KBOX issues Area Forecast Discussion (AFD) valid 2019-12-04T12:15:00Z 188 FXUS61 KBOX 041215 AFDBOX Area Forecast Discussion National Weather Service Boston/Norton MA 715 AM EST Wed Dec 4 2019 .SYNOPSIS... Chilly and unsettled weather is expected through Thursday. A passing disturbance aloft will increase clouds with some passing light snow showers or flurries today into early tonight, with another disturbance aloft passing south of Southern New England on Thursday. A cold front passing through on Friday with light snow showers will usher in dry, but unseasonably cold weather for Saturday. A significant moderation in temperatures occurs early next week with a period or two of rain Sunday night through Tuesday. && .NEAR TERM /UNTIL 6 PM THIS EVENING/... Dry in lower levels as shown by 8-15F dew point depressions at surface and mid-level clouds above. Lower clouds moving in from the west suggest some moistening this morning. Light echoes on BOX radar moving in with the clouds. Temperatures at 7 AM were mostly in the 20s. So any precipitation that reaches the surface may produce slippery spots. A Special Weather Statement will be reissued for the morning. Otherwise, what we see is what we get. Lots of clouds, spotty light precip mainly as light snow. No significant changes in the forecast. Previous discussion... Otherwise...A surface ridge of high pressure has built into Southern New England early this morning, though it is something of a "dirty ridge" as morning satellite depicts a considerable amount of warm advective cloudiness spreading eastward from the Hudson Valley and into the Poconos. These clouds are upstream of a compact but rather well defined shortwave trough seen in WV imagery rotating through eastern Ontario into western NY. Weak isentropic lift should result in an gradual lower/increase in clouds areawide today, and relatively limited insolation by midday will act to keep projected temperatures on the cooler side of temperature guidance. Regional radar across much of eastern NY into northern PA indicates some weak precip echoes (associated with trace one-hourly precip being reported by most METAR observations) advancing eastward. Only the NAM-based guidance simulated reflectivity brings some spotty coverage of echoes eastward through the first part of the day. Though model soundings show there is enough, albeit pretty shallow, moisture to be wrung out for some precip to fall, due to a partly- saturated ice crystal activation layer seen in the 00z ALB sounding and model forecast soundings, it's questionable in what form that may take. Though I think flurries or very light snow showers would predominate in most locations today, there's a low probability that very light freezing drizzle could result where temps are below freezing. Prospect of freezing drizzle is too low-probability at the moment for any statements, but could be needed if it looks more likely. Model QPF per consensus is next to nil areawide, and that meshes with what's occurring upstream. Continued cloudy, though trending dry for the second half of the day. With clouds through the day and snowpack around in most areas, albeit with modest warm advection, expect temps to climb a bit slower with highs mainly in the mid to upper 30s, and near freezing over the Berkshires where snowpack is deeper. && .SHORT TERM /6 PM THIS EVENING THROUGH 6 PM THURSDAY/... Tonight: Lead upper level disturbance lifts northward into Quebec, with another such disturbance right on its heels. However this one digs southward into the waters south of Long Island by Thursday morning. Cyclonic flow aloft is maintained across Southern New England regardless, with light WSW surface winds veering to WNW through tonight and overnight. A considerable amount of clouds will remain, with temperatures a bit slower to fall given the anticipated extent of cloud cover. Could even see some lake effect flurries late into the east slopes of the Berkshires, though better potential is on the other side of the ridgeline. Kept lows in the mid/upper 20s, though lower 20s in the Berkshires/NW MA. Thursday: Another generally weak upper-level disturbance and weak frontal system will pass to our north. This feature will mainly just keep mostly to partial cloudiness across the area for the first part of the day versus produce any precip. Best chance for any snow showers is mainly to our N and W in lake effect bands. NWly pressure gradient becomes tighter during the second half of the day and with cold advection, most areas should turn breezy/blustery. Looking for gusts in the lower 20s over the North Shore, but into the 25-30 mph range across northern CT/northern RI northward into central/western MA, with 30-35 mph gusts out on the Cape. Milder lows in the upper 30s to low 40s, with a few low-mid 30s highs in the Worcester/Tolland hills and into the Berkshires. && .LONG TERM /THURSDAY NIGHT THROUGH TUESDAY/... Big Picture... Two jet streams, north and south, in a near zonal flow with shortwaves in each stream moving through the flow. A shortwave ridge builds over us over the weekend, resolving the split jets into a single jet which moves up through the Ohio and St Lawrence Valleys early next week. Normal 500-mb heights in this area in early December are in the upper 540s and 550s. Forecast heights are in the 530s and lower 540s Friday, but moderate to the 560s Sunday and early next week. Expect cooler than normal deep-layer temps Friday-Saturday, moderating to above normal temperatures by Monday. Mass fields and thermal fields in the models are similar through Monday but show some model-to-model differences next Tuesday. This means moderate to high forecast confidence through the weekend, with moderate confidence on Tuesday. Daily Concerns... Thursday night... Upper trough moves off to the east. Surface high pressure builds in with subsidence and resulting drying. Dew points will be 15-25F with winds becoming light enough under the high to allow min temps in the upper teens and 20s. Friday-Saturday... Zonal flow prevents any substantial moisture from moving north into our area. A shortwave moves east from the Great Lakes, pushing a cold front across New England Friday night. Moisture cross-sections show shallow moisture, mainly below 700 mb, and PW values around 0.25 to 0.50 inches. Lift in the moist layer is mediocre. Expect increasing clouds Friday with a few light snow showers possible. The front moves off to the east Friday night with winds shifting from the northwest and clearing skies. Mixing reaches to 925 mb, where level temps of -4C to -6C support max sfc temps in the 30s. The air then dries Friday night with dew pts falling to the teens away from the coast. Temps should fall to 15-25F. High pressure builds in Saturday with dry air and mixing again to 925 mb. Winds in the mixed layer will be around 20 kt, so expect gusts to that speed. Temps in the mixed layer will support max sfc temps in the mid 20s to lower 30s. A few mid clouds Saturday night, as the leading edge of milder air moves north above the surface. Colder air lingers at the surface with readings falling to the teens and lower 20s. Sunday through Tuesday... Surface high pressure moves offshore Sunday. Upper shortwave ridge builds over our area early, then shifts offshore Monday and Tuesday. This suggests sufficient subsidence for a dry sunny day Sunday. Warm advection will limit the depth of the mixed layer, but low level temps aloft support max sfc temps of 35 to 45. Moisture increases in the developing south-southwest flow aloft during Sunday afternoon, so expect increasing clouds at that time and cloudy skies Sunday night through Tuesday. After an evening cool- down, expect increasing clouds and warm advection to bring rising temperatures Sunday night. Moisture deepens Monday and Tuesday with PW values climbing over 1 inch. With the lack of a ridge or other feature to hold in the cold air, this looks to be primarily rain. But if the rain moves in before the cold surface air can be removed, then there could be an initial period of sleet or freezing rain in Western and Northcentral MA Sunday night before a change to rain. As this is well in the future, it will need to be monitored. && .AVIATION /12Z WEDNESDAY THROUGH SUNDAY/... Forecaster Confidence Levels... Low - less than 30 percent. Moderate - 30 to 60 percent. High - greater than 60 percent. Short Term /through Thursday/...High confidence. Today: Mid level cloud bases in the east, lower ceilings moving out of the Berkshires. Expect a lowering trend during the morning and midday. May see light snow showers or flurries but not expected to cause restrictions. Tonight through Thursday: Continued BKN/OVC VFR ceilings. W-WSW winds initially early tonight 5-8 kts become WNW/NW tonight. WNW winds then increase to 8-12 kts with gusts 20-25 kts Thursday, near 30 kt towards the Cape. KBOS Terminal...High confidence in TAF. KBDL Terminal...High confidence in TAF. Outlook /Thursday Night through Sunday/... Thursday Night: VFR. Windy with local gusts up to 30 kt. Friday: MVFR/IFR conditions possible. Breezy. Chance SHSN. Friday Night: Mainly VFR, with local IFR possible. Windy with local gusts up to 30 kt. Slight chance SHSN. Saturday: VFR. Windy with local gusts up to 30 kt. Saturday Night through Sunday: VFR. Breezy. && .MARINE... Short Term /through Thursday/...High confidence. Small crafts still continue on the outer waters, where buoys report 5-7 foot seas. Winds are mostly below 25 kt. Expect conditions to lighten through the day, with the remaining small craft headlines eventually dropped/expired. Today: Initial WNW/W winds 10-15 kts gusts to 25-28 kts offshore, shift to WSW and decrease to 10 kts by afternoon, gusts to 15 kts. Seas initially 5-8 ft, though steady decreasing trends to 2-4 ft late in the day. Tonight: W-SW winds around 10 kts early become northwest around 10-15 kts, gusts 20-22 kts on southern waters. Seas 2-4 ft. Thursday: NW winds increase on Thursday. Likely to need small craft advisories for Thursday for most waters once existing SCAs drop. Low prob of gale-force gusts southern waters Thursday afternoon. Seas build to 5-7 ft on offshore waters, 2-4 ft coastal waters. Outlook /Thursday Night through Sunday/... Thursday Night: Low risk for gale force winds with gusts up to 35 kt. Local rough seas. Friday: Low risk for Small Craft Advisory winds with gusts up to 30 kt. Areas of seas approaching 5 ft. Chance of rain showers, chance of snow showers. Local visibility 1 to 3 nm. Friday Night: Low risk for gale force winds with gusts up to 30 kt. Areas of seas approaching 5 ft. Chance of snow showers. Visibility 1 to 3 nm. Saturday: Low risk for gale force winds with gusts up to 30 kt. Local rough seas. Saturday Night: Low risk for Small Craft Advisory winds with gusts up to 25 kt. Seas locally approaching 5 ft. Sunday: Moderate risk for Small Craft Advisory winds with gusts up to 30 kt. Local rough seas. && .BOX WATCHES/WARNINGS/ADVISORIES... CT...None. MA...None. RI...None. MARINE...Small Craft Advisory until 11 AM EST this morning for ANZ250- 254. Small Craft Advisory until 6 PM EST this evening for ANZ255- 256. && $$ SYNOPSIS...WTB/Loconto NEAR TERM...WTB/Loconto SHORT TERM...Loconto LONG TERM...WTB AVIATION...WTB/Loconto MARINE...WTB/Loconto