wxsniss

6z NAM says let’s refresh again

4.5” near Coolidge Corner

Loving the fresh powdery white piles and deep tundra feel

What a great event in an increasingly respectable winter 

… a mix of cool rare meso processes

… a widespread refresher for the region

… a fraud five actually delivering

Along with the broad brush positive bust Jan 25 with its massive dendrite finale, this winter’s got some character 

17 minutes ago, Henry's Weather said:

Perfect huge dendrites, West Concord

Awesome hoping that swings east more than south 

I’m in Brookline just west of city, finally heavy snow gotta be 1-2”/hr at least

Hoping we can benefit from the N-S orientation of this band whipping through city

Some 35-40 dbz stuff approaching south shore

@CoastalWx

13 minutes ago, CoastalWx said:

Yea crazy event. Not gonna lie, jelly of the north shore right now. So close yet so far. 

100%
Good Harbor Beach should be epic right now… very jealous but I’ll take the great refresher 

Hopefully metro area fills in 12-3p

Congrats @Ginx snewx@mattm4242folks RI/Orh/North shore getting absolutely crushed!

Awesome event so many mesoscale processes

HRRR has more filling in metrowest 12-3pm let’s hope we get better rates and growth

22 minutes ago, 40/70 Benchmark said:

Looks like the east trend has stopped....HRRR also nudged slightly westward.

RAPs 0z-1z-2z also ticked west

14 minutes ago, 40/70 Benchmark said:

I'm like a caveman with soundings, so I use the archaic Coolwx.com site, but even on that it was obvious how elite the OMEGA profile was for KLWM, KBOS and KBEV.

A museum exhibit... I think I've seen something like this during a Dec 17 2020 WAA event:

4 minutes ago, ORH_wxman said:

That’s prob 2-3” per hour easy. Might get a clap of thunder if that verified. 

What's also amazing is it's not just a passing hour... like 15z-21z

This is peak, but close to this for hours 15z-20z

KBOS from 0z HRRR...

Nice AFD, not a mundane setup:

-AO and -NAO teleconnections yielding a very suppressed polar jet
this period, with storm track well south of New England. However,
this supports a potent, high latitude closed low to descend from
James Bay into New England this weekend. This feature will be
accompanied by an arctic front Sat, which will yield a complex snow
event for SNE. This vigorous closed low induces cyclogenesis well
southeast of the 40N/70W benchmark Sat. However, such strong forcing
for ascent results in an inverted/Norlun trough from the offshore
low extending northwest into SNE Sat. This will result in narrow
bands of snow Sat moving west to east across CT/MA/RI.

Model data reveals a combination of steep low level lapse rates (up
to 9C/km in 0-2 km layer), deep saturated layer, strong convergence
and at least modest omega in the snow growth region, will yield a 2-
4 hr period of moderate to perhaps heavy snow at times, from late
Sat morning into the afternoon. The snow squall parameter ramps up
rapidly between 18-22z across eastern MA Sat afternoon. 12z NAM
bufkit for KBVY, indicates up to 20 u/bars of lift in the -10C to -
15C layer 15z-18z. This should yield favorable SLR and low vsbys.
This is also supported by HREF offering 30-50% probs of hourly
snowfall rates exceeding 1" and 20-30% for greater than 2" per hour,
with these high rates focused across eastern MA westward into RI and
the Worcester Hills. Duration of strong lift, deep saturation and
steep low level lapse rates is brief (2-4 hr window) and this will
limit accumulations to mainly 2 to 4 inches, with highest amounts
across RI and central-eastern MA.

This boundary becomes more enhanced as it enters eastern MA, with
multiple models generating over 0.50 inches of qpf across Essex
county, with some ocean effect enhancement contributing to the
higher totals. In addition, the 13z NBM (v5.0) snow probs have 20-
30% probs of 8+ inches across Essex county, esp the Cape Ann area,
and 50% prob of 6+ inches of snow. Given multiple models have strong
low level convergence, steep low level lapse rates, a deep saturated
layer and at least modest forcing for ascent in the snow growth
region, will hoist a small winter storm watch area for Essex county.
Snowfall projections here will be for 4-8", with even a low prob of
isolated higher amounts pending the evolution of this mesoscale snow
band associated with the inverted/Norlun trough. Keep in mind, these
mesoscale snow bands are very difficult to forecast and this is a
situation where snowfall amounts can vary greatly over short
distance. Thus, continue to check back and monitor our updated
forecast. For the remainder of coastal eastern MA, given the OES
component, isolated amounts of up to 6 inches is possible.

8 minutes ago, Ginx snewx said:

NAMs give me .39 qpf that is 5 to 7 Euro and EPS HRRR don't know what else they need 

12z GFS and RGEM also like RI, soundings there looked good too

They're probably accounting for ocean enhancement for higher confidence to warning amounts North Shore

Box adjusted, appropriately imo...

This is pretty close to HREF... some guidance had another max in south RI but not enough support:

13z NBM... not sure how useful these are if at all... they were very good in the Jan 25-26 event. (EDIT in case people are wondering: these get released for 1z/7z/13z/19z... I assume because they incorporate a blend of the 0z/6z/12z/18z models)

This is likely overdone, and not sure how appropriate a "blend" of models is for events that are heavily impacted by meso features for which models have lots of variation, but the trend has been steadily up. It's assuming mean SLRs 15-17:1 for the event. 

19 minutes ago, 40/70 Benchmark said:

What is the yellow there, 6"?

A prime nowcast setup, but there’s strong consensus at 12z on North Shore max (and a shot at warning)

RGEM, GFS, NAMs, RRFS,

…and this is the HREF:

Rather dynamic pattern for Fri night into Sat as deep mid level
trough amplifies across New Eng and eventually takes on negative
tilt, with 700 mb low eventually developing south of New Eng Sat
afternoon. Strong height falls developing off the coast will lead to
cyclogenesis well offshore on Saturday, but given the strong
upstream trough there will be an inverted trough that develops and
extends across New Eng. Increasing forcing for ascent ahead of the
trough and low level convergence with the inverted trough combined
with deepening moisture plume with near saturation through the mid
levels will result in widespread snow developing from late evening
through Saturday. The snow should taper off in the afternoon in
western New Eng behind the arctic front, while becoming more focused
in eastern MA. Interesting set up as snow will be moving in from the
west ahead of the trough, but hi-res guidance shows also bands of
snow moving into eastern MA from the ocean later tonight through
Sat from convergent NE flow ahead of the arctic front.

The bands moving in from the ocean and the inverted trough are
wildcards to the snow forecast. These inverted troughs are often
associated with narrow mesoscale bands of enhanced snowfall where
the low level convergence sets up. We also have potential for
localized snow squalls along the arctic front. There are decent
model signals given strong low level frontogenesis, relatively steep
0-3km lapse rates and some low level CAPE. Snow squall parameter is
highlighted across eastern New Eng Sat afternoon. In addition, model
soundings show very favorable snow growth for a few hours in eastern
New Eng with 20-30 units of omega in the DGZ suggesting locally
brief heavy snowfall rates of 1"+/hr within any enhanced snow bands.
However, forecasting the exact location and axis of where these
heavier snow bands is very challenging often leading to low
confidence forecasts in location of potentially higher impact
snowfall. This is a situation where snowfall amounts can vary
greatly over short distances.

19z NBM ticked up again

NBM 13z Feb 5 output

Not sure what this is worth and take guardedly for now (at least the distribution of highest amounts that will be pretty dependent on meso features), but it did pretty well with the Jan 25-26 storm... also notable that it has steadily ticked up the past 4 cycles:

3 minutes ago, CoastalWx said:

These stupid lows keep messing up what could have been a decent warning event at least to BOS. But they might be real since you have tremendous instability and forcing over the gulf stream. Sometimes these have some near term surprises...but guidance is fairly locked in. 

ngl, I (and I'm sure most here) check every NAM run for hint of a surprise, even though we've declared this dead.

Without those runaway lows... you look at H5 and surface at 6z Sunday and think how can this miss a better hit on eastern areas.

One thing that keeps me intrigued: yes the convection and buckshot lows further east are consistently run after run spoiling a hit, but where that convection and lows pop up is pretty inconsistent.

9 hours ago, wxsniss said:

Not saying this happens here but there is some resemblance: only from the perspective of the "shrapnel of vorticity" reminds me a bit of the March 13 2018 snowstorm... there too models struggled with multiple lows developing +/- chasing convection east +/- fujiwara of a low into New England...

At the time I called it the "mogwai" effect of all these pieces of vorticity spawning small surface lows once they hit the water off the Carolinas, and models struggling to resolve how to handle

Ultimately the stew of surface lows did gyrate close to BM with a big hit on much of region

Commented earlier today, here are some images... definitely some echoes of this on 18z Euro... models gonna struggle... 

EDIT: these images are from a NAM run before March 13, 2018: