255 FXUS61 KBOX 171129 AFDBOX Area Forecast Discussion National Weather Service Boston/Norton MA 629 AM EST Tue Nov 17 2020 .SYNOPSIS... A cold front approaches today and moves through Srn New England late today or tonight. This will bring colder than normal temperatures tonight through Wednesday night. High pressure south of New England brings a warming trend later this week, with temperatures rebounding to above normal Friday through the weekend. && .NEAR TERM /UNTIL 6 PM THIS EVENING/... 7 am update... Mid clouds are on the increase early this morning as the shortwave moves overhead. Very light returns showing up on radar but not expecting much makes it to the ground this morning...better chance late morning/afternoon. No major changes with this update. Previous Discussion... OVERVIEW...Broad upper trough over the Eastern USA and broad upper ridge over the Plains/Rockies. These features shift east today and Thursday. Today... Upper trough moves over New England today. Although jet support for generating lift remains well south of the region, a shortwave moving through the trough turns surface winds from the northwest this afternoon, drawing colder air over Srn New England and destabilizing the airmass. Precipitable water values, while only around 0.4 inches, are close to climate normals. Expect developing clouds. The chance of precipitation, while low, does exist with the best chance along the East Slope and CT River Valley regions. Mixing is expected to 850-mb, especially once the cold advection gets started. Temperatures at that level will be -6C to -8C, which would support max sfc temps in the 40s, possibly near 50 where sufficient solar heating may occur. && .SHORT TERM /6 PM THIS EVENING THROUGH 6 PM WEDNESDAY/... Tonight... Upper trough moves overhead and then off to the east. This pulls a cold front through the region. Clouds and widely scattered showers with the cold front during the start of the night, but with a drying trend overnight. Expect clearing and colder temperatures overnight, with min temps in the upper teens and 20s. Wednesday... High pressure builds over Srn New England. The pressure gradient leading the high and cold advection in the northwest flow will bring a period of increased winds, with gusts 20 to 25 kt. Expect a smaller mixing depth, roughly 900-mb, with temperatures that support max temps in the 30s. We then get into strong cold advection for Tues night, with 925 mb temps falling from around 0 to -4C early to values by daybreak near -8 to -11C. This is the leading edge of the shot of below- normal temperatures advertised for midweek. Strength of the CAA would indicate that gusts around 20 mph should still be possible even into the evening, and thus this factor may mitigate stronger temperature cooling. Shown lows in the upper teens to mid 20s for most (upper 20s/near 30 for the Cape). However, wind chills plummet into widespread teens, coldest by sunrise. Quite a shock compared to what we experienced last week. Will also see enough instability over the ocean for ocean effect cloudiness/precip across our waters. && .LONG TERM /WEDNESDAY NIGHT THROUGH MONDAY/... 430 PM update... Highlights... * Moderating temperatures late this week with highs approaching 60, peaking Sunday or Monday * Dry weather prevails until the beginning of next week Details... Temperatures... Quiet and increasingly mild weather is the story for most of the extended period under the influence of a few high pressure centers. First, starting Wednesday night into Thursday the ridge of high pressure that was over NY state will be suppressed south and east of southern New England. This will direct warmer, southwesterly flow around the high and into the region. This warm air advection will help moderate highs by about 10 degrees on Thursday compared to Friday. So, we're looking at temps in the mid 40s. The low pressure far to the north that suppresses the high, though, will tighten the pressure gradient and bring the return of gusty winds Thursday which will make it feel colder than it is. These gusty winds stick around Friday but temperatures will be warmer. By then 850 mb temps will reach the range that they'll stay in through the weekend, around 5- 7C. This translates to surface highs in the upper 50s to near 60! Monday may be the warmest day of the period as we get a strong surge of southerly warm air ahead of our next cold frontal system. Precipitation... Not a whole lot to speak of in the extended period under prolonged influence from high pressure. Some increased cloudcover Thur/Fri as the system passes well to our north, then again Sunday as moisture is pulled north ahead of the incoming trough. The next chance of rain comes with a frontal system/trough approaching late Sunday/Monday, but timing and details are uncertain at this time. && .AVIATION /12Z TUESDAY THROUGH SATURDAY/... Forecaster Confidence Levels... Low - less than 30 percent. Moderate - 30 to 60 percent. High - greater than 60 percent. 12Z update... Today... VFR but with periods of sky cover more than 50 percent. Widely scattered rain/snow showers possible, with focus on the East Slope and CT Valley in the afternoon. West- northwest winds gusting to 20 kt in the afternoon. Tonight...Clouds with heights 4000 to 5000 feet will linger the early part of the night. Clearing skies after midnight. Wednesday... VFR with mostly clear skies. Northwest winds gust 20 to 25 kt. BOS TAF...High confidence in TAF. Northwest winds Noon to 4 PM with occasional gusts to 25 kt. BDL TAF...High confidence in TAF. Outlook /Wednesday Night through Saturday/... Wednesday night through Friday Night: VFR. Breezy. Saturday: VFR. && .MARINE... Today...A cold front approaches during the day, bringing increasing clouds and scattered light showers. West winds turn from the northwest late in the day, bringing colder air and northwest wind around 20 kt. Seas diminish to less than 5 feet. Tonight...The cold front moves off to the east, with colder air flowing across the waters. This may generate clouds and scattered ocean-effect rain/snow showers. Northwest wind gusting 20 to 25 kt will build seas to 5 feet over the open waters. Small Craft Advisory will be needed on some or most of the waters. Wednesday... Northwest winds gust to 20 kt, maintaining seas of 5-6 feet on the exposed waters. Small Craft headlines may be needed on some waters. Outlook /Wednesday Night through Saturday/... Wednesday Night: Winds less than 25 kt. Seas locally approaching 5 ft. Thursday: Low risk for Small Craft Advisory winds with gusts up to 25 kt. Seas locally approaching 5 ft. Thursday Night through Friday: Moderate risk for Small Craft Advisory winds with gusts up to 25 kt. Local rough seas. Friday Night: Winds less than 25 kt. Seas locally approaching 5 ft. Saturday: Winds less than 25 kt. && .BOX WATCHES/WARNINGS/ADVISORIES... CT...None. MA...None. RI...None. MARINE...Small Craft Advisory until 1 AM EST Wednesday for ANZ235-237. Small Craft Advisory until 7 PM EST Wednesday for ANZ250- 254>256. && $$ SYNOPSIS...WTB/BW NEAR TERM...WTB/BW SHORT TERM...WTB LONG TERM...BW AVIATION...WTB/BW MARINE...WTB/BW