Area Forecast Discussion
Issued by NWS Boston, MA

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000 FXUS61 KBOX 270220 AFDBOX Area Forecast Discussion National Weather Service Taunton MA 1020 PM EDT Mon Sep 26 2016 .SYNOPSIS... A cold front will bring a period of showers to the area late tonight into Tuesday morning as it moves into the region. The front will stall near the coast keeping the threat of showers along the south coast and Cape/Islands through Tuesday night. Low pressure dives from the Great Lakes to Virginia midweek, pauses, then lifts northeast through New England during the weekend. This will support showers, fog, and drizzle through the weekend. && .NEAR TERM /UNTIL 6 AM TUESDAY MORNING/... 1005 PM Update... Mid and high clouds have continued to progress eastward over the last few hours, covering most of the region except for S coastal Mass, Cape Cod and the islands at 02Z. Lower clouds have reached into central and western areas, with bases down to 3500-4500 feet, as seen on latest obs as well as IR satellite imagery, and continue to move steadily E-NE. Leading edge of an area of showers extended from W of KGFL-KMSV- KABE, which was moving E-NE. Appears that most of the showers will push into northern New England, but the southern extent out of S central PA should push into the E slopes of the Berkshires by 07Z-08Z then continue across the region through daybreak. Current forecast was in pretty good agreement with satellite and radar trends, though a bit slow. Have updated to bring conditions current and incorporated trends into the overnight forecast. Temps were running a few degrees milder than forecast, while dewpts were a bit low. Have adjusted those trends as well. However, with the S-SW wind flow in place, expect temps and dewpts to slowly rise overnight, which was the previous forecast trend. Previous Discussion... Modest low level jet develops which will advect anomalous PWAT plume around 1.75" northward into SNE. Excellent moisture transport along the low level jet which is somewhat coupled with right entrance region of upper jet will result in deep layer forcing for ascent so expect a band of widespread showers to move across the region, 06-10z western New Eng and 10-14z eastern New Eng. Models do show some elevated instability south of New Eng so cant rule out isold thunder, mainly near the south coast. && .SHORT TERM /6 AM TUESDAY MORNING THROUGH TUESDAY NIGHT/... Tuesday... Approaching shortwave energy ahead of the mid level low in the Great Lakes will push a cold front into the region Tue morning with the front likely stalling near the south coast as flow aloft becomes parallel. Showers continue along and ahead of the front, but quickly diminish behind the front. Rapid mid level drying behind the front so showers will end quickly Tue morning and likely by daybreak in the west followed by partial sunshine late morning through the afternoon from west to east. However, clouds and a few showers will likely linger through the afternoon near the south coast and especially the islands where deep moisture plume persists near stalled front. QPF should average 0.25-0.50" with locally higher amounts near the south coast and Cape/Islands. In addition, may see some southerly gusts to 30 mph in the morning over the Cape/Islands as the low level jet moves through. A blend of the temp guidance suggests highs reaching 70-75 degrees and it will be somewhat humid in the coastal plain with dewpoints climbing into the low/mid 60s ahead of the front. Tuesday night... The northern edge of the elevated PWAT plume and deep moisture axis will remain over the outer Cape/Islands where showers will likely continue off and on through the night, especially over the islands. As upper low digs south through the Great Lakes, the flow aloft over New Eng will back somewhat and this may push deeper moisture further inland across SNE. However, the northward extent of possible showers is uncertain as models differ on how far NW deeper moisture extends. We will confine PoPs to SE New England with highest at ACK where best chance of rain. Patchy drizzle will be possible further north as abundant low level moisture moves in from the NE below dry air aloft. && .LONG TERM /WEDNESDAY THROUGH MONDAY/... Big Picture... Maritime trough moves off out to sea, allowing West Atlantic upper ridge to build to our east. Closed upper low over the Northern Great Lakes dives south to the mid-Appalachians and then stalls as it runs into the West Atlantic ridge. The associated surface low will linger in and around Virginia. The closed low sits for a spell. The entire system then slowly lifts northeast through New England over the coming weekend. Models show some differences in detail, but overall pattern agreement through at least Saturday. The largest differences are in the breakdown and ejection of the closed low through New England Sunday and Monday. The GFS is progressive while the ECMWF is farther west and lifts the system through New England about a day later. Details... Wednesday through Saturday... Closed low to our west and building high to our east should result in a stalled pattern over Southern New England from midweek to the weekend. Upper flow starts from the southwest and turns from the south by late week. Meanwhile surface low near Virginia and high pressure over Quebec and the Maritimes will mean a surface flow from the northeast possibly turning east by late week. Given the forecast configuration, one would at least expect an extended period of low clouds, fog, and drizzle. Models show deep lift on Thursday morning topped by divergence at 250 mb, and an extended period of low level lift through Saturday generated by divergence between 700-800 mb. Expect periods of showers embedded in the clouds/drizzle during this time. Sunday-Monday... Upper low and surface low eject up the coast over New England, driven by a 100 knot upper jet. This moves a coastal surface low past us either Sunday morning per the GFS or Sunday night per the ECMWF. Expect showers with this passage. The upper low then passes overhead either Monday morning per the GFS or late Monday night per the ECMWF. Vertical totals associated with the cold air pool at 26- 28. So partly to mostly cloudy skies during this phase, and scattered or widely scattered showers are possible. && .AVIATION /02Z TUESDAY THROUGH SATURDAY/... Forecaster Confidence Levels. Low - less than 30 percent. Moderate - 30 to 60 percent. High - greater than 60 percent. Short Term /through Tuesday Night/... Overnight and Tuesday...Moderate confidence, mainly due to timing. Expect VFR through at least 2 AM/06Z. Gradually expect lowering to MVFR-IFR from W-E especially 08Z-12Z as band of showers move through. Showers will exit 12Z-15Z followed by CIGS gradually lifting to VFR. However, occasional showers and lower CIGS/VSBYS will persist over the islands and possibly across the Cape through the afternoon. Brief southerly wind gusts to 25 kt possible over the outer Cape/Islands Tue morning. Tuesday night...Moderate confidence. Expect MVFR/IFR conditions to redevelop overnight as stratus expands across the region. Patchy fog as well. Occasional showers expected to continue across Cape and Islands. The showers may push inland overnight but uncertain how far north and west they get and may be confined south and east of I-95 corridor. KBOS TAF...High confidence in trends but lower confidence in timing. A period of IFR cigs possible Tue morning. KBDL TAF...High confidence in trends but lower confidence in timing. Outlook /Wednesday through Saturday/... Moderate confidence. CT and Western MA... VFR with brief MVFR possible in showers. Local IFR in late night/early morning fog. RI and Eastern MA including ORH... MVFR in low clouds and showers. Areas of VFR possible in the midday and afternoon. Areas of IFR in fog and drizzle at night. && .MARINE... Forecaster Confidence Levels. Low - less than 30 percent. Moderate - 30 to 60 percent. High - greater than 60 percent. Short Term /through Tuesday Night/...High confidence. Overnight into Tuesday...Expect increasing southerly winds after midnight and Tue morning as low level jet moves across the waters. Low level inversion is not that strong so we do expect some mixing. A period of 25-30 kt gusts possible, diminishing in the afternoon as the low level jet exits. In addition, seas will build to 5-6 ft over the southern waters. Small craft advisories will continue. Vsbys will lower late tonight into Tuesday in areas of showers and patchy fog, lingering over the SE waters. Low risk for a t-storm over southern waters. Tuesday night...Winds turn NE through the night as low pres tracks well to the south with ridging nosing down from the north. May see a pulse of 20-25 kt gusts over NE MA waters late. Vsbys reduced in showers and patchy fog, especially SE waters. Low risk for a t-storm SE waters. Outlook /Wednesday through Saturday/... Moderate confidence East to Northeast winds through the period. Winds to 25 knots, either sustained or frequent gusts, Wednesday through Friday then diminishing Saturday. The persistent Northeast wind will push Gulf of Maine waters toward the coast and build seas of 6 to 9 feet. Expect Small Craft Advisory for much of this period. Also period of poor vsbys in fog and showers/drizzle. && .BOX WATCHES/WARNINGS/ADVISORIES... CT...None. MA...None. RI...None. MARINE...Small Craft Advisory from 4 AM to 2 PM EDT Tuesday for ANZ232. Small Craft Advisory until 8 PM EDT Tuesday for ANZ235-237- 254>256. Small Craft Advisory until 2 PM EDT Tuesday for ANZ250. && $$ SYNOPSIS...WTB/KJC/EVT NEAR TERM...WTB/EVT SHORT TERM...KJC LONG TERM...WTB AVIATION...WTB/KJC/EVT MARINE...WTB/KJC

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