textproduct: Gray - Portland

WHAT HAS CHANGED

Confidence is increasing in radiational cooling bringing about cold temperatures that may allow for a patchy frost overnight Friday and early Saturday morning in the north.

KEY MESSAGES

1. Gusty winds subside this evening, then dry weather continues through Friday. Cold temperatures overnight Friday and early Saturday morning may allow for patchy frost development in northern zones.

2. Saturday will be dry but the next cold front will lead to some low impact showers on Sunday. Mainly dry weather is then expected once again to start the work week.

DISCUSSION

KEY MESSAGE 1 DESCRIPTION... The pressure gradient that is resulting in today's gusty winds relaxes tonight as the low pressure center continues eastward over Atlantic Canada. Winds will subside, but enough of a breeze may persist to keep valley fog from forming. This breeze along with passing clouds are also likely to be enough to keep most locations from decoupling tonight, but it will still be chilly. Those north of the mountains should see temperatures fall into the low to mid-40s, while the foothills and points south range from the upper 40s to the mid-50s.

Upper level flow turns northwesterly Friday as troughing takes hold over the area. There will also be a slight tightening of the pressure gradient again as high pressure begins to build in from the west, with northwesterly wind gusts only expected to be around 20 mph this time around. The cooler airmass is going to be apparent as high temperatures only climb into the low to mid-60s north of the mountains, and into the low to mid-70s in the south. High pressure begins to crest overhead Friday night, so strong radiational cooling is all but guaranteed to bring about a cold night. I tailored the forecast heavily toward MOS guidance which handles these situations better than the NBM which results in widespread mid to upper 30s north of the mountains. A patchy frost is possible, but also isolated enough that I don't feel an advisory is warranted at this time. However, anyone with sensitive plants or crops outside is going to want to take precautions to keep them from harm. Valley fog is also likely. South of the mountains temperatures are going to fall into the low to mid-40s.

KEY MESSAGE 2 DESCRIPTION...

High pressure will start to slide off into the Atlantic on Saturday as an upper level ridge axis moves overhead. These conditions will lead to fair weather and highs mainly in the 70s.

We transition back into southwest flow aloft Saturday night as the next trough/front starts to approach and clouds increase. Warm air advection and cloud cover may lead to some relatively warmer lows so did blend in a little bit of NBM90 to better account for these trends. Lows will mainly range from the lower 50s north, to the mid/upper 50s along the Main Coast, and up to the lower 60s across southern New Hampshire.

The next low to medium rain chances (20 to 50 percent) then return to the forecast Sunday morning in the form of scattered rain showers ahead of the cold front. Sunday's rain continues to appear low impact as NBM probabilities for a quarter inch of rain or more only max out around 50 percent over southern New Hampshire and far southwest Maine. Probabilities for a half inch or more max out around 40 percent over the same area and only 10 to 20 percent for an inch or more. Thus, we shouldn't see a wash out. The better chance for showers shifts north Sunday afternoon and into the evening along the cold front as it moves into the forecast area. Some of the models are suggesting some modest instability so a rumble or two of thunder is not out of the question in the afternoon or early evening. Any lingering showers will likely move out by early Monday.

Ridging then moves across the region, leading to mostly dry conditions to start the work week but the next front will bring more low impact precipitation chances mid to late week.

AVIATION /18Z THURSDAY THROUGH TUESDAY/

Through 18Z Friday...Wind gusts subside tonight with enough a breeze remaining to hinder the development of widespread valley fog. However, it can't be completely ruled out at HIE and LEB if winds do go calm for a period of time. VFR conditions continue through Friday with northwesterly wind gusts 15-20 kts at most terminals.

Outlook:

Friday night: IFR is anticipated at HIE and LEB as widespread valley fog is likely to develop under calm conditions

Saturday: Mainly VFR. Valley fog possible at HIE and LEB Saturday morning.

Sunday-Monday: Generally VFR, with MVFR possible in showers.

Tuesday: VFR expected.

MARINE

SCA wind gusts and seas continue through this evening, but subside tonight as the front moves offshore. High pressure than begins to build over the waters keeping conditions below SCA criteria through Friday. Wind direction will remain steady offshore.

Fronts moving off the coast early next week and toward the middle of the week may lead to some rough conditions, but this will depend on their strength.

GYX WATCHES/WARNINGS/ADVISORIES

ME...None. NH...None. MARINE...Small Craft Advisory until 8 PM EDT this evening for ANZ150>154.