textproduct: Gray - Portland

WHAT HAS CHANGED

Greater confidence in above normal heat lasting through Saturday for southern New Hampshire and far southern Maine. This will lead to heat accumulation, but humidity won't be as high as normal summer hot stretches.

KEY MESSAGES

1. Well above normal temperatures remain in the region through Friday (Saturday for at least southern and coastal locations), for an extended stretch of heat. While heat will be allowed to build, high humidity is not forecast and will prevent excessive heat impacts.

2. A slow moving cold front will bring the increasing chance for rain Saturday through at least Sunday morning, north to south. The front will bring a cooler airmass, with temperatures retreating back to normal early next week.

DISCUSSION

KEY MESSAGE 1 DESCRIPTION...

Large high pressure centered over the Ohio Valley will transit SE through the end of the week, with the anticyclonic flow providing northern New England with a dry continental airmass. Primary moisture advection in the CONUS will be positioned poleward across the Plains, lazily arching north of the Great Lakes. This is a long transit, and the distance from source moisture leads to high confidence in dry weather for the forecast area through Friday. Not only is this the case for precipitation chances, but also lower level moisture content.

Daily diurnal cycles should have deep mixing to at least 800mb, much of this with significant surface dewpoint depressions into midday. So while heat will be present, humidity values at the surface should be lower and less oppressive vs. mid-summer heat waves. This is reflected with forecast heat index values remaining near the forecast temperature. That said, the three to four days of above normal temps, daytime highs and overnight lows, will allow heat to accumulate.

This accumulation is most likely for southern NH through the Lakes Region of ME/NH and into the interior of ME. Westerly winds will allow daytime highs to push into the 80s along the immediate coast as well, especially into late morning and early afternoon. While low level winds remain westerly through the end of the week, the background intensity isn't very strong. Its possible, with the strong early season land/sea temp gradient, we see some mid to late afternoon relief in the way of onshore seabreeze along at least the Midcoast peninsulas and immediate southern ME/NH coast.

Thursday and Friday will see the most expansive coverage of highs in the upper 70s to upper 80s, but nearing cold front to the north on Saturday could limit highs to the mid 70s to lower 80s for the mountains and towards the Kennebec Valley. South of here, it looks to be one more hot day with values in the upper 80s for southern NH through far southern ME.

KEY MESSAGE 2 DESCRIPTION...

As the aforementioned high pressure across the Great Lakes sinks into the South, it will lose influence on the Northeast and moisture rounding the top will seep into northern New England. The first chance of showers arrives Saturday across the north, with showers developing in the afternoon into the foothills and perhaps interior. Ensemble mean MUCAPE comes in around 500 j/kg which is good for a chance of thunder, with plenty of time to refine this probability and strength. Still in a warm airmass, mid level lapse rates could keep a cap on intensity even if shear/instability align.

PWATs up to 1.3 inch continue to advect into the region as the high continues to collapse and push off the Carolina coast. Its here that the cold front's passage still has a wide spread in timing through guidance. But, rain chances seem to be greatest Saturday evening through early Sunday morning before exiting to the south. Jet dynamics aren't particularly strong, and will rely on the anomalous moisture plume, 30 kt LLJ, and warm cloud depth (~10-11kft) for what precip efficiency can be mustered. Current guidance mean precip through Monday morning looks to be a couple tenths either side of 1 inch, but expect this will ultimately depend where any convective elements enhance rates.

The resulting airmass behind the front will feature not as hot temperatures, settling closer to normal for early June in northern New England. The heat should break Sunday areawide, with highs around the lower 70s Sun and Mon.

AVIATION /18Z WEDNESDAY THROUGH MONDAY/

Through 18z Thursday...VFR. Dry airmass should prevent widespread valley fog development overnight. NW winds today become variable tonight, with a more westerly component Thursday. Weak sea breeze daily for coastal terminals, moreso PWM/RKD than PSM.

Outlook...

Friday and Friday night: VFR expected, clouds thickening and lowering.

Saturday and Saturday night: A cold front brings increasing chances for showers and storms and MVFR to IFR restrictions across the region, mainly for northern TAF sites Saturday morning and then generally areawide Saturday into Saturday night.

Sunday: Continued chances for showers/storms and IFR restrictions, especially the first half of the day on Sunday. Improvement from north to south is possible late in the day into Sunday, but this depends on how quickly the front pushes south.

Monday: Mostly VFR, but can't rule out a few lingering showers across southern NH.

MARINE

High pressure to the west will allow for conditions below SCA criteria through Friday night. This will generally result in gusts less than 15 kts and wave heights 1 to 3 ft. A cold front will cross the waters from the northwest Saturday into Sunday morning with an uptick in winds as they shift NW to NE after passage.

GYX WATCHES/WARNINGS/ADVISORIES

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