Area Forecast Discussion
Issued by NWS Boston, MA

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537 FXUS61 KBOX 171057 AFDBOX Area Forecast Discussion National Weather Service Taunton MA 657 AM EDT Thu Aug 17 2017 .SYNOPSIS... Dry and pleasant conditions continue today. A weather system moving through the Great Lakes Region towards the end of the work week will bring showers and scattered thunderstorms to southern New England during Friday and Saturday. High pressure returns with fair seasonable weather Sunday through Tuesday. A cold front approaches from the west Tuesday night and Wednesday, bringing our next chance of showers. && .NEAR TERM /THROUGH TONIGHT/... 7 AM Update... A few areas of fog, and patches of clouds. Otherwise fair skies. Expect the fog to dissipate this morning, with mostly sunny skies through the day. Very comfortable...morning dew points were ranged between 45 and 55. Temperatures are starting about 2-3F cooler than forecast in the interior, but on target otherwise. Previous discussion... Today... Transition day for southern New England. Surface high pressure crests overhead today resulting in light and variable winds. Thus sea breezes will develop along both coastlines. High temps will be a bit cooler today compared to yesterday thanks to the cool morning start and very little airmass change. Temps will be in the upper 70s to low 80s. Overall another pleasant summer day across the region. BEACH Forecast: Hurricane GERT is well east of the region. However there is still some lingering swell and thus anticipate a moderate risk for rip currents. Have contemplated about issuing a rip current statement out but decided to hold off as both waves, swells and the period will continue to drop through the day. Thus making on the morning hours more of a risk per the Surf matrix. Swimmers are urged to still use caution if heading into the surf today. Tonight... Quiet weather for the first half of the night. High pressure will move offshore as winds turn more to the south. Dewpoints will begin to rise through the overnight hours ahead of approaching warm front. Clouds will be on the increase within the WAA pattern limiting radiational cooling. Therefore temps will remain in the low to mid 60s. While most of the night will be dry, will have to watch for the development of a few showers moving into the region from overrunning in the higher theta-e. Best area will be across CT and western MA during the early morning hours on Friday. && .SHORT TERM /FRIDAY THROUGH FRIDAY NIGHT/... Friday and Friday Night... Overview: Mid-level ridge axis will slide through the region on Friday as a potent upper level trough moves into the Great Lakes. Upper level disturbance moving through the flow will push a warm front into the area on Friday, keeping southern New England in the warm sector into early Saturday morning when a cold front approaches from the west. Details: Warm, moist humid airmass will advect into the region early Friday. This will result in dewpoints rising into the low 70s with PWAT above 2 inches, which is 2 STD above normal. Quite anomalous for this time of year. Passing warm front will some precipitation in the morning. However, the focus is on later in the day when diurnal heating takes place, and elevated instability begins to move into the region. This combined with increasing 850mb LLJ could result in more widespread precipitation as well as a few thunderstorms. This could make for a messy evening commute. The potential for strong or severe storms will be limited as SB CAPE values are quite low with poor lapse rates. However, shear values are quiet strong and with the 70+ dewpoints, could help compensate the lack of instability. LI values and showalters do drop below 0 indicating some elevated instability so will keep the mention for a few thunderstorms. Main concern with any convection will be tropical downpours which could result in high rainfall rates. This is also suggested by WPC day 2 excessive rainfall outlook. Contemplated the idea of some sort of flood watch, but the surface and low level flow is moving so not expecting training cells. For now will hold off and let day crew get another look. Thermal axis will peak over the region Friday night as upper level trough begins to approach. Still anticipate ongoing showers and isolated thunder as the airmass remains unchanged. However upper level trough will begin to approach so the first half of the night may 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. This may be the focus for precip on Saturday. For now, anticipate ongoing precip and fog through the night with conditions slowly improving from west to east by the morning hours. && .LONG TERM /SATURDAY THROUGH WEDNESDAY/... Big Picture... Subtropical high remains in control across the Southern USA while a general zonal flow continues across Canada and the Northern USA. The high actually builds over the Western USA early next week, supporting a deepening of the wave pattern during next week. One shortwave moves through New England over the weekend. A second shortwave moves across from the Gulf of Alaska, forms a closed low over northern Canada, and then digs over Quebec midweek. 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 next. Model mass and thermal fields from the 00Z suite are in general agreement through Wednesday, building forecast confidence. Details... Saturday... Moderate confidence. Models hold the the cold front over western and central sections of Southern New England while moisture fields race east and drier air moves in during the afternoon. PW fields show 1.5 inch values move east of BOS-PVD by Saturday afternoon. CAPE fields show 1000-1500 J/Kg east of a ASH-ORH-IJD line in the afternoon. Winds aloft are marginally significant, with 25-30 knots at 850 mb and 35-40 knots at 500 mb. Stability parameters are mixed, with Totals in the mid 40s, except upper 40s ECMWF, and LI values subzero. Expect the main area of showers/heavy downpours to move east of Massachusetts during the morning, but with lingering scattered showers/thunder possible in the afternoon until the cold front moves past. With the flow becoming more southwest, expect deeper mixing than on Friday...and so a warmer airmass. Temps of 15-17C at 850 mb would support mid-upper 80s. Depending on the degree of clearing, temps could be a little lower than this. Expect max temps in the 80s. Surface dew points in the southerly flow will linger in the upper 60s and low 70s, so expect a warm and very humid day. Saturday night... Cold front moves offshore, and surface winds become northwest. Expect any showers to taper off. Skies will either clear or partially clear overnight. 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, with drier air closer to the surface. Expect diurnal clouds, probably a bit more than guidance is suggesting, but otherwise a fair day with less noticeable humidity. 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 mid to upper 80s Tuesday. Tuesday night-Wednesday... Closed low digs over Quebec and the Northeast USA with shortwave driving a cold front into/through New England. Expect showers/scatterd tstms with the front Tuesday night/Wednesday. && .AVIATION /12Z THURSDAY THROUGH MONDAY/... Forecaster Confidence Levels... Low - less than 30 percent. Moderate - 30 to 60 percent. High - greater than 60 percent. Short Term /through Friday Night/... Today...High confidence. VFR. Winds shift lightly W during the day except sea breezes along coast. Tonight...High confidence. VFR to start with deteriorating conditions after 09z as widespread rain approaches. Winds will turn more southerly overnight. Patchy ground fog is possible at typically prone terminals Friday into Friday night...Moderate confidence in trends, lower confidence on timing. Conditions will fall to MVFR/IFR through the day as rain moves from west to east and could be heavy at times restricting vsbys. Iso -TSRA is also possible on Friday. Southerly winds with gusts near 15 kts by the afternoon. Low clouds and precip will continue into the overnight hours. Slow improvements out west after 10z. KBOS Terminal...High confidence in TAF. Sea breeze development expected around 14-15Z. KBDL Terminal...High confidence in TAF. Outlook /Saturday through Monday/... Saturday... Moderate confidence. MVFR/IFR cigs and vsbys in showers and fog Saturday morning will improve to predominant VFR during the day. Continued MVFR ub scattered showers/thunder through afternoon especially in RI/Eastern Mass. Southerly winds during the day will shift from the west/northwest Saturday night after a cold front moves through. Areas of IFR cigs/vsbys in fog and low clouds later Saturday night. Sunday and Monday...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 Friday Night/...High confidence. Seas and swell have begun to subside early this morning across the southern waters. Thus have dropped the SCA. Otherwise, mainly quiet boating weather continued into tonight once seas subside. Approaching weather system on Friday will reduce vsbys in heavy rain. Waves will also build as southwesterly flow increases with gusts near 15 to 20 kts by Friday evening. SCA may be needed but confidence is low. Outlook /Saturday through Monday/... Saturday...Moderate confidence. Lingering potential for scattered showers/tstms through the day, diminishing west to east Saturday night. 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 later in the day. Sunday and Monday...Moderate-High confidence. High pressure builds over the waters. Winds remain 20 knots or less, and seas 4 feet or less. Leftover showers early, but these move off to the east during the morning. && .BOX WATCHES/WARNINGS/ADVISORIES... CT...None. MA...None. RI...None. MARINE...None. && $$ SYNOPSIS...WTB/Dunten NEAR TERM...WTB/Dunten SHORT TERM...Dunten LONG TERM...WTB AVIATION...WTB/Dunten MARINE...WTB/Dunten

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