textproduct: Gray - Portland

WHAT HAS CHANGED

A Heat Advisory has been issued for portions of southeast New Hampshire on Thursday.

KEY MESSAGES

1. Heat and humidity build through late week and persist through the weekend as a subtropical ridge of high pressure drives the weather pattern for New England.

2. There will be daily chances for storms Thursday through Saturday. Coverage will be limited while there is potential for a few strong storms and localized torrential rainfall.

DISCUSSION

KEY MESSAGE 1 DESCRIPTION...

The Bermuda High builds into the Eastern Seaboard through the weekend, bringing building heat and humidity possibly extending into early next week. This brings an extended period of seasonably warm temperatures, combined with anomalously high dew points.

Temperatures themselves won't be anything remarkable, with widespread highs in the upper 80s to low 90s across much of the area. Additionally, an afternoon sea breeze is likely along the coastline both Thursday and Friday, but then looks confined to the MidCoast for the weekend.

The dew points themselves will be the bigger story, with dew points pushing into the low 70s starting on Thursday across southern New Hampshire. These dew points continue to expand, and push into the low to mid 70s for much of the area from Friday through at least Sunday.

Increasingly, it looks like a stronger front that brings a return to cooler conditions takes longer to arrive, with next Tuesday to Wednesday looking like the more likely timeframe for a more substantial cool down. Before this, a couple of fronts make some progress into New England, but gradually wash out without bringing a noticeable cool down. One such front is likely on Saturday, and the clouds, showers and storms associated with it may serve as a spoiler for the higher temperatures expected on the other days around it.

With the heat and humidity building, a Heat Advisory has been issued for parts of southern New Hampshire on Thursday as heat indices push into the mid to upper 90s. It likely takes until Friday to see more widespread heat indices in the mid 90s push into the rest of the interior forecast area just back from the coastline.

KEY MESSAGE 2 DESCRIPTION...

New England will be positioned on the northwest flank of a sub tropical ridge Thursday through Friday with a long wave trough to the west over the central US. Short waves moving through the trough will help suppress the ridge by Saturday with Saturday having the highest chances for showers and storms through the period.

Deep southwest flow will advect anomalous moisture into the region Thursday with this moisture persisting into Saturday. Dew points will climb into the upper 60s across much of the area by Thursday afternoon with southern NH and SW Maine seeing dew points into the low 70s. PWATs will also climb above 1.75 inches across much of the area with pockets of PWATs greater than 2 inches. Surface heating combined with areas of dew points in the 70s will yield upwards of 2000 J/kg of SB CAPE amid 25-30 kts of deep layer shear Thursday afternoon. The proximity to the ridge and lack of surface forcing will limit chances for an isolated storm across New Hampshire Thursday afternoon. Given the CAPE and shear a strong storm cannot be ruled out and high PWATs combined with deep warm cloud depths would allow for torrential rainfall. Friday will be similar with the ridge close by and lack of surface forcing while expanding high dew points will allow for sufficient instability for storms across much of the area. Shear will be somewhat weaker around 25 kts or less and CAMs show little in the way of storm coverage. Given sufficient SB CAPE with no CIN have largely stuck with NBM PoPs Friday afternoon which are around 20-30 percent. High PWATs and deep warm cloud depths will again allow for any thunderstorm that does develop be prolific rain producers.

The 12Z model suite is in general agreement that short waves will start to suppress the ridge Friday night into Saturday. These waves will continue chances for showers and storms Friday night into Saturday morning. Forcing for ascent remains favorable through Saturday evening while cloud cover brings into question how much the atmosphere can destabilize for storms. Models generally suggest there will be sufficient CAPE and shear for storms with the continued theme of high PWATs and deep warm cloud depths allowing for storms to produce torrential rainfall. Shear will also be a bit stronger so instability trends will need to be watched for potential strong to severe storms. Daily chances for showers and storms diminish Sunday into Monday as flow aloft turns more zonal.

AVIATION /18Z WEDNESDAY THROUGH MONDAY/

Through Thursday Night: Mainly VFR tonight with valley fog likely bringing restrictions at LEB and HIE and potential for restrictions at CON. Light and variable flow tonight will become S at 5-15 kts on Thursday. VFR likely prevails through Thursday while isolated TSRA cannot be ruled out across NH. Increasing moisture will bring potential for patchy fog across much of the area Thursday night.

Outlook... Friday: Mainly VFR, although an isolated afternoon TSRA cannot be ruled out. Patchy fog is possible Friday night along with low cigs and SHRA.

Saturday: Scattered SHRA/TSRA will bring potential for localized restrictions. Cigs may also be around MVFR thresholds.

Sunday-Monday: Mainly VFR outside of isolated SHRA/TSRA.

MARINE

High pressure dominates over the waters Thursday and slowly shifts southeast into Saturday. Increasing moisture will bring potential for fog over the waters tonight into Saturday. A front approaches the waters late Friday into Saturday bringing chances for showers and storms. South to SW flow generally prevails through the period with winds and seas remaining below SCA thresholds.

GYX WATCHES/WARNINGS/ADVISORIES

ME...None. NH...Heat Advisory from 11 AM to 8 PM EDT Thursday for NHZ012-013. MARINE...None.