Area Forecast Discussion
Issued by NWS Boston, MA

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000 FXUS61 KBOX 210802 AFDBOX Area Forecast Discussion National Weather Service Taunton MA 302 AM EST Sun Jan 21 2018 .SYNOPSIS...
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A fast moving storm system will bring a period of heavy rain to southern New England Tuesday along with strong winds on the coast. This storm will be preceded by some mixed wintry precip Monday and a risk of light icing across the interior Monday night into early Tuesday morning. Blustery, dry and colder weather follows Wednesday through Friday, before above normal temperatures likely return next weekend.
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&& .NEAR TERM /UNTIL 6 AM THIS MORNING/... 105 AM Update... Appears the southern edge of the high clouds has been slowly drifting northward across the region, with clear or mostly clear skies being reported across S CT into the NYC area at 06Z as seen on GOES-16 Nighttime Microphysics RGB composite and fog product imagery. An area of clear or few cirrus clouds will cross the region out of central and S NY/E PA/N NJ for the next few hours. However, need to watch area of low clouds moving steadily eastward from W PA/WV/W NY, which should push into the CT valley during the morning hours to midday. Noting patchy fog has developed across the normally foggy locations of KEEN and KORE at 06Z. May see more patchy fog develop across the normally prone areas of the CT valley and possibly into N central and NE Mass through the remainder of the night. Where skies have cleared, temps have fallen to the 20s to lower 30s, while holding in the mid-upper 30s along coastal areas. Light/variable (mainly W-NW) to calm winds will continue through daybreak. Have updated previous forecast to bring conditions current. && .SHORT TERM /6 AM THIS MORNING THROUGH 6 PM MONDAY/... High pressure in place today with fair skies and light wind. Moisture fields do show some increasing mid and high level moisture, so expect some increasing clouds mostly in the afternoon/evening. With mixing to 950 mb, expect max temps in the mid to upper 40s, a few spots around 50. The clouds increase tonight. A cold front in Northern New England dips south and may slip through Southern New England. But with the upper flow west-east we do not expect a lot of push to it, and potential that it stalls either over or near us. The net change in the airmass is nil, so min temps Sunday night should be similar to tonight. Low level southerly winds reach only to Western NY/PA by 12Z Monday...still west winds over us at that time...which means little lift to be generated Sunday night. So the probability of precip should be low. If the precipitation were to move in faster, low level temperatures would support a mix with sleet and freezing rain. But that scenario looks unlikely at this time. Our forecast shows slight chance pops in Western MA/CT by 12Z Monday, and that may be overplaying the likelihood. && .LONG TERM /MONDAY NIGHT THROUGH SATURDAY/...
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Highlights... * Light icing risk Mon night across interior MA high terrain * Heavy rain Tue with a period of strong winds SE New Eng * Significant flooding issues are not expected Tuesday * Dry/blustery and turning colder Wed and especially Thu/Fri Monday night... Main concern for Mon night continues to be a risk of light icing over portions of the interior. Precip expected to be light as some mid level drying moves in while best forcing remains to the west. In fact, ice crystal growth region is drying which suggests more in the way of drizzle. There is a cold air damming signature with sfc ridging nosing down from the NE with low level northerly ageostrophic flow. Leaned toward 3km NAM for temps as higher resolution datasets should better capture localized cold air damming and it targets higher terrain in western and northern MA for near freezing temps, possibly down to higher elevations in NW Hartford county. This is where the greatest threat of some light freezing rain/drizzle exists with potential for a light icing event. This also is supported by hi-res ECMWF temps. Icy roads and walkways certainly a possibility Mon night over the higher elevations. Tuesday... Potential for some light freezing rain lingering over higher terrain on MA early, otherwise it`s all rain. Decent heavy rain signal for Tue with PWAT anomaly +2SD and low level wind anomaly up to +3SD. In addition, elevated instability at the nose of the low level jet will likely result in some convective elements with an isold t-storm possible. An area of heavy rain along the pre-frontal low level jet will move west to east across SNE with rain exiting the coast around evening. System is progressive which should limit window of moderate to heavy rainfall to about 6 hours. Rainfall amounts of 0.75 to 1.50 inches still looks reasonable with heaviest amounts in western New Eng. Given that most of the rain will probably fall in a 6 hour window, expect typical minor urban and poor drainage street flooding, but river flooding is not anticipated. The other concern is a period of strong winds across RI and eastern MA as 65-70 kt low level jet moves up along the coastal plain. Low level lapse rates are near moist adiabatic which will allow for some mixing, especially where temps climb into the mid 50s across RI and SE MA and in the vicinity of any heavier convective elements. In addition, models indicate impressive pressure falls on the order of 9 mb in 3 hours which will enhance isallobaric component of the wind. Potential exists for wind gusts up to 50 mph for a few hours in the afternoon across RI and SE MA. CIPS analogs also support this with fairly decent probs of 40+ kts across SE MA. Wednesday through Friday... Turning colder and blustery behind the storm with brisk NW flow. Temps near normal Wed, then trending below normal Thu/Fri as core of coldest air aloft settles over the region. Decent shot of cold air with -15C at 850 mb into SNE on Thu. Highs likely in the 20s Thu and blustery winds will make it feel considerably colder. Wind chills Thu night/early Fri morning dropping into the single numbers with subzero wind chills over higher terrain. Looks mainly dry during this period. Saturday... High pres moves off the coast with SE CONUS ridge building north into New Eng leading to a moderating trend with above normal temps likely. Dry weather.
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&& .AVIATION /08Z SUNDAY THROUGH THURSDAY/...
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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 Monday/... Through 12Z...High confidence. Mainly VFR. Patchy MVFR-IFR CIGS/VSBYS develop in patchy fog in normally prone areas of the CT valley, possible across portions of interior E Mass for a few hours prior to sunrise. Today...High confidence. VFR. Any leftover MVFR-IFR conditions should improve by around 14Z. W-NW winds around 10 kt or less as pressure gradient relaxes. Tonight...Moderate confidence in general trends, lower in timing arrival of precip. Mainly VFR through midnight, then MVFR-IFR conditions move into the CT valley in fog and mixed snow/sleet/freezing rain. KBOS Terminal...High confidence in TAF. KBDL Terminal...High confidence in TAF. Outlook /Monday Night through Thursday/... Monday Night: Mainly MVFR, with areas IFR possible. RA likely, chance FZRA interior northern MA. Tuesday: Mainly MVFR, with local IFR possible. Strong winds with gusts to 40 kt RI and SE MA. RA. Pockets of FZRA early interior northern MA. Tuesday Night: Mainly VFR, with local MVFR possible. Windy with gusts to 30 kt. Slight chance SHRA. Wednesday: VFR. Windy with gusts to 30 kt. Wednesday Night through Thursday: VFR. Windy with local gusts to 30 kt.
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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 Monday/...High confidence. Through 12Z... Winds have diminished below small craft levels across the near shore waters so the Small Craft Advisory ended at 10 PM. Still noting some gusts up to 20-25 kt and seas up to 5-6 ft over the open waters which will subside through the remainder of the night. Today and Tonight... High pressure over the waters should provide fair weather with light winds and seas below 5 feet. Clouds will increase overnight, but support for any precipitation will remain well to our west during the night. Outlook /Monday Night through Thursday/... Monday Night: Winds less than 25 kt. Areas of seas approaching 5 ft. Chance of rain. Tuesday: Low risk for gale force winds with gusts up to 40 kt. Rough seas up to 13 ft. Rain. Tuesday Night: Low risk for gale force winds with gusts up to 35 kt. Rough seas up to 10 ft. Chance of rain showers. Wednesday: Moderate risk for Small Craft Advisory winds with gusts up to 30 kt. Rough seas up to 10 ft. Wednesday Night: Moderate risk for Small Craft Advisory winds with gusts up to 30 kt. Areas of rough seas. Thursday: Moderate risk for Small Craft Advisory winds with gusts up to 30 kt. Rough seas up to 10 ft.
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&& .HYDROLOGY... A Flood Warning remains in effect for the Connecticut River at Middle Haddam, where ice jams are causing river fluctuations. Received new data from NERFC late tonight so updated the Flood Statement to provide the latest information. Not too much change from the previous forecast. 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... CT...None. MA...None. RI...None. MARINE...Small Craft Advisory until 6 AM EST early this morning for ANZ250-254>256. && $$ SYNOPSIS...KJC NEAR TERM...Frank/EVT SHORT TERM...WTB LONG TERM...KJC AVIATION...WTB/KJC/EVT MARINE...WTB/KJC/EVT HYDROLOGY...

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