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Sunday the 18th ... storm idea/early thoughts...


Typhoon Tip

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Overview...
WV imagery suggests that the arctic wave with Northwest
Territories origins is moving through WY/NE, and is phasing with
very weak S stream stretched vort-max. Even though this phasing
occurs, and the DT drops as low as 550mb, it continues to be
embedded within the progressive zonal jet, hence why between 12Z
and 00Z today it shifts from NE to the E coast and remains
open. The wave`s approach to the mid-Atlantic yields rapid
cyclogenesis and deepening offshore, as the resulting low pres
deepens at a rate roughly exceeding 1mb/hr. As it deepens,
models have settled on a track that takes the low pres generally
about 50nm S of the 40/70 Benchmark. Given the wave remains
open and embedded within the strong jet, it is very progressive,
hence why most impacts will fall during the overnight hours.
Given the feature has some S stream influence, it will carry
with it a modest PWAT plume around 0.75 inches (1 std deviation
above mid Feb climatological values). Also, with S flow, BL
temps will be key, as some marine influence could yield a change
to rain, and lower SLRs to the N, mainly along the S coast,
Cape/Islands. The key will be the two potential banding setups:
1) mid lvl f-gen/deformation within the core of the open wave,
yielding strong omega in the DGZ furthest N between H7-H8 and 2)
lower lvl coastal front, as marginal, marine-influenced temps
will try to shift inland. The former is likely to setup further
inland while the other could feature banding along the S coast,
and final location will help to pinpoint peak banding.

Timing...
With good synoptic scale agreement now, can generally agree
that precip onset occurs as mainly light snow between 01Z-03Z
(8pm-10pm), peaking mainly 03Z-09Z (10pm-4am), then tapering off
between 09Z-11Z (4am-6am). Timing of any change/shift to RA
will occur during the peak period.

Snowfall/Amounts...
With modest PWAT plume and potential for banding. Total QPF
amounts between 0.30 mainly across NW away from peak omega, to
+0.6. SLRs will not be as high as previous storms given the
influence of warmer marine air before precip onset and some
areas where lift is actually below the DGZ. In peak banding,
which lines up from central CT-central RI-SE MA, will use SLRs
12-15, with generally 12 or less on either side. This suggests a
risk for a narrow band of 6-8in in the region mentioned above
with 4-6in just outside, dropping toward NW MA. Watches will be
shifted to warnings as the areas where 6+ are forecast
generally fall within the current watch regime, with advisories
to buffer. Within this area where banding/omega signal is
strongest, will also likely see 1-2in/hr snowfall rates.

Lingering uncertainty...
With the possibility of at least two banding setups, the overall
area of 6+ inches could ultimately be slightly wider or even
slightly narrower than current warnings suggest. This is
especially true along the S coast, where marine influence will
lead to lower SLRs and snowfall totals as rainfall mixes in.

Impacts...
Given the wetter, heavier nature of the snow, cannot rule out
some isolated power outages especially along the S coast.
Snowfall rates of 1-2in/hr will likely lead to hazardous road
conditions when untreated.

 

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1 minute ago, gravitylover said:

So if you had to be in Littleton NH by 10am and were starting from near Danbury chances are it won't be the regular ~5hour trip so what time would you leave? Of course, the only snow/rotten driving conditions of the last few weeks and I have to be out in it :( 

 

Sorry for having to be in Littleton. 

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Just now, dendrite said:

Sorry for having to be in Littleton. 

Haha that's not fair :P I like it up there. Great mountain biking, not terrible skiing (really good if you're into the backcountry thing), good beer, affordable living and easy access out when you need it. It's a much more palatable place than it was even just a few years back.

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17 minutes ago, CoastalWx said:

Interesting how much colder the hi res RGEM is.

I’m usually on the warm side in here, but I think SE MA to the cape will paste here. Only issue is BL—strong VV should effectively mix the cold air/lower dews down to the surface. I think they stay all snow, but admittedly confidence low...

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Just now, jbenedet said:

I’m usually the warm side in here, but I think SE MA to the cape will paste here. Only issue is BL—strong VV should effectively mix the cold air/lower dews down to the surface. I think they stay snow, but admittedly confidence low...

I think they will, esp canal and just NW. Maybe one of those deals where if rates lighten they creep to 33-34 and don't accumulate much, but guidance has a good 3-4hr thump there to start. 

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2 minutes ago, CoastalWx said:

I think they will, esp canal and just NW. Maybe one of those deals where if rates lighten they creep to 33-34 and don't accumulate much, but guidance has a good 3-4hr thump there to start. 

Agreed. But part of the reason I think this verifies all snow down there is I don’t really see rates lightening. Looks like moderate + rates for duration. Timing obviously couldn’t be any better either...

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