Area Forecast Discussion
Issued by NWS Boston, MA

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000 FXUS61 KBOX 161455 AFDBOX Area Forecast Discussion National Weather Service Boston/Norton MA 1055 AM EDT Mon Apr 16 2018 .SYNOPSIS... A wind swept rain along with isolated thunderstorms will move through the region today. Precipitation will wind down by this evening. The mid level system will keep unsettled conditions across the region through mid week. As this system exits, another fast moving low will cross the region Thursday into Friday, bringing another round of showers. Dry conditions look to finally return across the region next weekend. Temperatures will remain below normal through most of this week, with some moderation possible next weekend. && .NEAR TERM /UNTIL 6 PM THIS EVENING/... 1030 AM update... * Heavy rainfall, isolated thunder, strong winds and areas of urban and poor drainage flooding Still a few areas across far northern and NW MA where temps are 31-32 degrees with pockets of icing. However, temps will be rising above freezing next 1-2 hours which will end icing threat. Main concern today is heavy rainfall and urban and poor drainage flooding, convection and risk for strong wind gusts. Heavy rainfall... Numerous showers, locally heavy moving across SNE but bigger concern is area of heavy rain and convective line lifting NE from SE NY through NJ along the nose of the low level jet. This jet is rather anomalous at 4+SD above normal and combined with 2SD PWATs will lead to heavy rainfall. Hi-res guidance shows this line moving through SNE 16-22z. Up to a 3 hour period of heavy rain with localized rainfall rates 0.5 to 1 inch/hr expected as this area moves through. MUCAPES 200-300 J/kg so isold t-storms are expected and lightning strikes noted across LI and SE NY. Areas of urban and poor drainage flooding likely although system is progressive which will reduce risk of very heavy rainfall amounts. Still expect widespread 1 to 2 inch amounts with localized 3 inch amounts possible. Heaviest rainfall should be tied to upslope areas of the Worcester hills and east slopes of the Berkshires. Strong wind... Already seeing east gusts 40+ mph along the coast. These gusts are a result of decent mixing from still cool low level temps. This afternoon, the risk for strong winds will come from low level jet which moves along the south coast 18-23z. Soundings do show a low level inversion and it appears temps will need to reach near 60 to erode the inversion enough to bring stronger winds down to surface. We expect temps to reach well into the 50s across from NE CT through RI and SE MA with a chance a few locations could hit 60. In addition, convective line moving through will increase risk of bringing these stronger winds down so we issued a wind advisory for southern RI and south coastal MA including the Cape/Islands for a brief period of gusts 45-50 mph. PREVIOUS DISCUSSION... Quiet an anomolous weather system for mid-April. Strong upper level low pressure that was over the Midwest will continue to move northeastward today. In response, a secondary low will take over near the triple point and move over the Hudson Valley and into western MA. Ahead of this system, strong PWAT plume will advect into the region nearing 2 to 3 STD above normal as a 925mb LLJ reaches close to 4 STD above normal. The combination of the strong jet and moisture plume will result in widespread heavy rainfall today. Luckily this system will be moving quickly and the heavy precipitation should be ending by the early evening hours. Depending on the exact location of the surface low and its associated fronts, temperatures could struggle to get above 45F, esp near route 2 today. Details on the hazards below. Temperatures... Precipitation should change to all rain by 10am as 925mb temps continue to warm. Still cannot rule out the potential for lingering ice across the high terrain near the MA/NH border. Surface low approaching from the west will help increase WAA esp near the surface as 925mb temps warm from 0C to 12-15C. Still a large gradient near route 2 so expect locations across the high terrain to remain in the 40s while locations south of the MA Pike warm into the mid 50s. Areas in between may only see a quick hour or two of temps in the 50s before quickly falling again as the warm sector will be short lived as surface cold front will quickly follow. Heavy rainfall... Strong PWAT plume will advect into the region while strong upper let jet will move overhead. PWATs will increase to near 1.3+ inches thanks to Gulf moisture influence. This amount of moisture combined with the LLJ jet will develop widespread heavy precip this morning into the afternoon. Timing seems to be on track from previous forecast. Heaviest rain will move into the west between 12-18z and then the eastern half of the region between 15-21z. There still remains a risk for elevated thunder as Showalters drop at or below 0 and K values increase above 30. In fact, dewpoints will warm into the 50s once we warm sector, esp south of the Pike. Thus believe the best region to see thunder today is across CT, RI and southeast MA. But because this system is quite strong, cannot rule out thunder reaching up to the North Shore. These seems to generally coincide with SPC`s general thunder risk. The risk for heavy rainfall across the region remains, esp with the risk for thunder. QPF totals will generally be between 1.5-2 inches with the classic shadowing possible in the CT river valley. Some of the hi-res guidance does spit out around 3 inches of QPF, but that is across the higher terrain due to upslope. While this is a lot of rain, rivers have been running a bit low so should be able to handle this much QPF. We may see some rises on the flashy streams but they should stay in their banks. Right now think that the urban and poor drainage flooding is the main threat. Gusty Winds... Tricky forecast when it comes to the winds as it depending on the depth of mixing today. Strong 925mb LLJ will increase to 60-70 kts today with the highest gusts across the southeast portion of the region. Winds appear to struggle to mix down, despite some warming today. BUFKIT soundings still show a stout inversion in place keeping most of the strong wind at bay. Still expect some mixing which will bring down around 30-40 MPH wind gusts. One region to watch is the east slope of the Berks and Worcester hills. The high terrain and upslope features could bring down some of these wind gusts. Esp since some of the model guidance shows lapse rates closer to 5-6 C/km. However, since temps will still be significantly cooler felt that the winds will struggle to mix down. Still cannot rule out 45 MPH wind gusts. Lastly, if any thunderstorm or heavy rain shower does develop, then gusty winds will be the main threat as they could tap into the strong wind aloft. Overall today will be a wind swept rain with the threat for isolated thunder and gusty winds. && .SHORT TERM /6 PM THIS EVENING THROUGH 6 AM TUESDAY/... Tonight... Conditions will begin to dry out tonight. However, strong upper level low will continue to spin over the Northeast. Cyclonic flow aloft as well as increasing mid-level moisture could develop some sct showers during the overnight hours. Area of best location will be across west, which is near the better dynamics. Temperatures behind the passing cold front will begin to fall through the overnight hours. Overnight low will bottom out into the mid 30s to low 40s. However, across the higher terrain lows will be near freezing so with any passing shower, expect some snow associated with it. Gusty winds will begin to die down the low level gradient begins to weaken. && .LONG TERM /TUESDAY THROUGH SUNDAY/... Overview... 165/00Z guidance remains in rather good agreement through Friday, with only typical minor differences further out in time. Will continue to favor a consensus approach for this portion of the forecast. Expecting an active pattern to persist through Thursday, with more quiet weather towards next weekend. Temperatures near to below normal for this part of the upcoming week. Details... Tuesday through Wednesday...Strong mid level cutoff low will slowly approach the Maritimes sometime Wednesday. With the continued cyclonic mid level synoptic flow, along S-SE winds, will keep decent low level moisture across the region, especially along the coast. Lift gets weaker as the core moves away from our region. Can`t rule out isolated to scattered showers both days. Wednesday night through Friday night... Another fast moving mid level shortwave heads towards the Great Lakes Wednesday night into Thursday. This wave looks potent enough to generate a clipper low pressure, and move it through our region Thursday into Thursday night. While this low pressure should be off to our east by Friday, still expecting to be stuck in cyclonic flow across southern New England. Looking at another round of wet weather during this time, mostly late Wednesday night into Thursday. Looks to be all rain this time, expect across the highest terrain of northern MA, where some wintry precipitation may mix in for a time during the night. Saturday and Sunday... Large high pressure will move SE from near Hudson Bay and central Canada, and will expand SE across New England to the mid Atlantic states through this weekend. && .AVIATION /15Z MONDAY THROUGH FRIDAY/... Forecaster Confidence Levels... Low - less than 30 percent. Moderate - 30 to 60 percent. High - greater than 60 percent. Short Term /through Tonight/... Today...Moderate confidence. Widespread IFR with areas of fog developing and spreading inland from the coast as higher dewpoint air moves in over cooler ocean. Period of heavy rain will move across the region with isold thunder possible. E gusts to 20-30 kts up to 40 kt Cape/Islands and Worcester hills. Tonight...Moderate confidence. Improving cigs to MVFR. Sct Rain/Snow showers out west. Drying trend towards dawn. Northeast winds will switch to the south and then southwest. Gusts falling to 15 kts near the coast. KBOS Terminal...Moderate confidence in TAF. Heavy rain late this morning/early afternoon. Low prob for thunder. KBDL Terminal...Moderate confidence in TAF. Heavy rain this morning through early afternoon. Low prob for thunder this morning. Outlook /Tuesday through Friday/...Moderate confidence. Tuesday: Mainly VFR, with local MVFR possible. Breezy. Slight chance SHRA. Tuesday Night through Wednesday: Mainly VFR, with local MVFR possible. Breezy. Wednesday Night: Mainly VFR, with local MVFR possible. Breezy. Chance SHRA, slight chance FZRA. Thursday: Mainly MVFR, with local IFR possible. Chance SHRA, slight chance FZRA. Thursday Night through Friday: Mainly VFR, with areas MVFR possible. Chance SHRA. && .MARINE... Forecaster Confidence Levels... Low - less than 30 percent. Moderate - 30 to 60 percent. High - greater than 60 percent. Short Term /through Tonight/...High confidence. Gale warnings will continue today and should drop to SCA by tonight. E/NE gusts increasing to 35-40 kt across south coastal waters this morning will transition to E gusts to 40-45 kt over eastern MA waters today. Strong waves over the near shore eastern waters, with near 6-12 foot seas right up to the shoreline. Vsbys will lower today as fog becomes more widespread with a period of heavy rainfall and isold thunder. Passing cold front this evening will switch the winds to the west towards dawn on Tuesday. Gusts should begin to subside allowing for seas to be relax. Outlook /Tuesday through Friday/...Moderate Confidence. Tuesday: Low risk for Small Craft Advisory winds with gusts up to 25 kt. Rough seas up to 10 ft. Tuesday Night: Low risk for Small Craft Advisory winds with gusts up to 25 kt. Areas of rough seas. Slight chance of rain showers. Wednesday: Low risk for Small Craft Advisory winds with gusts up to 30 kt. Areas of rough seas. Wednesday Night: Winds less than 25 kt. Local rough seas. Chance of rain showers. Thursday: Winds less than 25 kt. Areas of seas approaching 5 ft. Chance of rain showers. Thursday Night: Winds less than 25 kt. Local rough seas. Chance of rain showers. Friday: Winds less than 25 kt. Seas up to 5 ft. Slight chance of rain showers. && .TIDES/COASTAL FLOODING...
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1050 AM update... We expanded the coastal flood advisory south to include Boston and Suffolk county and Plymouth county. Tidal departure running around 1.5 ft at Boston. With seas already 13-14 ft this should be enough for areas of minor flooding of the typical vulnerable shore roads. Previous discussion... Looking at the coastline, feel that the Coastal Flood Advisory for coastal Essex county appears to be on track for the midday high tide. Thanks to strong easterly gales, seas will build to 15-20 ft just offshore from NE MA coast. Guidance continues to suggest a storm surge up to 1.5 ft. This combined with these waves may be enough for minor flooding of vulnerable shore roads along the Essex county coast where highest wave action expected. Tides are a bit higher for the Monday night tide just after midnight but wave action and surge a bit less. Especially as the wind will switch to a more southerly direction. At least splashover is possible, but feel that weak winds and offshore direction will lessen the risk. Along the south coast, it appears the strongest winds and surge will occur late afternoon between the high tides so this may limit minor flood potential.
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&& .CLIMATE... New record lowest max temperatures set for April 15 BOS 39 in 2018. Previous 40 in 1881. PVD 41 in 2018. Previous 42 in 1972. BDL 40 in 2018. Previous 41 in 1943. ORH 34 in 2018. Previous 39 in 1943. New record daily snowfall set for April 15 PVD trace in 2018. Previous trace in 2007. BDL 0.5 in 2018. Previous trace in 2014. ORH 0.4 in 2018. Previous 0.2 in 1988. && .BOX WATCHES/WARNINGS/ADVISORIES...
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CT...None. MA...Wind Advisory until 8 PM EDT this evening for MAZ017>024. Coastal Flood Advisory until 2 PM EDT this afternoon for MAZ007-015-016-019. RI...Wind Advisory until 6 PM EDT this evening for RIZ002>008. MARINE...Gale Warning until 7 PM EDT this evening for ANZ231>235-237- 255-256. Gale Warning until 2 PM EDT this afternoon for ANZ230. Gale Warning until 7 PM EDT this evening for ANZ236. Gale Warning until 10 PM EDT this evening for ANZ250-251-254.
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&& $$ SYNOPSIS...Belk/Dunten NEAR TERM...KJC/Dunten SHORT TERM...Dunten LONG TERM...Belk AVIATION...Belk/Dunten MARINE...Belk/Dunten TIDES/COASTAL FLOODING... CLIMATE...

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