593 FXUS61 KBOX 200151 AFDBOX Area Forecast Discussion National Weather Service Boston/Norton MA 951 PM EDT Sun May 19 2019 .SYNOPSIS... Scattered showers and possibly isolated thunderstorms are expected overnight. Summer- like heat and humidity is forecast for Monday. A cold front will trigger scattered strong thunderstorms Monday afternoon and early evening. This will be followed by dry, windy and cooler weather Tuesday. Dry weather continues Wednesday into Thursday afternoon with less wind and temperatures moderating back to seasonably mild levels. A period or two of unsettled weather may return late Thursday into Saturday, but a washout is not expected. && .NEAR TERM /UNTIL 6 AM MONDAY MORNING/... 950 pm update... Scattered showers and thunderstorms across NY State have weakened some over the last few hours, as SPC meso analysis indicates a less favorable environment near and especially east of the Berks. Nonetheless, still expect scattered showers and possibly isolated thunderstorms to work across the region overnight. Despite a marine influence on south to southwest flow, guidance indicates MUCapes increasing to between 500 to 1000 J/KG. This coupled with a southwest LLJ of 35 to 45 knots should be enough allow scattered showers/isolated t-storms entering the region overnight. While brief heavy rain is possible with the activity with Pwats over 1.5 inches, but do not expect severe weather overnight. The other concern will be for areas of fog developing along the south coast, Cape and Islands...some of which may be dense. This a result of high dewpoint air streaming in over the relatively cold ocean. Enough wind in the boundary layer should keep the fog threat mainly along the immediate south coast. Low temperatures will only drop into the lower to middle 60s, except for some 50s along the southeast New England coast with the marine influence. && .SHORT TERM /6 AM MONDAY MORNING THROUGH MONDAY NIGHT/... The main focus of the short term period will of course be the potential for our first severe weather event of the Spring. Monday southern New England will feel positively summer-like in the warm sector of a surface low moving across the Great Lakes region. Return flow tapping into subtropical moisture will bring temperatures in the 70s-80s and dewpoints in the low 60s. Winds are breezy out of the southwest. Model soundings indicate potential to mix down gusts of 25-30kts, peaking in the afternoon. Things continue to come together for a severe weather potential by the afternoon. First we'll start with the good, then mention a few inhibitors that are keeping some uncertainty in the forecast as to how intense and widespread any storms will be. Lift is there, if marginal. First and foremost, a cold front sweeps through from the west as our primary source of lift, but at 300 mb we're also placed beneath the RRQ of an upper level jet which will assist in some synoptic lift, though moreso to our north. Looking at 850-500mb omega Monday afternoon, values are decent, but again stronger over VT/NH. We'll have ample instability to work with by afternoon/evening. MUCAPE values on the order of 1000-2000 J/kg and low level lapse rates near 8 C/km are centered over western/central MA/CT, where the best chance of severe storms lies. For strong to severe storms we want sustained updrafts, and the necessary shear values are there. 0-6km bulk shear approaches 50kts with 0-1 km shear >20kts. That being said, HREF guidance indicates that the best updraft helicity will be centered over central NH, with lesser values extending over northern and northwest MA. This, together with the westward placement of greatest instability means those in western MA/CT would stand the best chance of seeing significant storms tomorrow. Low level lapse rates indicate a threat of strong damaging winds along with large hail. Though a low probability, the chance of a tornado exists, given said low level shear and 0-1 SRH values near 150. Finally, moisture is the biggest question for tomorrow. A necessary ingredient for widespread storms, it may be limited at the mid levels by the time the best instability moves in. The NAM has now joined the GFS and EC in showing drying aloft coincident with the best surface based instability. This could have the effect of keeping storms low topped and limiting intensity and areal coverage. That being said, even if coverage is sparse, with these convective parameters, any storms that to strengthen have the potential to be very damaging. Another thing we'll have to watch overnight into the morning hours will be how quickly the morning rain/clouds clear out. If we get another situation like today where showers linger longer than expected, it could limit our destabilization. Southeast of the BOS-PVD corridor, southwest winds off the 50 degree ocean will keep things more stable and storms weaker. The cold front passes Monday night bringing gusty winds out of the northwest and cooler lows in the 40s and 50s. && .LONG TERM /TUESDAY THROUGH SUNDAY/... Big Picture... The upper pattern through next weekend becomes mostly high- amplitude with a ridge over the Eastern USA and a trough/closed low over the Western USA, and another building ridge over the East Pacific. Shortwaves in such a flow will take the scenic route across North America...south along the Pacific Coast, then turning northeast across Northwest Mexico and the Plains/Great Lakes, then turning southeast across New England. Model consensus shows one shortwave moving across New England Tuesday, then off to the east. A second shortwave moves over the top of the ridge and then across New England Thursday night. A third shortwave moves over the top of the ridge and across Eastern Canada Saturday. Timing on these last two shortwaves is approximate. Upper heights/temperatures trend a little cooler Tuesday, but return to higher/milder levels through the weekend. This would suggest seasonably mild temperatures. There have been differences run-to-run on shortwave timing, which is not surprising. But the general pattern looks consistent through the week with differences among models developing next weekend. Confidence is high Tuesday-Wednesday, then moderate due to shortwave timing Thursday to Sunday. Concerns... Tuesday-Wednesday... Upper low and cold pool...-23C at 500 mb...move across Northern New England Tuesday with a supporting 150-kt jet moving across Southern New England. Cold pool moisture is concentrated in the north, but some extends south into at least areas north of the Mass Pike. Low level lapse rates over Srn New England become impressive, but mid level rates show little of interest. Perhaps no surprise that model rainfall is limited to ME/NH/VT with nothing for our area. We continue to expect diurnal clouds Tuesday, heaviest over the hills and near the NH border. Temps aloft support max sfc temps in the upper 50s and 60s. Min temps should be in the 40s and low 50s. Winds will be a concern for Tuesday. Cold advection aloft and daytime heating will promote the strong low-level lapse rates and promote mixing to 800 mb...which should bring northwest wind gusts around 35-kt to the surface. High pressure builds over the region Wednesday, so expect a fair-weather day with north winds gusting 15-20 kt. Temps aloft 4-6C should support max sfc temps in the 60s to around 70, cooler Cape and Islands. Min temps should repeat in the 40s and low 50s. Thursday to Sunday... As noted above, two shortwaves move through the New England area, and each is sufficient to support a period of showers. Current model timing would suggest one period is Thursday afternoon and night, the second period is Friday night and Saturday. Given this is in the late part of the forecast, the timing will likely shift a little and so should not be interpreted literally. Temperatures should be milder during this time with highs in the upper 60s and 70s and lows mostly in the 50s. && .AVIATION /02Z MONDAY THROUGH FRIDAY/... Forecaster Confidence Levels... Low - less than 30 percent. Moderate - 30 to 60 percent. High - greater than 60 percent. Short Term /through Monday Night/... Tonight...Moderate confidence. Marine clouds have spread over the region with ceilings 1500-2500 feet. Expect these clouds to linger through the night and possibly lower as higher humidity air works in from the south. Areas of fog, some of which might become dense are expected to mainly impact the immediate south coast, Cape and Islands overnight as high dewpoint air overspreads the region. Enough wind in the boundary layer should prevent much fog elsewhere. Scattered showers and possibly some isolated thunderstorms are also expected to work across the region overnight, resulting in brief localized lower conditions. Monday...Moderate confidence. Any leftover showers should move offshore early in the day. Most of the morning will feature low clouds breaking/lifting to VFR, with the possible exception of the Islands and immediate South Coast where low clouds/IFR may linger. Gusty south-southwest winds will be in place during the day with gusts to 25 kt. Thunderstorms are expected to redevelop Monday afternoon in Western New England. There is some hint of negative factors that would interfere with storm development, but enough positive factors to expect storms to form. Expect brief MVFR near any storms and IFR under them. Most likely timing would be from 17Z/1 PM to 01Z/9 PM. Monday night...Cold front moves through Southern New England, with showers/thunder diminishing after it moves through. Winds will shift out of the northwest with gusts 20-25 kt. Expect conditions to improve to VFR all areas. KBOS Terminal...Moderate to high confidence in TAF. KBDL Terminal...Moderate to high confidence in TAF. Outlook /Tuesday through Friday/... Tuesday: VFR. Windy with gusts up to 30 kt. Tuesday Night: VFR. Windy with local gusts up to 30 kt. Wednesday: VFR. Breezy. Wednesday Night: VFR. Thursday: Mainly VFR, with local MVFR possible. Slight chance SHRA. Thursday Night: Mainly MVFR, with local IFR possible. Chance SHRA, patchy BR. Friday: Mainly MVFR, with areas VFR possible. Slight chance SHRA. && .MARINE... Forecaster Confidence Levels... Low - less than 30 percent. Moderate - 30 to 60 percent. High - greater than 60 percent. Short Term /through Monday Night/... Tonight...High confidence. South to southwest winds will gust between 20 and 25 kt. Scattered showers and isolated thunderstorms. Small Craft Advisories end along the nearshore, but are in effect for all outer waters as seas build to 5 ft. Areas of dense fog may reduce visibility to below 1 nm, especially south coast overnight. Monday...High confidence. Small Craft Advisories continue for the outer waters with southwest winds gusting to 25 kts and seas building up to 6 ft in the southern outer waters. Scattered thunderstorms possible toward evening. Visibility reduced to 1 to 3 nm in fog, locally below 1 nm in the morning. Monday night...High confidence. Winds becoming NW gusting to 20kts. Seas 3-6 Outlook /Tuesday through Friday/... Tuesday: Low risk for Small Craft Advisory winds with gusts up to 30 kt. Areas of seas approaching 5 ft. Tuesday Night: Low risk for Small Craft Advisory winds with gusts up to 25 kt. Areas of seas approaching 5 ft. Wednesday: Low risk for Small Craft Advisory winds with gusts up to 25 kt. Seas locally approaching 5 ft. Wednesday Night through Thursday: Winds less than 25 kt. Thursday Night: Winds less than 25 kt. Areas of seas approaching 5 ft. Chance of rain showers. Friday: Winds less than 25 kt. Local rough seas. Slight chance of rain showers. && .BOX WATCHES/WARNINGS/ADVISORIES... CT...None. MA...None. RI...None. MARINE...Small Craft Advisory until 8 AM EDT Tuesday for ANZ230>236-250- 251-254. Small Craft Advisory from 8 AM Monday to 8 AM EDT Tuesday for ANZ237. Small Craft Advisory from 2 AM Monday to 8 AM EDT Tuesday for ANZ255-256. && $$ SYNOPSIS...WTB/BW NEAR TERM...Frank SHORT TERM...BW LONG TERM...WTB AVIATION...Frank/WTB/BW MARINE...WTB/BW