Area Forecast Discussion
Issued by NWS Boston, MA

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000 FXUS61 KBOX 171946 AFDBOX Area Forecast Discussion National Weather Service Taunton MA 246 PM EST Sat Feb 17 2018 .SYNOPSIS...
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Fast moving low pressure will bring a plowable snow to southern New England tonight, with the heaviest amounts near and south of the Massachusetts turnpike. The snow will be over by daybreak Sunday with rapidly improving conditions and temperatures rising well above freezing. Warm frontal showers lift N across the region for Monday followed by a warm-up for the mid week period, potential record breaking warmth. Return to more seasonable conditions by late week.
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&& .NEAR TERM /THROUGH TONIGHT/...
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*** Periods of heavy snow *** * Snow overspreads the region between 7 PM and 9 PM. * Periods of heavy snow with rates of 1-2 in/hr possible especially around and just NW of the I-95 corridor in MA and RI. Tricky forecast tonight for the immediate south coast. Still monitoring trends in the near term high resolution guidance. Only changes to the headlines early this afternoon was to convert the Winter Storm Watch to a Winter Weather Advisory. Expecting snow to mix with, or change to, some rain for a time overnight. This would be enough to keep average snowfall totals in the Advisory range. A few locations across the upper Cape may stay snow long enough to get locally higher totals approaching 7 inches. Still evaluating the need to expand a Winter Storm Warning farther north. 17/12Z models have increased QPF slightly, as well as shifted it ever so slightly more northward. Snow ratios of 12-15:1 are expected north of the Mass Pike, so the fluff factor could be enough to push snow totals over 6 inches. No question on precipitation type there. Decent lift and moisture within the dendritic growth zone between 10 PM Saturday and 4 AM Sunday. This should be the period of greatest snowfall. Timing will be crucial, a dry slot should work eastward along the south coast of New England after 1 AM. This would effectively end the snowfall. Snowfall rates of 1-2"/hr are possible in heavier bands. It would only take 3-4 hours of those snow rates to reach warning criteria. Still some uncertainty regarding the precise location of the heaviest banding, as well as the amount of snow along the immediate south coast, where the marine influence from southeast winds will be a huge factor.
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&& .SHORT TERM /SUNDAY THROUGH SUNDAY NIGHT/...
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The storm tonight will quickly depart within the progressive mid level flow. High pressure will build into the Mid Atlantic states, bringing drier and warmer conditions to southern New England during this period. Expecting a decent amount of snow melt Sunday afternoon. Good radiational cooling conditions Sunday night could lead to the formation of black ice on untreated surfaces.
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&& .LONG TERM /MONDAY THROUGH FRIDAY/...
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*/ Highlights... - Showers on Monday - Above average temperatures Tuesday and Wednesday - Following some rain, return to seasonable conditions late week */ Overview... Split flow parked out across the N-Central Pacific. S-stream dipping towards Hawaii into the ITCZ where anomalous westerlies prevail with a lingering phase 7 MJO. N-stream ridging into Alaska. The two under- going confluence into Western N America. N-stream energy sheared S capturing S-stream mild, moist westerlies before ejecting NE. A sub- tropical ridge builds off the SE CONUS as does the integrated water vapor transport into SE Canada, an anti-cyclonic Rossby wave break emerges into the N Caribbean. On the warm side of the thermal wind profile, looking at a surge of warmer than average conditions, shots of wet weather suppressed by mid-level subsidence, blustery SW winds throughout. Will hit on the targets of opportunity below. */ Discussion... Monday... Warm frontal light to moderate rain late and overnight. Isentropic ascent, convergent forcing. Precipitable waters +1-inch, expect higher outcomes N/W along SW-facing high terrain slopes, away from the building H5 sub-tropical ridge. Cold air eroding with breezy SW winds, gusts up to 30 mph initially before warmer air aloft inverts mixing to off. Both clouds and dewpoints increasing, potential fog issues given colder waters, ground. Non-diurnal temperature trend towards Tuesday morning as H925 temperatures warm above +10C. Tuesday into Wednesday... Potential record-breaking warmth, especially Wednesday. Please see the CLIMATE section below for record highs established on February 21st and for all of February since records began. H925 temperatures warming to +12-16C, H85 temperature anomalies +20C. Going with the cold front holding off till Wednesday night. A question of clouds hampering sunshine. Dry above H9, fair amount of low-level moisture to mix out below that given higher dewpoints. Breezy SW winds aiding in mechanical mixing along with anticipated daytime heating, expect cloud breaks over the interior while socked-in along the S/SE coast, especially evening and overnight periods. It`s amazing you only have to look back 1-year to get an idea on potential outcomes. Not the same synoptic pattern but can gain insight as to possible impacts. Stay conservative, hold highs in the 60s. SW gusts 30 to 35 mph possible. Wednesday night into Thursday Night... Showers at first along a sweeping cold front, suppressed in with its intensity by the sub-tropical ridge. Expect light outcomes. Then a cool down to more seasonable temperatures beneath high pressure. It could be into Friday morning we`ll see our coldest temperatures more so if radiational cooling materializes. Friday onward... If the sub-tropical H5 ridge weakens, thermal wind profiles shifting S/E, will be watching waves of energy out of the confluent base of a H5 trof over the W CONUS. On what side of the envelope and specific timing of individual waves, all pertinent on potential outcomes that at this time are uncertain. Preference to the EC ensemble mean.
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&& .AVIATION /19Z SATURDAY 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 Sunday Night/... Through 23Z this evening...High confidence. VFR. Winds around 10 kt veer through E to SE this afternoon. Tonight...High confidence in trends. Moderate confidence in timing. A quick moving winter storm passes SE of New England with -SN developing between 00Z and 02Z across all of New England, then periods of +SN with 1-2 in/hr snowfall rates continue from about 03Z-09Z, especially across portions of CT/RI and SE MA. Accums on runways expected. The SN then tapers off between 09Z and 12Z from W-E. Peak uncertainty across Cape/Islands, where some rain may mix in. Winds out of the S with a few gusts to about 20 kt across coastal areas. Sunday...High confidence. VFR. NW wind gusts up to 25 knots. Sunday Night...VFR. KBOS Terminal...High confidence in TAF overall, timing of lower categories this evening may be off a bit. KBDL Terminal...High confidence in TAF overall, timing of lower categories this evening may be off a bit. Outlook /Monday through Wednesday/...Moderate to High Confidence. Washington`s Birthday: VFR. Breezy. SHRA likely. Monday Night: Mainly VFR, with areas IFR possible. Breezy. SHRA likely. Tuesday: Mainly VFR, with areas IFR possible. Breezy. Slight chance DZ. Tuesday Night: Mainly VFR, with local IFR possible. Breezy. Slight chance DZ. Wednesday: Mainly VFR, with local MVFR possible. Breezy. Wednesday Night: VFR. Chance SHRA. Thursday: VFR. Slight chance SHRA.
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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 Sunday Night/... Poor visibility with gusty E-SE winds expected tonight in snow and rain. Gusts could approach 30 kt. Rough seas expected to build once more across the outer coastal waters Sunday. Gusty NW winds develop in the wake of a departing low pressure, too. Small Craft Advisories will likely be needed for some of the waters late tonight into Sunday. Still evaluating. Outlook /Monday through Wednesday/...Moderate to High Washington`s Birthday: Low risk for Small Craft Advisory winds with gusts up to 25 kt. Areas of seas approaching 5 ft. Chance of rain showers. Monday Night: Winds less than 25 kt. Seas up to 5 ft. Rain showers likely. Tuesday: Winds less than 25 kt. Local rough seas. Slight chance of drizzle. Tuesday Night: Winds less than 25 kt. Areas of rough seas. Slight chance of drizzle. Wednesday: Winds less than 25 kt. Areas of rough seas. Slight chance of rain showers. Wednesday Night: Winds less than 25 kt. Local rough seas. Chance of rain showers. Thursday: Winds less than 25 kt. Seas locally approaching 5 ft. Chance of rain showers.
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&& .CLIMATE...
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Record High Temperatures (Tuesday 20th / Wednesday 21st)... February.......20th......21st..... Boston.........68 (1930) 63 (1906) Hartford.......69 (1930) 63 (1930) Providence.....69 (1930) 63 (1930) Worcester......65 (1930) 59 (1930) Record Warmest Low Temperature... February.......20th......21st..... Boston.........46 (1930) 45 (1994) Hartford.......50 (1981) 49 (1981) Providence.....48 (1981) 50 (1981) Worcester......47 (1981) 43 (2002)
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&& .BOX WATCHES/WARNINGS/ADVISORIES...
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CT...Winter Storm Warning from 7 PM this evening to 7 AM EST Sunday for CTZ002>004. MA...Winter Storm Warning from 7 PM this evening to 7 AM EST Sunday for MAZ011>013-016>021. Winter Weather Advisory from 7 PM this evening to 7 AM EST Sunday for MAZ002>010-014-015-022-023-026. RI...Winter Storm Warning from 7 PM this evening to 7 AM EST Sunday for RIZ001>007. MARINE...None.
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&& $$ SYNOPSIS...Belk/Sipprell NEAR TERM...Belk SHORT TERM...Belk LONG TERM...Sipprell AVIATION...Belk/Sipprell MARINE...Belk/Sipprell CLIMATE...Sipprell

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