Area Forecast Discussion
Issued by NWS Boston, MA

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000 FXUS61 KBOX 200044 AFDBOX Area Forecast Discussion National Weather Service Taunton MA 719 PM EST Thu Jan 19 2017 .SYNOPSIS... High pressure will result in rather mild and tranquil weather Friday into Saturday, but there is a low risk for some spotty light freezing rain across interior Massachusetts Friday night. Otherwise, the main concern is the potential for a significant 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 seas to build to 20+ feet. && .NEAR TERM /UNTIL 6 AM FRIDAY MORNING/... 710 pm update... Not too many changes made to the going forecast. Trapped low level moisture will probably keep skies partly to mostly cloudy overnight. Low temps are tricky because if some brief breaks in the clouds develop, light winds will all temps across the normally coolest outlying locations to drop rather quickly. Overall though keeping lows in the upper 20s to lower 30s is the way to go for most of the region given a fair amount of clouds expected. Previous Discussion: Subsidence inversion remains in place through the overnight hours tonight, which continues to lock in a layer of moisture mainly between H9 and H8. Therefore, still some clouds on and off through the overnight. Not quite as concerned about fog potential as previous forecast suggests, near sfc dwpts continue to lower (drying). In fact even though pres gradient will slacken yielding near calm conditions, the trapped moisture beneath the subsidence inversion and resulting SCT-BKN clouds will limit radiational cooling potential and therefore mitigate fog development. A few patches are possible, but not as concerned about widespread fog with this update. Otherwise, given the lack of radiational cooling min temps should remain mostly in the upper 20s and low 30s. A few spots could dip into the low 20s where decoupling is typical as the early morning moisture layer dissipates. && .SHORT TERM /6 AM FRIDAY MORNING THROUGH FRIDAY NIGHT/... Tomorrow... Mid lvl ridge building keeps dry subsidence inversion in place. However lower lvl winds will veer to the E then S-SE through the day. This suggests modest warm advection and some increase in overall column moisture through the day. The primary result of these factors will be for another day of mixed sun and cloud (mainly all cloud by evening with the approach of shortwave and attendant warm front). However, clouds are likely to mainly be mid-high clouds except near E MA, where onshore flow could bring in some ocean SC onshore during the afternoon. Overall, with little change in airmass, expect highs once again in the low-40s, similar to those experienced today. Tomorrow night... Shortwave moving out of the great lakes looks acute enough to suppress continued mid lvl ridge building. This will combine with an increase in column moisture mainly across the W with PWATs nearing 2 std deviations above normal. Overall, enough lift expected for rain to build from SW during the evening and early overnight hours thanks to overruning ahead of the approaching warm front which will remain to the S. The rain will begin tapering as it pushes into Central S New England so highest POPs during the overnight are relegated to CT and W MA primarily. RI and areas E of the Worcester Hills could remain dry, or at most see a sprinkle before the rain shield dissipates entirely. This is thanks to the continued ridge building mentioned earlier. One note of consideration is the low risk for some brief freezing rain during the evening hours mainly for portions of NW and N-central MA. Dwpt depressions and initial cooling ahead of the precip shield could allow temps to drop to near or below freezing as the rain (what`s left of it at least) reaches this area. Given the precip may be ending or very light, marginal 2m temps and area sfc temps near or even above freezing, not expecting much in the way of ice accretion. Therefore, no headlines with this update, but we will need to monitor this trend over the next 24 hours. Temps mainly dip into the low-mid 30s thanks to the return flow and building clouds limiting any radiational cooling potential. && .LONG TERM /SATURDAY THROUGH THURSDAY/...
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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.
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&& .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...Moderate confidence. Subsidence inversion will probably keep a scattered to overcast CIGS between 2500 and 3500 feet. Therefore, expect VFR to marginal MVFR conditions tonight. Tomorrow...High confidence. Mainly VFR through the afternoon until some MVFR CIGS spill across portions of CT late, and SE MA during the afternoon with winds shifting to the E then SE through the afternoon. Tomorrow night...Moderate confidence. Mix of MVFR/IFR as a band of rain shifts into the region from the SW, while this is expected to dissipate as it moves E, areas across CT/W MA are likely to see wet runways. With temps near freezing some light ice also possible toward the Sat morning timeframe. KBOS TAF...Moderate to high confidence in TAF. KBDL TAF...Moderate to high confidence in TAF. Outlook /Saturday through Tuesday/... Saturday...Moderate confidence. Probably MVFR-IFR conditions to start the day, but improvement to high end MVFR-VFR probably occur by afternoon. 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 Friday Night/... Through early tomorrow...High confidence. Some lingering swell near 5-6ft will require Small Craft Advisories to continue tonight into early tomorrow before they slacken enough that they can be dropped. Otherwise, NW winds will remain generally 10-15 kt at the highest. Late day tomorrow and tomorrow night...High confidence. Winds will shift first to the E late tomorrow, then finally around to the S-SE and continue there during the overnight hours. Wind speeds remain 10-15 kt at most and seas should not exceed the 5 ft thresholds for small craft advisories. Some fog tomorrow night could yield occasionally low vsbys. Outlook /Saturday 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...
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CT...None. MA...None. RI...None. MARINE...Small Craft Advisory for hazardous seas until 1 PM EST Friday for ANZ250-254-255. Small Craft Advisory for hazardous seas until 10 AM EST Friday for ANZ251. Small Craft Advisory for hazardous seas until 6 AM EST Friday for ANZ256.
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&& $$ SYNOPSIS...Doody/Frank NEAR TERM...Frank/Doody SHORT TERM...Doody LONG TERM...Frank AVIATION...Doody/Frank MARINE...Doody/Frank

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