KBOX issues Area Forecast Discussion (AFD) valid 2019-12-06T08:17:00Z 177 FXUS61 KBOX 060817 AFDBOX Area Forecast Discussion National Weather Service Boston/Norton MA 317 AM 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 2 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 air for the first half of the week. Parts of the area could even reach 60 degrees by Tuesday. However, this would be accompanied by a widespread rain event from Sunday night through Tuesday. A cold front moves through Tuesday night, followed by a return to cooler temperatures for Wednesday. && .NEAR TERM /UNTIL 6 PM THIS EVENING/... ***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/... Highlights... Dry through the weekend with unseasonably cold temperatures on Saturday and Sunday AM. Temperatures rebounding on Sunday. Rain spreading in early next week with temperatures trending upward. Potential for strong winds on Monday and Tuesday. Cooler temperatures return on Wednesday. Daily Concerns... Saturday and Sunday... Cyclonic flow on Saturday with a trough lifting northeast of the region. Good agreement between deterministic models with cold northwesterly flow aloft. Mid level ridge building into the eastern Great Lakes. The surface response will be high pressure building just to the west of New England. Am a bit skeptical on the current sky forecast given the upper level pattern. Typically with cyclonic flow, cold air advection and day time heating there is more in the way of diurnal cu, but opted to leaned on the consensus amongst guidance which keeps it sunny. Something to keep in mind for future updates to the forecast. High temperatures range from the upper 20s across the higher elevations in western and central MA to the mid 30s along the coast. Moderate to high confidence in the forecast. High pressure builds in over New England Saturday evening into early Sunday. This will bring clear skies and light winds. Expect strong radiational cooling due to this setup. Bumped down temperatures toward the 10th percentile of guidance which brings low temperatures into the single digits above zero across the MA/CT/NH border. Low temperatures elsewhere will generally be in the teens, but it will be slightly warmer along the coast with lows in the low to mid 20s. High confidence in the forecast. High pressure shifts to the east as the day progresses on Sunday. Winds aloft will shift to a southwesterly direction and advect warm air into the region. Skies will be partly to mostly sunny with highs rebounding into the mid 30s along the MA/NH/VT border and the low 40s across the coast and SE MA. High confidence in the forecast. Sunday evening through Wednesday... Pattern change beginning late on Sunday through the middle of next week. Shifting from a zonal pattern to a more amplified pattern. A positively tilted trough will dig from the Pacific Northwest Sunday evening into the Central Plains by Monday evening. There are still some differences between the GFS and ECMWF/GEM. The ECMWF/GEM feature a northern and southern stream trough where the weaker southern trough lags over the Southwestern US and the northern becomes nearly neutrally tilted over the Central US. The GFS quickly absorbs the southern stream trough. This does not have major implications on how things evolve other than the GFS developing a deeper low than than ECMWF/GEM over the central Great Lakes late on Monday into Tuesday. The overall progression of the system is in good agreement through Tuesday. The low lifts from the central Great Lakes into eastern Ontario and Quebec. This keeps New England in storm warm air advection and anticipate that precipitation will remain all rain. There could be some snow mixing in across western MA as colder air filters in as things wind down late on Tuesday into early Wednesday. Still concerned about winds and wind gusts on Monday and Tuesday with a 40 to 60 kt 925 hpa low level jet developing. Held off from bumping winds up due to some disagreement between the ECMWF and GFS where the jet sets up, but may need to do so in future updates. Not out of the question that there are a few rumbles of thunder Monday into Tuesday. Temperatures will be warm on Monday and Tuesday due to the strong warm air advection. High temperatures will generally be in the 50s. Think there is a possibility for some 60 degree readings if strong winds develop and we tap into the mixed layer. Cooler temperatures return on Wednesday as the system lifts into Quebec. Flow will become northwesterly advecting cold air into the region. Highs on Wednesday drop into the low 30s across the Berkshires and the low 40s along the coast and SE MA. Low to medium confidence in the forecast. && .AVIATION /08Z 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. 06Z update... This morning ...VFR conditions with cigs around 40-80kft. Gusty winds gradually diminishing. Westerly gusts of 20-25 knots possible before the boundary layer decouples. Today...VFR initially, but cigs will lower to MVFR along with brief periods of light to moderate snow 18z-00z. 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. 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, with local IFR possible. Breezy. Patchy BR. Sunday: VFR. Breezy. Sunday Night: Mainly VFR, with local IFR possible. 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. 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. 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 6 AM EST early this morning for ANZ230-231-251. 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...BL/BW NEAR TERM...BW SHORT TERM...BW LONG TERM...BL AVIATION...BL/BW MARINE...BL/BW