Area Forecast Discussion
Issued by NWS Boston, MA

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000 FXUS61 KBOX 210201 AFDBOX Area Forecast Discussion National Weather Service Taunton MA 1001 PM EDT Wed Sep 20 2017 .SYNOPSIS... Jose continues northeast overnight, then turns east and circles well southeast of Nantucket through Saturday. This may be close enough to maintain rain and strong wind for a couple of days over Southeast Mass, favoring Cape Cod and Islands as well as the adjacent ocean waters. High pressure builds in from the Great Lakes with dry weather early next week. && .NEAR TERM /UNTIL 6 AM THURSDAY MORNING/...
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** Tropical Storm Warnings Continue For Some Waters, Cape Cod and the Islands. ** Jose is located about 145 south southeast of Nantucket and moving slowly northeast. Sustained winds to 34 knots are just south of our area. Radar shows showers across Nantucket and clipping the edges at the Vineyard and Chatham. Forecast temperatures and dew points were adjusted 1-3F based on observations. Trends on the radar and current observations line up well with the existing forecast. No significant changes planned. Earlier discussion follows... Jose rests, at the time of this writing, about 170 miles SSE of Nantucket and 90 miles SE of the 40/70 benchmark. Satellite presentation has actually improved diurnally, similar to yesterday with convection wrapping around the center, assuming a slightly more symmetrical shape. In fact, as of the latest advisory, NHC has been able to establish a max of 60 kt sustained. Jose will continue to slide ENE through the overnight hours before once again sliding slightly W given the lack of any synoptic pushing mechanism, riding right along the Gulf stream SST gradient, helping to maintain it. Given this close proximity, as has been forecast now for some time, Jose will continue to influence our sensible wx for some time, possibly until the weekend before completely letting go. NHC suggests extra-tropical conditions in 36-48 hours. Winds... Tropical Storm Warnings continue for mainly the waters open to the ocean and Cape Cod/Islands. These remain the only locations where sustained winds to the 39mph (34 kt) threshold are possible while other locations are likely to see gusts to this magnitude. Although some locations, particularly block Island and Nantucket, have already gusted to 40-45 mph with a few spots (Block Island again) reaching near 40 mph sustained. However, the strongest winds are yet to come. This is in part due to a slight amplification of the mid lvl ridge, which will allow the sfc anticyclone across N New England to respond and build slightly, enhancing the pres gradient between Jose and the high late tonight into early tomorrow. At the same time, slight northerly shift in isallobaric flow will allow drier air to filter into S New England from the N, which will help enhance the SFC-H92 lapse rates especially during daylight hours tomorrow, allowing a mix down of at least some momentum from the LLJ at H92 which peaks tomorrow morning between 45-55kt. All of this is an admittedly long winded means of stating that the peak winds actually won`t arrive until late tonight and linger through tomorrow night and slowly climb through tonight. At peak, sustained winds between 35-45 mph are possible mainly for those locations currently within Tropical Storm Warnings, with 25-35 along and SE of the I-95 corridor, and 15-25 for interior S New England. Gusts 45-60 mph, 35-50 mph, and 20-35 are possible respectively. Will continue, and actually extend Tropical Storm Warnings (TSW), Wind Advisories for I-95 and points SE not in TSW through tomorrow evening at least. Fully leaved trees will exacerbate the issue as we have already seen some spotty tree damage with lesser values. Rainfall... The biggest change this afternoon will be to drop the Flash Flood Watch (FFW) for Cape Cod. With the slight shift E, and tightening of the gradient/rain bands even as it shifts back W should keep the heaviest and convectively drive bands too far E for significant heavy rainfall over a short period of time. Not to say that additional 1-2 inches aren`t possible, but it would be over the next 24-48 hours. Meanwhile, Nantucket is still close enough to where guidance still suggests direct rain bands that it warrants keeping the watch up and extending it into tomorrow night as QPF bullseyes on meso-scale guidance do impact the island through tomorrow. Still holding on rain totals (inclusive of what has fallen) of 3-5 inches, which could be exacerbated by the inability to drain tidal surges over the next few high tide cycles. Interior showers and drizzle will taper off overnight as the winds shift N and allow drier air infiltrate some of the lower levels. High surf... With Jose now nearing its closest point to S New England, and expecting to linger nearby through late week, a building southerly swell, combined with strong N-NE flow will lead to building seas through tomorrow night at least, and then maintain high seas into the weekend. This will create high surf at local beaches outside of Boston Harbor which remains sheltered in this setup. 20-25 ft waves are possible well offshore. This will also lead to dangerous rip currents. High surf advisories continue into Friday for these conditions outside of Tropical Storm Warnings which cover this risk. For more on the coastal flooding and beach erosion risk, see that section below. Otherwise across S New England, temperatures will remain near normal across portions of W MA/CT where some breaks to sunshine are possible tomorrow and Friday. Meanwhile cloud shield will limit sunshine further E, leading to highs generally below normal.
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&& .SHORT TERM /6 AM THURSDAY MORNING THROUGH THURSDAY NIGHT/... Jose continues to slide back W allowing the maintenance of strong pres gradient. Will extend wind headlines into the overnight hours to match, and may need to push them into Friday given only a little change. Although do note some weakening of the NW quadrant LLJ by daylight on Fri. Otherwise, Jose continues to bring some rain to the extremities of Cape/Cod and Nantucket and the risk for Tropical Storm conditions, so these too will be maintained into early Friday. More to come in future updates. && .LONG TERM /FRIDAY THROUGH WEDNESDAY/... Highlights... * Gusty winds persist on Cape Cod and Islands Friday * Dry and seasonable over weekend and early next week * Monitor NHC forecasts as Maria heads through Caribbean With a tilted axis of high pressure building over the Great Lakes Friday, Jose will be pushed southwest but ultimately remain southeast of Nantucket. Dry conditions will return to the region through early next week. However, Tropical Storm Force winds will continue across the Cape and Islands through Friday. Winds will be slow to diminish as Jose remains to our south. A persistent ridge points to a warm period, especially Friday through Monday. Details... Friday through Wednesday... Both the ECMWF and the GFS are in agreement on the position of Jose through Monday morning. Jose will essentially sit at 40 N through 12z Monday, but depending on the position of the high pressure and shear aloft, Jose may considerably weaken and therefore have very minimal impact on the region. High pressure builds in from the west Friday, bringing clearing skies and lower humidity. Unfortunately, with Jose remaining in close proximity, an extended wind field will continue through Friday. The Cape and Islands will see gusts surpassing 40 knots well into Saturday. Models start to differ when it comes to Jose on Day 5. The ECMWF model now takes Jose further southeast and out to sea as a high continues to build to its west. The 12z run of the GFS however suggests that the high pressure currently building over the Ohio Valley will eventually push Jose to the southwest, making landfall along the New Jersey coast. Jose would be considerably weaker by this time period, likely a post-tropical depression. The eventual track of Jose will have impacts the track of Maria, currently located west Puerto Rico. If Jose does make landfall near the New Jersey coast, Maria will likely be forced out to sea and therefore having very little impact on Southern New England. Temperatures through the weekend will be well above normal with highs forecast to be in the mid 80s. The normal high for this time of year is 70 degrees. && .AVIATION /02Z THURSDAY THROUGH MONDAY/...
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Forecaster Confidence Levels... Low - less than 30 percent. Moderate - 30 to 60 percent. High - greater than 60 percent. Short Term /through Thursday Night/... Overnight...High confidence in trends, lower confidence in exact timing. Gradual improvement, but mainly MVFR CIGS E of Worcester hills while IFR CIGS remain across Cape/Island terminals. Low vsbys mainly confined to the Cape/Islands as well as SHRA/fog dissipates inland this evening. VFR W of the Worcester hills should generally stay from now on. Winds gradually increase through the night, but should peak mainly after sunrise. Sustained winds remain around 25-35 Cape/Islands tonight with gusts 35-45, 15-25 inland with gusts mainly 30-35 kt until sunrise. LLWS, especially E MA and RI with 40-50 kt off the deck. Thursday... Gradual improvement in ceilings from west to east with areas off- Cape becoming VFR by afternoon. Lingering MVFR cigs over Cape Cod and Islands. North-northeast winds continue to gust near 20 knots in the west, 20-30 knots in the central hills, and 30-40 knots in RI and Eastern Mass with some 45-55 kt possible Cape/Islands. KBOS TAF...High confidence in TAF trends, lower confidence in timing. Sustained NNE winds around 15-20 knots, and gusts increasing to 25-35 knots. KBDL TAF...High confidence in TAF trends, lower confidence in timing. Outlook /Friday through Monday/... High confidence. Thu night and Fri...Lingering MVFR ceilings in eastern MA, otherwise VFR. N winds gusting to 25-35kt on Cape Cod/Islands. Sat and Sun...VFR.
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&& .MARINE...
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Forecaster Confidence Levels... Low - less than 30 percent. Moderate - 30 to 60 percent. High - greater than 60 percent. Short Term /through Thursday Night/... Tropical Storm Jose passes southeast of the waters tonight, then lingers well offshore through Thursday. This will allow strong winds to linger over the waters through at least Thursday, with gusts of 35-45 knots over most of the waters, with gusts 25-30 knots on Narragansett Bay and Mass Bay/Boston Harbor. Building swells have already reached as high as 16 ft at buoy 44008 SE of Nantucket. Expect high swell will continue through Thursday, especially over the waters with a southern exposure. Showers and fog will also linger through Thursday, creating poor vsbys. Outlook /Friday through Monday/... Moderate confidence. N winds remain strong through Saturday morning, with gusts of 35-40kt on south coastal waters as Jose drifts S and away from southern New England. Winds diminish further as high pressure builds over region this weekend, but seas will take most of weekend to subside on open south coastal waters.
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&& .TIDES/COASTAL FLOODING... Our thinking in regards to coastal impacts remain generally unchanged. We anticipate the greatest impact to be along the Nantucket, Cape Cod, and Martha`s Vineyard shorelines where Tropical Storm Warnings remain in place. Our primary message remains the very long duration of beach erosion for this event. Jose is expected to remain quasi-stationary southeast of New England through the end of the week with slow weakening by the weekend. Relentless wave action will continue for the rest of this week. We anticipate some beach erosion along all ocean exposed sandy shorelines of MA and RI, but the most severe beach erosion is expected along the east side of Nantucket and the outer Cape ocean side from Eastham to Orleans to Chatham. Another area of significant beach erosion is expected to be the south sides of Nantucket and Martha`s Vineyard due to the impact of a persistent SE swell. As far as coastal flooding, the area of greatest concern remains the Nantucket Harbor area where we may experience a high end minor event late tonight and possibly moderate coastal flooding for the Thursday early afternoon and late night high tide cycles. Minor coastal flooding may linger along Nantucket Harbor for the Friday early afternoon high tide. Elsewhere across Cape Cod, Martha`s Vineyard, and the Plymouth County coastline, we anticipate areas of minor coastal flooding for the late night and early afternoon high tide cycles through at least Thursday night. We have extended the High Surf Advisory outside of the Tropical Storm Warning area through Friday. The advisory covers all ocean exposed coastlines in MA and RI outside of the Tropical Storm Warning and covers threats from both high surf and dangerous rip currents. We may end up having to extend the High Surf Advisory into at least part of the weekend. Swells from Maria may re-increase the surf and associated rip current risk across our ocean exposed south coast sometime during the first half of next week. && .EQUIPMENT... KBOX WSR-88D Z-R relationship remains in Tropical to get better rainfall estimates than with Convective. && .BOX WATCHES/WARNINGS/ADVISORIES... CT...None. MA...Tropical Storm Warning for MAZ022>024. High Surf Advisory until 6 PM EDT Friday for MAZ007-019-020. Coastal Flood Advisory until 4 AM EDT Friday for MAZ019. Wind Advisory until 6 AM EDT Friday for MAZ006-007-013>021. Flash Flood Watch through Thursday evening for MAZ024. RI...Tropical Storm Warning for RIZ008. High Surf Advisory until 6 PM EDT Friday for RIZ006-007. Wind Advisory until 6 AM EDT Friday for RIZ002>007. MARINE...Tropical Storm Warning for ANZ231>235-237-250-254>256. Small Craft Advisory until 4 PM EDT Friday for ANZ230-236-251. && $$ SYNOPSIS...Doody/Correia/Thompson NEAR TERM...WTB/Doody SHORT TERM...Doody LONG TERM...Correia AVIATION...WTB/Doody/Correia MARINE...WTB/Doody/Correia TIDES/COASTAL FLOODING... EQUIPMENT...

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