2010 CAPPING INVERSION

[Matt Gingery]

Matt,

I think if your trying to project the strength of the capping inversion over a whole season, that's a pretty tall order. I've never seen any studies re: inter-annual variations of the avg. cap strength, but that would be an interesting and challenging pursuit.

However, I agree w/ most of the previous replies that SST's in the Gulf don't have much to do with the cap. Let's take a step back and look at the bigger picture. For the areas most prone to severe weather in the spring, the source region for the (dry) air mass affecting the capping inversion is over the high terrains of the desert southwest and Mexican plateau - continental tropical air, not the maritime tropical air over the Gulf. You will often see this referred to as the elevated mixed later (EML).

Re: some of your other questions. Yes, the soil moisture content can directly effect surface dew points by evaporation and indirectly through transpiration. Although evapotranspiration can be important in overall surface moisture , advection from warm, moist regions is most often the "difference that makes the difference."

If it were like March 10th or so, and we had had weeks upon weeks of below-normal Gulf SSTs, I would be concerned about the quality of the source region of the moisture so important for severe weather over the plains. However, it's only January 8th, so it's too early to draw conclusions, IMO.

Actually I may have speculated a season capping issue. Actually, what I am looking at is the possibility of a ridge building into the midwest early in the season, and before the gulf rebounds. What adverse effects are we going to be looking at with a substantial warming period across the midwest prior to the gulf temp rebound?

Say we are in late Feb or early March and have a typical downslope pattern that provides substantial warming, with Lee Cyclogenisis becoming a factor, turning on the pumps and drawing in the cooler flow. I think it will be interesting to see how things transpire.

I don't think it is possible to draw long range season wide conclusions on a small scale subject such as capping. Early in the season Feb-April would be where I would be looking to see these effects.

Thanks for all the experienced input on this subject!!!

 

[Rich Thompson]

The Gulf SSTs are not strongly impacted by single frontal passages, namely because the large heat capacity of the water compared to air. The open Gulf cools from the mid 80s in late summer to the low-mid 70s by late Feb/early March in most years, thus the net impact of *everything* is about a 10-14F temperature drop. You'll see a much more dramatic impact over the continental shelf (within 100 miles of the coast) where upwelling is the main contributor to cooling (offshore flow behind fronts blows surface water away from the coast, and it's replaced by cooler water from below). The most recent SST anomalies for the Gulf run from about -1 C along the coast to near zero in the open waters (http://www.nhc.noaa.gov/tafb/atl_anom.gif ). The other factor in the seasonal water drop is the reduction of incoming solar radiation with shorter days and a lower sun angle in the winter.

There have been a few informal studies linking severe weather with water temperature anomalies - the cooler water years were also lesser severe weather years. However, the lag time in the response of the ocean could mean that the cooler temperatures are not necessarily a cause of the "problem", but simply a reflection of a pattern that supports lots of frontal passages and offshore flow. In other words, the same kind of pattern that reduces severe weather occurrences regardless of ocean temperatures.

The most important aspect of forecasting low-level moisture return is the path the returning air takes across the relatively warm ocean. Short paths originating in sinking air (within a surface ridge) over the continent will result in poor moisture, while long fetches from the Caribbean and southern Gulf provide the highest dewpoints. The situation is also complicated by the cold depth with the previous frontal intrusion.

Forecasting cap strength is dependent on the quality of the moisture return, as well as the source region of the elevated mixed layer. Cooler Gulf temperatures with air trajectories across the nrn Gulf, in combination with above normal surface temperatures across the southern Rockies, will result in the strongest cap situations. The more interesting chase scenarios are the result of Caribbean air reaching the Plains beneath a slightly cooler than normal elevated mixed layer. The depth of the elevated mixed layer also matters when you're worrying about CAPE.

In the end, it's nearly impossible to make any iron clad forecasts about how the upcoming season will turn out. If you happen to know the pattern evolution and storm track, you could probably make some reasonable guesstimates. Unfortunately, we aren't sure about the upcoming pattern, other than maybe some tendency for a stronger srn jet and storm track closer to the Gulf coast the first half of spring in conjunction with the ongoing El Nino. Still, the cold December-early January and dominant polar jet should be a strong indicator that a single influence (e.g., a "moderate" El Nino) is often far from the dominant factor in daily or monthly weather.