KBOX issues Area Forecast Discussion (AFD) valid 2019-12-06T17:20:00Z 993 FXUS61 KBOX 061720 AFDBOX Area Forecast Discussion National Weather Service Boston/Norton MA 1220 PM EST Fri Dec 6 2019 .SYNOPSIS... Clouds will increase ahead of a fast-moving clipper system that will push through with periods of light snow this afternoon and early evening. Best chance of seeing light accumulations is along and north of the MA turnpike. The East Slopes of the Berkshires could see upwards of 3 inches. Colder, drier air moves in Friday night and Saturday as high pressure brings dry weather for the weekend. The high moves offshore Sunday, starting a return flow of milder and moist air for the first half of the week. This will bring off and on rain for Monday and Tuesday. A cold front moves through Tuesday night, followed by a return to cooler temperatures for Wednesday and Thursday. && .NEAR TERM /UNTIL 6 PM THIS EVENING/... 1215PM Update... Light snow is moving across Western MA but has mostly not made it to the ground east of the Berkshires. Did note that KORE is now observing light snow. Temperatures have made it into the low 30s north of the MA turnpike and mid 30s south. This is on track with the forecast for light accumulating snow north of the turnpike and a coating at best south due to above freezing temperatures resulting in a rain/snow mix or just plain rain. High res guidance have hinted at wrap around moisture for northeast MA, including Essex and northern Middlesex counties. So have indicated the potential for up to 2 inches of snow in this area. Also hinted at the potential for 3 inches of snow for the highest elevations in Franklin county, MA. Not confident enough about the spatial extent of 3 inches plus so have opted not to issue a Winter Weather Advisory. Updated the Special Weather Statement to include higher snow totals for aforementioned areas. Previous Discussion... ***A CLIPPER SYSTEM IS EXPECTED TO BRING COATING TO AN INCH OF SNOW ESPECIALLY ALONG & NORTH OF THE MA TURNPIKE LATE THIS AFTERNOON INTO EARLY EVENING. LOCALLY 2-3 INCHES ARE POSSIBLE IN THE EAST SLOPES OF THE BERKSHIRES.*** Satellite shows clouds mostly clearing out early this morning, but it will be short lived. Looking upstream we see the deck of mid level clouds making its way toward southern New England. These will fill in from west to east by sunrise in western MA/CT and eastern MA by 9-10 AM. These clouds precede a quick hitting clipper type system that will strength over the Great Lakes in the next several hours before crossing directly over southern New England with its surface arctic front from 21-03Z (4-10 pm). This system brings our next shot of snow and rain. At the upper levels we have good jet dynamics in play beneath the RRQ of a modest upper jet which will help increase synoptic lift while in the lower levels speed convergence on the front of a 50 kt LLJ will enhance precip formation. The very progressive nature of the storm should limit snowfall to generally late morning through early evening, with accumulations of only a coating to 2 inches for most, mainly along and north of the MA Pike, and perhaps some spots of 2-3 inches in the highest elevations of the Berkshires. Nearly all guidance is in favor of this solution, with the latest EPS giving a 0% chance of >3" in southern New England, and the 00Z HREF mean guidance trending down toward the synoptic models. Given the amounts expected, we will continue to message this with a Special Weather Statement highlighting the impact that it may have to the evening commute. Those in eastern/central MA will continue to be impacted by snow showers during the Friday evening rush hour which could cause some additional travel hazards. Winds will also become breezy as the pressure gradient tightens south of the low, mainly along the south coast and over the southern waters where mixing is better; winds gust 25-30 mph. && .SHORT TERM /6 PM THIS EVENING THROUGH 6 PM SATURDAY/... Snow and rain showers should be tapering off in extreme eastern MA by 8-10 pm as the low center makes a hasty exit. Winds slowly but steadily decrease as a sprawling surface high moves in and the gradient relaxes. With this we also see post frontal NW winds begin to bring in much cooler air, though the breezy winds will likely limit much contribution from radiational cooling. Even so, temperatures dip into the teens to low 20s; warmer on the south coast. Saturday colder and drier air funnels into the region as high pressure builds in, though cyclonic flow lingers aloft. Subsidence under this high will keep things dry and skies mostly clear, though there is still some question as to whether we'll see more diurnal clouds than are currently forecast given that tendency under cyclonic flow with CAA and daytime heating. Overall a very nice, if cold, day with high temperatures in the upper 20s (higher elevations) to low 30s. && .LONG TERM /SATURDAY NIGHT THROUGH THURSDAY/... Big Picture... Zonal flow over the weekend, but this changes early next week as a digging upper trough sweeps south from the Canadian Prairies into the USA Plains and Great Lakes. This digging trough turns the flow over the Eastern USA out of the south and southwest, drawing milder air up over us as well as increasing moisture. The trough axis then moves across our area Wednesday with drier air moving in. Normal 500-mb heights in this area in early December are in the upper 540s and 550s. Forecast heights moderate to the higher/warmer side Sunday through Tuesday. These values then lower again Tuesday night through Thursday as the upper trough moves overhead. Expect a milder trend for early next week, and a return to colder temps midweek. Mass fields and thermal fields in the models are similar through Monday but then show some model-to-model differences midweek. This means moderate to high confidence through the early week, then diminishing confidence midweek. Daily Concerns... Saturday night and Sunday... High pressure builds over the region with sufficient subsidence to bring clearing skies and light wind. This will allow for radiational cooling Saturday night, With dew points in the upper single numbers and teens, that should allow temperatures to fall to similar values overnight...possibly around 20 near the coast. The high pressure slides offshore Sunday, making room for milder air to move north. The milder air will move earliest at the higher levels, which may bring some mid and high clouds Sunday afternoon. Overall a fair day. Temps at 925 mb and 950 mb were used to represent the lingering cold air near the surface. Those values will support max sfc temps in the 30s to low 40s. Increasing south to southwest flow Sunday night will bring increasing clouds. The flow itself will be strong with speeds increasing to 40-50 kt overnight. Not sure how much of this will reach the surface in gusts, but suspect the warm advection will work against any mixing. The 00Z GGEM brings light precipitation into our area late at night, while the 00z GFS and ECMWF keep precipitation just south of us. Guidance is more aggressive with POP values. The forecast will trim values below 15 pct for most of the night based on the dry air forecast in the nucleation zone for Sunday night, as seen in BUFKit soundings. With the warm advection in place, expect min temps in the early night followed by warming temps overnight. Temperatures will be above freezing all areas after midnight, so any late night precip should be as rain. Monday-Tuesday... Mild south-southwest flow will continue to bring warmer air and increased moisture. PW values will climb over 1 inch and RH fields show a deep column of moisture over us for Monday and Tuesday. Low level jet at 50-60 kt will support low level convergence and lift while also supplying the moisture. The GFS and ECMWF show a 150-kt upper level jet approaching on Monday. The GFS shows Srn New England in the left exit region for at least a part of the time while the ECMWF is a tad farther north. All of this points to good lift and good moisture both days, and a deep layer of above-freezing temps supports max sfc temps mainly in the 50s each day. Will forecast showers both days. Another concern will be winds. A low level southerly jet of 50-60 kt will linger overhead through the period and may bring gusts of 30 to 40 mph down to the surface each day. Gridded guidance is much more conservative. The forecast will increase winds from the grid guidance closer to 25 kt/30 mph, and it may need to go higher in later forecasts. The upper trough pushes a cold front through our area Tuesday night, with showers diminishing after passage. Wednesday-Thursday... Colder drier air moves in Wednesday. Cold advection will encourage mixing 35-45 mph winds to the surface in gusts. High pressure builds in by Thursday with fewer clouds and lighter winds. Mixing to 850 mb will tap -10C air Wednesday and -18C air Thursday. Will go with max temps 35-45 Wednesday and 20s to mid 30s Thursday. && .AVIATION /17Z FRIDAY THROUGH TUESDAY/... Forecaster Confidence Levels... Low - less than 30 percent. Moderate - 30 to 60 percent. High - greater than 60 percent. Short Term /through Saturday/...High confidence. Today...VFR initially, but cigs will lower to MVFR along with brief periods of light to moderate snow 19z-02z. Expecting mainly rain across the Cape and Islands and a period of a rain/snow mix at PVD. Potential for brief periods of IFR across northern MA, including BOS, with any heavier snow bands moving through. Precipitation ends from west to east Friday evening. Tonight...MVFR north of the Mass Pike, VFR to the south. Trend to VFR all areas between 00Z and 06Z with clearing skies after midnight. Saturday...VFR. Light NW winds. KBOS Terminal...High confidence in TAF. Moderate confidence after 18Z. KBDL Terminal...High confidence in TAF. Outlook /Saturday Night through Tuesday/... Saturday Night: Mainly VFR. Breezy. Patchy BR. Sunday: VFR. Breezy. Sunday Night: Mainly VFR. Windy with local gusts up to 30 kt. Patchy BR. Monday: MVFR/IFR conditions possible. Windy with local gusts up to 30 kt. Chance SHRA. Monday Night: Mainly MVFR, with local IFR possible. Windy with gusts up to 30 kt. SHRA likely. Tuesday: Mainly MVFR, with local IFR possible. Windy with local gusts up to 30 kt. SHRA likely. && .MARINE... Short Term /through Saturday/...High confidence. Today ... NW winds decrease and become SW through the morning and early afternoon before increasing again tonight. Winds on the southern waters gust 25-35 kts this evening into tonight as a clipper low moves across CT/RI and southeast MA today. Vsby will lower to 1 mile or less in snow. Seas 3-4 ft. Tonight... Gusty SW winds 25-35 kts decreasing through the night. Seas 8-10 ft on southern waters. Saturday... Clear skies and light winds. Seas 3-5 ft. Outlook /Saturday Night through Tuesday/... Saturday Night: Winds less than 25 kt. Seas locally approaching 5 ft. Sunday: Winds less than 25 kt. Seas less than 4 feet. Sunday Night: Low risk for Small Craft Advisory winds with local gusts up to 30 kt. Areas of seas approaching 5 ft. Monday: Low risk for gale force winds with gusts up to 35 kt. Local rough seas building to 5 to 10 feet. Chance of rain showers. Monday Night: Moderate risk for gale force winds with gusts up to 35 kt. Rough seas up to 13 ft. Rain showers likely. Tuesday: Low risk for gale force winds with gusts up to 35 kt. Rough seas up to 12 ft. Rain showers likely. && .BOX WATCHES/WARNINGS/ADVISORIES... CT...None. MA...None. RI...None. MARINE...Small Craft Advisory until 7 AM EST Saturday for ANZ232>235- 237. Small Craft Advisory until midnight EST tonight for ANZ236. Small Craft Advisory until 10 AM EST Saturday for ANZ250. Small Craft Advisory until noon EST Saturday for ANZ254. Gale Warning from 5 PM this afternoon to midnight EST tonight for ANZ255-256. && $$ SYNOPSIS...WTB/BW NEAR TERM...Frank/Chai SHORT TERM...BW LONG TERM...WTB AVIATION...WTB/BW/Chai MARINE...WTB/BW/Chai