000 FXUS61 KBOX 200149 AFDBOX Area Forecast Discussion National Weather Service Boston/Norton MA 949 PM EDT Sun Sep 19 2021 .SYNOPSIS... Large high pressure builds into the region today and overnight. This will bring dry weather with mild days and cool nights through mid week. A slow moving frontal system approaching from the west will be accompanied by showers at times Thursday into Friday. Improving conditions expected Saturday but a few showers could linger in eastern Massachusetts and Rhode Island. && .NEAR TERM /UNTIL 6 AM MONDAY MORNING/... 950 PM update... Chilly night ahead, especially outside of the urban areas where temps are already down in the 50s. 1024 mb high over VT is providing a dry airmass, light winds and clear conditions. Hence, ideal radiational cooling conditions. Previous forecast is for many suburbs to fall into the 40s overnight, with 50s in the urban areas. These temps remain on track, thus no change to previous forecast. Earlier discussion below. ================================================================ High pressure builds over the region tonight with clear skies and light winds. Conditions will be ripe for excellent radiational cooling so leaned heavily on colder MOS guidance to derive overnight lows in the mid 40s to low 50s, except for mid 50s in the urban centers and immediate coast. Might be a good idea to close the windows! && .SHORT TERM /6 AM MONDAY MORNING THROUGH MONDAY NIGHT/... Monday... Anomalous 592 Dm high builds into New England and becomes centered over our area. This is 99.5th percentile according to the NAEFS Situational Awareness Table. Nonetheless, daytime highs should be near or just above climatological normal due to southeast winds as 1028mb surface high slides just offshore. With 925mb temperatures +12 to +14C, expect low 70s near the coast and mid to upper 70s in the interior locations. Due to subsidence inversion, expect plentiful sunshine with a few mid to high level clouds filtering in from the southwest during the afternoon hours. That is associated with weak disturbances associated with the remnants of Tropical Cyclone Nicholas getting caught in the weakness between the 592 Dm ridge centered over the Northeast US and an approaching amplifying H5 trough from the Great Plains. An interesting statistic is that Boston has never failed to record a sub-70 degree high past September 20. The latest first sub-70 degree high heading into astronomical fall is September 20 1947. With southeast flow and SSTs in the mid 60s, it will be a close call whether Boston will officially hit 70 degrees, given that the climate site is just off the waters at Logan Airport. Have gone with a maximum forecast of 70 degrees, but it is a toss-up between 69 and 71F. We shall see if the record stands by the end of tomorrow! Monday night... Surface high slides further offshore, allowing winds to turn southerly. Dew points rise from the 40s into the 50s and along with increasing cloud cover, overnight lows will generally bottom out in the 50s, with mid to upper 50s near the coast and in the urban population centers. If cloud cover is less extensive, then upper 40s in the typical cool spots like the MA coastal plains are possible. && .LONG TERM /TUESDAY THROUGH SUNDAY/... Highlights... * Mainly dry and seasonably mild Tue/Wed * Scattered showers at times Thu into Fri and continued mild * Improving conditions Sat, but showers may linger in the east Tuesday into Wednesday... Strong mid level ridge over New Eng shifts to east on Wed while surface high remains sets up east of Nova Scotia with ridging extending west into SNE. Dry and seasonably mild weather Tue with sunshine mixing with some lower strato-cu as persistent SE flow brings a shallow layer of low level moisture. Then Wed, some of the guidance is indicating a few showers developing in the interior as low level moisture deepens a bit. Given that large scale forcing is limited with ridging nearby and lots of dry air in the mid levels, much of the day should be dry with just a low risk for a few showers in the interior. Temps Wed will be a bit milder with highs mid/upper 70s Thursday into Friday... Models are similar in the overall pattern with high amplitude trough/mid level low across the Gt Lakes opening up and lifting to the NE into eastern Canada as strong ridge persists to the east. As a result of the amplified pattern, there is consensus of a slow moving front approaching from the west with deep southerly flow across New Eng. Shower chances will increase during this period. Expect scattered showers Thu, especially in the west but given that best forcing remains to the west with neutral to slightly rising heights not expecting widespread showers and it may remain mostly dry in eastern New Eng Thu. The most likely period for numerous showers and a few t-storms will be Fri as low level jet and accompanying PWAT plume moves into region ahead of the frontal system. While some heavier rainfall is possible, the axis of heaviest rainfall is expected to be north and west of SNE. Temps will be in the 70s Thu/Fri with humid conditions as dewpoints will be well into the 60s. Saturday and Sunday... Low confidence forecast next weekend as there are differences on the amplitude of the upstream trough across the Gt Lakes which impacts the timing of the front moving offshore. GFS and ECMWF suggest improving conditions Sat, but many EC ensemble members have showers lingering on Sat with front stalling near eastern New Eng. Will maintain chc pops in the east on Sat but lots of uncertainty. For Sunday, we went with dry forecast in NW flow, but can't rule out more showers assocd with a shortwave passage. Seasonable temps expected next weekend as there is not push of colder air. In fact, ensemble mean 850 mb temps 10-12C. && .AVIATION /02Z MONDAY THROUGH FRIDAY/... Forecaster Confidence Levels. Low - less than 30 percent. Moderate - 30 to 60 percent. High - greater than 60 percent. 02z Update...no change to previous TAFs. Earlier discussion below. ============================================================ Tonight...VFR. Winds light and variable. Monday...VFR with light and variable winds, becoming southeast at 5-10 kt near the coast. Monday night...VFR to start but MVFR or localized IFR conditions may develop overnight in typical interior fog-prone terminals. Winds light and variable except for southeast winds 5-10 kt for Cape terminals. KBOS Terminal...High confidence. VFR. KBDL Terminal...High confidence. VFR. Outlook /Monday night through Thursday/ Tuesday: VFR. Tuesday Night: Mainly VFR, with local IFR possible. Wednesday: Mainly VFR, with local MVFR possible. Slight chance SHRA. Wednesday Night: Mainly MVFR, with local IFR possible. Breezy. Chance SHRA. Thursday through Thursday Night: Mainly MVFR, with areas VFR possible. Breezy. Chance SHRA. Friday: Mainly MVFR, with local VFR possible. Breezy. Chance SHRA, isolated TSRA. && .MARINE... Forecaster Confidence Levels. Low - less than 30 percent. Moderate - 30 to 60 percent. High - greater than 60 percent. 340 PM update... Rest of today...High forecast confidence. High pressure builds into New England, with NNE winds 10-15 kt and isolated 20 kt gusts diminishing by this evening. 5-7 ft seas and SE swells from Odette will diminish after midnight. Remaining Small Craft Advisories are scheduled to come down after midnight. Tonight and Monday...High forecast confidence. Large 1025+ mb high pressure builds over the maritimes, yielding fine boating weather with light winds, dry weather and good vsby. Monday night...Winds shift to the south at 5-10 kt with localized reduced vsby due to patchy fog possible. Outlook /Tuesday through Friday/... Tuesday: Winds less than 25 kt. Tuesday Night: Winds less than 25 kt. Seas locally approaching 5 ft. Wednesday: Winds less than 25 kt. Areas of seas approaching 5 ft. Wednesday Night through Thursday: Winds less than 25 kt. Seas up to 5 ft. Slight chance of rain showers. Thursday Night: Winds less than 25 kt. Areas of rough seas. Chance of rain showers. Friday: Winds less than 25 kt. Rough seas up to 7 ft. Chance of rain showers, isolated thunderstorms. && .CLIMATE... The latest first sub-70 degree high heading into astronomical fall for BOS is September 20 1947. It is possible we may break the record by tomorrow. The latest first sub-70 degree high heading into astronomical fall for PVD is September 26 2015. Both BDL and ORH have already recorded sub-70 degree highs earlier this month. && .BOX WATCHES/WARNINGS/ADVISORIES... CT...None. MA...High Rip Current Risk until 10 PM EDT this evening for MAZ007- 019-022-024. RI...None. MARINE...Small Craft Advisory until 10 PM EDT this evening for ANZ250. Small Craft Advisory until 1 AM EDT Monday for ANZ254. && $$ SYNOPSIS...KJC/Chai NEAR TERM...KJC/Nocera/Chai SHORT TERM...Chai LONG TERM...KJC AVIATION...KJC/Nocera/Chai MARINE...KJC/Chai CLIMATE...