Area Forecast Discussion
Issued by NWS Boston, MA

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000 FXUS61 KBOX 181151 AFDBOX Area Forecast Discussion National Weather Service Taunton MA 651 AM EST Sat Nov 18 2017 .SYNOPSIS... High pressure over New England this morning provides dry weather much of the day. A warm front approaches the region late in the day with rain overspreading the area toward sunset and into the evening. Strengthening low pressure moves across western NY state tonight with warm air, strong winds and showers impacting southern New England. The attending cold front sweeps across the area midday Sunday with strong winds ahead and behind the front. Dry and cold conditions move across the region Sunday night and Monday. Mild and dry weather Tue ahead of a cold front which brings a risk for showers Tue night and Wed, followed by dry and colder weather Thanksgiving day. && .NEAR TERM /UNTIL 6 PM THIS EVENING/...
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630 AM Update... Mainly clear skies noted across central and eastern areas via 11Z observations and latest GOES-13 IR satellite imagery. Leading edge of high clouds have started to push into the E slopes of the Berkshires and western CT River valley, which will continue to quickly push E through the day. Leading edge of precip has reach into western NY state/W PA, with a few reports of -RA over far W NY state and NW and W central PA where the higher radar reflectivities are noted on regional composite radar. With the very dry air in place and an approaching mid level disturbance weakens in the fast flow aloft, it should remain dry through late morning. Rather cold start to the day with the mainly clear skies, dewpoints down to the teens and light/variable or calm winds. Temps at 11Z were generally in the upper teens and 20s, except around 30 along some of the immediate S coast as well as near KBOS. Expect S-SW winds to pick up during the morning, with gusts up to around 20 kt along the S coast by midday. This will bring temps up rather quickly, as readings recover to around 50 degrees along the S coast, but only the mid 30s to lower 40s across N central and W Mass. Previous Discussion... Cold ridge of high pressure across the region this morning. Sunshine to begin the day, but clouds will quickly increase as mid level ridge axis moves offshore and flow becomes cyclonic with WAA pattern developing. Despite increasing clouds today low level WAA will provide a nice temp recovery from this morning`s cold with highs this afternoon 50-55 along the coastal plain, extending into the Boston-Providence-Hartford corridor. Farther inland shallow cool air will be more stubborn to modify and highs there rising into the mid and upper 40s. Area of rain showers across W PA/NY will weaken and mainly pass northwest of our region as this lead short wave trough deamplifies as it runs into the ridge over New England. Thus expecting mainly dry weather today with any rain holding off until 21z-00z and mainly confined to CT and western-central MA. Therefore expecting dry weather to prevail today across RI and eastern MA.
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&& .SHORT TERM /6 PM THIS EVENING THROUGH 6 PM SUNDAY/...
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Early AM update... *** Strong SSW Winds South Coastal MA and RI *** *** Possible Fine Line of low convection *** Tonight and Sunday Morning... Wind Threat... Strong low level jet overspreads the south coast tonight. Good agreement from models on magnitude of jet 55-60 kt at 925 mb. Model soundings initially show low level lapse rates favorable this evening for 35-40 kt transfer in gusts. But as the night progresses strong WAA steepens low level inversion with gusts becoming less of a factor and sustained winds becoming the main issue with speeds 25- 30 kt. This will be sufficient to support wind advisory for the entire south coast including Cape Cod and the Islands. Rain and Convective Threat... Periods of rain with warm front lifting north across the region. Impressive warm sector for mid to late Nov overspreads the area late tonight with dew pts surging to 55-60! Thus temps will rise overnight toward 60 in the coastal plain. This will set the stage for a potential fine line of low top convection toward daybreak Sunday from west to east. Short wave energy merges into a negative tilt trough approaching the region toward 12z Sunday. Also a strong upper level jet streak of 130 kt approaches from the southwest 12z Sunday with LFQ over southern New England, enhancing QG forcing. In addition very strong frontal convergence as low pres deepens to 982 mb over NY state. Thus very strong forcing for ascent in a high shear environment. With dew pts rising to near 60 a few hundred joules of CAPE advects across the area, anomalous for mid to late Nov. However limiting factor for convection is marginal low and mid level lapse rates. So question becomes will there be sufficient instability to yield a strong response at the surface in the form of fine low of convection. Unfortunately model timeframe here is 36-42 hrs, on edge of our mesoscale guid with HREFv2 only out to 36 hrs. However the NCAR ensembles, ARW and NMMB all hint at possible fine line in simulated reflectivity fields. For now will insert slight chance thunder with gusty winds and heavy downpours as PWATs climb to 1.5 inches (+2 standard deviations). Sunday Afternoon... *** Strong Post Frontal Winds likely *** 979 mb low traversing northern New England/St Lawrence River Valley will yield strong pres gradient, post frontal CAA and impressive pres rise-fall couplet for a period of strong WNW post frontal winds Sunday afternoon with speeds of 40 to 50 mph possible. Later shifts will have to evaluate model trends for possible issuance of wind advisories for Sunday afternoon. Drying trend behind the fropa along with temps falling from morning highs near 60.
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&& .LONG TERM /SUNDAY NIGHT THROUGH FRIDAY/...
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Highlights... * Strong W-NW winds linger Sunday night into Monday * Some question whether moisture tries to work up the eastern seaboard around the Tue night/Wed timeframe as cold front passes * Temperatures fluctuate day to day, from mild to cold back to mild again through most of the work week Overview... Overall mid level steering pattern across the lower 48 remains progressive through most of the long term periods. However, noting some members of the 00Z model suite try to dig a long wave H5 trough along the eastern seaboard around the late Tuesday into Wednesday timeframe, which may try to capture some southern stream moisture and push it up the eastern seaboard as a cold front passes across the region. The trough appears to retreat northward late Wed night or Thu, then becomes nearly zonal in the northern stream by the end of next week while another northern stream trough starts to dig into the Pacific northwest. Models and ensembles continue to signal fluctuating temperatures as progressive short wave troughs and ridges move across the region. So, will see mild temps ahead of approaching frontal systems, then a cool down after the systems pass. By the end of the week, temps may feel more like early winter than late autumn. Used a blend of available guidance for the majority of the forecast, though opted closer to a 00Z GGEM/ECMWF blend for Tue night/Wed with possible influx of moisture into the region ahead and with the approaching cold front. Details... Sunday night through Monday night... H5 short wave digs across into E PA/N NJ and steadily shifts E which will bring another shot of cooler air as well as gusty W-NW winds. As the strong surface low, on order of sub-980 hPa across N Maine into the Gulf of St Lawrence will exit with a tight pressure gradient in place. Also, both the GFS and NAM BUFKIT soundings showing deep momentum transfer, possibly up to 800 hPa across inland areas from Monday morning through early afternoon. There is also excellent low level lapse rates up to H85, up to 8-9C/km. So, could see gusts up to 35 to 45 mph, possibly higher across the higher inland terrain and along the coast at times. Have kept mention of the potential of strong gusts in the HWO for this timeframe. Expect dry conditions during this time, but some question whether any lake effect snow bands may reach into western areas during Monday on the strong W-NW winds. At this point, kept a dry forecast going. Strong cold air advection continues behind the departing low, with H85 temps down to -10C to -12C overnight into Monday morning and not changing much during the day Monday. So, expect highs possibly holding in the lower-mod 30s across the higher terrain of N central and NW Mass, ranging to 40-45 across the coastal plain. With the gusty winds, it will feel more like the upper teens to mid 20s inland to 30-35 along the shore. Tuesday... High pressure builds across the mid Atlantic and SE states, with northern ridging passing across the region. Winds will back to SW by Tue afternoon as the high pushes off the coast. Could see gusts up to 25-30 kt along the immediate S coast, Cape Cod and the islands late in the day. May see some mid and high clouds work into western areas Tue afternoon. Will likely see the mildest temps of the week during this time as highs are forecast to be in the 45-50 degree range inland and the mid 50s across N CT to the coastal plain. Tuesday night and Wednesday... Cold front approaches, so will see clouds increase. Associated low pressure will shift across southern Ontario and Quebec in the fast flow aloft. There are some questions as to the evolution of the eastern U.S. pattern during this timeframe, as there is wide model solution spread handling the H5 pattern. Some models trying to dig an H5 trough to the mid Atlantic or SE U.S. while one model member actually cuts off H5 low across S GA/FL. At this point, the front should push offshore, but the big question will be whether the timing of the front and a weakness in the H5 pattern may be enough to allow some southern stream moisture to work NE along the eastern seaboard across the region. At this point, have introduced CHC POPs Tue night into Wed, then shift E late Wed into Wed night. Have lower confidence due to wide solution spread. Expect highs on Wed to run close to or just above seasonal normals, though temps will start to fall as NW winds take over later Wed. Thanksgiving and Friday... At this point, looks like dry conditions will be in place at the end of next week as high pressure ridge builds in from the west as the northern stream steering flow becomes nearly zonal. Some clouds may push in across the Cape and islands on Friday as low pressure passes well SE of the region. Looks to be a chilly Thanksgiving day with highs only in the mid 30s to around 40 inland and the lower-mid 40s at the shore. It may be a bit milder on Friday, with highs mainly in the 40s, but some upper 30s may linger across the higher inland terrain.
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&& .AVIATION /12Z SATURDAY THROUGH WEDNESDAY/...
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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/... 1145Z Update... Today...High confidence. Expect VFR with dry conditions. The exception will be across western MA/N central CT where MVFR conditions in light rain will increase between 21z-00z. Light winds this morning increase sharply this afternoon from the S-SW with gusts approaching 30 kts by sunset (22z). Elsewhere S-SW winds increasing to 10-15 kt. Tonight...High confidence. VFR-MVFR to start in areas of light rain but trending toward IFR-MVFR after midnight. Strong S-SW 20-25G45kt winds along the south coast including Cape Cod and Islands. Elsewhere winds not as strong and this results in LLWS across much of RI and eastern MA including BOS. Sunday...Moderate confidence. Some uncertainty on probability of -TSRA and areal coverage along and ahead of strong cold front. MVFR-IFR with widespread showers, some with locally heavy downpours. Could be a fine line of low top showers with embedded thunder roughly 15z-18z. Gusty winds may accompany this convection. Strong SSW winds 20-25G45kt continue along the south coast including Cape Cod and Islands. Away from the south coast expect LLWS to continue thru the morning across RI and eastern MA. Then a strong cold front sweeps across the area late morning into the early afternoon, with improving conditions behind the front and a wind shift to the west. KBOS Terminal...High confidence in TAF thru 00z, then some timing issues on arrival of lower CIGS and rain. Period of rough weather tonight into midday Sunday with LLWS along with +SHRA. Isolated Thunder possible 15z-18z Sunday, then strong cold front sweeps across the area with improving conditions after 18z Sunday. KBDL Terminal...High confidence in TAF thru 21z, then some timing issues on arrival of lower CIGS and rain. Unsettled weather tonight into Sunday morning with +SHRA and possible isolated Thunder. Outlook /Sunday Night through Wednesday/... Moderate confidence. Sunday Night through Monday: VFR. Windy with gusts to 35 kt. Monday Night: VFR. Windy with gusts to 30 kt. Tuesday: VFR. Breezy. Tuesday Night: Mainly VFR, with local MVFR possible. Breezy. Chance SHRA, slight chance FZRA well inland. Wednesday: Mainly MVFR, with areas VFR possible. Breezy. 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/...High confidence. Today... Light winds this morning with high pressure overhead. S-SW winds increase this afternoon and approach Gale force by late in the day. Tonight... S-SW gales along with poor vsby in showers and fog. Sunday... Strong cold front sweeps across the region late morning and early afternoon, with SSW prefrontal gales and WNW gales behind the front. Showers, locally heavy in the morning along with fog but improving with the wind shift. Outlook /Sunday Night through Wednesday/... Moderate to high confidence. Sunday Night and Monday: Moderate risk for gale force winds with gusts up to 35-40 kt. Rough seas up to 12-14 ft. Monday Night: Low risk for gale force winds with gusts up to 35 kt. Areas of rough seas. Tuesday: Moderate risk for Small Craft Advisory winds with gusts up to 30 kt. Local rough seas. Tuesday Night: Moderate risk for Small Craft Advisory winds with local gusts up to 30 kt. Areas of rough seas. Chance of rain showers. Wednesday: Winds less than 25 kt. Local rough seas. Chance of rain showers.
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&& .BOX WATCHES/WARNINGS/ADVISORIES... CT...None. MA...Wind Advisory from 10 PM this evening to 5 PM EST Sunday for MAZ020>024. RI...Wind Advisory from 10 PM this evening to 5 PM EST Sunday for RIZ005>008. MARINE...Gale Warning from 5 PM this afternoon to 6 AM EST Monday for ANZ232>234. Gale Warning from noon to 11 PM EST Sunday for ANZ230. Gale Warning from 1 AM Sunday to 6 AM EST Monday for ANZ231- 251. Gale Warning from 7 PM this evening to 7 PM EST Sunday for ANZ236. Gale Warning from 3 PM this afternoon to 6 AM EST Monday for ANZ235-237-255-256. Gale Warning from 7 PM this evening to 6 AM EST Monday for ANZ250-254. && $$ SYNOPSIS...Nocera/EVT NEAR TERM...Nocera/EVT SHORT TERM...Nocera LONG TERM...EVT AVIATION...Nocera/EVT MARINE...Nocera/EVT

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