Area Forecast Discussion
Issued by NWS Boston, MA

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000 FXUS61 KBOX 231307 AFDBOX Area Forecast Discussion National Weather Service Taunton MA 907 AM EDT Sat Sep 23 2017 .SYNOPSIS... Jose southeast of Nantucket will slowly drift back to the southwest the next few days. This will maintain gusty winds, rain and rough seas today, which will diminish as the storm continues to weaken this weekend. Clearing skies will move in from the west tonight and Sunday. High pressure will bring unseasonable warm weather for Sunday into mid next week. Maria is expected to pass southeast of New England late in the week with strongest winds well offshore. && .NEAR TERM /UNTIL 6 PM THIS EVENING/...
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9 AM Update... Have issued a Dense Fog Advisory for Cape Cod and Nantucket for the remainder of the morning hours. Chatham, Hyannis, and Falmouth all were reporting 1/4 mile visibilities. Conditions were improving just north and west of the Cape and on Martha`s Vineyard, so we are hopeful that these will improve after about noon. Previous discussion... Outside of Southern New England, the big story is longwave high pressure with heights well about September normals. The surface reflection of this high is bringing fair dry weather from Northern New England to the Great Lakes and much of the South. We expect this high to build over Southern New England this weekend, bringing a trend to above normal temperatures. For today this means a clearing trend from west to east, except along Cape Cod and the South Coast where low clouds from Jose may hang tough through the day. Winds from the North-Northeast will diminish with lingering 20-25 knot gusts across the Cape and Islands, and 15-20 knots farther north and west. Lingering clouds and a north-northeast wind will slow down heating in Eastern Mass, while the CT River Valley should mix to between 900 and 850 mb. For the east, this means a mixing equivalent of 8-10C, supporting max temps in the 70s. In western areas the mixing equivalent depends on the depth...about 15-16C if mixing to 900 mb and 16-17C if reaching 850 mb. So temps there should reach at least the mid 80s and have the potential of approaching 90. Note that upstream the max temps yesterday reached the mid 90s in Michigan. Also note that record max temps for our area today are 90 to 96, so while a 90 in Worcester would tie a record, it would fall short at the other sites. Surf... Southeast swell of 4-5 feet lingers along the south and east facing exposed coastlines today. High surf and dangerous rip currents will be a concern. Swell is expected to diminish during the afternoon.
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&& .SHORT TERM /6 PM THIS EVENING THROUGH 6 PM SUNDAY/... Tonight... Winds diminish tonight, which with clear skies will allow temperatures to cool to dew point. We trended toward the mos values for min temp, which was a couple of degrees lower than the standard guidance. Radiation fog possible in the interior, and marine fog/stratus is possible along the Cape/Islands/South Coast. Sunday... High pressure over the region Sunday. Heights at 500 mb are forecast at or above 590 Dm, and 850 temps are forecast at 18-19C...the NAM shows 20C. Average September values per ESRL are 578 Dm for 500 mb heights and 10C for 850 mb temperature. So Sunday will trend toward very warm daytime temperatures. Moisture profiles show limited moisture around 850 mb and dry air below and above, so we expect a sunny day. Full mixing to 850 mb would support high temps around 90 or the low 90s, while mixing that falls just short would reach the mid to upper 80s. Light winds under the high pressure will allow daytime sea breezes, which will keep temps cooler along the shoreline. We will go with upper 80s and around 90. This would be either near or at record levels. Records are included in the Climate section near the end of this discussion. && .LONG TERM /SUNDAY NIGHT THROUGH FRIDAY/... Highlights... * Summer warmth expected to the middle of next week * Maria expected to sharply recurve out to sea southeast of New England late next week H85 temperatures 17C to 19C and impressive upper level ridge with H50 heights over 5850m support high temperatures well into the 80s across much of the region Monday and Tuesday and possibly for some of the region into Wednesday. A few interior spots may reach 90 degrees Monday, and perhaps even again Tuesday if the ECMWF 00Z. run verifies. Sea breezes may temper max temperatures a little along the coastal plain including Boston. Expansive outflow cloud shield from Maria and slowly lowering mid level heights may lower temperatures a few degrees on Wednesday but still above normal. Model guidance brings a risk of showers to southern New England late Wednesday. This is likely in response to a weakening cold front ahead of an approaching broad upper trough with a band of K indices above 30 with the 00Z operational GFS and ECMWF. This may provide enough of a trigger for at least scattered showers. Will keep POPs around 30 percent but any showers that do develop could have efficient rainfall given warm rain processes at work and precipitable water values near 2 inches. The latter part of the week features an amplifying upper level trough over the Great Lakes which would appear to provide the impetus for Maria to make a sharp right turn and pass southeast of New England. Model consensus keeps the track of Maria southeast of the benchmark and max winds on the east side. There may be some moisture entrainment into the approaching upper level trough so Maria could contribute to some rain if this is not counteracted by a band of subsidence outside of Maria`s circulation field. There is some uncertainty as to how this may play out and so for now will indicate chance POPs Thursday through Friday morning. Other impacts associated with Maria will likely be high surf and dangerous rip currents, mainly along ocean exposed south coast beaches. For several days, Maria tracks near a great circle route in our direction and hence will likely see significant long period swells from the S or SE propagate into our coastal waters most of next week with the subsequent result being high surf and rip currents. The slow speed of Maria for the next several days may enhance the swell activity during this period. && .AVIATION /13Z SATURDAY THROUGH WEDNESDAY/... Forecaster Confidence Levels... Low - less than 30 percent. Moderate - 30 to 60 percent. High - greater than 60 percent. Short Term /through Sunday/... Today...Moderate confidence. 9 AM update... Dense fog with LIFR ceilings/vsbys will persist on Cape Cod and Nantucket through 16Z. A Dense Fog Advisory is in effect there. Surrounding the Cape and Islands is a band of MVFR ceilings and pockets of IFR ceilings. Drier air filtering in should allow for gradual improvement through the morning, and by midday most of the area outside of the Cape and Islands is expected to have VFR conditions. A few northerly wind gusts to 20 to 25 knots will linger through early or mid afternoon across southeast New England. Tonight...Moderate confidence. Diminishing wind. VFR to start in most spots. Southeast Mass and RI will trend to IFR/LIFR in fog and low clouds. Areas of fog will also develop across the interior with IFR/LIFR conditions. Sunday... High confidence. Fog burns off during the morning. Most of the day will be VFR with light winds. Sea breezes expected along the coastlines, including BOS. KBOS TAF...Moderate confidence. Low MVFR cigs will gradually improve to VFR by late this morning and then clearing during mid to late afternoon. North to northeast winds 10 to 15 knots with a few gusts possible to 20 knots will diminish through the day. KBDL TAF...High confidence in TAF. Areas of fog may develop 06Z-12Z tonight, although the chance of this is stronger over valley locations north of Springfield. Outlook /Sunday Night through Wednesday/... Confidence...High. Sunday night through Tuesday...VFR expected. Tuesday night...Mostly VFR except MVFR in patchy stratus/fog some interior valleys and near the south coast. Wednesday...Generally VFR except MVFR ceilings/visibilities possible in scattered late day showers and patchy late night/early morning stratus/fog. && .MARINE... Forecaster Confidence Levels... Low - less than 30 percent. Moderate - 30 to 60 percent. High - greater than 60 percent. Today... With the remnants of Jose well south of the waters and moving slowly southeast, we expect conditions will improve over the waters this weekend. Gusty north winds will diminish through the day, with lingering 25-knot gusts this morning mainly on the southern waters. Seas at 5 feet or higher on all exposed waters this morning. These will subside, but with 5-6 foot heights lingering on the outer waters through this evening. Fog and drizzle this morning with some poor vsbys, but this will diminish through the day. A leftover 3 to 5 foot southeast swell will maintain high surf and rip tide hazards at the coastline, diminishing late in the day. Tonight... Fog lingers along the southern and southeast coasts in the evening, then moves to the coastline during the night. Expect periods of poor vsbys, especially on the waters south and east of Cape Cod. Winds will diminish to less than 15 knots, but seas will linger at 5-6 feet on the southern outer waters. Sunday... High pressure building over the region will bring light wind Sunday, with sea breezes developing at the shoreline. Poor vsbys in fog early, but these will improve during the morning. Seas of 5-6 feet will linger oin the southern outer waters, and may work north into RI Sound during the afternoon. A south swell sent north from Maria will start showing itself in our southern waters Sunday, with 3-4 feet swell heights in the afternoon. Outlook /Sunday Night through Wednesday/... Confidence...High Relatively light winds are expected through the period. However, swells from Maria will likely propagate into the southern coastal water, beginning late Sunday night or Monday. Small Craft Advisories may become necessary due to rough seas for the waters south of the MA and RI coast for the period Monday through Wednesday. The swells from Maria will likely produce another round of high surf and dangerous rip currents. && .TIDES/COASTAL FLOODING... Lingering high surf and additional beach erosion will still be a concern today given high seas just offshore. These values will continue to diminish. Swells from Maria will likely increase the surf and associated rip current risk across our ocean-exposed south coast starting late Sunday and continuing through the week. This will be true even though Maria may eventually recurve out to sea well southeast of New England. && .BOX WATCHES/WARNINGS/ADVISORIES... CT...None. MA...High Surf Advisory until 6 PM EDT this evening for MAZ007-019- 020-022>024. Dense Fog Advisory until noon EDT today for MAZ022-024. RI...High Surf Advisory until 6 PM EDT this evening for RIZ006>008. MARINE...Small Craft Advisory until 5 PM EDT this afternoon for ANZ232>234. Small Craft Advisory until 4 PM EDT this afternoon for ANZ231- 251. Small Craft Advisory until 10 AM EDT this morning for ANZ236. Small Craft Advisory until 11 PM EDT this evening for ANZ235- 237-250. Small Craft Advisory until 6 PM EDT Sunday for ANZ254>256. && $$ SYNOPSIS...WTB/Thompson NEAR TERM...Field/Thompson/WTB SHORT TERM...WTB LONG TERM...Thompson AVIATION...Field/WTB/Thompson MARINE...WTB/Thompson TIDES/COASTAL FLOODING...WTB/Thompson

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