OceanStWx

06z to 12z is like a 15+ degree swing in temps at PWM. That's going to be the thing to watch, is where the front sets up during the middle of the night tonight.

I'm not sure I've encountered a more difficult forecast than this one for wind. HRRR and NAM nest literally have a steady breeze at PWM but then one or two hours of 60 kt.

It's all going to depend on where the mid level front sets up. There won't be any really wind west of that. I mean that same NAM run has 8 knots at PWM and 63 knots at Cape Elizabeth.

3km has like gusts to 65 kt at Gardiner and a few miles away at AUG 7 kt.

What matters is where the mid level front is. The low level jet won't penetrate inland if the mid level front is parked right on the coast like some of the NAM runs are showing now. The farther west you can get that front, the higher the wind potential.

Since it's a blend of everything, it has the same biases as the models. So bad in CAD for instance. But it's bias correct individually at every grid point, so day to day it is quite good.

Which uses some combination of temp, RH, and lift in the DGZ. My preferred method, and available in Bufkit as an accumulation option. I believe the Max Temp in Profile would be Kuchera, Cobb has two study versions, and then there's the straight ratio slider bar option.

We're also trying to move models towards more explicit snowfall output. Like the NBM is going to start doing snowfall as a percentage of ptype. So if rain and snow is a 50/50 probability in the NBM, only half the QPF will be used as snow, multiplied by a forecast snow ratio.

Say you have a warm nose that change ptype over to sleet and that's about +2C. Kuchera would be ratio = 12 + 2(271.16-max temp). In this case 12 + 2(-4) or 4:1. Obviously much better than 10:1 sleet accumulation. Still to high but, better. But at Dendrite's place the warm nose is only -4C. Still warm enough to make some iffy snow growth, but cold enough to keep it all snow. Kuchera would be ratio = 12 + (271.16-max temp). In this case 12 + 2 or 14:1. And for each degree cooler than that you add an inch to the ratio. So you can see how it gets out of hand fast in cold environments with poor snow growth.

Kuchera will be better than 10:1 in the mixed zone, but where it is colder within the CAD it will be worse because ratios will still be about climo 10-12:1 and Kuchera will tend to make those 15+.

And that's usually going to be QPF x ratio for all frozen ptypes. Sleet? QPF x snow ratio. Rain/snow mix? QPF x snow ratio. That's why Kuchera attempts to bring in temps aloft to adjust those mixy situations down. HRRR also has a variable density snow accumulation which can work pretty well.

Always fun when the ensemble mean is 170 knots.

The flood watch and winter storm watch overlapping is great.

The current NAM/GFS forecasts just have the core of the LLJ so low that it's not hard to mix some of it down. Now there are seasonal differences between the two storms, but the Nammy is currently a more impressive forecast than it had for Oct 2017 right before the event (like 12 hour lead time). Obviously there were still leaves near the coast then, but just the magnitude of wind.

Just looking at the CT River, and it could rip for a couple hours there even, let alone VT. Nice lapse rates in the DGZ with lift still going. Could be an hour or two of snow, but could also be 2-4".

Modeling isn't my strong suit at all, so I don't know how they handle this aspect. But if you pull up any upper air map to hand analyze, they are plotted as if they are over the launch site. However we get winds from GPS calculations, so we know where the balloon has been at all times. It is possible they use the lat/lon pair when ingesting it, I just don't know for sure.

Welp, 12z NAM still insisting on a large area of 925 winds > 90 kt.

Even raobs aren't infallible, despite being an observation. They are plotted as if they are at a single location, but can be 100s of miles away from the launch site by the time it is sampling important upper level features. I do think there is a weighting system with data assimilation. Like raobs may get more weight than satellite measurements, so while satellite measurements have gotten better there may still be shifts in guidance once the upper air network gets a hold of it.

50ish is a pretty good middle of the road forecast right now, but I could see it being as high as 65 mph in the PSM area or as low as 40.

Inland extent is always the hardest forecast, but looking at the forecast soundings you're not out of the game for December of yore sou'easter. The LLJ doesn't really crank until it's passing the tip of the Cape, so I wouldn't be banking on parking your roof in your neighbor's yard, but I could buy an advisory.

The NBM mean wind gust at RKD is 70 mph with a standard deviation range of 60-80 mph. Overnight model runs really amped the wind threat up from when I left yesterday afternoon.

LOL That's the wildest 925 mb wind run I've seen in a while.

Yeah, it's right there with the 500 jet.

Places like CON really show the uncertainty. Bufkit says the NAM snows about 4.5 inches (compacts down to around 3), but for most of that time especially Sunday the snow growth zone isn't saturated. Hard to believe daytime snowfall accumulating in an environment like that.

I really like the look of the Euro/NAM postive snow depth change maps, which of course put it right on the razor's edge of warning for many.