Area Forecast Discussion
Issued by NWS Boston, MA

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000 FXUS61 KBOX 270242 AFDBOX Area Forecast Discussion National Weather Service Taunton MA 1042 PM EDT Sun Mar 26 2017 .SYNOPSIS... High pressure over Marine will retreat into the Maritimes, but provide cool weather across southern New England tonight into Monday. A warm front will approach the region during this time and result in periods of rain with areas of freezing rain across the high terrain. Wet weather continues Tue and Tue night as low pressure tracks over or near the region. Although Tue should be milder than Monday. High pressure brings dry but chilly weather Wed and Thu. More unsettled weather is possible late Fri into Sat. && .NEAR TERM /UNTIL 6 AM MONDAY MORNING/... 1040 PM Update... Only minor tweaks to the forecast this evening. Slowed the arrival of steadier precipitation by a couple of hours overnight. No changes to the existing Freezing Rain Advisory, yet. Any freezing rain will likely be confined to elevations at or above 700 feet. Will continue to monitor temperatures overnight. 7 PM update... A northwest to southeast band of showers moving across western MA/northeast CT and RI at 7 pm. This in response to WAA pattern across the region. Temps remain above freezing so precip is all rain. Periods of rain/showers combined with onshore flow has resulted in dew pts steadily rising this afternoon and now into the evening hours into the mid to upper 20s. Not much drier low level air to advect southward from central New England so threat of freezing rain/drizzle overnight may turn out to be less of a risk than earlier thought. Will help headlines up as is and let next shift evaluate trends for any possibly changes. Otherwise previous forecast on track. Earlier discussion below. Previous Discussion... */ Highlights... * FREEZING RAIN ADVISORY through 8 am Monday for the expectation of light icing on untreated surfaces in N/W portions of MA, especially across the higher terrain of the Berkshires and Worcester Hills. * SPECIAL WEATHER STATEMENT issued to address the freezing rain in the forecast however lower confidence of icing on untreated surfaces. */ Discussion... Into evening... Most of Southern New England has warmed above freezing as another band of precipitation approaches from the SW invoked by mid-level forcing and an increasing warm-moist airmass encroaching into the region. Increasing PoPs accordingly for S/W MA and CT behind the present lull in the weather. Temperatures holding steady with most locations seeing an E flow aside from the CT River Valley still observing some N funneling of winds. Tonight... Main focus of freezing rain impacts is mainly in Franklin / Northern Worcester County of Massachusetts, especially along the high terrain and with impact focus along the Route 2 and northern portion of the I-91 corridor, more importantly during early Monday morning commute. Will keep with the FREEZING RAIN ADVISORY giving greater exposure and awareness to the general public. SPECIAL WEATHER STATEMENT else- where where freezing rain is forecast highlighting the lesser confidence of icing on untreated surfaces. Seemingly such a marginal icing event. Absence of arctic air as well as one of considerable depth at the surface. The surface high setup more N/E rather than N/NW thus less N component of ageostrophic / isallobaric flow, more E onshore component off warmer ocean waters. While it`s late March, significant icing events are more favorable when temperatures are forecast in the 20s or below. Also noteworthy CIPS analogs are fairly quiet, not showing much of a strong signal. The SREF probabilities mainly confined to the high terrain of the Berkshires and Worcester Hills. Still a tough forecast given the synoptic setup especially with 2m surface temperatures. The warm front will be slow to creep N/E and given some uncertainty on timing, a forecast off by 1 to 2 degrees can make some difference. Could even hang for a period up against the N/E colder airmass, N funneling flow. Some inherent forecast challenges. So going into it, precipitation will overspread the area overnight per dominant low to mid level lift and forcing, more so against high terrain. While considering ice to be present within the column, the depth and magnitude of the warm layer around H85 above the shallow surface cold airmass favors any precipitation falling into a region of sub-freezing temperatures to be mainly in the form of freezing rain. Gradual change over to rain with a near-steady to non-diurnal temperature trend forecast, more than likely a combination of 1.) falling liquid through an increasing warm- layer, 2.) latent heat release of liquid freezing as ice on untreated surfaces, especially those elevated and exposed, and 3.) onshore E flow off the warmer waters ushering into the interior. Freezing rain impacts erode N/W towards Monday morning. Majority of the threat focused across the N/W high terrain. Particular focus to the N/W MA Route 2 and N MA I-91 corridor. Ice accretions mostly that of a glaze but the potential is there for up to a tenth of an inch. If having to travel this area tonight into the early morning hours of Monday, please be aware of the potential hazards for the area within the FREEZING RAIN ADVISORY. && .SHORT TERM /6 AM MONDAY MORNING THROUGH MONDAY NIGHT/... Monday... Mainly rain with, however, some lingering pockets of freezing rain during the morning for N/W MA, especially along the high terrain. Warm front lifts across the area with associated forcing mechanisms collocated with a +2 standard deviation precipitable water airmass. Yet notable the stronger QG-forcing and perhaps convergent focus is across Upstate NY into N New England. Thus some spread in the model forecast outcomes of more moderate rainfall. With little to no impact impacted with model forecast rainfall, will go with a consensus blend of the guidance. It`ll be a cold rain for some, especially those N/E still within a lingering airmass just above freezing, perhaps for a long period of time depending on the evolution N/E of the surface warm front, if it becomes hung up, which in turn will have implications on the surface wind forecast. Some challenges to be sure but think we`ll be okay otherwise. Will see temperatures warm into the 40s, lower 50s for locations S/W. E winds along the warm front becoming more S and a bit more breezy behind. Majority of the rain focus around the midday hours coming to an end into the evening. Monday night... Cloudy, dreary night. Lot of low level moisture given the recent rains plus the continued S flow off the waters, trapped beneath the continued stout warm layer inversion immediately aloft. Lots of low clouds, perhaps even some drizzle can be included. Given the over- cast deck, don`t expect temperatures to drop much overnight. Will remain mild, in thinking, with lows down around the 40s. && .LONG TERM /TUESDAY THROUGH SUNDAY/... Highlights... * Active weather pattern with precip likely Tue/Tue ngt & Fri/Sat * Dry but cool and blustery weather Wed and Thu Pattern Overview... Deterministic and ensemble guidance both support a split flow regime across the CONUS this period. Thus active weather pattern into New England this week and possibly into next weekend with a parade of moisture rich southern stream systems. Given it`s still late March, northern stream still showing appreciable amplitude so have to watch for the potential of precip type issues late Fri into Sat. Details... Tuesday/Tue night...moderate forecast confidence. Southern stream system currently located over the southern Plains will move across New England this period. All model guidance agrees that southern New England begins in the warm sector Tue so definitely a milder day than Monday. A model blend offers highs Tue between 55-60...cooler south coast. Could be warmer if clouds and precip are delayed until late in the day or at night. In addition, dew pts climbing to near 50 will help remove any coolness to the airmass. By Tue evening height falls shift frontal boundary south with low pres tracking over or just south of New England. Thus much cooler late Tue/Tue night as frontal wave passes and north-northeast flow overspreads the region. GEFS has PWATs climbing to about +2 standard deviations above climo. This combined with surface convergence from frontal boundary and surface wave could yield a period of moderate to perhaps heavy rain at times. As previous forecaster mentioned could see isolated thunder given weak elevated instability. Wednesday and Thursday...moderate to high forecast confidence Dry but cool weather as northern stream mid level closed low tracks from eastern Quebec thru Maine and then into the northern Atlantic. Wed will get off to a somewhat mild start with lows only in the upper 30s and lower 40s. However CAA overspreads the area by afternoon via gusty northerly winds. Clouds may be stubborn on Wed over the Cape and islands given northerly flow across the waters. In fact could have some ocean effect rain/snow showers over the outer Cape! A few degs cooler Thu given less blyr mixing with high pres overhead. However will be fairly pleasant given light winds and lots of sunshine. Friday/Saturday/Sunday...low confidence given time range and amplitude and interaction of northern and southern streams Deterministic and ensembles agree next southern stream system may impact our region in the late Fri-Sat time frame. Northern stream showing some amplitude downstream across the maritimes and this may result in cold confluent flow over or just to the north of the region. Thus a chilly rain with potential for wet snow/sleet at times on the northern portion of the precip shield. All guidance suggest system should be or moving offshore by Sunday, thus drying trend possible second half of the weekend. && .AVIATION /02Z MONDAY THROUGH FRIDAY/... Forecaster Confidence Levels... Low - less than 30 percent. Moderate - 30 to 60 percent. High - greater than 60 percent. Short Term /through Monday Night/...Moderate confidence. 1040 PM update... Increasing MVFR, with local IFR-LIFR possible late. Previous TAFs has this trend covered so no major changes planned. Earlier discussion below. Previous Discussion... Tonight... MVFR lowering to IFR-LIFR. E winds overall but remaining N within the CT River Valley. RA / FZRA coverage increasing across terminals. FZRA more likely for interior areas (E Central MA across to W MA, NW RI, N CT) with the highest risk across the high terrain of the Berkshires and Worcester Hills. Mainly trace accumulations but as high as a tenth of an inch. Main impacts after midnight into Monday morning, FZRA threat eroding to the N as well towards Monday morning. Monday... FZRA risk diminishes, lingering over N/W MA, especially the high terrain for the first few hours. Otherwise -RA/RA with MVFR-LIFR cigs with light E winds initially turning SE and increasing. Monday night... RA diminishing however IFR-LIFR cigs remain. Could see some DZ in addition. Light winds. KBOS TAF...Not expecting any FZRA impacts at the terminal with E onshore flow. Lowering MVFR down to LIFR after midnight Monday into Monday morning. KBDL TAF...FZRA threat around midnight Monday, yet expect spotty impacts. Not expecting any ice accretion, at most a trace as a worse case. Outlook /Tuesday through Friday/... Tuesday...Moderate confidence. MVFR conditions likely in areas of rain and fog with a trend lowering to IFR as frontal boundary and surface low approach. Low risk for thunder. Wednesday into Thursday...High confidence. Dry and VFR weather likely but MVFR in scattered rain/snow showers possible over the Cape and Islands Wed/Wed night. Gusty northerly winds Wednesday into Thu. Friday...moderate confidence. VFR and dry to start but likely lowering to MVFR or possibly IFR Fri night in rain. Low risk of rain mixing with snow/sleet interior Fri night. && .MARINE... Forecaster Confidence Levels... Low - less than 30 percent. Moderate - 30 to 60 percent. High - greater than 60 percent. Short Term /through Monday Night/... Warm front will lift across the waters tonight into Monday, along and behind which E flow may gust as high as 25 kts and build seas to 5-6 feet. Greater confidence for the outer waters than the inner waters, so opted to issue SCAs accordingly, will let the night crew evaluate whether it needs to be expanded. A weak frontal boundary sweeps the waters Monday night and winds turn westerly but remain light. This will allow seas to subside and SCAs to conclude. Outlook /Tuesday through Friday/... Tuesday...Moderate confidence. Light southerly winds with frontal boundary north of the region. However winds becoming northeast late Tue night as front slips south and wave of low pres tracks along the front south of New England. Vsby reduced at times in rain and fog. Wednesday into Thursday...High confidence. Gusty north winds near Wednesday into early Thu then diminishing as high pressure builds into the area later Thu. Friday...high confidence. Quiet weather with high pressure over the area early Fri. However increasing east-southeast winds Fri night as low pres approaches from the west. Also vsby lowering in rain and fog Fri night. && .BOX WATCHES/WARNINGS/ADVISORIES... CT...None. MA...Freezing Rain Advisory until 8 AM EDT Monday for MAZ002>004- 008-009-012-026. RI...None. MARINE...Small Craft Advisory from 2 AM Monday to 2 AM EDT Tuesday for ANZ250-254. Small Craft Advisory until 11 PM EDT Monday for ANZ255-256. && $$ SYNOPSIS...Nocera/Sipprell NEAR TERM...Belk/Nocera/Sipprell SHORT TERM...Sipprell LONG TERM...Nocera AVIATION...Nocera/Sipprell MARINE...Belk/Nocera/Sipprell

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