475 FXUS61 KBOX 201152 AFDBOX Area Forecast Discussion National Weather Service Boston/Norton MA 752 AM EDT Sat Jul 20 2019 .SYNOPSIS... Extreme heat and humidity is on tap for the weekend with dangerous heat indices between 105 and 115 degrees during both afternoons. Dry weather will remain through most of the weekend as high pressure dominates south of the waters. A cold front approaches the region, bringing the threat for showers and scattered thunderstorms Sunday into Sunday night. The front stalls south of New England Monday, with an area of showers and thunderstorms moving up along the front across our area later Monday through Tuesday. Eventually much cooler and less humid air moves into the region. Mainly dry Wednesday through Friday with seasonable temperatures and comfortable humidity levels. && .NEAR TERM /UNTIL 6 PM THIS EVENING/... 645 am Update Patchy fog at sunrise, especially in the coastal plain. This should burn off shortly. Also patchy mid to high clouds over Western and Central MA and adjoining CT. No changes expected to the forecast. Temperatures are starting warm and will get hot this morning. Humidity is high...many reports of 73 to 75 degree dew points, and the extreme values of dew point at the climate sites for this date have been 76. Expect similar dew point values through the day. Sufficient pressure gradient to maintain a west-southwest surface wind. Climate records are noted near the end of this discussion. Current conditions and forecast conditions indicate potential for several high temp records and high min temp records. Previous discussion... *** Excessive Heat Warning Continues into early Tonight *** *** Near Record High Temperatures *** Dry conditions weatherwise will continue today across the region with strong high pressure in place. Closest shower and thunderstorm activity noted across W NY into NW PA at 09Z. With the W-NW mid level steering flow in place, as well as strong subsidence and a mid level cap (H85 temps in the +13C range), should remain predominately through the day. Can't rule out a few brief, spotty showers mainly during the afternoon. The problem for the day will be the high heat and humidity that will prevail. Noting dewpoints in the lower-mid 70s at 09Z, which will continue through the day with a general W-SW surface wind flow, albeit light. However, expect highs to reach the 90s across most of the region, with the outer Cape and immediate S coast holding in the upper 80s. Can't rule out a few of the normal hot locations reaching 100 degrees. The combination of the heat and humidity will cause heat indices to rise to around 100 to 110 degrees during the late morning and afternoon hours. This will also cause problems with air stagnation, which can be problematic for folks that are sensitive to this. An Air Quality Alert remains in place across most of N CT as well as RI into SE Mass until 11 PM tonight. && .SHORT TERM /6 PM THIS EVENING THROUGH 6 PM SUNDAY/... Tonight... If any showers/thunderstorms do develop this afternoon, they won't last long into the evening. So, will see dry conditions weatherwise again tonight. Expect SW winds up to around 10 mph. However, it will be very uncomfortable night for sleeping with temps only falling back to the mid 70s to around 80 away from the coast, warmest in the urban centers. Could be a few degrees cooler along the immediate S coast. Expect generally light S-SW winds in place, possibly up to about 10 mph across Cape Cod and the islands. May also see patchy fog develop along S coastal areas, Cape Cod and the islands. Sunday... Expect another day of high temperatures and humidity. Highs may once again reach the mid and upper 90s, with a few spots possibly hitting 100. However, short range models signaling the possibility of a weak front beginning to slide SE toward the region. This could be the trigger for scattered showers and thunderstorms to develop starting around mid to late Sunday morning across the Route 2 area of N Mass, then may slide slowly S during the afternoon. Noting very high PW values in this tropical airmass, on the order of 2 to 2.3 inches, so this is a chance that some activity could produce locally heavy downpours. If the front pushes into N Mass late in the day, may see winds begin to shift to W-NW, but this will depend upon the timing of the front. && .LONG TERM /SUNDAY NIGHT THROUGH FRIDAY/... Big Picture... Upper ridge builds over the Western USA, an extension of the broad Subtropical ridge covering the Southern USA coast-to-coast. Upper low pressure sits over Hudsons Bay while another closed upper low sits over the Gulf of Alaska, and shortwaves are transmitted along the Canadian border from the latter to the former. Originally the first shortwave was supposed to interact with the Hudsons Bay to dig a trough over the Northeast USA...but now the models show the digging trough as a consequence of the second shortwave. The trough digs over the Eastern USA Sunday night or Monday, and lingers through most of the week. Heights at 500 mb start falling Sunday night and reach the mid-570s by Tuesday, a little below the seasonal average. Heights then trend back toward the average for the latter half of the week. Based on this we would expect a trend to normal summertime temperatures during the work week, most notable Wednesday through Friday. General big-picture agreement among the model mass and thermal fields through Thursday, but with differences evolving Wednesday through Friday. Forecast confidence is moderate-high overall, but edged more toward straight moderate for Monday-Tuesday due to differences in timing a shortwave through our area. Concerns... Sunday night through Tuesday... Weak cold front, associated with the first shortwave in the zonal flow, drops through Southern New England late afternoon/evening. The front then becomes aligned with the zonal flow as it moves off the South Coast, stalling the front just to our south. As the trough digs over the Great Lakes, the upper flow over us turns more from the southwest. Favorable jet dynamics remains off to the north/west much of Monday, but approaches our western areas during the afternoon and moves across during Monday night and Tuesday. This dynamics is expected to form a wave on the stalled front, with the wave approaching from PA late Monday and Monday night. There is uncertainty in timing the onset of this precipitation, with some guidance moving it in Monday morning while the rest suggests mid to late afternoon. We favor the slower timing, but will monitor. Cross sections show deep moisture moving overhead as the wave passes, as well as deep lift of 30-40 mb/hour (or 10-12 microbars per second). Uncertainty remains with the positioning of the wave, with some guidance bringing the wave over Southern New England while other guidance keeps it south. Either way, expect showers Monday night. The ECMWF and NAM show totals in the mid-upper 40s Monday night while the GFS is lower. Lifted index is subzero mainly along the South Coast but this may extend north across much of CT-RI- Southeast MA. Precipitable water values linger around 2 inches during this period. So expect showers, mostly Monday night and Tuesday morning, with local downpours possible and scattered or isolated thunder. The wave and its front move off to the south/east Tuesday, at which time the should taper off. Lower PWs indicate drier air moving in at that time. East winds Monday will bring cooler temperatures compared with the weekend. Inland heating will build a mixed layer to about 900 mb, where temps equiv to 850 mb would be 14-16C...supporting mid 80s. Cooler near the shore. Clouds and showers will limit Tuesday even further, with equiv temps around 10C supporting max temps 75 to 80. Wednesday through Friday... Upper trough moves overhead Wednesday, then transitional flow Thursday/Friday. Overall a dry period with a much less humid airmass. Some clouds possible Wednesday as the trough moves overhead. Mixing to 800 mb each day will tap 6-8C, equiv to 11-13C at 850 mb, which should support max temps in the upper 70s to mid 80s...warmest Friday. && .AVIATION /12Z SATURDAY THROUGH WEDNESDAY/... Forecaster Confidence Levels... Low - less than 30 percent. Moderate - 30 to 60 percent. High - greater than 60 percent. Short Term /through Sunday/...Moderate to high confidence. Today...Any MVFR-IFR conditions along the immediate S coast should improve by 14Z. Otherwise, expect VFR conditions. W winds 5 to 10 kt across most areas, except SW winds along the coast with gusts up to around 15 kt. Dry weather prevails. Tonight...VFR. Dry conditions continue. MVFR-IFR conditions in areas of fog by 01Z, except patchy LIFR conditions INVOF KACK. Conditions should improve to VFR by 06Z-07Z. W-SW wind 5-10 kt, with gusts to 15 kt this evening. Sunday...Mainly VFR. May see MVFR conditions in scattered SHRA/TSRA by around mid morning across N and W Mass, then slowly pushing S during the day. W winds up to 10 kt, may shift to W-NW late in the day across N Mass. Gusts up to around 15-20 kt in any thunderstorms. KBOS Terminal...High confidence in TAF. KBDL Terminal...High confidence in TAF. Outlook /Sunday Night through Wednesday/...High confidence. Sunday Night: Mainly VFR, with local MVFR possible. Slight chance SHRA, isolated TSRA. Monday: Mainly VFR, with areas MVFR possible. SHRA likely, chance TSRA. Monday Night: MVFR/IFR conditions possible. SHRA likely in the afternoon, chance TSRA, patchy BR. Tuesday: Mainly MVFR, with local IFR possible. Breezy. Chance SHRA. Tuesday Night: Mainly VFR, with local IFR possible. Patchy BR. Wednesday: VFR. && .MARINE... Forecaster Confidence Levels... Low - less than 30 percent. Moderate - 30 to 60 percent. High - greater than 60 percent. Short Term /through Sunday/...High confidence. Today and Tonight...W winds 10 kt or less, shifting to SW 10-15 kt with gusts up to 20 kt from midday today through this evening. Winds will diminish to around 10 kt from about midnight onward. Periods of low clouds and fog across the southern waters tonight. Seas 4 ft or less. Sunday...W-SW winds around 10 kt with gusts up to around 15 kt. May see low clouds and fog early Sunday, then should improve. Scattered showers and thunderstorms possible from late morning through the afternoon. Winds may shift to W-NW across the waters around Cape Ann late in the day. Outlook /Sunday Night through Wednesday/...High confidence. Sunday Night: Winds less than 25 kt. Slight chance of rain showers, isolated thunderstorms. Monday: Winds less than 25 kt. Rain showers likely, slight chance of thunderstorms. Monday Night: Winds less than 25 kt. Seas locally approaching 5 ft. Rain showers likely, chance of thunderstorms. Local visibility 1 to 3 nm. Tuesday: Winds less than 25 kt. Local rough seas. Rain showers likely, chance of thunderstorms. Tuesday Night: Winds less than 25 kt. Local rough seas. Slight chance of rain showers, patchy fog. Areas of visibility 1 to 3 nm. Wednesday: Winds less than 25 kt. Areas of seas approaching 5 ft. && .CLIMATE... Record highest min temps for Saturday July 20: BOS: 80 in 1977 BDL: 73 in 1977 PVD: 76 in 1977 ORH: 72 in 1977 Record highs for Saturday July 20: BOS: 99 in 1991 BDL: 100 in 1991 PVD: 101 in 1991 ORH: 95 in 1991 Record highest min temps for Sunday July 21: BOS: 81 in 1991 BDL: 74 in 1977 PVD: 77 in 1980 ORH: 73 in 1991 Record highest max temps for Sunday July 21 BOS: 102 in 1977 BDL: 101 in 1991 PVD: 102 in 1991 ORH: 95 in 1926 Last Occurrence of 100F or higher temperatures BOS: 7/22/2011 (103F) BDL: 7/18/2012 (100F) PVD: 7/22/2011 (101F) ORH: 7/4/1911 (102F) Notable high dew points in past years for July 20 and July 21: Saturday July 20 BOS 76 in 2013 (last of multiple years) BDL 76 in 2005 PVD 76 in 2013 (last of multiple years) ORH 76 in 2013 Sunday July 21 BOS 76 in 1994 BDL 78 in 1972 PVD 81 in 1977 (tied for highest all-time at PVD) ORH 76 in 2011 (last of multiple years) && .BOX WATCHES/WARNINGS/ADVISORIES... CT...Excessive Heat Warning until 8 PM EDT Sunday for CTZ002>004. MA...Air Quality Alert from 11 AM this morning to 11 PM EDT this evening for MAZ017>024. Excessive Heat Warning until 8 PM EDT Sunday for MAZ002>021- 026. Heat Advisory until 8 PM EDT Sunday for MAZ022-023. RI...Air Quality Alert from 11 AM this morning to 11 PM EDT this evening for RIZ001>008. Excessive Heat Warning until 8 PM EDT Sunday for RIZ001>007. MARINE...None. && $$ SYNOPSIS...WTB/EVT NEAR TERM...WTB/EVT SHORT TERM...EVT LONG TERM...WTB AVIATION...WTB/EVT MARINE...WTB/EVT CLIMATE...