370 FXUS61 KBOX 280822 AFDBOX Area Forecast Discussion National Weather Service Boston/Norton MA 422 AM EDT Sat Sep 28 2019 .SYNOPSIS... High pressure will continue to be in control for the first part of Saturday. A cold front will produce a period of showers areawide tonight, with a few thunderstorms in Connecticut and Rhode Island. This front will be followed by cooler but seasonable weather Sunday and Monday. Summer-like warmth returns later Tuesday into Wednesday ahead of a cold front. The front sweeps through late Wednesday or Wednesday night with showers and possibly a thunderstorm along the front. Much cooler weather follows for Thursday and Friday. && .NEAR TERM /UNTIL 6 PM THIS EVENING/... Center of surface high pressure is off the waters east of Nantucket, with a light southwesterly gradient flow across much of southern New England. This gradient tightens as one moves westward towards western NY into the lower Great Lakes where a cold front is responsible for organizing clusters of showers and weakening thunderstorms. This front will serve as a focus for sensible weather across Southern New England mainly after sundown. Early this morning, with another evening of good radiational cooling, localized areas of fog have formed in the northern CT and Deerfield River valleys in western MA per GOES nighttime microphysics RGB imagery. Fog has been less prevalent in the favored radiational fog areas in eastern MA. Any fog areas will still continue for a few more hours until winds pick up and the sun comes up. Overall looking at a pretty nice Saturday, with dry weather and quite mild temperatures under southwest winds with mostly sunny skies trending towards filtered sunshine late. By afternoon, southwest breezes to around 20-25 mph are expected for most areas. Dewpoints/moisture levels will also be on the rise moving into the afternoon hrs, to values in the upper 50s to mid 60s - not oppressive, but noticeable. This increase in moisture quality will also manifest itself in increasing precipitatable water values to around 1-1.5", highest west. Highs should reach well into the 70s to lower 80s inland, with mid 70s toward the southeast coast. && .SHORT TERM /6 PM THIS EVENING THROUGH 6 PM SUNDAY/... Tonight: Per most guidance, cold front looks to sweep across much of Southern New England after sundown. Driven mostly by low-level moisture advection along and ahead of the front, most models and the HREF ensemble depict a ribbon of 200-800 J/kg of most- unstable parcel CAPE (MUCAPE) even into the nighttime hours - this MUCAPE axis being maximized across much of CT into RI, and decreases with northern and eastern extent. Model soundings indicate this weak instability is also elevated in nature with roughly moist-adiabatic lapse rates. In addition, precipitable water values range from the 1.5 to 1.8" range with surface dewpoints in the low to mid 60s areawide. Think we'll end up seeing a cluster of thunderstorms from the Hudson Valley into mainly Hampden-Hampshire Counties in western MA into the Hartford metro area between 00-02z, and this cluster of storms will likely begin to weaken as they progress through the remainder of extreme south-central MA into eastern CT and RI through the 02-04z timeframe. At the moment, the biggest source of uncertainty in the forecast is the coverage of storms especially north of the Mass Pike towards the Merrimack Valley into the Boston suburbs. The 00z WRF-ARW and NMM depict a pretty robust line of heavy showers with possible embedded thunder further north/across a larger part of MA after 02z/10 PM. Though it's an outcome that can't be outright ignored, it doesn't have support from synoptic models, the 3-km NAM or the HREF ensemble mean. The official forecast keeps thunder mainly to CT and into RI. Given the high PWAT values, did include enhanced wording for heavy rainfall in this aforementioned corridor mainly for the potential for some urban/poor drainage flooding in heavy downpours. With the weak instability and that it is elevated, not expecting severe storms at this point (SPC's Day-1 outlook paints a Marginal Risk west of western MA/CT) - though a particularly strong core moving in from eastern NY could produce small sub-severe hail. Time of day and elevated nature to instability would limit any convective gusts to the surface. Any thunder is expected to weaken before reaching SE Mass, the Cape and the Islands with showers. Though localized downpours in thunderstorms could produce a half inch or so of rain, most areas will see a few hundreths to a tenth of an inch of rain. Should see clearing occur for the second half of the night across the interior as ridging noses in, though mostly cloudy skies and lingering showers towards SE Mass and the Cape. Enough of a northwest component to the winds should preclude fog even in spite of wettened ground. Pretty wide variation in lows, with low-mid 50s in NW Worcester and Franklin Counties, mid/upper 50s in interior CT, and upper 50s to mid 60s in RI/SE Mass, mildest towards the urban centers and on the Cape. Sunday: Still likely will have some lingering cloudiness towards the Cape, but cold and dry advection occurs across the interior as strong ridging builds in. Rather robust pressure gradient builds southeastward on the leading edge of the advancing ridge, with breezy north/north-northeast winds across the coastal waters, and gusts eventually developing across the Cape and the Islands as clouds clear and the PBL can mix deeper. Gradient slackens thru the afternoon, earliest in the interior and towards sundown towards SE Mass and the Cape and Islands. Looking at dry weather though more sun for the second half of the day, with highs into the mid/upper 60s towards the coast, and lower 70s inland. && .LONG TERM /SUNDAY NIGHT THROUGH FRIDAY/... Big Picture... Upper pattern through Monday shows a ridge over the Southeast USA and closed low and trough over the Western USA. Main jet path at this time will be from California to the Great Lakes and then across Eastern Canada. The closed low in the west ejects into Canada Monday night and merges with a closed low over Hudsons Bay. The large-scale trough in the west then moves east, reaching the Great Lakes Thursday and New England Friday. 500-mb heights are forecast in the mid to upper 580s, well above normal, for most of the forecast period, then dip through the 570s Thursday and Friday. So the deep layer should be warmer than normal through midweek. The surface layer is more variable, with a cooler flow Monday, a warmer southwest flow by midweek, then colder air Thursday and Friday. Model mass fields are similar through Thursday, then show differences as the remnants of the western trough move over New England Friday. High forecast confidence through Wednesday, then diminishing to moderate by Friday. Daily Concerns... Sunday night-Monday... Canadian high pressure extends over Southern New England and maintains dry air and light wind. As the ridge will be centered to our north, we expect a light east-northeast flow. With dew points below 50 and radiational cooling inland, we expect low temps mainly in the 40s with daytime highs in the 60s. Coolest temps along the east MA coast, warmest within the vicinity of Hartford. Tuesday... Jet max moves across Eastern Canada but stays well north of our area. Low level winds show a 30-40 kt west jet moving across the Great Lakes and Northern New England. This is where the strongest push of warmer air and resulting lift will be, likely in the form of a warm front. That area will have the most clouds and best chance of showers. Northern Massachusetts is on the edge of the support for clouds/showers and may see a few showers, but it is hard to support showers farther south. Dew points climbing to 45-50 and increasing clouds support morning min temps of upper 40s to low 50s. Daytime mixing to 925 mb and temps at that level near 16C support surface max temperatures in the 70s. Wednesday to Friday... Confidence in the model solutions begins to diminish at this point. But all show the southeast USA ridge diminishing and the 500-mb heights over us trending lower. This should help push a cold front south from Canada later Wednesday and Wednesday night. This would bring us a chance of showers as it moves through. The remnants of the Western trough arrive by Friday and would support a second round of showers. Some model guidance brings wet weather in on Thursday. We went with the drier Thursday solution based on the position of the upper trough at that time. Temperatures Wednesday will depend on timing of the cold front. Mixing to 925 mb and temps at that level around 20C in the warm sector would support max surface temps in the low 80s, but if the cold front is faster to move through then the high temps would be several degrees lower. Behind the cold front, winds turn from the northeast and east, which will bring colder temperatures to the region...clouds and showers Friday would maintain the colder temperatures. Expect daytime highs in the upper 50s and 60s, nighttime lows in the 40s. && .AVIATION /08Z SATURDAY THROUGH WEDNESDAY/... Forecaster Confidence Levels... Low - less than 30 percent. Moderate - 30 to 60 percent. High - greater than 60 percent. Short Term /through Sunday/...Moderate confidence. 09Z Update: Today...VFR. SW winds around 10-15 kts with gusts to 20-25 kts. Local sea breeze development not expected. Saturday Night: Mainly VFR, with local MVFR possible with cold frontal showers, possible TSRA. Best chance of SHRA/isolated TSRA & brief lower flight categories towards BAF, BDL between ~01-03z and PVD ~03-05z. Lighter -SHRA towards SE Mass, Cape and Islands after 06z. Windshift from SW to to W under 10 kts west of a BOS-PVD line thru 06z, and eventual wind shift toward Cape Cod after 06z. Sunday: Mainly VFR, brief MVFR possible at FMH/ACK/HYA. N winds become gusty on the Cape and Islands (20-30 kt) by mid-AM; N winds elsewhere mainly under 10 kts. KBOS Terminal...High confidence in TAF. Increasing SW winds today to preclude sea breeze development. Low chance at isolated TSRA ~02-04z. KBDL Terminal...High confidence in TAF. Better chance at isolated TSRA ~01-03z with possible brief visby restrictions. Outlook /Sunday Night through Wednesday/... Sunday Night through Monday: VFR. Monday Night: Mainly VFR, with local MVFR possible. Slight chance SHRA. Tuesday through Tuesday Night: Mainly VFR, with local MVFR possible. Breezy. Slight chance SHRA. Wednesday: Mainly VFR, with local MVFR possible. Breezy. Chance SHRA. && .MARINE... Forecaster Confidence Levels... Low - less than 30 percent. Moderate - 30 to 60 percent. High - greater than 60 percent. Short Term /through Sunday/...High confidence. Small craft advisories continue for the waters northeast of Mass Bay from southwest winds. On Sunday, strong pressure gradient will likely produce northerly winds to small craft levels on most of the eastern waters into the southern outer waters south of the Islands. Small craft advisories have been hoisted (and existing ones extended) for most of these waters until late Sunday afternoon for these expected conditions. Today...SW winds strengthen to 15 kts on most waters, gusting to near small craft levels by afternoon. Gusts to 25 kts expected in the waters north and east of Cape Cod. Seas build to 3-5 feet, greatest on the open waters northeast of Mass Bay. Good visibility. Saturday Night...SW winds less than 25 kt for much of the night. A cold front brings a period of showers to the waters after midnight tonight. Winds will shift to northwest and steadily increase to 20-25 kt. Seas locally approaching 5 ft. Sunday: N/NE winds 15-20 kts with gusts to 25-30 kts across most of the coastal and outer waters. Marginal small craft wind gusts towards the waters east of Montauk. Winds to slacken late in the day. Seas 3-5 feet. Outlook /Sunday Night through Wednesday/...Moderate confidence. Sunday Night through Monday Night: Winds less than 25 kt. Seas up to 5 ft. Tuesday: Winds less than 25 kt. Seas up to 5 ft. Slight chance of rain showers. Tuesday Night: Winds less than 25 kt. Areas of rough seas. Slight chance of rain showers. Wednesday: Winds less than 25 kt. Areas of rough seas. Chance of rain showers. && .BOX WATCHES/WARNINGS/ADVISORIES... CT...None. MA...None. RI...None. MARINE...Small Craft Advisory from 10 AM to 5 PM EDT Sunday for ANZ232- 255. Small Craft Advisory from 8 AM to 5 PM EDT Sunday for ANZ231- 251. Small Craft Advisory from 2 PM this afternoon to 5 PM EDT Sunday for ANZ250-254. && $$ SYNOPSIS...WTB/Loconto NEAR TERM...Loconto SHORT TERM...Loconto LONG TERM...WTB AVIATION...WTB/Loconto MARINE...WTB/Loconto