Area Forecast Discussion
Issued by NWS Boston, MA

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000 FXUS61 KBOX 100345 AFDBOX Area Forecast Discussion National Weather Service Taunton MA 1045 PM EST Fri Dec 9 2016 .SYNOPSIS...
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Mainly dry, but unseasonably cold temperatures are on tap for our region through mid afternoon Sunday. Low pressure will bring accumulating snow to much of the region Sunday night and Monday morning, which will likely change to rain on the coastal plain and perhaps a period of ice across the interior depending on the exact track. Another wave of low pressure may bring rain and/or snow to the region Wednesday, but that is uncertain. Regardless, a shot of arctic air appears to be in the cards to close out next week. .NEAR TERM /UNTIL 6 AM SATURDAY MORNING/... Not much change from the previous forecast. Northwest winds continue to slowly diminish. Minor tweaks to bring the forecast back in line with observed trends. Previous discussion... NW flow continues but beginning to relax as surface high pressure builds in from the W coincident with weak ridging aloft and drier air. Cold air advection slows with H85 temperatures bottoming out around -16C by Saturday morning. Mainly clear skies except for a few clouds sliding over the Berks and perhaps some ocean effect clouds brushing the outer-Cape at times. Low temps by daybreak will mainly be in the teens to around 20. Tack on the winds, looking at wind chill values down into the single digits.
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&& .SHORT TERM /6 AM SATURDAY MORNING THROUGH SATURDAY NIGHT/... Saturday into Saturday night... One more piece of mid-level energy through the prevailing cyclonic flow across the region. Some low to mid level decks and perhaps some flurry activity over the W portions of CT and MA. Otherwise thereafter a ridge of high pressure builds into the region yielding an increasing presence of surface high pressure. NW winds becoming light and variable late Saturday into Saturday night as conditions turn mostly clear. Highs barely getting above freezing, more than likely feeling like the teens and 20s given the NW breeze. Lows back down into the teens, possibly single digits, considering the light and variable flow, mostly clear conditions. Decent signs of radiational cooling and thus leaning with the coldest MOS guidance. Wicked cold. && .LONG TERM /SUNDAY THROUGH FRIDAY/... Highlights Sunday night and Monday... * Accumulating snow likely Sunday night/Mon am for most of SNE * Significant impact to Monday am rush hour possible especially northwest of a Boston to Providence line * Snow likely changes to rain across coastal plain and perhaps to a period of ice across the interior sometime Monday morning * Early Snowfall Estimates may shift higher or lower: - South Coast/Cape and Islands coating to 1" - Boston to Providence corridor 1-3" - Interior southern New England 3-6" Details Sunday and Monday... The main concern with this forecast package will be the first appreciable snow event for portions of southern New England. Large high pressure in control should keep dry weather in place through mid afternoon Sunday. As the high moves off to the east, warm advection over the cold dome should allow light snow to overpsread much of the region from west to east late Sunday afternoon and early evening. A very cold antecedent airmass with 925T between -6 and -8c, should allow initially for a period of light snow even on the south coast and Cape Cod. The forcing looks rather weak late Sun afternoon into most of Sunday evening, but moisture/weak lift may allow for up to an inch of snow accumulation. The main event will be late Sunday night into Monday morning. Low pressure will be lifting towards the Great Lakes, inducing a stronger level jet and enhancing the baroclinicity. At the same time, warmer air will be advancing northward in the 850 to 950 mb layer. The strength of the initial low pressure headed up towards the Great Lakes, secondary re-development off the, coast and exact positioning of the high across eastern Canada will determine snow amounts and ptypes which will discuss more below. The bulk of the precipitation should be over by early Monday afternoon, as a dry slot overspreads the region with just a few left over showers possible. Snow amounts and Ptype/Uncertainty... As mentioned in the previous paragraph, there are a lot of moving parts to this forecast. The ECMWF/GGEM are weakest with the primary low and hence the coldest, while the GFS and especially the NAM are about 10 mb stronger. This results in a stronger low level jet allowing the 850 to 950 mb levels warm quicker. We blended the guidance at this point given the uncertainty and came up with a coating to 1 inch south coast/Cape/Islands, 1 to 3 inches on the Boston to Providence corridor, and 3 to 6 inches across much of the interior. Keep in mind we are still 60+ hours out from the event. If the models trend colder, snow amounts will have to be increased with higher amounts into the Boston to Providence corridor. A milder trend would result in snowfall totals a few inches less, and also bring an increased risk for a period of light freezing rain across portions of interior southern New England. We tried to depict this in our most likely snowfall forecast, along with our reasonable worst/best case scenarios. Regardless of amounts a significant impact to the Monday morning rush hour, especially to the northwest of the Boston to Providence corridor where Ptype is less in question at the time. Tuesday through Friday... The majority of this forecast was focused on the impending winter weather event Sunday night/Monday morning. Appears seasonable and mainly dry weather follows Tuesday. Tremendous uncertainty Wed into Wed night, whether or not another wave of low pressure tracks close enough to bring us more rain and/or snow. Regardless of what happens, a shot of very cold to arctic air follow Thu/Fri. The extent of just how cold it gets remains to be seen, but the 12z ECMWF is the coldest and would indicate the potential for near record cold by the end of next week. && .AVIATION /04Z SATURDAY THROUGH WEDNESDAY/... Forecaster Confidence Levels... Low - less than 30 percent. Moderate - 30 to 60 percent. High - greater than 60 percent. Short Term /through Saturday Night/...High confidence. VFR conditions persist through Saturday night. Northwest wind gusts of 20 to 25 knots at times into Saturday in the typical vulnerable locations. KBOS TAF...High confidence in TAF. KBDL TAF...High confidence in TAF. Outlook /Sunday through Wednesday/... Sunday...High confidence in VFR conditions though at least mid afternoon. Sunday night and Monday...Moderate to high confidence in an extended period of MVFR-IFR conditions with localized LIFR conditions. Snow develops and changes to rain on the coast and perhaps a period of ice across the interior. The worst of the conditions will be late Sunday night into Monday morning. Tuesday...Moderate confidence. VFR conditions probably dominate. Wednesday...Low confidence. Uncertain if a wave of low pressure tracks close enough to bring us another round of rain/snow or we remain dry. && .MARINE...
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Forecaster Confidence Levels. Low - less than 30 percent. Moderate - 30 to 60 percent. High - greater than 60 percent. Short Term /through Saturday Night/...High confidence. Small Craft Advisories continue on most waters. Dropped the advisory for Narragansett Bay a bit early, where observations indicate gusts have diminished to less than 25 kt. Small craft wind gusts probably persist into Saturday, but should gradually diminish, and come to an end from west to east Saturday evening, as high pressure builds into the region. Outlook /Sunday through Wednesday/... Sunday...High confidence. Winds/seas remain below small craft advisory levels with high pressure overhead. Rain and snow develops Sunday evening and may reduce VSBYS for mariners. Monday and Monday night...Moderate to high confidence. Small craft wind gusts/seas develop as secondary low pressure develops. Winds shift from south to the west by Monday night behind the low pressure system. VSBYS reduced Monday morning mainly in rain and fog patches. Tuesday...Moderate to high confidence. Small craft northwest wind gusts continue with enough cold advection continuing over the relatively mild ocean. Wednesday...Low confidence. Winds/seas highly uncertain depending on track and potential strength of perhaps another low pressure system.
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&& .BOX WATCHES/WARNINGS/ADVISORIES...
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CT...None. MA...None. RI...None. MARINE...Small Craft Advisory until 2 PM EST Saturday for ANZ231>235- 237. Small Craft Advisory until 4 AM EST Saturday for ANZ230. Small Craft Advisory until 11 PM EST Saturday for ANZ250-254. Small Craft Advisory until 11 AM EST Saturday for ANZ251. Small Craft Advisory until 7 PM EST Saturday for ANZ255-256.
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&& $$ SYNOPSIS...Frank/Sipprell NEAR TERM...Belk/Frank/Sipprell SHORT TERM...Sipprell LONG TERM...Frank AVIATION...Frank/Sipprell MARINE...Belk/Frank/Sipprell

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