Area Forecast Discussion
Issued by NWS Boston, MA

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000 FXUS61 KBOX 200627 AFDBOX Area Forecast Discussion National Weather Service Taunton MA 127 AM EST Fri Jan 20 2017 .SYNOPSIS... High pressure will result in rather mild and tranquil weather Today into Saturday, but there is a low risk for some spotty light freezing rain across northern Massachusetts late tonight. Otherwise, the main concern is the potential for a significant coastal storm with heavy amounts of rain/ice and/or snow along with strong to damaging winds on the coast Monday into Tuesday. This will also be a dangerous storm for mariners with the potential for 50 knot wind gusts and seas building to 20+ feet. && .NEAR TERM /THROUGH TODAY/... 130 am update... Mid level ridge west of New England early this morning will move east over southern New England this afternoon. This anticyclonic flow and associated subsidence will tend to erode low level clouds across the area early this morning. While not expecting clouds to completely erode thinking there will at least by partial sunshine today. Model soundings suggest boundary layer extending up to around 925 mb this afternoon with temps about -2C at the top of this layer. This will support highs in the low to mid 40s this afternoon. Given weak pres gradient not expecting much wind today which will make it feel quite pleasant by late Jan standards. && .SHORT TERM /TONIGHT THROUGH 6 PM SATURDAY/... updated 130 am... Short wave trough currently moving across the OH valley will enter southern New England tonight. However given the short wavelength between the ridge over the area today and the approaching trough tonight...this short wave becomes sheared and deamplifies. Thus the highest chance of light rain/sprinkles tonight will be across CT and then diminishing across RI and eastern MA...where mainly dry weather may prevail. However as previous discussions have mentioned we will have to watch for any rain entering northern MA before drying up. Temperatures across northern MA especially northern Worcester county will be at or just below freezing. Therefore there is a low risk (less than 30 percent) for a brief period of light freezing rain across northern MA tonight. Keep in the mind given temperatures will be marginal along with little if any precip across northern MA the threat for freezing rain is low. Thus no headlines at the moment. Saturday... Some morning cloudiness associated with deamplifying short wave trough. Thus the trend will be for morning clouds giving way to afternoon sunshine. Airmass slightly milder than previous days along with light westerly pgradient to promote more of an offshore wind. This will support highs Sat in the mid to upper 40s with a low probability of few locations briefly hitting 50! So another very nice day by late Jan standards with partial afternoon sunshine, light winds and temps 45 to 50! Enjoy. && .LONG TERM /SATURDAY NIGHT THROUGH THURSDAY/... Highlights... * Dry/Unseasonably mild Sat * Potential for significant storm Sun night into Tue - Significant precipitation likely but ptypes/timing uncertain - Highest risk for significant snow/ice interior MA north of Pike - Strong to perhaps a period of damaging winds on the coastal plain Details... Saturday... Strong upper level ridging/anomalously high height fields should result in an unseasonably mild afternoon. Model cross sections indicate a lot of low clouds especially in the morning, but certainly may see some partial sun develop by afternoon. 850T near +4C should result in high temps in the low to mid 50s away from the immediate coast. Weak gradient may actually allow for seas breezes to develop on the coast holding temps in the 40s in these locations. Sunday through Tuesday... There continues to be potential for a significant storm late Sunday night into Tuesday, but timing/ptypes remain highly uncertain. The energy for this system still remains off the southern California coast and given this event is 4 to 5 days in the future, plenty of uncertainty remains. There are some ingredients though that have our attention. The GEFS anomalies indicate a 925 mb easterly jet 4 to 5 standard deviations above normal coupled with PWATS of 2 to 3 standard deviations above normal. That certainly indicates the potential for a significant storm. With that said potent southern stream energy loaded with moisture will result in developing coastal low pressure tracking northeast towards New England. At the same time, a modest surface high pressure system across eastern Canada will try to push down from the north with some low level cold air. This will increase the pressure gradient and given a moisture loaded southern stream system, certainly the potential for a significant precipitation event. Ptypes and timing though remain much more difficult. Will break it down below. 1) Timing: Developing northeast flow may allow some light precipitation to develop late Sun and particularly Sun night. While timing is still uncertain, latest guidance indicates the brunt of the heavy precipitation will occur Monday into early Tuesday as strong low pressure tracks up the coast. 2) Precipitation Types/Amounts: Probably the biggest aspect to this storm are the precipitation types. Unfortunately, that is where our confidence is lowest given very marginal thermal profiles. A subtle shift in the track of the closed upper level low/s can make for a big difference in specific precipitation types/amounts. The highest risk for a significant amount of snow and/or ice will exist in areas north of the MA turnpike and away from the coast. Whether the bulk of that falls as snow/sleet/freezing rain is uncertain. However, given models likely tracking surface low southeast of this region it would be hard to avoid at least modest amounts of snow or ice. Further southeast, near the Boston to Providence corridor a marginal airmass with strong easterly flow probably results in rain dominating. However, even in these locations some sleet and perhaps snow may be possible at the onset as cold air around 925 mb may press south into this region for a time. It is way too early to even try to give amounts of any snow/ice, particularly given this setup. We will say that given GEFS continuously showing an easterly low level jet of 4 to 5 standard deviations above normal, a significant precipitation event is likely. 1 to 3 inches of precipitation is certainly possible with even locally higher amounts possible. 3) Strong to Even Damaging Wind Gusts for the coastal plain: Given the anomalous easterly LLJ that is being signaled on the ensembles along with support from the CIPS Analogs/Bufkit soundings, a period of strong to perhaps damaging easterly winds on the coastal plain are a good bet. Timing uncertain, but highest risk Monday afternoon into Monday night with 45 to 55 mph gusts possible and perhaps even a low risk for up to 60 mph. Wind headlines will likely be needed for the coastal plain. Across the interior, low level cold air will likely limit mixing but still can not rule out a period of 25 to 40 mph gusts given magnitude of the LLJ. 4) Coastal Flooding Potential: Given potent easterly LLJ and period of strong wind gusts, expect seas to probably build to 20+ feet across the eastern waters late Mon into Tue. Fortunately, astronomical high tides are about as low as they get. The strongest winds Monday evening will occur with an astro high tide of only 8.2 feet in Boston, so the risk of coastal flooding is low. However, Tuesday morning the high tide is 9.4 feet. If winds are still blowing strong out of the northeast along with 20+ foot seas just offshore, minor coastal flooding will occur along the eastern MA coast with even a small chance of a low-end moderate coastal flooding. Wednesday and Thursday... Very limited time spent on this portion of the forecast. But will keep the weather mainly dry and tranquil for now. && .AVIATION /06Z FRIDAY THROUGH TUESDAY/... Forecaster Confidence Levels... Low - less than 30 percent. Moderate - 30 to 60 percent. High - greater than 60 percent. Short Term /through Saturday/... 130 am Update... Today... Other than some patchy MVFR cigs mainly VFR. Light winds. Tonight... MVFR with low risk of IFR. Very light rain or sprinkles possible western MA into CT. Very low risk of brief light icing across northern Worcester county into northern Franklin county. Saturday... MVFR likely in the morning with spotty IFR possible. The improving VFR by afternoon except in the higher terrain MVFR cigs will linger. KBOS TAF...Moderate to high confidence in TAF. KBDL TAF...Moderate to high confidence in TAF. Outlook /Saturday Night through Tuesday/... Sunday through Tuesday...Moderate confidence in trends but tremendous uncertainty on timing/ptypes. Extended period of MVFR- IFR conditions with the heaviest rain/ice and/or snow Mon into early Tue. Period of easterly wind gusts of 35 to 45 knots with a few gusts up to 50 knots possible along the coastal plain sometime Mon/Mon night. LLWS possible across the interior. && .MARINE... Forecaster Confidence Levels... Low - less than 30 percent. Moderate - 30 to 60 percent. High - greater than 60 percent. Short Term /through Saturday/... 130 am update... Today... Light winds, dry weather and good vsby. Tonight... Brief period of light rain or sprinkles possible. Otherwise light winds trending SW late. Saturday... Light winds, dry weather and good vsby. Outlook /Saturday Night through Tuesday/... ***Dangerous and potentially life threatening storm for Mariners Monday into Tuesday*** Saturday and Saturday night...Moderate to high confidence. Weak pressure gradient will keep winds below small craft advisory thresholds. However, easterly swell may result in some 5 foot seas across the eastern outer-waters. Sunday through Tuesday...Moderate to high confidence in trends but lower confidence in timing. Canadian high pressure combined with a strong coastal low pressure system will result in increasing northeast flow Sun/Sun night. Strongest winds appear to be Mon/Mon night when anomalous easterly LLJ should result in easterly wind gusts of 45 to 50 knots. Gale warnings are a slam dunk and a moderate risk for Storm Force Wind Gusts. Seas will likely build to 20+ feet across our eastern outer-waters given strong winds/long easterly fetch. This will be a dangerous and potentially life threatening storm for mariners. && .BOX WATCHES/WARNINGS/ADVISORIES... CT...None. MA...None. RI...None. MARINE...Small Craft Advisory for hazardous seas until 1 PM EST this afternoon for ANZ250-254-255. Small Craft Advisory for hazardous seas until 10 AM EST this morning for ANZ251. Small Craft Advisory for hazardous seas until 6 AM EST early this morning for ANZ256. && $$ SYNOPSIS...Nocera NEAR TERM...Nocera SHORT TERM...Nocera LONG TERM...Frank AVIATION...Frank/Nocera MARINE...Frank/Nocera

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