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 areas of strong to damaging winds tonight and Saturday.

- 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 areas of strong to damaging winds tonight and Saturday.

Highly amplified pattern results in an anomalously strong albeit quick-hitting storm system tonight and Saturday. System will bring periods of heavy rain and showers, but the bigger issue will be a period of strong winds on Saturday.

A period of strong to damaging winds continues to be of greatest concern with this system. Synoptically, strong winds will stem from a deepening area of low pressure and associated pressure rises as it pulls away. Guidance shows 3hr pressure rise values of 5-7 mb/3hr as the system winds up and pulls off the coast of MA Saturday afternoon. The latest HREF ensemble guidance greatly expanded the area of gusts 35-45 mph and now shows the potential isolated higher gusts (45-55 mph) well into the I-95 corridor. This seems reasonable given that the LLJ jet more likely shifts across east southern New England through the day. The degree of mixing has come into much better agreement among guidance with 1000-950mb lapse rates likely increasing to 8-10C/km Saturday afternoon! For these reasons, the Wind Advisory has been expanded further inland. Part of the rationale in expanding the advisories inland is increasing confidence in an apparent trend towards greater mixing in the lowest 100mb. Somewhat unusually, the core of the LLJ will be fairly close to the surface (between 925 and 850mb). Thus, the mixed layer won't have to be as deep to tap into the strong winds. The other change was to issue a High Wind Warning for the Cape and Nantucket where the LLJ will slightly stronger on the order of 55-65kt.

In terms of precipitation with this system we're looking at periods of showers that become more widespread after the evening commute into this evening and tonight. There could be a few embedded weak storms or downpours with some meager mid-level instability. Quick lull in the activity gives way to more widespread rainfall Saturday morning as low pressure rotates through. Showers slowly diminish as the low exits further offshore mid to late afternoon. It's possible for showers to linger as late as the evening for the outer Cape/Islands.

For rain totals, ensembles range rain totals 0.20" to 1.1" across the 25th to 75th percentile range with the higher amounts favored in northeast MA. While we are already in cool troughy flow, the passage of this deep upper low will shift a core of anomalously cold air aloft across the region Saturday. This will bring us a fairly cool day for late-May with highs in the upper 40s and low 50s. With the timing of the colder air and the showers, some model guidance hints at potential for a few snowflakes mixing in at the higher elevations. If the cold air arrives in a similar period of better precip rates some spots may cool enough to mix in some wet flakes. The NAM has shown the strongest signal for this, but also shows a colder airmass in place with a wider footprint compared to other guidance (Temps - upper 30s). So overall, can't rule out a wintry mix in the highest elevations of the interior although little impact 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 /18Z FRIDAY THROUGH WEDNESDAY/

Forecaster Confidence Levels...

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

18z TAF Update...

Today...High confidence.

Mainly VFR today outside the risk for a few showers mainly after 20z/21z. NW winds may gust to around 20 knots or so into mid afternoon...But sea breezes should kick in by mid afternoon along portions of the immediate coast.

Tonight and Saturday...Moderate confidence.

VFR this evening outside scattered showers especially east of the CT River and particularly eastern MA. Low risk for a rumble or two of thunder. We should then have a lull in the activity after midnight before the upper level low drops south. A shield of rain will drop south with the focus east of the CT River Valley in the 9z to 15z Sat time frame. Conditions will lower to MVFR with pockets of IFR conditions possible. too. N-NE winds will increase with gusts on the order 30-45 knots with the strongest of those along the coast. We may even briefly see some wet snow flakes mix in across the highest terrain. Gradual improvement expected Saturday afternoon and evening from west to east.

KBOS...High confidence in TAF but some uncertainty lingers on the specific timing of the sea breeze... likely between 1730-18z.

KBDL...High confidence in TAF.

Outlook /Saturday through Wednesday/...

Saturday: Mainly VFR, with local IFR possible. Strong winds with gusts up to 45 kt. Chance RA.

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

Sunday: VFR.

Sunday Night: VFR. Chance SHRA.

Monday: VFR. SHRA likely, chance TSRA.

Monday Night: VFR. Slight chance SHRA.

Tuesday through Tuesday Night: VFR. Chance SHRA.

Wednesday: VFR. Breezy. Chance SHRA.

MARINE

Forecaster Confidence Levels...

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

Tonight...

Generally tranquil with wave heights between 1 and 3 feet for most of the waters. Winds from the NW between 5 and 15 mph with gusts up to 20 mph. Winds and seas increasing from the north early Saturday morning.

Saturday into Saturday Night...

Storm warning for all the waters except Narragansett Bay and Boston Harbor where Gale Warnings are in effect on Saturday. A a strong area of low pressure system tracks across the region bringing gusty N-NE winds.

Outlook /Saturday through Wednesday/...

Saturday: Moderate risk for storm force winds with gusts up to 45 kt. Rough seas up to 13 ft. Chance of rain.

Saturday Night: Low risk for Small Craft Advisory winds with gusts up to 30 kt. Rough seas up to 12 ft.

Sunday: Winds less than 25 kt. Local rough seas.

Sunday Night: Winds less than 25 kt. Seas up to 5 ft. Chance of rain showers.

Monday: Winds less than 25 kt. Areas of seas approaching 5 ft. Chance of rain showers, slight chance of thunderstorms.

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

Tuesday: Winds less than 25 kt. Areas of seas approaching 5 ft. Slight chance of rain showers.

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

Wednesday: Winds less than 25 kt. Areas of rough seas. Chance of rain showers.

BOX WATCHES/WARNINGS/ADVISORIES

CT...None. MA...Wind Advisory from 8 AM to 8 PM EDT Saturday for MAZ004>007- 012>021-023-026. High Wind Warning from 8 AM to 8 PM EDT Saturday for MAZ022- 024. RI...Wind Advisory from 8 AM to 8 PM EDT Saturday for RIZ001>008. MARINE...Gale Warning from 2 AM to 8 PM EDT Saturday for ANZ230-236. Storm Warning from 2 AM to 8 PM EDT Saturday for ANZ231>235- 237-250-251-254>256-280>283.