Area Forecast Discussion
Issued by NWS Boston, MA

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000 FXUS61 KBOX 170756 AFDBOX Area Forecast Discussion National Weather Service Boston/Norton MA 356 AM EDT Thu May 17 2018 .SYNOPSIS...
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A weak cold front sweeps south from Canada today, pushing any leftover morning rain offshore. Friday will feature cooler weather with showers moving in by the later half of the day. Widespread heavy rainfall will move into the region Friday night into Saturday, followed by hit or miss showers/thunderstorms on Sunday. Dry weather returns by Tuesday of next week.
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&& .NEAR TERM /UNTIL 6 AM THIS MORNING/...
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Showers making some progress off to the east this morning, but not much progress north of the Mass Pike. A low pressure moving from the Great Lakes into the Saint Lawrence valley today will drive a dry cold front across southern New England. This will shunt any remaining showers this morning farther offshore through the day. It may take until early afternoon to clear the Cape and islands. Still thinking it will be a more clouds than sun kind of day, especially towards the south coast. Where the sun does break out, expecting temperatures well into the 70s. Unfortunately, temperatures this high will lead to seabreezes, and lower temperatures along the immediate coasts.
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&& .SHORT TERM /6 AM THIS MORNING THROUGH 6 PM FRIDAY/...
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High pressure to our n High pressure builds south behind the cold front and brings partial clearing to our area. Best chance for clearing will be north of the Mass Pike. Temps at 850 mb are expected to be 12-13C. With full mixing this would support max sfc temps in the lower 80s, but any clouds would keep temps a little lower than this. We started with guidance values and added a couple of degrees. Clouds linger Thursday night, especially in CT-RI-SE Mass, so we will go with min temps similar to tonight.orth will gradually have more of an influence on our weather tonight into Friday. Expecting mainly dry weather during this time. Although some question about how quickly rainfall will return late Friday. Kept a mention of a chance for showers late Friday, especially towards the south coast. Not expecting temperatures to change much from morning lows Friday, with an onshore flow and not much sunshine.
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&& .LONG TERM /FRIDAY NIGHT THROUGH WEDNESDAY/...
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Highlights... * Widespread heavy rainfall, flooding potential, Friday night into Saturday * Hit or miss showers/thunder on Sunday with warming trend * Dry weather returns by Monday of next week Details... Friday night into Sunday...Moderate confidence. A wet period will occur for the start of the weekend thanks to the Bermuda high off the southeast coast and a shortwave trough and associated low pressure system in the TN Valley. A stationary front extends from this low to the Mid-Atlantic. This front combined with the passing surface high in the Gulf of Maine will result in strong southerly winds resulting in a high PWAT plume from the Gulf of Mexico towards New England. This tropical connection will result in widespread heavy rainfall late Friday night and into Saturday. Friday evening and towards the overnight will remain dry for most of the region. The stationary front will begin to push northward during the overnight which could result in enough overrunning for precip along the south coast. The front will lift northward as a warm front during the day, however, appears that passing high pressure in the Gulf of Maine will still leave the region with easterly flow resulting in a cool, wet Saturday. Warm air advection and isentropic lift increases by Saturday morning allowing periods of rain to arrive from southwest to northeast. Given the plume of Gulf moisture riding up and over the frontal boundary, anticipate periods of moderate to heavy rainfall at times. In fact, PWATs will near 3-4 standard deviations above normal, reaching near 2.00. Models also suggest an increase 850mb LLJ reaching between 45-55 kts. This type of lift within the high PWAT airmass will result in high moisture transport as well as warm rain processes increasing confidence in heavy rainfall. Ensemble members continue to increase their probabilities of seeing over an inch of precip within a 24 hour time. Even the EPS are hinting at some low probs of 2 inches within 24 hours. This type of rainfall could lead to urban and poor drainage flooding. We may even need to watch the flashier streams as they could rise quickly with this much rain. Main stem rivers should remain in their banks as flows are currently at or below normal. Models are in agreement that the region will warm sector late Saturday into early Sunday. This will result in a non-diurnal temperature swing for the overnight. Sunday will be warm and humid as the region warm sectors leading to highs into the upper 70s. Approaching shortwave from the west could result in scattered showers and thunderstorms during the afternoon hours. Mid-level lapse rates near 6 C/km with showalters dropping below 0 and both surface and MU cape could be enough to result in a strong storm or two. Still some uncertainty due to the timing of the cold front that will approach from the west. Monday and beyond...Moderate confidence. Conditions should be improving as upper level trough pushes cold front through the region Sunday night/early Monday. Surface high pressure from Canada will move into the region resulting in dry weather. Zonal flow aloft, which could allow for any upper level disturbance to pass through during the week. Models are hinting at some weather around the Tuesday timeframe. So will insert a chc pop for now.
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&& .AVIATION /08Z THURSDAY THROUGH MONDAY/...
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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 Friday/...Moderate confidence. Through 12Z...MVFR and IFR cigs across most of southern New England. Light wind. Today...Light rain gradually moves offshore this morning with conditions improving to VFR during the morning hours. Light winds in the morning turn from the north-northwest by afternoon. Local seabreezes likely. Tonight...Mainly VFR. Clouds linger, especially along the south coast. Winds turn from the northeast with gusts near 20 knots along the coast after midnight. Friday...VFR. Breezy. Chance SHRA late in the day towards the south coast. KBOS Terminal...Moderate confidence in TAF. Main uncertainty is the timing of ceilings improving this morning. KBDL Terminal..Moderate confidence in TAF. Main uncertainty is the timing of ceilings improving this morning. Outlook /Friday Night through Monday/...Moderate confidence. Friday Night: Mainly MVFR, with areas VFR possible. Breezy. Chance RA. Saturday: Mainly MVFR, with areas IFR possible. Breezy. RA. Saturday Night: Mainly MVFR, with areas IFR possible. Breezy. SHRA, isolated TSRA. Sunday: Mainly MVFR, with areas VFR possible. Breezy. Chance SHRA, isolated TSRA. Sunday Night through Monday: VFR.
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&& .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 Friday/...Moderate confidence. Expecting an increasing NE flow late today, and continuing into Friday as pressure gradient increases from a high pressure passing by to our north. Seas slowly build in response, with 5 foot values expected on the outer waters Thursday night. Light rain showers across the southern waters this morning, possible into early afternoon. Areas of fog should reduce vsbys below 3 miles overnight. Dry weather much of tonight and Friday. Some risk for light rain across the southern coastal waters late Friday afternoon. Outlook /Friday Night through Monday/...Moderate confidence. Friday Night: Low risk for Small Craft Advisory winds with gusts up to 25 kt. Seas up to 5 ft. Chance of rain. Saturday: Winds less than 25 kt. Seas up to 5 ft. Rain likely. Saturday Night: Winds less than 25 kt. Areas of rough seas. Rain showers likely, isolated thunderstorms. Sunday: Low risk for Small Craft Advisory winds with gusts up to 25 kt. Areas of rough seas. Chance of rain showers, isolated thunderstorms. Sunday Night: Winds less than 25 kt. Local rough seas. Monday: Winds less than 25 kt. Areas of seas approaching 5 ft.
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&& .BOX WATCHES/WARNINGS/ADVISORIES...
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CT...None. MA...None. RI...None. MARINE...Small Craft Advisory from 4 AM to 9 PM EDT Friday for ANZ232>235-237. Small Craft Advisory from 2 AM to 11 AM EDT Friday for ANZ230. Small Craft Advisory from 2 AM to 1 PM EDT Friday for ANZ231- 251. Small Craft Advisory from 6 AM to 3 PM EDT Friday for ANZ236. Small Craft Advisory from 2 AM to 9 PM EDT Friday for ANZ250- 254. Small Craft Advisory from 5 PM this afternoon to 9 PM EDT Friday for ANZ255-256.
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&& $$ SYNOPSIS...Belk/Dunten NEAR TERM...Belk SHORT TERM...Belk LONG TERM...Dunten AVIATION...Belk/Dunten MARINE...Belk/Dunten

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