OceanStWx

The median values are like 10-11". The high end values are actually still quite high, because if it stays all snow there is still 18" potential. So their range just got wider.

What it means is that there are just enough models now spitting out the lower range values that the 10th percentile of the curve has lowered to around 3". Previously so many models were high QPF and cold that 9" was the lower end of that tail.

As evidence of how warm the 00z suite was, WPC's 10th percentile distribution on top of our snowfall (9 out of 10 time you should expect more than this number) for a location like ASH went from around 9" at 12z to 3" at 00z.

I was more gentle than I thought.

Well I'm having a hard time matching the snow forecasts to my west, but otherwise match BOX and CAR. But it was definitely a tick down from the previous forecast. There's the before, so people can chuck when the new one comes out.

Sorry, not a location they run these for. We have to pick and choose cities because the ensemble is run out of DTX and they don't have the server space for a bunch of locations.

Definitely a short term threat there. It's all about the overlap between +SN and winds increasing. Also a nice signal for a flash freeze in and around BOS too.

Trends for BOS are interesting. It doesn't change the high chances for 4", but the chances for 6 and 8" have been decreasing. 12" all but disappeared in the latest run.

That super ensemble is like a 90th/10th at PWM of 9-18" and at MHT 7-16" But those are the only two cities we have south of the mountains.

It should thump for a time. Given that this is a WAA regime on steroids, it should pound 1+/hr for a while in the good stuff.

We have some experimental ensemble guidance (basically time lag the shit out of everything available in the short term) that shows an interesting story for PWM. It has a very high likelihood (~70%) for 12" of snow, but a low (~30%) likelihood for 16". I think that's a really nice forecast right now. I feel comfortable we hit a foot, less comfortable we hit that 18" top of the range on our website.

Congrats Dendrite

NCEP featured a similar pattern. The driver of the uncertainty right now seems to be the northern stream.

I mean it was an error in that it was underforecasting heights, but it's not like the model was flat out wrong, if you get what I'm trying to say. Basically the 12z ensemble sensitivity patterns were a speed difference (about 50% of the variance) and a track difference (north shift 30% of the variance). To shift the track north the EPS members with lower heights across the ridging in western North America were the pattern to look for. All modeling was too low on heights out west (hence why models were farther north). So this could argue for the messenger SE shift in later "go time" runs.

Plenty of easterly wind at 925 though, which will be blocked by the terrain and help lock in cold.

FWIW, the entire Ohio Valley is verifying about 2-4C cooler at 850 mb than the GFS and NAM. So the entire leading edge of the WAA models are struggling with. As usual, the 12z Euro is actually performing better than the 00z Americans.

You don't need northerly components at 850 to hold the cold. You need easterly components below the tops of the mountains. But models can struggle at times with the depth of the cold air. If it's deeper than modeled, sometimes that modeled +5C ends up closer to 1C and it's sleet rather than freezing rain.

Tossing them right into the forecast package?

Far. There could be some microscale flips to freezing rain, but serious threat to accumulate would be well south. It should be more sleet, unless a petty large late shifts in modeling occur in the mid levels.

I don’t like to deviate too far from climo ratios until I see the 20:1 whites of its eyes.

The trend has definitely been down over the last few runs. Pretty classic trend honestly, rushing out to big QPF then settling towards reality. Will was banging 1-1.5” days ago, and that’s still the best forecast for all/mostly snow zones I think.

Yeah, WAA will be serious. But as Brian notes, even with that lift we’ll get some riming in the warm nose that will chip away at ratios. Probably also going to get some fracturing of flakes with such strong lift. 10-15:1 should be fine. More than that and I’m skeptical.

I’m looking at the storm in a couple ways. A classic SWFE is like a 6-10” storm. This is definitely amped up from typical, but how much higher should we be throwing snow totals? I wouldn’t want to be that far outside a 12-18” range. Also from strong WAA driven snow, it’s really hard to get ratios to deviate that much from climo.

Not sure I’ve seen a study on that specifically. Snow will absorb the rain before it freezes, so it can be equally damaging to the canopies. You probably get less run off with covered covered trees, but more true ice with bare trees.

I’ll say just check our first snowfall map we posted. I take great pride in the fact that I actually made a forecast that night and didn’t rip and read modeled QPF. Blends are great, but you also need to recognize model biases, and high medium range QPF is one.