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Area forecast discussion 
National Weather Service Boston/Norton Massachusetts 
655 PM EST Sat Jan 19 2019 


Synopsis... 


A significant winter storm will impact the region tonight into 
Sunday afternoon with heavy, wet snow, ice, and rain. Arctic 
air follows this system into early next week. Another weather 
system may bring rain and snow next Wednesday. 


&& 


Near term /through Sunday/... 


7 PM update ... 


Not too many changes at this point. Not wanting to wipe away 
immediately the 4 PM update. Evaluating the latest trends, the 
2m temperatures via the href are too cold whereas the latest 
hrrr and nbm are close to spot on. Thermal fields aloft continue 
to differ as does the low-level jet. 18z guidance has shifted 
ever so subtly but not in any one particular direction. At this 
point it'll be short-term / near-term updates going forward. 


*/ Highlights... 


- significant winter storm tonight into Sun afternoon 


- heavy rain and possible urban/poor drainage flooding southeast New 
England Sunday 


- flash freeze eastern Massachusetts/Rhode Island Sunday mid/late afternoon: very 
hazardous travel 


*/ overview... 


12z guidance suite has continued the northward trend and warmer 
scenario as we are dealing with a more amplified S stream short- 
wave. GFS remains on the western envelope of solutions but even 
the NAM/European model (ecmwf) has low tracking across southeast new eng on Sunday. We 
are leaning toward NAM/European model (ecmwf) solution. 


850/700 mb lows tracking mainly north and west of southern New England which is 
not a favorable track to keep deep cold air in place. Also, 
persistent mesoscale banding will be to the north of the mid 
level low track. Result will be a bit less snow due to rapid mid 
level warming, although still looking at a decent front end 
thump tonight. Highest risk for appreciable ice accum also 
shifting north across interior Massachusetts. 


This southern stream system will have abundant moisture with 
pwats up to 3sd above normal. Southerly low level wind 
anomalies also around 3sd above normal which will bring a heavy 
quantitative precipitation forecast event to southern New England. The main challenges are snowfall amounts 
tonight in the intense warm advection before ptype transition, 
location/extent of icing in the interior, timing of flash freeze 
eastern new eng Sun afternoon, and flood potential for southeast new 
eng. 


1) snow/ice accumulations and uncertainty... 


Steady snow develops this evening and should quickly ramp up 
with a period of heavy snow as a band of strong 850 mb 
frontogenesis lifts north across southern New England. Snowfall rates of 1-2 
inches/per hour are expected, especially in the interior. 
However, mid level warm layer expected to advance north across 
southern New England after midnight likely reaching New Hampshire border by 12z sun. This 
will result in snow Flipping to rain from the South Coast 
northward along the i95 corridor after 06z, with a changeover to 
sleet and freezing rain in the interior where low level cold air 
will persist. Much of the snow in northern Massachusetts will likely be 
over by 12z sun. Given that the warm layer aloft is more 
pronounced and low level cold layer is more shallow, we are 
expecting more freezing rain than sleet in the interior. The 
main risk of freezing rain on Sunday will be across interior Massachusetts 
west of i495 and possibly into the CT valley in northern CT. 


Our snowfall forecast has 1-3 inches along the South Coast, 
increasing to 3-6 inches across north CT and along the i95 corridor 
from north Rhode Island to Boston and coastal Essex County. The Max snowfall 
will be north of the Mass Pike where generally 6-10 inches are 
expected, highest amounts near the New Hampshire border, with a risk for a 
foot of snow across northwest Massachusetts. The exact timing of changeover will 
be critical to accumulation forecast as heavy snow will be 
falling during the transition time. Amounts could be off by 2 or 
3 inches if the timing changes by a few hours. 


Regarding ice accumulation, we shifted the axis of Max 
accumulation to the north, from northwest Hartford County to interior 
Massachusetts along and a bit north of the Pike. This is where locally 
0.25" to less than 0.50" ice accretion is possible. This would 
likely be in a narrow location but would increase power outage 
threat which would be significant problem given bitter cold air 
to follow Sun night. 


Winter headlines are largely unchanged but upgraded Hartford 
County to a warning for combination of 3-6" snow and up to 
0.25" ice. 


2) flash freeze across eastern Massachusetts/ri: very hazardous travel 


We have high confidence that a flash freeze will occur but 
lower confidence on timing. Given the further north trend in 
the guidance, much milder air is expected across the coastal 
plain with temps into the 40s with a chance of some 50s as well. 
However, a strong coastal front in the interior with temps in 
the 20s to the west will crash to the coast sometime in the 
afternoon with temps falling sharply into and through the 20s. 
Current timing is 3-6 PM from bos-pvd to the South Coast and a 
bit later for Cape Cod but timing could be off by a few hours. 
This is a big concern as any wet surfaces will rapidly freeze 
resulting in very hazardous travel. In addition, rain may end as 
a period of sleet or freezing rain here as deep cold air is 
lagging the low level cold air by a few hours. 


3) heavy rainfall and flood potential 


Given anomalous pwats and low level jet with strong forcing 
expect rainfall of 2-3 inches across Rhode Island and southeast MA, with locally 
1-2 inches possible in a 6 hour period 12-18z sun. This will 
result in areas of urban and poor drainage flooding so a Flood 
Watch will be issued. Some river flooding is also possible in Rhode Island 
if these rainfall amounts are realized. 


4) strong winds 


Strong low level jet will be moving across the cape/islands on 
Sunday. Thermal profiles are inverted which will limit gusts, 
but could see a few gusts to 45 mph so wind advisories will be 
issued for the Outer Cape and ack. 


&& 


Short term /Sunday night/... 


Sunday night-Monday... 


Weekend storm departs through the Maritimes Sunday night. High 
pressure over the Great Lakes and the storm in the Maritimes will 
generate a strong pressure gradient over New England, maintaining 
the cold advection and resulting mixing over our area. Strong north 
winds will turn from the northwest overnight and continue to draw 
Arctic air into southern New England. 


Temperatures upstream in the nrn plains started the past couple of 
days in the single numbers with dew points below zero. Expect Sunday 
evening temps in the teens, but falling into the single numbers by 
morning. A few spots in western and central mass and northern CT. 
Mixing from 950 mb suggests Monday Max sfc temps near 10f. We used 5- 
15f inland and around 20f on the cape and islands. 


&& 


Long term /Monday through Saturday/... 


Tuesday through Thursday... 


High pressure moves over New England Tuesday. Winds diminish with 
the high, which will generate radiational cooling Tuesday morning. 
The clear skies and light wind will allow min temps in the single 
numbers and below zero. 


Low pressure that moves onshore on the Pacific coast this weekend 
splits into two parts. One moves across Canada and the northern USA 
will move across the country next week, bringing rain or snow to 
southern New England Wednesday. The second part of the system dives 
southeast to the Gulf of Mexico by Wednesday, then moving up the 
East Coast Thursday or Friday maintaining a chance of rain or snow. 


&& 


Aviation /00z Sunday through Thursday/... 


Forecaster confidence levels... 


Low - less than 30 percent. 
Moderate - 30 to 60 percent. 
High - greater than 60 percent. 


Short term /through Sunday night/...high confidence on trends. Lower 
confidence on exact timing of precip type transitions. 


Tonight... 
widespread IFR / LIFR with -sn / snow developing, transitioning to 
pl / freezing rain towards the north and west with time, changing to rain along 
the bos-hfd line by 12z Sunday. With transition, may see improved 
vsbys, however cigs will remain IFR / LIFR. East winds increasing, 
gusts of around 35 kts towards morning with low level wind shear over southeast New 
England 2 kft agl 40 to 60 kts. 


Sunday... 
continued IFR / LIFR conditions. Pl / freezing rain over north/west Massachusetts through 
roughly 18z while rain / +ra for hfd-bvy and points S/E. Visibility 
issues with either. Low level wind shear for southeast New England through 21-0z at the 
latest, continued 2 kft agl 40 to 60 kts. East winds initially 
through roughly 18z with gusts up to 40 kts across east coastal Massachusetts 
terminals. Winds shifting west late, will see precip dissipate, 
conditions quickly improve, cigs lifting towards VFR. Concern 
for southeast New England with residual, standing water freezing on 
runways. 


Sunday night... 
blustery northwest winds, gusts upwards of 35 kts, conditions continuing 
to improve VFR. Residual water along runways over southeast New England 
likely freezing with Arctic air building in. 


Kbos terminal... 
snow ongoing likely to continuing into early morning by then will 
see a brief flip to a wintry mix during the morning push before 
quickly transitioning to rain. Have to watch low level wind shear closely for 
roughly around the 9-18z Sunday period. Blustery northwest winds Sunday 
night, could see flash freeze conditions on area runways. 


Kbdl terminal... 
snow through roughly after midnight, then we'll see the transition 
to sleet then to freezing rain. Freezing rain lingering through 
much of Sunday potentially, could see some significant ice 
accretion on runways. Current forecast amounts of around 0.2 to 
0.4 inches. Clearing out late Sunday with winds turning west / northwest 
and blustery. 


Outlook /Monday through Thursday/... 


Martin Luther King Jr day through tuesday: VFR. Windy with 
gusts to 35 kt. 


Tuesday night: VFR. Breezy. Chance snow. 


Wednesday: mainly VFR, with areas MVFR possible. Windy with 
gusts to 30 kt. Rain likely, chance sn, chance pl, freezing rain likely. 


Wednesday night: mainly VFR, with areas MVFR possible. Windy 
with local gusts to 30 kt. Rain likely, freezing rain likely. 


Thursday: mainly VFR, with local IFR possible. Strong winds 
with areas gusts to 40 kt. Rain likely. 


&& 


Marine... 


Forecaster confidence levels... 


Low - less than 30 percent. 
Moderate - 30 to 60 percent. 
High - greater than 60 percent. 


Short term /through Sunday night/... 


East winds increasing tonight with gale force gusts developing 
after midnight into Sunday. Winds shifting to S over south 
coastal waters Sunday, then near gale force north/northwest winds develop 
Sunday night behind Arctic front. Freezing spray developing late 
Sunday into Sunday night, becoming moderate to heavy further out 
in time. 


Outlook /Monday through Thursday/... 


Martin Luther King Jr day: strong winds with gusts up to 35 kt. 
Rough seas up to 12 ft. Freezing spray, chance of snow showers. 


Monday night: moderate risk for gale force winds with gusts up 
to 35 kt. Rough seas up to 12 ft. Freezing spray, slight chance 
of snow showers. 


Tuesday: low risk for gale force winds with gusts up to 35 kt. 
Areas of rough seas. Freezing spray likely. 


Tuesday night: winds less than 25 kt. Areas of seas approaching 
5 ft. Slight chance of snow. 


Wednesday: low risk for gale force winds with gusts up to 
35 kt. Areas of rough seas. Rain likely, chance of snow. 


Wednesday night: low risk for Small Craft Advisory winds with 
local gusts up to 30 kt. Areas of rough seas. Rain likely. 


Thursday: low risk for gale force winds with local gusts up to 
45 kt. Rough seas up to 11 ft. Rain likely. 


&& 


Tides/coastal flooding... 


An approaching deep low pressure will induce a strong easterly 
low level jet. 


By the time of high tide Sunday along the East Coast (roughly 
10 am local), surge should be approaching 2.0 ft, which 
combined with offshore wave action around 15 ft should lead to 
localized minor to moderate east coastal flooding given a high 
astronomical base tide in Boston around 11.6 ft MLLW. Given the 
high astro tide, continued a coastal Flood Warning for eastern 
Massachusetts and an advisory of the South Coast, cape, and islands. 


&& 


Box watches/warnings/advisories... 


CT...Winter Weather Advisory until 7 PM EST Sunday for ctz003-004. 
Winter Storm Warning until 7 PM EST Sunday for ctz002. 
Massachusetts...Flood Watch from 6 am EST Sunday through Sunday afternoon for 
maz017>022. 
Winter Weather Advisory until 7 PM EST Sunday for maz013- 
015>021. 
Coastal flood advisory from 8 am to noon EST Sunday for maz007- 
015-016-019-022>024. 
Winter Storm Warning until 7 PM EST Sunday for maz002>012-014- 
026. 
Coastal flood advisory from 5 am to 10 am EST Sunday for 
maz020-021. 
Wind Advisory from 5 am to 3 PM EST Sunday for maz022-024. 
Rhode Island...Flood Watch from 6 am EST Sunday through Sunday afternoon for 
riz002>007. 
Winter Weather Advisory until 7 PM EST Sunday for riz001>007. 
Coastal flood advisory from 5 am to 10 am EST Sunday for 
riz004>007. 
Marine...Gale Warning from 10 PM this evening to 7 am EST Monday for 
anz231>235-237-250-251-254>256. 
Small Craft Advisory from 10 PM this evening to 7 am EST 
Monday for anz230. 
Small Craft Advisory from midnight tonight to 7 am EST Monday 
for anz236. 


&& 


$$ 


Synopsis...kjc/sipprell 
near term...kjc/sipprell 



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