347 FXUS61 KBOX 131508 AFDBOX Area Forecast Discussion National Weather Service Boston/Norton MA 1108 AM EDT Mon Jul 13 2020 .SYNOPSIS... A slow moving front will slide eastward today. This will generate showers and thunderstorms this afternoon. A few thunderstorms may become strong to severe, with localized damaging winds and torrential rains. The heavy rainfall could lead to flash flooding. Clearing with cooler and drier conditions for Monday night. Upper low pressure brings more unsettled weather Tuesday, but high pressure then brings fair weather for Wednesday and possibly Thursday. Scattered showers and thunderstorms return on Friday through Sunday. && .NEAR TERM /UNTIL 6 PM THIS EVENING/... 11 AM Update: Following trends and reviewing the incoming high-res guidance, re- shaped PoPs/Wx a bit with this update. Though we are still expecting isolated to scattered strong to locally severe storms today, the area of potential threat seems to be narrowing towards areas bounded between roughly Fitchburg to Willimantic CT to the eastward to the eastern MA coast roughly to Plymouth/Marshfield. Have indicated solid Likely to lower Categorical pops given strong agreement in the high res CAMs between 18-22z (2-6 PM). Drew in a bit lower chances for stronger storms westward into the CT Valley where westerly flow and lower dewpoint mixing may lead to less instability and storm chances here. Thinking more isolated to widely scattered coverage here versus closer to scattered further east. Strong to damaging winds and torrential downpours capable of localized street flooding remain concerns with storms this afternoon. Will likely hoist an awareness SPS for the thunder threat, especially with these storms potentially coinciding with this afternoon's commute. The RI South Coast into Cape Cod and the Islands also appear to have a lesser shot at storms today. Otherwise, no other changes needed to the going forecast. Previous Discussion... * Isolated strong to severe storms possible today. Storms will bring threats of damaging wind gusts and heavy rainfall, which could produce localized flash flooding. Positively tilted shortwave trough lifts northeastward offshore and into northern New England by late this afternoon. A negatively tilted trough will dig into the eastern Great Lakes by late this afternoon. At the surface a cold front is extending from eastern Maine through central portions of southern New England into the Mid Atlantic. This front will slowly slide eastward today. Could have a few rain showers this morning with isolated thunder, but the better shot of precipitation is this afternoon into the evening. Will have dew points generally in the mid to upper 60s across the region, but expect some spots across the south coast to see readings in the low 70s. The lift necessary for thunderstorm development will be provided by the slow moving cold front. Should see MUCAPE values around 1000 to 1500 J/kg with bulk shear values in the 0-6 km layer initially around 35-40 kts, but decreasing to 25-30 kts by this afternoon. These factors combined together will bring southern New England the threat of strong to severe thunderstorms as highlighted by SPCs latest Day 1 Outlook. The main threat with any strong to severe storms that develop today will be damaging wind gusts and torrential rainfall, which may lead to localized flash flooding, please see the hydro section below for more details here. Will see nearly adiabatic low level lapse rates in place across the region, but mid level lapse rates remain poor with values around 5 to 6 degrees Celsius per km. CAMS suggesting and as highlighted in SPCs outlook a multicellular storm organization given the lower 0-6 km bulk shear values during the afternoon. Still seeing SREF probabilities of over 10,000 units of Craven- Brooks severe index parameter. The 00Z HREF also showing some 2-5 km Updraft Helicity across central and eastern MA. Increased high temperatures today as flow at 925 hPa will be westerly to northwesterly, which should promote some downsloping. This results in temperatures generally in the 80s across the region. && .SHORT TERM /6 PM THIS EVENING THROUGH 6 PM TUESDAY/... Tonight... Trough sliding into northern New England tonight, while the cold front gradually slides offshore. Still could have some strong to severe storms early on in this period, but overall expect activity to be diminishing. Will see slightly cooler dew points in wake of the front, but many locations will still be in the 60s so it still will feel muggy. VFR for the majority of the forecast. Conditions may deteriorate to local MVFR/IFR during the afternoon in SHRA/TSRA developing along a slowly moving front. Hard to pin point where these storms will be at this time, but time frame across central MA, northern RI and eastern MA is 18Z to 00Z. Any of these storms could bring heavy downpours and strong gusts. Brought visibilities down to MVFR in locations where am most confident in shower/storm activity. Winds S/SW becoming W late. Expect Cape Cod and the Islands to see low level stratus and fog move in later in the afternoon and into the evening. Not out of the question there is some fog development especially given winds will be light with clearing skies and recent precipitation. Have not included in the latest update given winds will be northwesterly advecting drier air in, but will likely need to add in areas where precipitation falls. Low temperatures will generally be in the 60s across the region tonight. Tuesday... Closed upper low will be in place across northern New England. Still will have 500 hPa temperatures between -10 to -15 degrees Celsius. This in combination with the July sunshine will result in diurnal showers and thunderstorms developing. Expect cooler temperatures across the region due to northerly/northwesterly flow at 925 hPa. This advects 10 to 12 degree Celsius 925 hPa air in. Will see high temperatures range from the mid 70s to the low 80s. The warmest readings will be found across the CT River Valley as there should be somewhat of a downsloping component in place. && .LONG TERM /TUESDAY NIGHT THROUGH SUNDAY/... Highlights... * Cooler Wed-Thu, especially eastern areas * Mostly dry until rain chances return Thursday night into Friday * Warmer temperatures and increased humidity Friday into the weekend Details... High pressure ridging moves in mid week keeping things dry while we'll get a break from the heat and humidity. Beyond that zonal steering flow sets up over much of the U.S. with a few shortwaves sliding through. This brings mostly dry conditions, except for Thursday night and Friday when a weak frontal system moves through to our north. While mostly dry, the weekend brings with it the return of more heat and humidity. Tuesday night showers and thunderstorms should be winding down with the loss of daytime heating as the low moves east offshore and heights are on the rise. Some low level moisture lingers overnight keeping some low to mid clouds around, but still temps will be a bit cooler than the previous night. Wednesday and Thursday will be mostly dry as high pressure builds south from Quebec at the sfc with mid level ridging aloft. A bit of uncertainty on Wednesday depending on how quickly the low pressure exits...lingering effects could keep a few isolated showers in the area but most will stay dry. We'll enjoy a nice break from the heat and humidity thanks to N/NE flow around the high which keeps western MA/CT in the low 80s; even cooler in the 70s for eastern MA into RI thanks to closer proximity to the marine influence. Our next chance of rain comes Thursday night as a warm front lifts north and WAA showers materialize ahead of an incoming shortwave. There remains significant disagreement among the guidance in timing the subsequent cold front around Friday, if it makes it this far south at all. Regardless, showers and thunderstorms are likely during the day on Friday. Beyond that confidence is low given the zonal flow and inherent difficulty in timing the disturbances. Trends would indicate potential for a renewed hot and humid airmass. && .AVIATION /15Z MONDAY THROUGH FRIDAY/... Forecaster Confidence Levels... Low - less than 30 percent. Moderate - 30 to 60 percent. High - greater than 60 percent. 15z TAF Update: Today: High confidence in trends, moderate on timing TSRA. Generally VFR thru 19z outside of the Cape and Islands where stratus should continue to prevail. After 19z, greatest threat for TSRA exists from roughly FIT-ORH-IJD line east to BVY-PYM, including Boston and Providence. Appears to be a lesser chance for the western TAFs. Any storm would be capable of brief ceiling/visby reductions, lightning and localized gusty to strong winds. Winds outside of storms mainly S/SW, though will note some potential for sea-breeze/onshore winds at BOS between 16-19z. Toight...Moderate confidence Showers and thunderstorms diminishing in coverage during the evening. Will see conditions across the interior improve to VFR, but not out of the question a few spot that had showers/storms see some patchy fog develop. Cape Cod and the Islands remaining in MVFR to LIFR stratus and fog. Tuesday...Moderate confidence VFR across the interior, but MVFR to LIFR across eastern coastal locations including BOS. Winds generally out of the north, but eastern coastal areas will see onshore flow. KBOS Terminal...Moderate confidence in TAF. Psbl sea-breeze between 16-19z, VCTS indicated 19z-00z for potential TSRA nearby the airport. KBDL Terminal...High confidence in TAF. VCTS 18-20z for possible TSRA. Outlook /Tuesday Night through Friday/... Tuesday Night: Mainly VFR, with local IFR possible. Slight chance SHRA. Wednesday through Thursday: Mainly VFR, with local MVFR possible. Thursday Night: Mainly VFR, with local MVFR possible. Breezy. Chance SHRA. Friday: Mainly VFR, with areas MVFR possible. Breezy. Chance SHRA, slight chance TSRA. && .MARINE... Extended the Small Craft Advisory across southern waters west and southwest of Marthas Vineyard through Tuesday. Expect seas to remain heightened at or above 5 ft through this period. Small Craft Advisories remain in effect for all waters. Lower clouds and fog may again develop on the southern waters this afternoon into today. Scattered thunderstorms may become strong Monday afternoon into early evening. Outlook /Tuesday Night through Friday/... Tuesday Night: Winds less than 25 kt. Areas of seas approaching 5 ft. Slight chance of rain showers, isolated thunderstorms. Wednesday: Winds less than 25 kt. Areas of seas approaching 5 ft. Wednesday Night: Winds less than 25 kt. Seas locally approaching 5 ft. Thursday: Winds less than 25 kt. Thursday Night: Winds less than 25 kt. Chance of rain showers. Friday: Winds less than 25 kt. Chance of rain showers, slight chance of thunderstorms. && .HYDROLOGY... 430 AM Update... As for torrential rainfall expecting PWATs of 1.5 to 1.75 which is close to if not just above the 90th percentile per SPCs mesoanalysis. Will also have a warm cloud layer depth of roughly 3 to 4 km. So will have a threat of torrential rainfall. Given the slow moving nature of the frontal boundary there is a threat of flash flooding. Looks like the highest risk of flash flooding is across central and eastern MA and as indicated by WPCs latest Day 1 Excessive Rainfall Outlook. 405 PM Update: We are looking at a couple of rounds of rain through Monday. The first would amount to light to moderate showers moving in overnight tonight into the first part of Monday. The second round comes in Monday afternoon into early night associated with scattered to numerous thunderstorms along a slowing cold front. Though some areas have received some rains of late, our area remains abnormally dry. This is reflected by high 1-hourly flash flood guidance which is anywhere from 2 to 2.75" of rain. However, PWAT values remain elevated at values ranging from 1.75-2 inches and support locally heavy downpours. High- res guidance blossoms scattered to numerous thunderstorms along a SW- NE oriented cold front. Signals of thunderstorms backbuilding on the frontal zone and training over the same areas as storms move eastward, could yield torrential rains in a short period of time. HREF neighborhood probabilities for 1" rains in 3-hours are very high at 70-80% from Windsor Locks to Fitchburg eastward to the Boston-Providence/I-95 corridor, with lower probs of 3" rains in 3-hours. If heavier rains fall over a typically vulnerable area, such as the larger cities where rain can go readily to runoff, a localized flash flood threat could materialize. Antecedent conditions and uncertainties in QPF and placement on the front precludes a Flash Flood Watch at this time. However it may need to be considered in later updates near or east of the frontal boundary, especially as storms are expected to coincide around or with the Monday PM rush hour. && .BOX WATCHES/WARNINGS/ADVISORIES... CT...None. MA...High Surf Advisory until 8 PM EDT this evening for MAZ023-024. RI...High Surf Advisory until 8 PM EDT this evening for RIZ008. MARINE...Small Craft Advisory until 8 PM EDT Tuesday for ANZ235-237-255- 256. Small Craft Advisory until 8 PM EDT this evening for ANZ254. && $$ SYNOPSIS...BL/BW NEAR TERM...BL/Loconto/BW SHORT TERM...BL LONG TERM...BW AVIATION...BL/Loconto/BW MARINE...BL/Loconto/BW HYDROLOGY...Team BOX