Area Forecast Discussion
Issued by NWS Boston, MA

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428 FXUS61 KBOX 070325 AFDBOX Area Forecast Discussion National Weather Service Taunton MA 1025 PM EST Tue Dec 6 2016 .SYNOPSIS... Two low pressure centers will miss Southern New England tonight, one to the northwest, the other well to the south. In between some light snow and rain will impact the region early in the morning tomorrow, giving way to drying by later in the day. A cold front crosses the region Thursday with little more than a brief passing rain/snow shower or two. Mainly dry but unseasonably cold temperatures follow Friday and Saturday. Low pressure may bring a period of mainly rain to the coastal plain with a wintry mix more likely across the interior sometime Sunday into Monday. && .NEAR TERM /UNTIL 6 AM WEDNESDAY MORNING/... 1025 PM update... Still dealing with rather large dew point depressions of about 5-10 degrees. The presence of this dry air has slowed the eastward progress of steadier precipitation. Forecast still appears to be on track, with most of the light snow falling across the higher terrain of interior southern New England, and mostly rain toward the coasts. Expecting a brief burst in the precipitation right around midnight, then gradually tapering off through the early morning hours. Tweaked the forecast to bring it back in line with observed trends. Still working on the timing and location of steadier precipitation. It`s been a moving target this evening. 655 PM update... Ptype will be mainly rain on the coastal plain given warm enough boundary layer temps. Probably a mixture of rain/snow across the lower elevations of the interior, but little if any accums across lower elevations of southwest MA/CT. Mainly snow expected across the higher terrain of the Worcester Hills and especially east slopes of the Berks. Accums generally a coating to 1 inch, but localized 2 inch amounts are possible across the highest terrain along east slopes of the Berks. This is where the special weather statement has been issued as untreated roads may be slippery for the Wed AM commute. Previous Discussion... Early overrunning and upper lvl moisture flux already evident in the form now nearly BKN-OVC CI across areas SW of the Worcester hills and points SW. Noting very wide precip shield mainly near and SW of the NYC metro, but bulk of this is sliding E rather than NE as it encounters the drier air at mid and low lvls thanks to subsidence inversion within remnant high pres nose. The bulk of the moisture field (where PWATS near or exceed 1.0 inches) will remain well S coincident with developing cyclone S of the Delmarva through 00Z. Still with upper lvl cutoff approaching from the NW, expecting the development of modest inverted trof and slight moisture flux where PWATs should approach 140 percent of normal by the early AM hours, as the column moisture loads slowly through the mid and low lvls. As this occurs, noting modest omega within the snow growth regime to suggest wet-bulb effect may occur as virga gives way to precip reaching the sfc. Areas inland of the coast wetbulb to near 30-32F where mins should reach. This initial wetbulb and omega within the snow growth suggest an initial thump of -SN inland beginning in CT near midnight and moving NE through about 4AM, however the omega weakens as the trof loses strength and the cyclone well to the S usurps some of the available moisture. Snowfall/Rainfall... With initial surge 10 pm-4AM local, and QPF values near 0.01-0.1 inches, areas inland of the coast should see mainly an inch or less of light snowfall during the early morning hours. Overall values should be generally less than we received Sun night into Mon. Marginal temps will limit these values and only within the E-slopes of the Berkshires do we expect 1+ inches, thanks to predominantly E flow enhancing the upslope flow. Near the shore, especially the E coast, mainly rain can be expected as the flow will be onshore, where SSTs hover in the upper 40s. Still some slippery areas for the AM commute in the interior high terrain of MA/CT where light snowfall could impact untreated roadways. Precip will be gradually ending due to the weakening and loss of moisture mentioned above after sunrise. Did lean a bit more widespread with at least a T of snow as mesoscale guidance is underdoing it with the omega available within the snow growth regime. && .SHORT TERM /6 AM WEDNESDAY MORNING THROUGH WEDNESDAY NIGHT/... Tomorrow... Early transition from snowfall/rainfall to mainly snizzle/drizzle due to loss of ice crystals in the column thanks to drying and cold advection early. Most of the precip should be winding down within a couple hours of sunset, and ending everywhere by mid day. This is thanks to ridge of high pres out of the SW regaining control and once again splitting the N and S streams. Still expect a fair amount of clouds even as precip ends mainly due to the cold advection aloft yielding some low-lvl steep lapse rates and trapped moisture. Therefore, leaned on the colder side of guidance in spite of the SW flow. Should only see highs in the upper 30s and low 40s (although a few breaks could allow for some mid 40s if they occur). Tomorrow night... Although ridging weakens through the overnight hours the continued flow of dry air should limit the impacts of warm advection under return SW flow which will veer to the W overnight. Trapped moisture will yield enough clouds to limit radiational cooling, therefore, will hold overnight mins in the upper 20s and low 30s predominantly, although some urban locations should stay above freezing during the overnight hours. && .LONG TERM /THURSDAY THROUGH TUESDAY/... Highlights... * Seasonable temps Thu with just a brief passing rain/snow shower * Unseasonably cold but mainly dry Fri into Sat * Period of mainly rain favored along the coastal plain with a wintry mix possible across the interior sometime Sun into Mon Details... Thursday... A cold front crosses the region on Thursday, but as is often the case the coldest air will lag a bit behind the front. High temps likely still recover into the upper 30s in the high terrain and lower to middle 40s on the coastal plain. Mainly dry weather anticipated, but a passing rain/snow shower or two are possible with the cold front particularly along the east slopes of the Berkshires. Thursday night and Friday... Much colder air will work into the region Thursday night and Friday as 850T drop to around -15C. Low temps Thu night should drop into the 20s in most locations with highs Fri remaining in the 30s. Northwest winds should also be gusty during the daylight hours Fri. Dry weather dominates Thu night/Fri, but a brief passing flurry/snow shower or two can not be ruled out with remnant Lake Effect Moisture. Highest risk should be along the east slopes of the Berks as is typically the case is in these situations. Friday night and Saturday... Large high pressure builds across the region Friday night into Saturday, bringing the coldest air of the season. Light winds/mainly clear skies should allow low temps to fall well down into the teens across most locales Friday night. Despite plenty sunshine Saturday, expect high temps to only reach the upper 20s to the lower 30s. Sunday into Monday... High pressure moves off the coast allowing for a return flow of milder air to work into southern New England. At the same time, low pressure will lift up towards the Great Lakes. This will generate southwest flow at the surface and allow the airmass to quickly modify along the coastal plain, without a lingering high pressure across eastern Canada. So while a brief period of snow can not be rule out along the coastal plain, odds would favor a quick change to rain especially given relatively mild SST this time of year. Across the interior, certainly a better shot for an extended period of snow/ice particularly if secondary low pressure develops along the coast. Nothing is set in stone at this time range, but the potential for at least a period of snow/ice exists across the interior sometime next Sun into Mon. Tuesday... Low confidence at this time range but will keep temps near seasonable levels. Ensemble guidance indicating potential for a shot of very cold air by next Wed/Thu, but still a ways out there. && .AVIATION /03Z WEDNESDAY THROUGH SUNDAY/... Forecaster Confidence Levels... Low - less than 30 percent. Moderate - 30 to 60 percent. High - greater than 60 percent. Tonight...Moderate confidence. MVFR CIGS overspread southern New England through 6z. Light precipitation arrives between 3z and 6z across portions of northern CT/western MA and elsewhere after 6z. Ptype mainly rain on the coastal plain, a mixture of rain/snow across the lower elevations of interior, and mainly snow across the high terrain. Mainly MVFR conditions through 12z, but some brief IFR conditions possible especially across the interior high terrain. Tomorrow and tomorrow night...High confidence. Most of the snow/rain ends between 12Z and 14Z (some may linger a bit later). Conditions remain mostly MVFR due to low cigs through the afternoon, after which some clearing/lifting should lead to more widespread VFR conditions with CIG heights mainly 030-060. Winds shift to the SW. VFR lingers into the overnight. KBOS TAF...Moderate to high confidence in TAF. No threat for any snow at the terminal. KBDL TAF...Moderate to high confidence in TAF. A mixture of rain/snow may develop between 3z and 6z, but little if any accums expected on runway. Outlook /Thursday through Sunday/... Thursday and Friday...Moderate to high confidence. Mainly VFR but a brief period of marginal MVFR CIGS possible in some strato-cumulus clouds and a brief passing snow shower or two. Northwest wind gusts of 25 knots expected Friday. Friday night and Saturday...High confidence in VFR conditions. Sunday...Low to moderate confidence. Conditions may lower to MVFR- IFR thresholds sometime Sun or Sun night in some rain and/or snow. && .MARINE... Forecaster Confidence Levels. Low - less than 30 percent. Moderate - 30 to 60 percent. High - greater than 60 percent. Short Term /through Wednesday Night/... Confidence...High Winds, although veering from the E tonight, back to the W by late tomorrow, should remain below small craft advisory thresholds into tomorrow night. May be a few gusts around 20 kt early tomorrow morning as low pres passes well S of the waters, but this will be the closest things get. Seas too, remain generally at or below the 4 ft mark. Outlook /Thursday through Sunday/... Thursday and Friday...Moderate to high confidence. West to northwest wind gusts of 30 knots expect to develop late Thu into Fri in the strong cold advection pattern behind a cold front. Brief gale force wind gusts can not be ruled out. Saturday and Sunday...Moderate to high confidence. Left over small craft wind gusts/seas gradually diminish Saturday as high pressure builds in from the west. High pressure moves across the waters Sun, keeping winds/seas below small craft advisory thresholds. && .BOX WATCHES/WARNINGS/ADVISORIES... CT...None. MA...None. RI...None. MARINE...None. && $$ SYNOPSIS...Frank/Doody NEAR TERM...Belk/Frank/Doody SHORT TERM...Doody LONG TERM...Frank AVIATION...Frank/Doody MARINE...Frank/Doody is the U.S. government's official web portal to all federal, state and local government web resources and services.