textproduct: Gray - Portland

WHAT HAS CHANGED

A slight northward trend in rainfall totals Sunday night through Monday.

KEY MESSAGES

1. High pressure maintains dry weather tonight and Sunday, with another seasonably cool night tonight.

2. Rain accompanies a passing system Sunday night 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.

3. Zonal mid-level flow should allow for mostly quiet weather next week. Tuesday and Wednesday should stay dry, but chances for showers and storms increase by the end of the week.

DISCUSSION

KEY MESSAGE 1 DESCRIPTION...

High pressure continues overhead tonight, with calm winds expected. Dry air through the column will also promote a mostly clear night sky, leading to excellent radiational cooling conditions.

Overnight lows could dip a few degrees lower than Friday night amid these conditions. This would again bring 40s to the mountains and northern valleys, with lower 50s across the interior and coast.

Sunday, the high will be exiting east with return flow improving moisture advection in the low levels with high clouds spilling in from the west. A mid deck of clouds will then increase SW to NE in the afternoon ahead of the next weather system bringing rain Sunday night into Monday morning.

KEY MESSAGE 2 DESCRIPTION...

Low pressure advances along the US/CAN border towards New England Sunday night into early Monday morning. The triple point along a frontal system will tap into an area of southern moisture, creating a period of showers and more stratiform rain after midnight.

Some showers will begin through the evening for western NH, but showers areawide should hold off until at least sunset.

The greatest rainfall rates look to pass through the area west to east late Sunday night into early/mid Monday morning. This is where above normal PWATs, a deep warm cloud layer, and forcing coincide to create a period of enhanced rainfall. Global ensembles seem to be in more agreement this morning, but have shifted the QPF max a bit further north across the mountains of northern NH and western ME. There could be some localized areas that see an inch of rain before sunrise Monday, but the lack of greater instability will isolate these to areas that see rainfall over a good portion of the overnight.

Remaining moisture lingers without much flow behind the disturbance, and this could lead to an expansive region of drizzle or light rain showers through the day Monday. Regardless, low clouds likely last through the afternoon.

Rainfall totals for the mountains and foothills of around 1 to 1.5 inch is forecast, with a general 0.5 to 0.75 inch elsewhere. A good soaking rain event, if the forecast coverage comes to fruition. One potential failure mode is convection from a wave through the Mid Atlantic limiting how much deeper moisture makes it to New England. Regardless, rain will still be likely given the low level jet dynamics and incoming front.

KEY MESSAGE 3 DESCRIPTION...

A relatively zonal pattern looks to setup over the entire CONUS next week. Low pressure over Canada will move eastward early next week, allowing for the development of a cold front as it crosses a region of upper- level divergence by Wednesday morning. This cold front will move through New England Wednesday night and could allow for the development of showers and thunderstorms along the front.

Wednesday also looks to be the warmest day of the week, with temperatures in the low to mid-80s forecast. Despite the passage of this cold front, low pressure and some instability could lead to more scattered showers and thunderstorms towards the end of next week.

The second half of next week should also feature cooler temperatures, with upper 70s and lower 80s in the forecast.

AVIATION /18Z SATURDAY THROUGH THURSDAY/

Through 18z Sunday...VFR through this evening with NW winds. A weak seabreeze will approach PSM/PWM/IWI, but may not arrive until mid to late afternoon. Winds become calm overnight. Again expect dense fog to develop along the CT River, resulting in LIFR conditions after midnight for LEB/HIE. This should lift around 12/13z Sunday with VFR continuing and more southerly wind.

Outlook:

Sunday Night: Clouds thicken and lower into the evening with SHRA then RA. This should lead to MVFR and IFR cigs, with periods of IFR vis when rain is heavier.

Monday: MVFR to IFR due to rain or drizzle.

Monday night: Conditions improve as a low exits the region, MVFR remains NW of the mountains.

Tuesday-Wednesday: VFR expected.

Thursday: MVFR due to showers and thunderstorms.

MARINE

High pressure passes overnight through tonight, with light north winds. Winds then become more onshore and southerly Sunday afternoon into the overnight.

A brief period of SCA-magnitude winds are likely Sunday night 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. Otherwise, winds and seas are bound to remain below SCA levels through the rest of next week.

GYX WATCHES/WARNINGS/ADVISORIES

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