Area Forecast Discussion
Issued by NWS Boston, MA

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000 FXUS61 KBOX 121202 AFDBOX Area Forecast Discussion National Weather Service Taunton MA 702 AM EST Thu Jan 12 2017 .SYNOPSIS...
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Mild and very wet conditions linger into the early overnight hours with a cold frontal passage clearing things out late into Friday. Mainly dry and much colder weather follows this weekend as strong high pressure builds into the region keeping low pressure well to the south. The high pressure moves offshore Monday, but maintains dry weather through the day. Low pressure moving out of the Southern Plains will bring milder temperatures and mostly rain beginning Tuesday.
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&& .NEAR TERM /UNTIL 6 PM THIS EVENING/...
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7 am update... As the previous forecaster noted, lots of low level moisture becoming trapped beneath a building inversion. Expect a trend of lowering cloud conditions if not so already and developing areas of fog with batches of light to moderate rain sweeping across the region. Magnitude of S winds is in question, as to the effective level of mixing. Feel the greatest risk of blustery SW flow is across the high terrain and for locations in SE New England. Can not rule out gusts up to 40 mph. Will see another surge in low level winds over SE New England ahead of the cold front which will sweep the area into the evening hours. Previous discussion... 50 kt LLJ already beginning to slide E of the region as of 8Z this morning leaving in its wake less widespread RA than we experienced much of the overnight and early morning hours. Still with PWATs remaining above 1.00 inches, and even as mid-upper lvls dry somewhat bulk of this moisture will remain trapped in the lower lvls under a building inversion through the morning. Therefore, will adjust from stratiform rain to mainly showers after sunrise. These showers will begin to diminish across the remainder of S New England as stronger LLJ and overall deeper column moisture remains trapped across N and N-central New England. POPs will reflect this thinking through the day until an actual cold front approaches by late evening and early overnight tonight. POPs will once again increase (although mainly to high chance) across the rest of the region late in the day. In spite of showers diminishing, the moisture trapped near the sfc beneath the inversion will likely maintain some fog, which may still yield reductions in vsbys through the day. In fact, would not be surprised if vsbys dip to a half mile or less at times requiring a potential dense fog advisory, and this may impact both this morning`s and this afternoon`s commute. The best chance for the fog to dissipate will be in higher wind gusts which we will discuss below. 40-50 kt LLJ will linger across the area through the day but peak in the lowest lvls this afternoon and evening across RI/SEMA. Any shra through the day could yield enough drag to overcome the deepening inversion, as H92 temps could reach or exceed +12C by the afternoon. Therefore, will need to monitor for gusty winds mainly in shra through the day across the W/NW. Otherwise, with H92 temps warming to +12C and low lvl temps reaching the mid to upper 50s especially across RI/SEMA this afternoon, gusts could approach wind advisory lvls at this time. However, given the strengthening inversion, am not confident enough for a wind advisory as mixing could still be just too limited. Will need to watch this today.
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&& .SHORT TERM /6 PM THIS EVENING THROUGH 6 PM FRIDAY/...
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Tonight... Cold frontal passage expected between about 10PM and 4AM local which will introduce much drier air to the region slowly from NW to SE. This should finally scour out the leftover moisture across S New England, leading to an end for shra and fog through the overnight hours. Temps will remain elevated through the FROPA, with MINs not expected until morning as dwpts drop and some slight cold advection occurs. Still, this will be delayed late enough that forecast MINs are above normal, still in the upper 30s and low 40s across much of the region. Friday... Strong cold advection marks the day. In fact, highs Fri are likely to be only be a few degrees warmer than Thu night MIN temps given diurnal warming will be little match for the cold advection aloft as H92 temps drop to -10C by 00Z Sat. In spite of sunshine, this suggests upper 30s to mid 40s, still mild for January, but not nearly as mild as highs today. Winds may be gusty during the day as the cold advection will likely increase mixing potential through the afternoon with plenty of sunshine expected. LLJ not as strong as previous days, hovering around 40 kt at H92. Therefore will cap most gusts at 25-35 mph, but these will last much of the day.
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&& .LONG TERM /FRIDAY NIGHT THROUGH WEDNESDAY/... */ Highlights... - Despite a slight chance of snow for the S-coast Saturday night, a warmer than average, wet pattern - Mainly nuisance-type weather, nothing blockbuster */ Discussion... A mild pattern marked by weak, wet-weather disturbances. There are two noteworthy signals: 1) +1-2 standard deviation in H5 heights across the E CONUS per ensemble means, and 2) concurrent confluent to anti-cyclonic flow within the mid to upper levels, progressive. Averaged out over the long-term period, there will likely be some shots of colder air in-between closer to seasonable values rather than of Arctic-origin. Synoptic disturbances are likely to be non- blockbuster, rather nuisance, progressive and quick-moving, with minor impacts more so with the onset of precipitation in the form of wintry weather transitioning to rain. Leaning with surface to H85 lows riding N/E mostly across the Great Lakes region ahead of which expect a pattern of gusty S winds and warmer air with temperatures potentially reaching 60 degrees at some point. So going into detail, will keep it dry through Tuesday morning. It is perhaps that we`ll see some flurry activity across the S-coast Saturday night. But given confluent flow aloft and forecast trends of favorable synoptics remaining S, plus ensemble and probabilistic signals, will go with slight chance PoPs if anything at all. Building upstream ridge across the N Pacific amplifies N flow rear- ward of a weak troughing signal towards the W CONUS. Energy shears equatorward into a closed, cut-off low across the SW CONUS into Mexico. Evolving as a deeper trough with the additional introduction of upstream Pacific-origin energy, begin to see shortwave impulses transition E across a building ridge and anti-cyclonic flow over the E CONUS. Isentropic focus within the low to mid levels, an overall evolving, persistent over-running setup brought about by successive waves of energy from the upstream trough, with SW to NE flow along a series of frontal boundaries. During onset with lead high pressure built S out of Canada anticipate an initial period of wintry precip, mainly focused over N/W Mass. Perhaps short-lived as surface lows are expected to track N/W of our region, will see a transition to rain with enhanced S flow and usherance of warm, mild air N. Individual impulses being weak as previously mentioned, confidence wanes beyond the midweek period as to specific outcomes with poor model handling. General feeling is that surface lows remain N/W of the region as we end up on the warmer side of the envelope. Targets of opportunity: blustery S winds and whether we may reach advisory criteria (low confidence), and with the usherance of anomalous warm, high dewpoint air N, depending on winds, the potential for fog impacts (moderate confidence should winds become light and given how and cold 2 cm soil temperatures). Too far out in the forecast to say with any certainty, just something to chew on and maintain a watch. && .AVIATION /12Z THURSDAY THROUGH MONDAY/...
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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 Friday/... 12z update... Through evening...Moderate confidence. Continued batches of -RA/RA along with breezy S winds with likely the high terrain terminals seeing the strongest gusts up around 35 kts, as well as possibly SE Mass. General 10 to 20 kt winds other- wise. Winds will be variable across most other terminals and as such there is the threat of LLWS given winds 2 kft agl will be out of the SW at roughly around 40 to 50 kts. Trending lowering conditions ahead of a cold front / wind shift later tonight. Lowering down to IFR with LIFR in spots. By late evening conditions clearing out as winds shift out of the W. Later tonight into Friday...High confidence. Winds shift to the W, expect improvement not long after, although timing may be off by a bit in TAFs. Once VFR, this will remain through the day on Fri. W winds gust 25-30 kt at times during the daylight hours Fri. KBOS TAF...VFR initially but lowering to IFR in advance of the cold front / wind shift with continued -RA/RA. S winds variable with occasional gusts. This presents a threat of LLWS at times. Not entirely confident on timing and trends with specificity. Given a SW wind, would expect MVFR to VFR conditions, but already the terminal has fallen to LIFR. Will hint at a return of low conditions subsequently so. KBDL TAF...MVFR down to LIFR mix ahead of the expected cold front later today into this evening. Batches of -RA/RA continue and winds are anticipated to remain light within the valley. There is the possible threat of LLWS, though the core of strong SW winds is forecast to remain to the S/W. Outlook /Friday Night through Monday/...Moderate confidence. Friday night... Diminishing NW winds shifting out of the W with increasing mid to high level cigs. Gusts up to 25 kts initially along the shores. Saturday into Saturday night... Light W/SW flow with low-end VFR cigs, MVFR across S CT and S MA with -SN. Low risk IFR mainly confined to the immediate S-coast Sunday through Tuesday... VFR through Monday. Lowering cigs into Tuesday down to IFR with RA and increasing SW winds. Low risk of LLWS impacts late in the period.
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&& .MARINE...
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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 Friday/... 7 am update... Through Tonight...Moderate confidence. The reason for lower confidence is mainly due to the possibility that afternoon/evening wind gusts could come very close to Gale Force out of the S-SW. Mixing will not be as good as previous days, however, so capped gusts to slightly below Gales and will maintain Small Craft Advisories well into the overnight. Will monitor this risk through the day. Feel the greatest risk is along the near-shore, especially around SE Mass. Late overnight winds may actually fall below small craft thresholds, especially during the early AM hours, although seas will remain elevated, mainly between 8-10 ft on the ocean waters. Otherwise, fog through the day and evening hours along with rain may lead to very low visibilities at times. Friday...High confidence. Small Craft Advisories have been extended but winds will shift mainly to the W. Gusts 25-30 kt are likely through much of the day. Although seas may diminish slightly due to a more W component to the flow, they will remain 5-7 ft on the outer waters. Outlook /Friday Night through Monday/...Moderate confidence. Blustery NW winds Friday night diminishing into Saturday allowing seas to subside. An offshore area of low pressure sweeping W to E allows for some light snow across the S waters, perhaps some lower visibility. Otherwise good boating weather Sunday with light N winds, shifting out of the S and slowly increasing into midweek along with chances of rain.
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&& .BOX WATCHES/WARNINGS/ADVISORIES... CT...None. MA...None. RI...None. MARINE...Small Craft Advisory until 7 PM EST Friday for ANZ231>235-237- 250-251-254>256. Small Craft Advisory until 7 PM EST this evening for ANZ230- 236. && $$ SYNOPSIS...Doody/Sipprell NEAR TERM...Doody/Sipprell SHORT TERM...Doody LONG TERM...Sipprell AVIATION...Doody/Sipprell MARINE...Doody/Sipprell

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