Area Forecast Discussion
Issued by NWS Boston, MA

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000 FXUS61 KBOX 260540 AFDBOX Area Forecast Discussion National Weather Service Taunton MA 140 AM EDT Mon Sep 26 2016 .SYNOPSIS... High pressure will center over New England tonight and allow for efficient radiational cooling. A low pressure area and associated front will bring showers to the area late Monday night into Tuesday morning. Considerable forecast uncertainty exists for the rest of the week. An easterly flow may persist for the rest of the week with a chance of showers or areas of drizzle from time to time, especially near the south coast. && .NEAR TERM /UNTIL 6 AM THIS MORNING/... 10PM update... Adjusted overnight mins/temps toward MET which was better handling the beginning stages of radiational cooling late this evening. This does not impact current Frost headlines so no need for adjustment there. Only impact is that frost may be just slightly more widespread than previously thought. Otherwise, CI may try to ruin the radiational cooling but not enough to trend toward these cooler temperatures. Forecast on track otherwise. Previous discussion... Surface high pressure crests over the area tonight and sets the stage for efficient radiational cooling under clear skies and with light winds. using minimum temperatures of 35F or lower as a guide, have expanded the Frost Advisory some from the initial issuance this morning to include far NW Middlesex County, Norfolk County, eastern Hampden and southern Worcester Counties. Feel reasonably confident about Norfolk County. Some of the other areas unlikely to be validated with ASOS data but think rural areas of eastern Hampden and southern Worcester County have a fairly good chance at temperatures near or a little below 35F for a few hours with virtually no mixing. Anticipate that we may experience patchy frost across other portions of more rural Middlesex County and our northern CT zones but probably not extensive enough to warrant an advisory. That can be reevaluated, however, with the next update. Low temperatures will make it to the 30s most areas outside the major urban areas and some immediate coastal areas. NW winds should diminish quickly in vicinity of sunset. && .SHORT TERM /6 AM THIS MORNING THROUGH 6 PM TUESDAY/... Monday should start out mostly clear with high clouds streaming in from the west during mid to late afternoon. High temperatures look to be similar to today`s values. A light gradient will set the stage for sea breezes along both east and south facing coasts on Monday. Short wave energy rotating about a long wave closed low approaching the NW Great Lakes will likely bring a baggy surface cold front or occlusion toward the area early Tue AM. Several factors gleaned from a consensus of numerical prediction models favor a high likelihood of showers with this system including divergence aloft, a near coupling of a strengthening low level jet and the right entrance region to the upper jet and precipitable waters over 1.5 inches. Anticipate showers moving into the CT River Valley soon after 06z Mon and reaching the MA east coast around 09Z Mon. Believe the moisture pooling and favorable dynamics could set the stage for some heavy showers, albeit progressive. A band or two of heavy showers could have some impact on the start of the Tuesday morning rush hour. Expect temperatures to remain nearly stationary or rise Mon night in response to cloud cover and warm advection. && .LONG TERM /TUESDAY NIGHT THROUGH SUNDAY/...
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HIGHLIGHTS... * Widespread showers Tue, especially coastal plain. Improving conditions west * Unsettled weather possible through end of week but low confidence * Temperatures near or slightly below normal Overview... Rather complex high amplitude pattern across the CONUS and Canada in the extended period will lend itself to low predictability. All revolves around the evolution of a cutoff low as it digs south from the Great Lakes with strong upstream and downstream in place. GFS has trended toward Ecmwf and there is agreement among the guidance that cutoff will set up to the SW of New Eng but exact location of the cutoff along with area of confluence to the north is uncertain. And spread increases significantly next weekend. Additionally, there is considerable spread with location/track of sfc lows to the south which will impact sensible weather. Given the complexity of the pattern it is certainly a low confidence forecast as predictability of this pattern at extended time ranges is quite low. Tuesday... Cold front will be moving across SNE Tue morning. Strong signal for a band of widespread showers along and ahead of the front. Good coupling between low level jet and right entrance region of upper jet will result in deep forcing for ascent acting on deep moisture axis with anomalous PWAT plume around 1.75". All suggests widespread showers and can`t rule out a few t-storms near the coast where elevated instability parameters are favorable. Still some uncertainty with timing of the front and showers but focus Tue morning appears to be across the coastal plain with improving conditions in western New England. In fact, expect developing sunshine in the interior during the afternoon. Front expected to get hung up near the coast as flow aloft becomes parallel to the front and this may keep showers going near the south coast and Cape/Islands much of the day. It will be somewhat humid with dewpoints climbing into the low/mid 60s and breezy near the coast in the morning as low level jet swings through with gusts to 30 mph possible. Tuesday night... There is uncertainty with how far south drying gets as front stalls south of New Eng. GFS and Ecmwf show drying making it to the south coast by evening. However, as upstream trof/upper low digs south from the Gt Lakes flow aloft over New Eng backs and this may bring moisture back north and keep threat of showers along the south coast. ECMWF and UKMET have showers near the coast and this is how we will play it. Wednesday through Sunday...Low Confidence. As cutoff low digs south of the Gt Lakes backing upper flow will bring back deeper moisture into SNE, but the extent of moisture is unknown as moisture will be battling upper confluence and high pressure to the north. Also, location and track of low pres to the south will be dependent on exact positioning of the cutoff low. Potential for a period of unsettled weather with showers from time to time, but it could end of dry for a good portion of the period if there is greater influence from upper confluence and high pres to the north. Lots of uncertainty so we have just low pops in the forecast. With persistent NE flow likely, temps expected to be near or a bit below normal.
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&& .AVIATION /06Z MONDAY THROUGH FRIDAY/...
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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 Tuesday/... Through today...High confidence. VFR. Winds generally light, although shifting toward the SW through the day. Sea breezed expected on coasts. Tonight and Tuesday...Moderate confidence, mainly due to timing. VFR to start the overnight. Gradually expect lowering to MVFR/IFR from W-E especially 06Z through 12Z, first due to lowering CIGS, then due to mix of RA and BR with lower vsbys. Expect this to gradually lift from W-E 12Z-20Z, although these lower categories may linger longer across the S Coast/Cape/Islands into Tue evening. KBOS TAF...High confidence in TAF through 06Z Tue, including sea breezes. KBDL TAF...High confidence in TAF through 06Z Tue. Outlook /Tuesday Night through Friday/... Tuesday...Moderate confidence. Area of showers with MVFR/IFR likely moving across SNE with main focus in the coastal plain. Improving conditions to VFR in the west. South wind gusts to 25-30 kt possible in the morning near the south coast. Wednesday through Friday...Low confidence. Areas of MVFR with showers possible at times, but VFR conditions if it is dry.
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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 Tuesday/...High confidence. Plan to let Small Craft Advisories play out as originally construed. Main issue moving into this evening will be residual rough seas as a combination of wind wave opposing a swell from Karl. The wind forcing should end by this evening, but the swell will linger into tonight across the outer waters. S to SW winds will likely frequently gust to/above 25 KT late Mon night and probably result in a new round of Small Craft Advisories. Areas of fog also likely late Mon night into early Tue AM. Outlook /Tuesday Night through Friday/... Tuesday...High confidence. Low level jet ahead of cold front will result in a period of pre-frontal southerly gusts to 25-30 kt in the morning, then diminishing winds int he afternoon. Vsbys reduced in showers and fog. Low risk for a few t-storms across the south coastal waters. Wednesday through Friday...High confidence in an extended period of E/NE winds with speeds mostly below SCA although occasional gusts to 25 kt possible. Vsbys may be reduced at times in showers but this is low confidence.
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&& .CLIMATE... Record low temperatures... September 26th (last set in...) Boston ....... 34 (1879) Hartford ..... 33 (1978) Providence ... 37 (1967) Worcester .... 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>013-017-018-026. RI...None. MARINE...Small Craft Advisory until 3 AM EDT early this morning for ANZ250-254-255.
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&& $$ SYNOPSIS...KJC/Thompson NEAR TERM...Doody/Thompson SHORT TERM...Thompson LONG TERM...KJC AVIATION...KJC/Doody MARINE...KJC/Thompson CLIMATE...

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