Scientific Forecaster Discussion

000 fxus61 kbox 190204 afdbox Area forecast discussion National Weather Service Boston/Norton Massachusetts 1004 PM EDT Thu Apr 18 2019 Synopsis... lifting warm front and showery conditions overnight makes way for warm, humid conditions into Friday. A period of heavy rain- fall is possible late Friday night into Saturday night as slow moving low pressure approaches from the west. The low pressure moves slowly across New England into early next week. Off and on rain chances, perhaps a midweek dry period before additional shower chances emerge late week into the following weekend. && Near term /until 6 am Friday morning/... 10 PM update... Scattered showers moving east across southern New England this evening and will exit the coast by midnight. Otherwise mid level drying lifts north across southern New England overnight which will result in mainly dry conditions. The exception will be near the South Coast where abundant low level moisture may produce patchy drizzle at times. Patchy fog this evening should become focused along the South Coast overnight as higher dewpoint air moves over cooler water. Warm front cutting across the coastal plain with temps upper 50s across southeast Massachusetts but holding in the mid 40s across northern northern Massachusetts and portions of CT valley. Expect the cooler interior air to persist into the overnight but temps should rise into the 50s late as the warm front lifts to the north. Previous discussion ... Continued scattered shower activity as the warm front continues to lift north. Low level isentropic ascent north brought per strengthening S/SW flow ushering in a coincident blob of 700 mb-9 higher Theta-E. Focused, lower cloud and visibility conditions sweeping North/East this evening with scattered showers before drier air advects in across the region just above the surface. Increasing S winds, non-diurnal temperature trend overnight, rising surface dewpoints. Expect improvement across the interior, however along the coast adjacent to cooler ocean waters, expect lower dew- point depressions, the persistence of patchy dense fog. Some drizzle possible as well. Winds will be brisk, hence a bit less confidence on any dense fog headlines. Thinking visibility will be around 1 to 2 miles especially for southeast-coastal New England that includes the cape and islands. && Short term /6 am Friday morning through Friday night/... trough digging over the eastern USA becomes a closed low Friday. This further turns the upper flow out of the south, drawing unseasonably humid air north. Precip water values of 1.5 to 1.75 inches are forecast. This would be Ho-hum for July, but not so for mid-April. Average values for mid-April are 0.6 inches, and high-end observed over the past 70 years have been around 1.4 inches. Model time cross-sections concur, showing deep moisture returning Friday afternoon and Friday night. Dynamics are less helpful. Zone of favorable upper jet dynamics remains over New York/PA during the period while low level jet and transport of moisture doesn't ramp up until Friday night. With this in mind, we will maintain low-end chance pops much of the day, ramping up during mid to late afternoon. Likely-to-categorical pops for Friday evening and night, with highest values over western Massachusetts and Hartford County CT. The low level jet, as mentioned, maintains a broad 50-knot zone just offshore during the day. A 60-knot maxima then moves up the coast late afternoon and evening with axis aimed mainly toward our western areas. This supports the idea of higher pops in western sections during Friday night, as well as the start of heavier rain especially in western Massachusetts/northern CT. Mixing to 950 mb would support Max sfc temps Friday in the mid 60s. Airmass over the southeast USA shows observed dew points in the 50s and lower 60s. We will expect this for our area Friday/Friday night in the warm sector south flow. So Friday night min temps would be upper 50s and low 60s. Gusty southwest winds continue due to the low level jet. Mixing is limited, but even from 950 mb we would see 25-30 knots. && Long term /Saturday through Thursday/... */ highlights ... - Saturday into Sunday, river flood concerns along with potential urban / poor drainage flooding ... also potential coastal flood issues along S-coastal New England - Monday into Tuesday, scattered shower activity along with cooler conditions prevails beneath the slow moving area of low pressure - Wednesday Onward, cooler conditions with shower chance could possibly prevail */ overview ... Soaking rains. Echoes of a S Greenland block (+nao) buckling the up- stream pattern as progressive North Pacific flow maintains a continuous feed of energy across the Continental U.S.. amplified 500 mb troughs, especially east of The Rockies, dipping S, tapping the sub-tropics, pulling poleward anomalous precipitable waters and much warmer air, slowed over the east Continental U.S.. 500 mb trough-ridge pattern, North Atlantic energy downshears by the weekend into early next week yielding a Rex-block that continues a slowed upstream pattern. Depending on the aforementioned North Pacific stream, could see a setup of on-again, off-again, decent rains that are potentially soaking through the end of April. Sweeping frontal boundaries, likely to see some temperature swings dictated by the direction of the wind. Spring continues to prevail. Details as to significant forecast concerns / threats and impacts below. */ Details ... Saturday into Sunday ... 1.) Interior flooding: High confidence mainstem CT per headwater snowmelt and forecast rain- fall. Mmefs concerns also signaling some smaller streams throughout S New England potentially seeing actionable to minor flood issues. But not highly confident on significant interior flooding elsewhere. Likely ponding on roadways and possible urban / poor-drainage flood issues, but widespread impact is seemingly less with the latest 12z suite of forecast guidance. Low undergoing full maturation towards occlusion, dry air wrapping in within the mid-levels, losing deep column moisture while it appears deep layer ascent is seemingly weak to diffuse. Widespread rain with embedded heavier showers and potential isolated thunderstorms does appear likely and it looks like generally around 1-2 inches rainfall with locally higher amounts somewhere in S New England. But not absolutely sure exactly where. Hopefully we'll gain greater confidence as convective allowing models become available as to where and when heaviest rainfall will occur. Focus Saturday, possibly lingering into Sunday, a slow moving system up against a developing Rex-block S of Greenland. Precipitable water surge up around 1.5 inches beneath mid-level ascent and broader 300 mb-5 diffluence aloft, but again, mid-level dry air could put the squash on heavier rain outcomes. Lift/Omega values up as high as around 20 Micro-bars per second. A feeling of a prolonged rain gradually adding up but not falling at once, not exceeding flash flood guidance. Flood Watch issued for the west-half of S New England in collaboration with wpc and other wfos. A focus on Saturday to start coincident with excessive rainfall out- looks per wpc. 2.) Coastal flooding: High astronomical tides plus longevity of S fetch, S-coast under the greatest threat of inundation issues. Mmefs signaling tidal issues at westerly, Rhode Island. Particular focus within inner bays and sounds with an eye on Narragansett and Buzzards Bay. Question whether if we push over a foot of surge, then there's concern. Continual monitoring, but right now it appears most spots will fall short. 3.) Winds: H925 S winds broadly speaking 50-60 mph over all S New England with possibly 70 mph along the immediate S coast. As to how effectively these winds mix-down to the surface will need to be watched closely, as to whether precipitation drag or isolated thunderstorms will bring momentum downward and pose wind-related hazards to the region. Much stronger inversion along the coast with cooler waters beneath much warmer air aloft, however the threat exists more so across the interior, especially the high terrain. Nevertheless, persistence of 15 to 25 mph S winds since Friday will ultimately aid in S surge of water along the S-coast and the threat of coastal flooding noted above. Monday into Tuesday ... Beneath the low, showers swirl. Winds subside thus the coastal flood threat subsides. Additional water into the system but should remain fairly light. Mainstem rivers remain in flood, perhaps some smaller streams as well. Lean cooler conditions as the Rex-block evolves to the immediate east blocking The Escape of the low east. Wednesday Onward ... Perhaps drying out, warming up ... or additional upstream Pacific energy continues the threat of scattered showers and cooler weather. But then again 18.12z indications that the eroding Rex-block keeps a persistent 500 mb trof lingering over southeast Canada, additional energy and shower chances dropping S along with cooler conditions. Forecast guidance has been adjusting, no true confidence. && Aviation /02z Friday 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/... Tonight...moderate confidence. Evening MVFR-IFR cigs and vsbys sweeping NE along with scattered rain showers activity. Winds turning S, becoming brisk. An overnight interior improvement, but likely IFR-LIFR settling in along the southeast-coastal terminals. Expect lower conditions fill back north across the interior, becoming more widespread into Friday. Low level wind shear threats as S/SW flow 2 kft agl will be increasing in excess of 40 kts, up to 50 kts towards morning, especially over southeast-coastal terminals. Friday...moderate confidence. Widespread MVFR/IFR with patchy -ra/-dz. SW gusts to 30 kt, more so for interior, high terrain terminals close to the coastline (i.E., Orh). Continued low level wind shear especially for the S-coast, cape and island terminals with S/SW winds 50-55 kt at 2 kft agl. Friday night...moderate confidence. Widespread MVFR/IFR. Increased likelihood of -ra/ra, especially from Worcester westward. Gusty winds with gusts to 30 kt. Low level wind shear continues. Kbos terminal... IFR ceiling / MVFR-IFR visibility issues through roughly 6z, thereafter an improvement to IFR however cigs remaining below 2 kft agl mostly, continuing through Friday. Low level wind shear threats through early Friday morning. Kbdl terminal...high MVFR-IFR may be quick to clear out during the evening hours per latest upstream trends and near-term forecast guidance. A brief period of VFR, towards morning could see widespread MVFR-IFR fill back in from the S with increasing S winds potentially gusting as high as 30 kts. Outlook /Saturday through Tuesday/... Saturday: MVFR/IFR conditions possible. Windy with gusts up to 30 kt. Ra, patchy br. Saturday night: MVFR/IFR conditions possible. Windy with local gusts up to 30 kt. Rain likely, patchy fog. Sunday: mainly VFR, with local MVFR possible. Chance ra, chance rain showers. Sunday night: VFR. Chance rain showers. Monday: mainly VFR, with local MVFR possible. Chance rain showers. Monday night through tuesday: VFR. Chance rain showers. && Marine... forecaster confidence levels... Low - less than 30 percent. Moderate - 30 to 60 percent. High - greater than 60 percent. Short term /through Friday night/...high confidence. A low level jet about 2000 feet over the waters will persist through Friday night. South-southeast winds this evening turn from the southwest overnight. Sprinkles/drizzle/fog tonight and Friday. Showers become more likely Friday night with reduced visibility in showers and fog. Rough seas develop tonight and linger through Friday night. Outlook /Saturday through Tuesday/... Saturday: moderate risk for Small Craft Advisory winds with gusts up to 30 kt. Rough seas up to 13 ft. Rain likely, patchy fog. Local visibility 1 to 3 nm. Saturday night: low risk for Small Craft Advisory winds with local gusts up to 30 kt. Rough seas up to 13 ft. Rain likely, patchy fog, slight chance of thunderstorms. Visibility 1 nm or less. Sunday: winds less than 25 kt. Rough seas up to 12 ft. Chance of rain, chance of rain showers, patchy fog. Local visibility 1 to 3 nm. Sunday night through monday: winds less than 25 kt. Areas of rough seas. Chance of rain showers. Monday night: winds less than 25 kt. Local rough seas. Chance of rain showers. Tuesday: winds less than 25 kt. Areas of seas approaching 5 ft. Chance of rain showers. && Tides/coastal flooding... with the strong southerly winds aligned with astronomical high tides due to the full moon late this week, there is a small chance for that a few locations along the South Coast may see minor coastal flooding around high tide tonight. Most information suggests some splashover but short of actual flooding. Will monitor for potential coastal flood headlines. && Box watches/warnings/advisories... CT...Flood Watch from late Friday night through Saturday evening for ctz002>004. Massachusetts...Flood Watch from late Friday night through Saturday evening for maz002>004-008>012. Rhode Island...none. Marine...Small Craft Advisory until 8 am EDT Saturday for anz231>237- 250-251-254>256. Small Craft Advisory from 6 am Friday to 8 am EDT Saturday for anz230. && $$ Synopsis...wtb/sipprell near term...kjc/sipprell short term...wtb

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