Area Forecast Discussion
Issued by NWS Boston, MA

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654 FXUS61 KBOX 171755 AFDBOX Area Forecast Discussion National Weather Service Boston/Norton MA 155 PM EDT Thu May 17 2018 .SYNOPSIS... 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. && .NEAR TERM /UNTIL 6 PM THIS EVENING/...
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150 PM update... Back edge of the rain now over the Islands and slowly eroding on the northern edge. Otherwise, sunshine developing across the interior with clouds prevalent near the coast. Partial sunshine will develop across RI and SE MA through the afternoon but clouds will linger along the south coast and especially Cape/Islands. Cold front pushing south from central VT/NH will move across SNE this evening and tonight. Models indicate some theta-e ridging and steep low level lapse rates developing ahead of the front across northern MA late today/evening so can`t rule out a few showers developing to the north toward evening. 925 mb temps around 18C this afternoon in the interior which normally would support highs into the lower 80s. However, given later arrival of sunshine, scaled back temps into the upper 70s interior. Cooler 60s closer to the south coast due to cloud cover.
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&& .SHORT TERM /6 PM THIS EVENING THROUGH 6 PM FRIDAY/... Clouds linger Thursday night, especially in CT-RI-SE Mass, so we will go with min temps similar to tonight. High pres to the north 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. && .LONG TERM /FRIDAY NIGHT THROUGH WEDNESDAY/... 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. && .AVIATION /18Z THURSDAY THROUGH TUESDAY/...
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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/... Through 00Z... IFR cigs near the south coast should temporarily improve to VFR 19-20z but low cigs and patchy fog lingering over the Islands. Rain ending over the islands by 19z. Tonight... Low confidence on northward extent of stratus and fog. Expect IFR stratus and patchy fog to redevelop along the south coast but uncertain how far north it expands. MVFR/IFR cigs may spread inland to the I-95 corridor for a time tonight but confidence is low. Higher confidence of VFR in the interior. Friday...Moderate confidence. MVFR cigs may linger through the morning near the south coast with patchy IFR over the islands. Otherwise VFR cigs. NE wind gusts to 25 kt along the coast. Dry weather. Friday night...Moderate confidence. Conditions lowering to MVFR/IFR along the south coast, then spreading northward toward daybreak. Rain developing from SW to NE well after midnight. KBOS Terminal...Moderate confidence in TAF. There is a risk of MVFR cigs redeveloping tonight. NE wind gusts 20-25 kt developing after 06z. KBDL Terminal..High confidence in TAF. 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... 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. && .BOX WATCHES/WARNINGS/ADVISORIES... 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 8 PM this evening to 9 PM EDT Friday for ANZ255-256. && $$ SYNOPSIS...Belk/Dunten NEAR TERM...Belk/KJC SHORT TERM...Belk LONG TERM...Dunten AVIATION...KJC/Dunten MARINE...Belk/Dunten is the U.S. government's official web portal to all federal, state and local government web resources and services.