Area Forecast Discussion
Issued by NWS Boston, MA

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000 FXUS61 KBOX 180812 AFDBOX Area Forecast Discussion National Weather Service Taunton MA 412 AM EDT Fri Aug 18 2017 .SYNOPSIS... Out ahead of a deep low pressure center lifting into Southern Canada, rain is to be expected, possibly heavy at times along with the threat of localized flooding, for today and tonight. A low risk of strong to severe thunderstorms mainly over S/W portions of MA and CT. A cold front moves through New England Saturday with a few leftover showers and thunderstorms, then moves off to the east Saturday night. High pressure returns with fair seasonable weather Sunday through Tuesday. Another cold front sweeps through New England Wednesday, followed by more dry weather Thursday. && .NEAR TERM /UNTIL 6 PM THIS EVENING/... 415 AM Update... Surface warm front remains south of the region as it is stretch across Long Island and beginning to move into southern CT. Some showers are beginning to ride along the front, with heavy precip back across eastern PA. Expect this precip to begin to fill in and move across the region through the morning hours. This precip could impact the morning commute, esp across the I-91 corridor. WAA is already pushing into the region as temperates have been warming the past few hours. Thus expect a mild start this morning and increasing humidity. Today... ***Heavy rain and flooding possible today*** ***Low risk for a strong to severe thunderstorm this afternoon*** Overview: Closed low pressure system over the northern Great Lakes will be the focus for wet weather today. Mid-level ridge over southern New England this morning will move offshore and build over the North Atlantic. Surface warm front will move from south to north of the region today and stall across NH. This will keep the region in the warm sector for much of the day before surface trough approaches from the west. Once thing to watch is how far north the front advects. If is slows down today, then it could be the focus for heavy precip and eventually flooding this afternoon. Details: This system appears to be focused on 2 different features. This first is the warm front this morning, with the second in the convection potential this afternoon. Weak shortwave will move through the southwest flow early this morning. This wave will continue to trigger precip across the region as the warm front lifts northward. Southerly flow will continue to advect moisture rich air into the region this morning as dewpoints rise in to the 70s. PWATs values will increase to above 2 inches indicating the potential for heavy rainfall. While at the surface, a warm front will lift through, most of the large scale dynamics is still to the north and west of the region. This is indicated by model QPF which is keeping the heavy rainfall amounts north and west of the region. However, cannot rule out some heavy precip moving across northern MA this morning. Because of the building ridge out east, QPF amounts across southeast MA will be less. Widespread precip this morning will become more scattered in nature as front should lift northward. Have noticed that the latest runs in the HRRR keep the front near the MA/NH border, which could be the focus for heavy precip through the afternoon hours. The focus then turns towards the afternoon hours. The region remains in a tropical moist atmosphere with PWATS near 2.2 inches, dewpoints well in the 70s and K values above 35C. Soundings continue to show warm cloud processes and with increasing LLJ at 925mb expect another round of showers and thunderstorms. Believe the NAM is a bit overdone with its CAPE values as cloud cover will limit destabilization today. However because of the high dewpoints and shear values between 30-40 kts a strong to severe storm is possible. Main focus will be heavy rainfall and gusty winds within any storms. Area of focus will be across the CT River Valley into western MA which is where the strongest shear and highest instability resides. NCAR ensembles are also pin-points this region with the best updraft potential. One thing to watch is the possibility for a spin-up in this tropical airmass combined with low LCLs. Several model guidances including the NCAR and SREF ensembles do show some rotation/helicity across this region and HRRR surface winds do back during the afternoon. BUFKIT hodographs are also quite impressive, so something to watch closely this afternoon. Heavy rainfall within any thunderstorm could have the possibility to lead to flooding. These high PWAT values are quite anomolous as they are near 3 STD above normal. Within any heavy thunderstorm, rainfall amounts could be near 1 to 2 inches per hours. Held off on Flash Flood watch as the flow is moving and storms do not appear to be training. But cannot rule out a few flash flood warnings being issued later today. Overall this is a messy day with a lot of mesoscale features occurring. Tried to trend the forecast with latest thinking, but expect rainfall this morning, a lull later in the day, with convection potential this afternoon. Western MA and CT seems to be the focus for convection as well as flooding. && .SHORT TERM /6 PM THIS EVENING THROUGH 6 AM SATURDAY/... Tonight... Thermal axis will peak over the region tonight as upper level trough begins to approach. LLJ will be on the increase early this evening. This combined with the warm, moist airmass and approaching upper level trough expect the first half of the night to be busy with ongoing precipitation. Conditions will slowly improve from west to east through the early morning hours, however the approaching cold front appears to stall either over or west of the region. Thus expect a very mild night as dewpoints will remain high, in the 70s. Could see a round of fog overnight as temp/dewpoint spread will be low. In fact, cannot rule out the dense fog potential. Overall, anticipate ongoing precip and fog through the night with conditions slowly improving from west to east by the morning hours. && .LONG TERM /SATURDAY THROUGH THURSDAY/... Big Picture... Subtropical high remains in control across the Southern USA while a general zonal flow continues across Canada and the Northern USA. The flow becomes less zonal next midweek as Alaska closed upper low digs south over the Gulf, subtropical upper ridge builds north over the Western USA, and upper low over Hudsons Bay digs south into the Northeast USA. One shortwave moves across New England Saturday night, with brief cooling aloft that destabilizes the airmass Sunday. A second shortwave crosses New England Wednesday as the Hudsons Bay low approaches and moves a supporting jet max across the region. Contour heights are normal to a little higher than normal through early next week. As the Quebec closed low deepens, heights over New England dip below normal midweek. Model mass and thermal fields from the 00Z suite are in general agreement through Wednesday, building forecast confidence through that time. Less agreement in the fields Thursday. Details... Saturday... Moderate confidence. Models hold the the cold front over Southern New England while moisture fields race east and drier air moves in during the late morning and afternoon. PW fields show 1.5 inch values across much of the region in the morning, but moving east of Cape Cod by evening. CAPE fields show 1000-1500 J/Kg over Easter CT/RI/Eastern Mass midday. Winds aloft are marginally significant, especially at 500 mb with speeds of 40-45 knots. Stability parameters are mixed, with Totals in the mid to upper 40s in the early night, and LI values subzero over RI and Eastern Mass. Expect the main area of showers/heavy downpours to move east of Massachusetts during the morning. Drier air moves in at low levels, with lingering moisture around 850 mb. This suggests at least some clouds during the day, but with potential for sunny breaks. This heating should push temps into the 80s, with enough instability to generate scattered showers/thunder along and ahead of the cold front. Saturday night... Cold front moves offshore, and surface winds become northwest. Skies will either clear or partially clear. Dew points will linger in the 60s, with min temps close to that level. Sunday... Upper shortwave and the coldest of the cold mid-level temps will be over New England. The more significant cold temps as well as cloud- level moisture will be over NH-VT. Cross-sections show Southern New England will have a moist layer based at 850 mb, but with drier air above and below that layer. Expect diurnal clouds, probably a bit more than guidance is suggesting, but otherwise a fair day with less noticeable humidity. Temps aloft 12-14C support max sfc temps in the upper 70s to mid 80s. Surface high pressure builds in with light variable wind Sunday night. With dew points in the upper 50s and around 60, expect min temps around this same range. Guidance temps were nudged a couple of degrees colder. Monday-Tuesday... High pressure builds over the region Monday and shifts offshore south and east by Tuesday. Cross sections show dry air in place at most levels through Tuesday evening, so expect dry weather both days. Increasing mid and high clouds Tuesday ahead of the next weather system. Temps aloft 14-15C Monday and 15-17C Tuesday, supporting low-mid 80s Monday and 85 to 90 on Tuesday. Tuesday night-Wednesday-Thursday... Closed low digs over Quebec and the Northeast USA with shortwave driving a cold front into/through New England. Expect showers/scattered tstms with the front Tuesday night/Wednesday. The airmass dries out Thursday, but lingering patches of moisture in the mixed layer should generate some diurnal sky cover. && .AVIATION /08Z FRIDAY THROUGH TUESDAY/... Forecaster Confidence Levels... Low - less than 30 percent. Moderate - 30 to 60 percent. High - greater than 60 percent. Short Term /through Tonight/... Before 12z...High confidence. VFR with MVFR/IFR CIGS not arriving until mainly after 10Z across the west. Winds generally light, shifting mainly S. Today and tonight...Moderate confidence. MVFR cigs moving in from west to east this morning as -RA/RA with embedded +RA with lower risk TSRA accompanies the dropping categories. Eastern terminals will be slower to drop, perhaps not til this afternoon. Through most of the day anticipate SCT RA/+RA/TSRA to develop into portions of MA and CT during the afternoon, dissipating into evening. Strong S winds around 10-15 kts with gusts up to 25 kts, strongest over S/SE New England. Potential LLWS overnight with 40-45 kt SW jet across S/SE New England. All conditions improving as winds turn W into early morning Saturday. KBOS Terminal...Moderate confidence. VFR with a gradual transition through the morning to MVFR/IFR along with a band of SHRA and occasional +SHRA. Low risk for occasional TSRA through the day. KBDL Terminal...Moderate confidence in TAF. Morning SHRA with IFR possible especially after 12Z. Some improvement is possible into the later-half of Friday prior to RA/+RA/TSRA encroaching from the S/W. Low confidence concerning timing. Outlook /Saturday through Tuesday/... Saturday... Moderate confidence. MVFR/IFR cigs and vsbys in showers and fog Saturday morning, then lingering showers/thunderstorms in Eastern Mass and RI in the PM. Trend all places will be to VFR. A cold front over the region will move offshore to the east by Saturday evening with winds shifting from the west. Areas of IFR cigs/vsbys in fog and low clouds later Saturday night. Sunday through Tuesday...Moderate-High confidence. VFR. Areas of IFR in early morning fog/low clouds, then improving to VFR by each mid morning and continuing the remainder of each day. && .MARINE... Forecaster Confidence Levels... Low - less than 30 percent. Moderate - 30 to 60 percent. High - greater than 60 percent. Short Term /through Tonight/...Moderate confidence. South winds will be on the increase, gusting up to 25 kts across the S/SE waters. SMALL CRAFT ADVISORY remain as winds will result in heightened seas up around 5 feet. Heavy rain with embedded thunderstorms sweeping W to E across the waters today and tonight. Will likely be some visibility restrictions. Outlook /Saturday through Tuesday/... Saturday...Moderate confidence. Lingering potential for scattered showers/tstms through the day, diminishing west to east late Saturday. Diminishing wind through the day. Some gusts 20-25 knots in the morning. Seas near 5 feet on the outer waters and on RI Sound, but trending lower through the day. Sunday through Tuesday...Moderate-High confidence. High pressure builds over the waters. Winds remain 20 knots or less, and seas 4 feet or less. && .BOX WATCHES/WARNINGS/ADVISORIES... CT...None. MA...None. RI...None. MARINE...Small Craft Advisory from 5 PM this afternoon to 9 AM EDT Saturday for ANZ231>234. Small Craft Advisory from 5 PM this afternoon to 2 AM EDT Saturday for ANZ236. Small Craft Advisory from 2 PM this afternoon to 11 AM EDT Saturday for ANZ235-237. Small Craft Advisory from 8 PM this evening to 2 PM EDT Saturday for ANZ250-254. Small Craft Advisory from 2 PM this afternoon to 8 PM EDT Saturday for ANZ255-256. && $$ SYNOPSIS...WTB/Dunten NEAR TERM...Dunten SHORT TERM...Dunten LONG TERM...WTB AVIATION...WTB/Dunten MARINE...WTB/Dunten

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