375 FXUS61 KBOX 212314 AFDBOX Area Forecast Discussion National Weather Service Boston/Norton MA 714 PM EDT Sat Mar 21 2020 .SYNOPSIS... High pressure will continue to dominate weather conditions across Southern New England this weekend, with mostly sunny conditions and seasonably cool and dry weather. Low pressure passing southeast of New England will bring snow to much of the area Monday afternoon, which should change to rain Monday evening, except across the higher elevations. Another coastal storm may affect the region Wednesday into Thursday, with some snow across the higher elevations and mainly rain elsewhere. && .NEAR TERM /UNTIL 6 AM SUNDAY MORNING/... 7pm Update... No major changes as forecast is still on track. With a sprawling surface high pressure in control, mostly clear skies and dew points in the single digits to low teens away from the coast, temperatures are expected to fall quickly after sunset. Short- term mesoscale guidance do show the surface high tracking east from the Great Lakes and strengthening somewhat, which should lead to an uptick in wind speed, particularly along the Eastern MA coast. This, combined with the higher RH in the lower levels is likely to lead to an increase in low to mid level clouds along Eastern MA around daybreak, particularly over Cape and the Islands. It is not out of the question that the low clouds (3-5k ceilings) could reach as far north and west as the Boston area. All other areas are expected to remain clear, albeit with a light E/NE breeze. Expect the lowest overnight lows to be in the East Slopes of the Berkshires, northern Worcester Hills and the usual cold spots on the coastal plains, where temperatures dip into the teens. Temperatures should hold steady in the mid to upper 20s near the coast due to the insulation effect of the clouds. 430pm Update... A large 1040 mb high pressure area is centered this afternoon over the U.P. of Michigan, extending a ridge from it across much of New England. This ridge in place will continue to govern sensible weather conditions across Southern New England tonight, with tranquil weather conditions expected. Clear skies and the very dry air mass in place will allow temperatures to fall rather quickly after sundown. Decent radiational cooling though it will be tempered by light northerly winds. Pressures do tend to rise a bit toward the second half of the evening which could allow for a modest uptick or freshening of the N to NE winds especially eastern MA, Cape Cod and the Islands. Around the pre-dawn hours, RH profiles in the 12z NAM, GFS and the 12z HREF low-level sky cover prog across Cape Cod, the Islands and southern Plymouth County indicate some lower clouds/stratocu streets. Many areas north and west of I-95 will stay clear. Lows tonight should range in the mid/upper teens into the Route 2 corridor and the east slopes of the Berkshires, and into the low-mid 20s across CT, RI, and into central and eastern MA. Stronger NE winds will keep temperatures mildest towards the Cape in the upper 20s. && .SHORT TERM /6 AM SUNDAY MORNING THROUGH SUNDAY NIGHT/... Sunday: By Sunday morning the center of the large high pressure should be positioned near central Quebec. This ridge then shifts ESE into the eastern offshore waters through the day. Subsidence related to deep- layer ridge will promote mostly clear skies and plenty of sunshine, though cold and dry advection will continue. Will be considerably more cloudy towards the South Shore, Cape Cod and the Islands with areas of ocean effect stratocu, however. Moisture within this stratocu layer is very shallow, though a few of the WRF-based CAMs do generate some flurries falling underneath the deck. Given how dry the low-levels are, this seems too be too low-prob to mention in the forecast but not something to be discounted. These lower clouds should also begin to thin out in coverage as we move into the afternoon. Winds look to be on the light side (around 5-10 mph) with more of an easterly onshore component into the afternoon as the high builds offshore keeping the coasts the coolest. Highs mid 30s near the eastern coast, to the upper 30s-mid 40s across Metrowest into much of CT/RI and western/central MA. Sunday Night: High pressure continues to shift eastward into the waters south of Nova Scotia, though a ridge will still hang back across Southern New England. Decent radiational cooling with sunset into the first part of the overnight, though southeast flow and warm advection should allow temperatures and moisture levels to rise moving into the second half of the night. Clouds will also be steadily increasing towards an OVC mid-level cloud deck by daybreak, with low-level (sfc- 700 mb) atmospheric column still rather dry/unsaturated per BUFKIT soundings. Indicated early lows in the low-mid 20s across interior MA, mid/upper 20s coastal MA into RI and CT. These rise a few degrees into the mid 20s to near 30 by daybreak. && .LONG TERM /MONDAY THROUGH SATURDAY/... Highlights... * Burst of snow Monday afternoon changes to rain Monday evening except across higher elevations. * Another round of rain/snow Wednesday into Thursday but snow should be limited to higher elevations. More active pattern taking shape next week as series of southern stream short waves deepen once they reach our latitude. How quickly they do so will impact the eventual track of each surface low, which ultimately affects precipitation type. It does look like we dry out for the end of the week. Temperatures should run above average for much of the week. Monday-Tuesday: Still uncertainty in the track of the surface low as it passes SNE with GFS farther east than other 12z guidance. However 12z ECMWF has trended a bit farther east and many of EPS members are also farther east (but there is still considerable spread). This uncertainty is due to how quickly mid level short wave deepens due to differences in strength of downstream ridge. Overall, however, larger scale pattern has a warm look to it, so this does not appear to favor widespread, heavy snow (even with a more offshore track). Initially airmass is marginally cold but also dry, so wet bulb cooling at onset should allow for snow across much of SNE. Exception should only be immediate South Coast, Cape, and Islands where a worst case would be rain/snow mix at onset. Models indicate potential for period of heavy snow late Monday afternoon/evening as main frontogenesis band lifts NE and strongest lift may end up in max dendritic growth zone. Once that band passes in the evening, warming takes over from SE to NW, forcing a change to rain for most of the region, except for the higher elevations near Berkshires and northern Worcester County which should remain all snow. Axis of EPS and NBM snow probs for >3" of snow is just to N of MA but both have fairly high probs in higher elevations near Berkshires and northern Worcester County. This is where we are most confident in accumulating snow, on the order of 3-6" with isolated 7". For hills farther S including N CT, should be a few hours of accumulating snow with 2-4" possible. Closer to coast, expect slushy inch or two, mainly on grass unless snowfall rate can overcome warm pavement temps (which may be tough to do). Based upon collaboration with WPC and neighboring offices we will hold off on any winter headlines since there is still uncertainty in track of low. If things remain the same on future model runs, we will probably be looking at an Advisory-level event. Wednesday-Thursday: Right on its heels, the next short wave undergoes similar strengthening near SNE. This time antecedent airmass is milder, such that rain should be the dominate p-type, although it may be cold enough for snow in the higher elevations. && .AVIATION /00Z SUNDAY THROUGH THURSDAY/... Forecaster Confidence Levels... Low - less than 30 percent. Moderate - 30 to 60 percent. High - greater than 60 percent. 00z TAF Update: Tonight: High confidence. VFR for most, though low-VFR to MVFR stratocu towards HYA/ACK after 08z. This deck may make it all the way to BOS but chance is low. NE winds 8-12 kt, with gusts 20-24 kt across Cape Cod and the Islands terminals particularly towards daybreak. Sunday: High confidence. Mainly VFR, with continued low-VFR/MVFR stratocu for ACK/HYA. Increasing onshore flow may allow the deck to spread further W and N into BOS and Cape Ann but confidence of inward extent is low. NE winds 8-12 kt, decreasing to 5-8 kt and becoming E into the afternoon. KBOS Terminal...High confidence in TAF. KBDL Terminal...High confidence in TAF. Outlook /Monday through Thursday/... Monday: Mainly VFR, with local IFR possible. Breezy. SN, chance RA. Monday Night: Mainly IFR, with local MVFR possible. Windy with local gusts up to 30 kt. RA likely, chance SN. Tuesday: Mainly VFR, with local IFR possible. Windy with local gusts up to 30 kt. Tuesday Night: Mainly VFR, with local MVFR possible. Chance RA. Wednesday: Mainly VFR, with areas IFR possible. Breezy. RA likely. Wednesday Night: MVFR/IFR conditions possible. Breezy. Chance RA. Thursday: Mainly VFR, with local IFR possible. Breezy. Chance RA. && .MARINE... Today... Northwest wind gusting 25 to 30 kt, except lingering gales on the waters east of Massachusetts this morning. Winds will turn from the north and diminish through the day, becoming less than 25 kt mid afternoon. Seas 5 to 7 feet on the exposed waters will subside through the day, with lingering 5 footers southeast of Nantucket late. A Gale Warning continues this morning, but will drop to Small Crafts at that time. Tonight... North to northeast wind 15 to 20 kt and seas 4 to 5 feet on the outer waters. Sunday... Northeast winds 15 to 20 kt and seas 4 to 5 feet on the outer waters. Outlook /Monday through Thursday/... Monday: Low risk for Small Craft Advisory winds with gusts up to 25 kt. Local rough seas. Rain, slight chance of snow. Monday Night: Moderate risk for Small Craft Advisory winds with gusts up to 30 kt. Rough seas up to 9 ft. Rain likely. Tuesday: Low risk for Small Craft Advisory winds with local gusts up to 30 kt. Rough seas up to 10 ft. Tuesday Night: Winds less than 25 kt. Seas up to 5 ft. Chance of rain. Wednesday: Low risk for Small Craft Advisory winds with gusts up to 25 kt. Seas up to 5 ft. Rain likely. Wednesday Night: Low risk for Small Craft Advisory winds with gusts up to 25 kt. Rough seas up to 10 ft. Chance of rain. Thursday: Low risk for Small Craft Advisory winds with gusts up to 25 kt. Rough seas up to 10 ft. Slight chance of rain. && .BOX WATCHES/WARNINGS/ADVISORIES... CT...None. MA...None. RI...None. MARINE...Small Craft Advisory until 8 AM EDT Sunday for ANZ250-254. Small Craft Advisory until 1 AM EDT Sunday for ANZ255-256. && $$ SYNOPSIS...Loconto/JWD NEAR TERM...Loconto/Chai SHORT TERM...Loconto LONG TERM...JWD AVIATION...Loconto/Chai/JWD MARINE...Loconto/JWD