Area Forecast Discussion
Issued by NWS Boston, MA

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000 FXUS61 KBOX 181756 AFDBOX Area Forecast Discussion National Weather Service Taunton MA 156 PM EDT Tue Apr 18 2017 .SYNOPSIS... High pressure maintains dry weather today along with a cool flow of air off the Gulf of Maine. A more active weather pattern is forecast for the late week period and again into early next week. Better chances of wet weather and cooler conditions with a series of low pressure centers, the first around Friday with a second around Tuesday into Wednesday. && .NEAR TERM /UNTIL 6 PM THIS EVENING/...
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155 pm update... High pressure over the Canadian Maritimes was generating a northeast flow of cool air across eastern New England. Temps at mid afternoon were mainly in the upper 40s along the eastern MA coast with a scattered to broken deck of clouds with even an isolated sprinkle or two. Further inland, across western/central MA, northern CT and much of RI plenty of sunshine will allow for highs well into the 50s with even some lower 60s in the lower CT River Valley through late afternoon.
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&& .SHORT TERM /6 PM THIS EVENING THROUGH 6 PM WEDNESDAY/... Tonight... Some uncertainty exists in cloud cover, since an easterly flow at night climatologically brings a higher risk of mostly cloudy or overcast skies. Model guidance is giving a mixed signal. For now, have gone close to a model blend that shows temperatures dropping into the 30s throughout forecast area. Those temperatures, however, could end up being somewhat low if more cloud cover should materialize. Wednesday... Expect much of the day to be dry, but there will be an increasing chance of showers from west to east toward evening as a relatively flat short wave trough progresses over the large scale eastern USA ridge and approaches southern New England. Onshore flow will keep temperatures cool in the east and probably extend far enough inland to drop temperatures a few degrees even in the CT River Valley. && .LONG TERM /WEDNESDAY NIGHT THROUGH MONDAY/... */ Highlights... - A wetting rain for the Wednesday night into Thursday timeframe - Better shot of wet weather around Friday AM - Quiet over the weekend though cooler - Potential coastal storm for early next week */ Overview... Continued dominance of progressive Pacific-origin disturbances. Per three features worthy of note: 1.) H5 Aleutian low, 2.) H5 low over N Canada, and 3.) a pseudo-developing rex-blocking feature / cut-off low over the N Atlantic, any energy emerging from the persistent, progressive Pacific jet into the W CONUS subsequently undergoes down- stream lee cyclogenesis and enters a region of favorable troughing from the Central to E CONUS dependent on the steering level and buffering influence per downstream features, namely the H5 low over Canada and features across the N Atlantic. This is reaffirmed via ensemble forecast of a driving negative WPO which signals cooler than average conditions extending from the N-Central CONUS and Great Lakes region into the NE CONUS. Indications of a more active weather pattern. Forecast confidence through the weekend, however lowered into early next week with considerable spread of steering features and possible outcomes associated with a potential coastal storm. */ Discussion... Wednesday night into Thursday... A wetting rain. Stalling frontal boundary across the region parent with weak mid-level troughing. Attendant low to mid level forcing, yet stronger dynamics generating deep layer ascent reside to the N. Thinking scattered, light showery weather emerging out of stronger dynamics upstream over Upstate NY, moving S/E with the mean layer flow. Enough moisture and lift present to generate some steady wet weather, though amounts likely not to be in excess of a tenth of an inch. Expecting enough rain to wet roadways. Clouds likely to be an issue, and that makes for a tricky temperature forecast. A consensus blend is preferred. Thursday night into Friday night... Better opportunity for wet weather. Potent cyclone ahead of which a combination of maritime/tropical continental conditionally-unstable airmass converges along a parent warm frontal boundary beneath both diffluence aloft/venting and deep-layer forcing. Seemingly initially a forward-thump of widespread rain with embedded heavier showers, perhaps even a rumble of thunder will give a decent dousing of rain- fall around the Friday AM timeframe. Some spatial uncertainty as to where heaviest rain will fall per uncertainty on synoptic features yet the signal is consistent with respect to wet-weather outcomes. Focus is away from model QPF fields, rather upon the low to mid level warm front and neighboring synoptics, particularly isentropic- convergent mechanisms. Another uncertainty is whether we`ll see activity along the follow- up cold front. Possibility of destabilization during the afternoon into evening, perhaps complicated by considerable dry-slotting. Will have to watch closely as uni-directional shear profiles oriented perpendicular to the cold front would present a wind threat should any line-convection materialize. These situations are tricky so far out in the forecast with lack of mesoscale guidance, especially as well as the morphology of the warm front is in question and whether we warm-sector. Overall, thoughts in the back of the mind for now going forward, will not mention any thunder for the moment. Weekend... High pressure behind the cyclone ushering cooler conditions over the region. An onshore flow as high pressure pushes E. Early next week... Indications of a coastal storm. At first glance, an occlusion is favored given forecast downstream influences slowing the longwave upstream pattern. Given this, expecting blustery northerly winds on the backside of the low driving a chilly rain across the region per an initial front-end thump followed by moisture trowaling rearward. Both spatial/temporal inconsistencies, so no certainty on specific outcomes. Just an atypical pattern that bears watching. && .AVIATION /18Z TUESDAY THROUGH SUNDAY/...
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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 Wednesday/... Rest of this afternoon...High confidence. Despite a scattered to broken deck of cloudiness across eastern New England, mainly VFR conditions will continue. However, brief MVFR cigs can not be ruled out across eastern MA. Tonight...Moderate to high confidence. Mainly VFR conditions but some marginal MVFR cigs develop after midnight across western MA/northern CT. Wednesday and Wednesday night...Moderate to high confidence. Mainly VFR conditions expected for the bulk of the daylight hours on Wed, but will have to watch for some MVFR cigs developing near the coast. More widespread MVFR conditions expected Wednesday night along with some scattered rain showers. KBOS TAF...High confidence in TAF. KBDL TAF...High confidence in TAF. Outlook /Thursday through Saturday/... Thursday through Friday night...Moderate confidence. MVFR/IFR mix with -RA at times. Higher confidence of visibility restrictions going into Thursday night, especially over the high terrain. Southerly winds turning easterly. Blustery with strongest gusts late Friday into Friday night as winds turn northerly. Gusts up to 35 kts possible for E MA coastline terminals. Will begin to clear out late Friday night into Saturday morning. Saturday into Saturday night...Moderate confidence. Low-end VFR/MVFR mix with higher confidence of MVFR along E MA and over the high terrain. N winds, blustery, an onshore flow for E MA coast. Gusts up to 35 kts possible.
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&& .MARINE... Forecaster Confidence Levels... Low - less than 30 percent. Moderate - 30 to 60 percent. High - greater than 60 percent. Short Term /through Wednesday/...High confidence. Allowed the Small Craft Advisory for the outer south coastal waters to expire at 2 AM and buoys near those zones indicate that seas have dropped below 5 feet. Did increase seas a tad above guidance for the east coastal waters due to a NE flow this morning and east to southeast flow late today into Wednesday. There may be enough gradient to build seas to near 5 feet east of Cape Ann Wednesday afternoon, but the confidence level on this is relatively low. Thus, have held off on any marine headlines for now. Outlook /Wednesday Night through Saturday/... Will see bouts of wet weather as S winds gradually turn E prior to becoming N ahead of an interior storm system pushing E strengthening over the Gulf of Maine. There is a risk of gales during this time- frame in association with the N winds that`ll push seas over the 5 foot mark, especially over the E/SE outer waters. High pressure building into the region Saturday will only exacerbate the N flow pushing seas higher around 6 to 8 feet prior to dampening Saturday night into Sunday morning. && .FIRE WEATHER... Still low relative humidities of 25 to 30 percent likely this afternoon in the CT River Valley, but relatively light winds are expected. We do not think a SPS should be needed today. Somewhat higher relative humidities are forecast throughout southern New England on Wednesday with minimum values generally in the 40 to 40 percent range across the interior and higher in coastal areas. && .BOX WATCHES/WARNINGS/ADVISORIES... CT...None. MA...None. RI...None. MARINE...None. && $$ SYNOPSIS...Sipprell/Thompson NEAR TERM...Frank SHORT TERM...Thompson LONG TERM...Sipprell AVIATION...Frank/Sipprell MARINE...Sipprell/Thompson FIRE WEATHER...Staff

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