NWS Forecaster Discussion
BOX
Area Forecast Discussion National Weather Service Boston/Norton MA.
000 FXUS61 KBOX 071419 AFDBOX Area Forecast Discussion National Weather Service Boston/Norton MA 1019 AM EDT Tue Jul 7 2020 .SYNOPSIS... A frontal boundary south of our region will result in dry and comfortable weather today. This front lifts north of our region tonight with a few showers, but its main impact will be to deliver summerlike warmth and humidity the rest of the week into the weekend. Showers/thunderstorms will be possible each day with some heavy downpours, but much of this time will also feature dry weather. A stronger system late Friday into Saturday brings heavy rain and potential flooding issues. && .NEAR TERM /UNTIL 8 PM THIS EVENING/... 1015 AM Update... Very pleasant weather this morning across the region with refreshing airmass in place with dew pts in the upper 50s to lower 60s. Lowest dew pt is at KACK with a reading of 55! Core of dry air associated with 1024 mb high over the Gulf of Maine. In face the SPC mesoanalysis has PWATs below one inch over Cape Cod, the Gulf of Maine into ME and northern NH/VT. With mid level ridge remaining over or just to the west of New England today, this drier, less humid airmass will remain over the region today, providing dry weather thru the day. The one caveat is deeper moisture is not far away with SPC mesoanalysis showing 850 mb and 925 mb moisture plume from Long Island extending northwest into NJ and the Lower Hudson Valley. Although with the flow in this layer expected from the SSW much of this deeper moisture will advect northward thru the Hudson Valley of NY. So expecting mainly dry weather for western MA/CT as well, however clouds will likely persist much of the day, however breaks of sunshine are expected from time to time. Thus given more clouds in the region may have to knock down temps a few degs for the afternoon. Despite cooler maritime flow via southeast winds into the region, strong July sun will warm this airmass into the upper 70s and lower 80s away from the coastline. Very comfortable as well with dew pts remaining in the upper 50s to lower 60s. Enjoy as it won`t last long. Previous Discussion... * Dry & Pleasant Weather Today High pressure across the Canadian Maritimes will keep the summer heat/humidity west and south of our region today. Across southern New England a modified maritime airmass will result in high temperatures in the upper 70s to the lower 80s under a mixture of sun and clouds. The bulk of the cloud will be in the CT River Valley on the eastern edge of the deeper moisture. Relative humidity values will be quite pleasant for July standards. The high pressure system across the Maritimes will keep the instability along with the shower/thunderstorm threat to the west and south of our region today. Therefore, will be going with a dry forecast today. && .SHORT TERM /8 PM THIS EVENING THROUGH 6 PM WEDNESDAY/... * A few strong to severe thunderstorms with heavy rain possible Wednesday afternoon/evening across western & northern MA Tonight... The high pressure system moves southeast of the Canadian Maritimes tonight. This will allow a return southerly flow of increasing low level moisture. There will be an increase in elevated instability and with just enough of a southerly LLJ a few showers with perhaps a rumble or two of thunder will be possible. Certainly not a washout, but the risk exists for a few showers particularly after midnight. We should also see low clouds and some patches of fog develop tonight with the increasing low level moisture. Overnight low temps should only drop into the middle to upper 60s. Wednesday... It will be warm on Wednesday, but more importantly become rather humid with dewpoints near 70. Afternoon high temps should reach into the 80s, but heat indices will reach the lower 90s in some locations. We may see a few showers Wednesday morning in the warm advection pattern, but not expecting a washout. The bigger concern will be from Wednesday afternoon and evening across western and northern MA. Given the high dewpoints in place, we should see 1500 to 2500 J/KG of Cape develop. A shortwave will also be dropping down in northwest flow aloft, which will trigger scattered showers and thunderstorms across northern New England. The better effective bulk shear and higher updraft helicity values courtesy of the HREF model are to the north of our region. This is where the highest risk for strong to severe thunderstorms exist. That being said, western and northern MA are certainly not out of the woods. Effective shear of 25 knots and Capes potentially exceeding 2000 J/KG is certainly enough for a few strong to severe Wednesday afternoon and evening across western and northern MA. Main risk would be localized strong to damaging wind gusts. In addition, Pwats approaching 2 inches would result in a heavy rain/localized street flooding threat with the strongest storms. We will have to examine the trends of the high resolution CAMs as there is some uncertainty on timing and how far south this threat will get. Nonetheless, it certainly will need to be monitored. && .LONG TERM /WEDNESDAY NIGHT THROUGH MONDAY/... Highlights... * Hot and humid conditions continue * Scattered showers and thunderstorms on Thursday * Widespread, heavy rain Friday into Saturday Details... A weak high pressure ridge keeps the hot and humid airmass in place on Thursday ahead of a deeper shortwave and closed upper low off the coast of the Carolinas which will bring widespread heavy rain Friday into Saturday. Aloft the mid level trough lags behind the coastal low, keeping shower and thunderstorm chances in the forecast through at least the weekend. Thursday southerly flow around the offshore high keeps temperatures above normal (in the upper 80s to near 90) with oppressive humidity (low 70s). Given this, we`ll have enough instability and moisture for scattered showers and thunderstorms, even though forcing is lacking. A cold front may come close enough to help but looks to stall somewhere to our NW. Wind fields aren`t supportive of very organized convection, but can`t rule out a strong to severe storm or two. Eyes then turn to the Friday/Saturday time period where, if guidance is any indication, significant rainfall is likely. This is thanks to a coastal low sliding up from the Carolinas, pulled north ahead of the trough digging into the Great Lakes. This has the potential to bring showers and thunderstorms with gusty winds and very heavy downpours. Confidence in rainfall amounts and placement is low, depending heavily on the eventual low track. However, heavy rainfall looks fairly certain with this system given a very potent tap of tropical moisture...PWATs may cross into the 2.5-3 inch range! That would be 2-4 standard deviations above normal for mid July and would lead us to start thinking about the possibility of some flooding issues. Temperatures cool a bit from Thursday, into the low to mid 80s. Confidence then drops a good deal in how things play out. After this system it looks to stay unsettled through early next week. The likely scenario is that the low lifts off into the maritimes while the mid level trough and cold pool aloft lag behind, keeping shower and thunderstorm chances around for Sunday ahead of yet another disturbance around Monday. && .AVIATION /14Z TUESDAY THROUGH SATURDAY/... Forecaster Confidence Levels... Low - less than 30 percent. Moderate - 30 to 60 percent. High - greater than 60 percent. 14Z update... Any lingering IFR over Cape Cod and the Islands will erode by midday and give way to VFR despite southeast winds. Elsewhere VFR with the exception across western-central CT/MA where MVFR clouds will be stubborn. However dry weather prevails across all terminals of MA/RI/CT today. Previous discussion below. ================================================================== Today...High confidence. MVFR-IFR conditions in some low clouds near the south coast and CT River Valley will improve to VFR this morning. Otherwise, VFR conditions continue elsewhere today.SSE winds of 10 to 15 knots develop with some gusts up to 20 knots by afternoon. Tonight...Moderate confidence. Increasing low level moisture will result in widespread low clouds and fog patches overspreading the region. Timing a bit uncertain, but widespread low end MVFR-IFR conditions should develop as the night wears along. A few showers and the low risk for a rumble of thunder possible too. Wednesday...Moderate confidence. MVFR-IFR conditions early should improve to mainly VFR by afternoon. A few spot showers possible in the morning, but perhaps a few strong thunderstorms near or after 18z across northern MA. Winds becoming SW at 10 to 15 knots with some gusts into the lower 20 knots possible. KBOS Terminal...High confidence in TAF. KBDL Terminal...Moderate confidence in TAF. Outlook /Wednesday Night through Saturday/... Wednesday Night: VFR. Slight chance SHRA, isolated TSRA. Thursday through Thursday Night: VFR. Chance SHRA, slight chance TSRA. Friday: Mainly VFR, with local MVFR possible. Breezy. Chance SHRA, slight chance TSRA. Friday Night: Mainly VFR, with areas IFR possible. Breezy. SHRA likely, slight chance TSRA. Saturday: Mainly VFR, with local IFR possible. Breezy. Chance SHRA, isolated TSRA. && .MARINE... Today through Wednesday...High confidence. High pressure over the Canadian Maritimes today will shift further southeast through Wednesday. The result will be SE winds of 10 to 15 knots shifting to the SW on Wednesday. Gusts to around 20 knots are possible this afternoon and perhaps some nearshore 20 to 25 knot gusts Wed afternoon. This may result in some choppy seas, but biggest concern will be the risk for a few strong t-storms Wed afternoon and evening across our eastern MA waters. Some patchy fog is expected across the southern waters early this morning and particularly overnight into Wednesday morning. Outlook /Wednesday Night through Saturday/... Wednesday Night: Winds less than 25 kt. Seas locally approaching 5 ft. Slight chance of rain showers, isolated thunderstorms. Thursday: Winds less than 25 kt. Thursday Night: Winds less than 25 kt. Slight chance of rain showers, isolated thunderstorms. Friday: Winds less than 25 kt. Local rough seas. Rain showers likely, slight chance of thunderstorms. Friday Night: Winds less than 25 kt. Areas of rough seas. Rain showers likely, slight chance of thunderstorms. Saturday: Winds less than 25 kt. Areas of rough seas. Chance of rain showers, isolated thunderstorms. && .BOX WATCHES/WARNINGS/ADVISORIES... CT...None. MA...None. RI...None. MARINE...None. && $$ SYNOPSIS...Frank/BW NEAR TERM...Frank/Nocera SHORT TERM...Frank LONG TERM...BW AVIATION...Frank/Nocera MARINE...Frank/BW