Area Forecast Discussion
Issued by NWS Boston, MA

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000 FXUS61 KBOX 281955 AFDBOX Area Forecast Discussion National Weather Service Taunton MA 355 PM EDT Sun May 28 2017 .SYNOPSIS... Isolated spot showers mainly across central and northern Massachusetts will come to an end early this evening. Otherwise, dry weather is expected tonight, but a fast moving low pressure system will bring a period of widespread rain and unseasonably cool temperatures to the region on Memorial Day. Upper low and cold pool linger during the mid and late week over the Great Lakes and Northeast USA while surface high pressure builds in. A couple of weak cold fronts are possible during this time bringing showers and possibly thunderstorms. Otherwise fair weather with near seasonable temperatures. && .NEAR TERM /UNTIL 6 AM MONDAY MORNING/... 350 pm update... Just enough diurnal heating coupled with low level moisture has allowed for a few spot showers to popup this afternoon, mainly across central and northern MA. Areal coverage will be quite limited and expect these brief isolated showers to come to an end early this evening with the loss of daytime heating. Otherwise, dry and tranquil weather in store for the region tonight with just an abundance of clouds. Low temps should bottom out between 50 and 55 in most locales. && .SHORT TERM /6 AM MONDAY MORNING THROUGH MONDAY NIGHT/... ***Period of widespread rain overspreads the region from west to east through early afternoon on Memorial Day with unseasonably cool temps*** Monday /Memorial Day/... A shortwave approaches the region on west to southwest flow aloft, which increases the forcing for ascent. This coupled with a modest ESE low level jet and Pwats 1+ standard deviations above normal, will allow rain to overspread the region. Timing still uncertain given model differences, but a period of widespread rain should overspread the region Monday morning through early afternoon from west to east. The rain should taper off to scattered light showers/drizzle late Mon afternoon/early evening. Elevated instability parameters look marginal for thunder, but there is a low risk for an isolated t-storm or two in our southern zones mainly along the south coast. It also will be an unseasonably cool Memorial Day as high pressure over the Canadian Maritimes will result in cool easterly surface winds, coupled with clouds and rain. High temps will only be in the 50s to near 60 and it will be breezy on the Cape/Islands. Monday night... Shortwave and deeper moisture will have exited the region by early Monday evening. However, moist northeast low level flow will probably allow scattered light showers/drizzle and fog to persist into the evening across eastern New England. While most of the scattered light showers/drizzle should come to an end by midnight, areas of low clouds and patchy fog may persist longer as low level moisture may remain trapped below the inversion. Low temps will mainly be in the upper 40s to the lower 50s. && .LONG TERM /TUESDAY THROUGH SATURDAY/... Big Picture... Longwave scale maintains a ridge west/trough east pattern for much of the long term period. Shortwave scale shows a closed low slowly moving from the Great Lakes into Eastern Canada, maintaining a cyclonic flow over the Northeast USA and especially New England through Saturday. Several shortwaves move through this flow. One additional shortwave dives south from the Canadian Arctic and over the Northeast USA Sunday as the closed low moves off. Mass fields are similar among the long-range models through Thursday, but differ in handling the Sunday shortwave. Thermal fields are similar through Thursday. This brings moderate-high confidence in the overall synoptic pattern through Thursday. Low confidence in events for next weekend. Details... Tuesday through Friday... Upper low and associated cold pool remain to our west Tuesday but move over us Wednesday through Friday. Multiple shortwaves move through the flow Tuesday through Friday, and exact timing at this stage is difficult. Already some model timing differences from the 12Z suite yesterday. At this time, the best chance of shortwave passages along with associated surface fronts/troughs would be Wednesday and Friday. Convection firing over Upstate NY and PA Tuesday could work into our area later Tuesday. Precipitable water values are highest Tuesday, then diminish Wednesday and Thursday. PW values return in surges Friday afternoon and Sunday afternoon. The cold pool will aid in destabilizing the airmass each day, with expected low level heating on Wednesday bringing the greatest destabilization. Convective parameters show increasing instability on Tuesday, greater instability Wednesday, stable air Thursday, and returning instability Friday. This suggests scattered showers and a few thunderstorms Tuesday, Wednesday, and Friday. We note that SPC shows a marginal risk of severe wx just to our west on their Day 3 outlook for Tuesday. Conditions do seem more favorable to our west and less favorable over our area. But Western Mass and adjacent CT are close to the outlooked area and will need to be monitored as Tuesday gets closer. It is possible a squall line or bow could develop to our west and then move into the adjacent areas late in the day. In such a case, the concern would be for straight-line winds late in the day. The Friday cold front stalls south of New England Friday night/early Saturday. Saturday-Sunday... High pressure brings a brief break in unsettled weather Saturday. Upper low sweeping south from the Canadian Arctic approaches our area on Sunday. This in turn generates a wave along the stalled cold front. The rain shield with this wave moves across our area on Sunday. As noted above, low confidence in this solution due to run- to-run variations. && .AVIATION /19Z SUNDAY THROUGH FRIDAY/... Forecaster Confidence Levels... Low - less than 30 percent. Moderate - 30 to 60 percent. High - greater than 60 percent. Short Term /through Monday Night/... Tonight...Low confidence. We kept things mainly VFR, but some of the guidance brings in a lot of IFR to even LIFR conditions. Certainly possible, but just did not have enough confidence to go that pessimistic given easterly flow and best moisture to our southwest. Later shifts will have to monitor trends closely this evening as confidence is rather low. Monday and Monday night...Moderate to high confidence. MVFR to IFR conditions overspread the region through the morning and into the early afternoon in a period of widespread rain. There also is a low risk for an embedded t-storm or two, mainly near the south coast on Mon. Easterly wind gusts around 20 knots are expected on the coast, with up to 25 knots across portions of the Cape/Islands. The rain will taper off to light showers/drizzle from west to east late Monday afternoon/early evening. However, MVFR- IFR conditions may persist for much of Mon night with the lowest conditions most likely on the coast. KBOS TAF...Moderate confidence in TAF. KBDL TAF...Moderate confidence in TAF. Outlook /Tuesday through Thursday/...Moderate confidence. Generally VFR. Potential for brief IFR in early morning fog. Potential for MVFR cigs/vsbys in afternoon/evening showers and scattered t-storms. Best chance for the latter will be in Western/Central Mass and Northern Connecticut. Least chance for showers/tstms will be Thursday. South winds Tuesday, becoming southwest Wednesday, west Thursday, and southwest Friday. All winds should be less than 20 knots. && .MARINE... Forecaster Confidence Levels... Low - less than 30 percent. Moderate - 30 to 60 percent. High - greater than 60 percent. Short Term /through Monday Night/... Tonight...High confidence. The ridge of high pressure will continue to move east of the waters. However, pressure gradient will remain weak enough to keep winds/seas below small craft advisory thresholds for most of the overnight hours. Monday...Moderate to high confidence. High pressure building across the Canadian Maritimes coupled with a wave of low pressure passing to our south will generate easterly wind gusts around 25 knots for many waters on Monday. Therefore, small craft headlines posted for many of our waters. Easterly fetch will also build seas between 3 and 6 feet across our open waters. The strongest wind gusts/seas will be across our southern waters. Rain and patchy fog is also expected to develop and an isolated t-storm or two can not be ruled out Monday across our southern waters. Monday night...Moderate to high confidence. Winds and seas will begin to diminish later Monday evening and especially after midnight as disturbance moves away from the region and pressure gradient weakens. Areas of fog may persist into early Tue am reducing vsbys for mariners. Outlook /Tuesday through Friday/...Moderate confidence. Tuesday through Friday... Winds less than 25 knots and seas less than 5 feet through the period. Scattered afternoon showers possible. Water temps may weaken most thunderstorms that approach the coastline, but one or two storms could briefly affect the waters. && .TIDES/COASTAL FLOODING... Latest observations continue to show a storm surge just over a half foot along the eastern MA coast. Another high astro tide of 12.0 feet occurs just after 2 am in Boston. While that is a bit lower than the last two nights, there will be a little more of an onshore component and probably a storm surge of a half foot or even a bit more. While no significant problems are expected, very minor nuisance coastal flooding is possible along the most vulnerable shore roads along the eastern MA coast. Therefore, have opted to go with another coastal flood statement. && .BOX WATCHES/WARNINGS/ADVISORIES... CT...None. MA...None. RI...None. MARINE...Small Craft Advisory from 9 AM to 8 PM EDT Monday for ANZ231>235-237. Small Craft Advisory from 11 AM Monday to 2 AM EDT Tuesday for ANZ250. Small Craft Advisory from 6 AM Monday to 2 AM EDT Tuesday for ANZ254>256. && $$ SYNOPSIS...Frank/WTB NEAR TERM...Frank SHORT TERM...Frank LONG TERM...WTB AVIATION...Frank/WTB MARINE...Frank/WTB TIDES/COASTAL FLOODING...Frank

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