Area Forecast Discussion
Issued by NWS Boston, MA

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000 FXUS61 KBOX 120247 AFDBOX Area Forecast Discussion National Weather Service Taunton MA 947 PM EST Thu Jan 11 2018 .SYNOPSIS... Multiple low pressure areas along an approaching cold front will bring heavy rainfall, flooding and possible gusty winds Friday into early Saturday. A sharp turn to much colder weather occurs rapidly late Friday night through Saturday from west to east. This may allow rain to change to a period of ice before ending, mainly in the interior along with a flash freeze across all southern New England. Mainly dry but very cold weather is expected Sunday and Monday, but will have to watch the potential for some snow and/or mixed precipitation Tuesday into Wednesday. && .NEAR TERM /UNTIL 6 AM FRIDAY MORNING/...
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945 pm update... Impressive southwest low level jet on the order of 40 to 50 knots will continue to allow low level moisture to increase overnight. Dewpoints in the 40s will also allow for areas of fog to develop...but whether it becomes dense remains uncertain. So far this evening...there is just enough wind mixing down from the low level jet to prevent dense fog. The MOS guidance generally keeps enough surface wind to prevent dense fog...but sometimes it is too strong with the winds and will have to watch for potential decoupling especially after midnight. Given dewpoints in the 40s and a deep snowpack...dense fog may form quite quickly "IF" winds decouple. No dense fog advisory yet...but will reissue the special weather statement highlighting the concern. Greatest risk is across the interior and near the south coast. As for precipitation...bulk of synoptic scale forcing/deeper moisture will remain west of our region. Therefore...expect dry weather to dominate but may see a few spot showers develop across the interior toward daybreak. Temperatures mainly in the 40s overnight...but readings may reach or exceed 50 across portions of the coastal plain by daybreak.
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&& .SHORT TERM /6 AM FRIDAY MORNING THROUGH FRIDAY NIGHT/... ***Unseasonably mild Friday with heavy rainfall and significant urban/street flooding threat for the late day/evening commute*** ***Freezing rain and icing possible toward daybreak Saturday across interior northern and western MA*** Heavy Rainfall and Flooding... Slow moving cold front approaching from the west with multiple waves of low pressure combined with highly anomalous PWATs and a good low level jet will bring heavy rainfall and flooding threat to SNE. Initial period of heavy rainfall assocd with a potent low level jet is expected to move across SNE during Friday from west to east with heaviest rainfall in the interior. A second wave approaching from the south as potent southern stream shortwave lifts NE toward New Eng will bring another round of heavy rainfall late Friday into Friday night. This will likely be focused somewhere from the I-95 corridor to Cape Cod as persistent low level jet sets up in this area. Highly anomalous PWATS of 1.50-1.75" which is 4-5 standard deviations above normal combined with +3SD low level jet is a strong signal for heavy rainfall. High confidence of widespread rainfall of 1.5 to 3 inches with localized 4 inch amounts possible. Isold thunder is also possible given the strong low level jet dynamics and marginal elevated instability. Heaviest rainfall amounts expected somewhere across eastern half New Eng with focus from I95 corridor to Cape Cod. Confidence is lower in axis of heaviest rainfall due to potential convection and mesoscale factors and it is possible this axis could shift. Temps will soar well into the 50s to even lower 60s across RI and SE MA with high dewpoints resulting in most if not all the snowpack rapidly melting. The rapid snowmelt combined with heavy rainfall will result in an increased risk for significant urban and poor drainage flooding, especially in vicinity of snow clogged drains. Expect sharp rises on rivers and streams but most rivers are expected to remain below flood stage. The NERFC forecast has the Assabet river at Maynard and the Pawcatuck river at Westerly reaching flood stage, but not until late Saturday night or early Sun. Strong wind... Potent low level jet with 60-70 kts down to 925 mb develops across SE New Eng later Fri into Friday night. Soundings not supportive of much low level mixing. However, where temps approach or exceed 60 degrees will enhance mixing and potential for stronger wind gusts 40-50 mph. The best chance for these stronger gusts will be across RI and SE MA where mildest temps expected. Wind advisories may eventually be needed if confidence increases in these higher gusts. Freezing rain and icing... Late Fri night, guidance indicates cold front will drop south across western half of SNE. Sharply colder air will move in behind the front with potential for rain changing to freezing rain with temps dropping rapidly into the upper 20s. There is still uncertainty with timing of front and low level cold air but best chance of icing before daybreak Sat will be across interior northern and western MA, mainly north and west of BAF- FIT. && .LONG TERM /SATURDAY THROUGH THURSDAY/... */ Highlights... - Icing hazards Saturday, combination of freezing rain and flash freeze - Cold Sunday through Monday, ocean-effect snows over Cape/Islands - Potential winter storm Tuesday into Wednesday */ Overview... Until a H3 jet impulse / H5 vort max is able to cascade E, the wave break / split flow regime lies offshore of the NE Pacific. H5 ridge prevailing over the W CONUS as the H5 Atlantic ridge and embedded traffic slow the downstream flow, a H5 trof pattern is preferred over the E CONUS through which both N and S stream energy rotate, influential upon the SW-NE baroclinic zone / cold front sweeping E through the region Friday into Saturday, stalling offshore against the prominent H5 Atlantic ridge. A series of storm systems expected into early next week until the flow seeming relaxes, moving towards a thawing pattern as mild, Pacific air comes ashore and sweeps the CONUS. Those storm systems and associated impacts addressed below. */ Saturday... Through the preferred H5 trof pattern, N/S pieces coming together aiding in mid-level ascent beneath the RRQ of the upper-level jet. Conveyor belt motions pulling together, anti-cyclonic WCB pulled to the W, the DCB drawn from the SW, as the CCB wraps in from the N/W. Evident anafrontal characteristics via upslope along the isentropes, SW-NE low to mid level frontogenetical banding rearward of a sharp surface cold front as airmasses clash and considerably colder air undercuts, the dendritic growth zone dried out. Freezing Rain... Assuming there is no ice within the column, colder air undercutting quickly with favorable lift / ascent aloft prior to dry air eroding the column, looking at a roughly 3-6 hour window of freezing rain / drizzle over N/W interior S New England, roughly W of interior Essex County MA, Worcester and Tolland Hills. There is the potential of reaching the I-95 corridor, some uncertainty on the speed at which colder air moves in with precipitation ongoing, but expecting the column to dry out into afternoon lessening the chance. Collaborating with WPC and neighboring WFOs, upwards of a tenth of an inch of ice accretion, the highest amounts in the Berkshires / NW MA, forecast. Majority of impacts around sunrise into early afternoon drying out thereafter, perhaps just freezing drizzle lingering before all is said and done. Held off on WINTER WEATHER ADVISORIES as we`re coming up to a potential flooding event, will let the overnight crew have another look as the 11.12z NAM / Canadian are a bit faster with the surface cold front and drying over global guidance. Flash Freeze... Considering a sharper temperature gradient, faster temperature drop, leaning with higher-res guidance from the NAM / Canadian. Looking at temperatures ranging from 40s to 50s around sunrise. By noon falling into the 20s and 30s for all but SE-coastal New England, teens for the Berkshires, then by evening single digits to teens, 20s along the coast. Residual wet surfaces, a 20-30 degree temperature drop in roughly a 6-hour period, expect any untreated surfaces to quickly freeze resulting in potential hazardous travel conditions. */ Saturday night through Monday... Arctic high pressure building across the region. Monitoring ocean effect snows over the Cape and Islands. Whether uni-directional N/NE flow within the boundary layer, relative humidity favorably at or above 70 percent, beneath cyclonic flow. Overall thinking outcomes will be light. Shear within the layer somewhat disruptive as lapse rates derive from temperature differences of 10-13 degrees. Noting the warm ocean temperatures of 40 degrees, the temperature of the top of the layer, ocean induced CAPE only as high as 200 J/kg, nCAPE up to 0.15. Would like to see higher values. Expecting light snow outcomes of tenths from Falmouth of Chatham given the wind direction throughout the forecast period, better chance seemingly Sunday night into Monday with mid-level ascent provided by an impulse rotating through the cyclonic flow. Coldest temperatures around Sunday, going to be in the freezer. Possible WIND CHILL headlines needed for the Berkshires otherwise expecting values as low as 10 below zero. */ Tuesday into Wednesday... Clipper low transferring energy offshore over the better baroclinic zone / stalled cold front up against the Atlantic H5 ridge. Coastal low developing per consensus of ensemble member / mean guidance. As to outcomes, still uncertain with respect to intensity, proximity, movement and timing, details which will be outlined as we get closer to the event. One thing worth noting is the positive-tilted H5 trof axis leading into the event becoming negative afterward. Seemingly developing across New England but deeper and more explosive down- stream. Looking like a rain / snow event, snow to liquid ratio dependent respective to regions of favorable lift / ascent, just too early to get on amounts and exact impacts. && .AVIATION /03Z FRIDAY THROUGH TUESDAY/...
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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 Night/... Tonight...Moderate confidence. Wide variety of conditions this evening will tend to deteriorate to at least low end MVFR along with IFR/LIFR conditions. Low clouds and fog will increase in areal coverage overnight especially across the interior. Biggest uncertainty is if the winds decouple enough to allow for areas of dense fog. Will continue to monitor closely. LLWS also a concern especially toward daybreak. Friday into Friday night...Moderate confidence. Widespread IFR-VLIFR. RA/+RA with FG along with increasing S winds with gusts up around 20-30 kts, strongest over interior SE New England terminals. LLWS with strongest S/SW 2 kft winds up to 60 kts over SE New England. -RA changing to FZRA/FZDZ towards Saturday sunrise over N/W MA and CT. KBOS Terminal...Moderate confidence in TAF. KBDL Terminal...Moderate confidence in TAF. Outlook /Saturday through Tuesday/... Saturday: Mainly VFR, with local IFR possible. Windy with gusts to 30 kt. RA, chance FZRA, areas FG, slight chance FZDZ. Saturday Night: Mainly VFR, with local MVFR possible. Breezy. Slight chance SN. Sunday through Sunday Night: Mainly VFR, with local MVFR possible. Martin Luther King Jr Day: Mainly VFR, with local MVFR possible. Slight chance SN. Monday Night: Mainly VFR, with local IFR possible. Breezy. Chance SN. Tuesday: Mainly VFR, with areas IFR possible. Breezy. Chance SN.
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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 Friday Night/... Tonight...High confidence. Southwest wind gusts of 20 knots expected with seas building to 5 ft over the southern waters. Friday into Friday night...Moderate confidence. Potent low level jet develops over the waters but a rather strong low level inversion is present which will prevent stronger winds from mixing down. We will certainly see SCA wind gusts, but best chance for gales will be over the nearshore waters and Boston Harbor and Narragansett Bay where best mixing anticipated. We will hold on to the gale watch over these waters and NE MA coastal waters as confidence not high enough for warnings, with SCA elsewhere. Risk for gales over the outer southern waters is low. Low vsbys expected in rain and fog. Outlook /Saturday through Tuesday/... Saturday: Low risk for gale force winds with gusts up to 35 kt. Rough seas up to 16 ft. Chance of rain, areas fog, chance of freezing rain, chance of freezing drizzle. Local visibility 1 nm or less. Saturday Night: Winds less than 25 kt. Rough seas up to 11 ft. Chance of snow. Sunday: Winds less than 25 kt. Areas of rough seas. Slight chance of snow. Sunday Night through Martin Luther King Jr Day: Winds less than 25 kt. Seas up to 5 ft. Slight chance of snow. Monday Night: Winds less than 25 kt. Areas of rough seas. Chance of snow. Tuesday: Low risk for Small Craft Advisory winds with gusts up to 25 kt. Rough seas up to 9 ft. Chance of snow. && .BOX WATCHES/WARNINGS/ADVISORIES... CT...Flood Watch from 6 AM EST Friday through Saturday afternoon for CTZ002>004. MA...Flood Watch from 6 AM EST Friday through Saturday afternoon for MAZ002>024-026. RI...Flood Watch from 6 AM EST Friday through Saturday afternoon for RIZ001>008. MARINE...Gale Watch from Friday afternoon through Friday evening for ANZ230>234-236-250-251. Small Craft Advisory from 3 AM Friday to 7 AM EST Saturday for ANZ235-237-254>256. && $$ SYNOPSIS...KJC/Sipprell NEAR TERM...Frank SHORT TERM...KJC/Sipprell LONG TERM...Sipprell AVIATION...Frank/KJC/Sipprell MARINE...KJC/Sipprell

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