textproduct: Boston / Norton

WHAT HAS CHANGED

The chance for rain for the Cape and Islands on Sunday has decreased since the last forecast update. Potential exists for heavy rainfall and perhaps even the risk of localized flash flooding beginning late Monday into the middle of next week.

KEY MESSAGES

- Pleasant and dry conditions expected through Monday except for far eastern MA, the Cape and Islands. Elevated rip currents possible for east facing beaches Sunday.

- Periodic chances for showers & t-storms late Mon-Wed perhaps lingering into Thu...heavy rain/localized flash flood risk.

DISCUSSION

KEY MESSAGE 1...Pleasant and dry conditions expected through Monday except for far eastern MA, the Cape and Islands. Elevated rip currents possible for east facing beaches Sunday.

Another cool night ahead as clear skies support radiational cooling. Thus, opted to blend in the NBM 10th percentile and MOS guidance for lows tonight. Lows range from the upper 40s across elevated portions of the interior, to near 60 in urban and coastal areas. High pressure moves offshore Saturday switching winds to the E with low relative humidity values. Expecting another dry and pleasant day with Highs across the interior likely range in the low to mid 80s while the onshore flow will keep temperatures slightly cool (mid- upper 70s) along the immediate eastern coastal areas.

By Sunday, heights fall as a weak trough approaches from the west. Most guidance shows a weak surface low tracking along a boundary well to the S of the region Sunday afternoon. Compared to the last forecast update, the NAM has backed off on the more robust POPs associated with a push of moisture across the Cape and E MA Sunday afternoon. Model soundings still indicate a shallow layer of moisture in the lower levels supported by the easterly surface flow with drier mid-level air. This could potentially point to a layer of low clouds and even some patchy drizzle in areas of eastern MA developing through the day on Sunday.

Outside of limited precip chances, there is higher confidence in a period of elevated rip current risk developing on Sunday with increasing easterly flow. Greatest threat for rough surf and rip currents will likely be across much of the east-facing portion of the Cape, MA, and the Islands.

KEY MESSAGE 2...Periodic chances for showers & t-storms late Mon-Wed perhaps lingering into Thu...heavy rain/localized flash flood risk.

The next shortwave trough approaches the region from the west in the late Mon-Wed timeframe. This will allow southerly flow to bring typical summerlike humidity back to southern New England. This will also bring a return to period chances of showers and thunderstorms to the region in the late Mon-Wed time frame...perhaps lingering into Thu if the shortwave is slower to depart. While there is quite a bit of spread in regards to timing and track...the ECMWF/GFS/UKMET indicating the potential for an approximately 995 mb surface low. That is pretty anomalous for July standards and the EPS ensemble situational awareness table indicates its strength outside the models climatology field. Pwats are also indicated to increase to 1+ standard deviation above normal on the order of 1.5 to 2". When you combined that with the potential for a 995 low pressure system...the potential is certainly there for a heavy rain/localized flash flood risk. And that may not even end up in our region depending on the track...But it certainly bears watching for the mid-Atlantic and northeast states.

We also should mention that the late Mon through Wed timeframe will not be a complete washout. Timing obviously uncertain at this point...But afternoons/evenings are favored given diurnal instability to work with as well.

Temperatures fairly seasonable for this time of year. Lows may be a bit above normal with highs a touch below normal given the increasing low level moisture and precipitation chances for the middle of next week.

AVIATION /00Z SATURDAY THROUGH WEDNESDAY/

Forecaster Confidence Levels...

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

00z TAF Update...

Tonight through Saturday...High confidence.

VFR. Light/calm winds tonight become ESE 5-12 knots on Sat.

Saturday night...High confidence.

Mainly VFR but some MVFR cigs/vsbys may impact parts of RI/SE MA toward daybreak Sunday. Greatest risk across parts of the Cape and Nantucket. Calm/light NE winds.

KBOS Terminal...High confidence in TAF. Sea breeze will kick back out to the light S/SSW 00-02z. Easterly winds between 6 and 12 knots redevelop by 13z/14z Sat.

KBDL Terminal...High confidence in TAF.

Outlook /Monday through Wednesday/...

Monday: VFR. Breezy.

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

Tuesday through Tuesday Night: Mainly VFR, with local MVFR possible. Breezy. SHRA likely, slight chance TSRA.

Wednesday: Mainly VFR, with local IFR possible. 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.

Through Saturday...High confidence.

Light winds and seas 1-4 ft Saturday. Increasing winds and seas Saturday night and Sunday with a low chance of SCA conditions across the eastern outer waters Sunday.

Moderate risk for rip currents over the weekend.

Outlook /Monday through Wednesday/...

Monday through Monday Night: Winds less than 25 kt.

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

Tuesday Night: Low risk for Small Craft Advisory winds with gusts up to 25 kt. Areas of seas approaching 5 ft. Rain showers likely, slight chance of thunderstorms.

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

BOX WATCHES/WARNINGS/ADVISORIES

CT...None. MA...None. RI...None. MARINE...None.