textproduct: Gray - Portland

WHAT HAS CHANGED

An Extreme Heat Watch has been issued for portions of NH and southwestern ME for Wednesday through Friday. Aviation section updated for 12Z TAFs.

KEY MESSAGES

1. High pressure brings fair weather today.

2. Hot and Humid conditions arrive this week, with near record breaking heat increasingly likely for mid to late week. Any preparations, such as installing air conditioning units, would best be completed early this week before the hottest conditions begin on Wednesday.

3. Building heat and humidity will provide fuel for thunderstorms starting Tuesday with chances continuing most days through the end of the week.

DISCUSSION

KEY MESSAGE 1 DESCRIPTION...

Upper trough axis pushes east early this morning with high pressure building in today from southeast Canada. This will allow for a pleasant day with highs in the 80s with relatively comfortable humidity levels. A weak gradient over the area will allow for a healthy sea breeze to develop by early afternoon that will be able to push well inland.

KEY MESSAGE 2 DESCRIPTION...

The warming trend continues on Tuesday with widespread highs in the 80s to lower 90s across the interior. Along the coast, onshore flow likely keeps highs mainly in the 70s for one more day. Dew points will generally be into the lower 60s and therefore heat indices will not differ significantly from actual air temperatures.

By Wednesday, the significant heat likely arrives into at least New Hampshire and western Maine as highs warm into the 90s. What makes this heat event notable is just how much moisture will accompany the heat. Dew points likely push into the low to mid 70s on Wednesday, pushing heat indices into the triple digits where temps reach the 90s. These high dew points continue through at least Friday. Overnight low temperatures on Wednesday through Friday night will only be into the 70s across many locations, which will result in accumulated heat stress.

One possible limiting factor for the heat remains the potential for rounds of convection originating near the Great Lakes, and riding around the ridge into New England. Should these arrive during the midday timeframe, they would serve to limit the heating potential. But arriving at any other time of the day, they will likely only serve to increase dew points. We'll likely have to wait until tomorrow to have a better handle on how these will effect temperatures in the Wednesday through Friday time frames, as the differences will be localized and variable.

Outside of any convection, the heat looks most likely to peak on Thursday and Friday. Modeled 850mb temps of 22-24C support the potential for near record high temps of 97-102 degrees, barring any hinderance from convection or sea breezes. It would be a misuse of time to tinker with which side of 100 temperatures will reach in the Thursday and Friday timeframe at this point in time. The main message remains that heat and humidity hazardous to health is increasingly likely as heat indices push 110 degrees by Thursday.

As a result, an Extreme Heat Watch has been issued for portions of NH and southwestern ME for Wednesday through Friday. The watch covers the locations most likely to see heat indices of 105 degrees or greater each afternoon. Heat advisories will likely be needed elsewhere as we get closer to the event and its possible that portions of the watch will later be converted to advisories. Given the increasing confidence for a potentially high impact heat event, wanted to go ahead with a watch now to help messaging and preparation.

Additionally, the trend over the last few model runs has been for an increasing chance of the heat to continue into at least Friday or Saturday. The models first trended warmer for Wednesday and Thursday over the last few days, and now look to be trending this way for Friday and Saturday as well. We will continue to watch the progression of this, with a trough and cooler conditions situated just to our east across Atlantic Canada.

KEY MESSAGE 3 DESCRIPTION...

Low pressure tracking northeast across Canada will lift a warm front into the area Tuesday afternoon with this front pushing through northern Maine Wednesday morning. This front will provide a focus for showers and storms. While the best instability looks to remain west of the area there will be sufficient deep layer shear near the front for organized convection. Several model solutions are hinting at the potential for an MCS to track southeastward across the Northeast late Tuesday with additional rounds of convection into Tuesday night. SPC has a Marginal Risk for severe storms across Upstate NY and Vermont that pushes into the CT Valley and could see this trend farther east based on latest CAM output.

On Wednesday there exists the potential for unusually high levels of CAPE for this part of the county as models continue to suggest a remnant EML will bring steep mid level lapse rates over the Northeast. While there will be a lack of surface forcing and mid level support for ascent any convective complexes that can ride atop the ridge could bring the threat for severe storms across the Northeast. The finer details on timing and coverage remain unclear Wednesday and beyond while the overall pattern may continue to support storms through Friday. Machine learning guidance out of CSU continues to have this signal.

AVIATION /12Z MONDAY THROUGH FRIDAY/

Through 12Z Tuesday...Mostly VFR expected with light northerly wind becoming southerly behind a sea breeze later today. Light and variable winds will prevail tonight along with some possible valley FG at KLEB and KHIE as well as along the coast.

Outlook:

Tuesday - Friday: Mainly VFR prevails at most terminals, but rounds of showers and storms will be possible each day. Marine fog will be possible at RKD.

MARINE

Marine stratus or fog seems likely to remain around the coastal waters for much of the coming week in between systems.

High pressure gradually settles south of the waters by midweek. Mainly fair conditions prevail, but some seas to near 5ft are possible Tuesday night in southwesterly flow.

CLIMATE

High temperature records at long term climate sites...

July 1st 2nd 3rd

AUG 92 93 94

PWM 93 98 95 CON 99 98 102

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.