Area Forecast Discussion
Issued by NWS Boston, MA

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000 FXUS61 KBOX 212220 AFDBOX Area Forecast Discussion National Weather Service Taunton MA 620 PM EDT Fri Oct 21 2016 .SYNOPSIS... Sub-tropical low pressure lifting north toward southern New England will bring numerous showers and isolated t-storms with locally heavy rainfall tonight. Additional showers are likely Saturday as the storm moves into northern New England along with strong winds developing late Saturday into Saturday night. Dry but cool and blustery conditions Sunday into early next week. && .NEAR TERM /UNTIL 6 AM SATURDAY MORNING/... 615 PM Update... We have been issuing Flood Warnings and Advisories due to some convective rainfall training taking place across portions of eastern CT, western RI, and central MA. This looks to continue for a while longer and will need to be watched closely in the event of the potential for any localized flash flooding. Prior Discussion... Active pattern in near and short term as high amplitude and negatively tilted upper trof swings through. Subtropical moisture becomes entrained this evening as reflected in a period of precipitable waters to 2 inches across southeast New England. The moisture plume is focused on the nose of a low level 40 knot ESE jet. High resolution models focusing heavy rain across eastern MA from south to north early to mid evening. This could result in urban street and poor drainage flooding. Isolated thunder this evening is not out of the question. Breaks in the clouds have allowed temperatures to rise into 70s across much of the area this afternoon. The temperatures will drop into the 60s toward evening but then likely hold given the high dewpoints. && .SHORT TERM /6 AM SATURDAY MORNING THROUGH 6 PM SATURDAY/... Notable diffluence aloft Saturday morning into early afternoon is anticipated just ahead of rigorous negative tilt 500 mb trof axis along with support of approaching left exit region of 110 to 120 kt upper jet. Have gone with likely POPs for a time Saturday in anticipation of a band or two of heavy showers that will probably rotate across CT, RI, and at least central/east central MA. Cold pool aloft will also introduce an element of instability in the afternoon in addition to a probably comma head wrap around. This is inducement to keep POPs elevated across western and central sections. Expect warmest temperatures to be in the early morning hours with a significant drop west to east during Saturday afternoon. This should be especially noticeable over the higher terrain of western MA. A surge of strong W to NW winds is anticipated toward evening. It will be first noted across the east slopes of the Berkshires. A Wind Advisory will likely be needed with a start time possibly as early as late Saturday afternoon. Per earlier coordination with surrounding offices, this will probably be considered with a subsequent shift after timing, extend, and magnitude become clearer. && .LONG TERM /SATURDAY NIGHT THROUGH FRIDAY/... Highlights... * Gusty winds expected Saturday night and Sunday * Temperatures drop below normal Sunday and continue dropping through Tuesday/Wednesday time frame * Mainly dry weather through this time with a few areas of showers possible on several days Models are in good agreement through much of the long term, particularly on the synoptic scale pattern. There are some discrepancies, but these are largely on the smaller scale details as well as timing. The pattern over the next week will be largely dominated by upper level troughing in the northeastern U.S. This will keep unsettled weather in the region for much of the time, along with below normal temperatures. Saturday night and Sunday...The main concern will be strong pressure rises combined with a tight pressure gradient and decent cold air advection that will be moving over southern New England. This will result in breezy conditions beginning sometime Saturday afternoon and continuing into Sunday. While winds will likely diminish a bit during the overnight period due to the loss of sunshine and a decline in mixing, the pressure rises and cold air advection will act to continue mixing and gusty winds will continue. Wind advisory criteria may be reached Saturday evening and again on Sunday morning but confidence is low in which areas and how frequently gusts may exceed criteria, so have opted to hold off on a wind advisory at this time. Temperatures will be seasonable through this period, though the drop in temperature from today added to the winds will make it feel much cooler. Monday through Wednesday...The upper level trough slowly moves offshore and low pressure moves into the Maritimes. Temperatures continue to fall with Tuesday and Wednesday likely the coldest of this fall so far. A shortwave moves through the upper level flow on Monday, possibly bringing a few showers to the region. High pressure starts building into southern New England Tuesday and Wednesday. Temperatures will be cool enough these days to possibly induce some ocean effect clouds and showers on the Cape and Islands. Thursday and Friday...A bit more uncertain with the timing here. The GFS is a bit slower with the progression of the upper level pattern than the ECMWF. Shortwave ridging will move over southern New England either Wednesday or Thursday before another shortwave trough moves through. This trough may bring another chance for some showers either late Thursday or Friday. Temperatures rebound a bit, climbing back to normal or even a few degrees above normal. && .AVIATION /22Z FRIDAY THROUGH WEDNESDAY/... Forecaster Confidence Levels. Low - less than 30 percent. Moderate - 30 to 60 percent. High - greater than 60 percent. Short Term /through Saturday/... Rest of this afternoon into tonight...High confidence. Anticipate general lowering of ceilings and visibilities due to high dewpoint air and numerous showers further saturating the lower atmosphere. Anticipate fairly widespread IFR conditions tonight with areas of LIFR conditions due to low clouds and fog/drizzle. Pockets of heavy rain appear likely this evening over eastern MA. Isolated thunder is possible as well this evening. Saturday...Moderate confidence. More showers, some heavy, are anticipated morning into early afternoon across central and northeast portions of the area. Still more showers are expected to expand underneath a developing comma head across western MA and NW CT during the mid and late afternoon. Increasing W to NW winds late Saturday afternoon to the W of a FIT- ORH-CEF-BDL line. The strong winds will move toward eastern sections toward evening. KBOS TAF...High confidence in trends but lower confidence in details. KBDL TAF...High confidence in trends but lower confidence in details. Outlook /Saturday Night through Wednesday/... Saturday night...Moderate confidence. Improving conditions to VFR. Westerly winds increase, gusting to 25 to 30 kts, possibly up to 35 to 40 kts on the south coast, Cape, and Islands. Sunday...High confidence. VFR. Gusty NW winds continue but will diminish slowly. Monday through Wednesday...Moderate confidence. Mainly VFR. Low confidence in MVFR/IFR conditions developing on the Cape and Islands in ocean effect clouds and rain showers Tuesday and Wednesday. && .MARINE... Forecaster Confidence Levels. Low - less than 30 percent. Moderate - 30 to 60 percent. High - greater than 60 percent. Short Term /through Saturday/...High confidence. Near term issue will be areas of fog including pockets of dense fog. There could also be some brief gusty winds in vicinity of showers this evening but in general below small craft advisory criteria. For Saturday, there will be increasing SW winds during the early/mid afternoon becoming W during late afternoon, and a Gale Warning is posted for all waters. Outlook /Saturday Night through Wednesday/... Saturday night and Sunday...High confidence. Westerly gale force winds will continue through this period with a tight pressure gradient between the departing low pressure system and building high pressure in the Great Lakes. Westerly winds will shift to the northwest on Sunday. Gale warnings have been extended for all waters through Sunday. Sunday night and Monday...High confidence. Small craft advisory conditions continue though both winds and seas will diminish gradually through the period as the pressure gradient slackens. Tuesday and Wednesday...Moderate confidence. Marginal small craft conditions continue, particularly on the outer waters Tuesday and Tuesday night. Winds and seas will be diminishing through this period as high pressure builds over the waters. && .TIDES/COASTAL FLOODING... We are now coming off the relatively high astronomical late afternoon tide and will let the Coastal Flood Advisory for Nantucket Harbor expire at 7 pm. Looks like the tide crested at about 4.9 feet, near the 5 foot flood stage level, with a surge near .9 feet at the time of the high tide. The next astronomical tide along both the south and east coasts is lower and do not anticipate any flooding. && .BOX WATCHES/WARNINGS/ADVISORIES... CT...None. MA...Coastal Flood Advisory until 7 PM EDT this evening for MAZ024. RI...None. MARINE...Gale Warning from noon Saturday to 6 PM EDT Sunday for ANZ231>235-237-250-251-254>256. Gale Warning from 2 PM Saturday to 6 PM EDT Sunday for ANZ230- 236. && $$ SYNOPSIS...RLG/Thompson NEAR TERM...Thompson SHORT TERM...Thompson LONG TERM...RLG AVIATION...RLG/Thompson MARINE...RLG/Thompson TIDES/COASTAL FLOODING...Thompson is the U.S. government's official web portal to all federal, state and local government web resources and services.