Area Forecast Discussion
Issued by NWS Boston, MA

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618 FXUS61 KBOX 242323 AFDBOX Area Forecast Discussion National Weather Service Boston/Norton MA 723 PM EDT Tue Apr 24 2018 .SYNOPSIS... Low pressure approaching from the southwest will bring a soaking wind swept rain to the region very late tonight and Wednesday with localized heavy downpours. This may result in some brief nuisance poor drainage street flooding. Other than a few brief showers possible mainly dry weather follows Thursday through Saturday along with seasonably mild temperatures. A brief cool down expected on Sunday, but this will be short lived as unseasonably warm temperatures are possible by the middle of next week. && .NEAR TERM /UNTIL 6 AM WEDNESDAY MORNING/...
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715 PM Update... Only adjustment to the forecast is to delay the onset of rain by a couple hours or so based on latest model and observed trends. Otherwise, the forecast remains on track. 400 PM Update... Dry weather prevails this afternoon as high pressure east of the region continues to influence the area. Plenty of mixing across the area which has resulted in temperatures warming into the upper 60s and low 70s. Southwest wind gusts have reached around 20-25 MPH, but should subside by sunset. Aside from a few minor tweaks, the forecast remains quiet and on track for this evening. Tonight... Southern stream low pressure system and approaching upper level trough from the upper Midwest will bring periods of rainfall late tonight and into tomorrow. Model trends have slowed the onset time just slightly for the onset of the warm advection rainfall. This is due to such a dry airmass in the mid-levels (700-800mb). BUFKIT soundings moisten the profile out west by 06z and over the east around 10z. Thus went ahead and slowed the timing. Aside, expect clouds to thicken and lower during the overnight hours before the precipitation begins to fall. Anticipate showers to start due to isentropic lift and in the WAA pattern. Heavier precip will fall by 09-12z from west to east towards the morning hours. Because of the increase cloud cover, overnight temperatures will be mild as radiational cooling will be limited. Lows will remain in the mid 40s.
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&& .SHORT TERM /6 AM WEDNESDAY MORNING THROUGH WEDNESDAY NIGHT/... Wednesday into Wednesday night... *** Heavy rainfall and isolated thunder for Wednesday *** Southern stream surface low will approach the region on Wednesday as northern stream trough swings into the area from southern Canada. This surface low will move across the region, allowing for the southern half of SNE to become warm sectored. This upper level trough will continue to dig and go negativly tiled pushing the surface low up into teh Gulf of Maine by Thursday morning. Rainfall... Steady rainfall will stream into the region by Wednesday morning as PWAT values increase to 1.3 inches, 1-2 STD above normal. Associated with this system, strong LLJ nearing 3 STD above normal will allow for strong moisture transport into the region. Right now appears that widespread 1-2 inches of rainfall will occur through Wednesday night. Think the highest amounts will occur across the Worcester hills and the east slope of the Berks where upslope flow will help create a bit more lift. Cannot rule out the potential for 2+ inches in that region, which is indicated by the ARW. Still with this much rainfall, appears the the main stem rivers will remain in their banks per NERFC`s forecast and MMEFS. Still, we will have to watch some of the flashy streams. Right now believe that urban and poor drainage flooding will be the main threat. Precip will come to an end by late Wednesday into the overnight hours as main jet lifts farther north. Models also show some drying into the mid to upper levels which could result in showery weather towards Thursday morning. Wind... As mentioned before, strong LLJ increases during the day on Wednesday reaching close to 3 STD above normal. In fact, 925mb winds increase between 50-60 kts. Believe that most of this wind will remain elevated as soundings show a strong inversion across the area. However, could see some mixing near the southeast coastal plain where the potential for thunder could bring down a few strong winds gusts. Also appears to be a narrow window where lapse rates may be steep enough. Believe that we could see somewhere near 25-35 MPH across the southeast coastal plain. Will hold off in any adv for this time as confidence is not that high. Thunder... A low risk for thunder during the day on Wednesday as we warm sector just a bit by the afternoon hours. Models indicate that showalters drop below 0 and there is some elevated CAPE nearing a few hundred jules. Like the timing of the SPC HREF MU CAPE so tried to incorporate that into the forecast. Biggest risk would be heavy rainfall and some gusty winds within any thunderstorm. Fog... As the system begins to wind down late Wednesday night into Thursday morning, we may have to watch for the potential for fog. High dewpoint airmass will still be over the region Wednesday night and with some drying in the mid-levels, low levels appear to remain saturated. Winds also appears to be light. So aside from some showers moving across the region, may have to watch for patchy dense fog, especially across the south coast. && .LONG TERM /THURSDAY THROUGH TUESDAY/... Highlights... * Other than a few possible brief showers mainly dry and seasonable Thu/Fri/Sat * Dry but noticeably cooler and blustery Sun * Moderating temps Monday and likely in the 70s Tue & Wed!!! Details... Thursday... Moist southern stream trough moving offshore around daybreak followed by a drying trend through the morning and especially the afternoon. The only wildcard will be a potent northern stream short wave that moves across NY state into VT/NH during the afternoon. Cold temps aloft (-20C at H5) combined with cyclonic flow will yield a risk for a few afternoon showers. However given the column dries significantly any shower threat will likely be confined to northern MA along the NH border where greatest moisture is available. Otherwise mainly dry weather prevails much of the afternoon with at least partial sunshine especially across CT/RI and southeast MA with more clouds over northern MA. Seasonably mild with highs in the 60s, cooler along the south coast given southwest winds off the cool ocean waters. It will be somewhat breezy with WSW winds 15-20 mph gusting up to 25 mph. Friday and Saturday... Friday`s forecast has the most uncertainty as southern stream short wave trough approaches from the southwest. This trough will be in the process of deamplifying. So despite deep layer moisture associated with this trough, forcing for ascent will be on a downward trend. Thus not expecting a washout but can`t rule out scattered showers. Seasonably mild with highs once again in the 60s except cooler along the south coast with SW winds off the cooler ocean. As for Sat, vigorous northern stream trough approaches from the northwest along with attending surface cold front. However limiting factor for rainfall is lack of deep layer moisture. Thus only expecting risk of a few showers with most locations possibly remaining dry. Mild once again with highs in the 60s, 50s south coast. Sunday... Dry weather but noticeably cooler and blustery behind frontal passage Sat night into Sunday with 850 temps down to about -2C (1 standard deviation cooler than climo). This will only support highs in the 50s but will feel cooler given blustery WNW winds. Monday and Tuesday... Deep mid level trough moves offshore with height rises and a warming trend developing as east coast ridging commences toward mid week. Dry weather much of this period with Monday the transition day, a cool morning giving way to a mild afternoon. Deterministic and ensemble guidance support highs in the 70s Tue and Wed. Normal high is 60-65. && .AVIATION /23Z TUESDAY THROUGH SUNDAY/...
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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 Wednesday Night/...Moderate to High confidence. Expect VFR into the early morning hours. Winds will shift to the east/southeast between 06Z and 09Z. MVFR dropping to IFR towards Wednesday SW to NE generally between 10Z and 14Z. Rain will quickly overspread the region as winds remain ESE. IFR/LIFR is possible during the late morning into the afternoon with +RA and iso thunder. Low risk for 35 kt gusts across the southeast coastal plain. Otherwise strong LLWS through much of the day. Wednesday night...IFR with possible LIFR across the area. Improving to MVFR towards dawn on Thursday. +RA remains trending towards showery weather after midnight. Could be low vsbys along the south coast in Fog through much of the night. KBOS Terminal...High confidence in TAF. May get the first part of the morning rush through in VFR, albeit lowering cigs. However, E to SE wind accompanied by rain likely keep cigs/low late morning into Wed night. Low risk for TSRA after 21z. KBDL Terminal...High confidence in TAF. Morning rush could be impacted with +RA. Outlook /Thursday through Sunday/...Moderate to High Thursday: Mainly VFR, with local MVFR possible. Breezy. Slight chance SHRA. Thursday Night: VFR. Friday: Mainly VFR, with areas MVFR possible. Chance SHRA. Friday Night: Mainly MVFR, with local IFR possible. Slight chance SHRA. Saturday: Mainly VFR, with areas MVFR possible. Slight chance SHRA. Saturday Night: VFR. Sunday: VFR. Breezy.
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&& .MARINE... Forecaster Confidence Levels... Low - less than 30 percent. Moderate - 30 to 60 percent. High - greater than 60 percent. Short Term /through Wednesday Night/... Tonight...High confidence. Approach surface low will switch winds to the ESE and gusts to near 25kts. Seas will build as a response resulting in SCA. Wednesday and Wednesday night...Moderate confidence. Small Craft Advisory winds with gusts up to 30 kt and a low risk for a few 35 knot gusts early in the evening across the southeast waters. Rough seas up to 13 ft. Chance of rain showers, areas fog, isolated thunderstorms. Visibility 1 to 3 nm. Outlook /Thursday through Sunday/...Moderate to High Thursday: Winds less than 25 kt. Rough seas up to 10 ft. Slight chance of rain showers. Thursday Night: Winds less than 25 kt. Areas of rough seas. Friday through Friday Night: Winds less than 25 kt. Seas up to 5 ft. Chance of rain showers. Saturday through Sunday: Winds less than 25 kt. Seas up to 5 ft. && .EQUIPMENT... The NOAA Weather Radio transmitter serving Providence is out of service due to phone line problems. The phone company has prioritized the issues and is working to get the weather broadcast back on the air as soon as possible. The transmitter serving Hyannis is back in service. && .BOX WATCHES/WARNINGS/ADVISORIES... CT...None. MA...None. RI...None. MARINE...Small Craft Advisory from 4 AM to 9 PM EDT Wednesday for ANZ231-232. Small Craft Advisory from 7 AM to 9 PM EDT Wednesday for ANZ233-234. Small Craft Advisory from 1 PM to 8 PM EDT Wednesday for ANZ230. Small Craft Advisory from 9 AM to 6 PM EDT Wednesday for ANZ236. Small Craft Advisory from 4 AM Wednesday to 9 AM EDT Thursday for ANZ235-237-254. Small Craft Advisory from 8 AM Wednesday to 9 AM EDT Thursday for ANZ250-251. Small Craft Advisory from 3 AM Wednesday to 9 AM EDT Thursday for ANZ255-256. && $$ SYNOPSIS...Nocera/Dunten NEAR TERM...Dunten/Thompson SHORT TERM...Dunten LONG TERM...Nocera AVIATION...Nocera/Dunten/Thompson MARINE...Nocera/Dunten EQUIPMENT...

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