Area Forecast Discussion
Issued by NWS Boston, MA

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577 FXUS61 KBOX 140239 AFDBOX Area Forecast Discussion National Weather Service Taunton MA 939 PM EST Sat Jan 13 2018 .SYNOPSIS... Arctic high pressure over the High Plains builds into New England through Monday night, providing a return to colder than normal temperatures. Low pressure may bring snow to the region from Tuesday into Wednesday night followed by another burst of cold weather Thursday through Saturday. && .NEAR TERM /UNTIL 6 AM SUNDAY MORNING/...
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935 pm update... Strong high pressure to our west will allow very cold air to continue to work into southern New England. Despite airmass not being that cold at 850mb...shallow cold air will continue to work into the region on NNW winds. Low temps by daybreak Sunday will be in the single digits to mid teens. By daybreak...some locations across interior southern New England may see 24 hour temperature changes exceeding 50 degrees. Quite the change back to winter. Mainly clear skies overnight with the exception being the far southeast New England coast...where some ocean effect clouds will impact the region. SST to 925 mb temperature differential not extreme...but NNW will be enough to generate some light ocean effect snow showers and flurries mainly east of Hyannis onto the outer-Cape. Any accums should be minimal...as activity should be light and transient with perhaps some neighborhoods picking up a coating.
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&& .SHORT TERM /6 AM SUNDAY MORNING THROUGH SUNDAY NIGHT/... 415 PM update... Sunday... Colder than normal temps continue with arctic 1040 mb high pres over NY state. 925 mb temps lower to -14C. This will only support highs in the 20s with upper teens across the high terrain. A modest northerly wind combined with this frigid temps will yield wind chills in the single digits and teens. Mostly clear skies Sunday but given the low Jan sun angle and arctic airmass will call it ineffective sunshine. Cape Cod and Nantucket may remain overcast given the moist NNE wind across the region along with scattered ocean effect snow showers continuing, but becoming more fragmented from diurnal heating providing enough disruption to the low level circulation. Sunday night... Dry but frigid with arctic high pres anchored across New England. Ridge axis across the interior and will provide ideal radiational cooling. Thus leaned toward the colder MOS temps here with lows zero to minus 5 across western MA. Not much of a wind chill here given the light winds. Much of the region will see mins in the single digits to about 10 degs in Boston and Providence urban areas to teens along the south coast. Not as cold here given NNE winds off the ocean. However this will create lower wind chills and also ocean effect snow showers spreading more onshore into Plymouth county and possible into Boston and the north shore. Could be some minor accumulations including the RT-3 corridor of Plymouth county. Will have to watch this going into the Monday morning commute. Not a lot of snow but perhaps a coating to an inch, enough for snow covered roads. Again will watch trends on Hi Res models. May not be enough for a winter weather advisory but perhaps a Special Weather Statement to increase Public Awareness on some slippery travel Sun night into Mon morning. && .LONG TERM /MONDAY THROUGH SATURDAY/... Big Picture... Longwave pattern through next Friday shows trough over the Eastern USA and a ridge over the Western USA. This trends to a flatter flow by next Saturday. Shortwave pattern shows a shortwave over the Canadian Arctic sweeping south this weekend and then digging over the Great Lakes. This trough then sweeps east, crossing New England midweek. Quieter flow for Friday-Saturday with building heights. Model mass fields are similar through Tuesday, but continue to show differences in the evolution of the eastern trough Wednesday- Thursday. The GFS and GGEM are more progressive with the trough, the ECMWF holds it closer to the coast. The GFS and ECMWF both develop secondary lows late Wednesday, but the GFS and 12Z ECMWF carry it well offshore while the 00Z ECMWF kept it close to the coast. Ensemble runs show solutions in between. The variations between models and between model runs do not generate confidence. We remain agnostic between the two opposing solutions at this time. Confidence in the forecast remains high through Tuesday night, then low Wednesday-Thursday and moderate Friday-Saturday. Details... Monday... Two weather systems...high pressure centered north of Maine and a storm well out to sea. These will combine to bring a northeast surface flow over Southern New England. Surface wind speeds of 10-15 knots and moisture below 850 mb suggest ocean effect clouds and scattered snow showers moving ashore over Cape Cod and much of east coastal MA. An inch of snow possible in the more significant snow showers. Clouds may extend to RI and Central Mass. Ridging from the high extends over Southern New England and shows a cold-air damming signal. So temperatures remain cold with 950 mb temps supporting max sfc temps mainly in the 20s and low 30s. Clouds and a light flow will keep temps a little less cold that the previous night. Based on forecast dew points, we well go with min temps in the teens inland and 20s along the coast. Tuesday-Wednesday-Thursday... Low pressure from the Canadian Arctic will sweep south to the Great Lakes Monday, then turn east and cross New England. This system will draw on precip water values on the high side of normal but not by much. Expect temperatures would suggest light snow. QPF of roughly 0.25 inches is possible, which would support a few inches snow. An advisory may eventually be needed for this event. As noted above, the ECMWF agrees with consensus on bringing this first system through on Tuesday-early Wednesday. The 00Z ECMWF has a slower-moving upper trough which places the supporting upper jet along the East Coast during Wednesday with a favorable left exit region of one jet segment over the Carolina coast. This develops a coastal low that moves up the coast and then near the 40N/70W benchmark Wednesday night. The GFS and 12Z ECMWF show similar cyclogenesis, but much farther east of North Carolina...near 70W longitude...and track the storm too far offshore to bring any pcpn to Southern New England except possibly Cape/Islands. The 00Z ECMWF scenario would bring a second burst of pcpn late Wednesday/Wednesday night ending Thursday morning with additional snow accumulation possible. The GFS/12Z ECMWF scenario would mean dry weather late Wednesday through Thursday. We are not confident on either solution at this time, but will keep our options open by mentioning pops through Wednesday night. Friday-Saturday... High pressure over the Southern USA builds north and brings dry weather to Southern New England. A weak shortwave races across during this time and could bring a period of clouds but no precipitation. Temperatures at all levels trend warmer with potential for above normal max temps and min temps during this time. && .AVIATION /03Z SUNDAY THROUGH THURSDAY/... Forecaster Confidence Levels... Low - less than 30 percent. Moderate - 30 to 60 percent. High - greater than 60 percent. Short Term /through Sunday Night/... Tonight... High confidence. VFR and dry weather with N winds 10-20 kt, highest winds over Cape Cod and Islands. One cloud deck moves over the Cape and Islands early tonight, with clearing by midnight. More clouds will develop over Massachusetts Bay as cold air flows in. This will generate more clouds with MVFR cigs, which will move into Cape Cod and Islands either late tonight or Sunday morning. Scattered ocean effect snow showers may be associated with these late clouds and may produce 3-4 mile vsbys. Sunday...high confidence. VFR and dry weather except MVFR in scattered snow showers over Cape Cod and Nantucket. Modest north winds 10-20 kt. Sunday night...moderate confidence. Some uncertainty on timing and areal coverage of MVFR conditions. MVFR in scattered snow showers continues over Cape Cod and Nantucket. However as winds shift from N to NE these MVFR conditions spread westward into eastern MA toward daybreak Mon. KBOS Terminal...High confidence in TAF thru Sunday then some uncertainty if snow showers make it westward into Logan Sun ngt. KBDL Terminal...High confidence in TAF. Outlook /Monday through Thursday/... Martin Luther King Jr Day: Mainly VFR, with local IFR possible in Eastern Massachusetts. Breezy. Chance SHSN. Monday Night: Mainly VFR, with local IFR possible in Eastern Massachusetts. Tuesday through Tuesday Night: MVFR/IFR conditions possible. Chance SN. Wednesday and Wednesday Night: MVFR/IFR conditions possible, improving to VFR during Wednesday night. Breezy. Chance SN. Thursday: VFR. Breezy. && .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 Sunday Night/... Tonight... NNW wind gusts between 20 to 25 knots with seas exceeding 5 feet across many open waters...where we will continue SCA headlines. Scattered snow showers and flurries will develop across eastern MA waters with some lower vsbys after midnight. As winds become northerly and combine with leftover southerly swells, seas will become very chaotic and choppy. Some light freezing spray possible. Sunday... NNE winds 10-20 kt with arctic high pres over NY state. Rough seas across the eastern MA waters. Scattered snow showers lower vsby at times over eastern MA waters. Some light freezing spray possible. Sunday night... NNE winds increase to 15-20 kt with arctic high pres anchored over southeast Quebec. Snow showers over eastern MA waters lowers vsby at times. Some light freezing spray possible. Outlook /Monday through Thursday/... Martin Luther King Jr Day: Winds less than 25 kt. Seas up to 5 ft. Chance of snow showers. Visibility 1 to 3 nm. Monday Night: Winds less than 25 kt. Local rough seas. Slight chance of snow showers. Areas of visibility 1 to 3 nm. Tuesday: Winds less than 25 kt. Areas of rough seas. Slight chance of snow. Areas of visibility 1 to 3 nm. Tuesday Night: Winds less than 25 kt. Local rough seas. Chance of snow. Visibility 1 to 3 nm. Wednesday through Wednesday Night: Winds less than 25 kt. Seas up to 5 ft. Chance of snow. Visibility 1 to 3 nm. Thursday: Low risk for Small Craft Advisory winds with gusts up to 30 kt. Local rough seas. Slight chance of snow. Areas of visibility 1 to 3 nm.
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&& .HYDROLOGY...
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Heavy rainfall and complete melting of our snowpack resulted in significant rises on rivers and streams. Flood warnings remain in effect for several rivers (please refer to our Flood statements and warnings). For details on specific area rivers, including observed and forecast river stages, please visit the Advanced Hydrologic Prediction Service /AHPS/ graphs on our website.
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&& .BOX WATCHES/WARNINGS/ADVISORIES...
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CT...None. MA...None. RI...None. MARINE...Small Craft Advisory until 7 PM EST Sunday for ANZ235-237-250- 251-254>256.
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&& $$ SYNOPSIS...WTB/Nocera NEAR TERM...Frank SHORT TERM...Nocera LONG TERM...WTB AVIATION...WTB/Nocera MARINE...Frank/Nocera/WTB HYDROLOGY...Staff

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