Area Forecast Discussion
Issued by NWS Boston, MA

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102 FXUS61 KBOX 231203 AFDBOX Area Forecast Discussion National Weather Service Taunton MA 703 AM EST Tue Jan 23 2018 .SYNOPSIS... Scattered showers along with areas of drizzle and patchy dense fog persist this morning with some light icing across northern MA. A fast moving storm system will bring a period of heavy rain and perhaps even an isolated thunderstorm or two later this morning and into the afternoon along with strong winds across portions of eastern MA and RI. Blustery, dry and colder weather follows Wednesday through Friday, then a warming trend next weekend. A cold front may bring showers next Sunday or Monday. && .NEAR TERM /UNTIL 6 PM THIS EVENING/...
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*/ Highlights... * Heavy rain/isolated thunderstorm risk * Brief poor drainage street flooding possible * Period of strong wind gusts along and southeast of BOS-PVD corridor */ 7 am update... Cold air damming continuing as far down as the S-coast of MA. Starting to see warmer air intrude along the Cape and Islands with improved ceilings and visibility, winds turning S. Will be interesting to see how the trend evolves, as to whether the cold air will only retreat once the low pressure center is upon S New England, the ageostrophic flow reverting southerly in the process. Otherwise stout northerly, a majority of S New England may remain socked in with low clouds and fog into midday. N locales of MA under the threat of freezing rain / drizzle towards the same timeframe, especially on up into the Merrimack River Valley. Expect slippery travel conditions and dense fog to persist through much of the morning, dense fog more so over CT / RI / Southeast MA and across the high terrain. Decent slug of rain approaching from the SE, timed into the region around 7-8a, along the nose of the H925 low level jet. Already some embedded thunder and rainfall amounts exceeding a half of inch in a short period of time upstream across E PA and NJ. Still feel the forecast rainfall amounts are in good order call for around an inch to an inch and a half in spots. Threats still anticipated with respect to urban / poor-drainage flooding, more so with any convective threats to which "heavy rain" wording was appended. May also need to append gusty winds but feel that is over-stating the obvious as a WIND ADVISORY is currently in effect. Blend of the HRRR / RAP model doing very well with wind and 2m temperature trends at this time. */ Previous update (425 AM)... Subfreezing Temps/Freezing Rain-Drizzle... Confluent flow over southeast Canada will continue to support 1033 mb high pressure at the surface with cold air damming (CAD) signature across much of the region early this morning. Pressure falls continue to advect from southwest to northeast across the region this morning, aiding in NNE drainage flow from NH and ME southward into our region. Plenty of shallow cold air upstream to replenish/offset warming from latent heat release associated with precip. Given this setup we have expanded the winter weather advisory to include coastal Essex county southward into Cambridge. 06z NAM appears to capture this low level cold air best so followed this guidance closely for this portion of the forecast. Scattered showers moving northward will yield a period of freezing rain for the I495 and possibly the I-128 corridor of northeast MA for the early morning commute. Thus expect some slippery travel in this region this morning. Dense Fog... Warm sector trying to advance northward with dew pts in the 40s across the Cape, islands and the immediate south coast. This warm air surging northward with shallow cold air entrenched inland resulting in areas of dense fog. Have expanded the dense fog advisory to account for latest trends supporting more widespread fog. Conditions will slowly improve later this morning and especially this afternoon. Heavy Rainfall... Closed low over the OH valley with emerging warm conveyor belt (WCB) advancing up the eastern seaboard. Strong jet dynamics associated with this feature as 925 and 850 mb wind anomalies increase to +3 SD this afternoon across this area. Subtropical moisture associated with this WCB increases to +3 SD as well. The combination of strong forcing for ascent, subtropical moisture and steepening mid level lapse rates supports periods of heavy rain along with the risk of a thunderstorm. The progressive nature of the system will limit heavy rainfall duration to about 6 hrs and also cap rainfall totals to about 0.75 to 1.25 inches. Strong Wind Threat... Impressive low level jet up to 85 kts at 925 mb develops this afternoon along the south coast. This results in very strong low level WAA and a stout inversion developing, limiting the risk of strong winds aloft from reaching the surface. However with dew pts rising into the mid 50s along with steepening mid level lapse rates models generating a few hundred joules of MUcape. In addition, most of the Hi Res guidance generating impressive model simulated reflectivity this afternoon with a broken line of low top convection with embedded stronger elements. This low top convection may result in an isolated T-storm with strong gusty winds aloft reaching the surface. Will continue with the wind advisory for the coastal plain of RI and southeast MA as warm sector likely overspreads this area yielding the greatest risk for strong winds aloft mixing to the surface. Temperatures... Very strong WAA yields +12C temps at 925 mb across RI and southeast MA this afternoon. Despite showers highs will likely approach 60 across RI and southeast MA. Record high for PVD today is 64/1906, thus there is a low probability of approaching this record.
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&& .SHORT TERM /6 PM THIS EVENING THROUGH 6 AM WEDNESDAY/... 425 AM update... Tonight... Frontal passage occurs this evening with strong post frontal drying, so expect decreasing clouds and a drying trend from west to east early. Temps may initially jump upward across the interior where shallow cold air was unable to scour out during the day, as post frontal winds mix down warm air aloft initially before CAA develops. However core of cold air is delayed until Wed night/Thu. Thus only seasonably cold tonight. && .LONG TERM /WEDNESDAY THROUGH MONDAY/...
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*/ Highlights... - Slightly unseasonable, breezy through the remainder of the week - Mild conditions along with wet weather return early next week */ Overview... Fairly benign pattern for the remainder of January. Progressive flow across the CONUS with colder, Arctic air locked up around the Hudson Bay / Davis Straight region. H3 jet streak with wave break out over the N-Central Pacific, H5 ridge into the Bering Straight, downstream H5 trof into the Gulf of AK into the W CONUS, but overall flat, an unimpeded flow downstream, not greatly amplified, so long as traffic does not bottle up across the N Atlantic. Milder pattern lies ahead of any synoptic disturbances, wet weather as well, followed by cold that is not as deep, slightly unseasonable, lacking a deeper Arctic connection. However, watching early February closely. Aforementioned H3 wave break progressing E, potential H5 ridge into the W CONUS, a nod to ensemble signals of a +PNA pattern emerging. Winter isn`t over just yet. */ Discussion... Through Friday... Quiet and cold. Cyclonic flow persisting aloft, additional mid-level energy rotating through, cold air advection enhanced and maintained, boundary layer mixing up around H9 through which NW flow prevails, looking at slightly below seasonable conditions (typically mid 30s for highs, around 20 for lows) and breezy conditions. Talking wind chills as low as the single digits, possibly below zero for the N/W high terrain. Scattered cloud decks, flurries over the Berkshires, NW wind gusts as high as 30 to 35 mph, otherwise quiet. Some thought as to refreezing ice jams, but cold not as deep, so perhaps river flows elevated by Tuesday rains are uninhibited as ice jams would not be expected to refreeze so quickly. Weekend into early next week... Expunging cold air prior to the onset of the next Pacific-origin disturbance. Question progressive nature and subsequent morphology with the potential downstream traffic. Certain Arctic air remains locked up around the Hudson Bay region, the approaching system warm- core, exhibiting inside-runner behavior along regions of better baro- clinicity N and W, looking at another period of mild conditions and wet weather over much of the NE CONUS followed by return colder air however not exceptionally deep. However, acknowledge the spread in ensemble member solutions, nothing is certain. Preference to slower EC/UKmet solutions, parent with GEFS beginning to signal anomalous S- influx trends. Cold ground prevailing, certain of fog / visibility issues, potential for flooding with the possibility of moderate to heavy rain, more so with swollen and/or ice jammed river basins. Some uncertainty how quickly cold air is scoured out prior to and with potential onset of wet weather.
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&& .AVIATION /12Z TUESDAY THROUGH SATURDAY/...
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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 Tonight/... 130 AM/0630Z update... Today... Widespread IFR/LIFR with DZ/FG though potential improvement with increasing S winds especially over S/E areas of New England. FZDZ over N/NE MA this morning, eroding with onset of -RA/RA 12-14z, RA/+RA around 15-21z. TSRA possible but held off with mention with the 12z TAF issuance. LLWS threat with 60-70 kt S jet moves across the S-coast 17z-23z W to E. Conditions improve 20z- 22z western CT/MA then 22z-01z RI and eastern MA with a wind shift from south to southwest. Tonight... Big improvement with any leftover IFR/LIFR eastern MA around 00z quickly improving to VFR by 03z or so. Elsewhere VFR with developing gusty SW winds. Any leftover showers over Cape Cod and Nantucket at 00z quickly moves offshore. Thus dry runways overnight. KBOS Terminal...high confidence in TAF on overall trends but lower confidence on exact details. Heavy showers with possibly embedded Thunder 17z-22 and LLWS. KBDL Terminal...high confidence in TAF on overall trends but lower confidence on exact details. Heavy showers with possibly embedded Thunder 15z-18z Outlook /Wednesday through Saturday/... Tuesday: Mainly IFR, with local MVFR possible. Strong winds with gusts to 40 kt. SHRA, areas FG, FZRA likely, isolated TSRA. Tuesday Night: Mainly VFR, with local IFR possible. Windy with gusts to 35 kt. Areas FG, slight chance SHRA. Wednesday: VFR. Windy with areas gusts to 30 kt. Wednesday Night: VFR. Breezy. Thursday through Thursday Night: VFR. Windy with local gusts to 30 kt. Friday: VFR. Breezy. Friday Night through Saturday: Breezy.
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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 Tonight/... Today... Southerly Gales develop with strongest winds along the waters of RI and southeast MA this afternoon. Showers and fog limit vsby. Tonight... Front sweeps across the region early this evening with winds shifting from S to west. Thus southerly gales diminish with the wind shift along with vsby improving. Outlook /Wednesday through Saturday/... Tuesday: Moderate risk for gale force winds with areas of gusts up to 45 kt. Rough seas up to 13 ft. Rain showers, areas fog, isolated thunderstorms. Visibility 1 nm or less. Tuesday Night: Low risk for gale force winds with gusts up to 35 kt. Rough seas up to 12 ft. Chance of rain showers, areas fog. Local visibility 1 nm or less. Wednesday: Moderate risk for Small Craft Advisory winds with gusts up to 30 kt. Areas of rough seas. Wednesday Night: Winds less than 25 kt. Local rough seas. Thursday: Moderate risk for Small Craft Advisory winds with local gusts up to 30 kt. Areas of seas approaching 5 ft. Thursday Night: Moderate risk for Small Craft Advisory winds with local gusts up to 30 kt. Local rough seas. Friday: Winds less than 25 kt. Seas locally approaching 5 ft. Friday Night: Winds less than 25 kt. Saturday: Moderate risk for Small Craft Advisory winds with gusts up to 30 kt. Areas of seas approaching 5 ft.
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&& .HYDROLOGY... The heavy rain will mainly just result in typical brief nuisance poor drainage street flooding. However...a Flood Watch has been issued for the Millers River near Athol where an ice jam remains frozen in place and has the potential to cause additional flooding. A Flood Warning also remains in effect for the CT River at Middle Haddam...where this heavy rain may result in increased flooding problems as a result of the ice jam. For details on specific area rivers, including observed and forecast river stages, please visit the Advanced Hydrologic Prediction Service (AHPS) graphs on our website. && .BOX WATCHES/WARNINGS/ADVISORIES...
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CT...Dense Fog Advisory until 9 AM EST this morning for CTZ002>004. MA...Wind Advisory from 11 AM this morning to 7 PM EST this evening for MAZ013-015>024. Dense Fog Advisory until 9 AM EST this morning for MAZ002-004- 008-009-012-017>021-026. Winter Weather Advisory until 9 AM EST this morning for MAZ002>008-012-014-026. Flood Watch until 6 PM EST this evening for MAZ003-004. RI...Wind Advisory from 11 AM this morning to 7 PM EST this evening for RIZ002>008. Dense Fog Advisory until 9 AM EST this morning for RIZ001>007. MARINE...Gale Warning until 8 PM EST this evening for ANZ232>235-237- 250-254>256. Gale Warning from 10 AM this morning to 6 PM EST this evening for ANZ230-231-236-251.
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&& $$ SYNOPSIS...Nocera/Sipprell NEAR TERM...Nocera/Sipprell SHORT TERM...Nocera LONG TERM...Sipprell AVIATION...Nocera/Sipprell MARINE...Nocera/Sipprell HYDROLOGY...WFO BOX

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