407 FXUS61 KGYX 291834 AFDGYX Area Forecast Discussion National Weather Service Gray ME 234 PM EDT Mon Jun 29 2026 .WHAT HAS CHANGED... A slight expansion of the Extreme Heat Watch into southern Grafton/Lebanon area with this forecast update. Saturday is trending hotter as well. && .KEY MESSAGES... 1. Heat builds into the region by Wednesday and continues through at least Friday. Saturday is also trending hotter as well. High dew points combine with the heat to bring hazardous conditions from Wednesday through at least Friday. To be completed ahead of Wednesday's heat, all AC installations and preparations should be completed by Tuesday. 2. Showers and thunderstorms will be possible most days through the holiday weekend. Given the ample heat and humidity any of these thunderstorms days could also see strong to severe storms. && .DISCUSSION... KEY MESSAGE 1 DESCRIPTION... Temperatures climb into the upper 80s to low 90s back from the coastline on Tuesday. With dew points still in the low 60s, heat indices remain close to the actual air temperatures, and remain below advisory levels. The significant heat arrives on Wednesday, at least for New Hampshire and western Maine. Highs climb into the mid to upper 90s here, with mid 80s to low 90s elsewhere. Wednesday likely still features a sea breeze along the immediate coastline, helping to keep highs in the 80s along the Maine coast. Dew points climb into the low to mid 70s, pushing heat indices into the 105 to 110 degree range for much of New Hampshire and western Maine. Dew points likely drop a little more than the NBM is suggesting through the Merrimack River Valley with afternoon mixing, so heat indices have been tapered down by a few degrees from the NBM. However, this would still be warning level heat. We are holding with the watch for now with uncertainty around possible convection, which is discussed in more detail in Key Message 2. Thursday continues to look to be the peak of the heat. With 850mb temps around 22C, high temperatures either side of 100 degrees looks likely for much of the interior, and especially for southern New Hampshire. There remains some question as to whether the coast will see a sea breeze develop of not, but this will also be influenced by the possible convection. Dew points look to push into the low to mid 70s again, with any rain Wednesday night only helping to push dew points higher. This pushes heat indices to near 110 degrees anywhere the air temps reach the upper 90s to low 100s. The story remains much the same for Friday, with temperatures likely within a couple degrees of Thursday's highs. Widespread upper 90s to near 100 degrees looks likely again. Dew points may start to come down slightly, but at this point still look to run in the upper 60s to low 70s. Saturday increasing looks hot, with heat likely continuing for at least New Hampshire. Model trends over the last couple of days have trended toward the ridge holding on for at least one more day. With this still being five days out, and daily convection playing a compounding roll over time on the airmass evolution, it's still too early to say how hot it will be and where for the 4th of July. However, it's likely a good idea to plan for another day of highs in the mid to upper 90s across much of the interior. Sunday looks likely to offer some relief from the heat, but doesn't look to be a major cool down at this point. Right now 80s to low 90s looks most likely, but will depend on how the preceding days evolve. Nighttime low temperature forecasts will be tricky Tuesday night through Friday night. Without storms, lows only making it down into the mid to upper 70s is likely across southern and central locations. However, with any thunderstorms temperatures would quickly drop into the upper 60s to low 70s. So the low temperature forecast tries to split the difference between these two outcomes, but acknowledges that there will be variability across the area. Without storms, lows around 80 degrees are likely through the Merrimack River Valley Wednesday and Thursday nights. The temperatures discussed each day are for outside of any convection or thunderstorms. There remains uncertainty on the extent and timing of any storms, with any daytime convection limiting the heat potential. This is discussed further in Key Message 2. KEY MESSAGE 2 DESCRIPTION... It will be a difficult convective forecast thru the extended. The long story short is that given the high temps and climbing dewpoints, CAPE will not be a problem. Any storms will have plenty of fuel. The 29.12z GFS forecast also shows a good example of how shear is rarely an issue around these parts too. Thru its forecast for the next week bulk shear remains at least 25 kt, which is enough to keep storms semi-organized. Looking at guidance from various sources, including the ECMWF EFI, the CAPE/shear combo remains seasonably strong into the weekend. So we have the shear, the instability, and the moisture. The remaining question is lift - we will need a trigger to produce thunderstorms. There are a few potential options for triggering convection thru the period. To get more widespread thunderstorms and severe weather would be to get a synoptic shortwave to swing thru the region. But model guidance does not really want to lower heights until Fri at the earliest. That means any height falls aloft will have to come from convectively induced shortwaves. Those will be difficult to predict with much more than 24 hour lead time. Finally local influences could also trigger storms, including differential heating the mtns and along any sea breeze circulation. So my general thought is that most days at least isolated severe thunderstorms will be possible, and if a well defined convective shortwave is approaching or the late week height falls materialize the threat could become more widespread in coverage. Looking at forecast soundings and taking conceptual models into account, the primary hazard from any storms will be first damaging wind. Large hail is possible but given how warm it will be that threat is somewhat lower than usual. Likewise without a strong synoptic influence the tornado threat is lower than usual. And as always any storm will be capable of torrential rain and frequent lightning. At least during the work week there will be the threat for both afternoon and nocturnal convection. CAM guidance continues to hint at ridge rolling MCSs approaching and decaying thru the region Tue and Wed. Thru the week that threat for storms should gradually shift to the south. && .AVIATION /18Z MONDAY THROUGH SATURDAY/... VFR conditions expected thru Tue. Winds turn southwest tonight and that should keep marine fog at bay for the next 24 hours anyway. Some guidance does hint at fog for LEB and HIE tonight, but given it did not happen in any widespread fashion last night I have held off from including in the TAF. Outlook: Wednesday Night: Conditions may begin to deteriorate as marine fog/stratus becomes more likely. Confidence is low at this time in MVFR or lower conditions. Local MVFR or lower also possible in showers/thunderstorms. Thursday - Saturday: Local MVFR or lower possible in showers/thunderstorms. Areas of IFR or lower possible at night with marine fog/stratus. Saturday Night - Sunday: Some clearing possible from the north as the heat and humidity gets pushed south. && .MARINE... Winds and seas expected to remain below SCA thresholds thru the holiday weekend. The main hazards to watch for will be developing marine fog, especially as winds become onshore ahead of any low pressure approaching, and then thunderstorms which will be possible most days into the weekend. && .CLIMATE... High temperature records at long term climate sites... July 1st 2nd 3rd AUG 92 (1971) 93 (1963) 94 (2002) PWM 93 (1971) 98 (1941) 95 (2002) CON 99 (1913) 98 (1966) 102 (1966) && .GYX WATCHES/WARNINGS/ADVISORIES... ME...Extreme Heat Watch from Wednesday afternoon through Friday evening for MEZ012-013-018>021-023>026-033. NH...Extreme Heat Watch from Wednesday afternoon through Friday evening for NHZ004-006>015. MARINE...None. && $$ DISCUSSION...Clair/Legro AVIATION...Legro MARINE...Legro