Area Forecast Discussion
Issued by NWS Boston, MA

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955 FXUS61 KBOX 201931 AFDBOX Area Forecast Discussion National Weather Service Boston/Norton MA 331 PM EDT Sun May 20 2018 .SYNOPSIS... After a few showers with a cold frontal passage this evening, drier weather returns through Monday night. Cooler with passing showers Tuesday afternoon into Tuesday evening as a warm front approaches. Mainly dry weather for the rest of the week into Saturday with temperatures averaging above normal. The risk for showers increases by Sunday. && .NEAR TERM /UNTIL 6 AM MONDAY MORNING/... SFC obs and MSAS mass fields suggest the cold front is currently draped from about Hartford-Worcester-Boston at the time of this writing and continuing the slide SSE. With the attendant upper shortwave weakening and flow aloft becoming more zonal, this front may slow somewhat, but current timing has it fully offshore of even the Cape/Islands after 00Z. SCT SHRA have developed where some late morning and afternoon clearing were able to force MU CAPE values to about +500j/kg, but with mediocre lapse rates and low ML Cape values they have struggled to form decent updrafts. No TS so far and in fact, will feature gradually dissipating POPS as the front and SHRA slide S as cloud cover has kept S CT, RI and SE MA even more stable. Otherwise, lingering coastal fog possible through the evening hours, but with developing NW flow behind the cold frontal passage these should clear on the mainland. Widespread stratus/fog to the W of MVY/ACK may linger later in the evening but these too should be clearing with time. NW drier flow should lead to clearing through the overnight, pres rises are relatively meager. Therefore, could be a cool morning with the possibility of localized decoupling, mins in the upper 40s to mid 50s. && .SHORT TERM /6 AM MONDAY MORNING THROUGH MONDAY NIGHT/... Tomorrow... 1025mb high pres noses in from the W through the day as modest mid lvl ridging returns. Dry wx prevails with some diurnal CU possible. H85 temps near +10C, but mixing may well exceed this value, therefore expecting widespread mid-high 70s for highs. A few exceptions, W downsloping should allow typical downslope locations like the CT and Merrimack valleys to reach around 80, and sea breezes are possible along the immediate coastlines, holding highs in the lower 70s. Comfortable with low, well mixed dwpts. Tomorrow night... Mainly dry as high pres slips E. Only issue will be incoming mid and high clouds during the AM hours. However, the precip shield from a remnant MCS should hold off until the daylight hours. Mins in the 50s most locations. && .LONG TERM /TUESDAY THROUGH SUNDAY/... Highlights... * Cooler with passing showers Tue into Tue night * Mainly dry weather with above normal temps Wed through Sat, coolest Thursday * Risk of showers increases by Sunday Overview... Zonal flow with weak shortwave passage Tue, then northern stream trough will amplify to the north and east Wed into Thu which will push a cold front through the region. Rising heights and shortwave ridging follow Fri into Sat then next northern stream trough approaches Sunday. Precipitation... Main focus for wet weather this week will be Tue into Tue night as shortwave passage with weak warm front approaching from the SW and moistening column will likely bring some showers during this time period. Best chance appears to be in the afternoon/evening. No surface instability but marginal elevated instability may support an isold t-storm late late Tue and Tue evening. Looks dry Wed into Sat. We have a cold front moving through Wed, but limited moisture and low level convergence will likely result in a dry fropa. Increasing risk of showers by Sunday with approach of next northern stream trough, but confidence is low due to timing uncertainty. Temperatures... Coolest day of the week will likely be Tue as SNE will be influenced by clouds and some shower activity on cool side of frontal boundary, but temps close to seasonable normals. Warmer air returns Wed ahead of a cold front then a bit cooler post-frontal airmass Thu. Potential for summer warmth with temps into the 80s interior Fri-Sun. && .AVIATION /20Z SUNDAY THROUGH FRIDAY/... Forecaster Confidence Levels... Low - less than 30 percent. Moderate - 30 to 60 percent. High - greater than 60 percent. Short Term /through Monday Night/... Through 03Z...Moderate confidence. Mainly VFR north of a line from HVN-PVD-GHG, but with a risk for SHRA between 19Z-23Z from N-S as they develop. IFR/LIFR S of this line lingers through the evening, with a risk for a spot shower as the line of SHRA dissipates. Winds shift to the W then NW through the evening. Overnight into tomorrow...High confidence. Winds continue to shift to the NW through the early AM hours this will allow for clearing of most clouds into the daylight hours Monday. VFR dominates, except maybe ACK where lingering fog is possible during the morning. Tomorrow night...High confidence. Mainly VFR. Light W Winds. KBOS Terminal...Moderate confidence in TAF, at issue is the exact timing for a SHRA/TSRA risk mainly 19Z through 21Z. Improvement thereafter. KBDL Terminal...Moderate confidence in TAF. Outlook /Tuesday through Friday/...Moderate confidence. Tuesday: VFR. Chance SHRA. Tuesday Night: Mainly VFR, with local MVFR possible. Chance SHRA. Wednesday: VFR. Slight chance SHRA. Wednesday Night through Thursday Night: VFR. Friday: VFR. Breezy. && .MARINE... Forecaster Confidence Levels... Low - less than 30 percent. Moderate - 30 to 60 percent. High - greater than 60 percent. Short Term /through Monday Night/...High confidence. Winds will gradually be shifting to the NW all waters late this afternoon and evening, but should remain below 25kt from now on. The winds remain below Small Craft thresholds through tomorrow night. Seas however were running about 5-7 ft on the SE ocean waters, so will continue Small Craft Advisory into tomorrow for these seas, at which point they too should drop below thresholds. A few showers cross the waters through the early evening hours. Otherwise, lingering fog possible mainly across the SE waters, these too should lift during the overnight. Outlook /Tuesday through Friday/...Moderate confidence. Tuesday through Tuesday Night: Winds less than 25 kt. Chance of rain showers. Wednesday: Winds less than 25 kt. Slight chance of rain showers. Wednesday Night through Thursday: Winds less than 25 kt. Seas locally approaching 5 ft. Thursday Night: Winds less than 25 kt. Friday: Winds less than 25 kt. Areas of seas approaching 5 ft. && .FIRE WEATHER... Spotty locations received as much as 0.5 inches of precipitation this weekend, but several locations received less. Given fuels were dry, this may not have been enough to fully moisten them. RH values Monday will drop as low as 15-25 percent inland, but winds should remain light out of the NW, around 10mph most of the day. && .BOX WATCHES/WARNINGS/ADVISORIES... CT...None. MA...None. RI...None. MARINE...Small Craft Advisory until 1 PM EDT Monday for ANZ254>256. Small Craft Advisory until 6 AM EDT Monday for ANZ235-237. && $$ SYNOPSIS...KJC/Doody NEAR TERM...Doody SHORT TERM...Doody LONG TERM...KJC AVIATION...KJC/Doody MARINE...KJC/Doody FIRE WEATHER... is the U.S. government's official web portal to all federal, state and local government web resources and services.