Area Forecast Discussion
Issued by NWS Boston, MA

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852 FXUS61 KBOX 212326 AFDBOX Area Forecast Discussion National Weather Service Taunton MA 726 PM EDT SUN AUG 21 2016 .SYNOPSIS... Widespread showers and thunderstorms with locally heavy rain will move through the region tonight with a cold front. A few strong thunderstorms are possible. Dry, cooler weather returns for Monday and Tuesday. They will be replaced by warmer and more humid weather Wed through Fri along with the risk of showers and thunderstorms by weeks end. Dry weather may return next weekend. && .NEAR TERM /UNTIL 6 AM MONDAY MORNING/...
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7PM update... Initial band of rain just making it into extreme W BOX CWA as of 630 local. Noting that a combination of dynamics shifting N, convection in the mid Atlantic and drier air remaining in place across E New England, has led to this band being mostly light- moderate so far with echos barely topping out at 40Z aloft. Given these factors, will only be mentioning heavy rain with this update as there is likely to be little extra destabilization ongoing through the evening. Main threat will be some urban type of flooding as the rain moves through after dark. One caveat, will be to watch for secondary development offshore in the area of LLJ near the current band of convection upstream, this may provide just enough extra dynamic forcing to increase the late night risk for convection. Stay tuned to updates. Previous discussion follows... Tonight...Low pressure moving through Quebec ahead of a shortwave will bring a cold front through southern New England. Ahead of this front, a line of showers and thunderstorms is expected to move across the region. Tropical moisture, drawn in on southerly winds, will bring in precipitable waters over 2 inches. This will result in periods of heavy rainfall with these showers and storms. While the storms and front are expected to move quickly across the area, such heavy rainfall may result in some urban and poor drainage street flooding, especially in typical areas. The best chance for heavy rainfall will be across western MA and northern CT where the jet is stronger. In addition, surface based and mixed layer CAPEs are about 500 J/kg and 0-6km shear values between 40 and 50 kts, is enough for a risk of strong storms. This is not a setup that usually produces much severe weather, but the risk for some isolated wind damage is there. The models are not developing a secondary low pressure system over southern New England as strongly as they were overnight and yesterday. That said, should one develop, there is a very low probability of a brief tornado or waterspout developing. However, this is a very low probability.
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&& .SHORT TERM /6 AM MONDAY MORNING THROUGH MONDAY NIGHT/... Monday...Most, if not all, of the overnight precip should be offshore by Monday morning. If anything, expect perhaps a few showers lingering on the Cape and Islands. As the cold front moves offshore during the morning, gusty NW winds will increase. A decent low level jet, good mixing, and a tight pressure gradient between the low exiting to the maritimes and the high building over the OH valley will be responsible for these gusty winds. In addition, a marked decrease in dewpoints and relative humidities as well as temperatures a few degrees cooler will result in a cooler feeling day. High temperatures expected to be in the upper 70s to lower 80s for most locations with a few cooler readings across NW MA. Monday night...High pressure builds more over southern New England. This will result in light winds and clear skies and excellent radiational cooling conditions. With dewpoints in the upper 40s to lower 50s for much of the area, expect low temperatures to be quite brisk and fall like Monday night. && .LONG TERM /TUESDAY THROUGH SUNDAY/... Highlights... * A touch of fall-like temps Tue * Warmer and more humid Wed thru Fri * Dry much of next week except a risk of T-storms Fri Overview and model preferences... A slight taste of fall early leads into gradual warming and more seasonal late August conditions by late in the week. The cooler/drier stretch is courtesy of a Hudson Bay vortex which will advect Canadian prairie air into New England Mon and Tue. After, thanks to better dynamic digging of a trof across the W CONUS, the resulting downstream ridge across the E and NE allows influence of the subtropical high to impact the region. This will bring about slightly more humid and summer-like conditions by the end of the week. One last forecast note is a weakening trof and upper lvl shortwave which may bring about some wet/unstable wx Fri. Models continue to show good agreement through much of the long term, so have no problems continuing to use a consensus blend that also includes persistence with this update. Details... Tue... After a cool start early Tue morning mid lvl (H85) temps reach back to near +10 to +12C. While this could suggest warmer highs, the lower start suggests a peak near in the upper 70s and low 80s. These pleasant numbers are capped by afternoon dwpts remaining in the upper 40s and low 50s. Wed and Thu... Continued warming trend but under the influence of high pres which will be shifting E. The combination of moisture/heat advection will lead to slightly more humid/slightly warmer conditions each day. H85 temps reach near +15C Wed then +16C Thu. This translates with full mixing to mid-upper 80s with a few spots possibly touching 90F. Dwpts begin shifting back into the 60s by Thu afternoon. Fri... Weakening shortwave ejects from the developed longwave trof across the W CONUS. Models continue to align fairly well on timing of the attendance cold front across new England, bringing it through in the later hours of Fri/Fri evening. The issue is how much stability/moisture is left when this front arrives. ECENS/GEFs probabilities are rather low for the near 2.00 inch PWAT mark, and likewise QPF probabilities are low for numbers more than 0.50 inches. This is likely a result of the weakened wave interacting with the relatively anomalous high pres just offshore. Remnant subsidence will also limit moisture increases. In fact, noting that the operational ECMWF and even some ensemble members are now completely dry with this frontal passage. For now, will continue with afternoon/evening pops in the slight to chance range as there is still much time to better resolve these more mesoscale features. Next weekend... Another round of ridging and drier air moves in from the NW behind the exiting front. While a return to drier wx looks likely, it is not expected to be as cool or dry as the airmass moving in tomorrow. Looking at highs near normal with a return to more comfortable dwpts. && .AVIATION /00Z MONDAY THROUGH FRIDAY/...
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Forecaster Confidence Levels. Low - less than 30 percent. Moderate - 30 to 60 percent. High - greater than 60 percent. Short Term /through Monday Night/... Overnight...moderate confidence, mainly due to timing uncertainties. Gradual reduction from all those currently VFR to IFR/MVFR with low cigs and vsbys in rain/fog. This process goes through 03Z-05z or so. Once down, expect terminals to remain down in these lower categories for 3-4 hours until rain moves through. Occasional TS also possible, with heavier rain. Improvement back to VFR begins mainly after 08Z and continues through 12Z from W-E. Monday and Monday night...High confidence. Any locations not VFR will improve to VFR quickly Monday morning. VFR conditions will continue through the period. Gusty NW winds are likely Monday. KBOS Terminal...Moderate to high confidence in TAF. Main risk for heavy showers and embedded thunderstorms between 05Z and 09Z. KBDL Terminal...High confidence in TAF. Main risk for heavy showers and embedded thunderstorms between 01Z and 05Z. Outlook /Tuesday through Friday/... High confidence in a stretch of mainly dry VFR weather. Sea breezes most likely mid-late week.
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&& .MARINE... Forecaster Confidence Levels. Low - less than 30 percent. Moderate - 30 to 60 percent. High - greater than 60 percent. Short Term /through Monday Night/...High confidence. Quiet boating weather is expected through the period, with a slight rise in winds and seas ahead of a cold front tonight. Otherwise, a chance of showers and thunderstorms tonight will be the main hazard to boaters. Keep an eye on the weather tonight and remember that lightning and gusty winds are always a possibility in any thunderstorm. Quiet weather returns for Monday, though gusty NW winds may reach close to SCA criteria during the afternoon. Outlook /Tuesday through Friday/...High confidence. Tuesday through Thursday ... High confidence. Quiet boating weather as slow moving high pressure builds over the waters. By Thu SSW winds will likely become gusty near shore. Friday...Moderate confidence. Increasing SSW flow and seas, may just touch small craft criteria. Otherwise, late day risk of showers and thunderstorms over the waters. && .BOX WATCHES/WARNINGS/ADVISORIES... CT...None. MA...None. RI...None. MARINE...None. && $$ SYNOPSIS...Doody/RLG NEAR TERM...Doody/RLG SHORT TERM...RLG LONG TERM...Doody AVIATION...Doody/RLG MARINE...Doody/RLG is the U.S. government's official web portal to all federal, state and local government web resources and services.