textproduct: Gray - Portland

WHAT HAS CHANGED

Model guidance has remained consistent on keeping most of the rainfall with tonight's system north of the mountains. Some CAMs have shown the potential for some backbuilding of showers across far northern New Hampshire and northwestern Maine. Updated rainfall amounts to reflect these changes in model guidance. Otherwise, the forecast remains on track.

KEY MESSAGES

1. Rain accompanies a passing system tonight into Monday morning. A period of moderate to heavy rainfall after midnight and early Monday morning could result in nuisance flooding or ponding in poor drainage areas for the morning commute.

2. Low-impact weather expected mid to late week, with a couple chances for light rain.

DISCUSSION

KEY MESSAGE 1 DESCRIPTION... This evening, low pressure moves in from the Great Lakes. The low could allow for the potential to see numerous showers overnight. The storm environment looks to be very moisture-lade, with PWATs between 1.5 and 1.8". PWATs of this magnitude sit near the 90th percentile, and could allow for very efficient rain making showers. However, it is not an impressive environment convectively, with poor lapse rates and very low CAPE across the region. Some thunder can't be ruled out but mainly expecting heavy showers. CAMs have the most consistent and heaviest swaths of showers across locations north of the mountains, and has been the trend for the past few model cycles as well. 70+ kts of effective layer wind shear will allow for showers to move fairly progressively over the area, but some showers may also backbuild up north. Flash flooding can not be ruled out with any aforementioned backbuilding showers.

As mentioned earlier, showers should be more numerous across northern NH and northwestern ME. 3 hour Flash Flood Guidance across this region is quite low, and is between 1.80-2.50" of rainfall. So it also shouldn't take much rainfall across this region to start noticing some nuisance flooding impacts.

Speaking of, rainfall amounts look to be fairly hit or miss, with locations north of the mountains likely to see at least a half inch. However, higher totals locally of 1.5-2.5 inches are possible as well north of the mountains. Showers are possible south of the mountains but they look to be more isolated, as some moisture gets channeled to an adjacent low pressure system over the Mid- Atlantic. Wetting rains are likely down there.

The low exits the region Monday morning. The environment could still be fairly moist, with dewpoints in the upper 60s. Instability from daytime heating may allow for the development of a couple isolated showers and storms along the CT River Monday night. In addition, fog and low stratus is likely to move onshore and allow for gloomy and foggy conditions through the 2nd half of Monday and continuing into Tuesday morning.

KEY MESSAGE 2 DESCRIPTION...

Tuesday, low level moisture may still be entrenched along the coast and interior. It will depend on proximity to a low as it exits southern New England and pivots towards/just outside of the Gulf of Maine. Would expect northerly flow behind the low to whittle this away, but just 10 kts at 850mb won't make it quick.

Guidance has been initiating some diurnal showers or storms Tuesday afternoon across portions of the interior where this moisture overlaps enough daytime heating. Model profiles show moisture in the lower levels, but also feature a very dry mid to upper level. It is possible warm air aloft also advects in, creating a cap and ultimately limiting a unstable layer. As a result, went with lower precip chances at this range. Not ruling out showers and storms, but think they could be more isolated in nature given the prominent dry air aloft and ceiling on instability.

Warmer for Wednesday as southerly flow kicks in ahead of an elongated cold front. Highs should bump up into the low 80s, a few degrees warmer than Tuesday. While the approaching front will carry precipitation with it, lift is in question as the parent surface low is well north, east of Hudson Bay. Thus precip into New England may be dependent on daytime instability if timing aligns.

A southern stream disturbance may pass just to the south Thursday, exiting across the Gulf of Maine or open Atlantic. This could bring NW periphery showers, but we could also see a deep enough dry layer to negate much of this. High pressure then looks to return to the region late week, perhaps in time for a sunny start to next weekend.

AVIATION /18Z SUNDAY THROUGH FRIDAY/

TONIGHT THROUGH 12Z TUESDAY...VFR is expected through the evening. Conditions start to degrade overnight tonight as low pressure moves in from the west, bringing scattered showers. MVFR restrictions continue through the day on Monday, with scattered showers and thunderstorms forecast. Conditions could degrade down to IFR as marine fog builds in during the afternoon. Although showers and storms look to generally become more isolated by Monday evening, restrictions will stay down as fog continues to spread across NH and western Maine. Fog looks to mix out with diurnal heating Tuesday morning.

OUTLOOK:

Tuesday: Improvement to VFR through the morning west to east. Some SHRA possible.

Tuesday night and Wednesday: VFR, no sig wx.

Wednesday Night: SHRA possible for the mountains, MVFR/VFR.

Thursday: SHRA possible along the coast, mainly VFR.

Friday: VFR, no sign wx.

MARINE

A brief period of SCA-magnitude winds are likely overnight tonight through the end of the day Monday. Seas should mostly remain below SCA levels through this timeframe, with seas of 2-4ft expected. Some 5ft waves can not be ruled out in the open waters over 25 miles offshore.

Winds and seas likely remain below SCA levels through the rest of the week. A cold front approaches the waters midweek, as a wave of low pressure passes south of New England and into the Gulf of Maine or open Atlantic. High pressure could return late week.

GYX WATCHES/WARNINGS/ADVISORIES

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