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