AL012015 (Ana) Off the GA/SC Coast

The US National Hurricane Center has started tracking the system off the SEUS as “Subtropical Storm Ana”.  Here is the current forecast wind swath using my Taru(tm) model and the NHC forecast track:

Winds could reach tropical storm strength right along the coast where this think moves onshore, with widespread areas (seen in blue) of gust winds.  Impacts are forecasts to be light; it’s likely any efforts to prepare extensively (especially evacuations, shutdowns, etc.) would cost more than the storm itself, but it’s a good idea to review your hurricane plans, get dead limbs out of the way and clean up possible debris, prepare for some scattered power outages, those kinds of things.  Consider it a test run for the real thing.

So just what is a “subtropical storm”?  First some quick abbreviated definitions.  A tropical cyclone (the family that is commonly called hurricanes or tropical storms) is defined as warm core low pressure system with winds above a given threshold (34 knots).  Tropical low pressure systems have a relative, the extratropical cyclone or mid-latitude cyclone.  Extratropical cyclones are fairly common, but can form nor’easters, which are intense, cold-core low pressure systems, sort of the cold cousin of the hurricane.  Subtropical cyclones are hybrids.  The have a cold or cool core but are taking on some tropical characteristics such as the development of thunderstorms near the center of circulation. There is a spectrum of low pressure systems with various characteristics like wind fields, temperatures, and driving mechanisms. Like many things in nature, weather systems sometimes don’t fit into nice neat categories.

Snow means it’s warmer???

Not a lot of major disasters lately (although the small ones obviously hurt the folks in the way), and I’ve been busy doing a climate analysis project for the UK DFID. The real time site always has the latest earthquakes, volcanoes, and hurricanes/tropical cyclones situation.

The recent snowstorms across the US have triggered the usual snarky comments from those who don’t accept the *fact* of anthropogenic climate change.  But anyone who has lived in the far north or arctic knows that it has to be “warm” to get snow.
You read that right, but “warm” is a relative term.  Take a look at this graph, from Danial Cobb, science officer of the NWS/WFO in Caribou, Maine (click to embiggen):

Not the biggest/fluffiest/deepest snow rates will be between -14 and -18 C.  OK, -16C (3.2F for you folks stuck in the 17th Century) isn’t warm if you are from the south, but it’s actually pretty “warm” for places that routinely see temperatures below zero F.  The other big issue is that the colder it gets, the less moisture the air can hold.  So colder air is naturally drier, and can’t hold as much snow.  So the “optimum” for both quantity and depth is somewhere between freezing and zero F.  Ironically, in the arctic, to get a blizzard, it has to warm up!  The other big issue is transporting moist air into the colder areas.  The models have predicted (and we seem to be seeing) more “latitudinal” (eg north-south) flow, which would cause more stormy and snowy weather – even though the overall temperatures are actually “higher” in relative terms.

Climate change is a very complex phenomena, intermixing human and natural changes and activities.  Far too many people on both sides of the debate do the discussion a disservice by simplistic, apocalyptic explanations.