Area Forecast Discussion
Issued by NWS Boston, MA

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086 FXUS61 KBOX 211404 AFDBOX Area Forecast Discussion National Weather Service Taunton MA 1004 AM EDT Sat Oct 21 2017 .SYNOPSIS... High pressure centered over Pennsylvania Saturday will move offshore Sunday, bringing warm, dry weather to our region. A cold front with possibly waves of low pressure will bring showers to the area Tuesday afternoon into Wednesday. Drier and cooler weather is expected for late in the week. && .NEAR TERM /UNTIL 6 PM THIS EVENING/... 10 AM Update... Shallow cool surface layer from last night has mixed out thanks to WSW winds. 10 am temps already jumped into the low and mid 60s across most of eastern MA and RI including Boston to Providence. This will set the stage for another top ten day with temps on their way into the mid and upper 70s, except closer to 70 along the south coast including Cape Cod and the Islands where SSW winds off the ocean will keep these locales slightly cooler. This is courtsey of deep layer ridge across PA/NY into Ontario and Quebec, providing dry NW flow aloft and associated subsidence. 1028 mb surface high over central PA will advect eastward to just south of New England. This will support SSW winds across the area, however a seabreeze may briefly develop from 11 am to 2 pm across the eastern MA coast before flipping back to the SSW mid afternoon. It will feel even warmer than the mid and upper 70s given full sunshine and light winds. Very comfortable too with dew pts in the 40s. Previous forecast captures these details so no major changes planned with this update. && .SHORT TERM /6 PM THIS EVENING THROUGH 6 PM SUNDAY/... Center of a high pressure continues to move east of southern New England during this time. Dry weather with mostly clear skies will continue, with only some passing high clouds at times. South to southwest winds also continue, meaning temperatures remain well above normal. && .LONG TERM /SUNDAY NIGHT THROUGH FRIDAY/... Highlights... * Heavy showers and gusty winds are possible ahead of a cold front Tuesday afternoon into Wednesday There is strong consensus in medium range models that the strong ridge over the eastern United States will break down for at least a time this coming week. There is fairly high confidence in one more warm, dry day on Monday as well as a high amplitude trough bringing an anomalous plume of moisture and low level southerly jet into the region for the Tuesday into Wednesday time frame. Thereafter, confidence drops as medium range models begin to diverge with respect to both mass and thermal fields for the latter part of the week. Most of the focus for this forecast period is on the Tuesday through Wednesday time frame. Monday...Fairly confident of one more day with lots of sunshine and an air mass capable of high temperatures again in the 70s. May start to see some high clouds spill over the upper ridge during the afternoon. Tuesday through Wednesday...There is fairly good medium model consensus and ensemble support for the approach of a high amplitude upper trough and associated surface cold front approaching the east coast. One or more surface waves of low pressure may form along this slow moving front. A low level southerly jet of 50 to 60 knots is anticipated for Tuesday night with precipitable waters of 1.5+ inches. Some instability is indicated in the models with K indices above 30, showalters near to a little below zero, and even total totals in the upper 40s. The instability combined with the low level jet forcing in an anomalously moist air mass may set the stage for heavy showers/downpours as well as isolated embedded thunder. In addition, a warm thrust in the lower levels may allow some mixing of the low level jet energy to the surface, even if at at night. Thus, gusty winds possibly strong enough for headlines may be also be an issue. The initial surge will probably pass east or northeast of the area during the day Wednesday with showers diminishing or ending across at least most of the area. Thursday...Have kept chance POPs in the forecast due to upper trough axis probably passing across with a cold pool aloft and to some extent for the sake of forecast continuity. It is possible that any showers Thursday may be widely scattered. The ECMWF 00Z operational run seems to have trended closer to the last couple of GFS runs. Friday...For now Friday looks dry with the upper trough passing east of the region and at least a short wave ridge moving across. Unlike most of the month, we may see temperatures below normal with H850 temperatures likely only 0C to 4C. The latest ECMWF run is now more in line with the GFS operational run and holds off the warm advection until Saturday. && .AVIATION /14Z SATURDAY THROUGH WEDNESDAY/... Forecaster Confidence Levels... Low - less than 30 percent. Moderate - 30 to 60 percent. High - greater than 60 percent. Short Term /through Sunday/...High confidence. 14z update... Not much change from 12z TAFs. Only item to watch is potential brief seabreeze for eastern MA coastline from 15z-18z when winds will briefly switch from SSW to SSE then flipping back to SSW after 18z. Otherwise previous discussion below. ================================================================= High pressure provides VFR, dry weather and relatively light winds through Sunday. Main issue in the near term is if a sea breeze is possible at BOS and other east coast locations. The surface gradient is certainly slack enough to suggest a sea breeze but there looks to be enough vertical mixing to keep the surface wind offshore, albeit light. KBOS Terminal...High confidence in TAF. KBDL Terminal...High confidence in TAF. Outlook /Sunday Night through Wednesday/... Sunday Night: VFR with local IFR in fog. Monday: VFR. Monday Night: VFR, with local IFR possible in fog. Tuesday...Conditions lowering to MVFR ceiling and visibilities in showers spreading west to east. Tuesday night...MVFR ceilings/visibilities with areas of IFR/LIFR in showers and fog. A period of low level wind shear followed by strong southerly surface gusts possible. Wednesday...IFR persisting across the Cape and Islands. Elsewhere, MVFR improving to VFR from west to east. && .MARINE... Forecaster Confidence Levels... Low - less than 30 percent. Moderate - 30 to 60 percent. High - greater than 60 percent. Short Term /through Sunday/...High confidence. Very tranquil boating weather for late Oct with high pressure moving across the waters through Sat night. Expecting light winds, dry weather and good vsby through this period. Outlook /Sunday Night through Wednesday/...Moderate confidence. Increasing southerly winds Tuesday may contain gale force gusts late Tuesday through early Wednesday. Will mention in HWO. Seas in excess of 12 feet possible in the outer southern coastal waters due to long fetch of winds possibly gusting to gale force. && .FIRE WEATHER... Dry weather continues through at least Sunday with minimum RH values Saturday down to 25 to 35 percent. However with high pressure nearby winds will be very light. Thus fire weather parameters remaining below headline criteria. && .CLIMATE... Record high temperatures for Saturday, October 21: BOS 82 in 1920 BDL 82 in 1920 PVD 81 in 1920 ORH 80 in 1920 && .BOX WATCHES/WARNINGS/ADVISORIES... CT...None. MA...None. RI...None. MARINE...None. && $$ SYNOPSIS...Belk/Thompson NEAR TERM...Belk/Nocera/Thompson SHORT TERM...Belk LONG TERM...Thompson AVIATION...Belk/Nocera/Thompson MARINE...Belk/Thompson FIRE WEATHER...staff CLIMATE...staff

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