textproduct: Gray - Portland

WHAT HAS CHANGED

No major changes were needed for this forecast package. Initial showers early this afternoon will continue to move east and diminish while some thunderstorms develop later this afternoon and into the evening. A few of these storms could become strong to marginally severe over southern/central New Hampshire.

KEY MESSAGES

1. An expansive upper low will lead to some scattered showers and thunderstorms this afternoon/evening with more isolated activity expected on Monday.

2. The next chance of widespread rain comes late in the week, bookended by periods of dry weather through midweek and over the weekend.

DISCUSSION

KEY MESSAGE 1 DESCRIPTION...

An expansive closed upper low is currently located over southwestern Ontario and will gradually meander up into central Quebec by Monday afternoon. This pattern will lead to unsettled weather conditions to start the new week.

An initial batch of rain showers continues to move east as a warm front lifts over New Hampshire. This activity should start to diminish as it approaches western Maine and it loses upper level support. A few of the latest CAMs are suggesting that we could see a few modest breaks in the clouds behind the early shower activity, mainly over portions of western New Hampshire in the late afternoon hours. Storms should be ongoing over Vermont along a cold front that will approach the border around 00z this evening. While most CAMs suggest that these storms will be weakening on their approach (diurnal timing is less than favorable), HRRR and RAP soundings are still showing ~500 to 700 J/kg of MLCAPE and plenty of 0-6km bulk shear in the 40 to 50 knot range. While a few isolated storms will be possible ahead of the front, the best chance for a strong to marginally severe storm or two appears to be in the 22z to 03z time frame over southern or central New Hampshire. Gusty winds, frequent lightning, and small hail will be the main threats.

Rain chances diminish overnight, but another round of widespread fog is again likely. As we remain in cyclonic flow aloft, more clouds and showers/storms are possible on Monday, especially over the mountains. Storm coverage and instability will be more limited than today but forecast shear would support a stronger storm or two depending on how the environment evolves.

KEY MESSAGE 2 DESCRIPTION... An upper level low continues to slowly move through Quebec Tuesday leaving us under the influence of the trough for one more day. This means mostly cloudy conditions and a few showers are likely for the mountains with high temperatures in the low to mid- 70s. South of the mountains should see more sun, but northwesterly flow dropping 850 mb temperatures to +10C is going to limit high temperatures to the upper 70s to low 80s. With less of a chance of seeing any precipitation due to downslope flow. Clearing skies and calm winds are going to allow temperatures to fall into the 50s Tuesday night.

Another trough approaches later in the week, but in between we see some brief ridging bringing drier weather and a warming trend Wednesday through most of Thursday. Wednesday remains comfortable with high temperatures just a few degrees warmer than Tuesday, but continued southwesterly flow is going to make Thursday feel a bit stickier as dewpoints slowly climb to the mid-60s with temperatures generally in the 80s.

Thursday night into Friday looks like the next best chance for widespread precipitation as a broad trough once again pushes into the region with a series of fronts accompanying it. Nothing too noteworthy at the moment, but long range models do already have a 30 to 50% chance for rainfall exceeding a half inch. It would be nice to see trends go up with this one, so we might be able to get some beneficial rain before we head into Fall which is our usual dry period. As of now it looks like the fronts clear by Saturday morning with high pressure then crossing and making for a mostly dry weekend.

AVIATION /18Z SUNDAY THROUGH FRIDAY/

Scattered SHRA are possible spreading west to east through the day. Some improvement to MVFR and VFR is possible by evening. TSRA is possible between 20z this afternoon to 03z this evening, mainly across New Hampshire terminals. Storms will be capable of locally strong winds and possible GR. IFR to LIFR is likely at most TAF sites in low CIGs and reduced visibility in fog overnight. Conditions then improve to MVFR and VFR from west to east during the day on Monday, with showers and storms possible Monday afternoon across the mountains towards LEB.

Outlook:

Monday night: VFR conditions expected.

Tuesday-Thursday: Generally VFR conditions expected

Thursday night-Friday: Generally MVFR conditions expected with widespread showers. IFR conditions possible in fog at night.

MARINE

Conditions remain below SCA levels through Monday as a slow moving cold front approaches the waters and passes through on Monday. Reduced visibility in marine fog is likely through midday Monday.

Generally sub-SCA conditions expected through the weekend as the Gulf of Maine remains on the periphery of Atlantic high pressure.

GYX WATCHES/WARNINGS/ADVISORIES

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