Area Forecast Discussion
Issued by NWS Boston, MA

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444 FXUS61 KBOX 272253 AFDBOX Area Forecast Discussion National Weather Service Taunton MA 653 PM EDT FRI MAY 27 2016 .SYNOPSIS... Humid weather continues tonight with any lingering showers diminishing but fog possible overnight. A backdoor cold front will bring cooler temperatures to eastern MA Sunday but it will remain very warm across the interior, where scattered showers and thunderstorms likely develop during the afternoon. The potential exists for a period of heavy rain very late Sunday night into Memorial Day. Dry weather likely follows Tuesday through Thursday, with above normal temperatures trending back to seasonable levels by late in the week. A cold front approaches on Friday with a chance of showers. && .NEAR TERM /UNTIL 6 AM SATURDAY MORNING/... The remaining showers should diminish this evening. Concern now turns to fog formation overnight. Current dew points 55 to 65 while water temps east and south are in the 50s. Satellite imagery continues to show fog along the Maine coast and farther offshore. With light winds and increasing moisture in the air, we are likely to have another round of fog. This will favor coastal areas as well as typical inland fog spots. The fog may be locally dense, and may require a dense fog advisory once confidence is higher in its location. Otherwise, mins only fall into the low-mid 60s again tonight thanks to high crossover temps and lingering clouds. && .SHORT TERM /6 AM SATURDAY MORNING THROUGH SATURDAY NIGHT/... Tomorrow... Mid lvl ridging continues with rising heights across the region and plenty of sunshine. No trouble mixing to at least H85 where temps will approach +16C by afternoon peak heating. Weak return flow will limit sea breezes to mainly the south coast. Therefore, still expecting probably the hottest day we have seen thus far with highs topping out in the low-mid 90s across much of the region away from S coastal areas. Dwpts will remain elevated as well, mainly in the low-mid 60s even during peak mixing. The other issue will be convective potential. Plenty of SFC/mixed lvl CAPE would be available, except the rising heights and modest subsidence it/s associated with is likely to yield a slight capping inversion and limit updraft development. Lapse rates are modest, mainly approaching 6.0C/KM in the mid lvls, but this is most likely a reflection of the mid lvl dry air. Even the latest 12Z ECMWF now is almost NIL for QPF during the afternoon and evening hours. Definitely not going NIL given the rising PWATs and potential energy available if the cap is broken, but will limit it mainly to slight and low end chance across the terrain and along developing sea breezes where at least some lift will be available. T-storms will be mainly airmass-type as shear is also quite limited. Tomorrow night... Continued increase in low-mid lvl moisture will actually help to erode the cap and allow for some maintenance of any shra/t-storms that develop, so will be slow to drop overnight POPs. Otherwise, the increase in moisture beneath the inversion increases the risk for overnight fog development which once again has the risk of being locally dense at times and may require headlines. Otherwise, another mild night with mins only falling back into the 60s. && .LONG TERM /SUNDAY THROUGH FRIDAY/... Big Picture... Longwave scale shows East Coast upper ridge over the weekend. The ridge axis shifts west and deamplifies Sunday and Monday. Pattern shifts to ridge West and trough East during mid to late week. Shortwave scale shows East Coast ridge Sunday shifting out to sea and opening a moist flow up the coast from a disturbance off Florida. All models hold some of this disturbance off the SE USA coast early week while sending pieces of it up the coast and past New England early week. Timing of these pieces remains in question but model consensus would point to Monday-Monday night passage. Northern jet stream shows one shortwave racing past New England Monday night, and a second approaching next Friday. Details... Sunday... Maritime high pressure builds and pushes a backdoor front into Southern New England. With the upper ridge shifting overhead the surface front will at some point be parallel to the upper flow and may stall over the region. Stability parameters such as the Total- Totals and the Lifted Index become marginally unstable by the afternoon across Western and Central MA. Any sunshine heating this airmass together with the low level convergence along the front may lead to thunderstorm development. Best chance would be in Western/Central MA, but this could expand across Northern CT as well depending on placement of the backdoor front. Precipitable water values near 1.5 inches, well above the May 29 average of 0.88, suggest some potential for local heavy downpours. The backdoor front shifts north Sunday night plunging our area more fully into mild and humid air. Temperatures at the top of the mixed layer are expected to be equiv to 15-16C at 850 mb. Full mixing of this would support highs in the 80s. East/marine flow on the cool side of the backdoor front may keep temps in Eastern MA in the 70s. Increasing clouds and dew points of 55 to 65 should keep Sunday night temps no lower than 55 to 65. Monday... Surface low pressure, part of the system off the SE USA coast, moves up the coast and spreads a good supply of moisture into New England. The coastal low will bring some dynamic support for lift. At the same time, a shortwave moves east from the Great Lakes and drives a cold front toward New England. This shortwave will move New England under the lift-friendly right entrance region of its upper jet. It will also draw plenty of moisture north with precipitable water values at 1.75 inches and potentially near 2 inches. As with the Sunday values, this is well above normal for the Memorial Day weekend. This will mean more showers, scattered thunder, and some locally heavy downpours. Much of the concern for Monday will be the developing potential for local flooding from any heavy downpours. Tuesday through Friday... High pressure brings drier air in on Tuesday and lingers through the week. Westerly winds Tuesday will allow temps to reach the low to mid 80s. Later in the week, as the high shifts offshore and the surface flow becomes south to southeast, max temperatures will be held to the 70s with some 60s at the coast. By Friday a cold front in the St Lawrence Valley will bring showers in New York and Northern New England. The question will be if any of those showers can reach Southern New England. For now we will go with consensus values with slight chance pops during the day and chance pops in some locations Friday night. This timing will likely change. && .AVIATION /00Z SATURDAY THROUGH WEDNESDAY/... Forecaster Confidence Levels. Low - less than 30 percent. Moderate - 30 to 60 percent. High - greater than 60 percent. Tonight...moderate confidence. Mainly VFR through about 06Z inland, then patchy fog possible especially at typically prone airports with localized MVFR/IFR conditions. Another risk for LIFR along Cape Cod and Island terminals overnight into Saturday morning. Saturday and Saturday night...moderate confidence. Mainly VFR. Local MVFR-IFR conditions in patchy fog through mid morning. Low probability of brief MVFR conditions in SCT afternoon SHRA/TSRA across the interior. Another risk for overnight fog tomorrow night. KBOS TAF...High confidence in TAF. KBDL TAF...High confidence in TAF. Outlook /Sunday through Wednesday/... Sunday...Moderate Confidence. VFR. Areas of MVFR in afternoon showers and scattered thunderstorms. Sunday night-Monday...Moderate confidence. Conditions lowering to MVFR and IFR in fog overnight. MVFR-IFR conditions in showers late Sunday night through Monday. Locally heavy downpours. Winds become east Sunday night and South on Monday. Tuesday-Wednesday...Moderate to high confidence. VFR conditions except local IFR in fog each late night and morning. && .MARINE... Forecaster Confidence Levels. Low - less than 30 percent. Moderate - 30 to 60 percent. High - greater than 60 percent. Short Term /through Saturday Night/...High confidence. Tonight... Winds and seas remain uneventful tonight. There will be some fog to contend with. The fog could be quite dense at times and may take some time to burn off even after sunrise. Saturday and Saturday night... Seas on the southern outer waters may briefly reach 5 feet at times but should diminish again at night. South to southwest winds will remain less than 20 knots. More fog may develop Saturday night. Outlook /Sunday through Wednesday/... Sunday...High confidence. Winds and seas remain less than Small Craft Advisory thresholds. Locally poor vsbys in fog Sunday morning. Sunday night-Monday...Moderate confidence. Winds and seas increase but still remain below SCA thresholds. Areas of poor vsbys in rain and fog. Monday night through Wednesday... Seas build to 5-6 feet on the exposed Southern Waters Monday night and Tuesday. South winds near 20 knots early Monday night shift out of the West and diminish overnight. Winds remain below SCA thresholds through Wednesday. A Small Craft Advisory may be needed for hazardous seas Monday night and Tuesday. && .CLIMATE... Still expect highest temperatures of the week on Saturday. Some record highs may be challenged. However, might even see temps getting close to records today as well. Record highs for Saturday May 28... Boston...92 set in 1931 Providence...91 set in 1931 Hartford...93 set in 1977 Worcester...88 set in 1911 and 1929 Milton/Blue Hill...90 set in 1929 && .BOX WATCHES/WARNINGS/ADVISORIES... CT...Air Quality Alert from 11 AM to 11 PM EDT Saturday for CTZ003- 004. Air Quality Alert until 11 PM EDT this evening for CTZ002>004. MA...Air Quality Alert from 11 AM to 11 PM EDT Saturday for MAZ004>007-009>018-026. Air Quality Alert until 11 PM EDT this evening for MAZ002>006- 008>013-026. RI...Air Quality Alert until 11 PM EDT this evening for RIZ001>005. MARINE...None. && $$ SYNOPSIS...WTB/Doody NEAR TERM...WTB/Doody SHORT TERM...Doody LONG TERM...WTB AVIATION...WTB/Doody MARINE...WTB/Doody CLIMATE...

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