textproduct: Boston / Norton

WHAT HAS CHANGED

Upgraded the Wind Advisory to a High Wind Warning for Cape Cod and Nantucket. Expanded the Wind Advisory to include all of Eastern Massachusetts and Rhode Island.

KEY MESSAGES

- Low pressure system brings rain and strong to damaging to portions of southern New England today.

- Cooler than normal temperatures early next week with hit-or- miss shower/thundershower chances during the daytime hours Monday and Tuesday.

- Monitoring possible offshore low development around midweek, which could pass far enough south to allow for warming temperatures, or lead to a continuation of cooler than normal temperatures and possible rains with a closer pass.

DISCUSSION

KEY MESSAGE 1...Low pressure system brings rain and strong to damaging to portions of southern New England today.

The forecast remains on track for a deep trough of anomalously cold air and an associated strong low-pressure system to impact the region today. While this system will bring widespread showers and isolated storms, the primary concern is strong, potentially damaging winds.

Model guidance has remained consistent regarding this wind threat. As a deepening low rotates offshore, a 45-60 kt LLJ at 925 mb will sweep across eastern portions of southern New England. Model guidance signals distinct pressure rises on the west side of the exiting low. Analyzing from a vertical profile perspective, model soundings show how close the 45 kt+ winds are to the surface. With steep lapse rates, these are good indicators for a good portion of the winds are favored to be mixed down to the surface. A good portion of high-res guidance and the ECMWF and its ensemble show a signal for strong to damaging gusts. The NAM/GFS have leaned on the lower end with gusts. Inland areas can expect widespread gusts 35-45 mph. HREF means support potential for gusts 45-55 mph near the coast as the LLJ passes overhead. This seems reasonable given that the strongest core of the LLJ passes through east southern New England. The upper percentiles of the HREF even suggest a low probability for isolated 60 mph gusts across the Cape and Islands. Winds will ramp up from north to south, increasing after 7 AM for northern locations and peaking for the south coast and Cape and Islands in the afternoon.The current Wind Advisories and High Wind Warnings are currently on track based on latest guidance.

As the system tracks offshore, it will bring another batch of precipitation through the region today. Following a brief lull early this AM, showers will spread southward after 6 AM, likely not reaching the Cape until the early afternoon. Very marginal instability may support a few embedded brief downpours and isolated thunderstorms. Rain will gradually decrease from west to east mid- afternoon into the evening as the low pulls away. Ensembles indicate rain totals between 0.20" and 1.2" is the more likely range with the higher end totals favored for northeast MA.

In tandem with the strong wind signal, this deep trough is pulling down an anomalously cold airmass that will linger all day, keeping afternoon highs on the chilly side for end of May. Highs likely stay in the upper 40s and low 50s. Given the timing of the cold air and precipitation, higher resolution guidance, particularly the NAM, suggests some snowflakes mixing in at the higher elevations. It has backed off a little in recent runs, likely due to timing of the steadier precip rates (need that for efficient dynamic cooling). However, it still highlights the north MA border for a trace of light snow. All that said, we could still see a few flakes mix in for the higher elevations of the interior this AM if the timing is right with the higher risk for the high elevations of the northern MA border. No travel impacts expected.

KEY MESSAGE 2...Cooler than normal temperatures early next week with hit-or-miss shower/thundershower chances during the daytime hours Monday and Tuesday.

Ensemble means continue to point to 500 mb troughing governing Southern New England's weather late this weekend into a good portion of next week, in a slow-evolving, blocky 500 mb pattern. There are a couple of upper level lows that will be moving into and/or meandering around Southern New England.

The first of these closed upper lows settles in Sunday night and into Monday, before moving offshore into Tuesday. Really not much change in terms of sensible weather each day, with diurnally-driven, hit-or-mostly-miss showers or low-topped thundershowers popping up with the cool pocket of air aloft associated with the closed low. This type of showery pattern also means no one area can be guaranteed to be rain-free, but there will be several dry periods in between. Temperatures trend slightly cooler than early-June normals, with values in the mid 60s to mid 70s with comfortable humidity levels. Climatological normals run around the mid/upper 70s for highs.

KEY MESSAGE 3...Monitoring possible offshore low development around midweek, which could pass far enough south to allow for warming temperatures, or lead to a continuation of cooler than normal temperatures and possible rains with a closer pass.

More uncertainty during the mid to latter part of the upcoming workweek with regard to the next surge of vort energy diving southward from the Canadian Maritimes. This feature digs south/south- southwest to a position generally south of 40N/70W into a wound-up, closed mid/upper circulation and associated surface low. The placement of this cyclone, when it begins to exit further offshore and if we see any rainy conditions associated with its north/northeast passage are all unclear at this time. Besides just rain chances, this feature's location will also dictate temperatures as some of the further-offshore/drier solutions bring in shortwave ridging aloft north of it extending into New England, with potential for drier weather and a considerable warmup in temps for at least interior Southern New England. On the other hand, a closer pass to Southern New England would bring cooler than normal temperatures and perhaps some light rains. We offered to not make many changes to NBM at this point and let trends dictate subsequent adjustments to the forecast.

AVIATION /08Z SATURDAY THROUGH WEDNESDAY/

Forecaster Confidence Levels...

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

06z TAF Update:

Through 12Z: High confidence in TAF/trends but moderate on timing.

Line of moderate SHRA with a some embedded thunderstorms. Tempo MVFR visibilities possible in any storms. Brief lull in most shower activity by 09Z. Winds shift NW at 10-15 kts with gusts 20-25 kts.

Toward 12Z, ceilings trend toward VFR-MVFR range northern MA with moderate rain nearing the VT/NH/MA border, with winds becoming north and increasing to 15-18 kt with gusts 25-30 kt. For BDL- PVD-Cape airports, VFR should still govern, with NW winds around 10-13 kt / gusts to 20 kt.

Today: Moderate confidence.

Mainly MVFR ceilings. IFR is possible for the east terminals, though lower confidence. Rain spreading south from northern MA; more persistent rains from ORH east. Low chance for a few embedded thunderstorms though lower confidence on location.

NE winds increase significantly to 18-25 kt, with gusts 30-45 kt, strongest in eastern MA. Will take until early afternoon (17-20z) for strong NE winds and moderate rain to develop towards the Cape and Islands.

N-NE wind gusts and rain decrease generally from west to east after 20Z with ceilings trending VFR as well. BOS should start seeing improvements around 22Z and 00-04Z for the Cape and Islands.

Tonight: High confidence.

MVFR with moderate rain and NE winds 15-25 kt / gusts 35-40 kt Cape and Islands early, but conditions improve to widespread VFR by 04z Sunday. Winds shift to N/NW and decrease significantly to around 5-10 kt, occurring soonest in northern MA.

KBOS...Moderate confidence in TAF. Steady moderate rain with MVFR visby and MVFR-IFR ceilings develops 11-13z, with strong NE winds increasing to 20-25 kt sustained/gusts to 45 kt most of Sat.

KBDL...Moderate confidence in TAF. Light rain arrives 13-15Z with MVFR ceilings and N winds gusting to 30-35 kt most of Sat. Rain/winds decrease after 20Z with improving ceilings.

Outlook /Sunday through Wednesday/...

Sunday: VFR. Slight chance SHRA.

Sunday Night: VFR. Chance SHRA.

Monday: Mainly VFR, with local MVFR possible. Chance SHRA.

Monday Night: VFR.

Tuesday: VFR. Chance SHRA, slight chance TSRA.

Tuesday Night: VFR. Slight chance SHRA.

Wednesday: VFR. Breezy. Slight chance SHRA.

MARINE

Forecaster Confidence Levels...

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

*Dangerous Marine Conditions Expected Today into Early Tonight*

A powerful storm system is expected to bringing strong northeast winds, rough seas, and moderate rain to the waters today.

NE winds increase this morning across the northern waters, eventually increasing across the southern water this afternoon. Winds 30-35 kts with frequent gusts 45-50 kts. Offshore seas build to around 12-14 ft, with 5-8 ft nearshore. NE winds come around to northerly this evening into the Gale force range, decreasing further tonight. Storm Warnings for most of the waters, with the exception of Boston Harbor and Narragansett Bay where there are Gales.

High winds and built up seas will bring high surf to the surf zones today along the east facing beaches. If you are venturing out to the beach, stay a safe distance for jetties.

Outlook /Sunday through Wednesday/...

Sunday: Winds less than 25 kt. Local rough seas. Chance of rain showers.

Sunday Night: Winds less than 25 kt. Areas of seas approaching 5 ft. Chance of rain showers.

Monday: Winds less than 25 kt. Seas locally approaching 5 ft. Slight chance of rain showers.

Monday Night through Tuesday: Winds less than 25 kt.

Tuesday Night: Winds less than 25 kt. Seas locally approaching 5 ft. Slight chance of rain showers.

Wednesday: Low risk for Small Craft Advisory winds with gusts up to 25 kt. Local rough seas. Slight chance of rain showers.

BOX WATCHES/WARNINGS/ADVISORIES

CT...None. MA...Wind Advisory from 8 AM this morning to 8 PM EDT this evening for MAZ004>007-012>021-023-026. High Wind Warning from 8 AM this morning to 8 PM EDT this evening for MAZ022-024. RI...Wind Advisory from 8 AM this morning to 8 PM EDT this evening for RIZ001>008. MARINE...Gale Warning until 8 PM EDT this evening for ANZ230-236. Storm Warning until 8 PM EDT this evening for ANZ231>235-237- 250-251-254>256-280>283.