textproduct: Gray - Portland

WHAT HAS CHANGED

No significant changes to the forecast were needed.

KEY MESSAGES

1. Mainly dry weather continues through tonight with cooler and unsettled weather this weekend.

2. A slow moving system will continue shower chances into early to mid week, with widespread measurable rainfall unlikely.

DISCUSSION

KEY MESSAGE 1 DESCRIPTION...

Mostly dry conditions are expected to continue through tonight as high pressure sits overhead this afternoon. The high will then begin shifting to the northeast tonight as it gives way to multiple low pressures that will move across Northern New England this weekend.

For tonight, other than the coast and southern NH, skies should stay mainly clear, allowing temperatures to fall into the 50s south of the mountains with some 40s to the north. More valley fog is likely along the CT River, and I do have some low PoPs (20%) in southeast NH as some models indicate a few showers are possible overnight.

The first low then approaches from the south Saturday morning with skies becoming mostly cloudy, generally in a south to north progression through the day as the low carries higher moisture into the area. It will also bring scattered showers (with dry periods as well), although models have been struggling on a run to run basis regarding timing, coverage, and location of showers. Looking at latest guidance, global models indicate broad lift over the Gulf of Maine approaching the coast and southeast NH in the morning and early afternoon, which will the primary areas to watch for showers. Lift spreads northward through the afternoon and evening (mostly across western ME), bringing increasing chances for showers, and the 12Z NAMNest and HRRR are generally on board with this scenario. Rainfall accumulations are expected to remain light, but parts of western ME could pick up over 0.25" of rain through Saturday evening.

More uncertainty exists Saturday night into Sunday morning. Some models favor a lull in the showers due to some mid-level drying while the low-levels remain nearly saturated, suggesting fog (possibly dense) and precipitation that falls more in the form of drizzle. Others indicate another wave of low pressure, which would bring more in the way of showers and less in the way of fog. Due to the uncertainty, PoPs of 20-40% continue to be advertised.

Sunday remains cloudy and dreary for most with scattered showers possible at times in the morning. Another low pressures tracks across Quebec that will send a cold front in from the west, resulting in another increase in shower chances and possibly a storm or two in the afternoon and evening hours. Some showers continue into Sunday night, but these should diminish south of the mountains from west to east overnight behind the front.

KEY MESSAGE 2 DESCRIPTION...

Closed low moving east of the Great Lakes will continue to pivot into southern Quebec Monday. Showers in the morning exit northeast across southern ME, with thinning clouds across NH. An embedded wave in the low's circulation will lift north through the morning, with building instability through mid afternoon. This could result in a rejuvenated shower/thunderstorm chance.

The thunderstorm parameter space has some ingredients going for it, but also limitations. 0-6km shear of around 50 kts is abundant on the periphery of the broad low, and this overlaps with marginal surface CAPE early Monday. Low and mid level lapse rates would both be supportive of convection, however its possible dry air aloft will have increased significantly. Will keep the mention of iso to sct showers/thunder in the forecast at this time.

Tuesday may see further influence of the closed low, with a return of moisture producing a continuation of cloud cover with the chance of showers. These showers take advantage of daytime heating, likely more diurnally driven than associated with a front. They may also be more widespread than the current forecast carries, but will see how timing and influence of the closed low trends. The low finally loses its grasp on New England come midweek as it nudges northeast into the Canadian Maritimes. This would provide a break from the dreary conditions as temperatures remain seasonable.

AVIATION /18Z FRIDAY THROUGH WEDNESDAY/

Through 18Z Saturday...VFR the rest of the day today and at least into the first part of tonight. LIFR to IFR valley fog is likely at HIE and LEB again tonight with the forecast more uncertain elsewhere. Southern NH and coastal terminals may start seeing MVFR to IFR ceilings as early as overnight into Saturday morning as well as a few showers. However, it's possible restrictions hold off until beyond the TAF period, so these have been left out of the TAFs due to uncertainty.

Outlook:

Rest of Saturday-Saturday night: Increasing chances for showers, and a lowering of ceilings from south to north is likely with many sites reaching at least IFR, but LIFR is possible, especially going into Saturday night. Fog is also likely Saturday night. Timing of restrictions remains uncertain, but most likely time to start seeing IFR is Saturday afternoon into Saturday evening.

Sunday: Fog lifts in the morning, except may linger at RKD much of the day. Ceilings may remain IFR to even LIFR for most sites, especially in western ME, although NH terminals could reach MVFR by the afternoon. Showers remain likely over much of New England.

Sunday night: IFR to LIFR restrictions remain likely due to fog, low stratus and showers. Some improvement from west to east is possible overnight, but that remains uncertain.

Monday: MVFR cigs possible, improving towards VFR as the day continues. SHRA possible across the interior.

Monday Night: Less SHRA chances, VFR to MVFR cigs

Tuesday: SHRA develop during the day with MVFR cigs possible as a closed low moves overhead.

Tuesday Night and Wednesday: Improvement to VFR with no sig wx.

MARINE

Through Sunday night...High pressure will lift northeast of the waters through tonight as multiple weak low pressures or surface troughs approach from the south and move across the waters this weekend, bringing increase chances for showers and fog. Northeast winds may briefly approach SCA levels Saturday but are otherwise expected stay below SCA levels. Another low crosses Quebec Sunday night and is expected to send a cold front into the waters overnight and toward daybreak Monday.

Closed area of low pressure will push towards New England into the beginning of next week bringing a broad wind field. Gusts should remain below SCA criteria, but expect gradually building waves 2 to 4 ft through midweek.

GYX WATCHES/WARNINGS/ADVISORIES

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