Nantucket Sound Marine Forecast
|Overnight...Sw Winds 10 To 15 Kt. Gusts Up To 20 Kt. Seas 2 To 3 Ft. Patchy Fog. Vsby 1 To 3 Nm.|
|Sun...Sw Winds 10 To 15 Kt. Gusts Up To 20 Kt In The Afternoon. Seas 2 To 3 Ft. Patchy Fog. Vsby 1 To 3 Nm.|
|Sun Night...W Winds 10 To 15 Kt With Gusts Up To 20 Kt, Becoming Nw 5 To 10 Kt After Midnight. Seas 2 To 3 Ft.|
|Mon...N Winds Around 5 Kt, Becoming E In The Afternoon. Seas Around 2 Ft. Showers Likely With A Chance Of Tstms.|
|Mon Night...S Winds 5 To 10 Kt. Gusts Up To 20 Kt After Midnight. Seas 2 To 3 Ft. Showers Likely With A Chance Of Tstms. Vsby 1 To 3 Nm.|
|Tue...W Winds 5 To 10 Kt With Gusts Up To 25 Kt. Seas 2 To 4 Ft. Showers With Tstms Likely. Vsby 1 To 3 Nm.|
|Tue Night...Nw Winds 5 To 10 Kt With Gusts Up To 20 Kt, Becoming N After Midnight. Seas 2 To 3 Ft. A Chance Of Showers. Vsby 1 To 3 Nm.|
|Wed...S Winds Around 5 Kt. Seas 2 To 3 Ft.|
|Wed Night...Sw Winds Around 5 Kt. Seas 2 To 3 Ft.|
|Thu...N Winds Around 5 Kt. Seas 2 To 3 Ft.|
|Thu Night...Sw Winds Around 5 Kt. Seas 1 Foot Or Less. Winds And Seas 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
740pm EDT Sat July 20 2019
Synopsis: Extreme heat and humidity continues into Sunday with dangerous heat indices between 100 and 110 degrees during the afternoon and early evening. The heat and humidity may trigger a few isolated showers/thunderstorms from late Saturday through Sunday. Otherwise most locations experience dry weather. Low pressure then tracks along a frontal boundary into southern New England Monday and Tuesday, and will be accompanied by showers and thunderstorms with tropical downpours and strong winds possible. Eventually much cooler and less humid air moves into the region late Tuesday into Wed. Mainly dry Wednesday through Friday with seasonable temperatures and comfortable humidity levels.
Near Term - Until 6am Sunday Morning
730pm update... Focus this evening is on continued warm temperatures, not expected to dip below the upper 70s/low 80s tonight. Watching some thunderstorms over eastern NY as they approach. Thinking is that they should begin to weaken as they approach and we losing heating with sunset, but the environment is quite unstable over SNE at the moment so something to watch as they approach. Hi- res guidance would have the more extreme CAPE values seen on mesoanalysis (4500 J/kg over SE Mass, more like 3000 J/kg elsewhere) decrease sharply with sunset, weakening these storms as CIN increases. However, cloud tops continue to cool and we can't rule out one of these storms remaining strong into our western zones.
Previous Discussion... *** Excessive Heat Warning Continues into Sunday *** *** Near Record Warmth Tonight ***
Fairly capped environment with 589 dam heigheights across the region along with +12C temps at 700 mb. However a few isolated showers/T-storms have fired in CT along the seabreeze boundary and this should be the focus for any new convection moving forward into early this evening. Given the higher dew pt air here (mid to upper 70s) highest 0-3km CAPEs are across CT/RI and southeast MA. However with shear values less than 30 kt, storms will pulse down as quickly as activity pulses upward. Also with mid level lapse rates only 6C/KM, this will temper updraft strength.
Otherwise remainder of the region remains dry this evening and overnight.
Given dew pts are in the low to mid 70s with a few upper 70s, only expecting temps to cool off into the mid to upper 70s overnight. Even warmer in the urban centers of Boston, Providence and Hartford where temps may only fall into the low 80s, which is near record warmth for overnight mins
Short Term - 6am Sunday Morning Through Sunday Night
415pm update ... Sunday... *** Excessive Heat Warning Continues Sunday *** *** Near Record Highs Possible ***
Still capped environment Sunday with only subtle height falls and warm temps persist aloft. Not much cyclonic curvature to the mid level flow, thus little if any synoptic scale lift. However deep layer shear increases a bit from 15 kt today to about 25 kt Sunday afternoon, still a high CAPE low shear environment. Although with blyr winds becoming west and increasing to 15-20 kt in the afternoon, this will limit surface convergence and also dry out the boundary layer with dew pts falling into the mid and upper 60s. So a notch less oppressive tomorrow but this will also lower CAPE values down to 1000-2000j/kg, plenty of instability to support deep convection but definitely less instability than today. Conversely mid level lapse rates more impressive tomorrow as EML currently over the Great Lakes and OH Valley advects across SNE. Thus mixed signals for convection tomorrow but more unfavorable than favorable parameters. As a result most of the CAMs models suppress convection Sunday.
Otherwise another very hot day with temps perhaps a few degs higher than today given west winds will increase tomorrow to 15-20 mph, providing more downslope adiabatic warming. So while temps may be a few degs high tomorrow, dew pts will likely fall into the upper 60s and lower 70s, so not quite as oppressive humidity. Nevertheless the combination of highs tomorrow 95-100 coupled with dew pts in the upper 60s to lower 70s yielding heat indices of 100-110 will continue with current headlines.
Sunday night... Still very warm but not as oppressive as tonight with slightly drier airmass over the region. Any isolated late day storms should dissipate by late evening. Otherwise dry weather prevails
Long Term - Monday Through Saturday
Highlights... * Showers/thunderstorms likely Monday into Tuesday with eventually much cooler less humid air moving into the region
* Mainly dry with seasonable temperatures Wednesday through Saturday with comfortable humidity
Details... Monday and Tuesday... The big idea for Monday and Tuesday will be a very wet storm system which moves through bringing heavy rainfall and some scattered thunderstorms to the region. Beginning Monday mid level flow amplifies, digging a deep trough over the eastern US with a large ridge over the intermountain west. As this deepening trough approaches southern New England flow becomes southwest pulling in deep, moisture rich air ahead of it (PWATs near 2"). The front stalled south of the coast which brought some relief from the heat and humidity will lift north again on Monday putting a portion of SNE in the warm sector, though there is still considerable uncertainty in just how far north that moves. Best chance now would be that it stays along or south of the MA Pike, and this is where we'll see the best instability and chance for thunderstorms Monday afternoon and overnight. With instability on the order of 1500 J/kg CAPE and good deep layer shear (40-45 kts) there is potential for a few strong to severe thunderstorms in the warm sector. However, the greatest risk will likely be from heavy rain and potential flooding as detailed below.
As the upper trough digs a surface low is generated along the stalled frontal boundary, strengthening over eastern PA on Monday before moving into New England, potentially as a double barrel low. This would bring two rounds of very heavy rain to the region. Plenty of moisture is present as mentioned earlier. This with approaching frontal convergence, the RRQ of a 110kt 300mb jet, and a 30-35kt LLJ means we have potential to see a prolonged period(s) of heavy rain from late Monday through Tuesday. WPC currently has western MA/CT in a Slight Risk of Excessive Rainfall, with a Marginal Risk for the rest of the area. We'll have to monitor for the potential of flooding with this system given deep layer moisture and strong unidirectional shear. The low should lift out late Tuesday bringing rain to an end. Cooler and drier air moves in behind the system which along with cloudcover should keep high temps on Tuesday in the 70s.
Wednesday through Saturday... A cooler, more seasonable, and drier period is on tap for mid week into the weekend. The upper trough moves overhead with building high pressure at the surface. 850mb temps around 10C temperatures will make surface temps feel cool relative to this weekend, in the mid 80s; should feel drier as well, with PWATs <1" and dewpoints in the 50s and low 60s.
Forecaster Confidence Levels... Low - less than 30 percent. Moderate - 30 to 60 percent. High - greater than 60 percent.
Short Term /through Sunday Night/...High confidence.
415pm update ... Tonight ... mainly light to modest SSW winds. Patchy fog possible overnight. Any isolated T-storms remain close to shore.
Sunday/Sunday night ... SSW winds becoming WNW late Sunday into Sunday night. Isolated T-storm possible. Patchy fog possible Sunday morning but improving.
Outlook /Monday through Thursday/...High confidence.
Monday: Winds less than 25 kt. Chance of rain showers, slight chance of thunderstorms.
Monday Night: Winds less than 25 kt. Seas locally approaching 5 ft. Rain showers likely, chance of thunderstorms.
Tuesday: Low risk for Small Craft Advisory winds with gusts up to 25 kt. Local rough seas. Chance of rain showers, chance of thunderstorms.
Tuesday Night: Winds less than 25 kt. Local rough seas. Chance of rain showers.
Wednesday: Winds less than 25 kt. Areas of seas approaching 5 ft.
Wednesday Night: Winds less than 25 kt. Seas locally approaching 5 ft.
Thursday: Winds less than 25 kt
Record highest min temps for Saturday July 20:
BOS: 80 in 1977 BDL: 73 in 1977 PVD: 76 in 1977 ORH: 72 in 1977
Record highs for Saturday July 20:
BOS: 99 in 1991 BDL: 100 in 1991 PVD: 101 in 1991 ORH: 95 in 1991
Record highest min temps for Sunday July 21:
BOS: 81 in 1991 BDL: 74 in 1977 PVD: 77 in 1980 ORH: 73 in 1991
Record highest max temps for Sunday July 21
BOS: 102 in 1977 BDL: 101 in 1991 PVD: 102 in 1991 ORH: 95 in 1926
Last Occurrence of 100F or higher temperatures BOS: 7/22/2011 (103F) BDL: 7/18/2012 (100F) PVD: 7/22/2011 (101F) ORH: 7/4/1911 (102F)
Notable high dew points in past years for July 20 and July 21:
Saturday July 20
BOS 76 in 2013 (last of multiple years) BDL 76 in 2005 PVD 76 in 2013 (last of multiple years) ORH 76 in 2013
Sunday July 21
BOS 76 in 1994 BDL 78 in 1972 PVD 81 in 1977 (tied for highest all-time at PVD) ORH 76 in 2011 (last of multiple years)
NOAA Boston MA Office: Watches - Warnings - Advisories
CT...Air Quality Alert until 11pm EDT this evening for CTZ002>004. Excessive Heat Warning until 8pm EDT Sunday for CTZ002>004. MA...Air Quality Alert until 11pm EDT this evening for MAZ017>024. Air Quality Alert from 11am to 11pm EDT Sunday for MAZ020>024. Excessive Heat Warning until 8pm EDT Sunday for MAZ002>021- 026. Heat Advisory until 8pm EDT Sunday for MAZ022-023. RI...Air Quality Alert until 11pm EDT this evening for RIZ001>008. Air Quality Alert from 11am to 11pm EDT Sunday for RIZ003>008. Excessive Heat Warning until 8pm EDT Sunday for RIZ001>007. MARINE...None