Area Forecast Discussion
Issued by NWS Boston, MA

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000 FXUS61 KBOX 262322 AFDBOX Area Forecast Discussion National Weather Service Taunton MA 722 PM EDT Wed Jul 26 2017 .SYNOPSIS... High pressure will remain over the region tonight providing dry cool conditions. The high moves offshore Thursday giving way to slightly more humid conditions along with the risk for a few showers and an isolated thunderstorm, but by no means a washout. A drying trend likely develops Thu night into Fri as a cold front moves through the region and then stalls near the south coast Fri. A coastal storm will push off the mid Atlantic coast, which will bring showers and a few thunderstorms, along with gusty onshore winds with cool temperatures Friday into early this weekend. Dry and seasonable conditions return by early next week. && .NEAR TERM /UNTIL 6 AM THURSDAY MORNING/... 710 PM Update... Diurnal cumulus clouds tending to dissipate as sun lowers in the sky this evening, as seen on latest visible satellite loop, with mainly clear skies along the coast with the light onshore winds. Expect skies to become mainly clear by around 02Z-03Z, though mid and high clouds will start to approach NW Mass as leading edge of frontal system approaches. Temps at 23Z mainly in the 70s across the region, except the 60s with light S-SW winds along the S shore of Cape Cod as well as Nantucket and Martha`s Vineyard. Near term forecast pretty much on track. Have updated to bring conditions current and incorporated into the evening forecast trends. Previous Discussion... The only issue will be with dry airmass /dew pts in the 50s - lowest inland/ in place coupled with light winds, temps will fall quickly with sunset. So expect cool conditions with sunset. Otherwise tranquil weather prevails. Later tonight mid and high clouds will overspread the region from NW to SE and this will stabilize temps from falling much more. However given the ideal radiational cooling conditions the first half of the night sided with the cooler MOS temps for mins. Thus another cool night with lows in the 50s except around 60 in the urban areas. && .SHORT TERM /6 AM THURSDAY MORNING THROUGH THURSDAY NIGHT/... Thursday... Increasing clouds ahead of next approaching short wave. Weak low level WAA will result in surface dew pts climbing into the low and mid 60s, so it will feel more humid than today. Airmass is slightly warmer than today but with increasing clouds highs tomorrow will be close to values of today, 75-80. It will be slightly warmer along the eastern MA coast/beaches than today given SSW winds tomorrow. Overall a typical summer day, perhaps a few degs below average. As for the risk of showers/T-storms, approaching short wave provides modest mid level moisture advection (K index low 30s) along with weak forcing/height falls for ascent. This should be sufficient for isolated to widely scattered showers. However T-storms especially strong to severe appears very low as NW flow precludes true warm sector airmass from advecting northeast into southern New England. This is evident on all model guidance offering less than 500 j/kg of MUcape. This combined with weak mid level lapse rates (H5 temps only cool from -9C to -10C) will result in weak updrafts and stronger storms likely remaining in the warm sector across NYC/NJ/eastern PA. After collaboration with SPC the slight risk was removed from southwest CT for Thu. So isolated to widely scattered showers and T-storms Thu afternoon, however many hours of dry weather too, so by no means a wash out. Thu night... Good drying thru the column per decreasing K index values as short wave trough moves east of the area and mid level anticyclonic flow and associated subsidence take over. This will result in a drying trend from north to south. So any evening showers/T-storms will shift south and then offshore. However front will not reach the south coast until Fri morning. So expect warm and humid conditions to persist much of the night. This moist low level airmass will increase the risk of patchy fog as well. && .LONG TERM /FRIDAY THROUGH WEDNESDAY/... Highlights... * Risk continues for showers and a few thunderstorms possible Fri night and Sat, best chance S coast * Gusty E-NE winds possible along coastal areas, highest along S coast, Cape Cod and the islands late Fri night into Sat evening * Dry conditions likely Sunday into early next week Overview and model preferences... Continue to see rather wide model solution spread for track, strength and timing of low pressure moving off the mid Atlantic coast and S of New England. The big question is how far N the precip shield moves into the region and how strong the E-NE winds will be especially late Fri night through into Saturday night, especially along the S coast, Cape Cod and the islands. Rather strong H5 short wave associated with this system, especially pronounced on the ECMWF across S NJ during Sat. Could also see gusty winds along the coast, especially the S coastal areas late Fri night into early Sat night, but timing is still an issue. Models showing a bit better agreement after the low exits Sat night or even Sun morning, with dry conditions as high pressure builds across. Will see leftover cool temps on Sunday, then becoming more seasonable by the beginning of the work week. However, still noting broad H5 long wave trough developing across the eastern U.S. while another high amplitude ridge builds across the western states. Some question as to timing of another cold front moving SE out of southern Quebec/Ontario, but appears not a lot of upper support with this system. Timing and movement still very much in question. Details... Friday... Will start off dry and seasonable early Friday, then some showers may start to push NE by midday well ahead of low pres moving off the S NJ coast along a stationary front S of New England. Clouds will increase, along with the chance for showers. May see some marginal instability for a few thunderstorms Fri afternoon. Light/variable winds become light easterly, though may start to pick up along the S coast by evening. Expect highs to run a few degrees below seasonal normals due to the developing onshore wind. Friday night-Saturday... Best chance for showers especially from central Mass southward. Have likely POPs mentioned there for most of this period. Marginal instability remains across the region, so have kept in slight chance for thunderstorms mentioned as well. Big question will be the track of the low as it deepens into a rather anomalously strong system as it passes S of New England Friday night to mid day Saturday. Very strong H925-H9 jet associated with this system, with E-NE winds up to 40-50 kt S of eastern Long Island and Block Island by around midday Saturday. How much mixes down is in question, but can not rule out gusts to 30-40 kt especially over the coastal waters Could see gusts that high across Cape Cod and the islands during Saturday as well. Something to keep a close eye on. Still noting convective feedback issues, but rather good slug of precip moves across. Noting PWATs up to 2 to 2.2 inches moving across Cape Cod and the islands Sat, so could see some heavier rain there late Fri night through midday Sat. Will be a rather sharp line where heaviest precip falls, mainly along S coast, with about 0.25 inches or less from about N CT/RI to S of Boston northward. At this point, could see some improving conditions Saturday afternoon/evening from W-E as the low shifts offshore, but timing is still in question. Saturday will be another chilly day by July standards, with highs only in the lower-mid 70s, about 10 degrees below seasonal normals. Saturday night through Monday... Mid level long wave shifts offshore during Sat night, with NW flow in place aloft into early next week which may shift into Quebec. Meanwhile, at the surface, high pres builds eastward out of the Ohio Valley to the Appalachians. Good ridging and dry conditions build in this period. Will remain on the cool side Sunday, with highs running around 5 degrees below seasonal normals, but will warm on Monday especially across the interior. Tuesday-Wednesday... Another cold front starts to approach during Tuesday, though timing and strength in question. Not a lot of moisture associated with the front with the general W-NW flow aloft in place. Broad long wave trough sets up across the eastern U.S., but not much energy to work with. At this point, may see a few showers Tue, then should be dry on Wed but timing of features in question. Low confidence. && .AVIATION /22Z WEDNESDAY THROUGH MONDAY/... Forecaster Confidence Levels... Low - less than 30 percent. Moderate - 30 to 60 percent. High - greater than 60 percent. 710 PM Update... Tonight...Sky cover SCT-BKN at around 6-7kft through 01Z-02Z, then becoming mainly CLR-SCT mid to high level clouds. Light S-SE winds becoming calm to light/variable, then S-SW toward morning. Thursday...VFR with BKN CIGS 040-050 in the afternoon along with isolated to widely scattered T-storms. Thursday night...VFR but trending toward MVFR especially across CT/RI and southeast MA including Cape Cod and the Islands where patchy fog will form. Scattered showers and isolated T-storm possible in the evening hours. KBOS Terminal...High confidence. Isolated to widely scattered showers/T-storms Thu afternoon/early evening. Strongest storms will likely remain well southwest of the region across NYC/NJ/PA area. KBDL Terminal...High confidence. Isolated to widely scattered showers/T-storms Thu afternoon/early evening. Strongest storms will likely remain well southwest of the region across NYC/NJ/PA area. Outlook /Friday through Monday/... Fri...High confidence. Mainly VFR. May see patchy fog with local MVFR-IFR VSBYS, should improve by 14Z-15Z. Scattered showers and a few isolated TSRA push into S coastal terminals then slowly move N during the day. Local MVFR conditions possible. Fri night and Sat...Moderate confidence. Potential for IFR-MVFR conditions in showers. Patchy fog during the nighttime hours. Slight chance for thunderstorms, best chance S coastal terminals. E-NE winds which could gust 25-35 kt mainly near the coast from KGHG southward Sat. Rainfall, especially along S coast may be heavy at times, though still uncertain how close the heavy rain shield will come to the coast. Conditions should improve from W-E Sat afternoon/evening. Saturday night through Monday...High confidence. VFR. N-NE wind gusts to around 30 kt across Cape Cod and the islands early Sat night, then diminishing. && .MARINE... FORECASTER CONFIDENCE LEVELS... Low - less than 30 percent. Moderate - 30 to 60 percent. High - greater than 60 percent. 710 PM Update... Short Term /through Thursday Night/...High confidence on wind and waves, but lower confidence on probability of showers/T-storms. Tonight...Light onshore wind this evening becoming S-SE less than 10 kt. East swell of 2-4 ft continues across the ocean waters, waves 2 ft or less near shore. Dry conditions and good visibilities prevail. Thursday...SSW winds 10-15 kt possibly increasing to 20 kt in gusts late in the day. Isolated shower/T-storm possible. Good vsby continues. Thursday night...SSW winds 10-15 kt shifting to WNW toward daybreak Friday. Isolated shower/T-storm however fog may become fairly widespread but improving toward morning across the northern waters given the wind shift from SSW to WNW. Outlook /Friday through Monday/... Friday...High confidence. Light/variable winds shift to E-SE but remain light. Seas 3 ft or less. Friday night into Saturday night...Moderate confidence. Low pres passes S of New England while it intensifies. Still questioning how close the low will approach, but winds shift to E-NE and pick up after 06Z Sat. Could see sustained wind reaching 20-25 kt by 12Z Sat across the SW waters. E-NE winds could gust to 30-40 kt during Sat, likely highest across the southern waters to N and E of Cape Cod, with small craft winds elsewhere. Seas may reach as high as 8-12 ft on the open ocean waters. Showers with isolated thunderstorms, with periods of heavy rain on the southern outer waters. Winds slowly diminish Sat night, down to small craft levels. Seas will also slowly subside but remain at or above 5 ft. Sunday and Monday...High confidence. Improving conditions as Leftover small craft winds/seas diminish by Sunday evening, giving way to quiet boating conditions. && .BOX WATCHES/WARNINGS/ADVISORIES... CT...None. MA...None. RI...None. MARINE...None. && $$ SYNOPSIS...Nocera/EVT NEAR TERM...Nocera/EVT SHORT TERM...Nocera LONG TERM...EVT AVIATION...Nocera/EVT MARINE...Nocera/EVT

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