textproduct: Gray - Portland

WHAT HAS CHANGED

Heat Advisories have been expanded for Friday to include coastal Cumberland county and the Midcoast region. Opted to begin them as soon as the Extreme Heat Warnings end (8 PM), as accumulated heat stress from warm overnight lows remains a factor.

KEY MESSAGES

1. Heat continues to build and then lingers into at least Friday. Overall the heat remains on track, with minor tweaks to temperatures and heat indices over the next few days as dewpoints will be trending cooler.

2. Isolated strong to severe storms possible this evening (Level 1/5 for severe weather) and again Friday evening.

3.The ridge responsible for the extreme heat breaks down late Friday through the weekend allowing for gradually cooling temperatures. Shower and storm chances are greatest Saturday and then again by Monday/Tuesday of next week.

DISCUSSION

KEY MESSAGE 1 DESCRIPTION... Not much has changed in regards to the temperature forecast Friday as +18-21C heights will mix down to surface temperatures in the mid to upper 90s for locations south of the mountains (mid to upper 80s will be more the story to the north). West winds should fight off the seabreeze so these warm temperatures will be felt right down to the water once again. These continued west winds are likely going to make ambient temperatures a degree or two higher in spots, but we will also see dewpoints start to trend cooler, so despite this the apparent temperatures likely end up a hair cooler than what is being experienced today. For this reason, I don't think its necessary to continue Extreme Heat Warnings for coastal Cumberland county and the Midcoast region and instead have added them into the Heat Advisory. Areas currently slated to remain in the Extreme Heat Warning are unchanged, as even if we don't quite hit apparent temperatures of 105F the accumulated heat stress over the last several days justify continuing it. A front is still expected at some point on Saturday which will bring cooler dewpoints, but summer temperatures will stick around through the holiday weekend with mid to upper 80s south of the mountains and upper 70s to low 80s to the north. The advantage is that with less humidity it will feel more like the ambient temperature and less like the upper 90s and low 100s. The only exception may be southern New Hampshire on Saturday, depending on the progress of the front, where apparent temperatures may climb into the upper 90s but will hold off on any advisories for now due to the uncertainty.

KEY MESSAGE 2 DESCRIPTION... The signal for any convective activity is rather low both this evening and again Friday evening with forcing rather weak as we sit on the northern edge of deep layer ridging over the Mid Atlantic and southeastern states. That being said...the environment will support strong/severe storms if a storm is able to initiate. CAMs suggest the best potential for this comes in the evening hours both this evening and again Friday evening. While coverage is likely to be low...there will be moderate deep shear....moderate MLCAPE and rather robust DCAPE to support an isolated severe storm with damaging wind gusts the most likely hazard followed by a lower potential for hail. These chances will be best over NH given the ridge location and recent CAM guidance.

KEY MESSAGE 3 DESCRIPTION... A shortwave trough brings a more substantial cool frontal passage Saturday with an afternoon round of showers and thunderstorms possible before we get a more robust push of cooler/drier air for Sunday with temperatures still above normal Sunday and then closer to early July norms for Sunday. Beyond this...there is overall model consensus on another...slower moving upstream shortwave with has the potential to bring a round of unsettled weather Tuesday if the associated surface low tracks close enough to the region. There is roughly a 50/50 split amongst the ensemble guidance on the wetter/cooler solution at this time with the greatest threat for rainfall being over southern NH and SW ME. Regardless of the Tuesday forecast...there is more confidence that the forecast will turn drier and warmer for Wednesday and the end of the week.

AVIATION /18Z THURSDAY THROUGH TUESDAY/

Through 18Z Friday...VFR conditions are expected to prevail through Friday afternoon. Isolated to scattered showers remain possible this afternoon and evening and confidence is low on where exactly they will pop up, but if they do pass over a terminal expect brief MVFR conditions. Any instances of fog tonight should also be very isolated.

Outlook... Friday night-Monday:Isolated-scattered showers/storms are possible Saturday afternoon and evening. Otherwise...VFR conditions are expected to dominate the period.

MARINE

Westerly winds will continue through the day Friday, fighting off the seabreeze. Wind gusts and seas remain below SCA levels through much of next week.

CLIMATE

High temperature records at long term climate sites...

July 3rd

AUG 94 (2002)

PWM 95 (2002) CON 102 (1966)

GYX WATCHES/WARNINGS/ADVISORIES

ME...Heat Advisory until 8 PM EDT this evening for MEZ007>009. Extreme Heat Warning until 8 PM EDT Friday for MEZ012-018>020- 023-033. Heat Advisory until 8 PM EDT Friday for MEZ013-014. Extreme Heat Warning until 8 PM EDT this evening for MEZ021- 022-024>028. Heat Advisory from 8 PM this evening to 8 PM EDT Friday for MEZ021-022-024>028. NH...Heat Advisory until 8 PM EDT this evening for NHZ001>003. Extreme Heat Warning until 8 PM EDT Friday for NHZ004>015. Air Quality Alert until 10 PM EDT this evening for NHZ013-014. MARINE...None.