Area Forecast Discussion
Issued by NWS Boston, MA

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000 FXUS61 KBOX 181325 AFDBOX Area Forecast Discussion National Weather Service Taunton MA 925 AM EDT Tue Oct 18 2016 .SYNOPSIS... A backdoor front will slip south across northern MA this morning before lifting to the north this afternoon. A weak cold front will drop south across the region during Wednesday. Unseasonably warm conditions are expected through Wednesday with the potential for a few near record high temperatures. Unsettled weather likely at times Thursday and Friday. Not as warm late week but still mild for mid Oct. A drying and cooling trend likely next weekend. && .NEAR TERM /UNTIL 6 PM THIS EVENING/... 7 AM update... Remnant convective shra dissipate this morning as primary moisture source and upper lvl shortwave moves offshore. However, backdoor cold front continues to slide WSW and is now draped mainly from the Worcester hills to Plymouth. Expect it to stall and then slowly begin to lift N through the late morning and early afternoon as the S LLJ becomes established. This continues to make temp forecasting a bit tricky, but it appears latest meso-scale guidance is capturing current frontal movement well, so adjustments this morning lean heavily on their solutions. Mainly used the HRRR/WRF runs. Previous discussion... Showers assocd with weakening convective complex are moving across NE MA and will clip portions of SE coastal MA and Cape Cod before moving offshore around daybreak. These showers will help to push a weak backdoor front south across northern MA which will make for a difficult temp forecast across portions of NE MA and a sharp gradient across the region. Much of the hi-res guidance holds temps in the 60s for much of the day across NE MA while increasing SW flow to the south and west of the boundary will push temps into the upper 70s to lower 80s. The forecast challenge is whether the increasing SW flow will eventually win out and allow temps to spike into the 70s by late day in NE MA. As low level jet develops we think the increasing SW flow and warmer air will eventually make it into NE MA but this is lower confidence given the hi-res guidance and later forecasts may need to adjust temps. At the very least, the cooler air will likely persist into early afternoon in NE MA. Otherwise, cloud cover moving across the region early this morning will move out with clearing and increasing sunshine developing from the west and southwest. Dewpoints will climb into the lower 60s today making for humid conditions and it will become rather breezy by mid/late afternoon as low level jet develops. SW gusts to 25-30 mph expected. && .SHORT TERM /6 PM THIS EVENING THROUGH 6 PM WEDNESDAY/... Tonight into Wednesday... Weak cold front moving east from the Great Lakes will drop south across northern New England tonight and move into SNE during Wed. Forcing for ascent is limited but deep moisture axis moves in ahead of the boundary with PWATs briefly increasing to 1.5". This may support a few brief showers moving north to south across the region late tonight into Wed morning within the axis of deeper moisture. The front will bring increasing cloud cover which will linger into Wed before clearing moves in from the north during the afternoon. will be a mild night tonight with low temps only in the upper 50s to lower 60s. Max temps Wed a bit tricky due to the cloud potential but clearing should be moving in from the north in the afternoon. Low level temps are still quite mild with 17-19C at 925 mb and winds will be shifting to the west so expect another warm day with highs upper 70s to lower 80s across much of the region. In addition it will be quite humid with dewpoints reaching the low to mid 60s in the morning, especially south of the Pike but drier air will move in from north to south during the afternoon with dewpoints falling through the 50s. && .LONG TERM /WEDNESDAY NIGHT THROUGH MONDAY/... Highlights... * Widespread showers possible both Thu and Fri * Drying and cooling trend as the Overview and model preferences... Zonal flow gradually gives way to digging trof across the E CONUS and Canada as cutoff low currently in the Canadian prairie shifts toward the E. Initial low pres development in the OH valley will likely shift W of the region as an inside runner. However, will need to monitor a tropical disturbance N of Dominica which could be captured by the developing trof, increasing moisture and dynamics, and slowing the overall progression. Current models vary, with the ECMWF most progressive, and the NAM least. While the operational GFS falls in between these two solutions, it too is much slower than the ECMWF and several ensemble members from both the GEFs and ECENS. Given the difficulty guidance often has with these mid-late period tropical phasing situations, will likely need a bit more time to fully nail this down. However, given the current forecast, will put more weight on the middle solutions from the GFS and ensemble means. Details... Wed night... Warm front stalled to the S will attempt to make a push to the N. However, am concerned it may struggle a bit to push N as guidance does show building high pres to the NE with inverted ridging into SNE. Bulk of moisture remains upstream and the upper lvl jet RRQ remains to the west through the overnight hours, therefore, aside from increasing clouds expect mainly dry conditions. Mins mainly in the low-mid 50s. Thu and Thu night... Developing low pres will move out of the OH valley and shift under the RRQ of the strong upper lvl jet streak as it shifts into NY state. Steering flow defined by the digging upper trof suggests the core of low pres should pass to the W by Thu night. Warm front finally shifts into N New England with warm sector LLJ energy pushing moisture across SNE as it passes. With PWATS nearing 1 std deviation above normal, expect widespread shra to accompany this warm frontal passage. Daytime conditions cool and wet, thanks to E flow with the warm front, shifting to traditional warm sector warming overnight. Total precip values will vary, but around a quarter inch is possible. Temps mainly in the mid 50s to low 60s. Fri into early Sat... Upper lvl trof may capture leftover tropical disturbance offshore. The timing of this capture will define how impact SNE will ultimately see. PWATS associated with this feature are about 2 std deviations above normal. So while there is the potential for widespread mod-hvy rainfall it will depend on how far offshore the phasing occurs. ECMWF is the latest, hence why it is most progressive. Will need to continue to monitor, as it could be a wet period with a good soaking (1.00+ inches) of rainfall thanks to the tropical influence. Probabilities favor the east half of the region at this time. Otherwise, expect blustery NW winds on the backside of this feature early Sat which may require a localized wind advisory. Temps Fri in the mid-upper 60s with lows Fri night dependent on how quickly rainfall/clouds clear out. Rest of Sat and Sun... Fall returns with gusto. Cold advection aloft will allow H85 temps to drop to -3C to -5C while H92 temps hover between 0C and +2C by Sat night into Sun morning. This suggests a definite cool down from the anomalously warmer temperatures this week. Also, with cold advection could yield some shra development even in NW flow thanks to decent low-mid lvl lapse rates. At the very least, a lot of diurnal CU can be expected along with brisk NW winds each day. Highs will remain in the 50s and while overnight mins could dip into the 30s across much of the region, especially if, by Sun night, the pres gradient slackens enough to allow for radiational cooling. Mon into Tue... Secondary reinforcing shortwave passage. While QPF will be dependent on column moisture, it may come through mainly dry. However, it will prolong the fall like conditions thanks to another round of cold advection with the wave passage. && .AVIATION /14Z TUESDAY THROUGH SATURDAY/... Forecaster Confidence Levels. Low - less than 30 percent. Moderate - 30 to 60 percent. High - greater than 60 percent. Short Term /through Wednesday/... Today...High confidence. Early morning mix of IFR/MVFR and VFR will give way to all VFR as sites currently in the lower categories lift through 15Z. Timing may be off by an hour or so in the TAF, but the overall trend is expected. Otherwise once VFR, expect VFR to remain until late evening. Winds shift through the day, giving way to primarily S by late afternoon. Tonight...Moderate confidence. Mainly VFR with 4-8K FT cigs developing. Low risk of a brief shower late tonight, especially north. Low confidence on potential for IFR fog. Diminishing wind. Wednesday...Moderate confidence. Mainly VFR cigs with clearing moving across northern areas during the afternoon. Low risk of a brief shower south of the Pike in the morning. Outlook /Wednesday Night through Saturday/... Wed night into Fri night...Moderate confidence. Although somewhat uncertain, mainly due to timing concerns, periods of IFR/MVFR certainly possible through this entire period thanks to a mix of low clouds and occasional rainfall. Some fog also possible. Easterly flow until late Fri, at which point, winds may shift to the S. Sat and Sun...Moderate confidence. Improvement to VFR all terminals during the daytime Sat. However, strong NW winds could gust 25 to 35 mph. && .MARINE... Forecaster Confidence Levels. Low - less than 30 percent. Moderate - 30 to 60 percent. High - greater than 60 percent. Short Term /through Wednesday/...High confidence. Today into tonight...Increasing SW winds expected this afternoon as low level jet develops. Gusts to 25-30 kt developing which will linger into the evening before diminishing. Seas building to 5-7 ft over the outer waters. Wednesday...Expect SW winds less than 20 kt with diminishing seas. Outlook /Wednesday Night through Saturday/... Wed night into Thu night...High confidence. Although some showers may spread over the waters, the east flow should remain light enough to limit wave development. Therefore, not expecting headlines through this period. Fri and Fri night...Moderate confidence. Uncertain, primarily whether low pres from the S will join with low pres across the mainland over or east of the waters. In any case, building swells and shifting winds could lead to small craft advisory conditions. Waves nearing 7 ft on the ocean waters are possible, along with E winds shifting to the S and SE with gusts to 20 kt. Sat and Sun...Moderate confidence. Winds shift to the NW and could reach near Gale force Sat afternoon into Sun. Seas maintain 6-9 ft. At the very least small craft advisories are likely. However, Gales may be needed as well. && .TIDES/COASTAL FLOODING... 9 AM UPDATE... Boston Harbor gauge has tidal departure of 1.1 feet, and at time of Astronomical High Tide. While this may come down a little through midday, expect enough water along the East MA Coast to upgrade the statement to an advisory. We have also added a coastal flood statement to the South Coast of RI and MA. Previous discussion... Astronomical tides are quite high with BOS tide at 12.3 ft at 121 pm. Current tidal departure is 0.7 ft at 4 AM and ESTOFS shows up to a 0.5 ft departure around high tide. This combined with seas up to 4 ft just offshore and NE winds developing behind front across NE MA will likely result in minor splashover along most vulnerable shore roads across east coastal MA. We issued a coastal flood statement to highlight this potential. && .CLIMATE... Record High Temperatures: Tuesday October 18th Wednesday October 19th Boston 82 in 1947 Boston 84 in 1945 Worcester 85 in 1908 Worcester 81 in 1963 Providence 85 in 1908 Providence 81 in 1945 Hartford 80 in 1968 Hartford 82 in 1963 && .BOX WATCHES/WARNINGS/ADVISORIES... CT...None. MA...Coastal Flood Advisory until 3 PM EDT this afternoon for MAZ007-015-016-019. RI...None. MARINE...Small Craft Advisory from 2 PM this afternoon to 8 AM EDT Wednesday for ANZ232>235-237-255-256. Small Craft Advisory from 4 PM this afternoon to 11 PM EDT this evening for ANZ230. Small Craft Advisory from 4 PM this afternoon to 2 AM EDT Wednesday for ANZ231-251. Small Craft Advisory from 2 PM this afternoon to 2 AM EDT Wednesday for ANZ236. Small Craft Advisory until 8 AM EDT Wednesday for ANZ250-254. && $$ SYNOPSIS...KJC/Doody NEAR TERM...WTB/KJC/Doody SHORT TERM...KJC LONG TERM...Doody AVIATION...KJC/Doody MARINE...KJC/Doody TIDES/COASTAL FLOODING... 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