Area Forecast Discussion
Issued by NWS Boston, MA

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000 FXUS61 KBOX 280131 AFDBOX Area Forecast Discussion National Weather Service Taunton MA 931 PM EDT WED JUL 27 2016 .SYNOPSIS... One more day of heat and humidity Thursday before a cold front slowly drops south across the region, possibly bringing scattered showers and thunderstorms. An area of low pressure along with rain and possible thunderstorms is forecast around Friday which could be heavy at times. With that there is the potential for localized flooding. Seasonable and dry for Saturday. Shower chances return for Sunday into Monday. Looking dry with a moderating warming, muggy trend through the remainder of the week. && .NEAR TERM /THROUGH THURSDAY/... A few patches of cirrus, but otherwise clear skies as high pressure moves across. Expect this to continue overnight. Light winds will allow for radiational cooling. Dew points range from the mid 50s in Northern MA to the upper 60s over Cape Cod and Islands. So min temps should range from the upper 50s in low lying areas in the interior to around 70 in urban centers and along the coast. && .SHORT TERM /THURSDAY NIGHT THROUGH 6 PM THURSDAY/... SW flow gets underway Thu as high pressure moves offshore and cold front approaches from northern New England. Some of models such as 12z NAM/GFS want to bring showers and storms into southern New England during afternoon and front draws closer. However, it appears most of the instability and better lift stays north of our area, which is where we expect most of activity to be favored. This is in agreement with most ensemble solutions, including high-res NCAR ensemble, which show better chances for storms across VT and NH. It`s certainly possible we see few storms manage their way into western or north central MA late in day, so will keep chance PoPs in these areas. Not looking at severe threat given lack of strong 0-6km shear and weak mid level lapse rates. We are more confident in one more day of heat and humidity as highs reach 90s in many locations (aside from higher terrain and immediate South Coast). Even E MA beaches should reach 90 given SW flow. Dewpoints will be a bit higher than today, getting into upper 60s to near 70, which results in heat index into low to mid 90s. && .LONG TERM /THURSDAY NIGHT THROUGH WEDNESDAY/... */ HIGHLIGHTS... - Low confidence forecast for Thursday night through Friday night - Still some possibility of heavy rain, flooding - Seasonable and dry for Saturday - Chance of wet weather for Sunday into Monday - Warming and becoming more muggy through the remainder of next week */ DISCUSSION... Thursday night into Friday night... Overall, an interesting synoptic setup atypical with respect to the pattern as of late depending on which camp of solutions one prefers. GFS/UKMET vs. NAM/EC/Canadian/SREF? S-stream sub-tropical energy brought N captured within the N-stream trough pattern and cyclonic flow undergoes surface cyclogenesis as the trough undergoes a negative-tilt. Maturing, closing off beneath H7, conveyor-belt motions strengthen drawing N rich sub-tropical moisture with pwats up to 2.5 inches while undergoing deep-layer synoptic forcing ahead of mid-level vortex energy and beneath the right-rear-quadrant of the upper-level-jet. Much of the sub-tropical moisture focused within the forward-leading anticyclonic conveyor belt flow, looking at a front-end thump with particular attention to the NE quadrant of the H85 low where frontogenesis and deep-layer forcing combine along a SW to NE axis. It is here heavy rain is anticipated with rainfall amounts at least 1-inch, locally higher with around 2 to 3 inches possible. But challenges remain: Are there complications of and between n- stream and s-stream systems? The depth, timing and orientation of the H5 trough axis? Where and when will the forecast heavy rain axis setup? Will it impact S New England? Will thunderstorms be an issue? Over how long will this event take place, if it does impact S New England? And the biggest question, are there any flood threats? Evaluating ensemble probabilistics, CIPS analogs, and SREF guidance, a considerable spread of outcomes lending to low confidence. Notable differences in the 27.09z SREF run with ARW core solutions wetter than NMB, while exhibiting a considerable spread per plumes in both camps. But when looking looking at the past 4 SREF solutions, S of the Mass-Pike has trended with a considerably wetter forecast, while there are indications of a N-S gradient of rainfall. GEFS plumes too has a considerably large spread with the operational GFS run towards the bottom (driest) of solutions for S New England. Collaborating with WPC, they`re going with a GFS/UKMET blend holding with a quicker solution with the attributing vortex energy becoming sheared within the overall flow, that all other solutions which are slower subsequent of the deeper developing surface low are outliers. But they too have low confidence. While the 27.12z EC has trended S from its prior run, it still has significant outcomes for S New England. Will go ahead with a 50/50 blend of the two camps. Upon doing so, likely PoPs prevail for all of the region. Will keep with chance thunderstorms. Conservative with precipitation amounts. An overall low confidence forecast. In closing, there are some points to consider depending on which camp of solutions are preferred: 1) Thunderstorms will be possible given elevated instability with moist adiabatic, conditionally unstable profiles. 2) Atmospheric column is forecast to be above freezing below H6 (give or take 12-13 kft) in areas of heaviest rain. 3) And storm motion is roughly E/NE at 20 to 30 mph. All combined, it is likely within areas of forecast heaviest rain that warm-rain processes should be anticipated and this could yield significant rainfall rates on the order of 1-inch, possibly as high as 2-inches, per hour. Thus the concern with this synoptic setup with regards to timing, strength, and specifically where the area is of greatest risk for possible flooding. Will highlight this in the hazardous weather outlook while preparing for all possible outcomes. Saturday... High pressure in control with a slightly cooler airmass aloft with H85 temperatures around +12-14C. Looking at seasonable temperatures with light and variable winds likely yielding sea-breezes along the shores. Thinking just some scattered mid to upper level cloud decks from upstream convection and troughing over the Ohio River Valley, otherwise quiet weather. Sunday... Not ready to call it completely dry. While high pressure exiting to the E with a measure of dry and air low to mid level anticyclonic flow lingering, there is a fairly potent mid-level H5 trough axis and attendant vortex energy sweeping the NE CONUS. Tapping into S moisture and likely more favorable destabilization S/W of our region feel some rainfall is possible for S New England late into the overnight period. Monday... Timing of the mid-level H5 trough critical. There may be a window of opportunity in where the e-half of New England destabilizes ahead of synoptics. Thus PoP chances continue but low confidence as forecast solutions differ on outcomes and timing. Some measure of rainfall does seem possible so long as we can tap into S moisture rather than being robbed by a possible emerging Mid-Atlantic low. Rest of the week... Keep it dry though diurnally forced convection is not out of the question. Greater confidence to the N/W as NW flow aloft persists pushing cooler temperatures aloft across the region. Looking at a period of warming temperatures with a return of muggier conditions. Next system worth of note may not be till Thursday. && .AVIATION /02Z THURSDAY THROUGH MONDAY/... Forecaster Confidence Levels. Low - less than 30 percent. Moderate - 30 to 60 percent. High - greater than 60 percent. Short Term /through Thursday/...High confidence. 0Z major changes. VFR. Local MVFR/IFR in patchy valley fog overnight. Continued S/SW flow Thursday. Scattered TSRA possible after 18z mainly across western and north central MA. KBOS TAF...High confidence. KBDL TAF...High confidence. Outlook /Thursday Night through Monday/... Thursday night through Friday night...Low confidence. +RA possible with TSRA at times. Accompanying reductions in visibility, in some instances due to BR/FG. MVFR to LIFR impacts. S winds backing out of the N/NW. Saturday...Moderate confidence. VFR. Light W/NW winds with sea-breezes developing along the shores. Sunday into Monday...Low confidence. Chance SHRA with S winds. Will hold VFR. && .MARINE... Forecaster Confidence Levels. Low - less than 30 percent. Moderate - 30 to 60 percent. High - greater than 60 percent. Short Term /through Thursday/...High confidence. Winds remain below 25 knots and seas below 5 feet through the period. Light S/SW flow develops overnight and perks up a little on Thursday. Outlook /Thursday Night through Monday/... Thursday night through Friday night...Low confidence. Rain likely, heavy at times, with possible with thunderstorms. S winds with gusts up to 15 to 20 kts backing out of the N/NW as the low passes near or across the S waters. Wave heights forecast below 5-feet. Soupy conditions and fog expected with rain reducing visibilities at times. Saturday...Moderate confidence. Light W/NW winds with sea-breezes developing along the shores. Waves below 5 feet. Good boating weather. Perhaps some showery weather lingering over the SE waters during the morning. Sunday into Monday...Low confidence. Chance showers with winds out of the S. Possible thunderstorms. Will hold waves below 5 feet. && .FIRE WEATHER... Dry conditions will persist Thursday, except possibly in western and north central MA where scattered showers/storms may bring localized rainfall Thu afternoon. Minimum RH values of 25 to 35 percent are expected Thursday across much of southern New England, except 40 to 60 percent on the South Coast, Cape Cod, and the Islands. South to southwest winds will stay under 20 mph. && .BOX WATCHES/WARNINGS/ADVISORIES... CT...Air Quality Alert from 11 AM to 11 PM EDT Thursday for CTZ002. MA...None. RI...None. MARINE...None. && $$ SYNOPSIS...Sipprell/JWD NEAR TERM...WTB/Sipprell/JWD SHORT TERM...JWD LONG TERM...Sipprell AVIATION...Sipprell/JWD MARINE...Sipprell/JWD FIRE WEATHER... is the U.S. government's official web portal to all federal, state and local government web resources and services.