Read their AFD.
Very interesting thoughts from them.
Low pressure developing off the Carolina coast early Sunday morning
will rapidly lift northeast and pass near the 70W/40N Benchmark
Sunday afternoon/early evening. Snow will rapidly overspread the
region from southwest to northeast Sunday morning and continue
Sunday afternoon. Precipitation type with this system should be
mainly snow away from the immediate coast. The Outer Cape and
Nantucket will likely see mainly rain, while rain/sleet mixing
should cut down the snow totals across Eastern MA coast,
including parts of Boston. Further west towards the East Slopes
and Hartford County, they will be on the northwest edge of the
heaviest precipitation, so have issued a Winter Weather Advisory
for these areas. Block Island is a tough call because they are
west enough to be removed from the strongest warm advection (as
opposed to Nantucket and Martha`s Vineyard) so have put them in
an Advisory as well.
Dynamics couldn`t be more perfect for heavy snow where mixing is
not an issue. Omega is maximized in the -12 to -18C Dendritic
Growth Zone (as low as -10C for the coastal area due to
abundance of salt nuclei) along with 700-925mb speed/directional
convergence will result in a 3 to 4 hour window of impressive
frontogenesis. This is a strong signal for convective snow and
where mixing is not an issue, could easily see 2 to maybe even 3
inches per hour snowfall rate. So even giving the progressive
nature of the system, snow could easily pile up and accumulate 7
to 10 inches in the most persistent mesoscale bands.
Key issue that we have been grappling with is where the rain/snow
line would set up. With such a robust system, the thinking is that
it will bring in quite a bit of warm air out ahead of it to preclude
snow for most of the time for the Outer Cape and Nantucket. The 00z
and 12z HREF have both been very bullish with intense snowfall rates
over the Cape and Islands. The global ensemble guidance is also very
bullish with the probability of 3 and even 6 inch plus snowfall over
the Cape and Islands. However, wet bulb temperature bottoms out
around 32-34F across the Cape and Islands even according to the
NAM. So thinking at best high-end Advisory/very low-end Warning
snowfall for the Inner Cape and possibly no more than a couple
inches for the Outer Cape and the Islands. The fact that the
snow is falling during the daytime hours also makes it harder to
accumulate. In addition, I did lower the snow totals over the
immediate Boston area and much of the Eastern MA coast. With the
winds initially out of the E and NE during precip onset, snow
ratios will be in the 5-7:1 range initially but gradually
increase to around 10:1 as winds back to the N by 20-21z. As for
the interior, higher SLRs are expected generally 12-15:1. QPF
wise, expect generally 0.3 to 0.5 inches west of Worcester and
0.5 to 1 inch over SE MA and RI. Overall, highest snowfall
totals should be from NE CT to Worcester to Metrowest Boston in
the order of 6 to 9 inches where the mesoscale banding coincides
with a thermal profile that minimizes mixing.
Temperatures wise, have used Hi-Res Ensemble Forecast (HREF) to
populate the hourly temperatures given that the most reasonable
way to get around uncertainty with the thermal profile is using
an ensemble approach. It should be noted here that the forecast
temps by the HREF does not correspond with the most likely
precip type (snow) across SE MA, which is a red flag against
forecasting too much snow especially over the Outer Cape and the
Islands.
https://forecast.weather.gov/product.php?site=BOX&issuedby=BOX&product=AFD&format=CI&version=1&glossary=1&highlight=off
