Area Forecast Discussion
Issued by NWS Boston, MA

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000 FXUS61 KBOX 191414 AFDBOX Area Forecast Discussion National Weather Service Boston/Norton MA 1014 AM EDT Thu Apr 19 2018 .SYNOPSIS... Low pressure over Virginia will pass south of Nantucket this afternoon and pass south of Nova Scotia tonight. This will bring a period of chilly rain today. The rain may mix with wet snow, especially in the higher elevations. Drier air moves in on Friday, but with isolated showers possible. Building high pressure will bring gradually warming temperatures and dry weather into early next week. Storm system expected for mid to late week. && .NEAR TERM /UNTIL 6 PM THIS EVENING/... 10 AM update... Low pressure near 1000hPa had moved eastward, and was centered south of Long Island. Precip shield associated with a combination of mid lvl f-gen and overrunning crossed through southern New England this morning in advance of the this developing low. Radar indicating that the back edge of the initial widespread shield of precip had pushed eastward into eastern MA and RI at 10 am. HREF seems to pick this up nicely, bringing a bit of a break in the precip for late this morning/early this afternoon before more light precip moves in this afternoon. Have updated pops to reflect trends on radar and latest hi- res guidance. Looking at dual pol/cc there is still some sleet mixed in with the light RA/SN within eastern MA and possibly RI, the sleet should move offshore with main shield of precip late this morning. Then we transition to rain for primary precip-type for most of the area although can`t completely rule out a bit of light SN. Exception to this is -SN/-RA along the Worcester Hills and the east slopes of the Berkshires, a coating of accumulation possible. Early morning discussion follows... Low pressure along the Mid Atlantic coast will feed upon favorable dynamics and a sharp baroclinic zone covering the Mid Atlantic and Southern New England. This favors strong lift, with models showing 40-50 mb/hr rates during the day. Cross sections show moisture reaching to 500 mb. While not deep, this is probably deep enough. Favorable lift and moisture move over our area starting between 09Z and 12Z and reach maximum values during the morning, then diminish during the afternoon. Also of concern is precip type. Temperatures start the day mainly in the 30s while dew points will be in the upper 20s and lower 30s. As precip rates increase this morning, this may induce wet bulb cooling which may bring temps down a few degrees. Forecast profiles of temperature show borderline rain/snow profiles, and any additional cooling would only increase the potential for snow. Profiles most favor snow over the highlands as well as across Western and Central Mass, and maybe the Merrimack Valley. An inch or two accumulation is possible in the highlands, with an inch or less elsewhere in the favorable area. Mainly rain expected in CT/RI/Southeast Mass except in the highlands along the CT/RI border. The low passes Nantucket around 18Z/2PM, at which time the favorable lift starts moving off to the northeast. Surface winds will start shifting from the northwest, bringing in drier air. Low level temps aloft support max sfc temps in the low to mid 40s. The wet bulb cooling may keep some areas in the highlands in the 30s. && .SHORT TERM /6 PM THIS EVENING THROUGH 6 PM FRIDAY/... Tonight... Even as the surface low moves off to the east tonight, the upper trough will be swinging overhead. The axis is sharp, and carries a -24C to -26C cold core. This may be sufficient to maintain clouds and a chance of showers through the evening. The trough moves off after midnight, after which we may see some clearing especially in southern sections. Friday... Cyclonic flow around the upper low continues on Friday. The moisture aloft diminishes through the day, which should allow at least partly sunny skies. The diminishing moisture will reduce the development of showers, but we expect at least widely scattered showers during the afternoon. The cold advection and cold temperatures aloft should allow mixing to at least 850 mb. Winds in this layer should reach 25 knots, so we expect gusts of 25-30 mph. Temperatures in the layer support mid to upper 40s, and if mixing goes above 850 mb then highs in the low 50s are possible. && .LONG TERM /FRIDAY NIGHT THROUGH WEDNESDAY/... */ Highlights... - Near-seasonable, dry conditions over the weekend - Warmer conditions, dry, Monday into Tuesday - Mid to late week storm system Overview and model preferences... A couple of slow meandering cutoffs to monitor through the long term. The first, is the one that will most effectively define sensible wx details through the weekend, formed in N New England and shifting into the Maritimes. The slow movement maintains below normal H5 height anomalies through early next week, yielding temps near or below normal in spite of the continued modification. As this first cutoff shifts E, riding will build in its wake thanks to warm advection from the W. This ridge will dominate the forecast through the weekend and into early next week. The second cutoff meandering through the S stream all weekend lead to a merger point for Pacific energy mid-week yielding another risk for wet and more unsettled wx. Given synoptics are in fair agreement between both operational and ensemble data, will continue to use a blend for this forecast update. Discussion... Weekend... Upper lvl cutoff and attendant trof remain close enough to S New England that upper lvl flow is cyclonically curved through the period. This may assist in diurnal cloud development. Combine this with H85/H92 temps a full std deviation below normal (H85 roughly -4C to -6C while H92 temps run near -2C), and expect both highs and lows to remain below seasonal normals. Mon and Tue... Moderating temperatures as ridging gains control. H85 temps shift above 0C by Mon afternoon, with H92 temps approaching +4 to +6C. This will allow highs to reach near or even above seasonal normals, limited in part by the strengthening subsidence inversion as high pres crests across the region. Also, coastal areas limited by sea breezes thanks to low lvl flow SSTs still in the 40s. Overnight lows cool thanks to radiational cooling setup. Mid-late next week... A potential merger of Pacific, S stream and even N stream energy will redevelop longwave trof across the E CONUS, agreed upon by most ensembles as mean height anomalies once again drop below normal. This will yield another round of wetter and more unsettled wx. Timing could be as early as Wed for a new round of wet wx, but exact timing/details will be better defined as we approach. && .AVIATION /14Z THURSDAY THROUGH MONDAY/... Forecaster Confidence Levels... Low - less than 30 percent. Moderate - 30 to 60 percent. High - greater than 60 percent. Short Term /through Friday/... Today and tonight...Moderate confidence. Areas of MVFR spread E (low CIGS and VSBYS in precip) through the morning with a risk for IFR (mainly due to VSBY in SN) across the higher terrain of interior MA/CT. This changes to rain all TAF sites through the morning, with mainly MVFR conditions. However, through the afternoon will see some improvement from NW to SE although the SHRA/SHSN risk will linger into the evening hours. Note that the timing of the improvement may be off in the TAF. Winds shift to the N-NW through the day, peaking during the evening hours. Friday...High confidence. VFR. Northwest winds will gust to 25 knots during the day. Widely scattered showers during the day, and some of this could briefly cause MVFR cigs/vsbys. KBOS Terminal...High confidence in TAF. Precipitation mainly rain, but some snow/sleet possible for the first few hours of the TAF. Then mainly rain and MVFR conditions until some improvement during the evening hours. KBDL Terminal...High confidence in TAF. Light precipitation develops around 09Z/5 AM, increasing to max intensity 13Z to 18Z. Lowest cigs/vsbys expected at that time, with some improvement during the mid afternoon, then briefly lower again during the evening. Any snow accumulation is expected to be confined to grassy surfaces. Outlook /Friday Night through Monday/...Moderate confidence. Friday Night through Saturday: VFR. Breezy. Saturday Night through Monday: VFR. && .MARINE... Forecaster Confidence Levels... Low - less than 30 percent. Moderate - 30 to 60 percent. High - greater than 60 percent. Short Term /through Friday/...High confidence. Today... A coastal weather system moves northeast from the Delmarva coast and passes near the benchmark mid afternoon, then moves south of Nova Scotia tonight. Rain will lower vsbys to around 2-3 miles today. Winds remain below 25 knots and seas below 5 feet. Tonight... Increasing northwest winds tonight as the offshore weather system moves off past Nova Scotia. Winds will reach 30 knots at times. Seas will build on the southern waters late at night, with five to seven foot heights overnight. Small craft advisory has been issued for all waters. Friday... Northwest winds gusting to 25 knots, with seas near 5 feet on the outer waters. Small Craft Advisory in effect. Outlook /Friday Night through Monday/...Moderate Friday Night: Low risk for Small Craft Advisory winds with gusts up to 25 kt. Areas of seas approaching 5 ft. Saturday through Monday: Winds less than 25 kt. && .BOX WATCHES/WARNINGS/ADVISORIES... CT...None. MA...None. RI...None. MARINE...Small Craft Advisory from 2 AM to 3 PM EDT Friday for ANZ231>234-251. Small Craft Advisory from 4 AM to 6 PM EDT Friday for ANZ230. Small Craft Advisory from 4 AM to 3 PM EDT Friday for ANZ236. Small Craft Advisory from 9 PM this evening to 3 PM EDT Friday for ANZ235-237. Small Craft Advisory from midnight tonight to 3 PM EDT Friday for ANZ250. Small Craft Advisory from 9 PM this evening to 8 PM EDT Friday for ANZ254>256. && $$ SYNOPSIS...WTB/Doody NEAR TERM...WTB/Doody/NMB SHORT TERM... LONG TERM...Doody AVIATION...WTB/Doody/NMB MARINE...WTB/Doody is the U.S. government's official web portal to all federal, state and local government web resources and services.