Area Forecast Discussion
Issued by NWS Boston, MA

Current Version | Previous Version | Graphics & Text | Print | Product List | Glossary On
Versions: 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50
-- Highlight Changed Discussion --
-- Discussion containing changed information from previous version are highlighted. --
000 FXUS61 KBOX 191938 AFDBOX Area Forecast Discussion National Weather Service Taunton MA 238 PM EST Thu Jan 19 2017 .SYNOPSIS... High pressure remains in control through Saturday, although some light rain is possible in the west tomorrow night. Developing low pressure over the Southern Plains will bring rain and possibly some initial ice to New England Monday, with the rain lingering into Tuesday. High pressure builds Wednesday with drier air. && .NEAR TERM /THROUGH TONIGHT/... A case of the sun working against itself as modest cold advection between H92 and H85, combined with early clearing thanks to increased mixing and some moisture trapped beneath a building subsidence inversion. This has allowed the previous clearing to give way to a mix of SCT-BKN clouds between 2-4kft. Still even with the BKN clouds temps have rebounded nicely into the low 40s with H92 temps cooling only to around 0C. Subsidence inversion remains in place through the overnight hours tonight, which continues to lock in a layer of moisture mainly between H9 and H8. Therefore, still some clouds on and off through the overnight. Not quite as concerned about fog potential as previous forecast suggests, near sfc dwpts continue to lower (drying). In fact even though pres gradient will slacken yielding near calm conditions, the trapped moisture beneath the subsidence inversion and resulting SCT-BKN clouds will limit radiational cooling potential and therefore mitigate fog development. A few patches are possible, but not as concerned about widespread fog with this update. Otherwise, given the lack of radiational cooling min temps should remain mostly in the upper 20s and low 30s. A few spots could dip into the low 20s where decoupling is typical as the early morning moisture layer dissipates. && .SHORT TERM /FRIDAY THROUGH FRIDAY NIGHT/... Tomorrow... Mid lvl ridge building keeps dry subsidence inversion in place. However lower lvl winds will veer to the E then S-SE through the day. This suggests modest warm advection and some increase in overall column moisture through the day. The primary result of these factors will be for another day of mixed sun and cloud (mainly all cloud by evening with the approach of shortwave and attendant warm front). However, clouds are likely to mainly be mid-high clouds except near E MA, where onshore flow could bring in some ocean SC onshore during the afternoon. Overall, with little change in airmass, expect highs once again in the low-40s, similar to those experienced today. Tomorrow night... Shortwave moving out of the great lakes looks acute enough to suppress continued mid lvl ridge building. This will combine with an increase in column moisture mainly across the W with PWATs nearing 2 std deviations above normal. Overall, enough lift expected for rain to build from SW during the evening and early overnight hours thanks to overruning ahead of the approaching warm front which will remain to the S. The rain will begin tapering as it pushes into Central S New England so highest POPs during the overnight are relegated to CT and W MA primarily. RI and areas E of the Worcester Hills could remain dry, or at most see a sprinkle before the rain shield dissipates entirely. This is thanks to the continued ridge building mentioned earlier. One note of consideration is the low risk for some brief freezing rain during the evening hours mainly for portions of NW and N-central MA. Dwpt depressions and initial cooling ahead of the precip shield could allow temps to drop to near or below freezing as the rain (what`s left of it at least) reaches this area. Given the precip may be ending or very light, marginal 2m temps and area sfc temps near or even above freezing, not expecting much in the way of ice accretion. Therefore, no headlines with this update, but we will need to monitor this trend over the next 24 hours. Temps mainly dip into the low-mid 30s thanks to the return flow and building clouds limiting any radiational cooling potential. && .LONG TERM /SATURDAY THROUGH WEDNESDAY/... Big Picture... Large scale pattern initially shows trough in the West and ridge in the East. Shortwave ejecting from Pacific NW storm Friday digs as it sweeps across the Southern USA over the weekend. This carves out a negative-tilt trough over the Eastern USA by Monday. This trough then slowly pulls out to the east early next week. The remainder of the Pacific NW storm digs into the desert Southwest as high pressure rebuilds behind the eastern storm. This rebuilds a semblance of the trough-west/ridge east pattern, but with lower/colder heights. The GFS and ECMWF mass fields remain similar through Sunday, then diverge on the details Sunday night through Wednesday. The position of the East Coast storm is closer to the coast and slower on the ECMWF, while the GFS and GGEM are faster in moving it past on Tuesday. As such, forecast confidence is good through Sunday, then low early next week. Expect confidence to remain low until the first part of the Pacific storm moves ashore Friday night or Saturday. Details... Friday night through Sunday... High pressure ridge builds over New England. Even with this in place, two shortwaves move through the region. The first, a southern stream system, slides through on Friday night. Cross sections of relative humidity show deep moisture will be over CT and aided by marginal lift at best. Will mention chance pops over CT and parts of RI and Western MA. A second shortwave moves south from Quebec on Saturday night. This will bring limited moisture and no support for lift. The alignment on this system resembles a backdoor cold front. Expect some clouds but no pcpn Saturday night. The main effect will be for colder air to settle south into New England. Cross sections show moisture lingering below 850 mb through the weekend. So expect a best-case of partly cloudy, with periods of mostly cloudy skies. Sunday night-Monday-Tuesday... The main concern for weather takes place during this period. Low pressure takes shape over the Southern Plains, sweeps to the Southeast coast, then moves up the coast and spreads precipitation into New England Sunday night or Monday. High pressure building south Sunday night will bring the typical signal for cold air damming. Low level ageostropic flow is currently less revealing. It shows strong drainage flow Sunday night, but from the NW rather than the usual direction of North. Meanwhile, the building of the ridge over Gaspe and New Brunswick is in a location that could block the northward movement of any coastal system from the south. Because of the high, and because the upper low is forecast to become negative tilted Monday, we have slowed the onset of pcpn over our area. Even so, we indicate chance pops Sunday night and the best pops for Monday and Tuesday. Temperature profiles suggest that Western and Central MA and parts of Northern CT may see a period of freezing rain or sleet Sunday night before milder air starts working in on Monday. Eastern MA and RI will be facing an increasing East wind which should keep temps in the coastal plain above freezing. Also expecting a windy period. The low level jet will shift north up the coast Monday. Winds in the core of the jet are forecast to reach 60 knots. So coastal areas could be seeing surface gusts of 30 to 40 knots. Most at risk would be Cape Cod and Nantucket. As the surface system moves up, it will nudge warmer air into our area. We expect temperatures to rise much of Monday night, with coastal plain temps reaching the lower 50s Tuesday while the interior is in the 40s. The storm moves off to the northeast Tuesday night. At this point, expect precipitation to wind down, although with lingering clouds and drizzle much of the night. Wednesday... High pressure builds over the region with sunshine and drier air. && .AVIATION /19Z THURSDAY THROUGH TUESDAY/... Forecaster Confidence Levels... Low - less than 30 percent. Moderate - 30 to 60 percent. High - greater than 60 percent. Short Term /through Friday Night/... Through tonight...Moderate Confidence. W-NW winds will peak this afternoon with 10-20 kt gusts possible at times. Otherwise, mix of VFR and MVFR due to occasional low CIGS especially across W MA/CT and into the Worcester hills. Some SCT-BKN MVFR CIGS are possible during the overnight hours tonight, but these will mix with VFR conditions. Tomorrow...High confidence. Mainly VFR through the afternoon until some MVFR CIGS spill across portions of CT late, and SE MA during the afternoon with winds shifting to the E then SE through the afternoon. Tomorrow night...Moderate confidence. Mix of MVFR/IFR as a band of rain shifts into the region from the SW, while this is expected to dissipate as it moves E, areas across CT/W MA are likely to see wet runways. With temps near freezing some light ice also possible toward the Sat morning timeframe. KBOS TAF...Mainly VFR through the overnight, although periodic MVFR CIGS could move over the terminal at times this evening and overnight. More MVFR late tomorrow. W-NW winds peak 10-20 kt today but then shift tomorrow first to the E, then to the S-SE. Moderate confidence in TAF, mainly due to timing of the periodic MVFR CIGS. KBDL TAF...Moderate confidence. Main issue are the periodic MVFR CIGS which may break some this afternoon, but return at times again during the overnight hours. Outlook /Saturday through Monday/... Moderate confidence. Friday night through Sunday... Plenty of low level moisture for clouds, but not much for any precipitation. Areas of MVFR cigs, but unrestricted vsbys. Winds from the south Friday morning shift out of the west by Saturday and then out of the east Sunday. Sunday night through Monday... Ceiling lowering to IFR/LIFR. Rain and possible freezing rain inland Monday, with rain along the coast. Vsbys becoming 2 to 4 miles in rain and fog. East winds increase with potential gusts to 35 knots Monday. && .MARINE... Forecaster Confidence Levels... Low - less than 30 percent. Moderate - 30 to 60 percent. High - greater than 60 percent. Short Term /through Friday Night/... Through early tomorrow...High confidence. Some lingering swell near 5-6ft will require Small Craft Advisories to continue tonight into early tomorrow before they slacken enough that they can be dropped. Otherwise, NW winds will remain generally 10-15 kt at the highest. Late day tomorrow and tomorrow night...High confidence. Winds will shift first to the E late tomorrow, then finally around to the S-SE and continue there during the overnight hours. Wind speeds remain 10-15 kt at most and seas should not exceed the 5 ft thresholds for small craft advisories. Some fog tomorrow night could yield occasionally low vsbys. Outlook /Saturday through Monday/...Moderate confidence. Friday night through Sunday... Variable wind becomes WSW but with speeds less than 20 knots. Winds shift Sunday from the ENE with speeds near 20 knots. Seas remain 4 feet or less. Sunday night-Monday... Rain moves up from the south as a deepening coastal storm approaches and moves up against high pressure over the Maritimes. This will bring increasing winds from the east. Winds at 2000 feet above the surface will reach 60 knots, bringing a potential for 35-40 knots at the surface. This wind will build seas, with heights up to 15 feet possible on the outer waters. Rain and fog will bring occasional poor vsbys on the waters. Based on expected winds, a gale watch/warning may eventually be needed for this time period. && .BOX WATCHES/WARNINGS/ADVISORIES... CT...None. MA...None. RI...None. MARINE...Small Craft Advisory for hazardous seas until 1 PM EST Friday for ANZ250-254-255. Small Craft Advisory for hazardous seas until 10 AM EST Friday for ANZ251. Small Craft Advisory for hazardous seas until 6 AM EST Friday for ANZ256. && $$ SYNOPSIS...WTB/Doody NEAR TERM...Doody SHORT TERM...Doody LONG TERM...WTB AVIATION...WTB/Doody MARINE...WTB/Doody is the U.S. government's official web portal to all federal, state and local government web resources and services.