textproduct: Gray - Portland

WHAT HAS CHANGED

There is a localized threat for torrential rainfall late tonight and again Saturday afternoon. Isolated instances of flash flooding are possible.

KEY MESSAGES

1. Isolated thunderstorms from the White Mountains towards coastal Maine will bring localized torrential rainfall into this evening.

2. More widespread showers and storms overspread the area late tonight through Saturday. Locally heavy rainfall is likely and isolated instances of flash flooding are possible.

3. Heat and humidity continue through at least early next week, followed by a gradual cool down through the end of the week.

DISCUSSION

KEY MESSAGE 1 DESCRIPTION...

Hot and humid conditions have allowed for the atmosphere to destabilize with isolated slow moving showers and storms developing along an area of convergence from the White Mountains towards the Mid Coast. High PWATs are in place with KGYX 12Z sounding recording 2.20 inches, allowing for torrential rainfall to occur within any cell that develops. Recent runs of hi res guidance suggest activity will remain isolated along this convergence zone into this evening with activity diminishing with the loss of heating. Given rainfall rates of around 2 inches per hour with cells that have developed an isolated instance of flash flooding cannot be ruled out if slow moving storms persist over a given location.

KEY MESSAGE 2 DESCRIPTION...

Better forcing for ascent will overspread the area tonight through Saturday as the ridge to the east breaks down and subtle short waves are able to traverse the area. CAMs are agreement that falling heights associated with one wave late tonight will allow for enough instability for convection while the placement and coverage is variable amongst models. Some model solutions show little in the way of convection overnight while others show isolated convection could bring localized rainfall in excess of 3 inches in 3 hours which would exceed flash flood guidance. Therefore it is encouraged that people have multiple ways to receive a warning overnight tonight because isolated instances of flash flooding are possible.

The first wave exits east after day break with another wave poised to cross Saturday afternoon. Despite potential cloud cover, CAMs are in good agreement that there will be enough instability for convection as this second wave crosses. Shear will be lacking reducing the threat for strong to severe storms while high PWATs will support torrential rainfall and bring the threat for localized flash flooding. CAMs remain variable in the placement of convection while most generally agree that localized amounts of 3 inches in 3 hours may occur somewhere across the forecast area through Saturday evening. A multi model consensus places the highest PoPs across the mountains and foothills. Will have to monitor areas that receive rainfall through tonight as these areas will be more susceptible to the heavy rainfall threat through Saturday evening. Convection will wane Saturday evening as this second wave passes east with little in the way of rainfall expected Saturday night.

KEY MESSAGE 3 DESCRIPTION...

The heat and humidity bounce back on Sunday with a return to more sunshine to close out the weekend. The muggy airmass keeps the chance for showers and storms a possibility as a weak front dissipates while crossing New England. Temperatures return to the upper 80s to low 90s south of the mountains again on Sunday and Monday.

Dew points climb into the low to mid 70s on Sunday, pushing heat indices into the low to mid 90s. But the dissipating front does look likely to begin the trend toward some drier air by Monday. Dew points look more likely to remain in the 60s on Monday, lessening the moistures' impact on the apparent temperatures.

The gradual trend toward drier and relatively cooler conditions looks to continue through the remainder of the week. Rather than one big front bringing a change of airmasses, a series of weak fronts from early to mid week looks to gradually bring temps back close to average by the end of the week. The dew points also look to drop with each passing front, falling back into the 50s by late in the week.

Widespread 80s still look likely for much of mid to late week. But with a drier airmass, heat indices don't look to be a concern, and nighttime lows will trend more significantly cooler with a return to 50s at night by late week.

AVIATION /18Z FRIDAY THROUGH WEDNESDAY/

Through Saturday night...Variable conditions are likely tonight through Saturday night as multiple rounds of SHRA/TSRA overspread the area along with patchy fog and marine influences. Valley fog will bring the potential for restrictions tonight at LEB and HIE. Marine stratus or fog will bring potential for MVFR to IFR at PSM, PWM and AUG and possible LIFR at RKD. MHT and CON likely stay VFR overnight. MVFR cigs may linger near the coast Saturday otherwise mainly VFR outside of TSRA Saturday afternoon and evening. Low level moisture lingers Saturday night bringing the potential for patchy fog.

Outlook...

Sunday: Mainly VFR prevails, but brief restrictions in afternoon showers and storms are possible.

Sunday Night - Tuesday Night: Mainly VFR prevails, with a slight chance for brief restrictions in pop up showers and storms. Nighttime valley fog is possible at LEB and HIE each night.

Wednesday - Thursday: Mainly VFR prevails, but restrictions in any showers and storms remain possible.

MARINE

Disturbances crossing the waters late tonight and Saturday afternoon will bring potential for thunderstorms over the waters. Humid conditions remain in place continuing the potential for fog over the waters tonight through Saturday night. Otherwise, winds and seas remain below SCA thresholds.

Mainly fair conditions prevail from the end of the weekend through midweek next week as the Bermuda High remains in control of the weather pattern across the Gulf of Maine. Areas of marine fog will remain possible into Monday. A series of weak cold fronts bring a gradual shift of winds from southwest to more westerly through the week.

GYX WATCHES/WARNINGS/ADVISORIES

ME...Heat Advisory until 8 PM EDT this evening for MEZ014-018>028- 033. NH...Heat Advisory until 8 PM EDT this evening for NHZ010-012>014. MARINE...None.