Area Forecast Discussion
Issued by NWS Boston, MA

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224 FXUS61 KBOX 232353 AFDBOX Area Forecast Discussion National Weather Service Taunton MA 753 PM EDT Thu Mar 23 2017 .SYNOPSIS... High pressure along the eastern seaboard will move offshore later tonight bring windy and milder conditions Friday. A cold front will push south across the region Saturday, then stall south of New England early next week. Weak low pressure waves forecast to push along this front, with the potential for periods of rain and mixed precipitation. Another stronger front may approach around the middle of next week. && .NEAR TERM /UNTIL 6 AM FRIDAY MORNING/... 730 PM Update... Mainly clear skies, except for a few strips of thin cirrus seen across New England and down the eastern seaboard on latest IR satellite imagery at 23Z. Leading edge of lower and mid level clouds working into the eastern Great Lakes and along the NY/Canadian border, moving steadily SE. Based on the satellite trends, expect high clouds to approach western areas by around midnight, then move steadily SE with mid deck clouds pushing in around or after 07Z-08Z. Noting light precip moving out of southern Ontario into western NY state on latest NE regional mosaic 88D radar, though observations beneath the obs not reporting precip at 23Z. Very dry airmass across the region, with dewpts in the single digits either side of zero, so will take quite a while to saturate the airmass, as dewpts upstream mainly in the 5-10 degree range. With the much lower dewpts and mainly clear skies for a few more hours than previously forecast, will need to monitor temps which could go into freefall with quick radiational cooling once winds drop off. Current forecast in pretty good shape, with dry conditions expected through the night. Needed to update dewpts, which were too high based on observations. Remainder of forecast brought current. Previous Discussion... We expect radiational cooling the first part of the night, shutting down later tonight as the clouds thicken. Dew points start in the single numbers below zero and climb into the teens by late night. We favored going a couple of degrees below guidance with a range from the high single numbers in the interior to near 20 along the coastline. Most areas would be in the teens. && .SHORT TERM /6 AM FRIDAY MORNING THROUGH FRIDAY NIGHT/... Friday... High pressure offshore will provide a flow of milder air. Winds above the surface will climb to 30-40 knots by midday. That will provide a flow of moist air with precipitable water values reaching near an inch by Friday evening. Weak overrunning created by this southwest flow will create clouds and light precipitation starting in the morning. Temperatures will be cold enough at the start for snow, but quickly moderate to rain by afternoon if not sooner. The transition period may feature some sleet. The best chance for this precipitation will be in Northern and Western Mass. But frozen amounts will be light, with any snow/sleet accumulation less than an inch. Have high confidence in the overall trend, but only moderate confidence in the precipitation types. The strong southwest low level jet will also provide the momentum for gusty winds, especially during the afternoon when mixing is greatest. It still appears the greatest risk for gusts over 30 mph to be along the Boston-Providence corridor and southeast from there. Max temperatures should make a 5-10 degree rise from today, but values in the mixed layer Friday are supportive of highs in the low to mid 40s so still below March normals. Friday night... Clouds and a southwest flow of milder air will keep temperatures from falling too much, and most of that should be in the evening. Temps may rise a little overnight. We maintained a chance of showers in far Northern Mass, but low values for pops. && .LONG TERM /SATURDAY THROUGH THURSDAY/... Highlights... * Seasonal temperatures on Saturday with periods of light rain * Unsettled weather at times Sunday-Tuesday with some rain, sleet and/or freezing rain possible during the overnight periods * Low pressure and a cold front push across around mid week, then improving conditions around next Thursday Overview... While the 12Z operational model suite and ensembles continue to signal an active weather pattern across the northeast U.S. during most of this timeframe, noting some improving conditions toward the end of the long term. Still noting a mainly amplified but progressive steering pattern through most of the long term period. Noting a fast flow aloft to start this weekend while cutoff H5 low sits across the central Mississippi valley. This will keep weak low pres waves moving across the northeast while a front wavers across southern New England. The cutoff low will lift NE while it opens up to a short wave over the weekend into early next week, while large high pressure builds southward out of Quebec across Maine and N NH. Some question as to whether the eastern extent of the stationary front over the region will shift south for a time during the weekend. At this point, the ridging may push a bit too far E to bring somewhat drier conditions. The cutoff low will bring another shot of lift and moisture and, with some marginally cooler air trying to work in from the passing ridge to the E, may see a wintry mix of snow, sleet and/or freezing rain Sun night into early Mon morning. This should pass quickly as yet another H5 short wave works E from the central U.S early next week. Another surface low and associated cold front should shift sometime in the Tue-Wed timeframe, though wide model solution spread lends to lower timing confidence. May see improving conditions push in around late Wed or Thu, but again exact timing is still very much in question. Details... Saturday-Saturday night...Moderate confidence. Will be a mild day on Saturday as H85 temps start off from +5C to +8C. However, as high pressure builds SE out of Quebec into N NH and Maine during the day. Will see periods of rain develop during the day along an advancing cold front that will sink S of the region. Temps will top off near seasonal normals, from the mid-upper 40s across N Mass, ranging to the mid 50s across S coastal areas. Saturday night-Sunday...Moderate confidence. As temps fall steadily behind the front, readings will approach the freezing mark across N central and W Mass into far N CT. Partial thickness pattern suggests that precip will change over to sleet/freezing rain mainly across the higher terrain of N central and W Mass, with some snow also mixed in for a time overnight. QPF will be light, generally 0.1 inches or less, but any icing could be problematic. Will continue to monitor this closely. Temps will slowly rise, so any mixed precip should change back to light rain. However, temps may not reach freezing until late morning across some of the higher terrain of the Worcester hills and Berkshires near the MA/NH border. Another shot of steadier precip will push out of NY state during the afternoon. It will be a cooler day with highs only in the mid-upper 30s well inland to the lower 40s across the coastal plain. Sunday night-Monday...Moderate confidence. Will see yet another shot of QPF and lift move across the region during this timeframe. Also noting increasing PWATs in the S-SW flow aloft, on order of 0.8 to 1 inch. QPF amounts through 18Z up to 0.5 to 0.6 inches are possible, with the best shot between 06Z-12Z Mon but mainly S of where the colder temps will be. However, this is still another more problematic situation for possible winter weather headlines, especially for portions of central and western Mass into far N CT. Have highlighted this in the hazardous weather outlook. Temps will again slowly rise during Mon morning, but precip will continue through the day. Expect readings to top off from around 40 degrees across the higher terrain to the upper 40s across S coastal areas as milder air starts to push toward the region. Monday night through Thursday...Low confidence. Noting somewhat better general forecast agreement through most of this timeframe, but timing and track of specific features still in question. Could see another round of mixed precip late Mon night mainly around the Route 2 area of the Worcester hills and Berkshires with marginally cold temps with some residual cold air damming, which should not last too long as temps rise through the 40s on Tuesday. May see a brief break in the precip as well as a weak ridge builds across. Another H5 short wave/surface low will shift NE Tue night/Wed. Again, appears another round of rain will move in ahead of associated cold front. Some model spread with the frontal passage, with faster timing shown on the ECMWF, and somewhat slower on the 12Z GFS. Held on to chance POPs through midday Wed, then should start to see improving conditions Wed afternoon/night. As ridging develops at the surface and aloft across the Great Lakes/western Quebec by 12Z Thursday, looks like a dry but cool day. && .AVIATION /00Z FRIDAY THROUGH TUESDAY/... Forecaster Confidence Levels... Low - less than 30 percent. Moderate - 30 to 60 percent. High - greater than 60 percent. Short Term /through Friday Night/... Tonight...VFR. High confidence. NW winds with gusts up to 20 kt through around 02Z, then becoming light and variable or calm for a time tonight. Winds become light south to southwest late tonight. Area of high clouds pushes into western areas by around 03Z-04Z, steadily shifting SE. Friday and Friday night...Moderate confidence. VFR with some MVFR mainly north of the Mass Pike. Increasing and lowering clouds during the morning. Areas of MVFR cigs/vsbys in light snow or sleet, with best chance along or north of Boston and the Mass Pike. All frozen precip should change to rain during late morning/early afternoon. Southwest winds will gust 25-30 knots during the day, highest along the coast and in higher terrain. Winds diminish Friday night. KBOS TAF...High confidence in TAF. KBDL TAF...High confidence in TAF. Outlook /Saturday through Tuesday/... Saturday-Saturday night...Moderate confidence. Periods of light rain through Saturday with areas of MVFR CIGS and local MVFR- IFR VSBYS. May see brief period of mix of sleet/freezing rain overnight Sat night across higher terrain of Central and W Mass into N CT. NE wind gusts to around 20 kt Sat night across Cape Cod and the islands. Sunday-Monday...Low to moderate confidence. Areas of MVFR CIGS/VSBYS with local IFR conditions through most of the period. May briefly improve for a time during the daylight hours. Areas of rain, sleet and/or freezing rain possible late Sun night into Mon morning, which may linger most of Mon across the higher terrain. E-SE winds may gust to around 20 kt along the immediate E coast Monday. Tuesday...Low confidence. Areas of MVFR CIGS/VSBYS with local IFR at times. Should see mainly patchy light rain, though can not rule out brief freezing rain early Tue morning and again late Tue night with marginally cold temps. && .MARINE... Forecaster Confidence Levels... Low - less than 30 percent. Moderate - 30 to 60 percent. High - greater than 60 percent. Short Term /through Friday Night/... Tonight...High confidence. Diminishing NW winds becoming light SW overnight. Seas near 5 feet on the outer waters east of Massachusetts early, but subsiding during the night. Seas elsewhere less than 5 feet. A Small Craft Advisory lingers on those Eastern Outer Waters for a few more hours until seas subside. Friday...Moderate confidence. High pressure moves well offshore, bringing southwest winds to the waters. Potential for wind gusts 25-30 knots. This will build seas with 5 foot heights on many of the waters by the afternoon. Patchy light rain may bring brief visibility restrictions. Small Craft Advisories are being issued for most waters. An advisory for the eastern outer waters will be issued once we move clear of the existing headline. Friday night...Moderate confidence. Southwest winds 25 to 30 knots may become west toward morning. Seas of 5-7 feet will linger through the night on the outer waters and all southwest-exposed waters such as RI Sound. Small Craft Advisory will lower on Boston Harbor and Narragansett Bay, but linger on the remaining waters. Outlook /Saturday through Tuesday/... Saturday...High confidence. NW winds gusting to around 25 kt Sat morning, then diminish and veer to NE by Sat night. Gusts may approach 25 kt on the southern outer waters Sat night. Seas 5-6 ft on the southern waters early then subside. Sunday...Moderate confidence. E winds gusting to around 25 kt, become SE Sun night. Small Crafts may be needed. Seas build to around 5 ft over the outer waters E and S of Cape Cod and the southern outer waters. Monday-Tuesday...Low to moderate confidence. E-NE winds gusting to 25 kt. Seas building to 4-7 ft over the eastern open waters Mon night-Tue. && .BOX WATCHES/WARNINGS/ADVISORIES... CT...None. MA...None. RI...None. MARINE...Small Craft Advisory from 10 AM Friday to 6 AM EDT Saturday for ANZ232>235-237-255-256. Small Craft Advisory from noon to 8 PM EDT Friday for ANZ230. Small Craft Advisory from 10 AM Friday to 3 AM EDT Saturday for ANZ231-251. Small Craft Advisory from 10 AM to 10 PM EDT Friday for ANZ236. Small Craft Advisory until 2 AM EDT Friday for ANZ250-254. && $$ SYNOPSIS...WTB/EVT NEAR TERM...WTB/EVT SHORT TERM...WTB LONG TERM...EVT AVIATION...WTB/EVT MARINE...WTB/EVT is the U.S. government's official web portal to all federal, state and local government web resources and services.