Area Forecast Discussion
Issued by NWS Boston, MA

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000 FXUS61 KBOX 282316 AFDBOX Area Forecast Discussion National Weather Service Taunton MA 716 PM EDT Fri Apr 28 2017 .SYNOPSIS... Another mild night tonight with a risk for showers and thunderstorms for some in the morning. Warm conditions follow once again for the daytime Saturday. After a cold frontal passage Saturday night 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. && .NEAR TERM /THROUGH SATURDAY/... 7 pm Update... Patchy clouds moving in from the west. Meanwhile, the final visible satellite images showed two areas of fog/stratus moving up on MVY and ACK. The 7 pm observation at BID also reported the start of fog/stratus. Expect these low clouds to linger much of the night and possibly expand up to the South Coast. Not sure that the poor vsbys will reach the mainland, but likely some vsby reduction during the night. No significant changes to the forecast. Fair for most of the night with a chance of showers late night, especially along the South Coast. Previous discussion... Not confident enough to hoist an advisory given that the W component of the flow is typically not conducive to marine fog over land, but it will need to be watched. Otherwise, increasing mid-upper clouds and high dwpts from the washed out front this afternoon will limit overnight mins to the upper mid and upper 50s. The one key to the forecast for the overnight and early tomorrow is a burst of elevated instability associated with the remnant cold pool from a developing MCS across the lower OH valley late this evening. Noting an injection of moisture into the column from the remnants of the MCS, and with steady lapse rates leftover from this afternoon/evening near 6.0-6.5C/km, this will yield MU/elevated CAPE profiles near 500j/kg between 09Z (5AM local) and 15Z (11am local). This is also associated with a modest LLJ around 40 kt yielding about 45-50 kt of 0-6km shear. Therefore, the ingredients are available for a second round of early AM elevated convection. One caveat as is often the case with upstream MCS development is steering of the convection. MCS is likely to turn somewhat S, following the gradient of highest sfc based instability, but given the increase in elevated stability further N (over SNE), the threat risk remains. Could be a two part type of event, one watching the actual remnants of the MCS relative to its cold pool, and the second further N along a weak shortwave/LLJ. Given these factors, have focused the highest POPS/chances for TSRA across CT/RI and SE MA similar to this morning, however the risk is definitely non-zero further N. Severe risk is low given the bulk of the instability is elevated but the influence of the LLJ does suggest a few gusty winds are possible if stronger TSRA are observed.j Otherwise, after the shortwave/instability shifts E in the late morning, expect another gradual clearing with only diurnal SC through the afternoon. Airmass similar to today suggests another round of highs in the upper 70s to low 80s, warmest away from the S coast. A bit more breezy however, as the lingering LLJ could yield some wind gusts to around 25 mph. && .SHORT TERM /SATURDAY NIGHT/... More typical setup for spring overnight tomorrow. Lingering clouds but more room for cooling as drier dwpts begin to filter in through the afternoon and evening tomorrow. However, expecting slow backdoor cold front progression during the late night into Sun timeframe. Therefore, looking at mostly lows in the upper 40s and low 50s. Which is closer to seasonable normals than we have experienced with the moisture of the past few days. && .LONG TERM /SUNDAY 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 /23Z FRIDAY THROUGH WEDNESDAY/... Forecaster Confidence Levels... Low - less than 30 percent. Moderate - 30 to 60 percent. High - greater than 60 percent. Short term...through Saturday night. Through 04Z (midnight local)...Moderate confidence. VFR for all TAF sites except ACK where LIFR in fog expected. This fog may expand into FMH and HYA...while becoming IFR these sites will likely have better vsbys than ACK. After 04Z through tomorrow...Moderate confidence. As noted, IFR likely along or close to coastal RI and S coastal MA in low CIGS/fog. Otherwise VFR mainly inland but with a risk for some SHRA/TSRA. Best chance for these would be across CT/RI/SE MA between 09Z and 14Z from W-E. Lower risk further N. 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 kt during the afternoon. Tomorrow night...High confidence. W flow and mainly VFR. KBOS TAF...Generally high confidence in TAF. If sea breeze occurs this afternoon/evening it would be relatively short lived. KBDL TAF...High confidence in TAF. Outlook /Sunday through Wednesday/... Moderate confidence. Sunday... High confidence. VFR. 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. && .MARINE... Forecaster Confidence Levels... Low - less than 30 percent. Moderate - 30 to 60 percent. High - greater than 60 percent. Short Term /through Saturday Night/...High confidence. Lingering offshore swell continues on the waters, holding at 5-6 ft. Therefore, offshore small craft advisories will continue until this swell subsides a bit more. Otherwise, another round of fog tonight especially across the southern waters and then another risk for showers or a thunderstorm on the waters during the early morning hours. After the showers/storms and fog dissipate during the morning hours tomorrow, W winds become a bit more breezy, especially near shore with gusts to 20 kt, however feel these will remain below small craft advisory thresholds throughout the day. There is a low risk for slightly stronger W gusts during the overnight hours along with seas reaching near 5 ft, but this is very marginal and doesn`t warrant another advisory at this time. Later shifts can re-evaluate. Outlook /Sunday through Wednesday/... Moderate confidence. Sunday... East winds less than 20 knots and seas less than 5 feet. 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. && .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... CT...None. MA...None. RI...None. MARINE...Small Craft Advisory until 11 PM EDT this evening for ANZ250. Small Craft Advisory until 2 AM EDT Saturday for ANZ254>256. && $$ SYNOPSIS...WTB/Doody NEAR TERM...WTB/Doody SHORT TERM...Doody LONG TERM...WTB AVIATION...WTB/Doody MARINE...WTB/Doody TIDES/COASTAL FLOODING...

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