Area Forecast Discussion
Issued by NWS Boston, MA

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000 FXUS61 KBOX 170457 AFDBOX Area Forecast Discussion National Weather Service Taunton MA 1257 AM EDT Tue Oct 17 2017 .SYNOPSIS... High pressure will maintain dry weather this week along with a gradual warming trend. && .NEAR TERM /UNTIL 6 AM THIS MORNING/... 100 AM Update... Northerly flow is keeping clouds locked in around Cape Cod and Islands, and even few light rain showers appearing on radar on outer Cape (with light rain at PVC). Trend will be for any showers to diminish overnight, followed by clearing toward sunrise as upper flow becomes more cyclonic and N flow eases. Otherwise clear skies overnight. Still some concern that temperatures will stay just warm enough to prevent widespread frost, as many locations may stay closer to 35-37 degrees, but for now we`ll leave the Frost Advisory as is. && .SHORT TERM /6 AM THIS MORNING THROUGH 6 PM WEDNESDAY/... Tuesday... Center of high pressure moves to the Mid Atlc region but surface ridging will be in control with abundant sunshine and light winds. After a chilly start, temps will recover to the mid/upper 50s which is about 5 degrees below normal. Tuesday night... Surface ridging shifts south of New Eng with SW flow developing. Clear skies and light winds will result in another cool night but temps not as cool as tonight as weak low level warm advection commences. Lows ranging from mid/upper 30s in the colder outlying locations to the 40s along the coast. && .LONG TERM /WEDNESDAY NIGHT THROUGH MONDAY/... 355 AM update... * Highlights... - Warmer than normal this period with highs in the 70s Thu-Sat-Sun - Dry weather through the weekend Overview and model preferences... The long term is marked by a mean nearly zonal jet squeezed between longwave arctic vortex pinwheeling near the NW territories and unforgiving ridging across the S and SE CONUS which has remained a player for a few weeks now. As previous forecaster noted, there are a couple of weak shortwave perturbations which will push through this flow, however given the confluence implied within the jet, they generally remain weak and nearly open. Also, their origins within a CP airmass suggest little moisture attendant to each wave. Therefore, a mainly dry week-weekend pattern is favored, with a gradual reinforcement of the ridge to the S allowing for progressively warmer conditions, until a mid CONUS longwave is able to dry deeper moisture by mid next week. Both operational and ensemble guidance is on board, so feel a consensus blend will work for this update. Details... Wed and Thu... +1025hPa high pres begins a slow slide to the ESE across S New England. This allows overall winds to shift to more of a return flow setup, ie WSW or purely SW. Implied weak warm advection allows H85 temps to warm from an average around +8C on Wed to +10C on Thu with H92 temps near +13C at peak on Thu. BL mixing should be able to tap these values, so expecting highs to reach into the upper 60s low 70s Wed followed by low-mid 70s on Thu in spite of a few clouds from the N associated with a weak front/shortwave. Afternoon mixing should also keep dwpts in the mid 40s, which suggest that lows each night, given weak flow and little cloud cover, to reach the mid 40s in the coolest spots, warmer in the urban heat islands. Fri... Weak cold frontal passage with barely a PWAT spike to 0.5 inches early Fri, and given the lack of overall moisture only a few clouds are likely to result. This will put a modest damper on warming, as H85 temps drop back closer to +8C, and as such highs are likely to remain in the mid-upper 60s rather than the 70s experienced mid-week. Still dry, however as the moisture starved column is unlikely to produce any rainfall in spite of the lift implied by the early day frontal passage. Sat and Sun... Continued warming as H5 heights reach nearly 2 std deviations above normal, along with H85 temps 10-13C warmer than mid October normals. This is thanks to a gradual longwave trof development well upstream. The resulting nearly 1030hPa high pres continues the dry spell, while the rising heights/WAA suggests another surge in warmth through the weekend. At this time, H85 temps hover near +13C Sat, followed by +15C on Sun. This should allow highs to reach well into the low to mid 70s Sat and mid to even upper 70s possible on Sun, exacerbated in spots where W downsloping can be maximized. Dwpts within the high remain relatively low-seasonable, mainly in the mid 40s to around 50, so lows each night given weak flow and mainly clear skies should be able to dip to these values as well. Early next week... Deeper longwave trof upstream may finally be able to trap Gulf of Mexico moisture and advect it poleward toward the NE CONUS. A frontal boundary, although slow as it becomes parallel to the mid lvl flow may approach sometime Mon-Tue and allow for some wetting precipitation. It is with this gradual shift, and how deep or progressive the upstream wave ultimately is, where guidance begins to split more significantly. Therefore, will just need to watch as solutions converge as all the players become better sampled by remote sensing. && .AVIATION /06Z TUESDAY THROUGH SATURDAY/... Forecaster Confidence Levels... Low - less than 30 percent. Moderate - 30 to 60 percent. High - greater than 60 percent. Short Term /through Wednesday/...High confidence. MVFR ceilings near Cape Cod and Islands will scatter out toward sunrise, leaving VFR conditions today into Wed. N winds gusting to 20-25kt on Cape Cod and Islands this morning will diminish by afternoon as winds turn to W/SW areawide. KBOS Terminal...High confidence in TAF. Brief weak sea breeze possible 15z-19z but not included in TAF since it looks marginal. KBDL Terminal...High confidence in TAF. Outlook /Wednesday Night through Saturday/...High confidence. Mainly VFR through the period except some patchy ground fog at typically prone terminals each morning. && .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. Mid level trough passage tonight will bring another pulse of gusty northerly winds. A period of 25 kt gusts expected tonight with SCA for all waters. Winds diminish during Tue with light winds Tue night as high pres builds across the waters. Outlook /Wednesday Night through Saturday/... Wed into early Thu...High confidence. High pres builds over the waters with mainly quiet boating weather expected. Late Thu into Fri...Moderate confidence. A cold front will cross the waters. This will bring about a period where W winds reach around 25 kt at times, mainly late Thu and Thu night. Seas will reach 5-7 ft mainly on the E open ocean waters. Small craft advisories may be needed for at least a portion of the waters. Sat...High confidence. High pres once again returns to the waters with quiet boating weather expected. && .BOX WATCHES/WARNINGS/ADVISORIES... CT...Frost Advisory until 8 AM EDT this morning for CTZ002>004. MA...Frost Advisory until 8 AM EDT this morning for MAZ005-006- 009>014-017-018-020-021-026. RI...Frost Advisory until 8 AM EDT this morning for RIZ001-003-006. MARINE...Small Craft Advisory until 8 AM EDT this morning for ANZ231>235-237-251. Small Craft Advisory until 6 AM EDT early this morning for ANZ230-236. Small Craft Advisory until 2 PM EDT this afternoon for ANZ250. Small Craft Advisory until 5 PM EDT this afternoon for ANZ254- 255. Small Craft Advisory until noon EDT today for ANZ256. && $$ SYNOPSIS...KJC/Doody NEAR TERM...JWD SHORT TERM...KJC LONG TERM...Doody AVIATION...JWD MARINE...KJC/Doody

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