Area Forecast Discussion
Issued by NWS Boston, MA

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000 FXUS61 KBOX 191503 AFDBOX Area Forecast Discussion National Weather Service Taunton MA 1003 AM EST Thu Jan 19 2017 .SYNOPSIS... High pressure builds over the region into Friday. Weak disturbances, one from the southern stream and one from the southern stream, will race through New England this weekend, ultimately allowing colder air to drain into parts of Southern New England by Sunday. Developing low pressure over the Southern Plains will bring rain and possibly some initial ice to New England Monday, with the rain lingering into Tuesday. High pressure builds Wednesday with drier air. && .NEAR TERM /UNTIL 6 PM THIS EVENING/...
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10 am update... The combination of a building subsidence inversion and diurnal mixing this morning is allowing much of the low lvl cloud cover to erode. This trend will continue, although a few spots mainly in the coastal plain and lower valleys may hold on to some clouds/fog for a few more hours before things can completely scour out. WX/Skies will be updated. Otherwise, did adjust temps/dwpts which were cooler due to the slower cloud erosion. 2M temps generally better than MOS so leaned mostly on these solutions.
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&& .SHORT TERM /6 PM THIS EVENING THROUGH 6 PM FRIDAY/... Tonight... A tricky forecast depending on which model solution you wish to believe. High pressure parked across the region with an attendant warm-dry inversion up around H9 is well agreed upon per consensus. What differs is the amount of low level moisture beneath, as to whether we`ll get low clouds and/or fog. Interrogating SREF probabilities, confidence is weighed more N/W in regards to low cloud / fog threats where snow pack resides that will have likely melted to some degree earlier in the day despite some uncertainty on cloud cover as highs are forecast around the low 40s. So believe we`ll be dealing with a combination of both: low clouds below 1 kft agl with subsequently the higher elevations becoming socked in with fog, and perhaps with some cold air drainage into the valleys we`ll see fog. A first guess, ran with a local fogger tool procedure wherever the dewpoint depression was 1 or less and winds were at or near calm. This put out patchy fog in the CT River Valley and across the high terrain of the Berkshires. Will monitor it closely. May have to make some adjustments depending on how conditions evolve during the day, perhaps if it ends up being drier with lower dewpoints. Have left sky cover as scattered to broken for the time being. May need to lean more overcast N/W. Friday... Ridge of high pressure in place and again a tricky forecast. The inversion remains stout around H9 as suggested by some of the high- res guidance. However the SREF guidance as of 19.03z seemingly suggests that we mix out. Will lean with scattered to broken cloud decks during the day mostly. However, will begin to see return S flow associated with the ascending branch of the warm-moist conveyor belt in tandem with a S stream shortwave trying to break through the stout ridge across the NE CONUS. Isentropic over-running along a warm frontal boundary lifting NE through the Mid-Atlantic, subsequent moisture looks to dry out as it approaches. Will keep the forecast period dry with only a slight chance for far SW Hartford County CT. Sprinkles if anything. Otherwise another mild day with highs around the low 40s with light S winds. && .LONG TERM /FRIDAY NIGHT THROUGH WEDNESDAY/...
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Big Picture... Large scale pattern initially shows trough in the West and ridge in the East. Shortwave ejecting from Pacific NW storm Friday digs as it sweeps across the Southern USA over the weekend. This carves out a negative-tilt trough over the Eastern USA by Monday. This trough then slowly pulls out to the east early next week. The remainder of the Pacific NW storm digs into the desert Southwest as high pressure rebuilds behind the eastern storm. This rebuilds a semblance of the trough-west/ridge east pattern, but with lower/colder heights. The GFS and ECMWF mass fields remain similar through Sunday, then diverge on the details Sunday night through Wednesday. The position of the East Coast storm is closer to the coast and slower on the ECMWF, while the GFS and GGEM are faster in moving it past on Tuesday. As such, forecast confidence is good through Sunday, then low early next week. Expect confidence to remain low until the first part of the Pacific storm moves ashore Friday night or Saturday. Details... Friday night through Sunday... High pressure ridge builds over New England. Even with this in place, two shortwaves move through the region. The first, a southern stream system, slides through on Friday night. Cross sections of relative humidity show deep moisture will be over CT and aided by marginal lift at best. Will mention chance pops over CT and parts of RI and Western MA. A second shortwave moves south from Quebec on Saturday night. This will bring limited moisture and no support for lift. The alignment on this system resembles a backdoor cold front. Expect some clouds but no pcpn Saturday night. The main effect will be for colder air to settle south into New England. Cross sections show moisture lingering below 850 mb through the weekend. So expect a best-case of partly cloudy, with periods of mostly cloudy skies. Sunday night-Monday-Tuesday... The main concern for weather takes place during this period. Low pressure takes shape over the Southern Plains, sweeps to the Southeast coast, then moves up the coast and spreads precipitation into New England Sunday night or Monday. High pressure building south Sunday night will bring the typical signal for cold air damming. Low level ageostropic flow is currently less revealing. It shows strong drainage flow Sunday night, but from the NW rather than the usual direction of North. Meanwhile, the building of the ridge over Gaspe and New Brunswick is in a location that could block the northward movement of any coastal system from the south. Because of the high, and because the upper low is forecast to become negative tilted Monday, we have slowed the onset of pcpn over our area. Even so, we indicate chance pops Sunday night and the best pops for Monday and Tuesday. Temperature profiles suggest that Western and Central MA and parts of Northern CT may see a period of freezing rain or sleet Sunday night before milder air starts working in on Monday. Eastern MA and RI will be facing an increasing East wind which should keep temps in the coastal plain above freezing. Also expecting a windy period. The low level jet will shift north up the coast Monday. Winds in the core of the jet are forecast to reach 60 knots. So coastal areas could be seeing surface gusts of 30 to 40 knots. Most at risk would be Cape Cod and Nantucket. As the surface system moves up, it will nudge warmer air into our area. We expect temperatures to rise much of Monday night, with coastal plain temps reaching the lower 50s Tuesday while the interior is in the 40s. The storm moves off to the northeast Tuesday night. At this point, expect precipitation to wind down, although with lingering clouds and drizzle much of the night. Wednesday... High pressure builds over the region with sunshine and drier air.
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&& .AVIATION /15Z THURSDAY THROUGH MONDAY/... Forecaster Confidence Levels... Low - less than 30 percent. Moderate - 30 to 60 percent. High - greater than 60 percent. Short Term /through Friday/...Moderate Confidence. 9z update... Today...IFR cigs in Southeast MA and patchy spots elsewhere. Also patchy MVFR cigs in the west and north. Light fog lingers in a few spots, but most areas are at VFR or will be there this morning. High pressure builds in during the day. Partial clearing has already moved into the Greater NYC area, and this should spread across Southern New England by afternoon. Tonight...Monitoring for the potential development of MVFR-LIFR cigs and vsbys more so across N/W MA and CT terminals, including the CT River Valley. Low confidence at this time. Will keep it mostly VFR for the time being. Light winds. Friday...Possibly some MVFR-LIFR conditions in the morning though with uncertainty given the low confidence forecast earlier. Should at least see increasing mid to high level clouds out of the SW during the day, lowering down to low-end VFR. A possible sprinkle into central CT. Light S winds. KBOS TAF...Moderate confidence. Conditions gradually improving toward daybreak. Exact timing uncertain. KBDL TAF...Moderate confidence. Conditions gradually improving late tonight. Exact timing uncertain. Outlook /Friday Night through Monday/... Moderate confidence. Friday night through Sunday... Plenty of low level moisture for clouds, but not much for any precipitation. Areas of MVFR cigs, but unrestricted vsbys. Winds from the south Friday morning shift out of the west by Saturday and then out of the east Sunday. Sunday night through Monday... Ceiling lowering to IFR/LIFR. Rain and possible freezing rain inland Monday, with rain along the coast. Vsbys becoming 2 to 4 miles in rain and fog. East winds increase with potential gusts to 35 knots Monday. && .MARINE... Forecaster Confidence Levels... Low - less than 30 percent. Moderate - 30 to 60 percent. High - greater than 60 percent. Short Term /through Friday/...Moderate Confidence. Winds have diminished, so small craft advisories were adjusted to account for hazardous seas. The headline will continue through Friday morning by which point the presence of building high pressure and light winds should allow seas to subside below advisory thresholds. Outlook /Friday Night through Monday/...Moderate confidence. Friday night through Sunday... Variable wind becomes WSW but with speeds less than 20 knots. Winds shift Sunday from the ENE with speeds near 20 knots. Seas remain 4 feet or less. Sunday night-Monday... Rain moves up from the south as a deepening coastal storm approaches and moves up against high pressure over the Maritimes. This will bring increasing winds from the east. Winds at 2000 feet above the surface will reach 60 knots, bringing a potential for 35-40 knots at the surface. This wind will build seas, with heights up to 15 feet possible on the outer waters. Rain and fog will bring occasional poor vsbys on the waters. Based on expected winds, a gale watch/warning may eventually be needed for this time period. && .BOX WATCHES/WARNINGS/ADVISORIES... CT...None. MA...None. RI...None. MARINE...Small Craft Advisory for hazardous seas until 1 PM EST Friday for ANZ250-254-255. Small Craft Advisory for hazardous seas until 10 AM EST Friday for ANZ251. Small Craft Advisory for hazardous seas until 6 AM EST Friday for ANZ256. && $$ SYNOPSIS...WTB/Sipprell NEAR TERM...Doody SHORT TERM...Sipprell LONG TERM...WTB AVIATION...WTB/Sipprell MARINE...WTB/Sipprell is the U.S. government's official web portal to all federal, state and local government web resources and services.