Area Forecast Discussion
Issued by NWS Boston, MA

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000 FXUS61 KBOX 290756 AFDBOX Area Forecast Discussion National Weather Service Taunton MA 356 AM EDT Sat Apr 29 2017 .SYNOPSIS...
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Mild start for today with showers and isolated thunderstorms moving through a portion of the area in the morning. Warm conditions follow once again for the daytime. After a cold frontal passage tonight high pressure builds over New England Sunday with dry but cooler air. Sprawling low pressure in the Plains will push a warm front through our area Monday, then swing a cold front through on Tuesday. Another storm moves up from the Gulf of Mexico late Thursday or Friday.
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&& .NEAR TERM /UNTIL 6 PM THIS EVENING/...
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400 AM update.... Mild start this morning with many spots still in the 60s. Showers are beginning to move across western portion of the area. Main focus is on the overnight convection moving into NJ. This area of thunderstorms will most likely maintain itself and move across the south coast including the southern waters. This area of convection is in line with an area 850 mb divergence and upper level jet streaks and is moving along the 925 mb moisture line. Guidance and SPC meso analysis continues to show that there is still enough instability as well as steep mid-level lapse rates across the southern half of the region to maintain the convection. Will have to watch this area closely for the potential for a few strong storms with heavy downpours as PWATs increase to 1.5 inches. This afternoon... Cold front will sweep across the region this afternoon turning the flow to a more westerly direction. Still some lingering moisture in the mid-levels will result in just clouds moving through. Still need to keep an eye on the potential for a pop-up shower/t-storm ahead of the cold front. Best are for this to occur is across the south coast where guidance is hinting as some surface instability as well as dewpoints in the 60s before the front pushes through. Lapse rates are also somewhat steep. Aside from clouds as the potential for a rogue storms, westerly winds aloft will increase between passing shortwave in southern Canada and Bermuda high to the south. Anticipate gusty winds near 20 MPH during the afternoon. Cooler temps aloft will also lag behind the front, so another warm day will be in store across the region with highs back into the 80s.
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&& .SHORT TERM /6 PM THIS EVENING THROUGH 6 PM SUNDAY/...
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Tonight and Sunday... Stalled front to the south of the region with zonal flow aloft. Some of the models including the NAM are showing another decaying MCS/thunderstorms moving across PA into southern NY tonight. Believe that these storms will follow along the stalled boundary and should stay south of the region. High pressure to the north will begin to slide down into southern New England. This will switch the winds from the west to a more northerly direction and eventually east by Sunday. Dry air will spill in the area resulting in the potential for radiational cooling. Temps overnight will drop into the mid 40s to low 50s. During the day, large temperature spread is possible with high temps in the mid 60s for the CT river valley to low 50s along the MA east coast. Depending on cloud cover temps out west could be to optimistic. Approaching warm front south and west of the region may result in some overrunning precip during the later half of the day on Sunday. Low confidence if this will occur as model soundings are quite dry at the low levels. Heights aloft are also building which will lowers chances. However, if this were to occur then the wester half of the region has the best shot, but for now kept pops low on Sunday.
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&& .LONG TERM /SUNDAY NIGHT THROUGH FRIDAY/... Big Picture... Longwave pattern shows ridges off each coast with a trough over the Midwest. This pattern starts shifting east late next week. Shortwave pattern shows two systems of interest in the long term. The first is a closed low over the Southern Rockies during the weekend that ejects through the Great Lakes Tuesday and is absorbed into the Hudson Bay low thereafter. This draws a strong upper jet across New England Monday night and Tuesday. The second system is a shortwave south of the Aleutians this weekend that reaches the Wash/BC coast Monday and digs a closed low over the Southern states by Friday. Model mass fields are similar through Wednesday then disagree on the late week system...the GFS shows a coastal storm while the ECMWF shows it tracking north through Central NY. The GGEM shows a little of both. Also large run-to-run changes. As the system is well out over the North Pacific, we also note time and sampling issues. This means at least moderate confidence in the early week system but low confidence in the late week system. Details... Sunday... High pressure builds over the region with a light east to southeast wind. Warm front stalled over the Mid Atlantic could combine with the east flow to spread some clouds into the area, but airmass looks otherwise dry. Expect coolest temps at the East MA shore, warmest in the CT River Valley. The high shifts offshore Sunday night and the warm front shifts north. This should bring an increase in sky cover. Can/t rule out scattered showers at this time from the onshore flow, but these should be locally generated as the southwest low level jet will be too far away to generate lift over the front. Monday-Tuesday... Low pressure in the Great Lakes lifts northeast, pulling the warm front north through New England Monday or early Monday night. Questions remain regarding timing of the warm front and associated wind shift. Because of this we have nudged guidance temps about 2 degrees cooler. Some lingering showers especially north of the Mass Pike. But expect most of this to move off to the north. Low level southwest jet moves up the coast with 40 knot winds at 2000 feet, helping generate gusts to 20 knots. Cold front sweeps east from New York Monday night. Second low level southwest jet moves in with 50-60 knot winds at 2000 feet. Upper jet with 110 knots moves over New England after midnight and provides upper venting as the front moves through and provides low level convergence. Stability parameters look favorable for thunder with forecasts of Totals 48-50 and LI 0 to -2. Precip water forecast at 1.7 inches. Expect a period of showers and scattered thunder with the front Monday night and early Tuesday. Locally heavy showers possible. Drier air moves in on Tuesday. Some uncertainty as to how deep the mixing will be behind the cold front, although model cross sections show it as high as 800 mb. Temps in this layer will support max surface temps to at least 70F and possibly to the mid 70s. Winds in the layer reach as high as 30 knots, so a gusty afternoon in store. Wednesday... Surface low over the Maritimes and cloud shield into Northern New England. Upper shortwave lingers over Northern Maine along with cold pool over Northern New England. This should generate some diurnal clouds, especially north of the Mass Pike. Temps aloft support max sfc temps in the 60s. Thursday-Friday... As noted above, long range models seem to agree that a storm will affect the Northeast USA but disagree on how it will do so. This and the fact that this is a Day 6/7 forecast will limit forecast pops to 30-40 pct. Model consensus on timing would bring rain into our area Thursday mid to late afternoon, but we do note the ECMWF is six hours faster than consensus. Best chance looks to be Thursday night and Friday. && .AVIATION /08Z SATURDAY THROUGH WEDNESDAY/...
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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 Sunday. Today...High confidence. VFR with a risk for some showers this morning. CT/RI/SE MA site shave the best chc for -TSRA between 09Z and 14Z from W-E. After these move through expect SCT-BKN low end VFR clouds through the day. Winds shift from SW to W in the morning with a few gusts around 20 to 25 kts during the afternoon. Tonight...High confidence. Westerly flow to start with VFR conditions. Tomorrow...High confidence. Flow will turn east with VFR conditions. Low prob of a passing showers during the later half of the day. KBOS TAF...High confidence in TAF, low prob of -TSRA around 12z. KBDL TAF...High confidence in TAF, low prob of -TSRA around 9z. Outlook /Sunday Night through Wednesday/... Moderate confidence. Sunday night to Tuesday... Moderate confidence. Ceilings lower to MVFR/IFR Sunday night and the start of Monday in low clouds and fog/drizzle. Brief improvement to VFR possible Monday, then a cold front moves through Monday night and early Tuesday with showers and scattered thunder. Low level wind shear leading the cold front with southwest winds of 50-60 knots at 2000 feet AGL. The cold front moves offshore Tuesday morning followed by clearing skies and southwest winds gusting 25-30 knots. Wednesday... Moderate confidence. VFR. West winds gusting to 20 knots.
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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 Sunday/...High confidence. Small craft adv has been issued for the outer waters this afternoon as passing frontal system combined with lingering swell will push waters back to 5 feet. Also have issued a SCA for Boston Harbor and Narragansett Bay for wind gusts near 25 kts. There is a chance that this may be expanded for the other near shore waters, but confidence was to low to issue it at this time. High pressure from the north will slide across the region by Sunday. This will allow for both wind gusts and seas to relax below SCA. Outlook /Sunday Night through Wednesday/... Moderate confidence. Monday... Warm front moves north of the waters late Monday. East winds turn from the southeast and eventually from the south but remain less than 25 knots. Seas less than 5 feet most of Monday, then build 5-8 feet Monday night on the exposed southern waters. Winds 2000 feet above the surface will increase to 50-60 knots, so potential for gusts to 30 knots Monday night. Showers and possibly thunder Monday night. A Small Craft Advisory will probably be needed. Tuesday... Cold front moves across the waters early Tuesday. Winds will shift from the southwest after the front moves through. Showers/thunder end after the wind shift. Southwest winds may gust 25-30 knots Tuesday. Seas remain 5 to 8 feet on the southern waters exposed to a southwest flow. A Small Craft Advisory will probably be needed. Wednesday... West winds will be 20 knots or less. Seas of 5-6 feet linger on the southern outer waters.
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&& .TIDES/COASTAL FLOODING... * Isolated minor coastal flooding impacts during astronomically high tide tonight along the east coast The astronomical high tides remain very elevated into this weekend. Given a lingering swell offshore and its remnant surge, with high tide values only a few tenths shy of typical minor flooding levels, a Coastal Flood Statement continues for the high tide near 2 AM tonight for east coastal locations. At this time, we anticipate only minimal impacts at typically prone locations. Also, due to the breadth of the fetch, we may encounter enough wave action with lengthening periods on top of an elevated tide to cause pockets of erosion tonight along the Martha`s Vineyard south coast, Nantucket south and east coasts, and the outer Cape ocean side from Truro to Chatham. Boston High Tides (flood stage at Boston 12.5 feet)... 12.14 feet / Saturday 12:56 am && .BOX WATCHES/WARNINGS/ADVISORIES...
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CT...None. MA...None. RI...None. MARINE...Small Craft Advisory from noon today to 2 AM EDT Sunday for ANZ254>256. Small Craft Advisory from 1 PM this afternoon to 8 PM EDT this evening for ANZ230-236.
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&& $$ SYNOPSIS...Belk/Dunten NEAR TERM...Dunten SHORT TERM...Dunten LONG TERM...Belk AVIATION...Belk/Dunten MARINE...Belk/Dunten TIDES/COASTAL FLOODING...Staff

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