textproduct: Boston / Norton

WHAT HAS CHANGED

No significant changes.

KEY MESSAGES

- Rain arrives from west to east through the morning continuing overnight.Rain exits most areas Monday morning with the exception of a few lingering showers near the Cape and Islands.

- High pressure will yield mainly dry conditions and cool temperatures the first half of the week. Potential for an offshore coastal low late week. There is still uncertainty in the track and strength at this time.

DISCUSSION

KEY MESSAGE 1...Rain arrives from west to east through the morning continuing overnight.Rain exits most areas Monday morning with the exception of a few lingering showers near the Cape and Islands.

Expect the second-half of the weekend to be a soggy one. A shortwave trough coupled with an 850mb jet will advect in ample moisture and increase lift across the region. Precipitable water values increase to 1.3-1.8" which equates to 160-190 percent of normal. Despite the anomalous moisture, weak forcing and a lack of instability will limit heavy rain rates, keeping the flash flood threat low. This event will primarily feature a prolonged period of more stratiform- like rain driving by the WAA rather than convective downpours that are fueled by instability.

High-resolution guidance shows rain likely arriving mid to late morning for most areas as rain spreads from west to east. Rain may not arrive in the more eastern parts of southern New England until around noon. Rain continues overnight, coming to and end between 2AM- 8AM for most areas. High-res guidance hints at the potential for showers to linger for the Cape and Islands through the afternoon (latest).

As mentioned earlier, there isn't a strong signal for widespread heavy rain/downpours with this system. However, there are indications of very marginal instability along the south coast which may be enough to support an embedded rumble of thunder and a brief heavier rainfall. Areas of increased convergence will also be favored for enhanced rain rates. There are a couple areas of enhanced convergence as well that have been highlighted across model guidance with areas along the Mass Pike and higher terrain in western and central southern New England. These areas would at least be more favorable for more enhanced rates and thus higher end totals.

For total amounts, HREF PMM shows a range of 0.50 to 1.2" with the higher end of the range being more localized and in those favored areas discussed above (terrain, south coast). HREF ensemble max shows localized amounts of 1.5-1.75" which would be a low probability (< 10%) given it is a max of that guidance. Global/ensemble guidance leans slightly lighter overall with amounts 0.30-0.70". Outside of the localized moderate to briefly heavier rates, the lower rain rates should keep the flash flood threat low. Can't rule out some minor ponding in the usual poor drainage spots, but otherwise this will be prolonged soaking rain.

KEY MESSAGE 2...High pressure will yield mainly dry conditions and cool temperatures the first half of the week. Potential for an offshore coastal low late week. There is still uncertainty in the track and strength at this time.

There is good agreement across model guidance with a fairly strong surface high shifting over Quebec Tuesday into late-week. This will put the region under a cooler than normal airmass with consistent NE flow. This will yield cooler than normal highs in the 60s through the week. There is a low probability that a system just brushes southern New England Tuesday bringing a chance for showers mainly confined to the immediate south coast and Cape/Islands.

Deterministic and ensemble guidance continues highlight potential for a slow moving offshore coastal storm developing Wednesday into late-week. There is a general consensus with an upper low and trough first tracking eastward across the upper mid-west, although cluster analysis does highlight some variance in the amplitude of the trough and track of the low. As this pushes to the east coast, a coastal low develops in the Atlantic then tracking northward toward the northeastern coastline. As with most coastal storms this far out, there is still a notable amount of variance across guidance in how this low tracks and its strength. This will directly feed into the uncertainty in elements such as winds, rain chances/amounts, and coastal erosion/flooding concerns.

With a strong high to the north and an incoming upper low, the pressure gradient will tighten Tuesday through the week. This will result in strengthening winds/gusts across the coastal waters, especially the southern waters in closer proximity to the stronger gradient. There is some question on how strong the gradient gets and where the strong part of the gradient positions and this will likely depend on the track and strength of the low. All that said, ensemble guidance is signaling potential for gale force gusts for the southern waters. Strong gusts may develop as early as Tuesday night, but more likely Wednesday morning. Strong winds are likely to stay as the coastal low tracks northward; however, the magnitude will depend more on the track and strength of the low itself which remains less certain at this time. For land, winds will be gusty as well although not as high in magnitude as the coastal waters(25-35 mph) Wed- late week. Cape and Islands will likely be on the higher range of the gusts. If the low ends up tracking more west, periods of rain will be possible sometime in the Thursday to Sat timeframe.

AVIATION /08Z SUNDAY THROUGH THURSDAY/

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.

VFR with increasing mid-level clouds. Light ESE wind with some areas light and variable.

Sunday...High confidence in trends, moderate confidence in timing

VFR to start, trending downward to MVFR from west to east as rain overspreads the region 13-17Z. Potential for IFR ceilings to develop this afternoon-evening although coverage is lower confidence. The threat for LLWS has lessened, although can't be ruled out as a weaker low level jet shifts across southern New England between 18Z-06Z. Otherwise winds generally SE to ESE at 5-10 kt. Chances for embedded thunderstorms are very low to near zero.

Sunday Night...High confidence in trends, moderate confidence in timing and ceilings.

IFR ceilings with LIFR possible in areas. Visibilities 2-5SM with -RA continuing overnight. -RA ends from west to east mainly between 06Z-12Z. Winds shift to the N at 8-12 kt.

KBOS Terminal...High confidence in TAF.

KBDL Terminal...High confidence in TAF.

Outlook /Monday through Thursday/...

Monday: Mainly VFR, with local MVFR possible. Breezy.

Monday Night: VFR. Breezy.

Tuesday: VFR. Windy with local gusts up to 30 kt. Slight chance SHRA.

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

Wednesday: Mainly VFR, with local MVFR possible. Windy with gusts up to 35 kt.

Wednesday Night: VFR. Windy with gusts up to 35 kt.

Thursday: VFR. Strong winds with local gusts up to 40 kt. Slight chance SHRA.

MARINE

Forecaster Confidence Levels...

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

Through Monday...High Confidence.

A low pressure system will bring rain today arriving into the coastal waters in ther afternoon, continuing into Monday There is a low chance for a few rumbles of thunder, primarily across the southern waters. The bulk of the rain ends early Monday morning, though a few showers may linger through the afternoon across the waters. Seas remain between 2-4 ft. Southeast winds today less than 25 kts. Front moves through early Monday morning shifting winds to the N/NE. Gusts increase into the 20-30 kt range with the highest gusts across the southern waters Monday morning into the evening. Seas increase to 5 ft across the southern and eastern outer waters. Small Craft Advisory issued 12Z-00Z for those areas.

Outlook /Monday through Thursday/...

Monday: Low risk for Small Craft Advisory winds with gusts up to 25 kt. Seas up to 5 ft. Chance of rain showers.

Monday Night: Moderate risk for Small Craft Advisory winds with gusts up to 30 kt. Seas up to 5 ft. Slight chance of rain showers.

Tuesday: Strong winds with gusts up to 35 kt. Seas up to 5 ft. Chance of rain showers.

Tuesday Night: Strong winds with gusts up to 40 kt. Rough seas up to 11 ft. Slight chance of rain showers.

Wednesday: Strong winds with gusts up to 40 kt. Rough seas up to 14 ft.

Wednesday Night: Strong winds with gusts up to 45 kt. Rough seas up to 16 ft. Slight chance of rain showers.

Thursday: gale force winds with gusts up to 45 kt. Rough seas up to 20 ft. Chance of rain showers.

BOX WATCHES/WARNINGS/ADVISORIES

CT...None. MA...None. RI...None. MARINE...Small Craft Advisory from 8 AM to 8 PM EDT Monday for ANZ232>235-237-254>256.