Area Forecast Discussion
Issued by NWS Boston, MA

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601 FXUS61 KBOX 181958 AFDBOX Area Forecast Discussion National Weather Service Boston/Norton MA 358 PM EDT Wed Apr 18 2018 .SYNOPSIS... A low pressure area from the Midwest will produce a period of chilly rain potentially mixing with wet snow during Thursday. Showers linger Friday. Building high pressure weekend into early next week, gradual warm up forecast. Storm system expected for mid to late week. && .NEAR TERM /UNTIL 2 AM THURSDAY MORNING/... 400 PM Update... Closed low up in southern Canada will continue bring cyclonic rotation to the region. Lingering moisture around 850mb combined with steep low level lapse rates has resulted in a broken cloud deck across much of southern New England. Still some break of sun resulting in good mixing up to 850mb. This has resulted in surface temps warming into the low to mid 50s and wind gusts between 25-35 MPH. Highest gusts near the coastline. Clouds will begin to erode and wind gusts lessen as we near sundown. This is due to the lost of mixing and diurnal heating. Overall a dry weather day will remain into the overnight. Tonight til 2AM... Weak ridging will occur across the region this evening as a vigorous vort max and surface low pressure system approaches from the west. Cloud cover will increase ahead of this potent system as WAA increases. There appears to be enough WAA and isentropic lift to result in some showers breaking out ahead of the main system. These showers will be confined to the south coast where the best low level f-gen will set up overnight. Temperatures will slowly fall during the night as cloud cover will limit rational cooling. Regardless, below normal temperatures will remain for the area. && .SHORT TERM /2 AM THURSDAY MORNING THROUGH THURSDAY NIGHT/... After 2AM Thursday into Thursday night... Vigorous upper low in the Midwest will move eastward across the lower Great Lakes tonight. Its track will be deflected southward by a stout vort max moving south from Hudson Bay on the edge of our northeast upper low. The two systems will interact with each other resulting in the surface low passing south of southern New England and strengthening through the day. Precipitation will begin to overspread the region after 06z and closer to 09z once the profile saturates. Pretty good omega at the onset of this system which will result in a good burst of precip. Models and especially the hi-res guidance shows a narrow swath of heavy QPF, which is located just north of the 925mb temperature boundary. Expect lower vsbys in this swath. Biggest focus was on the temperature profile for this system. Temperatures at the onset will be near 35 degrees and with this burst of precip, anticipate temperatures to wet-bulb resulting in snow for the higher terrain. Cannot rule out a mix of rain and snow north of the Pike, but am not expect much to accumulate. Across the higher terrain, because temperatures will be cooler expect an inch or two of snowfall. Because of the timing, snowfall will coincide with the morning commute north of the Pike, and especially across route 2. Lastly, there is a weak warm nose trying to impend into the region during the morning. This could result in some sleet pellets. Main surface low will pass north of the Benchmark and head towards the Maritimes by the afternoon. Bulk of the heavy precip will begin to subside, however, appears a secondary area of rain and snow will occur by the afternoon hours. This is noted by the strong QG forcing, saturated low level moisture and omega in the snow growth region. While this secondary area of precip won`t be as strong as the morning, expect widespread showers with some snow flakes mixed in as low levels are quickly being cooled. Temperatures across the region will still be below average during the day. Went a bit cooler than guidance as precip will drag the cooler air aloft down to the surface limiting high temps to get above 45F. Upper level low continues to spin aloft tomorrow night. Still enough low level moisture and forcing aloft to result in some sct showers during the overnight. Best areas will be across the high terrain. Mixing does begin to increase along the coastline so winds will begin to gusts near 20-25 mph toward morning. && .LONG TERM /FRIDAY THROUGH WEDNESDAY/... */ Highlights... - Lingering showers Friday - Near-seasonable, dry conditions over the weekend - Warmer conditions, dry, Monday into Tuesday - Mid to late week storm system */ Overview... Aleutian low persistence. With maturation to occlusion, deepening, immediate downstream energy sheared S, cut-off, closed off near the SW CONUS. With death, energy shears E, Pacific wave train opens, pattern marches on, energy pushes E, pulling from polar and sub- tropical latitudes, towards a preferred H5 trof pattern over NE North America. Tandem advective processes, an ebb and flow pattern is in the cards, mixing, airmasses may not be exceptionally warm, exceptionally cold. Aside, weekend chill followed by early week warmth ahead of midweek active weather. Lots of moving parts, unknowns. Downstream traffic / blocking, snow pack / depth N and W, N/S-stream energy interacting / phasing? Confidence lowers further out in time. EPS doesn`t note a pattern-break, continued indications of H85 temperature anomalies into early May, H5 troughing over NE N America persisting, cooler weather maintaing its grip at times, shots of warmth, ebb and flow. */ Discussion... Friday... Cyclonic curvature of lingering moisture, colder low-levels, steep lapse rates, daytime heating, breezy NW winds, likely showers, more so along leading edges of rotating vort-lobes. Minimal impact, dry air working in, rising heights, threat diminishing towards late. Weekend... Beneath lingering trof, colder air aloft, NW flow diminishing along with scattered cloud cover, abundant sunshine, near-seasonable, dry, gradual warming trend. Noting weak vort-lobe energy rotating through the trof, despite a general trend, can`t rule out some exacerbation with a bit more clouds, slightly faster winds. Timing crucial via daytime mixing. Monday / Tuesday... High pressure, light winds, three opportunities: 1.) daytime sea- breezes yielding sharp coastal-interior temperature gradients, 2.) overnight radiational cooling, and 3.) when high pressure shifts E allowing S/SW warmer air. Suppressing ridge remaining in place, ascending cloud cover per approaching midweek storm holds off, Tuesday is the warmer of the two. Mid to Late Week... Aforementioned cut-off, closed-off SW CONUS low kicked by N-stream Pacific train shearing S via upstream ridging, continued stream of energy. Series of vort-lobes, watching closely how well dug S and manifestation / draw S colder air to the surface, what extent and depth of snow pack exists N/W. Can`t rule out NE CONUS wintry weather. && .AVIATION /20Z WEDNESDAY THROUGH MONDAY/... Forecaster Confidence Levels... Low - less than 30 percent. Moderate - 30 to 60 percent. High - greater than 60 percent. Short Term /through Thursday Night/... No significant changes with 12Z TAF update... Before 00z... High confidence. VFR. West winds gusting to 25 knots, possibly near 30 knots around Nantucket. Tonight...High confidence through midnight, moderate confidence after midnight. VFR to start the night, lowering to MVFR cigs/vsbys towards daybreak. This is due to widespread rain across much of the region and snow in the higher terrain. Tomorrow and Tomorrow night... Moderate confidence. MVFr falling to IFR across the region in snow for the higher terrain and rain across the coastal plain. Vsbys look to lift by the afternoon as precip turns more to showers. Cigs will still be MVFR/IFR lifting to VFR during the overnight. Lingering MVFR across the east coast due to ocean effect showers. KBOS Terminal...High confidence in TAF. Morning push looks to be impacted with heavy rainfall/MVFR conditions. KBDL Terminal...High confidence in TAF. Morning push looks to be impacted with heavy rainfall/MVFR conditions. Outlook /Friday through Monday/...Moderate confidence. Friday: VFR. Breezy. Slight chance SHRA. Friday Night through Saturday: VFR. Breezy. Saturday Night through Monday: VFR. && .MARINE... Forecaster Confidence Levels... Low - less than 30 percent. Moderate - 30 to 60 percent. High - greater than 60 percent. Short Term /through Thursday Night/...High confidence. Tonight...Diminishing winds and seas. Lingering rough seas over the southern and southeast outer waters. Small Craft Advisory lingers in those areas. Thursday and Thursday night... A coastal weather system brings rain to the waters, limiting visibility to 1 to 3 miles. Winds remain less than 25 knots during the day and most of the night. Seas will increase above 5 feet or higher by Thursday night, especially along the southern outer waters. SCA will be needed. Low risk for gales, but wind gusts appear to under 30kts toward daybreak. Outlook /Friday through Monday/...Moderate Friday: Low risk for Small Craft Advisory winds with gusts up to 25 kt. Local rough seas. Slight chance of rain showers. Friday Night: Winds less than 25 kt. Seas locally approaching 5 ft. Saturday through Monday: Winds less than 25 kt. && .BOX WATCHES/WARNINGS/ADVISORIES... CT...None. MA...None. RI...None. MARINE...Small Craft Advisory until 10 PM EDT this evening for ANZ231>234. Small Craft Advisory until 2 AM EDT Thursday for ANZ235-237- 250. Small Craft Advisory until 6 AM EDT Thursday for ANZ254>256. && $$ SYNOPSIS...Dunten/Sipprell NEAR TERM...Dunten SHORT TERM...Dunten LONG TERM...Sipprell AVIATION...Dunten/Sipprell MARINE...Dunten/Sipprell is the U.S. government's official web portal to all federal, state and local government web resources and services.