Area Forecast Discussion
Issued by NWS Boston, MA

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000 FXUS61 KBOX 200801 AFDBOX Area Forecast Discussion National Weather Service Taunton MA 301 AM EST Mon Feb 20 2017 .SYNOPSIS... A brief return to seasonably cool weather occurs today through Tuesday with high pressure in control. Strong low pressure developing across the Central Plains slowly lifting towards the Great Lakes will result in an extended period of unseasonably mild temperatures Wednesday through Saturday. The cold front will cross the region on Saturday, likely bring some rain followed by more seasonable temps returning by Sunday. && .NEAR TERM /UNTIL 6 PM THIS EVENING/... Mid level trough axis amplifies east of southern New England as additional shortwave energy crosses the region. This will result in dry northwest flow today, but decent cold air advection will allow 850T drop to between -8C and -10C by afternoon. Column is fairly dry so plenty of sunshine. This combined with a rather mild start should allow highs in the upper 30s across the higher terrain...to mainly the lower to middle 40s elsewhere. Bufkit soundings show 20 to 25 mph wind gusts developing later this morning and afternoon as mixing increases. && .SHORT TERM /6 PM THIS EVENING THROUGH 6 PM TUESDAY/... Tonight... Large high pressure builds across southern New England tonight. The result will be mainly clear skies, except for some ocean effect clouds across the Cape/Nantucket as winds become northerly. Otherwise, light winds/mainly clear skies elsewhere should allow for overnight lows in the teens across the outlying locations with lower to middle 20s in the urban heat islands of downtown Boston/Providence. Tuesday... High pressure overhead Tuesday morning will slowly shift offshore by late in the afternoon. Plenty of sun expected during the morning...but some mid/high level cloudiness should move across the region from west to east during the afternoon. Mid level temperatures will be warming as height fields rise, but a chilly start and relatively weak mixing will keep high temps mainly in the lower to middle 40s. Weak pressure gradient should allow sea breezes to develop along the eastern MA coast and may keep highs in the upper 30s for those locations. && .LONG TERM /TUESDAY NIGHT THROUGH SUNDAY/... Highlights... * Seasonably cool Tuesday * Light rain/pockets of freezing rain Tue night into Wed * Thursday looks to be our warmest day * More likely of widespread rain Friday night into Saturday Overview and model preferences... The initially amplified and complex synoptic pattern across the E half of NOAM will gradually trend toward a more zonal (although with SW-NE orientation) toward the end of the week. This is mainly a response to an arctic influenced wave which should remain to our N as it meets the building heights within ridging across the northeast and Maritimes. Still noting good agreement amongst both ensemble and deterministic guidance that positive 500hPa height anomalies remain through the long term, suggesting the influence of warmer air and a quieter pattern overall. This is indeed the case amongst guidance save for a stronger wave arriving by late Fri-Sat. Given there is fair agreement across the board, will continue with the consensus blend used over the last few updates until some of the subtle, mesoscale features are better resolved. Details... Tue... Quiet wx remains thanks to high pres which will crest early Tue morning. Nearly isothermal profile with both H92 and H85 temps between 0C and +3C suggests warmer temps, in the mid 40s for the most part. Southerly flow with increasing moisture will yield more clouds through the latter portions of the day. Tue night into Wed... The differences in timing of a weak warm frontal passage and its attendant overrunning continue with the 19.12Z guidance update. GFS continues to be the faster model, but also the weakest and driest while, out of the operational runs, ECMWF/CMC are slower and slightly more robust. Noted a modest trend toward the latter solution in both GEFS/ECENS members, so timing/impacts will be more reflective of the ECMWF/CMC. Modest overrunning begins overnight and continues through the first few hours of daylight Wed. Isothermal profiles remain, with low lvl temps mainly near 0C to +3C. Bulk of the moisture (PWATs only around 0.7 inches) relegated to the lowest 200hPa. Initial wet-bulb processes could yield a mix of sleet/wet snow, but this will shift to mainly rain. Pockets of freezing rain possible, especially N Central and W MA, as temps may drop near or below freezing before precip onset, and wetbulbs will only be lower. Not expecting much in the way of ice, but enough that overnight, early AM commutes could be slippery on untreated roadways. Otherwise, improvement on Wed outside of some lingering clouds. Warming of H92 temps to nearly +6C to +8C by 00Z Thu suggests temps will rapidly rebound and may even approach the low 50s in spots especially if enough afternoon sunshine is observed. Thu and Fri... Modest clipper will move through N New England Thu. Although little to no impact expected (outside of some clouds), the brief increase in low-lvl temps in response, combined with a W component to the mean flow, may yield the warmest day of the week. H92 temps average +10C to +12C (as a comparison, today`s H92 temperatures are closer to +6C to +8C. Remaining snowpack, additional clouds may limit mixing somewhat, but all conditions point toward a day with widespread 60s possible. Ensemble probabilities are supportive of this thinking. Some slight cooling on Fri, especially with increasing clouds ahead of a warm front suggests highs mostly in the 50s at best. Timing of overrunning precip looks to hold until Fri night thanks to initial mesoscale ridging possible. Next weekend... Sharper wave currently wrapped up in the Aleutian vortex will be forced E and may start taking on a neutral or even negative tilt as it arrives in the OH Valley. Good agreement that parent low pres remains an inside runner, but it will have some connection to Gulf of Mexico moisture with PWATS approaching 1.3+ inches (3+ standard deviations above normal). Within the warm sector (Saturday) LLJ 40-05 kt, TT values near 50 and mid lvl lapse rates steeper than moist-adiabatic, therefore, will need to watch for pockets of heavy rainfall at times along with this feature passage. Can`t even rule out a rumble of thunder or two if conditions line up. Conditions improve Sun, with a return to much more seasonable cool temps following in the wake of this stronger system. && .AVIATION /08Z MONDAY THROUGH FRIDAY/... Forecaster Confidence Levels... Low - less than 30 percent. Moderate - 30 to 60 percent. High - greater than 60 percent. Short Term /through Tuesday/... Today...High confidence. Mainly VFR but some brief marginal MVFR CIGS are possible with the highest risk across Cape Cod. Northwest winds will gust to 20 knots later this morning and afternoon with up to 25 knots across portions of the Cape/Nantucket. Tonight and Tuesday...High confidence. VFR except some marginal MVFR CIGS possible into Tuesday morning across portions of Cape Cod. KBOS TAF...High confidence in TAF. KBDL TAF...High confidence in TAF. Outlook /Tuesday Night through Friday/... Tue night into Wed...Moderate confidence. Overnight low-moderate risk for some MVFR/IFR CIGS along with occasional MVFR vsbys in light rainfall. Low risk for some light icing with light freezing rain, mainly Worcester Hills and Western MA. Improvement to VFR gradually after sunrise on Wed. Winds shift to the W-SW. Thu and Fri...High confidence. Mainly VFR. Low risk for some mixed MVFR CIGS by late day Fri, but these may hold off until Fri night. && .MARINE... Forecaster Confidence Levels... Low - less than 30 percent. Moderate - 30 to 60 percent. High - greater than 60 percent. Short Term /through Tuesday/... Today...High confidence. Northwest wind gusts of 20 to 30 knots in the cold air advection pattern. The strongest of those winds gusts will occur across our eastern waters. Small craft headlines posted for all waters except Boston Harbor and Narragansett Bay. Tonight...High confidence. Lingering marginal small craft wind gusts/seas across our eastern waters this evening should diminish after midnight as high pressure builds in from the west. Tuesday...High confidence. High pressure overhead will result in light winds and flat seas. Outlook /Tuesday Night through Friday/... Tue into Wed...Moderate confidence. Quiet boating wx remains in place on Tue. Accompanied by some light rain early Wed morning, W-SW wind gusts may reach near the 25 kt threshold for small craft advisories, but then diminish by about mid day on Wed. Seas remain generally 5 ft or less, so there is a low risk that a brief period of small craft advisories may be needed Tue night into Wed morning. Thu and Fri...High confidence. Generally quiet boating weather even as winds shift from mainly W-SW to E by Fri. && .BOX WATCHES/WARNINGS/ADVISORIES... CT...None. MA...None. RI...None. MARINE...Small Craft Advisory until 7 PM EST this evening for ANZ231- 232. Small Craft Advisory until 1 PM EST this afternoon for ANZ233>235-237. Small Craft Advisory until 1 AM EST Tuesday for ANZ250-254-255. Small Craft Advisory until 9 AM EST this morning for ANZ251. Small Craft Advisory until 3 PM EST this afternoon for ANZ256. && $$ SYNOPSIS...Frank NEAR TERM...Frank SHORT TERM...Frank LONG TERM...Doody AVIATION...Frank/Doody MARINE...Frank/Doody

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