textproduct: Gray - Portland

WHAT HAS CHANGED

Confidence continues to increase, despite a few complicating factors, for mid to late week heatwave potential.

KEY MESSAGES

1. Seasonable temps and scattered showers continue through Sunday.

2. A hot airmass moves into the region this week, this time accompanied by humid conditions. This combination will bring the potential for extreme heat Wednesday and Thursday for interior/coastal Maine and the southern half of New Hampshire. Prepare cooling systems early this week, and if plans include strenuous outdoor activities, consider rescheduling for the morning when heat isn't as intense.

DISCUSSION

KEY MESSAGE 1 DESCRIPTION...

With a trough lingering across northern New England through the end of the weekend, the weather pattern remains similar through Sunday. Scattered showers and an isolated thunderstorm this afternoon taper off through the evening with the loss of daytime heating.

Tonight sees another seasonably cool night with lows generally dropping into the 50s. Some patchy fog is likely again overnight, and will be most prevalent along the coast and through the river valleys.

Tomorrow features another day of morning sunshine giving way to pop up showers and a few thunderstorms in the afternoon hours. These will be more widespread across the higher terrain of Maine, and stand a better chance of reaching the coastline than today's showers. High temps warm a little more tomorrow, with generally mid to upper 70s expected across the north and along the coast, and low 80s elsewhere.

Showers dissipate again Sunday evening, with another seasonably cool night expected as lows bottom out in the 50s. A light west is expected Sunday night. This keeps the fog coverage less overnight, and it will most likely remain confined to the river valleys.

KEY MESSAGE 2 DESCRIPTION...

The main takeaway going into this week is pairing heat with humidity to potentially bring a summer heatwave. The combination featuring periods of 97 to 99th percentile precipitable water plus around 99th percentile 850mb temperatures instills greater than normal confidence at this range. The outcome would be days (and nights) worth of cumulative heat, leading to building heat stress.

After a pleasant weekend with temperatures in the 70s to lower 80s, a warming trend starts the week, with hot conditions settling over the area by mid to late week. Tuesday night, anomalously warm 850mb temperatures begin to advect eastward through the Great Lakes and Ohio Valley with high pressure set up across the South. Temperatures at this level could push into the lower 20s (C) by midday Wednesday, well positioning daytime surface temperatures to climb into the 90s. At the same time, greater moisture collocated with the warm airmass should lead to increased dew point temperatures, and thus higher humidity to meet the heat. Will continue to monitor these ingredients, their magnitude, coverage, and timeliness, but current forecast daytime heat indices would rise above 95 and towards the triple digits Wed and Thurs, potentially lasting into Friday. Overnight lows are not expected to provide relief Wed/Thurs night, settling in the upper 60s to lower 70s.

The spatial coverage of 20C+ temps at 850 does allow greater confidence in this forecast, however there are a few items that muddy forecast temps. The first will be the generally unstable airmass accompanying the heat and humidity. Showers/thunderstorms could result, leading to inconsistent heating for the day. It will be possible to receive convective systems from the west, rounding the ridge and anticyclonic flow. Either of these would provide more cloud cover or precipitation, limiting max heat potential. Second, locations near the coast, barring strong enough offshore flow, will be susceptible to afternoon seabreeze that could provide marine air relief. Lastly, there is some uncertainty how long 20C+ 850 temps linger or potentially consolidate to the south quicker later in the week.

On the thunderstorm threat: greater moisture and low level warming typically increases thunderstorm fuel via CAPE if a EML or greater mid level lapse rates are in place. Various machine learning convective guidance is suggesting Wed/Thurs as days to watch for potential strong to severe thunderstorms, but it remains unclear if these will develop overhead or arrive from the west. Thus predictability and confidence remain low, but should be monitored in the coming days if the threat increases.

AVIATION /18Z SATURDAY THROUGH THURSDAY/

VFR prevails this afternoon, with brief showers possible across interior terminals. Fog develops again tonight, with coastal terminals likely seeing IFR to LIFR conditions by late this evening, and interior terminals after midnight. The fog and low stratus gradually gives way to VFR early to mid morning on Sunday. Some scattered showers are possible again Sunday afternoon. VFR prevails at most terminals Sunday night, but valley fog is likely at LEB and HIE after midnight.

Outlook:

Monday: Lower ceilings/vis should consolidate to the coastal waters, with inland terminals becoming VFR.

Monday night: Return of lowered ceilings possible overnight for coastal ME. MVFR to IFR possible.

Tuesday and Tuesday night: Improvement to VFR in the morning, but SHRA arrive from the west in the afternoon, these may continue into the overnight for all terminals.

Wednesday and Wednesday night: SHRA with some TS possibly develop inland through the afternoon. These should reduce in coverage overnight.

MARINE

Marine fog lingers across the southern waters today, and likely develops near the coastline again tonight. Patchy dense fog lingers through the day on Sunday, and then dissipates with a west wind Sunday night. Otherwise, fair conditions prevail with broad high pressure slowly building across the waters.

High pressure continues to reside overhead through Tuesday morning. A weak warm front may drape over the waters through midweek with strong high pressure remaining over the Deep South.

GYX WATCHES/WARNINGS/ADVISORIES

ME...None. NH...None. MARINE...None.