000 FXUS61 KBOX 200553 AFDBOX Area Forecast Discussion National Weather Service Boston/Norton MA 153 AM EDT Tue Apr 20 2021 .SYNOPSIS... High pressure builds offshore tonight. A cold front slides through on Tuesday and could bring a few isolated showers. A warm front lifts across southern New England early Wednesday and a cold front brings rain, wind, and rumbles of thunder during the afternoon. Thursday is a transition day, before more pleasant weather arrives for Friday and Saturday. && .NEAR TERM /UNTIL 6 AM THIS MORNING/... 1 AM Update... Have added a bit more fog to the CT River Valley based on Nighttime Microphysics Satellite and surface observations. Otherwise there haven't been any major changes to the forecast. 10 PM Update... Have introduced patch fog roughly from central MA/RI westward. Expecting skies to become clear and winds light. Given the recent precipitation this should result in radiation fog developing as indicated by the HREF guidance and makes sense. Rest of the forecast is on track. 750 PM Update: Outside of a few scattered showers, mainly across western MA/CT, it has been dry for most. An isolated shower did pass through the Boston area around 730pm with a trace of precipitation. Those remaining showers across western zones will slowly collapse with the loss of the daytime heating. The only minor change to the forecast were to the winds - which were to bring them more in line with the current observations. Otherwise the forecast remains on track and you can read the prior discussion below. Discussion from 4 PM... Some scattered showers with an isolated thunderstorm are possible through early this evening. Then as the weak H5 shortwave departs, expect mainly dry conditions overnight as weak ridging builds back into Southern New England. Clearing skies along with light winds should allow for good radiational cooling. Some radiation fog may be possible in the typical fog-prone valley locations. Overnight lows generally in the upper 30s to low 40s. && .SHORT TERM /6 AM TUESDAY MORNING THROUGH TUESDAY NIGHT/... Tuesday... Tuesday is expected to be the pick of the week as a H5 ridge briefly builds into Southern New England. With 925 mb temperatures between +11 to +13C and good southwest flow (and resultant downsloping), expect highs to be around 70 to 75, except cooler along the immediate south coast and the Cape and Islands. With deep mixing up to 800-850mb, sea breezes should be kept at bay for much of the Eastern MA coast. While most of us will see plenty of sunshine and dry weather, cannot rule out an isolated shower especially north of the Mass Pike as a shortwave trough passes well to our north. In addition, there could potentially be an elevated risk for fire danger with minimum relative humidity between 25 to 30 percent away from the immediate south coast as well as the Cape and Islands. The recent rain may help to mitigate the risk, however. Tuesday night... By Tuesday evening, our attention turns to an amplifying mid level trough and strengthening frontal wave crossing the Great Lakes. Expect clouds to be on the increase, which will help keep temperatures milder, with lows generally in the mid 40s. CAM guidance suggests that most of us should stay dry with better chances for precipitation across our Northwestern zones latter half of the night. && .SHORT TERM /6 AM THIS MORNING THROUGH 6 PM WEDNESDAY/... Tuesday... Tuesday is expected to be the pick of the week as a H5 ridge briefly builds into Southern New England. With 925 mb temperatures between +11 to +13C and good southwest flow (and resultant downsloping), expect highs to be around 70 to 75, except cooler along the immediate south coast and the Cape and Islands. With deep mixing up to 800-850mb, sea breezes should be kept at bay for much of the Eastern MA coast. While most of us will see plenty of sunshine and dry weather, cannot rule out an isolated shower especially north of the Mass Pike as a shortwave trough passes well to our north. In addition, there could potentially be an elevated risk for fire danger with minimum relative humidity between 25 to 30 percent away from the immediate south coast as well as the Cape and Islands. The recent rain may help to mitigate the risk, however. Tuesday night... By Tuesday evening, our attention turns to an amplifying mid level trough and strengthening frontal wave crossing the Great Lakes. Expect clouds to be on the increase, which will help keep temperatures milder, with lows generally in the mid 40s. CAM guidance suggests that most of us should stay dry with better chances for precipitation across our Northwestern zones latter half of the night. && .LONG TERM /WEDNESDAY NIGHT THROUGH MONDAY/... Highlights... * Showers & embedded thunderstorms possible Wednesday as a cold front sweeps across the region. * Cool & raw Thursday with breezy conditions. * Quiet & comfortable conditions for Friday and Saturday. Unsettled conditions return Sunday. Wednesday... Model guidance continues to support a deepening mid-level trough and surface low pressure system moving across the northeast through Wednesday. A warm front noses north during the morning hours which will put much of southern New England into the warm sector of the system - allowing many to climb into the low and middle 60s and dewpoints in the low 50s. Expecting gusty southwesterly winds ahead of the cold front as a 925mb low level jet passes overhead - BUFKIT soundings support deep mixing to over 900mb with lapse rates ranging between 7.0 & 9.0 C/KM during the early afternoon hours. Depending on the timing of the cold front with the movement of the LLJ storms during the afternoon could produce isolated wind gusts to 50mph in embedded showers and t'storms. SPC places southern New England outside of a marginal risk for thunderstorm - something we will keep an eye on moving forward. The cold front will move across the region from west to east during the mid to late afternoon hours with a widespread soaking rain. Wednesday Night and Thursday... The cold front passes and we are placed on the backside of the area of low pressure. The center of the low moves northeast towards Nova Scotia and deepens. Southern New England is in the middle of a tight pressure gradient, the low to our northeast and an area of high pressure to our south. Given the strong CAA steepening lapse rates even into the overnight, gusty winds are expected which could be strong enough to consider wind headlines. CAA overnight will drop temperatures into the low 30s across much of southern New England and the upper 20s for the higher terrain of northwestern MA. Northwest flow will continue through the day and will keep temperatures in the 40s to 50 degrees. A few wrap around showers especially across the higher terrain aren't out of the question across western MA/CT. Friday and Saturday... High pressure builds in from the south and allows for quiet and dry weather. Temperatures return the the mid and upper 50s for Friday. Saturday we reach well into the 60s. Sunday... Models suggest a coastal low pressure system will move off shore impacting southern New England with rain and wind. Still too early to dive into details at this time, but we will continue to monitor. && .AVIATION /06Z TUESDAY THROUGH SATURDAY/... Forecaster Confidence Levels. Low - less than 30 percent. Moderate - 30 to 60 percent. High - greater than 60 percent. Through 12Z...High confidence. Most areas will remain VFR with light SW to W winds. The only exception is across the CT River Valley and other spots across the interior where they received some rain today. The combination of clear skies and light winds will result in patchy radiation fog. Should see vsbys range between MVFR to IFR. Today...High confidence. VFR conditions expected with gusty WSW to SW winds. Most areas will see speeds of 10-15 kts and gusts of 25-30 kts. Not completely out of the question for a few 35+ kt gusts especially if any heavier showers move through. Though am thinking the shower activity will be isolated in nature. Have not included a mention in the latest set of TAFs as am uncertain on how widespread the coverage will be. Expecting the mixed layer to decouple between 22-01Z, which will bring an end to the gusty winds. Tonight..High confidence. VFR conditions expected with increasing clouds. Should see cloud bases generally between 5-10 kft. Scattered showers possible after midnight for far western areas after midnight. Light winds shifting to the S. Wednesday...High confidence in trends moderate in timing. Conditions deteriorating from VFR to start to MVFR/IFR and maybe some localized LIFR as showers and thunderstorms move through. S winds of 10-15 kts and gusts of 25-30 kts. Winds shifting to the SW/W across western areas late in the day as a cold front slides through. KBOS TAF...High confidence in TAF. KBDL TAF...High confidence in TAF. Outlook /Wednesday Night through Saturday/... Thursday: VFR. Windy with gusts up to 35 kt. Thursday Night: VFR. Windy with areas of gusts up to 35 kt. Friday through Saturday: VFR. Breezy. Saturday Night: Mainly VFR, with local MVFR possible. Chance SHRA. Sunday: Mainly MVFR, with local VFR possible. Windy with local gusts up to 30 kt. Chance SHRA. && .MARINE... Tonight...High Confidence. Weak pressure gradient will keep winds and seas below small craft advisory thresholds. Tuesday...High Confidence. A good setup for gusty nearshore winds during the afternoon. Good surface heating should yield SW wind gusts of 25 kt and choppy seas, particularly near shore. Have opted not to hoist Small Craft Advisories at this point as the 25 kt gusts should be fairly localized and transient. Tuesday night...High Confidence. Winds diminish well below SCA threshold but seas may approach 5 ft for the southern outer waters. Outlook /Wednesday Night through Saturday/... Thursday through Thursday Night: Strong winds with gusts up to 30 kt. Rough seas up to 10 ft. Friday: Moderate risk for Small Craft Advisory winds with gusts up to 25 kt. Areas of rough seas. Friday Night: Low risk for Small Craft Advisory winds with gusts up to 25 kt. Local rough seas. Saturday: Winds less than 25 kt. Seas locally approaching 5 ft. Saturday Night: Winds less than 25 kt. Areas of seas approaching 5 ft. Chance of rain showers. Sunday: Winds less than 25 kt. Areas of rough seas. Chance of rain showers. && .BOX WATCHES/WARNINGS/ADVISORIES... CT...None. MA...None. RI...None. MARINE...None. && $$ SYNOPSIS...BL/Chai/Gaucher NEAR TERM...BL/Chai/Gaucher SHORT TERM...Chai LONG TERM...Gaucher AVIATION...BL MARINE...Chai/Gaucher