Area Forecast Discussion
Issued by NWS Boston, MA

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000 FXUS61 KBOX 251830 AFDBOX Area Forecast Discussion National Weather Service Taunton MA 230 PM EDT Tue Apr 25 2017 .SYNOPSIS... Low pressure will slowly track up the east coast today and Wednesday, bringing periods of rain, gusty winds, and the risk of minor coastal flooding. Drier and milder conditions will briefly return Thursday. A cold front may bring scattered showers and possibly a few thunderstorms on Friday and stall across the region on Saturday. A backdoor cold front is possible on Sunday. && .NEAR TERM /UNTIL 6 PM THIS EVENING/... 215 PM Update... Southerly plume of moisture combined with easterly low level flow contributing to showers moving through southern New England, and advancing towards our area from the south. Could still have a a rumble of thunder within RI/southeast MA prior to 00Z. East/Northeast winds continue through this afternoon, with gusts 25 to 30 mph along east coastal MA, southeast MA and southern RI. Overall temps remaining nearly steady in the mid 40s to low 50s this afternoon, with some mid 50s in the CT River Valley region from Springfield thru Hartford. Early Morning Discussion follows... High confidence in trends, moderate confidence in timing. A low pressure off the Mid Atlantic coast will try to move north today. However, a weak surface ridge over southern New England will prevent it from making much northward progress today. Despite that, moisture will stream north into our region, resulting in plenty of clouds, and eventually periods of rain, across all of our region. Precipitable water values approach 1.25-1.50", which is about 2-3 standard deviations above normal. Rainfall could be heavy at times, especially late this afternoon. Not thinking this rainfall will lead to river flooding, but will need to monitor the poor drainage areas, especially the urban centers. Dew point depressions across northern MA remain large early this morning, which means it will be a while longer before significant rainfall reaches the ground. Previous forecast had this covered well, so only tweaked the onset timing by an hour or so to reflect observed trends. Still thinking at least light rainfall will develop over the region by 9-10 AM. East winds will mean another raw spring day, especially along the coasts. Max temperatures today are expected to be around 10 degrees below normal. && .SHORT TERM /6 PM THIS EVENING THROUGH 6 PM WEDNESDAY/... Tonight...High confidence. Widespread rainfall will fall tonight as weak ridge over southern New England breaks down. A strong southeast low level jet will also approach, especially after midnight. Not a lot of instability, but marginal mid level lapse rates could result in a few thunderstorms tonight into Wednesday morning. The most likely area where this could occur remains the Cape and islands, due to their proximity with the low level jet. Temperatures should remain steady or slowly rise tonight, especially across RI and SE MA. Gusty easterly winds will continue tonight along the coast. Wednesday...Moderate confidence. Low pressure continues to move slowly north along the coast. Still expecting this low to remain south of our region through Wednesday evening. Despite that, the mid level low associated with this system should be overhead, opening the door for mid level drying to bring an end to the widespread rainfall. Thinking this is more likely to happen during the afternoon hours, but do not have a lot of confidence in the exact timing. Even so, scattered areas of rain, or perhaps drizzle, may persist for much of the afternoon. Low risk for thunderstorms continuing, mainly across the Cape and islands, and the adjacent coastal waters. && .LONG TERM /WEDNESDAY NIGHT THROUGH MONDAY/... Highlights... * Active weather pattern through the period * Moderating temperatures towards the end of the work week * Possible back door cold front on Sunday Pattern Overview... 00z guidance is in good agreement for the synoptic scale, does have issues at the mesoscale level that will need to be resolved. An active weather pattern will continue into the extended as cut-off low pressure will begin to move towards the Maritimes Thursday morning. Long wave trough over the central CONUS will result in a narrow short wave ridge axis which will move across the region during the later half of Thursday. Approaching cold front on Friday appears to be stalled/weaken as Bermuda high builds into the region. This puts the flow/mid- level boundary right over the region with several waves moving out of the Plains, this could result in on/off showery weather Saturday and into Sunday. There is the potential for a backdoor cold front due to passing wave over the Maritimes on Sunday keeping things a bit cooler Sunday into Monday. Potent cold front moving across the Great Lakes late Sunday will result in strong ridging for Monday with cold frontal passage on Tuesday. Details... Thursday into Friday...High confidence. Upper level low still lingers early Thursday before moving towards the Maritimes during the later half of the day. Both low and mid- levels are still fairly moist so anticipate clouds to stick around as well as hit or miss showers thanks to onshore flow. Thursday will start on the cooler side as 850 mb temps fall to about 8-10 C thanks to weak CAA. However later in the day, strong WAA returns from narrow short wave ridge which will dry out the mid-levels and 850 mb temps warm to near 13C. Still anticipate a few rogue showers if not sprinkles during the day. Soupy conditions for the overnight with fog as dewpoints remain high. Approaching cold front on Friday will keep clouds around the area as well as strong southwesterly flow. Dewpoints could reach into the mid to upper 50s during the day, and depending on the amount of sunshine, we could see enough CAPE for a rumble of thunder. Still some uncertainty with this system as mid-level ridge from Bermuda high will be increasing heights ahead of approaching cold front. This will stall and/or weaken the front as it arrives. There is also some timing issues with the EC/CMC a bit more progressive with this frontal system versus the GFS. Still cannot rule out some showery activity with a rumble of thunder. Some of these showers could contain heavy downpours as PWAT values increase to above an inch to near 1.5 inches. The weekend...Moderate confidence. Bermuda high continues to build with ridging over the eastern CONUS. Potent southern Plains trough will dig and slowly move eastward through the period. This will keep the region in a continuous southwesterly flow as Gulf moisture continues to stream into the area thanks to Bermuda High. A few pieces of shortwave will move through the flow which could trigger a few showers on Saturday. Temperatures will be warm with Saturday appearing to be the pick of the weekend with highs in the 70s to low 80s, despite the rain/cloud cover. Cannot rule out a rumble of thunder. Upper level ridge over the region, will flatten overnight and into Sunday as northern wave shortwave across Canada drops a back door cold front. Still some uncertainty on timing and how strong the front will be but both GFS and EC dropped surface temps be 15-20F compared to previous runs. Thus have a lower confidence in this forecast. Next week...Moderate confidence in trends, low confidence on specifics. Potent trough across the central CONUS will approach the region by early next week. High PWAT airmass will continue to stream into the region resulting in showers well ahead of cold frontal passage. Still some timing with the EC still a bit more progressive versus the GFS and its 12Z run. But overall good model consensus that system will moving into the region Monday/Tuesday. && .AVIATION /18Z TUESDAY THROUGH SUNDAY/... Forecaster Confidence Levels... Low - less than 30 percent. Moderate - 30 to 60 percent. High - greater than 60 percent. Short Term /through Wednesday/... This Afternoon and Tonight...High confidence in trends. Moderate confidence in timing. Conditions deteriorating to IFR/LIFR during this afternoon with showers and fog. Potential exception of BDL/BAF vicinity, where MVFR conditions are expected into early evening, then deteriorating to IFR. E wind gusts 25 to 30 kt over Cape/Islands with winds veering to SE tonight with similar gusts. LLWS developing Cape/Islands this evening as SE/S low level jet at 50+ kt develops, especially after midnight. LLWS may extend as far north as BOS. Isolated TSRA possible tonight. Wednesday...Moderate confidence. MVFR-IFR CIGS continue with areas of -RA/RA and patchy fog, especially across the eastern half of southern New England. Reduced VSBYS likely, down to IFR in spots. May see areas of LIFR CIGS across the higher inland terrain and along the coast. SE winds gusting up to around 25 kt across Cape Cod and the islands. Low risk of TSRA early in the day, especially the S coastal terminals. LLWS impacts for the Cape and islands, and possibly as far north as BOS, during the morning hours. KBOS TAF...Moderate confidence in TAF. Potential for LLWS 08Z- 14Z with SE low level jet. KBDL TAF...Moderate confidence in TAF. Outlook /Wednesday Night through Saturday/... Wednesday night into Thursday...Moderate confidence. Showers continue with MVFR/IFR conditions. Cannot rule out LIFR in fog and DZ during the overnight hours. Improving conditions later Thursday. Friday...Moderate confidence. MVFR/IFR possible during the overnight hours in fog. Showers moving from west to east resulting in possible MVFR conditions. Iso TSRA is possible with heavy downpours. Saturday...Moderate confidence. VFR with possible MVFR in passing showers. && .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. Persistent easterly flow across the coastal waters this afternoon into tonight. Gusts 25 to 30 kt possible, especially across the southern coastal waters. Widespread rain continues this afternoon, and is expected to be heaviest tonight into Wednesday morning. Plenty of visibility restrictions in both rain and fog expected. Low risk of thunderstorms, mainly on the southern waters. Extended Small Craft Advisories across most waters through Wednesday. Outlook /Wednesday Night through Saturday/... Thursday...High confidence. Lingering southeast swell will help keep seas up on Wed PM into Thursday. Conditions begin to relax/improve, but Small Craft Advisory will still be needed. Friday...Moderate confidence. Southwesterly flow with outer seas near 5 feet. Moderate confidence on if Small Craft Advisory will be needed or not, as showers pass over later in the day. Saturday...Moderate confidence. Southwesterly flow continues with hit or miss showers. Seas lingering near 4-5 feet. Low confidence on if Small Craft Advisory will be needed. && .TIDES/COASTAL FLOODING... * Potential minor flooding impacts during astronomically high tides Tonight through Wednesday along the east and south coasts Not anticipating any flooding impacts for the high tide later today, as winds, surge and seas will not have built up enough. Minor flooding possible during the high tide cycles tonight and Wednesday along both coastlines. A storm surge around 1 ft with low risk of 1.5 ft surge on top of high astronomical tides and building seas may lead to minor inundation of vulnerable shoreline roads. Some beach erosion is also possible, especially for east and southeast facing beaches. Boston High Tides (flood stage 12.5 feet)... 11.47 feet / Tuesday 11:20 pm 11.15 feet / Wednesday 11:50 am Providence High Tides (flood stage 7 feet)... 5.98 feet / Tuesday 8:11 pm 5.53 feet / Wednesday 841 am && .BOX WATCHES/WARNINGS/ADVISORIES... CT...None. MA...None. RI...None. MARINE...Small Craft Advisory until 3 PM EDT Wednesday for ANZ231>234. Small Craft Advisory until 8 PM EDT this evening for ANZ230- 236. Small Craft Advisory until 8 PM EDT Wednesday for ANZ235-237- 250-251-254>256. && $$ SYNOPSIS...Belk/EVT NEAR TERM...Belk/Dunten/NMB SHORT TERM...Belk LONG TERM...EVT AVIATION...Belk/EVT/NMB MARINE...Belk/EVT TIDES/COASTAL FLOODING... is the U.S. government's official web portal to all federal, state and local government web resources and services.