waking up to a morning valley fog is a little more common (still hard to find during the summer).

Precipitation induced fog and calm weather fog have both been ruled out, leaving just two categories. This already ruled out about 40% of the data. One is an orographically forced fog where wind blows up a slope, adiabatically cools to its saturation point, and causes condensation. A majority of the little bean events stemmed during warm days and were from a southerly wind flow. A moisture loaded southerly flow that persists for several days suggests the Bermuda High was involved, and the High Country was positioned around the High's western edge. Not surprising, many of these days also had precipitation from afternoon instability.

The second type of fog was much harder to find. In fact, I could only find TEN examples of it during a 5 year time period. It is a summer time cold air damming event. This fog is noted by a surface high located northeast of the High Country. For it to be a true cold air damming, the high had to be the primary component driving the dam. If there was a low pressure system funneling in cold air, then this was ruled out. A cold air damming event is where sinking air from a high northeast of the High Country funnels in cool, stable, and moist air southwest into the Piedmont. Because the air is so stable, it is unable to sufficiently rise up and over the mountains, thus creating a shallow stratocumulus cloud deck.

Cold air damming fog, from L'Heureux's Weather

Connecting the cold air damming events to the teleconnection patterns did not yield clear results. I was surprised the North Atlantic Oscillation seemed to have no clear connection to summer related cold air damming events but retrospect, it does make sense now. The NAO is calculated as a pressure gradient between the Icelandic Low and the Azores High, both of which are in the central to east Atlantic. A pattern that is more associated with the west Atlantic could be counteracted by the eastern sector and thus nullified in the NAO.

A true cold air damming event, with a low level cloud deck induced by a surface cold dome

There DID seem to be a great positive change of the PNA prior to a damming event but during the summer the role of the PNA is diminished and is no longer connected to events in the Atlantic. However, that study focused on the PNA syncing up with the NAO, and as indicated in the last paragraph, the NAO is focused on the EAST Atlantic. The Northern Annular Mode and North Atlantic Oscillation focus a little more on this but there is still no clear numerical index that represents a sector based North Atlantic Teleconnection Index. We have examples for ENSO, such as Nino 1, Nino 2, Nino 3, Nino 3.4 and Nino 4. For NAO, we have one universal index representing the basin instead of a sector oriented NAO index (west NAO, central NAO, eastern NAO). The index is generally sufficient because one would not expect to see strong geopotential height anomalies persist in an area and have it be confined to a small sector.

NAO height anomalies, from the Climate Prediction Center

Geopotential Height Anomalies for summer cold air damming events

   The low anomaly off the Carolina coast is from low pressure systems attached to a backdoor cold front. The high pressure off of southeast Canada is what I am really interested in, as this is our source of the cold air being funneled into the southeast.

Surface winds for a true cold air damming event

And just to make sure my sample means are true cold air damming events, the instability needs to be really low, since the air mass involved is very stable.

 And to drive the point home, the percentage of low cloud cover. The numbers should drop rapidly just west of the Blue Ridge, and they do!

 And just to prove that there is surface fog....the surface relative humidity.

But how does the fog actually stack up to pre-existing indices? Not well at all. If we took the BEST correlated index, the PNA, the results are less than stellar. The June - August PNA correlates rather poorly to the height levels over the High Country over the winter. It does translate better for the interior of Canada, southeast Canada, and the southwest but the anomaly over the west Atlantic can be discarded, due to a poorly linked PNA pattern to height levels across the Atlantic during the summer. However, there is no sector on this map that passes a 95% confidence interval. The positive values mean that the higher the index number, the higher the pressure in the winter. The negative values mean that the lower the index number, the higher the pressure in the winter. This would suggest a positive PNA, exactly the opposite found for the summer cold air damming events, would be more conducive for a cooler winter. Again, I am really questioning anything outside of central Canada due to the lack of a PNA influence on the Atlantic during the summer.

PNA summer correlation to winter geopotential heights

The NAO actually does worse, with no discernible correlation to ANYTHING located within its sector. However, there is one thing to note: the negative anomaly over southeast Canada (suggesting that a negative NAO would result to higher height levels). Again the correlation coefficient fails the 95% confidence interval. And since there was no clear relationship of the NAO to the cold air damming events, this has to be ruled out.

NAO summer correlation to winter geopotential heights

And this plot above fails the winter test. The summer of 2011 was largely a negative NAO yet the winter was absurdly warm. A big culprit was a NEGATIVE height anomaly over southeast Canada, and the correlation plot has that completely wrong.  At least the plot correctly shows that a positive phase NAO increases the strength of the southerly flow during the summer. And it is the positive NAO phase that likely led to the majority of the light orographic morning fog events (little beans).

NAO correlation to winds

The weak positive ENSO this summer and the negative Pacific Decadal Oscillation have largely canceled out one another.

Conclusion: Even constricting the foggy days, there is no clear link to the foggy days in the summer and severity of the winter, let alone to the number of winter storms. When focusing on "cold foggy" or "warm foggy" (cold air damming and moist southerly flow), neither had an impact on the winter. Cold air damming events were not influenced by the NAO phase and the important sector that the PNA influenced was questionable. Both the negative NAO and negative PNA influenced an interesting sector, and that would be Southeast Canada. But the lack of any empirical evidence all but rules out the idea that foggy days correlate to winter storm frequency.

For current forecasts though, remember how the negative PNA seemed to favor cold air damming events during the summer? Bingo. The plots below are for Friday, and this is a true cold air damming event because the northeast wind flow is confined to a low level. Saturday will not be a true damming event as the same ridge will actually be pushed towards the southwest and we will soon be enveloped by a trough.


One more thing. This surface low off the coast of South Carolina intrigues me. We have already had two named tropical storms this season form from the same type of system. Will that low be our third? My thought - the frontal boundary sits in place for several days with shear values lessening. It has an opportunity to become subtropical before it rushes to the northeast into the Atlantic by the middle of next week.

www.weather.unisys 500mb heights (color) and surface pressure contours.

www.weather.unisys 1000mb level wind and temperatures

General Weekend Forecast

The map above (from CIMSS) shows the shear tendency and total tropospheric wind shear. We can actually see both the polar jet and the subtropical jet. The environment is just too hostile right now to support tropical cyclone activity.

 It would be an understatement saying there is a lot of rainfall coming next week, but when (and why)? As this lovely map above shows, the bulk of the rainfall will be confined to the Gulf Coast and areas to the west of the Carolinas. Here is a map of the upper level low that will eventually act as kerosene on a burning fire for storm activity.

The upper level low is over Texas on Sunday (300mb level winds, from www.weather.unisys.com)

And the 500mb vorticity for Sunday, from www.weather.unisys.com

Saturday still has a very strong dry layer aloft, which all but crushes rainfall chances across the Carolinas. Note the pre-existing upper level high across south Florida and the moisture plume aloft it is carrying towards the Gulf States.

Saturday's 500mb moisture content

The charts on unisys do not make it clear but the upper level low over Texas eventually drops down towards the Gulf Coast as a trough in the northerly branch of the jet stream deepens. This trough deepens as it traverses down the Rockies. This stretches out the upper level low component to it, contracting the trough and making a more favorable environment downstream. The upper Midwest will have a stormy day on Sunday. The deepening trough will shunt the southern branch farther south as well. Thus, even Sunday is precipitation free for the Piedmont but we will see the cloud cover roll in. There is a chance of a popup shower in the Mountains on Sunday.

Sunday's 300mb winds

 While the precipitation is confined to the southern states on Sunday, a now synoptically supported upper level flow in conjunction with a surface moisture flow off the Gulf will REALLY enhance thunderstorm activity across the Gulf Coast. Now, we have a very long line of thunderstorm and rain activity extending along the Mississippi River.

Sunday's upper level relative humidity.

With the deepening trough for the northern branch of the jet stream, the moisture plume from the upper level low over the Gulf Coast (and from the southern jet) are both carried much farther into the United States.

Sunday's 500mb vorticity

The subtropical jet will provide additional moisture on Monday. Hence, this is the kerosene on the fire and when we will start to see our rainfall and thunderstorms. I think the model output has the rainfall as too high, but I can see between a half inch and 3/4" for Monday alone.

Monday 300mb winds

 The precipitation doesn't make it into the Piedmont until Monday morning. I think the models are pinging too high a rainfall amount but I do support the end result of there 1-2" up to Wednesday.

Monday upper level relative humidity

Tuesday upper level humidity