Area Forecast Discussion
Issued by NWS Boston, MA

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000 FXUS61 KBOX 190200 AFDBOX Area Forecast Discussion National Weather Service Taunton MA 1000 PM EDT Sat Mar 18 2017 .SYNOPSIS...
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An ocean storm will pass far enough E of our region to prevent significant snowfall across SE New England through Sunday but it still will be very windy across the Cape and Islands. The highest risk is across SE MA, especially Cape Cod and the Islands. Drier and warming temperatures follow Monday and Tuesday with another arctic cool mid to late week.
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&& .NEAR TERM /UNTIL 6 AM SUNDAY MORNING/...
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10 pm update... This continues to be a very tough and challenging forecast. In brevity, evaluating low to mid level forcing especially within H85-7and in regards to vorticity and enhanced ascent. This coupled with H6-8 moisture availability and mesoscale lift in tandem with moisture trowaling and increasing deformation within the N/W quadrant of the developing offshore low with associated mesoscale snow banding seemingly remaining far enough E of our region. Continued signs of outcomes across the Cape and Islands, especially, developing after midnight and continuing through the morning hours. It would appear that only the 18.18z 3 km NAM and 18.21z SREF are handling present outcomes well enough. Latest WSR-88D reflectivitity shows the mid-level energy shooting through the lower Hudson Valley into CT likely to usher the deepening process in regions of more favorable baroclinicity offshore, enhancing energy already rotating N from the Mid- Atlantic within deep-layer column troughing pattern. So am confident the banding signature is going to begin developing and kick off. However still uncertain is the proximity to Cape Cod and the Islands as well as its magnitude. The HRRR is keeping a tight gradient along the I-95 corridor with impacts over Plymouth and Bristol Counties encompassing all of the Cape and Islands. However the 18.18z 3 km NAM and even the HRRR time-lagged ensemble is keeping outcomes mainly confined to the Cape and Islands, further offshore. Will keep with the going forecast for now, evaluate 19.0z guidance. If something really stands out then will make changes accordingly. Right now there is not much to work with given the continued spread even within the near-term high-res guidance. Light snow continues to impact mainly CT but will see this area transition S/E later evolving into the forecast NE-SW banding signature anticipated over Cape Cod and the Islands. Little if any impact with the initial light snow, only a coating of accumulation is expected with some minor reductions to visibility.
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&& .SHORT TERM /6 AM SUNDAY MORNING THROUGH 6 PM SUNDAY/... Highlights... * Snow potential has diminished across southeast New england very late tonight/Sunday with any impact likely to be low * Strong to locally damaging wind gusts still possible across the Cape/Islands on Sunday The vast majority of the 12z models guidance has trended further east with the developing ocean storm very late tonight into Sunday. The big outlier was the RGEM...which continues to indicate a significant wet snow event for the extreme southeast New England coast/Cape and Islands. Based on the rest of the model guidance...we consider the 12z RGEM a very low probability event and sided with the vast majority of the guidance. Diminished Snow Potential.... Appears the shortwave energy dropping south into the Mid Atlantic will not dig far enough to the west and south. This allows the developing ocean storm to escape further east and should spare the southeast New England coast heavy wet snow. While some light to briefly moderate snow may affect southeast MA and especially Cape/Islands on Sunday it probably results in low impact. Given the strongest forcing likely remaining offshore...coupled with a relatively mild boundary layer/increasing March sun angle it probably will be difficult for snow to accumulate particularly on paved surfaces. An inch or two of wet snow is certainly possible...but the bulk of that will probably be confined to grassy surfaces resulting in a low impact. Precipitation may also be mixed with light rain at times across portions of the Cape/Nantucket given mild boundary layer. We may also have some snow showers at times further inland across eastern MA/RI...but likely with little impact as surface temps should be above freezing. Winds... Despite a further offshore track...NNE winds and steep lapse rates from 950 mb to the surface will allow for some decent winds across the Cape and Islands. The strongest winds look to occur from late Sunday morning through Sunday afternoon. We went with a wind advisory for the Cape/Marthas Vineyard for expected 45 to 55 mph wind gusts. Opted to maintain the High Wind Watch for Nantucket as the potential exists for 50 to 60 mph wind gusts. Since this is not expected until the 2nd period...felt there was enough time for the next shift to examine and decide on a potential upgrade to a warning or to just go with an advisory. && .LONG TERM /SUNDAY NIGHT THROUGH SATURDAY/... Highlights... * Moderating temps Mon and Tue with 50 possible for some Tue! * Arctic blast with near record cold returns middle of next week * Moderate warming returns late week-weekend Overview and model preferences... Noting that differences from previous runs and all runs from 18.12Z are agree well. Therefore, will not be leaning too heavily on any given model with this update, save for maybe ensembles from both GEFs and ECEN to well represent the totality of difference solutions. Neutral/positive PNA with positive AO/NAO one might miss the downstream blocking which is fed by the sharp trof/cutoff moving E Mon. This allows for overall deepening of an arctic wave moving out of the NW Territories by mid week, which leads to another round of anomalous arctic cold for mid-late March. Therefore, the brief early week warm-up is short lived followed by another very cold outbreak similar to last weekend. Will need to watch a baroclinic zone stalled to the S as this upper shortwave settles across New England, while frontal wave energy looks to be weak and too far S for any impact on S New England, the one minor difference in latest models is a slight shift N of the precip shield. For now will continue with persistence in highlighting a mainly dry forecast. Details... Sun night into Mon... Deepening low pres shifts further E ending any remaining inland snows save for some ocean effect snows lingering for the Cape Mon night. Ocean enhancement is minimal as H85-SST delta-T values are generally around 10C or less, but still provide enough lingering lift to yield light SHSN and the continuation of clouds across the Cape/Islands into Mon. Pres gradient remains strong too across the SE, suggesting winds still gust 30-40 mph at times. Meanwhile inland regions clear such that Mon is a mainly sunny day. Min temps drop to 20s Sun night but max temps rebound into the 40s on Mon thanks to H85 temps warming to -2C or 0C. Tue... Arctic cold front will be making a late day passage (although cold advection aloft begins by afternoon). With H85 temps still near 0C or only as cold as -2C temps could make another run into the mid 40s with a few spots possibly reaching near 50. Timing of the cold advection and snowcover will play a role in this, therefore, the best chance for any one location reaching 50 will be SE MA and RI where snowpack is at a minimum to null. The FROPA is mainly dry as the column suggests little moisture but a brief sprinkle or flurry cannot be ruled out given the steepening of low-mid lvl lapse rates in response to the cold advection. Wed and Thu... Reinforcing arctic shortwave will need to be watched, mainly due to weak inverted trofing possible as frontal wave develops along the stalled arctic front to the S. For now, best F-gen and deformation remains well S of the 40 parallel, which suggests maintaining the dry forecast. Still a slight shift N is apparent with this set of runs which bears watching for early Wed AM. Confidence is very high however, that H85 temps will be dropping on their way to -20C (+2 std deviations below climatological normal as noted by the previous forecaster, and near or even below the CHH H85 temps for March 22-23). This will yield another round of temperatures well below seasonable normals with the risk for lows and low-max records to be reached or broken. Highs in the mid-upper 20s and low 30s each day with lows in the single digits are possible. More refined as we approach. Fri and Sat... Trof will shift to the E allowing gradual ridge building with connection to the Gulf of Mexico. Although an inside runner is currently suggested the wave may actually weaken as it interacts with the ridge. Will be watching for a some wet precipitation, but confidence in exact timing is low at this time. Once again, however, confidence in temps rising to near or above normal. H85 temps avg near +6C by late Fri and +8C by Sat amongst ensembles. This suggests highs in the 50s and even low 60s are in play. && .AVIATION /02Z SUNDAY THROUGH THURSDAY/...
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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 Sunday/... 2z update... Overnight...Moderate confidence. VFR lowering to MVFR after midnight. Some -SN initially with some minor visibility impacts. Both linger into the early morning hours prior to -SN focusing across the Cape and Islands roughly 07Z-10Z where visibility occasionally drops to MVFR. Increasing E/NE winds with gusts up to 30 kts for KACK. Sunday...Moderate confidence. MVFR cigs and wet SN for the Cape and Islands mixing with -RA. Any accums probably confined to grassy surfaces. May also see some SHSN further northwest towards the I-95 corridor but with little impact. NNE winds should increase to between 35 and 45 knots across Cape/Islands by late morning and continue into the afternoon. A few gusts near 50 knots are possible across KACK. KBOS TAF...Moderate confidence in TAF. A few SHSN possible but expect runways to remain wet. KBDL TAF...Moderate confidence in TAF. SHSN ongoing, dissipating after midnight. Little to no impact. Wet runways. Outlook /Sunday Night through Thursday/... Sun night into Mon...High confidence. VFR develops inland, with MVFR and occasional IFR lingering Cape/Islands mainly due to low CIGS, but some lowered vsbys in light SN may linger as well. Winds inland drop yielding gusts mainly 20-30 kt. Meanwhile Cape/Island gusts may hold 30-45 kt into Mon morning. Tue and Wed...High confidence. Mainly VFR. W-NW wind gusts 20-35 kt mainly on Wed, but occasional gusts to this magnitude possible Tue as well. Thu...High confidence. VFR. Lighter winds.
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&& .MARINE...
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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 Sunday/... Tonight and Sunday...Moderate confidence. Developing ocean storm coupled with high pressure across E MA will allow winds to increase very late tonight and especially Sunday. Strongest of these winds will be from mid morning Sunday into the afternoon. Therefore...left Storm Watches up for Cape Cod Bay / Nantucket sounds points S/E where its marginal for a period of gusts up to 50 knots. Across our other waters converted Gale Watches to Gale Warnings...except maintained Watches across Boston Harbor / Narragansett Bay. Outlook /Sunday Night through Thursday/... Sun night through Mon night...High confidence. Northerly gales continue with gusts 35-45 kt at times only dropping below 34 kt by late Mon morning. Small craft advisories will follow with gusts to 25 kt into Mon night. Seas peak late Sun night. 14-16 ft on the ocean waters, dropping below 10 ft by Mon afternoon. Seas will take until Tue to fully drop below small craft advisory thresholds. Tue...Moderate confidence. Westerly winds may approach 25 kt and allow seas on the ocean waters to hold near 5-6ft. Therefore Small Craft Advisories may be needed for at least a portion of the waters on Tue. Wed and Thu...Moderate confidence. NW winds once again gust 25-30 kt with seas around 5-7ft Wed. Small Craft Advisories may continued. Thu, winds and seas may finally drop below Small Craft Criteria for a bit by mid day. Otherwise, very cold temperatures could yield some light freezing spray at times Wed into Thu, but these will be dependent on final wind speeds.
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&& .TIDES/COASTAL FLOODING... Coastal flooding appears unlikely from this storm. The main concern would be Cape Cod Bay with strong NNE wind gusts early Sunday evening with a 2 foot storm surge. However...relatively low astronomical high tides should prevent any problems. && .BOX WATCHES/WARNINGS/ADVISORIES... CT...None. MA...Wind Advisory from 10 AM to 8 PM EDT Sunday for MAZ022-023. High Wind Watch from Sunday morning through Sunday evening for MAZ024. RI...None. MARINE...Storm Watch from Sunday morning through Sunday evening for ANZ231-232-254-255. Gale Warning from 5 AM Sunday to 6 AM EDT Monday for ANZ233>235-237. Gale Watch from 5 AM EDT Sunday through Sunday evening for ANZ230-236. Gale Warning from 5 AM Sunday to 8 AM EDT Monday for ANZ250- 251-256. && $$ SYNOPSIS...Frank/Doody NEAR TERM...Frank/Doody/Sipprell SHORT TERM...Frank LONG TERM...Doody AVIATION...Frank/Doody/Sipprell MARINE...Frank/Doody/Sipprell TIDES/COASTAL FLOODING...WFO BOX Staff

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