Area Forecast Discussion
Issued by NWS Boston, MA

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607 FXUS61 KBOX 201123 AFDBOX Area Forecast Discussion National Weather Service Boston/Norton MA 723 AM EDT Fri Apr 20 2018 .SYNOPSIS... An upper level disturbance over New England moves east to the Maritimes today. This may generate isolated light rain/snow showers, mainly over the higher hills. High pressure builds just west of New England today and Saturday band will bring gradually warming temperatures and dry weather into early next week. Storm system expected for mid to late week. && .NEAR TERM /UNTIL 6 PM THIS EVENING/... 7AM update... For the most part forecast remains on track this morning. Initial clearing will give way to typical diurnal CU. This will be especially true late this morning and afternoon as shortwave currently in S Quebec/N New England rotates through the cyclonic flow. Spot shower still possible with this, but too isolated to warrant increase in POPs. Previous discussion follows... Broad trough across the Northeast USA maintains cyclonic flow over Souther New England. A shortwave embedded in this flow moves south from the St Lawrence Valley, crossing our area during the morning. Similar timing on this feature on both the ECMWF and GFS. Moisture cross-sections show decreasing values and depth this morning and through the day. The initial lift from the shortwave may be enough to generate clouds, but less confidence on precipitation. Best chance for precip would be over the Berkshire East Slopes and possibly to the Connecticut River. Farther east, sprinkles if anything reaches the ground. Otherwise a dry day. Cold advection and cold temperatures aloft will allow mixing to around 850 mb, possibly a little higher. Winds in this layer will be 25-30 mph, so expect similar gusts developing late morning through afternoon. Temperatures at the top of the mixed layer will be -4C to -6C, suggesting max temps in the upper 40s and lower 50s. Clouds may keep the hills in the mid 40s, and if the coastal plain mixes a little deeper then temps there could reach 53F or 54F. && .SHORT TERM /6 PM THIS EVENING THROUGH 6 PM SATURDAY/... Tonight... High pressure builds to our west, maintaining a light pressure gradient over us and building subsidence over the region. Expect mostly clear skies and diminishing northwest winds. Dew points will be in the mid 20s to low 30s, so expect min temps in the low to mid 30s in Eastern Mass and Eastern RI, with 25-30 farther inland. Saturday... High pressure continues to build over the region. Skies should remain mostly clear. Mixing may bring afternoon wind gusts of 15-20 mph. Temperatures in the mixed layer may be a degree or two warmer aloft, so expect max temps in the upper 40s northwest and low to mid 50s most places. && .LONG TERM /SATURDAY NIGHT THROUGH THURSDAY/... Highlights... * Moderating temperatures and dry weather will prevail through the weekend and into next week * Next batch of rainfall will occur during the mid-week Overview and model preferences... Under the influence of eastward meandering cutoff through the weekend, the combination of below normal height, H85 temp and H92 temp anomalies suggest that, in spite of dry wx, temps will remain near or below normal until early next week. By Mon, ensembles support height anomalies above normal, and temps do respond, finally shifting to near/above normal as well as ridging follows in the wake of the exiting cutoff. By the middle portion of next week, a second cutoff slowly moving E in weak flow on the N periphery of the subtropical ridge looks to merge with some combination of both Pacific and arctic energy, allowing for the redevelopment of a longwave trof across the E, and especially the NE, CONUS. Ensembles/deterministic guidance in fair agreement, so will continue to baseline the forecast with a blend of these. Details... Sun... The remnants of cyclonic curvature and H85 temps averaging near -5C (about 1 std deviation below climatological normal values) finally begin to shift E, but the effects will continue to be felt. Even with full mixing and enough sunshine, temps are likely to be held mainly in the mid and upper 50s. In spite of the cyclonic flow, soundings show that subsidence is already in place, likely limiting any cloud cover. Mon and Tue... Moderation finally occurs as the height/temp anomalies finally shift to the E. In fact, H85 temps finally shift above 0C, to an average of +2C on Mon and +4C by Tue. This should allow temps to easily make it into the 60s each day (low risk for 70s with some downsloping). The only caveat will be sea breezes, flow will be weak with high pres cresting. Given SSTs in the 40s, coastal areas will definitely feel the difference. Ridging and strong high pres in control suggest mainly dry conditions linger. Wed and Thu... The redevelopment of a longwave trof is looking more likely by the mid-late week time frame. PWATs increase closer to 1.00 inches (per ensemble means) and slow occluding low pres approaches from the SW. Guidance in fairly good agreement that precip develops during the daylight hours Wed, and as previous forecaster noted ensemble probabilities continue to suggest high probs of at least 0.50in. Thermal profiles look warm, in spite of the risk for secondary coastal development. In any case, a transition back to wetter and unsettled wx is looking more and more likely each run. Late week... The redevelopment of the longwave trof looks to maintain the wet/unsettled conditions beyond mid week while also allowing cooler air to filter back into the region. && .AVIATION /12Z FRIDAY THROUGH TUESDAY/... Forecaster Confidence Levels... Low - less than 30 percent. Moderate - 30 to 60 percent. High - greater than 60 percent. Short Term /through Saturday/... High confidence. Today... VFR. Afternoon CIGS SCT/BKN 040-060 at times. Otherwise, spot SHRA possible, but very isolated. NW flow sustained 10-15 kt with gusts to around 25 kt. These winds will preclude seabreeze development. Tonight...VFR with skies becoming mostly clear. Northwest winds diminish. Saturday...VFR. Mostly clear skies and winds less than 20 knots. KBOS Terminal...High confidence in TAF. VFR. Gusty NW winds around 25 knots. KBDL Terminal...High confidence in TAF. VFR. Outlook /Saturday Night through Tuesday/...High confidence. VFR. && .MARINE... Forecaster Confidence Levels... Low - less than 30 percent. Moderate - 30 to 60 percent. High - greater than 60 percent. Short Term /through Saturday/...High confidence. Today...Northwest winds gusting 25-30 knots with seas 5 to 7 feet, mainly on the outer coastal waters. Small Craft Advisories are in effect. Tonight...Northwest winds continue to gust around 25 knots early in the night, then diminish after midnight. Seas will subside through the night, generally less than 5 feet. Small Craft Advisory continues through the early part of the night. Saturday... West to northwest winds will be 20 knots or less. Seas will be less than 5 feet. Outlook /Saturday Night through Tuesday/...High confidence. Winds less than 25 kt and weak seas. && .BOX WATCHES/WARNINGS/ADVISORIES... CT...None. MA...None. RI...None. MARINE...Small Craft Advisory until 2 AM EDT Saturday for ANZ231>234- 250-251-254>256. Small Craft Advisory until midnight EDT tonight for ANZ230-235- 237. Small Craft Advisory until 10 PM EDT this evening for ANZ236. && $$ SYNOPSIS...WTB/Doody NEAR TERM...WTB/Doody SHORT TERM...WTB LONG TERM...Doody AVIATION...WTB MARINE...WTB/Doody is the U.S. government's official web portal to all federal, state and local government web resources and services.