textproduct: Gray - Portland

WHAT HAS CHANGED

Thunderstorm activity has diminished and have cancelled the Severe Thunderstorm Watch. Aviation section has been updated for the 00Z TAFs.

KEY MESSAGES

1. Chances for showers and thunderstorms increase this afternoon. Some storms could be strong to severe with damaging winds and hail as the primary hazards. A Severe Thunderstorm Watch has been issued for Central and Southern New Hampshire and far southwest Maine. Fog could then be dense in some areas tonight.

2. A trough exits New England Sunday through Monday.

3. A substantial warming trend follows through the remainder of the week next week as a ridge builds across the eastern half of the US. The potential for hazardous heat is increasing for mid to late week next week.

DISCUSSION

KEY MESSAGE 1 DESCRIPTION...

Earlier rains have shifted off to the east as broad low pressure approaches from the west this afternoon. Increasing lift and instability will allow additional showers and thunderstorms to form this afternoon, and this is already underway just upstream across upstate NY and approaching VT.

Diving in a little deeper, latest SPC Mesoanalysis depicts roughly 1000 J/kg of SBCAPE and 40-50 kt of effect shear across much of NH and portions of far western ME, that will be sufficient to support some of these storms becoming strong to severe as the develop or move into the area. Damaging winds and hail 1" or larger in diameter will be the primary hazards, but a brief tornado or two can't be completely ruled out. A Marginal Risk (Level 1/5) from the SPC continues for all of NH and into portions of western ME, but within this Marginal Risk area, the higher risk appears to be roughly along the CT River Valley extending into central and southern NH and far southwestern ME where the better overlap in shear/instability is forecast to be SPC recently upgraded this higher risk area to a Slight Risk level 2/5, which is also where a Severe Thunderstorm Watch has been issued until 9PM this evening.

Another potential hazard is the risk for flash flooding within slow-moving persistent showers/storms or where multiple rounds occur. One area to watch in particular is in and around the NH Lakes Region, where 1-2" of rain already fell overnight and this morning around Lake Winnipesaukee.

Showers and storms will lessen in coverage and intensity toward sunset, but the HRRR suggests a couple of showers or storms may for an hour or two after. Other than a few upslope showers in the mountains overnight, the rest of the area is expected to remain dry. With light winds and saturated grounds from today's rain, most of the area has fog in the forecast. Fog could be dense in some areas with the higher probabilities of this occurring in western ME.

Drier conditions are forecast for Saturday with light winds and seasonable temperatures in the 70s to lower 80s. Aloft, weak troughing will still be overhead, so as daytime instability builds scattered showers and isolated storms will develop from late morning and through the afternoon hours before waning toward sunset. A couple of storms could produce small hail. There are also indications of slower-moving storms and heavy downpours, so the next shift may to take another look at potential hydro issues.

Light winds Saturday night will again promote patchy fog, especially in valley and areas that receive rain during the day. Overnight temps will be in the 50s.

KEY MESSAGE 2 DESCRIPTION...

The influence of the lingering trough will still be felt on Sunday with increased chances for pop up showers and storms during the afternoon hours. The best chance for these will be across the higher terrain and central Maine, with generally lower chances toward western New Hampshire. Temperatures also begin to rebound with relatively more sunshine on Sunday. Central and southern New Hampshire likely warm into the low to mid 80s, with mainly 70s expected elsewhere.

By Monday, chances for showers drop off significantly as the trough exists to the east, and the warm up begins. Highs are expected to warm into the 80s across most of the region, with a sea breeze knocking back temps along the coast by the afternoon hours.

KEY MESSAGE 3 DESCRIPTION...

The main warm up then gets underway by Tuesday as a highly amplified ridge builds across the Midwest and Great Lakes. New England looks to be positioned on the eastern portion of this ridge, but the trend since yesterday has been for the ridge to set up a little further east. This progression has been supported by the full suite of the Euro, Euro Ensembles, and AIFS models. The GFS has been a cool outlier, but looks to be following the way of the Euro on it's latest run. We'll continue to monitor for any changes in trends over the next few runs.

This pattern supports at least a couple days of widespread highs in the 90s, and potentially more days should the ridge remain amplified enough to ward any backdoor fronts or retrograding trough from the Canadian Maritimes. Tuesday looks like the potential first day of 90s reaching into southern New Hampshire. Wednesday and Thursday then bring the potential for the most widespread heat with highs into the 90s across much of the area. This heat also brings with it some deep moisture originating from the Gulf of Mexico, and transiting through the Midwest and Great Lakes. This brings dew points into the upper 60s and 70s, combining with highs in the 90s to bring heat indices potentially into the triple digits.

This pattern progression is a reversal from the trends seen yesterday, but appears substantial and consistent enough across most of the model suites to be believed. This weekend would be a good time to get any air conditioners and fans set up before the most substantial heat and humidity of the year arrives.

AVIATION /00Z SATURDAY THROUGH WEDNESDAY/

Through Saturday...Thunderstorms have diminished with lingering SHRA at AUG, PWM, CON, PSM, and MHT for the next hour or two. Fog has already pushed into RKD where LIFR will continue into Saturday morning. Elsewhere tonight, IFR from fog and low stratus will be likely over much of western ME and NH.

Improvement to VFR is expected Saturday morning, which will prevail most of the day. TEMPO MVFR possible from scattered SHRA and isolated TSRA in the afternoon.

Outlook:

Saturday night: Mainly VFR. IFR/LIFR possible from valley fog at HIE, LEB, and CON.

Sunday: Mainly VFR, but brief restrictions in showers and storms possible across northern and eastern terminals.

Sunday Night - Tuesday: Mainly VFR, but nighttime valley fog possible.

Tuesday night - Thursday: Mainly VFR, but rounds of showers and storms possible at times.

MARINE

Through Saturday night...Conditions will remain below SCA levels through Saturday night. Southerly flow continues into this evening as low pressure approaches from the west. This low will bring an ongoing chance of rain showers into this evening along with the possibility of offshore-moving thunderstorms, especially from around Casco Bay southward. Winds briefly turn light and north to northwest tonight and then will be more light and variable in direction Saturday and Saturday night.

Extended...Generally fair conditions are expected as broad high pressure gradually build across the waters early to midweek next week.

GYX WATCHES/WARNINGS/ADVISORIES

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