Area Forecast Discussion
Issued by NWS Boston, MA

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000 FXUS61 KBOX 251948 AFDBOX Area Forecast Discussion National Weather Service Taunton MA 348 PM EDT Thu May 25 2017 .SYNOPSIS... * Near- and Short-Term updates only... Low pressure and its attendant warm front will bring a steady rain and drizzle to the region through tonight along with cool northeast winds. The rain will be heavy at times tonight into Friday morning as the low tracks across Southern New England. Big improvement to start of the holiday weekend as weak high pressure delivers dry weather with mild days and cool nights both Saturday and Sunday. However by Sunday night and into Monday another coastal low may bring the risk of showers, although a washout is not expected. && .NEAR TERM /UNTIL 6 AM FRIDAY MORNING/... 345 pm update... */ Highlights... - Widespread light rain / drizzle - Moderate-heavy rain towards midnight into early morn Friday - Increasing onshore easterly winds, cooler air off waters - This all prior to a warm front and low pressure sweeping over S New England Friday morning */ Latest Discussion... Into this evening... Cool and damp. Continued broadscale, isentropic ascent, i.e., over- running, over the warm-moist conveyor belt beneath diffluence aloft is lending to widespread light to moderate rain. The dry slot slowly advancing as discerned via water vapor imagery, should see rain change over to mainly drizzle. This as the low deepens across the Mid-Atlantic as it begins its transition from the mature to dying phase as it begins to undergo occlusion. The increasing pressure gradient resultant, already seeing an increasing onshore E-draw of cooler air N of the stalled surface warm front off the coast of NJ and S of Long Island. That combined with the saturated atmosphere, we are seeing temperatures cool to their respective wet-bulb giving way to the development of areas of mist / fog. Temperatures mainly steady through the remainder of today, in and around the low 50s. Not very representative for late May. Tonight... Moderate to heavy rain expected with the possibility of a rumble of thunder. Low occlusion sweeping NE across S New England overnight. Crux of deep-layer synoptic forcing out ahead of the low through a fairly moist profile well up to H3 beneath the left front quadrant of the upper level cyclonic jet streak. Strong omega forcing not out of the question that will easily and efficiently wring out moisture, rather the nature and magnitude of said moisture wrapping into the occlusion, its associated precipitable waters / theta-E. Confident as to moisture pooling along the low to mid level warm front lifting gradually N across the region. Conditionally unstable moist profile aloft, and given strong omega, can not rule out thunder. The system itself is quick-moving, short duration, the bulk of the rain falling around midnight into the 6a timeframe. But there`s still spread, even between the GEFS and SREF members, with the SREF exhibiting members with higher amounts up around 2.0 to 2.5 inches. Quite an anomalous system with winter-like characteristics, taking nothing for granted, and as such am leaning more towards the high- res guidance as consensus of ensemble probabilities along with CIPS Analogs signal a low-prob of greater than 3-inches storm total rain- fall over E/SE MA and RI. Can not rule out an inch of rainfall in the roughly 6-hour period noted earlier. Potential urban and poor drainage issues, mainly inland as low tide proceeds after midnight into morning along the shores. Not thinking any impact to the rivers rather nuisance issues along area roadways, low-lying spots and culverts notorious for having issues. Focusing on the I-95 corridor and immediate roadways surrounding. && .SHORT TERM /6 AM FRIDAY MORNING THROUGH FRIDAY NIGHT/... Friday... Scattered shower activity lingering within the wrapping comma-head occlusion. Influx of cooler air rearward as the pressure gradient tightens with the low lifting E with a weak ridge of high pressure building from the W, will see a steepening boundary-layer profile allowing for both moisture and momentum to mix out. So thinking bulk of wet-weather activity will be early on, with the later-half mostly dry as winds increase out of the NW. Clouds breaking, more sunshine, given the time of year and the fact that the low lacks a more pole- ward connection of colder air, with the cooler air an artifact of being dragged down from aloft, should turn out near-seasonable with highs around the mid to upper 60s, warmer SW CT while cooler NE MA. Friday night... Drying out. Occlusion continues to lift E out to sea. Still a N flow on the backside with cooler air, the boundary layer remains well- mixed beneath rising heights from the W and an inversion around H8. Some scattered to isolated shower activity initially, diminishing through the overnight hours. Low clouds linger given boundary-layer mixing and moisture pooling beneath the inversion. With the warm warm blanket in place, keeping it mild with lows down into the upper 40s to low 50s. && .LONG TERM /SATURDAY THROUGH THURSDAY/... */ Highlights... * Mainly dry weather w/mild days and cool nights Saturday and Sunday * Risk of rain returns Sun night into Monday * Seasonably warm Tue & Wed but low risk of scattered showers */ Details... Friday night... Improving weather as closed mid level low over the Gulf of Maine exits seaward and allows rising heights along with mid level anticyclonic flow to move across New England. This will promote a drying trend overnight. Temps will be seasonably cool with lows in the upper 40s and low 50s. Saturday... WNW flow aloft with mid level mean ridge axis across the eastern Great Lakes will provide dry weather and seasonable temps across southern New England. Models do differ on cloud cover Sat with GFS showing much more low level moisture. Given the WNW flow aloft thinking at least partial sunshine will verify. This combined with light winds will provide very pleasant conditions with highs 70-75 CT river valley and 60s elsewhere. Weak pressure gradient will support cooling afternoon seabreezes for the coastline. Sunday... Deep layer ridging lingers across the area promoting dry weather much of the day. However model time sections and moisture profiles indicate lots of clouds likely on Sunday. 00z deterministic and ensemble guidance in good agreement that rain/showers associated with upstream system should arrive into southern New England either late in the day or after sunset. So as of now the bulk of the daylight hours of Sunday appear dry, especially across RI and eastern MA. Given the expected cloud cover highs should be a few degs cooler than Saturday. Fortunately SSE winds should be fairly light but will keep the immediate coastline cooler. Monday... Weak coastal low may pass south of New England along with its precip shield impacting the area. Doesn`t look like a washout but enough support to keep showers in the forecast. Somewhat cool with highs in the 60s given cloud cover and light onshore winds. Tuesday/Wednesday... Not as cool as parent low pressure over the Great Lakes provides warmer southwest flow into New England. With the milder temperatures comes the risk of scattered showers as mid level flow becomes increasingly cyclonic. && .AVIATION /20Z THURSDAY THROUGH TUESDAY/... Forecaster Confidence Levels... Low - less than 30 percent. Moderate - 30 to 60 percent. High - greater than 60 percent. Short Term /through Friday Night/...Moderate confidence. 18z update...No major changes to the forecast. Minor tweaks to account for trends. Otherwise, on track. Today... Into midday -RA/RA transitioning to DZ with increasingly lower visibility with FG. Lowering CIGs S to N with time towards IFR. N/NE winds becoming more E/NE, gusty at times potentially as high as 30-35 kts though thinking isolated. Mainly 20-25 kts. Tonight... IFR-LIFR CIGs and VSBYs. RA/+RA potential mainly from 6-12z with embedded TSRA possible. E winds continue with mainly 20-25 kt gusts. Friday... IFR/MVFR CIGs with SCT-BKN -SHRA, on and off through the day. Conditions may improve to VFR across western terminals in the late afternoon. Northwest winds for all terminals, becoming gusty into the later half of the day. Friday night... SHRA dissipating while improving VFR. NW winds blustery initially, relaxing. CIGs becoming BKN to SCT. KBOS TAF... IFR-LIFR through overnight. Winds a challenge. Strong easterly sustained at first, becoming light overnight while turning counter-clockwise NE to NW with low passage. Closely watching the 6-12z Friday timeframe for +RA potential and possible TSRA, whether there will be impacts to the AM push. Adverse weather may impact prior. KBDL TAF... Hold N winds funneling through the CT River Valley, turning NW with low pressure passage overnight. FG/DZ into evening, more RA and possible +RA/TSRA overnight 6-12z. IFR-LIFR, especially with CIGs through Friday morning. Outlook /Saturday through Tuesday/...moderate confidence. Saturday and Sunday daytime... VFR. Light winds Sat with seabreezes at the coast. Winds Sunday more from S-SE direction at fairly light speeds. Sunday night-Monday... Cigs/vsbys trending to MVFR/IFR in showers. Winds from the southeast, trending from the south Monday. && .MARINE... Forecaster Confidence Levels... Low - less than 30 percent. Moderate - 30 to 60 percent. High - greater than 60 percent. Short Term /through Friday Night/...High confidence. Tonight... Rain and fog, possible thunder. This as low pressure lifts N/E across S New England ahead of which along a warm frontal boundary E/NE winds will be sustained around 15 kts initially, becoming somewhat light with low passage and beginning to turn N/NW. Seas 6 to 8 feet on the outer waters. Friday into Friday night... Low pressure exiting behind which showers will linger as winds will be initially blustery out of the NW with sustained flow around 15 kts, gradually diminishing with time as a weak ridge of high pressure builds in from the W. Outlook /Saturday through Tuesday/... Saturday and Sunday... Fairly light winds and tranquil seas as high pres crest over the area. Dry weather and good vsby too. Sunday night and Monday... A series of weak lows may develop and pass south of New England, near the 40N/70W benchmark. && .TIDES/COASTAL FLOODING... */ Tonight... Combination of a very high astronomical tide and onshore flow will result in widespread minor coastal flooding tonight, mainly for the E MA coast. Persistent easterly flow around 15 mph will result in a surge of 0.8 to 1.0 feet along the E MA coast. In addition to very high astronomical tides will result in minor flooding. Greatest concern continues to be from Salisbury to Cape Ann. This is because this is where the strongest easterly flow is forecast along with a combination of E swell and wind wave direction. Greatest risk of both minor coastal flooding and beach erosion. A Coastal Flood Advisory continues for the MA E coast including Cape Cod and Nantucket for tonight`s high tide. Waves will not be a major factor but looks like enough low level wind gradient to produce 6 to 8 foot waves in the near shore waters in Ipswich Bay, about Cape Ann, and Massachusetts Bay could be an issue. Some erosion is likely along the Salisbury and Plum Island shorelines where wave action will be somewhat more significant. Elsewhere along the MA and RI coasts, the combination of the high astronomical tide and a tidal departure near a half foot may be enough to cause pockets of minor coastal flooding as has become more common during such king tide cycles. But our confidence continues to be low and will not be issuing a statement for the remaining coastline. */ Friday night into very early Saturday morning... A surge of 0.2 to 0.4 feet is forecast and combined with the peak of astronomical tides (Boston at 12.41 feet), should see at a minimum coastal flooding of low-lying locations notorious for becoming inundated during such tides (i.e., Morrissey Boulevard in Boston). Additional coastal flood statements may be necessary. Will continue to monitor and make updates after this evenings tide cycle. && .BOX WATCHES/WARNINGS/ADVISORIES... CT...None. MA...Coastal Flood Advisory from 10 PM this evening to 3 AM EDT Friday for MAZ007-015-016-019-022-024. RI...None. MARINE...Small Craft Advisory until 4 PM EDT this afternoon for ANZ231>234. Small Craft Advisory until 6 AM EDT Saturday for ANZ235-237- 250-254>256. Small Craft Advisory until 6 PM EDT Friday for ANZ251. && $$ SYNOPSIS...Nocera/Sipprell NEAR TERM...Nocera SHORT TERM...Sipprell LONG TERM...Nocera AVIATION...Nocera/Sipprell MARINE...Nocera/Sipprell TIDES/COASTAL FLOODING...WFO BOX Staff

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