Karen weakens, but still has sleeper potential for strengthening shortly before landfall
I woke up this morning to snow in Denver (here's a quick video) from the back end of a storm that's producing an early-season blizzard in Wyoming and South Dakota. That might not seem related to Tropical Storm Karen -- nor to extreme wildfire danger in California, nor a significant threat of severe weather in the Midwest later today, possibly including violent tornadoes in Iowa -- but actually, they're all connected.
Eric Holthaus explains:
These events comprise a crowded weather map that is actually the manifestation of a single continent-scale choreography of weather: high pressure out west is helping to steer and strengthen an intense low pressure system over the upper midwest that in turn is pulling the tropical storm northward towards the coast. It’s a perfect picture of the physics of the atmosphere, working seamlessly together.
The western high-pressure creates the intense offshore Santa Ana winds that have the forecasters in the National Weather Service's Los Angeles office shouting from the rooftops about the worst fire danger in five years. The midwestern low pressure fuels both the blizzard and the severe weather and tornado threat, and is also responsible for shearing apart Tropical Storm Karen and, as Holthaus said, pulling it toward the coast.
According to Dr. Jeff Masters, "Wunderground weather historian Christopher C. Burt has done some research to see the last time a blizzard, major severe weather outbreak, tropical storm, and extreme fire danger all threatened the U.S. at the same time, and has not been able to find such an event in past history."
The tornado and wildfire potential might well be today's biggest threat, moreso than Karen, to be perfectly honest:
But this blog's charge is to cover hurricanes and tropical storms that are threatening the United States, so let's focus on Karen. (For ongoing coverage of the other severe weather events, I'd recommend Mike Smith's blog and Amy Sweezey's "Wx Tweeps" Twitter list. I'll also continue to tweet and RT relevant information as I see it about all of these weather events on Twitter at @brendanloy.)
Karen has been relatively resilient in the wake of significant wind shear, but it nevertheless weakened further overnight, and is now down to 50 mph winds. The storm's circulation center is exposed, with the thunderstorms sheared off to the northeast -- leading to a dramatic visible satellite image this morning at sunrise of the thunderstorms casting a shadow over the center:
As I wrote last night, Karen continues to look unlikely to ever become a hurricane. The "outlier" intensity models, which last night were still insisting on intensification to hurricane strength, have seemingly relented. That said, the 11am EDT National Hurricane Center discussion implies that there's still a chance:
THE ENVIRONMENT DOES NOT LOOK FAVORABLE FOR SIGNIFICANT INTENSIFICATION...WITH MODERATE SHEAR EXPECTED TO CONTINUE FOR THE NEXT DAY OR TWO. HOWEVER...IF THE SHEAR DOES LESSEN...EVEN FOR A SHORT PERIOD OF TIME...DEEP CONVECTION COULD RE-DEVELOP CLOSER TO THE CENTER AND ALLOW FOR SOME INTENSIFICATION. IN ADDITION...BY 48 HOURS UPPER-LEVEL DIVERGENCE AHEAD OF AN APPROACHING MID/UPPER-LEVEL TROUGH COULD ALLOW FOR SOME STRENGTHENING. THE NEW NHC INTENSITY FORECAST HAS BEEN ADJUSTED DOWNWARD AND SHOWS LITTLE CHANGE IN THE NEXT 24 HOURS AND A STRENGTHENING TO 55 KT BY 48 HOURS.
The strong upper level winds have been taking a toll on Karen but despite the relentless shear, deep convection keeps trying to form right over the well defined center of circulation. Water temps are plenty warm and all it will take is a relaxation of the shear for Karen to intensify and in quick fashion. This is discussed very well in the NHC forecast which notes that even the ECMWF model which has been the least "enthused" about Karen shows a 10 millibar drop in pressure before landfall. ...
If Karen is strengthening right up until landfall, this will also mean that the wind will be likely be more dramatic. A convectively active storm or hurricane tends to have more downburst winds than one that is weakening. This is something we will need to watch closely over the weekend as Karen approaches the coast.
I agree. But most likely, this will be primarily a rainmaker for the areas it affects, rather than a major destructivewind and storm surge event. That said, as the recent devastating floods here in Colorado reminded me, flooding rains are nothing to take lightly. Sudduth again:
One of the bigger issues here is going to be heavy rain for a fairly prolonged amount of time. Since Karen is not moving very fast, once the rain shield makes its way onshore, we could be looking at nearly a foot in some areas. Even half of that amount is enough to cause serious flooding concerns and dangerous driving conditions for the region affected.
As for the exact track, that remains uncertain.
And it may remain uncertain for a while, as the NHC discussion explains:
THE FUTURE TRACK WILL BE QUITE SENSITIVE TO THE STRUCTURE OF KAREN OVER THE NEXT COUPLE OF DAYS. IN THE SHORT TERM A WEAKER SHALLOWER SYSTEM WILL BE STEERED MORE TOWARD THE LEFT BY THE LOW-LEVEL FLOW...WHILE A DEEPER MORE VERTICALLY COHERENT CYCLONE WOULD TURN NORTHWARD MORE QUICKLY DUE TO A MID-LEVEL RIDGE TO THE EAST.
In other words, as I wrote in my initial update, further west = weaker storm; further east = stronger storm. Also...
ALL OF THE GUIDANCE SHOWS A NORTHEASTWARD TURN IN 36 TO 48 HOURS...BUT WITH LARGE DIFFERENCES IN THE LATITUDE AT WHICH THE TURN OCCURS AND SIGNIFICANT SPREAD IN WHERE THE CENTER CROSSES THE COAST.
So, although the computer models are fairly confident about the general concept of the forecast, the timing and angle of approach are suc that relatively minor variations within that conceptual consensus will have large ramifications from the perspective of coastal residents who (quite understandably) want to know whether or not they will get hit.
Having said that, don't focus too much on the exact track "line," because this storm is so asymmetrical right now that the "center" isn't nearly as meaningful as in, say, a major hurricane. Indeed, barring significant last-minute strengthening and storms "wrapping around" the center, the worst weather will likely be somewhat east of the eventual track "line" -- which could mean that the Florida panhandle may be the most likely area for significant rain and wind, even if the center stays a bit further west.
P.S. Capital Weather Gang has an excellent overview on the situation with Karen.