Marine Weather and Tide Forecast for Topsham, ME

NEAR TERM /THROUGH SUNDAY/
800 PM Update... Have allowed the Severe Thunderstorm Watch to expire. Otherwise, no notable changes at this hour.

Previously...
715 PM Update... A weakening line of thunderstorms is pushing eastward across central NH. This line will continue to weaken as it approaches western ME where a marine airmass is locked in place. The greatest lingering potential for perhaps a gusty storm through the remainder of this evening will be across southern NH where temperatures are still into the lower 70s. Any storms will produce locally heavy rainfall, which could result in some minor flooding. Will allow the Severe Thunderstorm Watch remain in place through its expiration at 8PM.

Previously...
Based on surface observations, the warm front appears to be draped roughly from Lebanon, NH to near Concord, NH as temperatures along and to the south have reached 70s. The warm front may lift a bit farther to the north and east over the next few hours, and this area as well as farther north along the CT River will be the regions to watch for strong to severe storms going into this afternoon and evening. SPC Mesoanalysis indicates upwards of 1000-1500 J/kg of SBCAPE over southwest NH and not quite as farther north along the CT River, although Coos county is clearing out per latest visible satellite. As the better forcing approaches the area, this environment will be supportive of large hail in excessive of 1" in diameter and damaging winds gusts. There is also enough 0-1km SRH to support a brief tornado in discrete cells. The 0-3km shear vector is also in excessive of 30 kt, and this could also aid in a brief tornado within line segments where it becomes more perpendicular to the line. A Severe Thunderstorm Watch is in effect until 8 PM for the Grafton, Sullivan, and Cheshire counties. Based on near-term trends the watch may need to be extended into Coos county.

There is some uncertainty in how far east the severe risk extends but does appear to be marginally supportive of strong to severe thunderstorms across central and eastern NH (so it's possible an eastward expansion to the watch may be needed). The environment then becomes much less favorable toward the Seacoast and into ME where the marine layer is well established, but elevated CAPE could support thunder and maybe even small hail.

Lastly, will have to watch for Flash Flooding, and please refer to the hydrology section below.

The threat for strong to damaging wind gusts diminishes this evening as surface-based CAPE wanes with the loss of daytime heating.
However, forecast soundings show pretty steep lapse rates aloft, which may prolong a threat for hail a little longer, and at the very least sufficient enough for thunderstorms. This is primarily through this evening across NH and into SW ME, and again the convection should then encounter a less favorable environment as it progresses eastward farther into ME through the late evening or early overnight hours, although it's possible there could still be small hail and some rumbles of thunder.

Fog is again likely tonight, especially along the Maine coast and interior, but will also be possible over areas that see rainfall today. Additional showers may also develop overnight as the upper low continues to approach.

SHORT TERM /SUNDAY NIGHT THROUGH 6 PM SUNDAY/
What's left of the fog should mostly clear out by mid-morning on Sunday except could linger near the Midcoast a little longer.
Otherwise, the upper low will cross overhead during the day with showers likely across northern areas with lower chances (30-50%)
with southward extent. Steepening lapse rates as temperatures aloft cool do also support enough instability for heavy downpours and maybe a couple of storms. Highs will be mostly in the 60s.

For Sunday night, waves of energy rotating around the upper low will continue a chance of showers with the higher chances again favoring the mountains and northern areas where low-level upslope flow will also be present. Northwest winds will also increase overnight, especially across NH where gusts of 30-35 mph will be possible.

LONG TERM /SUNDAY NIGHT THROUGH SATURDAY/
The upper low crossing the region this weekend will slowly exit into the western Atlantic Monday and Tuesday. As this system exits another upper low approaches from the Great Lakes Wednesday night into Thursday. This upper low will cross the region Friday through the holiday weekend. Chances for rain will be on the downward trend Monday into Tuesday before increasing Wednesday with the highest chance for widespread soaking rainfall occuring Thursday into Friday. As the upper low crosses overhead next weekend there will remain chances for showers, while no one day looks to be a washout.
The unsettled pattern will lead to temperatures averaging below normal through the period.

Global models are in good agreement that a vertically stacked low pressure system will be over Nova Scotia by Monday morning with drying northwest flow over developing the forecast area.
Showers will remain likely across the mountains and north through the day while areas south of the mountains will see lower chances and breaks of sunshine. Highs will range from the upper 40s north to the mid 60s across the south. Tuesday looks to be mostly dry outside of the mountains with highs mainly in the 50s to low 60s. High pressure tries to build in from the north late Tuesday into Wednesday for a cooling trend and an extension of mostly dry conditions. Steady east to northeast winds will keep highs Wednesday in the low to mid 50s.

As the next upper low approaches from the Great Lakes a surface low will develop over the Mid Atlantic Wednesday night. Models diverge on the track of this low with some solutions taking the bulk of the precipitation south of the area and others track the system into the Gulf of Maine Thursday bringing a widespread soaking rainfall. Have mainly stuck with the NBM PoPs that bring 70-80 percent chances of rain Thursday with PoPs diminishing to 40-50 percent Saturday.

AVIATION /00Z SUNDAY THROUGH THURSDAY/
Short Term...TEMPO IFR to MVFR restrictions possible with showers and storms into this evening and tonight especially over NH with lesser chances for storms going into ME. However, the ME terminals are more likely to see IFR to LIFR restrictions from fog and low stratus. Expect fog to clear out Sunday morning with mostly VFR outside of showers during the day Sunday.
However, LEB and HIE could see more persistent MVFR ceilings through Sunday night.

Long Term...VFR likely prevails south of the mountains Monday, while upslope clouds and showers brings restrictions to KHIE. VFR likely prevails across all TAF sites Tuesday into Wednesday. Low pressure approaches Wednesday night bringing the threat for restrictions Thursday.

MARINE
Short Term...Southeast winds will persist through this evening and will then become more west to southwest overnight into Sunday morning as a weak low pressure crosses the waters. Fog is expected to linger through tonight, but visibilities are expected to improve from west to east through the day Sunday as the W/SW flow increases but remaining below SCA levels. As the low pressure becomes centered near the Bay of Fundy Sunday evening Sunday night, NW winds may increase enough to where an SCA will be needed.

Long Term...Offshore winds will prevail Monday and Tuesday with gusts generally below 25 kts. High pressure builds in from the north Tuesday night and Wednesday shifting winds out of the northeast and then east. Low pressure approaching from the Mid Atlantic Thursday will bring enhanced easterly flow with winds and seas reaching SCA criteria.

HYDROLOGY
Multiple rounds of showers and storms are likely through this evening and could pose another threat of flash flooding. The threat is highest from the foothills and points north on Saturday where WPC has highlighted a Slight Risk of Excessive Rainfall.

GYX WATCHES/WARNINGS/ADVISORIES
ME...None.
NH...None.
MARINE...None.