textproduct: Boston / Norton

WHAT HAS CHANGED

No significant changes.

KEY MESSAGES

- Widespread showers and embedded thunder with locally heavy downpours this evening, exits SE New Eng overnight.

- Warmer Sunday with a few afternoon showers or an isolated t-storm possible.

- Best chance of rain Sun night into Mon is expected to be along the south coast.

- Dry day Tuesday, increasing chance for widespread rainfall mid to late week.

DISCUSSION

KEY MESSAGE 1...Widespread showers and embedded thunder with locally heavy downpours this evening, exits SE MA overnight.

Mid level shortwave moves into SNE this evening with a modest low level jet, focused across RI and eastern MA, developing ahead of the shortwave. Deep moisture plume and decent forcing for ascent has resulted in widespread showers from NE PA into western MA. Expect numerous showers to develop across SNE through early evening and become more convective as weak elevated instability develops at the nose of the LLJ. This will lead to locally heavy downpours and can't rule out an embedded t-storm as well. HREF indicating 30 percent probs of 1+ inch rainfall in 3 hr period late afternoon and evening near the south coast through SE MA so localized rainfall amounts over an inch are possible with any persistent convective showers. Otherwise, looking at 0.25 to 0.50" of rain. Showers will move off the coast around midnight but areas of stratus and fog will linger into the morning.

KEY MESSAGE 2... Warmer Sunday with a few afternoon showers or an isolated t-storm possible.

Improving conditions Sunday as weak height rises develop in between systems. Morning stratus and patchy fog will burn off giving way to partly to mostly sunny skies. However, a weak cold front will move into western New Eng during the afternoon. An area of low-mid level moisture associated with the front and marginal instability combined with weak convergence may be enough for a few afternoon showers developing and perhaps an isolated t-storm, mainly in the interior. 925 mb temps increase to 12-14C which support highs into the low-mid 70s, but 60s along the immediate south coast with SW flow.

KEY MESSAGE 3...Best chance of rain Sun night into Mon is expected to be along the south coast.

Upper trough moves across eastern Gt lakes Sun night into Mon. The cold front moving through SNE will shift south of the coast Sun night with a surface wave developing along the mid Atlc coast then tracking well south of New Eng Mon. The recent trends in the guidance have shifted south with the moisture and rainfall as flow aloft remains relatively flat. Best chance of some showers will be along the south coast and especially the Islands, but it appears bulk of rainfall from this system will remain to the south. Cooler day on Monday as 925 mb temps drop to 6-8C. Highs will be mostly 60- 65F.

KEY MESSAGE 4...Dry day Tuesday, increasing chance for widespread rainfall mid to late week.

Heights build Tuesday as a mid level ridge builds over the region. At the surface, high pressure sets us up for a dry day. Deep Northwest flow should lead to a mostly cloudless sky. Despite full sun, expecting a somewhat cool day as 850 mb temps fall to around - 1C, or about 10-15 degrees below average. Highs will generally be in the lower 60s across the coastal plain with upper 50s possible at higher elevations.

Unfortunately, the key descriptor for dry weather this upcoming week is "brief" as ensemble guidance shows the return of troughing and well below average heights starting midweek. First chance for appreciable rain appears to be later Wednesday afternoon through Thursday as a weakening system tracking across Southern Canada drapes a frontal boundary across New England. Exact details are somewhat TBD at this time, but the possibly exists for secondary low pressure formation as guidance places the region under under the left-exit region of a 90-100kt jet ahead of an approaching trough Wednesday night. Another detail to be resolved will be the potential for the low to quickly become captured by the upper level trough and meander over the area for 1-3 days. This is admittedly a low confidence forecast with many moving parts, but there is a signal for a prolonged period of wet weather from mid to late week.

AVIATION /18Z SATURDAY THROUGH THURSDAY/

Forecaster Confidence Levels...

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

18z TAF Update:

Through 00z Sunday: Moderate confidence.

Initial OVC VFR bases begin to deteriorate to MVFR/pockets IFR ceilings with 4-6 SM -RA, spreading east between 18z western MA and 22z to the east coast. Possible heavier downpours, rumble or two of thunder for PVD and the Cape airports which were TEMPO'd. Rain should be coming to an end at BDL around 00z Sunday but will continue into Saturday night elsewhere. Easterly winds to shift to SE/S and increasing to around 10 kt, except northerly at BDL.

Tonight: Moderate confidence.

Periods of -RA, some heavy at times, to continue thru 01-04z west to east. MVFR/IFR ceilings with MVFR-IFR visby in rain. Ceilings then deteriorate to a widespread IFR-LIFR condition as rain comes to an end; some of the guidance wants to hit fog potential hard as ceilings lower, but am skeptical as forecast soundings hint at enough mixing to support stratus vs fog. S winds under 10 kt, with areas of low level wind shear across the Cape.

Sunday: High confidence in trends, moderate on timing.

IFR-LIFR ceilings persist thru 13-15z, then begin to scatter to VFR levels. There could be periods of MVFR-IFR ceilings around the South Coast, Cape and Islands as shallow moisture looks to be locked in over the waters. Isolated to widely scattered SHRA 18-23z but development is uncertain. SW winds around 8-12 kt, with gusts up to 20 kt.

KBOS Terminal...Moderate confidence in TAF. VFR, generally dry thru at least 20z. Then a period of MVFR SHRA thru 02z, rumble of thunder possible but not as likely as southeast MA. Ceilings then drop to IFR levels once rain ends. Winds start off ENE around 5-8 kt, but winds shift to SE/S and speeds increase to around 10-12 kt this afternoon, then becoming light south as rain ends.

KBDL Terminal...Moderate confidence in TAF. MVFR SHRA thru 00z, rumble of thunder possible. Ceilings then drop to IFR levels once rain ends. Winds start off light northerly, then flip to SE by mid- morning with increase to around 10-12 kt this afternoon, then becoming light south as rain ends.

Outlook /Monday through Thursday/...

Monday: VFR. Chance SHRA.

Monday Night: VFR.

Tuesday: VFR. Breezy.

Tuesday Night: VFR.

Wednesday through Wednesday Night: VFR. Breezy. Chance SHRA.

Thursday: Mainly VFR, with local IFR possible. Breezy. SHRA.

MARINE

Forecaster Confidence Levels...

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

Through Sunday Night...High confidence.

A low level jet will bring some southerly wind gusts to 25 kt over nearshore south coastal waters into early this evening then diminishing tonight as the LLJ moves to the east. SW winds less than 20 kt Sun shifting to N Sun night into Mon morning.

Outlook /Monday through Thursday/...

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

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

Tuesday through Tuesday Night: Winds less than 25 kt. Seas locally approaching 5 ft.

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

Wednesday Night: Winds less than 25 kt. Seas locally approaching 5 ft. Chance of rain showers.

Thursday: Winds less than 25 kt. Areas of seas approaching 5 ft. Rain showers likely.

BOX WATCHES/WARNINGS/ADVISORIES

CT...None. MA...None. RI...None. MARINE...Small Craft Advisory until 2 AM EDT Sunday for ANZ231>235-237- 251. Small Craft Advisory until 8 PM EDT this evening for ANZ236. Small Craft Advisory until 8 AM EDT Sunday for ANZ250-254>256.