Really good discussion, including the 6Z NAM, this morning from the NWS in NYC. How does a met even forecast with such divergent info, other than maybe saying, ok, let's stick with continuity and the current forecast and wait to see if there is any move towards the hi-res models at 12Z or not or if they move back towards the globals. Tough job, as we know...Well, the NWS went partway to what I thought they might, issuing advisories for the rest of SNJ and watches for all of NYC, but surprisingly, no advisories for Mercer/Middlesex or NE NJ (Union up to Bergen), even though the maps show 2-4" of snow.
And since these were issued, the 6Z NAM (both the 3 km and 12 km NAMs) came out showing 12-20" of snow from 95 eastward and 6-12" within 20-30 miles west of 95 and the hi-res HRDPS (kind of like the 3 km NAM, a high res model0 shows 6-12" from 95 east and 3-6" west of 95, but the 6Z GFS and RGEM only show maybe 2-4" for I-95 and a bit more for the coast. Very hard to make forecasts with model runs like that. Maybe the NAM is just a hiccup or maybe it's on to something. I have no clue. Anyway, here's the map of watches/advisories warnings and snowfall maps are below that. Gotta work today, so not many posts expected unless the 12Z runs look like the NAM.
National Weather Service New York NY
637 AM EST Wed Jan 3 2018
.SHORT TERM /6 PM THIS EVENING THROUGH THURSDAY NIGHT/...
** Heavy snow with strong winds expected for Suffolk County
and much of Southern CT Wed Night through Thu - Blizzard Conditions
Possible **
** Light to Moderate snow accumulations and strong winds expected
Wed Night through Thu for remainder of the region - Moderate to
Heavy snow accumulation possible **
Polar trough will continue to dig into the SE US, aided by one
strong shortwave/jet streak diving through the Gulf Coast this
morning, and by a second separate vigorous shortwave/jet streak
diving through the lower Mississippi River Valley this morning.
As mentioned over the last few days, there are still subtle
model differences in the timing/location of phasing of the
above shortwave energies as they round the base of the digging
trough this evening/tonight, as well as a final interaction with
a third shortwave/jet streak (diving through the Upper
Mississippi River Valley this morning) off the SE US coast late
tonight into Thursday morning. This interaction will affect the
track and timing of explosively deepening low pressure. The
other factor appears to be models having difficulty with
convection development and vorticity feedback off the US east
coast as this intense mid/upper level energy interacts with a
sub-tropical moisture plume, which also could play a
significant factor in low pressure track.
Operational models have already shifted about 50 miles closer
to LI with track of low pressure by late Thursday than 24 hrs
ago, with consensus track of a 955 to 960 mb low pressure
system tracking over or just se of the 40/70 lat/lon benchmark.
The exact phasing evolution will determine any additional
forecast track adjustment, with GEFS/ECMWF/SREF still showing at
least 150 miles of spread in members within 24-36 hrs. This
range of potential shift would have significant implication on
westward expanse of frontogenetically forced heavy snow banding,
and strong to damaging wind fields with this intense low. In
fact 06z NAM, has taken a significant shift westward from 00z
with its frontogenetic forcing, showing potential for 1 ft+ snow
across the entire area. For this reason, NHC will be doing a
reconnaissance flight and several dropsondes this
afternoon/evening off the Fl/SE US coast to better capture this
phasing and convection latent heat release in the models.
For now, based on above and consensus 00z operational/ensemble
track and uncertainty, expectation for snow to develop from S/SE
to N/NW tonight as polar and southern stream jet energy phase.
This will place the region under deep lift of right rear of 150
kt jet streak, with subtropical moisture being fed by cold
conveyor into the region. Then as phasing shortwave energy and
rapidly deepening low pressure move up the coast tonight, LI
and SE/SC CT are expected to fall under strong low- mid level
forcing late tonight into Thu afternoon. Heavy snow banding is
typically under and just to the NW of that. Also potential for
enhanced updraft/convection through CSI as indicated by negative
epv above the frontogenetic forcing. SPC SREF/SPC HREF
corroborate this through indicating a moderate to high potential
for 1 per hour snowfall rates during this time, and possibly 2
inch per hour. Once again though, this banding could
expand/shift westward by 50 or more miles, which is in the realm
of QPF spread seen in GEFS and SREF, which have a reasonable
range of 1/10 to 1 inch across NYC!
So overall, the probability for a light to moderate accumulating
snowfall event for the entire area is high late tonight into Thu
eve, with a heavy snowfall expected for much of LI and CT. Potential
still exists for the entire region to see a moderate to heavy snow
event.
A good middle of the road compromise for qpf with the
track/banding uncertainty is staying close to GEFS/SREF/ECMWF
ensemble means. This was blended in with WPC for consistency.
This results in 1/2 to 1 inch qpf across LI/SE CT, 1/4th to 1/2
inch across NYC/NJ metro and SW CT, and up to 1/4 inch to the
NW. Snow ratios are a bit tricky with strong omega in the
saturated snow growth and eventually in the warmer riming
saturated region, especially for eastern and coastal areas. So
a blend of snow ratio algorithms and WPC was used that appear to
capture this. This points towards a likelihood of 6 to 12 inch
snowfall for E LI/SE CT, 4 to 7 inches westward to the Hudson
River, and 1 to 4 inches to the NW. As mentioned, this remains a
fluid situation based on model spread, with potential for these
numbers to go down, but still seems moreso upwards based on
model trends and mode of SREF/GEFS/ECMWF ensemble members
compared to operational runs. This higher trend is reflected in
latest WFO/WPC snow probs, with high probs of greater than 6
inches of snow across E LI/SE CT and low to moderate probs to
Hudson River. A significant jump in the reasonable worse case
has also been noted, with 1 in 10 chance of seeing more than
12-16 inches of snow across much of LI/CT, 8 to 12 inches of
snow across NYC/NJ metro to Hudson River, and 4 to 8 inches N &
W still in play. In fact, the 06z NAM has come in with 1 1/2 to
2 1/2 inches of liquid QPF for much of the region, which is
within SREF ensemble spread.
The other hazard with this storm continues to be strong winds,
with a blend between GFS/NAM profiles pointing to likelihood for
30 to 40 mph gusts along the coast Thu aft/eve, and 45-50 mph
gusts for eastern coastal areas. This will lead to blowing and
drifting snow, possibly near whiteout conditions in the warned
areas, as well as some downed tree limbs and power lines.
Based on above, have upgraded to winter storm warning for Suffolk
and SE CT with 6 to 10 inches of snow expected and potential
for 1 ft or more. No blizzard warnings at this time, as
despite likely high winds, heavy snowfall and low vbsy, but low
confidence that sub 1 ft snowfall will have signifcant enough
impacts. If expected snowfall amounts increase today, and
therefore high end imapcts, a blizzard warning will be hoisted.
A Winter storm watch issued for NYC metro, Nassau and SW CT with
3 to 7 inches expected and potential for widespread in excess
of 6 inches. To the N&W of this region an advisory is likely,
but not enough confidence in 6+" for watch. Have held off on
headlines after collab with surrounding offices in this region
due to 3rd period timing and uncertainty. Again, if 6z NAM is
pre-cursor, we may see the warning upgraded another tier
westward or possibly more.
