textproduct: Gray - Portland

WHAT HAS CHANGED

No major changes were needed for this forecast package, but did significantly increase cloud cover for the rest of the afternoon given widespread cumulus.

KEY MESSAGES

1. A slow moving upper-level trough and will trigger more isolated to scattered showers across the forecast area the rest of today and into the evening, followed by a dry Wednesday under high pressure.

2. A series of fronts brings widespread showers and maybe a few thunderstorms late Thursday and Friday.

3. Dry weather and lower humidity returns for the weekend. Chances for showers return early next week.

DISCUSSION

KEY MESSAGE 1 DESCRIPTION...

An upper-level trough axis is currently located over the region which has led to isolated/scattered over much of the higher terrain across New Hampshire and western Maine. This activity should increase in coverage through the afternoon. A few thunderstorms will also be possible with some weak instability in place. SPC mesoanalysis suggests MLCAPE will only max out in the 250 to 500 J/kg range with very weak shear. Thus, any isolated storms that do form should be fairly tame. That being said, brief gusty winds will be possible under any showers or storms that move overhead.

Any showers or storms will diminish quickly with the loss of daytime heating, giving way to more valley fog overnight and maybe some marine fog. Lows will mainly be in the 50s.

High pressure moves in overnight into Wednesday as an upper- level ridge axis moves into the region. This will lead to dry conditions and warmer temperatures. Highs will mainly range from the low/mid 70s north, to the low/mid 80s across the south and along the coast.

KEY MESSAGE 2 DESCRIPTION... Clouds build and dewpoints rise during the day Thursday as a trough approaches the region. Most of the area should remain dry as the 25.12Z NAM Nest and RRFS suggest just a chance for some isolated to scattered showers as the warm front approaches. The best chance is over New Hampshire as Maine looks to remain on the periphery of high pressure departing to the east. High temperatures climb into the low 80s just about everywhere, with dewpoints in the mid-60s making it feel somewhat sticky.

The first of two cold fronts attempts to make a pass Thursday night which will increase PoPs for the western half of the CWA, but most of the eastern half of and coast hold on to 30-40% PoPs as the NAM and RRFS suggests this front washes out before making it too far. Depending on how this shakes out Friday morning could feature plenty of fog and drizzle as it wont to much to budge dew points. This along with lingering cloud cover may act to stunt development of anything during the day Friday. Interestingly, the RRFS which tends to be an aggressive model is not currently suggesting much more than scattered showers and storms. Long range probabilities of rainfall greater than a half inch continue to fall with much of the area given a 25 to 35 percent chance by the NBM and only about a 10 to 15 percent chance among the ECMWF and GFS blends. Higher probabilities exist in the mountains where orographic lift will help rainfall rates. Otherwise, it looks like that, outside of a good thunderstorm, many locations are only going to see around a tenth to a quarter of an inch. Certainly not the trend we wanted to see drought-wise, but I can't say I am surprised as this has been a persistent trend all Summer. CSU's machine learning model maintains low probabilities for severe storms Friday, but as mentioned if little instability is able to develop, that won't be much of a concern.

KEY MESSAGE 3 DESCRIPTION... The front is progged to clear the area by Saturday morning and it's progression is going to have an impact on Friday nights low temperatures. If the front clears the mountains overnight, we could see temperatures radiate down into the upper 40s. The foothills may see temperatures fall into the low to mid- 50s, and the coastal plain is less likely to clear out much and more likely to see temperatures only fall into the upper 50s and even low 60s on the immediate coast. Drier air does quickly fill in Saturday with high pressure building in at the surface. This makes for a mostly sunny and precipitation free weekend with high temperatures in the mid-70s to low 80s and dewpoints in the 50s across much of the area.

Global models suggest at least one upper low moves through Quebec Sunday making its closest pass Sunday night which may drag a front through the region early next week. There is still plenty of uncertainty among the global models on exactly how this plays out, but I think the chance PoPs from the NBM are fair.

AVIATION /18Z TUESDAY THROUGH SUNDAY/

VFR conditions are generally expected through the remainder of the day across most places but a few isolated to scattered showers may lead to brief restrictions through the early evening, especially over the mountains and interior. A stray rumble of thunder or two is also possible. Some valley fog will once again be possible tonight which could lead to IFR visibilities or lower through early Wednesday morning at CON and LEB. IFR or LIFR ceilings will be possible at HIE overnight, but fog is more uncertain here.

Outlook:

Wednesday - Wednesday night: VFR conditions expected. Local IFR or lower at night in valley fog.

Thursday: Generally VFR conditions expected

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

Saturday-Sunday: Any early MVFR/IFR conditions should quickly improve to VFR Saturday with VFR the prevailing condition through the weekend.

MARINE

Winds and seas are expected to remain below SCA thresholds through midweek. Until the upper trough can cross the Gulf of Maine, there will be areas of marine fog/stratus that form, especially at night.

A period of near SCA conditions are possible in southerly flow ahead of a cold front on Thursday into Thursday night. High pressure then builds across the waters for the weekend, bringing an extended period of fair conditions into early next week.

GYX WATCHES/WARNINGS/ADVISORIES

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