Area Forecast Discussion
Issued by NWS Boston, MA

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000 FXUS61 KBOX 120005 AFDBOX Area Forecast Discussion National Weather Service Taunton MA 705 PM EST Wed Jan 11 2017 .SYNOPSIS...
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A period of rain will move in tonight along with slowly rising temperatures. Expect unseasonably mild conditions into Thursday. An approaching cold front will bring a chance of additional showers Thursday afternoon and evening, then turning colder and blustery Friday behind the front. Mainly dry and much colder weather follows this weekend as strong high pressure builds into the region keeping low pressure well to the south. The high pressure moves offshore Monday, but maintains dry weather through the day. Low pressure moving out of the Southern Plains will bring milder temperatures and mostly rain for Tuesday and Wednesday.
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&& .NEAR TERM /THROUGH THURSDAY/...
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7 pm update... Radar shows echoes over NJ and southern PA, but observations show pcpn farther back, just crossing the Mason-Dixson line. This is moving northeast and trends to CT about 03Z/10 PM, and to the remainder of the region by Midnight. So the existing timing remains in line. A couple of sites...TAN and BED...had radiational cooling this evening and dropped into the 30s. With clouds moving in from the southwest any cooling should end early tonight and temps will even trend up a few degrees during the night. No other changes. Previous discussion... Next low pressure will be moving through the Gt Lakes tonight and will result with another SW low level jet 60-65 kt developing late tonight through Thu morning. This will result in deepening moisture plume across the region with PWATs increasing over 1" which will bring a period of rain later tonight, mainly after 03z. System is progressive and only looking at a 3-6 hour period of steady rain which will limit amounts to 0.25-0.50". Nose of the jet focused south of the Mass Pike and especially RI and SE MA where heaviest rainfall is expected. Rain will be exiting the region before daybreak. Areas of fog expected which may be locally dense, but increasing wind will likely limit areal extent of dense fog. Another period of gusty SW winds will develop in the coastal plain late tonight. The jet is a bit weaker than this morning and inversion is stronger as 950 mb temps increase to 8-10C compared to +6C this morning. This should keep gusts below wind advisory criteria, with gusts to 30-35 mph late tonight. Low temps occur this evening then rising temps overnight through the 40s and near 50 along the coast by daybreak.
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&& .SHORT TERM /THURSDAY NIGHT/... Thursday... Cross sections indicate low level moisture will be locked in which will keep clouds through the day. Cold front will be dropping south across northern New Eng and models suggest sct showers will develop, especially north of the pike. In fact, there is some instability with KI over 30, TT near 50 and SWI near zero so some of the showers will be convective in nature with brief heavy rain possible. Despite cloud cover it will be another unseasonably mild day as 950 mb temps near 10C. With sunshine, would see highs into the 60s but cloud cover and shallow mixing depth will limit heating but still temps well into the 50s away from the south coast with a few locations possibly touching 60. Gusty winds will continue in the coastal plain. 950 mb winds 40-50 kt and there is some concern that if temps get warm enough mixing could increase enough to tap some of this wind. Wind gusts 30-40 mph expected in the coastal plain but low risk for 40-50 mph gusts in eastern MA and portions of RI if low level inversion is weakened. Confidence not yet high enough for a wind advisory but this will need to be further evaluated. Thursday night... Lingering showers possible into the evening as cold front moves south into the region, then expect partial clearing from north to south after midnight as front moves to the south and drier W/NW flow develops. Lows mainly in the 30s interior but lower 40s near the coast. && .LONG TERM /FRIDAY THROUGH WEDNESDAY/... Big Picture... Flat flow aloft for much of the long term period. Closed upper low initializing over the Pacific Northwest sweeps down the coast to Baja California by Saturday, then turns the corner and moves into the Southern Plains Sunday. This upper low then ejects through the Great Lakes by midweek. Lowest height contours are in place over the weekend, followed by building heights early next week in advance of the ejecting upper low. This supports coldest temps over the weekend followed by a trend to milder values during the first part of next week. Model mass fields are similar through Monday, then diverge Tuesday and Wednesday. This is shown by the differing positions of the ejecting low with the GGEM fastest and ECMWF slowest. This lends to uncertainty with pcpn onset next week with the GGEM bringing pcpn to our area Monday night, the ECMWF Tuesday, and the GFS Tuesday night. Low confidence in timing this Day 6/Day 7 storm, but better confidence that a weather system will be approaching sometime during that time period. We used a general blend of model data. Details... Friday... High pressure builds in with fair weather. Sufficient pressure gradient for a brisk northwest wind. Mixed layer reaches 950 mb, with 25 to 30 knot winds in the layer. Expect wind gusts in this range during the midday/afternoon. Cold advection by this northwest wind will mean cooling temps in the mixed layer through the day. Temperatures in the morning support highs in the 40s, but the cooler temps moving in would support highs only in the 30s. We will go with max temps 35 to 45. Pressure gradient lingers early in the night, then diminishes. While isobars continue to indicate a northwest wind, the spread is greater. Expect winds to diminish, with potential for radiational cooling overnight. Dew points will be in the single numbers, so cold spots inland may also dip below 10 while the remainder of the region is in the teens. Saturday through Monday... High pressure over the region means fair weather. Temps aloft Saturday support max sfc temps in the 20s, then trend milder Sunday and Monday. Min sfc temps also trend milder through the period, from 15-25 Saturday night to 25-35 Monday night. Jet streak moves across the Great Lakes/Mid Atlantic Saturday. The GFS and ECMWF show the right entrance region of this jet spinning up a coastal low off North Carolina Saturday night. With the upper flow remaining westerly, would expect this to move out to sea. Model consensus agrees with this assessment. Clouds from this system should move into areas south of the Mass Pike Saturday night, but all models agree on pcpn remaining offshore. We will need to keep an eye out for any drift north in the storm track. It seems a low probability at best, but never say never. Monday night through Wednesday... Weather system coming across the Plains to the Great Lakes. As noted above, there are timing differences between the long range models with the GGEM fastest and GFS slowest. We adjusted the blended timing of pops toward the middle solution...the ECMWF...which shows pcpn onset on Tuesday in the west and Tuesday night in the east. This suggests mainly a rain event, but with a chance of snow or ice in northwest MA later Tuesday night before a change to rain. && .AVIATION /00Z 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/... Tonight...High confidence in trends. Lower confidence in timing. Conditions lower to MVFR/IFR 05-08z as rain overspreads the region and becomes more than sprinkles. Local LIFR possible in fog. Rain exits before daybreak. SW wind gusts to 30 kt developing coastal plain 09-12z along with areas of LLWS as SW winds at 2K ft increase to 45-55 kt. Thursday...Moderate confidence. MVFR/IFR in the morning improving to MVFR. Scattered showers developing, especially interior in the afternoon. SW gusts to 35 kt possible in the coastal plain. Thursday night...Moderate confidence. Areas of MVFR, but improving conditions overnight from north to south. A few evening showers possible. SW winds diminishing and becoming W/NW overnight. KBOS TAF...High confidence in trends. Lower confidence in details. A period of IFR possible late tonight but confidence is low so kept cigs in MVFR category. KBDL TAF...High confidence in trends. Lower confidence in details. A period of IFR possible late tonight. Outlook /Friday through Monday/...Moderate confidence. VFR through the period. Gusty northwest winds Friday with speeds reaching 25 to 30 knots. Gusts will diminish Friday night. An ocean storm passes south of New England Sunday. There is low chance that the storm track shifts farther north, which could move MVFR cigs/vsbys into our South Coast region.
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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 Thursday Night/... Tonight and Thursday...Moderate confidence. Increasing SW winds after midnight and continuing Thu as low level jet develops. Low level inversion should limit gusts to 30 kt but low risk for gale force gusts during Thu for Boston Harbor and Narragansett Bay where low level mixing will be greatest. Thursday night...Moderate confidence. Gradually diminishing SW winds becoming west overnight. SCA gusts will continue into the evening. Outlook /Friday through Monday/...Moderate confidence. Friday... Northwest winds with gusts around 30 knots, and seas 5 to 8 feet. The higher seas will be on the outer waters and possibly RI sound. Winds and seas will diminish Friday night as high pressure builds over the waters. Small Craft Advisory will be needed. Saturday through Monday... High pressure moves over the waters with shifting winds less than 20 knots. Seas will mostly be 2 feet or less through the period. An offshore weather system off North Carolina will race east and stay well south of the waters Saturday night and Sunday. This may bring clouds during that time, but any snow should stay to the south. If the forecast track shifts north the next couple of days then this would be more of a concern, but there is no sign of this happening at present. && .BOX WATCHES/WARNINGS/ADVISORIES... CT...None. MA...None. RI...None. MARINE...Small Craft Advisory from 1 AM Thursday to 3 AM EST Friday for ANZ231>234-251. Small Craft Advisory from 3 AM to 7 PM EST Thursday for ANZ230- 236. Small Craft Advisory until 3 AM EST Friday for ANZ235-237. Small Craft Advisory until 7 AM EST Friday for ANZ250-254>256. && $$ SYNOPSIS...WTB/KJC NEAR TERM...WTB/KJC SHORT TERM...KJC LONG TERM...WTB AVIATION...WTB/KJC MARINE...WTB/KJC

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