Area Forecast Discussion

WHAT HAS CHANGED

Flood Watches have been issued for western portions of Southern New England for Tuesday afternoon into part of Wednesday evening, with an increase in rainfall totals in interior Southern New England.

KEY MESSAGES

- Flood Watch issued for interior western New England, with heavy rainfall leading to potential street and flash flooding Tuesday afternoon through Wednesday evening. Heaviest rain in eastern New England is expected on Wednesday, with street flooding possible but less favored than further inland.

- One final push of rain Thursday with isolated flash flooding possible. Slow-moving system may keep periodic showers around into Friday.

- Potentially unsettled conditions linger into the weekend, although confidence remains lower.

DISCUSSION

KEY MESSAGE 1...Flood Watch issued for interior western New England, with heavy rainfall leading to potential street and flash flooding Tuesday afternoon through Wednesday evening. Heaviest rain in eastern New England is expected on Wednesday, with street flooding possible but less favored than further inland.

An anomalously strong upper-level cyclone for late July is expected to close off into a slow-moving upper low over central/south-central New York Tuesday into early Wed, then is forecast to slowly meander ESE through the lower Hudson Valley Wed into Wed night. This system will draw in rich subtropical moisture (e.g. PWAT values around 1.5- 1.7") along/ahead of a frontal boundary, a boundary will tend to slowly lift or pivot E/ENE Tue night/Wed. Additional enhancements to rainfall in an easterly upslope flow in interior Southern New England are also anticipated. Put all that together and we are looking at a soggy, dreary midweek with rounds of rainfall, some of which could turn heavy at times. Because of the slow eastward progression and timing, we'll break this down regionally below.

For interior portions of Southern New England: moderate to heavy rain with some embedded rumbles of thunder into expected to develop Tuesday afternoon and continue into Wednesday across western New England, especially into the CT Valley and northern CT. Heaviest rain is expected to fall late Tuesday night/overnight and continue into Wednesday. With a longer duration of this type of rain, the risk for street flooding as well as rises on creeks and streams looks to be the greatest in this general area. We've issued a Flood Watch to best message this potential for the CT Valley in MA into Hartford and Tolland Counties in CT, and this runs from Tuesday afternoon through early Wednesday evening. Although it is still uncertain what the maximum rainfall will be, this general area seems more primed to be at least in the vicinity of seeing that maximum; we'll also need to monitor where a pivot point ("deformation zone", in the weather vernacular) develops as the frontal boundary lifts northeastward as an area potentially favorable for heavier rain accumulations. This is also reflected in the coarser-resolution global ensemble QPF probs from the ECMWF, Canadian GEM and the GEFS, which show high probs of 2+ inches of rainfall, with low (<20%) probs of 4 or more inches of rain. We're also not yet really into the higher- resolution guidance yet, so we still have to digest that data as we move through the next 24-36 hours, but the synoptic-scale picture paints the potential for locally significant rainfall accumulations in western New England. This is also reflected in the ECMWF Extreme Forecast Index (EFI) and Shift of Tails guidance and justifies the Watch. Although a slight risk for excessive rainfall was collaborated with WPC, a zone of moderate risk has now been introduced for points just to our south and west into the lower Hudson Valley in New York, which speaks to the risk for flooding. Additional, but lighter rain is expected even into Thursday. I've shown rain totals through 7 AM Thursday which range from 2 to 3 inches, with locally higher totals nearing 4 inches along the east slopes of the Berkshires.

For eastern portions of Southern New England, roughly Worcester and Windham Counties eastward into RI and eastern MA: outside of isolated to widely scattered rain showers associated with a northward-moving warm front Tuesday morning, much of this region ends up being mostly dry on Tuesday with a relative minimum in PoPs (chance or less). The frontal boundary will be gradually moving in either late Tuesday night (after midnight), but more likely by early Wednesday. It looks like Wednesday is the period of moderate to heavier rainfall for this portion of Southern New England, only slowly decreasing in intensity as we move into Wednesday afternoon to early evening. There could be some risk for street flooding here too, especially in the typically more vulnerable urban areas (not limited to Boston, Providence, Worcester, Fall River and New Bedford), but felt the risk was less than into western portions of Southern New England who are poised to see a longer duration of moderate to heavy rain. Later shift(s) could consider expanding the Flood Watch into some of these locations if rain totals continue to increase, but it seems more conditional/marginal. Rain totals through 7 AM Thursday range from 1.5 to 2.5 inches, with the highest rainfall totals in hillier Worcester County as well as into the Merrimack Valley.

In terms of river response...obviously one of the bigger challenges when it comes to projecting impacts is that we're still dealing with a stubborn drought where streamflows in several locations are still below-normal. Right now there are no rivers forecast to rise into minor flood stage per NERFC forecasts, although rises can be expected over the next few days.

The persistent rain and cloud cover with easterly onshore flow will keep diurnal temperature ranges narrow, with milder than normal low temperatures but cooler than normal high temperatures.

KEY MESSAGE 2...One final push of rain Thursday with isolated flash flooding possible. Slow-moving system may keep periodic showers around into Friday.

Rain continues into Thursday, with the potential for one final surge of heavier rainfall during the second half of the day. The unsettled weather is driven by a slow-moving area of low pressure that becomes vertically stacked through the mid levels, with little eastward progression due to blocking over the northwest Atlantic. While Thursday lacks the robust upper-level jet dynamics of the previous day, PWATs increase to around 1.7 inches (roughly one standard deviation above climatology), and strong mid-level vorticity will support periods of widespread rainfall. Rainfall rates should generally be lower than Wednesday's, with precipitation becoming more stratiform than convective. Even so, antecedent wet soil conditions warrant continued monitoring for isolated flash flooding, particularly in urban and poor-drainage locations where heavier rain bands develop. The system is slow to depart, resulting in periodic showers lingering into Friday. Confidence in the exact timing remains limited, but Friday does not currently appear to be a washout.

Both days will feature below-normal temperatures due to abundant cloud cover and persistent east to northeast winds, with little diurnal temperature range. Highs on Thursday are expected to reach the lower to middle 70s, warming modestly into the middle and upper 70s on Friday.

KEY MESSAGE 3...Potentially unsettled conditions linger into the weekend, although confidence remains lower.

Forecast confidence decreases heading into the weekend, with conditions largely dependent on how quickly the downstream ridge breaks down and allows the slow-moving low pressure system to finally exit the region. Model consensus remains poor at this lead time, resulting in lower confidence in the forecast. Given the 5 to 6 day forecast horizon, additional changes are likely.

That said, periodic showers remain possible through the weekend. With a continued trend toward greater cloud cover, significant heat appears unlikely, and temperatures should remain near seasonal normals.

AVIATION /00Z TUESDAY THROUGH SATURDAY/

Forecaster Confidence Levels...

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

00z TAF Update:

Rest of Today...High confidence.

Isolated SHRA/TSRA towards the western terminals persist early this evening, fading post sunset. Outside of these, VFR conditions should prevail across the region for the rest of today. SE winds 5-10 kt through this afternoon and evening.

Tonight...Moderate confidence.

VFR to start, then the general trend is deteriorating ceilings toward low end VFR/MVFR, with localized IFR/LIFR ceilings possible. Confidence regarding lower ceilings is still somewhat more moderate, but generally have them rolling in around 08z in TAFs. Widely scattered showers possible overnight, though not enough confidence to include in TAFs at this time. Potential for widespread precipitation increases for Tuesday. Light S winds.

Tuesday...Moderate confidence.

Chances for widespread rainfall ramp up Tuesday with lower end VFR through much of the day, then deteriorating to MVFR/IFR towards the afternoon. Chances for embedded TSRA are low though not zero at this time. Breezier SE winds 10-13 kt with gusts to 20 kt possible in the afternoon.

Tuesday Night...Moderate confidence.

IFR/MVFR. The most significant rainfall is expected to overspread the region heading into Tuesday night-Wednesday morning. IFR to LIFR ceilings across much of the interior with low end MVFR eastward. Persistent SE winds 6-10 kt with gusts to 20 kt possible towards the interior.

KBOS Terminal...High confidence.

KBDL Terminal...High confidence.

Outlook /Tuesday Night through Saturday/...

Wednesday: Mainly MVFR, with areas IFR possible. Breezy. SHRA, isolated TSRA.

Wednesday Night: Mainly VFR, with local IFR possible. Breezy. SHRA.

Thursday: Mainly VFR, with areas IFR possible. Breezy. SHRA, chance TSRA.

Thursday Night: Mainly VFR, with local IFR possible. Chance SHRA, slight chance TSRA.

Friday: VFR. Chance SHRA, slight chance TSRA.

Friday Night: VFR. Slight chance SHRA.

Saturday: Mainly VFR, with local MVFR possible. Slight chance SHRA, slight chance TSRA.

MARINE

Forecaster Confidence Levels...

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

Through Tuesday Night: High confidence.

Winds and seas to remain below small craft advisory levels through the daytime hours on Tuesday. Southerly winds around 10 kt will be increasing to around 15-20 kt Tuesday, with seas building to 3-4 ft. A few showers are possible on southern waters during the Tuesday daytime hours.

Small craft advisories have been issued Tuesday evening into Wednesday as south to southeast winds increase to around 25 to 30 kt and seas in the 4 to 7 ft range, highest offshore. Showers and embedded thunderstorms can be expected, mainly into Wednesday.

Outlook /Tuesday Night through Saturday/...

Wednesday: Moderate risk for Small Craft Advisory winds with gusts up to 25 kt. Seas up to 5 ft. Rain showers, isolated thunderstorms.

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

Thursday Night: Winds less than 25 kt. Seas up to 5 ft. Chance of rain showers, slight chance of thunderstorms.

Friday: Winds less than 25 kt. Seas up to 5 ft. Slight chance of rain showers, slight chance of thunderstorms.

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

Saturday: Winds less than 25 kt. Areas of seas approaching 5 ft.

BOX WATCHES / WARNINGS / ADVISORIES

CT...Flood Watch from Tuesday afternoon through Wednesday evening for CTZ002-003. MA...Flood Watch from Tuesday afternoon through Wednesday evening for MAZ002-003-008>011. RI...None. MARINE...Small Craft Advisory from 2 AM to 8 PM EDT Wednesday for ANZ230-231-250-251. Small Craft Advisory from 8 PM Tuesday to 8 PM EDT Wednesday for ANZ232>237-254>256.