Area Forecast Discussion
Issued by NWS Boston, MA

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000 FXUS61 KBOX 221120 AFDBOX Area Forecast Discussion National Weather Service Boston/Norton MA 720 AM EDT Sun Apr 22 2018 .SYNOPSIS... High pressure remains in control through Tuesday bringing dry weather with moderating temperatures. Wet weather returns Wednesday into Thursday as low pressure moves through our region. Unsettled weather continues into the weekend, but not a washout. && .NEAR TERM /UNTIL 6 PM THIS EVENING/... 7am update... SKC conditions. Increased daytime MAX temps a bit based on both current trends and expected mixing, which is likely to exceed H85 away from sea breezes. Otherwise, forecast on track. Previous discussion follows... High pressure builds over the region today. Moisture cross sections show a moist layer between 750 mb and 850 mb this afternoon. This may be enough to generate a few clouds above 5000 feet, but otherwise a sunny day. The light pressure gradient will mean light winds today, but will also be favorable for sea breezes developing along the coast during the late morning/midday and continuing through afternoon. Mixing yesterday reached to nearly 750 mb, per 00Z soundings. Model cross sections show mixing to 800 mb today. Temperatures at that level will be -5C to -6C, which is equivalent to 0C to -1C at 850 mb. Fully mixed, this would favor max sfc temps in the upper 50s and around 60. Mixing a little deeper would support max temps 60-65. Cooler temperatures near the coast due to the sea breeze. && .SHORT TERM /6 PM THIS EVENING THROUGH 6 PM MONDAY/... High pressure remains overhead tonight and Monday. Expect radiational cooling tonight, bringing another chilly night with areas of frost, even some temps below freezing. Sunny skies on Monday with light wind, which will again allow developing sea breezes. Similar deep mixing to today, but with temperatures a few degrees warmer and supporting sfc max temps in the low to mid 60s away from the cooler coast. && .LONG TERM /MONDAY NIGHT THROUGH SATURDAY/... */ Highlights... - Wet and breezy midweek - Perhaps a reprieve Friday - More showers possible Saturday Overview and model preferences... Phasing S stream and Pacific waves will lead to gradual height falls and reduction of the ridge across NE throughout the day on Tue and into Wed. Another longwave trof gradually develops across the CONUS as a result during the latter half of the week. Following the initial development, a second stronger wave connected to the Aleutian vortex will likely deepen the trof by next weekend, yielding a return to cooler, and maintaining the unsettled and wet conditions that look to begin on Wed. Noting a better agreement between deterministic runs in the phasing process and deepening of the wave for mid week than previous runs. Therefore, although ensemble means will still be used, will blend in more operational guidance with this forecast update. Details... Tue... High pres slips E but remains in control through the day and much of the evening. However, a combination of developing return flow, and increased moisture loading aloft will likely yield increasing clouds. Dry WX prevails. H85 temps near +4C should be mixed too, so expecting another day in the 60s in spite of the clouds. S shore sea breezes likely, with a risk on the E coast as well, but increase pres gradient may preclude this. Wed and Thu... Low pres develops out of the remnant cold pool from convection across the SE, as S stream wave develops. Guidance in fair agreement on track, E of the spine of the Appalachians as low pres deepens enough to draw it more N of E. With it, PWATs increase to nearly 200 percent of normal. As the low deepens, an LLJ of 40 kt also develops through the morning Wed. This will lead to a period of rain, potentially heavy at times as K indices exceed 30 and soundings become conditionally unstable. Initial overrunning followed by mid low and mid lvl convergence will lead to rain most of the day. With the wave continuing to deepen, the low pres will deepen and slow, leading to wet conditions persisting into the overnight hours, with wrap around SHRA possible into Thu, but with some improvement. Probs of QPF exceeding 1.00 inches rather high. Clouds along with a dip in H85 temps will yield cooler temps each day, with highs mainly in the 50s and lows in the 40s. Fri and Sat... Although trof remains in place, a brief period of rising heights looks possible ahead of a secondary/acute wave traversing the CONUS from the Aleutian low and NW Territory origins. Therefore, a period of more dry-than-wet conditions looking more likely per ensemble/operational guidance. Still with cyclonic flow and some moisture clouds and a spot shower can be ruled out. Temps near seasonal normals, although potentially cooler if more cloud than sun is observed. Early next week... The aforementioned wave reinforces the trof across the E, with a return to cooler and wetter wx possible. Still several details to iron out, but confidence rises based on ensemble means/operational runs showing at least some synoptic agreement. && .AVIATION /12Z SUNDAY THROUGH THURSDAY/... Forecaster Confidence Levels... Low - less than 30 percent. Moderate - 30 to 60 percent. High - greater than 60 percent. Short Term /through Monday/...High confidence. VFR through the period with light wind. Coastal seabreezes will develop midday/afternoon, dissipate in the evening, then redevelop Monday midday/afternoon. KBOS Terminal...High confidence in TAF. Moderate-high probability of a sea breeze midday/afternoon. KBDL Terminal...High confidence in TAF. Outlook /Monday Night through Thursday/...High confidence. Monday Night through Tuesday: VFR. Tuesday Night: MVFR/IFR conditions possible late. Breezy. Chance RA. Wednesday: Mainly MVFR, with areas IFR possible. Breezy. RA. Wednesday Night: Mainly MVFR, with areas IFR possible. Breezy. Chance SHRA, chance RA. Thursday: Mainly MVFR, with areas VFR 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 Monday/...High confidence. Light winds and seas. Winds turning onshore near the coast midday/afternoon and again Monday midday/afternoon as sea breezes develop. Outlook /Monday Night through Thursday/...High confidence. Monday Night through Tuesday: Winds less than 25 kt and weak seas. Tuesday Night: Winds less than 25 kt. Seas locally approaching 5 ft. Chance of rain. Wednesday: Small Craft Advisory winds with gusts up to 25 kt. Areas of rough seas. Rain and fog. Wednesday Night: Winds less than 25 kt. Rough seas up to 12 ft. Chance of rain showers, chance of rain. Thursday: Winds less than 25 kt. Rough seas up to 9 ft. Slight chance of rain showers. && .BOX WATCHES/WARNINGS/ADVISORIES... CT...None. MA...None. RI...None. MARINE...None. && $$ SYNOPSIS...WTB/Doody NEAR TERM...WTB/Doody SHORT TERM...WTB LONG TERM...Doody AVIATION...WTB/Doody MARINE...WTB/Doody

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