OceanStWx

It's the same as here. <=1/4SM, 35+ mph for 3+ hours.

Ideally to get really big totals you would like to see the surface and mid level fronts get closer together to increase the lift.

The bulk of the 12z sensitivity (73%) is in the north/south (some timing) position of the low pressure. The southern (read: cold) scenario which we want is tied to uncertainty that grows out of both the Arctic and central Pacific (which is a change from 00z last night which grew mainly out of the Arctic). If we get a deeper southern stream, and shallow out the northern stream we end up seeing the surface low stay south.

Not hard to find the mid level forcing and surface warm front just from the QPF.

Another way to look at it is positive and negative energy (you take/need positive energy to melt, you release/lose energy to freeze). At 12z Sunday at BDL the NAM has almost twice as much negative energy than positive. So you are more likely to totally refreeze the hydrometeors in that scenario. Once positive energy > negative the hydrometeors won't have enough negative energy to refreeze completely.

That's pretty much the range of what we have now. Could be one of those fairly uniform spreads where dendrites are falling in the low QPF to the north, while the beefy QPF is lower ratio.

We're already pushing 2" QPF for southern zones, so I don't anticipate going higher than what we have already. Still plenty of snow in the grids though.

If you look at a Wilks 2006 reliability diagram for QPF you'll see that all models tend to have a wet bias for QPF. They too often are too wet. I'm trying to find a specific North American diagram with recent stats, but no luck. For Europe the EPS is something like 80% of the forecasts for heavy QPF, it actually occurs about 50% of the time. I suspect North America would be no different in that regard.

NAEFS was a slight increase in the strength of the IVT plume at 12z today, maybe a once every other year type event. EPS was definitely stronger, with the return interval more like 5 to 10 years.

So will BDL be 25 like the NAM says Sunday morning, or 45 like the GFS? But honestly as much as Kevin wants to lose every tree in his yard to ice, the NAM is a lot of sleet. The cold layer is cold and almost 3500 ft deep.

I'll say better below than above. Right in the heart of it is obviously the best, but if it's below at least there's a hope it will extend upwards and tickle the DGZ.

The 2013 "ice storm" for the GYX area was a situation where you had stout CAD and a surface front well to the south, but the mid level warm front blew through the area and so neither provided a single deep forcing for ascent. Now the mid level front in this case is unlikely to blast through, but if the surface front is displaced far enough you could see a similar situation. December 2008 and January 1998 are probably the two best recent examples for the southern GYX CWA.

There is like no spread from the North Americans SE of the low track, but a 15 mb spread NW. The 16.00z suite basically was the perfect solution, with the range of outcomes being this mean track or NW.

I mean it actually is kind of fun to run and re-run the grids to see what falls out. A lot of times the first run on snowfall is because snow ratio is almost always modeled too high.

This is a tough forecast. It's still pretty far out there, but it was a pretty significant shift colder by the 00z guidance. With the tools at my disposal that creates a whiplash forecast to significant snow, at the risk that things trend back slightly warmer with future runs. So I had to artificially blend in the 18z GFS to warm things up and provide a step down but more gradual that solely blending the previous forecast with 00z.

If the high remains in that position, you take the under on modeled temps right now. Your area of cold in the sounding would definitely outweigh the area of warmth = sleet. But a threat of a period of freezing rain would definitely be on the leading edge of the cold tuck as it undercuts the warm air aloft. Near the coast from like NE MA northward could also see a period of freezing rain (verbatim) because we shouldn't see temps warm above freezing (for reason stated above) but have a little warmth aloft near 925.

Given that the amped solutions are driven by northern stream phasing, and that's a historically poorly sampled region, a few more swings in the next 48 hours would not shock me.

If I'm referencing vorticity in Quebec does that mean I can go bilingual? Add it to the list of words I need to add to the AFD: misery mist, freshet, tourbillon.

I have to say, vorticity sounds like a fancy word, but tourbillon really rolls off the tongue at Meteocentre.

http://meteocentre.com/numerical-weather-prediction/forecast-systems.php?lang=en&amp;map=na&amp;mod=ukmet Still isn't updated there yet either.

I mean the NAEFS has a pretty stout IVT plume (originating in the Gulf of Mexico). High QPF around the warm front shouldn't be out of the question.

To be fair, the NAM always leads the 00z runs because it comes out first.

That's what 12z ensemble uncertainty was showing. The NW solutions were driven by northern stream features. Speed was the "southern" stream, but that shortwave is out over the open Pacific. With 48-60 hours to go before that's over the West Coast there could be some additional swings, but maybe GOES-17 is providing a little better sampling and data ingest.

Casual 22 degree (26 to 4) 2 m temp difference from PWM to GYX (17 miles as the crow flies).