textproduct: Boston / Norton

WHAT HAS CHANGED

Increased confidence for a dusting to an inch of snow across interior MA overnight. Confidence is increasing in a significant winter storm Sunday into Monday but specific details remain uncertain.

KEY MESSAGES

- A weak clipper system brings some light snow to the higher terrain areas and light rain for the coastal plain tonight.

- Temperatures surge above normal Thursday with gusty winds 25-35 mph.

- Arctic front moves through late Fri into Fri evening, bringing dangerously cold wind chills Fri night into Sat night. Low risk for a few snow showers or squalls with the passage of the front.

- A significant winter storm may bring heavy snow to southern New England Sunday into Monday, but specific details are uncertain.

DISCUSSION

KEY MESSAGE 1...A weak clipper system brings some light snow to the higher terrain areas and light rain for the coastal plain tonight.

Latest suite of model guidance has come into better agreement with respect to precipitation amounts associated with a weak Clipper system moving over The Northeast this evening. QPF amounts currently consistently coming in between 0.05 to 0.15 " of liquid across most of southern New England. Interior MA, RI, and CT will be cold enough to support snow. Accumulations will be minimal with only a coating to an inch forecast. Some locations over the higher terrain of The Worcester Hills may see as much as 2 inches. Further south and east across the coastal plain, temperatures will be more marginal, so expect rain mixing in after midnight. This will result in little to no accumulations for these areas. South coast, southern RI, and Cape/Islands should only expect rainfall given overnight temperatures in the mid to upper 30s.

KEY MESSAGE 2...Temperatures surge above normal Thursday with gusty winds 25-35 mph.

Moderate south/southwest winds develop Thursday morning and persist through Thursday evening. This will support low-level WAA which will allow 925 hPa temps to warm to between -3 and 1 degree C. This will be an increase of 5 to 10 degrees C from Wednesday afternoon. Thus, expect warmer afternoon temps on Thursday in the upper 30s to low 40s under partly cloudy skies. Sunshine should support enough surface heating for efficient diurnal mixing. As a result we could see gusty winds at times with gusts between 20 and 30 mph possible.

KEY MESSAGE 3... Arctic front moves through late Fri into Fri evening, bringing dangerously cold wind chills Fri night into Sat night. Low risk for a few snow showers or squalls with the passage of the front.

Arctic front will be dropping south into SNE Fri afternoon and reaching the south coast by evening. While mod els are not showing much if any QPF, the signals in the guidance suggest there could be a few snow showers or squalls with the arctic front passage. There is an area of enhanced moisture and lift along the boundary as well as steep low level lapse rates. This suggests a few snow showers or squalls will be possible although areal coverage will be limited. Something to watch as we approach Friday.

The bigger issue will be the arctic air that pours into the region Fri night into Sat on gusty NW winds. 850 mb temps bottom out around -25C late Fri night and early Sat before slowly moderating during Sat. Lows Fri night will drop to zero to -5F over the higher terrain and single numbers elsewhere. Highs Sat will only reach the single numbers higher terrain and low to mid teens in the coastal plain. The strong cold advection will be accompanied by strong wind gusts Fri night as soundings show potential for a few gusts to 35-45 mph. The gusty winds and arctic cold will result in wind chills of -10 to -20F across much of SNE late Fri night and early Sat, recovering to only -10 to +5F during Sat afternoon. Another very cold night Sat night with lows zero to 10 above but winds will be less of a factor.

Additionally, the combination of gusty winds and very cold air will produce widespread moderate freezing spray on the waters FrI night into Sat night with low risk for heavy freezing spray.

KEY MESSAGE 4... A significant winter storm may bring heavy snow to southern New England Sunday into Monday, but specific details are uncertain.

The northward trend in the guidance continues resulting in increasing confidence for a significant winter storm. Despite the northward trend, there is still a fair amount of track spread in the ensemble guidance with the EC members furthest north and GFS members on the southern edge of the guidance envelope. In fact, there are almost as many EC members north of the benchmark as there are to the south while most GFS members are south of the benchmark. As a result it is best that we remain probabilistic as this is still a day 4-5 event and it will likely change going forward.

Using the 25th percentile snowfall as a low end suggests at least advisory snowfall near the south coast, while the 75-90th percentile would indicate potential for a major snowstorm for SNE. These snowfall accums are based on 10:1 ratio. Given deep cold air in place, higher ratios are certainly likely but we can't just assume it'll be a 15-20:1 ratio as this depends on deep saturation and strong omega through the snow growth region for getting dendrites and achieving these higher ratios. And these are details that can't be determined in a day 4-5 forecast. I do think it is fair to assume a 12-15:1 ratio as a preliminary estimate. So based on all this we have high confidence of at least an advisory event in SNE, especially near the coast, with a risk of a major snowstorm for SNE if this northward trend locks in. Interesting that the ProbWSSI is showing high probs (90%) of moderate impacts which aligns closely to winter storm watch/warning criteria, and moderate probs (50-60%) of major impacts. This is based on a WPC 60 member ensemble from the latest model cycle so this will change with updated guidance.

So what can go wrong? This is a day 4-5 event and snowfall in SNE is very track dependent which can't be determined with high confidence at this time range. I would expect wobbles in track going forward and these wobbles can result in significant changes in impacts. Also, there is a lot cold dry air to the north which makes us susceptible to snowfall gradients on the northern edge if storm wobbles southward aND it does appear we could be near that gradient. So probably won't be able to lock in to a forecast with confidence until Friday.

So bottom line, while there is certainly a risk of a major snowstorm for SNE this is a day 4-5 forecast and a subtle shift back to the south would keep the significant impacts to the south.

Timing wise, it appears snow could develop by Sun morning with heaviest snow expected in the afternoon and Sunday night. Heaviest snow should be over by Monday, however strong upper trough and shortwave approaches from the west with inverted trough at the surface and deep moisture in place so expect light snow to continue for much of Monday with additional minor accums.

Beyond Monday, well below normal temps will continue with arctic air remaining in place.

AVIATION /06Z THURSDAY THROUGH MONDAY/

Forecaster Confidence Levels...

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

Today: High Confidence.

-RA/SN clears out between 06-09Z from west to east. Conditions improve to VFR with SW winds gusting up to 25 kts.

Thursday Night: High Confidence.

VFR. Light west winds, 5-10 knots. Except the Cape and Islands remain gusty through the night.

Friday: High Confidence.

VFR. WSW winds around 10 knots steadily increase through the morning. Gusts 25-30 knots by the afternoon. KBOS Terminal...High Confidence.

Lingering -RASN and MVFR cigs through 08-09z with conditions improving to VFR 09-10z. Gusts up to 25 knots likely through the afternoon before calming tonight.

KBDL Terminal...High Confidence.

Lingering -SN through 08-09z with improving conditions 09-10z. Gusts up to 25 knots likely through the afternoon before calming tonight.

Outlook /Friday Night through Monday/...

Friday Night: VFR. Windy with gusts up to 30 kt.

Saturday: VFR. Windy with local gusts up to 30 kt.

Saturday Night: VFR. Breezy. Slight chance SN.

Sunday: Mainly VFR, with areas IFR possible. Breezy. Chance SN.

Sunday Night: Mainly IFR, with areas MVFR possible. Windy with local gusts up to 30 kt. SN likely.

Monday: MVFR/IFR conditions possible. Breezy. SN likely.

MARINE

Forecaster Confidence Levels...

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

Tonight through Thursday Night

Winds increase out of the west/southwest tonight with sustained wind between 10 and 20 knots over the coastal waters with gusts between 20 and 30 knots. These winds will support SCY criteria through Friday morning. Seas tonight into tomorrow will be 6-8 feet over the outer marine zones and 3-5 feet for the near-shore zones. Expect seas to gradually subside to 2-5 feet over all the marine zones between Thursday night and Friday morning.

Outlook /Friday Night through Monday/...

Friday Night: Strong winds with gusts up to 35 kt. Areas of rough seas. Freezing spray, slight chance of snow.

Saturday: Moderate risk for Small Craft Advisory winds with gusts up to 30 kt. Areas of rough seas. Freezing spray.

Saturday Night: Winds less than 25 kt. Areas of seas approaching 5 ft. Chance of snow.

Sunday: Low risk for Small Craft Advisory winds with gusts up to 30 kt. Seas locally approaching 5 ft. Snow likely, freezing spray. Visibility 1 nm or less.

Sunday Night: Strong winds with gusts up to 35 kt. Seas up to 5 ft. Snow likely, freezing spray. Visibility 1 nm or less.

Monday: Moderate risk for Small Craft Advisory winds with gusts up to 30 kt. Seas up to 5 ft. Chance of snow, freezing spray. Local visibility 1 nm or less.

BOX WATCHES/WARNINGS/ADVISORIES

CT...None. MA...Extreme Cold Watch from Friday evening through Saturday afternoon for MAZ002-008-009. RI...None. MARINE...Gale Watch from Friday evening through Saturday morning for ANZ231>235-237-250-251-254>256. Small Craft Advisory until 7 PM EST this evening for ANZ232- 233-235-237-250-254>256.