Area Forecast Discussion
Issued by NWS Boston, MA

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318 FXUS61 KBOX 241338 AFDBOX Area Forecast Discussion National Weather Service Taunton MA 938 AM EDT Mon Oct 24 2016 .SYNOPSIS... A wave of low pressure tracking along the coast will move offshore this morning with rain quickly exiting the region. Otherwise, winds become blustery out of the W/NW beneath a cool, dry airmass which continues through midweek. A period of rain is likely Thursday into Friday as low pressure crosses the region. Seasonable temperatures are expected for next weekend. && .NEAR TERM /UNTIL 6 PM THIS EVENING/... No major changes to the forecast this morning. Rain has moved well away from southern New England, already. Will need to keep an eye out for some stray lake-effect showers spilling east of the spine of the Berkshires in the northwest flow. Still expecting mainly clouds, though. Tweaked temperatures and sky cover this morning to reflect observed trends. Previous discussion... Rain is quickly moving into SNE south of the Pike. Hi-res guidance and NAM/GFS/RGEM are in good agreement and are targeting CT/RI and SE MA for a brief burst of rain through 6-8 am. Open wave at the surface tracks near the south coast. However, robust mid level shortwave with with area of mid level frontogenesis producing a narrow band of strong mid level omega which quickly moves near the south coast early this morning within an area of deep moisture. In addition, TT 50+ and SWI near zero moves near and south of the coast so low risk of an isold t-storm. System is quick moving and rain will exit northern CT around 6 am and Cape/islands 8-9 am. Excellent mid level drying moves in this morning with clouds quickly giving way to mosunny skies from west to east through the morning hours. However, a secondary shortwave moves in later today with 500 mb temps dropping to near -30C by 00z. The column is dry but some low level moisture will lead to sct-bkn strato-cu developing during the afternoon, and cant rule out an isold sprinkle in the interior. High temps mid/upper 50s except closer to 50 over higher elevations. Good cold advection will lead to gusty NW winds developing with gusts 25-30 mph. && .SHORT TERM /6 PM THIS EVENING THROUGH 6 PM TUESDAY/... Tonight into Tuesday... Series of shortwaves rotating through the flow will reinforce the mid level trof across New Eng with cold mid level temps and cyclonic flow aloft. Generally partly cloudy skies tonight, then increasing diurnal clouds developing late morning into the afternoon on Tue. Mainly dry conditions as the column is quite dry. Below normal temps. Regarding temps, expect lows tonight in the low/mid 30s across much of the region, milder outer cape/islands. For those areas where the growing season is still active, temps will remain above freezing and frost is not a concern given large enough T/Td spreads with dewpoints falling into the 20s and modest wind. So no frost/freeze headlines. Chilly day on Tue with 850 mb temps around -6C and 925 mb temps 0-1C. Highs upper 40s to near 50 coastal plain and lower 40s higher terrain. Gusty NW winds to 25-30 mph Tue. && .LONG TERM /TUESDAY NIGHT THROUGH SUNDAY/... Highlights... * Unseasonably chilly but mainly dry Wed other than a few ocean effect rain showers across Plymouth County/Cape/Islands * Lows should drop well down into the 20s Wed night/Thu am * Period of rain likely sometime Thu into Fri, low risk for wintry mix early Thu if several conditions are met * Confidence in weekend forecast low Overview and model preferences... Labrador trof will continue to impact the region as ridging builds from the great lakes through early Thu. Clipper system, which looks to be undergoing an occlusion process as it approaches S New England then brings more wet/unsettled wx for Fri, especially as the trof takes on a slight negative tilt. Models are in good agreement through this point, enough such that a general blend of deterministic guidance can be used as a baseline. However, into the weekend there are disagreements at the handling of a secondary trof which may/may not phase with the Hudson`s Bay vortex. GFS appears like the biggest outlier, with a much deeper trof than both its own ensemble mean, ECMWF and some of the ECENS members. Now it is too early to tell how this phasing will occur, but given the fact that the GFS solution seems far different from many of the others, will lean more heavily on the ECENS/GEFS and ECMWF beyond Fri. Details... Tue night through Wed night... The coolest portion of the period. H85 temps dive well below 0C (as low as -7C by Wed morning to be exact) while H92 near -2C. This, combined with continued NW unmodified flow, should yield highs mainly in the 40s and low 50s. Overnight mins Tue night will dip into the 30s, but not expecting widespread frost as the pres gradient should mitigate its development. However, good radiational cooling ahead of CI deck on Wed night should yield several locations below freezing (and in the 20s). This suggests frost in locations where the growing season continues, so stay tuned if you continue to have growing interests. Otherwise, ocean effect rain showers likely for the Cape/Islands and immediate coastal Plymouth County MA thanks to N flow and delta-T values between H85 and the sea sfc exceed 20C. Thu into Fri... Clipper low pres undergoing slow cold occlusion process will be approaching on Thu as the attendant trof gradually shifts toward a negative tilt. Precip shield out ahead of the occlusion may take until late Thu to initiate across the W as soundings show a fair amount of moisture in the lowest lvls that will need to be overcome. By Fri night, as the initial low pres shifts N toward Canada, secondary (triple-point) low may form and traverse directly over S New England into Fri, enhancing the rainfall process. PWATS are modest, mainly around or even below 1.00 inch, so it will take this secondary low for moderate-heavy rainfall. QPF values around 0.5-1.00 inches are possible should this occur. Otherwise, temps closer to seasonal normals thanks to warmer nights under cloud cover (mainly 40s and low 50s. Highs in the 50s and low 60s. One other minor note here, there is a slight chance for wintry mixed precip at the onset early Thu. This is highly dependent on timing, how efficiently the veering winds are able to scour out the low valleys, and dynamic cooling potential. It would really need all these things to line up right (or wrong depending on your perspective) for this to occur. However, feel it is at least worth noting here given the cool airmass and low dwpts (wetbulb process) in place before the precip shield arrives. Sat and Sun... Removing the GFS with this update, and using thermal wind (mean- jet) arguments, noting steering should be mainly S of New England with any additional vort-maxes and/or mid lvl F-gen. Therefore, will allow for the high pres to the N to dominate the pattern across the region, with the primary focus for additional rainfall well to the S. Will yield NIL pops for now, but maintain that this remains an uncertain forecast as the wave in question has yet to be even sampled. Temps will be very close to seasonal normals, as H92 temps average +4 to +6C. Although overnight mins would be colder with the crest of high pres and radiational cooling. Early next week... Low confidence. Even using the blends for the weekend yields different results as the upper flow pattern suggests zonal flow and fast moving (albeit weak) shortwaves moving through. Therefore, will suggest seasonable conditions with a low risk for occasional shra, but at least at this point, it does not look like a true washout. && .AVIATION /14Z MONDAY THROUGH FRIDAY/... Forecaster Confidence Levels. Low - less than 30 percent. Moderate - 30 to 60 percent. High - greater than 60 percent. Short Term /through Tuesday/... Today...W to NW winds pick up through the late morning and afternoon, with gusts 20-25kt, at times, expected. Tonight and Tuesday...High confidence. Sct-bkn VFR cigs. Occasional NW gusts 20-25 kt, although diminishing wind tonight, except over higher terrain and along the immediate coast. KBOS TAF...High confidence in TAF. KBDL TAF...High confidence in TAF. Outlook /Tuesday Night through Friday/... Tuesday night into Thursday morning...High confidence. Mainly VFR. 20-25 kt winds out of the NW near shore continue Tue night into early Wed. Thursday into Friday...Moderate confidence. Low risk for some wintry mix when precip starts on Thu. Otherwise expect rain lingering into Fri with IFR/MVFR conditions. Winds veer from NE to SE and finally to S by Fri. Sat...Moderate confidence. Mainly VFR with some improvement. && .MARINE... Forecaster Confidence Levels. Low - less than 30 percent. Moderate - 30 to 60 percent. High - greater than 60 percent. Short Term /through Tuesday/...High confidence. Small craft advisories in effect through Tue. Today...Winds turn NW today and stay blustery with gusts to 25 kt. Tonight into Tuesday...Persistent NW winds with gusts 25-30 kt. Seas 5-7 ft over the outer waters. Outlook /Tuesday Night through Friday/... Tue night into Wed...High confidence. Last day of continued Small Craft Advisory conditions. Winds out of the N-NW will continue to gust to around 25 kt into the morning hours on Wed. Seas will gradually recede form 5-7ft, dropping below 5 ft by Wed evening. Wed night into Thu...High confidence. Mainly quiet boating weather regarding seas (which should only top out at 3-5 ft), while winds will gradually shift from NE to SE through the day. Some rain overspread the water by evening Thu. Thu night into Fri...Moderate confidence. As areas of rain overspread the waters, winds out of the E-SE will increase, gusting 25-35 kt at times, this suggests a risk for Gales, although at the very least small craft advisories will be needed. Seas build to 8-10 ft potentially, if low pres passes directly over the waters. The winds shift to the S on Fri, but remain elevated. Sat...Low confidence. It`s possible conditions subside through the overnight hours Fri night, such that mainly quiet boating weather is experienced by Saturday. However, there is also a risk for winds and seas to remain at low end SCA thresholds. && .BOX WATCHES/WARNINGS/ADVISORIES... CT...None. MA...None. RI...None. MARINE...Small Craft Advisory until 8 PM EDT Tuesday for ANZ231>235-237- 250-251-254>256. Small Craft Advisory until 8 PM EDT Tuesday for ANZ230-236. && $$ SYNOPSIS...KJC/Doody NEAR TERM...Belk/KJC/Doody SHORT TERM...KJC LONG TERM...Doody AVIATION...Belk/KJC/Doody MARINE...Belk/KJC/Doody is the U.S. government's official web portal to all federal, state and local government web resources and services.