Area Forecast Discussion
Issued by NWS Boston, MA

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000 FXUS61 KBOX 160749 AFDBOX Area Forecast Discussion National Weather Service Boston/Norton MA 349 AM EDT Sat Oct 16 2021 .SYNOPSIS...
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Unseasonably warm and humid conditions will continue today. A strong cold front brings breezy conditions along with showers and possible thunder moving west to east across the region late today into tonight. Mainly dry weather with seasonably cool temperatures follows for Sunday into much of next week.
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&& .NEAR TERM /UNTIL 4 PM THIS AFTERNOON/...
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Most of the showers and isolated t-storms early this morning are focused across northern NY into northern New Eng where best low level jet dynamics and marginal instability are located. A few showers may move NE across western and northern MA through about daybreak. Otherwise, deeper moisture plume lifts to the north today ahead of amplified mid level trough moving across the Gt lakes. Warm sector dry slot will prevail across SNE through at least mid afternoon with dry conditions. Cold frontal convective band not expected to reach western MA zones until after 4 pm. Unseasonably warm and humid warm sector airmass across SNE today with a mix of sun and clouds. 925 mb temps 16-18C will support highs in the mid/upper 70s with dewpoints low/mid 60s. Developing low level jet will bring gusty southerly winds this afternoon. Soundings support gusts to 25-35 mph.
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&& .SHORT TERM /4 PM THIS AFTERNOON THROUGH 6 PM SUNDAY/...
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Late today through tonight... Strong cold front will enter western MA toward 00z and sweep across SNE tonight, moving east of the Cape after 06z. Robust mid level trough and favorable upper jet dynamics will result in a strongly forced convective line moving east from eastern New York State late this afternoon, with the line of showers and embedded t-storms moving into western MA/CT 5-8 pm. All the hi- res CAMS are suggesting the convective line will weaken as it moves into western new Eng as instability diminishes to near zero. Wind gust potential will be limited by the lack of instability and low level jet outrunning the cold front a bit. Current thinking is there could be isolated gusts 35-45 mph as the line moves through western New Eng. However, will need to closely monitor instability trends. If instability overachieves, potential for a few damaging wind gusts would increase. Anomalous PWATs support brief heavy downpours, but the quick movement will limit rainfall amounts to less than one inch. The convective line is expected to continue to weaken as it moves east across SNE tonight as it gets further removed from shortwave energy lifting to the north. However, a plume of instability with 200-400 J/kg will be moving up from the ocean across eastern New Eng, aided by mid 60s dewpoints, so a few t-storms may accompany the showers moving east across rest of SNE. Showers may linger a few hours after the frontal passage, then good post-frontal drying will result in partial clearing overnight from west to east. Lows will drop into the upper 40s over the higher terrain and 50s elsewhere. Sunday... Deep upper trough settles over New Eng with good mid level cooling as 500 mb temps cool to -22 to -24C. The cold pool aloft combined with a plume of low and mid level moisture moving in from the west will result in the development of afternoon diurnal clouds and can`t rule out a spot shower. Otherwise, dry weather is expected. 925 mb temps 8-10C support highs low/mid 60s, except upper 50s over higher terrain which is close to the seasonable normals. W/NW gusts to 20 mph expected within a deep and well mixed boundary layer.
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&& .LONG TERM /SUNDAY NIGHT THROUGH FRIDAY/...
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Highlights... * Unsettled, blustery conditions but generally dry seasonable weather through Tues. * Warming trend into Wed with a frontal system Thurs or Fri that may bring showers. * Uncertain weekend outlook, but watching for possible coastal wave low development near mid-Atlantic waters Sat. Details: Sunday Night through Tuesday: Deep longwave trough and an associated cold pool of air aloft establishes itself over Southern New England in this period. Taking a wider view, an omega blocking pattern takes shape over the northern Atlantic. Such a regime favors slow eastward translation to the trough aloft, and there`s some level of uncertainty on when this pattern breaks down. Thus, a continued blustery, unsettled period but also a generally dry one. Given the cold pool of air aloft, we should see partly to mostly cloudy conditions as shallow moisture blossoms into diurnal cumulus. 925-850 mb thermal trough axis over Southern New England on Monday, then it and the trough at 500 mb shifts just offshore enough for more sunny breaks on Tuesday. Temperatures in this period are seasonable if slightly cooler than normal for mid-October. Highs on Monday in the mid 50s to lower 60s, feeling like the mid 40s across the Berkshires/Worcester Hills under 20-25 mph NW gusts. Highs on Tuesday only slightly milder in the upper 50s to lower to mid 60s. Lows each night in the 40s, with upper 30s possible in the sheltered valleys in northwest MA pending wind decoupling. Wednesday: Though some uncertainty in the timing, looking like the second half of Wednesday into Wednesday night heralds a transition back to above- normal temperatures as shortwave ridging builds eastward. 850 mb temps look to rise into the upper single digits to lower teens Celsius range, bringing highs back well into the 60s with mostly sunny conditions. Thursday through Saturday: By Thursday, 500 mb pattern aloft becomes governed by a prominent closed low near north-central Canada and an overall rather active northern stream. Shortwave disturbance aloft swings a frontal system ENE from the Gt Lakes to the Northeast states either later Thursday or on Friday. Most of the guidance indicates primary low pressure and mid-level forcing passes well to our north. Though this frontal system offers the next chance for showers, guidance isn`t too bullish on QPF and some are a shutout entirely. Still time to see how this system evolves but lack of deeper moisture/continental flow could favor a drier solution. Temperatures trend slightly above normal. Lot more uncertainty in the details looking into next weekend, though the dominant feature in mid-levels is the potent closed low near Hudson Bay. Indications are for a possible coastal low to develop along leftover baroclinic zone around Sat, as trough energy digs into the lower OH Valley and central Appalachians. But lots of model variation on where this feature develops and how far offshore any development would occur. Temps however still look fairly seasonable.
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&& .AVIATION /07Z SATURDAY THROUGH WEDNESDAY/...
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Low - less than 30 percent. Moderate - 30 to 60 percent. High - greater than 60 percent. 06Z TAF Update... Through 12z... Mostly MVFR cigs with patchy IFR possible east of CT valley. Fog will be limited. Today...High confidence. Areas of MVFR cigs early improving to VFR. Mainly dry, but line of locally heavy showers and embedded t-storms moving into western MA/CT 21-00Z. S wind gusts to 25-30 kt developing. A brief W wind gust to 35-40 kt possible as convective line moves into western New Eng. Tonight...High confidence. Showers and a few embedded t-storms moving east across SNE with MVFR conditions, exiting the CT valley by 06z and the Cape/Islands 09-12z. Improving conditions to VFR with partial clearing after the showers exit. Gusty S winds shift to W/NW and diminishing 02-08z from west to east. Sunday...High confidence. VFR with broken cigs 4-6k ft developing in the afternoon. A spot shower possible. W/NW wind 10-20 kt. KBOS Terminal...High confidence in trends. KBDL Terminal...High confidence in trends. Outlook /Sunday Night through Wednesday/... Sunday Night: VFR. Breezy. Monday: VFR. Breezy. Slight chance SHRA. Monday Night: VFR. Breezy. Tuesday: VFR. Windy with gusts up to 35 kt. Tuesday Night: VFR. Breezy. Wednesday: VFR.
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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. Pre-frontal southerly winds increase today with gusts to 25-30 kt developing this afternoon into this evening with building seas. Winds shift to W/NW overnight into Sunday behind the cold front, with gusts to 20 kt. Showers and scattered thunderstorms tonight. SCA for the waters continues. Outlook /Sunday Night through Wednesday/... Sunday Night: Low risk for Small Craft Advisory winds with gusts up to 25 kt. Seas up to 5 ft. Slight chance of rain showers. Monday: Moderate risk for Small Craft Advisory winds with gusts up to 25 kt. Seas up to 5 ft. Slight chance of rain showers. Monday Night: Moderate risk for Small Craft Advisory winds with gusts up to 25 kt. Local rough seas. Slight chance of rain showers. Tuesday: Moderate risk for Small Craft Advisory winds with gusts up to 30 kt. Areas of rough seas. Tuesday Night: Low risk for Small Craft Advisory winds with gusts up to 25 kt. Seas up to 5 ft. Wednesday: Winds less than 25 kt. Seas up to 5 ft.
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&& .BOX WATCHES/WARNINGS/ADVISORIES...
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CT...None. MA...None. RI...None. MARINE...Small Craft Advisory from 10 AM this morning to 4 AM EDT Sunday for ANZ232>235-237. Small Craft Advisory from 10 AM this morning to 2 AM EDT Sunday for ANZ230-231-236-251. Small Craft Advisory from 10 AM this morning to 2 PM EDT Sunday for ANZ250-254>256.
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&& $$ SYNOPSIS...KJC/Loconto NEAR TERM...KJC SHORT TERM...KJC LONG TERM...Loconto AVIATION...KJC/Loconto MARINE...KJC/Loconto

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