Narragansett Bay Marine Forecast
|Today...S Winds 5 To 10 Kt, Increasing To 10 To 15 Kt This Afternoon. Gusts Up To 25 Kt. Waves 2 To 3 Ft. Patchy Fog This Morning. A Slight Chance Of Showers This Morning With Vsby 1 Nm Or Less.|
|Tonight...Sw Winds 5 To 10 Kt. Gusts Up To 20 Kt In The Evening. Waves Around 2 Ft. Patchy Fog. Vsby 1 Nm Or Less.|
|Thu...S Winds 5 To 10 Kt. Gusts Up To 20 Kt In The Afternoon. Waves Around 2 Ft. Patchy Fog. Vsby 1 To 3 Nm.|
|Thu Night...S Winds 5 To 10 Kt. Gusts Up To 20 Kt In The Evening. Waves Around 2 Ft.|
|Fri...Se Winds 5 To 10 Kt. Waves 2 To 4 Ft. Showers Likely With A Chance Of Tstms.|
|Fri Night...Se Winds 5 To 10 Kt. Waves 3 To 5 Ft. Showers Likely With A Chance Of Tstms.|
|Sat...S Winds 5 To 10 Kt With Gusts Up To 20 Kt. Waves 3 To 5 Ft. Showers Likely.|
|Sat Night And Sun...Sw Winds 5 To 10 Kt, Increasing To 10 To 15 Kt With Gusts Up To 20 Kt. Waves 2 To 4 Ft. A Chance Of Showers.|
|Sun Night...Sw Winds 5 To 10 Kt. Waves 2 To 4 Ft. Winds And Waves Higher In And Near Tstms. Seas Are Reported As Significant Wave Height, Which Is The Average Of The Highest Third Of The Waves. Individual Wave Heights May Be More Than Twice The Significant Wave Height.|
Area Forecast Discussion|
National Weather Service Boston/Norton MA
709am EDT Wednesday July 8 2020
Summer-like warmth and humidity takes control of our weather today and persists through the weekend. A few showers/thunderstorms will be possible each day with some heavy downpours, but much of this time will also feature dry weather especially on Thursday when heat and humidity peak. A stronger system late Friday into Saturday brings heavy rain and potential flooding issues.
Near Term - Until 6pm This Evening
Hi-res guidance has underestimated the eastward extent of the low stratus deck this morning, so the main change with this update was to extend cloudcover. We should start to see some clearing in the next hour or two, though. Otherwise the forecast is on track with just a few spotty showers this morning.
Previous Discussion... * A few strong-severe t-storms possible between 4 and 10pm across northern MA and interior SNE with a low risk of flash flooding
Low clouds along with patches of fog were impacting the region early this morning. This a result of deep low level moisture making its return into southern New England. A southwest LLJ has also allowed for some widely scattered showers to develop early this morning, mainly across RI/SE MA. A few showers are possible just about anywhere into mid morning, but dry weather will dominate.
The strong July sun angle coupled with southwest flow should allow for partial sunshine to develop later this morning and especially this afternoon. 925T on the order of 19C to 22C should yield afternoon high temps in the middle to upper 80s across many locations. The bigger issue though will be dewpoints near 70 resulting in quite humid conditions. This will allow for heat indices to reach the lower 90s in some locations.
The main concern will revolve around the potential for a few strong to severe thunderstorms later today into this evening. Diurnal heating coupled with near 70 dewpoints will generate 1-2K J/KG of Cape this afternoon. Meanwhile, shortwave energy across northern New England will drop down in NW flow aloft. This combined with the instability should trigger scattered showers and thunderstorms. High resolution CAMs seem to be struggling with timing issues. While a few isolated t-storms are certainly possible by early afternoon, feel the main risk will be roughly from 4 to 10pm this evening across northern MA and parts of interior southern New England when better forcing arrives. That being said, the limiting factor will be marginal wind fields with effective shear only 25 to 30 knots. The higher risk may be to our north where effective shear will be higher. Nonetheless, instability and shear are certainly enough for a few strong to severe thunderstorms and northwest flow aloft events can sometimes over achieve a bit.
So in a nutshell, a few strong to severe thunderstorms are possible roughly between 4 and 10 PM. Main risk appears to be northern MA and into parts of interior southern New England, where the HREF and other high resolution guidance indicates best 2-5KM updraft helicity values. The main risk will be locally strong to damaging straight line wind gusts with modest low level lapse rates.
Lastly, there is a low probability for localized flash flooding. PWATs (Precipitable Waters) around 1.75 inches will result in torrential rainfall with the strongest t-storms. While they will be moving southeast, certainly could see localized 2"+ rainfall amounts in 45 minutes. If this falls over a vulnerable area localized flash flooding would be possible.
Short Term - 6pm This Evening Through 6pm Thursday
* Hot/Humid on Thu with heat indices in the middle to upper 90s
Tonight... Scattered showers and thunderstorms early this evening should weaken as they attempt to enter RI/SE MA and out run the main dynamics. Otherwise, mainly dry weather is anticipated later tonight but it will remain muggy. Low temps will only drop into the upper 60s to near 70 with the high dewpoints in place. Patchy dense fog may also develop overnight especially in the typically prone locations that receive rainfall today.
Thursday... Upper level ridging overhead will result in hot and humid weather on Thursday. 925T between +22C and +24C should yield high temperatures in the upper 80s to the lower 90s. Dewpoints near 70 will result in afternoon heat index values in the middle to upper 90s in many locations. Mainly dry weather is anticipated on Thursday given upper level ridging overhead and lack of synoptic scale forcing. While an isolated spot shower/t-storm is possible given modest instability, the vast majority of the region will remain dry. Therefore, just carried slight chance pops.
Long Term - Thursday Night Through Tuesday
Highlights... * Temps cool a bit but humidity remains elevated
* Widespread, heavy rain at times around late Friday/Saturday
* Unsettled weather continues Sunday into next week but looking drier for the new workweek
Details... The oppressive humidity is here to stay, with no truly dry air in the forecast through the end of the forecast period. At the same time temperatures remain near or above normal through early next week. Good news, though, as we'll catch a relative break in the heat Friday and Saturday as mid and upper level cloudcover overspreading southern New England Friday will help keep temperatures in the upper 70s to low 80s (5-10 degrees cooler than Thursday).
This relief in the heat, though, comes as a coastal low slides up from the coast of the Carolinas. This system, which is currently nothing more than a disorganized area of showers over the southeast, has a 40% chance of tropical cyclone formation from the National Hurricane Center. Regardless of classification, though, this system will almost certainly bring a period of very heavy rain and some thunderstorms to southern New England. The extremely moist slug of air that moves overhead through the day on Friday into Saturday morning contains PWATs (Precipitable Waters) potentially surpassing 2.5". If realized this would near or surpass the July record for Chatham observations. Currently we're not certain when the heaviest period of rain and potential flooding issues will be. 00Z EC guidance has slowed the system down significantly bringing the heavier rain in Saturday while the GFS is much quicker, more of a Friday/Friday night thing. At this point what we know is that with this kind of moisture plume someone is going to get a lot of rain. Where and when that is will have to be settled in the coming days.
This, of course, lowers confidence in the Sunday-Tuesday forecast period quite a bit, since it will depend on how the system plays out. Unsettled weather remains likely, though, as another lobe of energy rotates around the trough with a surface disturbance on Sunday.
Today...High confidence. SW winds of 10 to 15 knots may gust to around 25 knots this afternoon along the nearshore southeast New England coastal waters. This a result of strong heating/mixing over the land. Opted to issue a SCA (Small Craft Advisory) for this afternoon for gusty nearshore winds/choppy seas. Also, a few strong t-storms are possible late this afternoon/early evening across our northeast MA adjacent waters.
Tonight...High confidence. Winds/seas should remain below SCA (Small Craft Advisory) thresholds. S-SW winds of 10 to 15 knots and some fog may develop across the southern waters with high dewpoint air advecting over the ocean.
Thursday...High confidence. High pressure east of our waters will generate southerly flow of 5 to 15 knots. Gradient will be weak enough to keeps winds/seas below SCA (Small Craft Advisory) thresholds.
Outlook /Thursday Night through Sunday/... Thursday Night: Winds less than 25 kt. Slight chance of rain showers, isolated thunderstorms.
Friday: Winds less than 25 kt. Areas of seas approaching 5 ft. Chance of rain showers, slight chance of thunderstorms.
Friday Night: Winds less than 25 kt. Areas of rough seas. Rain showers likely, slight chance of thunderstorms.
Saturday: Winds less than 25 kt. Areas of rough seas. Chance of rain showers, slight chance of thunderstorms.
Saturday Night: Winds less than 25 kt. Local rough seas. Slight chance of rain showers, isolated thunderstorms.
Sunday: Winds less than 25 kt. Seas up to 5 ft. Slight chance of rain showers, isolated thunderstorms.
NOAA Boston MA Office: Watches - Warnings - Advisories
CT...None. MA...None. RI...None.
Small Craft Advisory from noon today to 6pm EDT this evening for ANZ231>234.