textproduct: Gray - Portland

WHAT HAS CHANGED

Increased QPF in Western Maine as CAMs continue to show a corridor of higher rainfall totals associated with storms this afternoon.

Also added patchy valley fog to the forecast through this morning as observations and microphysics satellite imagery show it's development.

KEY MESSAGES

1. A cool airmass brings the chance for frost across the north and parts of the interior this morning.

2. A shortwave brings scattered showers and thunderstorms for the afternoon hours today. A few more showers are possible Monday.

3. Seasonable temperatures and mostly dry for the remainder of the week.

DISCUSSION

KEY MESSAGE 1 DESCRIPTION...

With light winds and clearing skies this morning, it should become quite cool across much of the forecast area. Low temperatures will range from the low to mid 30s across the north and into portions of the interior, to the upper 30s and lower 40s south and along the coast. This will lead to areas of frost across the north and portions of the interior. The Frost Advisory continues to be in good shape.

KEY MESSAGE 2 DESCRIPTION...

The next shortwave travels down the highly amplified meridional flow aloft with ribbons of vorticity forecast to traverse the forecast area from late morning through early evening. This feature and a weak surface low will lead to widespread showers and thunderstorms across most of the forecast area. Highs will mainly range from the upper 50s to upper 60s and dewpoints will only be in the 40s, but cold air aloft will lead to widespread MUCAPE values in the 300 to 600 J/kg range. Effective shear will be fairly weak in general but could approach the 35 to 40 knot range over southern New Hampshire. All that said, severe weather is not expected but the strongest cores could lead to some gusty winds and small hail given the cold temperatures aloft. Most of the thunder chances will diminish with the setting of the sun but a few showers may linger into the overnight hours.

The bulk of the shortwave will push east and south of the area on Monday. However, lingering cyclonic flow aloft should lead to some diurnal showers once again but with much less coverage than today. The RAP suggests that we will have some instability present again, so some thunder is not out of the question. Highs on Monday will be a little bit cooler than today, mainly in the upper 50s to the mid 60s. Monday night lows will mainly be in the low to mid 40s.

KEY MESSAGE 3 DESCRIPTION...

The upper-level troughing finally releases its grip on New England by Tuesday. This happens as the omega block pattern breaks down, and the anomalous ridge over Canada is squashed southeast into the Northeast U.S. For local sensible weather, this should result in a multi-day period of tranquil conditions and near seasonable temperatures as a 1025 mb high pressure crests over the region. Flow will be light so sea breeze formation will be possible each day Tuesday onward. Much of the week will feature highs in the 60s-70s, but there is a chance temperatures could top 80 degrees by Thursday-Friday as the airmass becomes warmer aloft.

As indicated, this overall looks like a rain-free period. The only days to keep an eye on would be Tuesday and next weekend. On Tuesday, the remnants of the weekend's cold pool will still be in the vicinity, which may contribute to a few isolated diurnal showers. By next weekend, some guidance shows another shortwave approaching the region, but detail/timing remain highly uncertain with this being close to a week out.

AVIATION /06Z SUNDAY THROUGH THURSDAY/

Any fog and low ceilings will lift early with VFR conditions through the remainder of the morning. Periods of MVFR to brief IFR visibilities are then possible through the afternoon and evening hours as scattered showers and thunderstorms are possible at all terminals. A few of the stronger storms could produce gusty winds and small hail. A few showers will remain possible Sunday night with some scattered MVFR ceilings.

Outlook:

Monday: Isolated showers and storms may lead to localized IFR to MVFR conditions for brief periods of time. Winds are expected to be easterly at 5 to 10 knots turning south and light by the evening.

Monday night: VFR expected.

Tuesday: VFR with isolated showers possible. Light north winds becoming southerly on the coastal plain during the afternoon.

Tuesday night: VFR expected.

Wednesday/Wednesday night: VFR with light winds.

Thursday/Thursday night: High pressure is favored with VFR conditions.

MARINE

SCA conditions continue along the coastal waters and into Casco Bay as seas remain above 5ft most of the morning. Seas gradually fall below 5ft from north to south through this afternoon. High pressure passes through the Gulf of Maine on today, then a weak low pressure system crosses through tonight.

Unsettled pattern persists into Monday while winds and seas generally stay below SCA thresholds. Fair weather Tue-Fri with winds and seas remaining below SCA thresholds.

GYX WATCHES/WARNINGS/ADVISORIES

ME...Frost Advisory until 7 AM EDT Sunday for MEZ007>009-012-033. NH...Frost Advisory until 7 AM EDT Sunday for NHZ001>004. MARINE...Small Craft Advisory until 5 AM EDT Sunday for ANZ150-152-153. Small Craft Advisory until 2 PM EDT Sunday for ANZ154.