Area Forecast Discussion
Issued by NWS Boston, MA

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582 FXUS61 KBOX 250520 AFDBOX Area Forecast Discussion National Weather Service Taunton MA 120 AM EDT Sun Sep 25 2016 .SYNOPSIS... Cool and dry weather this weekend will continue into early next week. Low pressure may bring showers to southern New England some time late Monday night into Wednesday morning. Otherwise, seasonable and mostly dry weather is expected much of next week. && .NEAR TERM /UNTIL 6 AM THIS MORNING/... 10pm update... Minor updates this evening. Guidance actually too warm for radiational cooling ongoing as several sites already reporting calm winds. Dropped mins by a degree or two across the board. Not enough to expand any current headlines. Previous discussion... Tonight...High pressure building into the region from the north and west. Radiational cooling setup as the boundary layer decouples allowing for winds to become light beneath clear conditions. Though still a decent pressure gradient along the E/SE coast yielding some coastal and offshore NW winds thereby making it milder. Forecasting lows around the upper 30s with coldest conditions over N/W MA with the likelihood of some frost, warmest over the E/SE shore. Focus upon areas which are prone to radiate. && .SHORT TERM /6 AM THIS MORNING THROUGH 6 PM MONDAY/... Sunday...Unseasonably cool and mostly sunny. Highs topping out around the mid to upper 60s as the coldest of airmass at H85 sweeps through the region, but still stepped up a degree or two given the sun angle and dry conditions across the region. Quite the autumn feel. Worth nothing, such an H85 airmass, around +2C, close to the record lowest observed of just slightly below freezing per Chatham MA sounding climatology. Overall light winds but still a bit breezy along the E/SE coast. Went with deep-layer mixing of the boundary layer, so expect a mix down of drier air with dewpoints possibly into the 20s and breezy N winds along the E/SE coast with gusts up to 20 mph possible. Sunday Night...Given a chilly day and high pressure settling in over central New England overnight, am going to lean very low and close to record low temperatures given the synoptic setup. This gives a forecast low temperature range around the low to mid 30s for much of Southern New England. See the climate section below for September 25th and 26th record low temperatures. Clear conditions, light winds overall. Am concerned. Thinking an anomalous situation given dry conditions across the region coupled with the radiational cooling setup. Wonder whether guidance is capturing this rather well. A hunch that perhaps there`s an opportunity here to undercut guidance by a couple of degrees. Feel Sunday morning lows, if lower than initially thought, will weigh on confidence and forecast thinking going into Monday morning. Rather than going with double-headlines, have kept with the frost advisories for tonight going into Sunday morning, while highlighting the Monday morning impacts in the hazardous weather outlook. Could be looking at headlines over much of southern New England as frost conditions are possible in the E/SE interior of Massachusetts. && .LONG TERM /MONDAY NIGHT THROUGH SATURDAY/... HIGHLIGHTS... * Seasonable temperatures expected much of the long term * Low pressure may bring a chance of showers Mon. night to Wed. * Quite a bit of uncertainty in the pattern for late next week Operational models show quite a degree of spread in the synoptic pattern for next week. In addition, both the GEFS and the EPS members are spread over a couple of solutions. Much of this lays in how progressive the pattern will be, particularly across the NE. The slower solutions, including the ECMWF, keep the NE out of the westerly flow, allowing for the upper trough in the Great Lakes Monday morning to stall over the eastern US for much of the week. The more progressive models, including the Canadian, keep things moving, allowing the trough to move through southern New England and offshore. The GFS finds itself in the middle of the ensemble means, being a fairly good compromise on the speed. However, all of this spread leads to quite a bit of uncertainty especially Wednesday and beyond. Monday...High confidence. High pressure over southern New England will be the main influence for the weather. After a chilly morning, temperatures will rebound to near normal for late September. Still a shock after all the warm weather we`ve been having. Pleasant, dry, with full sun, it should be a nice day as the high slowly moves off to the south and east. Monday night through Wednesday morning...Moderate confidence. As the high moves offshore Monday night and a broad upper level trough moves into the Great Lakes, low pressure and a cold front will have some effect on southern New England. There is already some spread in the models for this time frame with the GFS again a bit more progressive than the ECMWF. The primary low pressure will be over the Great Lakes but both models indicate a secondary low developing somewhere around southern New England. The exact timing and placement of this development will have some effect on the timing and location of showers. However, there is expected to be some showery weather sometime during this time frame. The GFS is a little speedier, moving it out Tuesday night, while the ECMWF keeps a chance of showers continuing into Wednesday, particularly on the south coast. Will stick with high end chance to low end likely PoPs for much of this time frame. Otherwise, temperatures will continue to be seasonable with highs in the upper 60s to around 70 and lows in the upper 40s to mid 50s for most. Wednesday afternoon through Saturday...Low confidence. As mentioned above, much of this time frame will depend on how quickly the upper level trough is allowed to exit the east coast. The ECMWF is one of the slowest solutions, keeping the upper trough over the eastern half of the CONUS through Saturday. The GFS is one of the faster operational solutions, but in the midst of the ensemble members, and moves the upper trough out by Friday morning, allowing for some upper level ridging to begin. The main difference in sensible weather will be either a prolonged period of northeasterly winds and clouds and periods of showers, per the ECMWF. Or it will be drier with high pressure building into the region. Temperatures will remain seasonable either way. Will stick with a model blend for this time period, with a bit more weight given to the GFS given its place in the midst of the ensemble members. && .AVIATION /05Z SUNDAY THROUGH THURSDAY/...
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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 Monday/...High confidence. VFR. A few diurnal cu. Otherwise SKC. Some 15-20 kt NW winds during the daylight hours today, diminishing after sunset. KBOS TAF...High confidence in TAF. KBDL TAF...High confidence in TAF. Outlook /Monday Night through Thursday/... Monday night through Wednesday morning...Moderate confidence. Mainly VFR with periods of MVFR in scattered showers. Wednesday afternoon through Thursday...Low confidence. Most likely VFR for much of this time, but model uncertainty could mean periods of MVFR in lower clouds and scattered showers.
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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 Monday/...High confidence. This afternoon... Small craft boaters should be aware of the persistence of N winds upwards of 20 kts through much of the afternoon, settling towards evening under the influence of high pressure. Seas holding around 5 feet thus will continue the small craft advisories where the threat of 5 foot seas and gusts up to 25 kts is more likely. Tonight into Sunday... Resurgence of NW winds to near 25 kt will likely require near shore waters to once again have a small craft advisory, and continue the ocean waters. Seas will be the issue tomorrow, dropping from about 6-7 ft through the day. Outlook /Monday Night through Thursday/... Monday...High confidence. Quiet boating weather expected with winds and seas remaining below small craft criteria. Monday night and Tuesday...Moderate confidence. Winds and seas increase as low pressure approaches southern New England from the west. Small craft conditions likely. There is a low probability of southerly gale force gusts being reached on Tuesday. In addition, visibilities may be reduced at times in showers and fog. Wednesday and Thursday...Low confidence. Small craft conditions may continue, particularly on the outer waters for a time. It will depend on which model solution verifies. See long term section for more meteorological explanation.
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&& .CLIMATE... Record low temperatures... September 25th / 26th (last set in...) Boston ....... 38 (1950) / 34 (1879) Hartford ..... 30 (1963) / 33 (1978) Providence ... 36 (1963) / 37 (1967) Worcester .... 31 (1944) / 28 (1925) && .BOX WATCHES/WARNINGS/ADVISORIES...
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CT...None. MA...Frost Advisory until 8 AM EDT this morning for MAZ002>004-008. RI...None. MARINE...Small Craft Advisory until 11 AM EDT this morning for ANZ231>235-237-251. Small Craft Advisory until 3 AM EDT Monday for ANZ250-254-255. Small Craft Advisory until 4 PM EDT this afternoon for ANZ256.
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&& $$ SYNOPSIS...RLG/Sipprell NEAR TERM...Doody/Sipprell SHORT TERM...Sipprell LONG TERM...RLG AVIATION...Doody/RLG MARINE...RLG/Sipprell CLIMATE...

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