textproduct: Boston / Norton

WHAT HAS CHANGED

Rainfall coverage and amounts have trended lower for late tonight into Monday.

KEY MESSAGES

- Seasonably warm with increasing humidity today. Sunshine this morning gives way to increasing clouds this afternoon ahead of an approaching system.

- Showers develop late tonight into Monday, though coverage and rainfall amounts have trended lower from previous forecasts.

- Warm temperatures and moderate humidity return through the middle of next week, followed by increasing shower chances late in the week.

DISCUSSION

KEY MESSAGE 1...Seasonably warm with increasing humidity today. Sunshine this morning gives way to increasing clouds this afternoon ahead of an approaching system.

High pressure begins to shift offshore today as a developing low pressure system moves across Quebec and another area of low pressure develops across the Mid-Atlantic. Warm air advection aloft will increase cloud cover through this afternoon, while temperatures remain seasonable, ranging from the upper 70s to lower 80s. Humidity gradually increases today, with dew points returning to the lower 60s and becoming more noticeable compared to Saturday. Most, if not all, of the daylight hours remain dry, with showers arriving late this evening into the overnight hours.

KEY MESSAGE 2...Showers develop late tonight into Monday, though coverage and rainfall amounts have trended lower from previous forecasts.

Over the past several nights, we have been monitoring the potential for a more widespread rainfall event late tonight into Monday. However, the trend continues toward less rainfall, particularly across southern New England. The 00Z guidance generally depicts the strongest forcing associated with a developing weak surface low across the Mid-Atlantic and another area of low pressure near northern New England and southern Quebec. With southern New England positioned between these areas of stronger forcing, it may be difficult to fully capitalize on the higher PWATs of 1.8 to 2.0 inches across the region.

Nevertheless, periodic showers will develop late tonight and continue into Monday. There remains high confidence that most of today remains dry. The majority of the 00Z CAMs keep southern New England dry through roughly 8 PM, though a few solutions bring showers into western portions of the region earlier. The most favorable period for numerous light to moderate showers appears to be after sunset through roughly 4 to 6 AM Monday.

The threat for heavy rainfall has also decreased. While an isolated heavier downpour cannot be ruled out, widespread heavy rainfall is no longer expected. Forecast rainfall totals have generally trended to less than a half inch across Connecticut, Rhode Island, and southeast Massachusetts, with amounts closer to a quarter inch or less possible farther north across Massachusetts. Given this trend, the Marginal Risk for Excessive Rainfall no longer includes southern New England.

Showers should become more scattered during the day Monday, potentially resulting in a drier, though cloudy and humid, afternoon. A cold front then pushes through the region late Monday and may initiate an isolated thunderstorm. However, extensive cloud cover and limited instability should limit thunderstorm development and intensity. Given the expected cloud cover, showers, and easterly flow, Monday will be cooler, with highs ranging from the lower to mid-70s across eastern Massachusetts and Rhode Island to around 80 degrees towards the Connecticut River Valley. Dew points remain elevated, resulting in rather sticky conditions.

KEY MESSAGE 3...Seasonably warm temperatures and moderate humidity return through the middle of next week, followed by increasing shower chances late in the week.

Drier air advects in from the northwest late Monday night into Tuesday, leading to another dry stretch Tuesday through Wednesday, and possibly into the first half of Thursday, as surface high pressure builds back into the region. Temperatures remain seasonable, with highs in the lower to middle 80s and tolerable humidity as dew points remain in the lower to middle 60s. Ensemble guidance continues to signal a transition toward more unsettled weather late in the week, with shower chances returning and temperatures potentially trending slightly below normal.

AVIATION /12Z SUNDAY THROUGH THURSDAY/

Forecaster Confidence Levels...

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

12z TAF Update...

Today...High confidence.

VFR. Light and variable winds early this morning become S at 6 to 12 knots by mid afternoon...except for sea breezes along portions of the immediate eastern MA coast.

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

Mainly VFR conditions persist this evening. MVFR with localized IFR ceilings over spread the interior between 04z and 08z and reach the I-95 corridor by 12z. Scattered showers may also accompany the low clouds. S winds generally 6-12 knots.

Monday...Moderate confidence.

Low end MVFR to IFR ceilings should dominate Monday with perhaps even some localized LIFR conditions. Scattered showers especially in the morning but not a washout of a day. An isolated t-storm or two is also possible...but nothing widespread is expected. SE winds 8-16 knots may shift to light N-NE across the interior as the day wears along.

KBOS Terminal...High confidence in TAF. Sea breeze develops by 13z/14z and should kick out as winds shift to the S in the 20z-22z time frame. VFR persists today and most of tonight...but MVFR with localized IFR conditions should arrive toward the Monday morning push.

KBDL Terminal...High confidence in TAF.

Outlook /Monday Night through Thursday/...

Monday Night: Mainly VFR, with local IFR possible. Slight chance SHRA.

Tuesday: Mainly VFR, with local MVFR possible.

Tuesday Night through Wednesday: VFR.

Wednesday Night: VFR. Slight chance SHRA.

Thursday: VFR. 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 Monday...High confidence.

High pressure shifts east of the coastal waters today ahead of a low pressure system arriving tonight into Monday. Southerly winds today increase to around 20 kt tonight, then shift to southeast Monday morning and east by late afternoon, with gusts of 20 to 25 kt. Seas remain relatively calm today under high pressure but build to 5 to 8 ft by early Monday morning as the low approaches. Small Craft Advisories remain in effect through Monday night.

The risk for rip currents increases Monday along south-facing beaches, then Tuesday along east-facing beaches, as rough seas and long-period swells settle in behind the departing system.

Outlook /Monday Night through Thursday/...

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

Tuesday: Winds less than 25 kt. Seas up to 5 ft.

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

Wednesday through Wednesday Night: Winds less than 25 kt.

Thursday: Winds less than 25 kt. Chance of rain showers, slight chance of thunderstorms.

BOX WATCHES/WARNINGS/ADVISORIES

CT...None. MA...None. RI...None. MARINE...Small Craft Advisory from 8 AM to 11 PM EDT Monday for ANZ231- 232-250-251-254. Small Craft Advisory from 2 AM Monday to 2 AM EDT Tuesday for ANZ233>235-237-255-256.