textproduct: Gray - Portland

WHAT HAS CHANGED

No significant changes made to the going forecast this evening.

KEY MESSAGES

1. Mostly dry weather with seasonable temperatures continues into Thursday, although some uncertainty remains if southern NH and SW Maine will see more in the way of light showers later in the day on Thursday. Fog could also be locally dense across portions of western ME through tonight.

2. The weather trends cooler through the weekend with the potential for a system to bring rain, late in the week. Temperatures trends back warmer for the first part of next week with the overall pattern remaining somewhat unsettled.

DISCUSSION

KEY MESSAGE 1 DESCRIPTION... A slow-moving front is situated in the vicinity of the eastern Great Lakes and St Lawrence River Valley this afternoon. Aloft, a 500mb shortwave will intersect this front and result in the development of isolated to scattered showers, mainly in and around the mountains this afternoon and evening. Elsewhere, partly sunny conditions prevail with afternoon temps in the mid 70s to low 80s.

Also keeping an eye on an area of marine fog and stratus on latest satellite imagery. This could bring reduced visibilities as early as this afternoon along the Maine coastline before potentially spreading inland this evening and tonight, especially along and east of the I-95 corridor from Portland to Augusta and Waterville, where fog could be locally dense. Aside from this area, patchy fog could develop in many areas tonight, especially within and near river valleys. Overnight lows will range from the 50s to lower 60s.

Fog will lift by mid-morning Thursday, and another seasonable day is expected temperature-wise with highs in the mid 70s to lower 80s. The previously mentioned front will push into New England, and while most will stay dry, a few showers may develop during the afternoon and evening while clouds increase across the area. A somewhat better chance of showers will be across southern NH and perhaps far SW Maine as a more-defined wave moves across southern New England. The last couple runs of the HRRR have trended toward this while other models keep the precip more suppressed to the south. For now PoPs are 20-30% in this area, but may need to be raised or lowered with how model trends go. Either way, amounts are expected to be light and a tenth of an inch or less.

If the showers materialize, they should end by or shortly after midnight Thursday night. More valley fog possible overnight, and lows will be a tad cooler than tonight with most 50s, except 40s in some of the northern valleys.

KEY MESSAGE 2 DESCRIPTION... Shortwave ridging builds in Friday behind the departing trough with high pressure looking to move in and dominate in our forecast area. For Friday, temperatures will generally hold in the mid to upper 70s, with a few places in Southern New Hampshire flirting with the 80 degree mark. Once again, as for most of the summer, a seabreeze should kick in during the afternoon, dropping temperatures near the coastline a few degrees come time early evening. Overnight temperatures should stick to the mid to upper 50s as well. Looking ahead to the weekend, models are mixed on when the first batch of precipitation moves into our area for Saturday. Here, a 500 mb low from our west joins up with approaching moisture from our south. Some deterministic models, are a bit unsure of the positioning of this trough and its associated moisture with long- range solutions like the Canadian and European bring precipitation into southern areas of our CWA by Saturday afternoon, while the GFS holds back on widespread precip overall. Knowing this, it would be better to see some agreement in the models regarding precip because, as of right now, it is too early to tell when this potential batch of precipitation moves in, but trends will be monitored in the coming days to figure out whether Saturday will be dry or wet. At the moment, PoPs are generally in the 20-40% range over our portions of our CWA by late in the day on Saturday. In terms of temperatures, highs will be in the upper 60s to low 70s.

As stated above, rain chances could further increase Saturday night into Sunday as the 500mb works its way in and covers most of the Eastern Seaboard. A strong signal from ensembles shows our area seeing measurable rain with models hinting at greater than a quarter of an inch for most of our CWA, with the potential for heavier amounts of rainfall along the immediate coastline and at higher elevations. At the moment, this 500 mb trough looks to remain an open wave, but this could change if it closes off, to which a slower onset and end of precipitation will likely be the result. If the trough closes off, the slower onset and end could come to fruition, where in this case, precipitation could have trouble ending by the start of next week. Overnight Sunday, rain could be clearing out away from the coastline, but upslope showers could continue through early Monday morning.

Drying conditions should generally dominate going into early next week, with long range models still hinting at chances for upslope showers continuing into the middle of next week. These upslope shower chances will wane as the week progresses, with some models showing near zero rain probabilities as early as Tuesday, along with high pressure returning to our area. On the temperature side starting next week, highs look to be average for this time of year staying in the mid to upper 70s with nightly lows in the mid 50s to low 60s.

AVIATION /00Z THURSDAY THROUGH MONDAY/

Through 18Z Thursday...Mainly VFR, but an area of marine fog/stratus brings IFR or LIFR to RKD through tonight. For tonight, the marine fog could bring IFR or lower conditions to PWM and AUG but is of lower confidence. Elsewhere (especially LEB and HIE), valley fog could bring IFR or lower late tonight into early Thursday. Conditions should improve to VFR by 14Z Thursday.

Outlook:

Rest of Thursday: Mostly VFR, except MVFR possible due to -SHRA at MHT, CON and PSM, mainly after 20Z.

Thursday night: -SHRA and MVFR remain possible across southern NH through around 06Z. Valley fog could then bring IFR or lower restrictions for LEB and HIE.

Friday-Saturday night: Generally VFR during the day with fog possible at night. Some brief MVFR restrictions are possible at coastal terminals Saturday if showers materialize.

Sunday-Sunday night: Generally MVFR conditions with widespread rain expected. Rain may taper off Sunday night giving way to areas of fog that would likely bring about IFR conditions.

Monday: Conditions return to VFR, but there may still be some MVFR and/or IFR conditions early.

MARINE

Through Thursday night...SCA conditions are not expected. Winds briefly turn west to northwest tonight, but remaining light, as weak boundary moves into the waters. Winds return to more southerly Thursday as a weak front approaches, and these will switch to north/northwest Thursday night behind the front. Fog will also remain possible at times, especially from around Portland eastward.

Generally sub-SCA conditions are expected to prevail late week into early next week, but there is potential for a coastal system to bring some brief SCA conditions over the weekend. Areas of fog are also likely through much of this period.

GYX WATCHES/WARNINGS/ADVISORIES

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