textproduct: Boston / Norton

WHAT HAS CHANGED

Introduced a slight chance of showers/t-storms this afternoon though most will stay dry.

KEY MESSAGES

- Mainly dry conditions through Sunday with the exception of an isolated shower or storm today. Unsettled weather arrives Sunday night.

- Cold front brings showers and thunderstorms with heavy rain Monday. Street flooding is possible. A brief break on Tuesday before turning active and unsettled again Wednesday.

- Still unsettled late in the week with periods of showers and storms.

DISCUSSION

KEY MESSAGE 1...Mainly dry conditions through Sunday with the exception of an isolated shower or storm today. Unsettled weather arrives Sunday night.

Day Cloud Phase satellite imagery shows development of shallow cumulus this afternoon with a few isolated showers driven by the diurnal heating. Expect any showers to diminish around sunset. Quiet weather overnight with weak high pressure overhead. Moisture advects in within the southerly flow tonight similar to last night. Model soundings show a signal for moisture in the low levels under an inversion with the strongest signal for the south coast and Cape/Islands. This will increase the potential for stratus and perhaps some fog. HREF has shown potential for the stratus to push northward after midnight although there is lower confidence how far north it extends. Patch fog is also possible in the CT Valley overnight.

Similar pattern for Sunday with the mid-level ridge and surface high offshore as a shortwave trough to the west of the region. Weak high pressure stays put over the area keeping conditions mainly dry. Can't rule out an isolated shower or storm in the afternoon, mainly in western MA. High temperatures range in the low to mid 80s for most with southerly winds. Skies will be mainly cloudy.

A low pressure system moves through the Ohio Valley as the mid-level trough pushes further east across western southern New England Sunday night. An accompanying LLJ pushes northward into the region. This will help advect in a plume of above normal moisture into the region ahead of the approaching trough. A mix of isentropic lift and jet dynamics will provide synoptic support for rain developing Sunday night. There is a signal for marginal instability which may support a few embedded moderate showers or brief downpours in localized areas with the higher risk for western MA/CT. The higher risk for the heavier rainfall will be Monday (see next key message).

KEY MESSAGE 2...Cold front brings showers and thunderstorms with heavy rain Monday. Street flooding is possible. A brief break on Tuesday before turning active and unsettled again Wednesday.

Still monitoring the potential for heavy rain into the early and possibly into the middle portion of next week. This rain and embedded thunderstorms looks to be forced by an injection of subtropical moisture on SWly low-level jet maxima of upwards of 45 kt Monday, ahead of an anomalously strong mid-level low stalling over the lower OH Valley Monday and remaining more-or-less in place into early next week. This strong meridional flow of air brings precipitable water values to 2-2.25", corresponding to anomalies which are 2 to 2.5 standard deviations above normal! A slow-moving south-to-north surface cold frontal boundary will interact with this subtropical moisture plume and favor rounds of showers and thunderstorms. Monday looks to be a washout with moderate (40-60%) model-ensemble probabilities of 24-hr rainfall totals at/over 1", with very low (<10%) chances of 24-hr rain totals at/over over 2". Will have to watch for possible street and poor drainage flooding in heavy downpours, as well as rises on rivers, creeks and streams as USGS streamflow data over the last 7 days (e.g. coming out of the very wet stretch we've had of late) show most rivers running above- to well-above normal. WPC's excessive rainfall outlook shows as much as a Level 2 of 4 (Slight) Risk for Excessive Rainfall for Monday. While not a zero-risk, severe weather potential looks to be a more limited, secondary risk in the form of water-loaded downdrafts given the juiced airmass.

It looks like on Tuesday that the northern end of the frontal boundary shifts the axis of highest PWAT values offshore, and allows for weak subsidence to return - put another way, it looks like Tuesday is a general minimum in rain chances, but it's still unsettled as the mid-level low to our south could still allow for northward-moving showers and storms in the northern Mid-Atlantic to clip portions of western Southern New England. A better chance for another round of showers and storms mainly into western/central Southern New England returns by Wednesday. All told a pretty muggy and unsettled weather pattern with several opportunities for showers and storms.

Temperatures should run above normal, especially with regard to the low temps in a humid, muggy airmass.

KEY MESSAGE 3...Still unsettled late in the week with periods of showers and storms.

The mid-level low over the Ohio Valley weakens and washes out into Thursday. Unfortunately an active northern stream portion of the jet continues, so it still ends up looking unsettled with showers/storms which are too difficult to provide more distinct periods when they become more likely. With stronger mid-level westerly flow associated with the stronger northern stream, there could be the potential for some storms to become strong but whether or not this comes to fruition remains to be seen. Temperatures continue to run above normal as we move into late week.

AVIATION /20Z SATURDAY THROUGH THURSDAY/

Forecaster Confidence Levels...

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

18Z TAF Update:

This Afternoon: High confidence.

VFR. SW winds around 5-10 kt and sea breeze around 10 kts at BOS. Low chance for an isolated shower or weak storm through 00Z. Sea breeze kicks out of BOS 23-00Z returning to SSW flow.

Tonight: Moderate confidence.

VFR to start. Main challenge for tonight will be ceilings. There is potential for re-development of a stratus deck across the Cape/Islands potentially extending to the south coast closer to 03Z. This could bring a range of LIFR to IFR ceilings with FG possible for the Cape/Islands. There is considerable uncertainty in the northward extent of this stratus deck. There is a low end potential for this deck to extend northward close to BOS after 09Z. Low ceilings appear in TAFs with higher confidence for the stratus (Cape/Islands) while the other terminals have either FEW/SCT decks hinting at the lower cigs for now. Will have to re-assess for the 00Z TAF.

KBOS Terminal...Moderate confidence in TAF. Sea breeze this afternoon around 10 kts. Kicks out 23-00Z to SSW flow less than 10 kts for remainder of night. SCT 4000-5000ft decks this afternoon with an isolated shower possible. If a shower were to develop it would more likely be west of the terminal.

There is low probability for a stratus deck to extend north enough to bring lower MVFR/IFR ceilings to or near BOS 09-13Z. Confidence too low to include in TAF. Messaged as a FEW deck at this time. Will need to monitor for a stronger signal for 00Z TAF.

KBDL Terminal...Moderate confidence in TAF.

Potential for MVFR/IFR ceilings after 06Z through around 12Z with BR. Confidence lower in the extent/coverage of a low stratus deck.

Outlook /Sunday through Thursday/...

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

Sunday Night: MVFR/IFR conditions possible. Breezy. SHRA likely, slight chance TSRA, patchy BR.

Monday: MVFR/IFR conditions possible. Windy with local gusts up to 30 kt. SHRA, chance TSRA.

Monday Night: VFR. Breezy. Chance SHRA, slight chance TSRA.

Tuesday through Tuesday Night: VFR. Slight chance SHRA.

Wednesday: VFR. Chance SHRA, slight chance TSRA.

Wednesday Night: VFR. Slight chance SHRA, slight chance TSRA.

Thursday: VFR. Breezy. Chance SHRA, slight chance TSRA.

MARINE

Forecaster Confidence Levels...

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

Winds and seas below small craft advisory levels through Sunday. Winds and seas increase late Sunday night into early Monday morning across the southern and eastern waters. Seas 5-8 ft for the outer coastal waters, especially south. 5-7 ft seas stretch into portions of the inner souther waters. SW winds 15-20 kts with gusts around 30 kts. SCAs have been issued for the southern and eastern waters included the Buzzards Bay, Vineyard Sound, and Nantucket Sound starting early Monday morning. Elevated seas will keep SCAs around through Tuesday for the southern waters.

Outlook /Sunday through Thursday/...

Sunday: Winds less than 25 kt.

Sunday Night: Low risk for Small Craft Advisory winds with gusts up to 25 kt. Local rough seas. Chance of rain showers, slight chance of thunderstorms, patchy fog.

Monday: Moderate risk for Small Craft Advisory winds with gusts up to 30 kt. Rough seas up to 9 ft. Rain showers likely, slight chance of thunderstorms.

Monday Night: Low risk for Small Craft Advisory winds with gusts up to 25 kt. Rough seas up to 8 ft. Rain showers likely, slight chance of thunderstorms.

Tuesday: Winds less than 25 kt. Local rough seas. Slight chance of rain showers.

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

Wednesday: Winds less than 25 kt. Seas locally approaching 5 ft.

Wednesday Night: Winds less than 25 kt.

Thursday: Winds less than 25 kt. Seas locally approaching 5 ft.

BOX WATCHES/WARNINGS/ADVISORIES

CT...None. MA...None. RI...None. MARINE...Small Craft Advisory from 8 AM Monday to 2 AM EDT Tuesday for ANZ232>234. Small Craft Advisory from 8 AM Monday to 2 PM EDT Tuesday for ANZ235-237-254. Small Craft Advisory from 5 AM Monday to 8 PM EDT Tuesday for ANZ255-256.