textproduct: Gray - Portland

WHAT HAS CHANGED

Another Coastal Flood Statement has been issued for Portland and points south for tonight's high tide cycle.

KEY MESSAGES

1. High astronomical tides continue into the middle of the week with minor flooding possible around the times of high tide.

2. Expect slightly warmer temperatures on Wednesday with some isolated to scattered showers and thunderstorms during the afternoon, mainly over the mountains.

3. A strong storm system crosses the region on Thursday, likely bringing a soaking rainfall along with the potential for a few strong to severe storms in the afternoon.

4. The weather pattern remains unsettled Friday into the weekend with daily chances for afternoon showers and a few storms.

DISCUSSION

KEY MESSAGE 1 DESCRIPTION...

High astronomical tides will continue through the middle of the week. Wave action is not expected to be an issue until at least the latter half of the work week, and by then tides will be decreasing in magnitude. With surge expected to to be around 0.5 feet again late tonight/early Wednesday morning, went ahead and issued another Coastal Flood Statement for Portland and points south where we could see some minor flooding. We could end up seeing one more very high tide late Wednesday night/early Thursday morning as residual surge around 0.5 feet remains in place. However, it appears that tides will be just below minor flood stage during this time frame.

KEY MESSAGE 2 DESCRIPTION...

Upper level ridging will start to build tonight and into Wednesday which will lead to some warmer temperatures. Afternoon highs will range from upper 60s and lower 70s north, to the upper 70s and lower 80s south and along the coast.

Later in the afternoon, a weak shortwave will approach which will lead to some 20 to 40 percent chances of showers and maybe an isolated thunderstorm or two. Guidance suggests most of this activity should generally be confined to the higher terrain and should diminish with the loss of daytime heating.

A potent upper low and longwave trough then starts to approach overnight Wednesday and we could see the next round of precipitation entering southwest New Hampshire late. Overnight lows will mainly range from the upper 40s to mid 50s.

KEY MESSAGE 3 DESCRIPTION...

A potent low pressure more typical of fall or spring will move across the St Lawrence River Valley on Thursday and drag a cold front toward New England. Strong lift associated with this system will likely result in a widespread soaking rain overspreading the region during the morning as a warm front lifts into northern New England. Prior to the onset of rain, there are signals that fog could be locally dense into a portion of the morning commute, especially along the western ME coast and interior.

For the afternoon, onshore flow and if the rain and low stratus can clear will play a key roll in how things evolve as the cold front approaches from the west. As it looks right now, the onshore flow, rain, and marine layer will keep much of western ME stable in the low levels while NH, particularly the southern half, stands a better chance (but still very uncertain) of getting into the warm sector with partial clearing and gaining some instability, upwards of 500 J/kg of SBCAPE per some CAMs. There will be no shortage of wind shear (possibly too much with limited instability) with this system, but if more surface-based instability can be realized, there is potential for storms to be strong to severe with damaging winds gusts and and even a tornado or two. With instability the big question mark and potential limiting factor, agree with the SPC Marginal Risk (level 1 out of 5). Rainfall rates will also have to be monitored with the strong forcing and high PWATs, and we are currently in a Marginal Risk for Excessive Rainfall.

Worth mentioning: There are also a couple of areas to watch for gusty winds (outside of storms). The first is those that get in the warm sector and start mixing could gust 30-40 mph. The other is along the coastline as a 925mb 50-55kt LLJ works into the Gulf of Maine.

Patchy fog is possible Thursday night in some areas if winds become light enough, but in general a clearing trend is expected behind the front with the boundary layer remaining fairly mixed.

KEY MESSAGE 4 DESCRIPTION...

From Friday through the weekend, we'll have broad cyclonic flow across northern New England with an upper-level low centered to the north or northeast. This lift along with waves rotating around the upper low and building daytime instability will result in diurnal cumulus along with scattered showers and a few storms in the afternoon and early evening hours each day...with the higher chances toward the mountains. Based on forecast soundings, a couple of storms could contain small hail. Friday and Saturday will be on the breezy side Friday will be in the 70s and 80s, and then mostly in the 70s this weekend.

Global models are in reasonable agreement for early next week in depicting a shortwave trough moving across the Great Lakes with the possibility of a surface low developing as the trough moves toward New England. This would be the next opportunity for more in the way of widespread precipitation, but location of the trough and low is still uncertain at this point.

AVIATION /00Z WEDNESDAY THROUGH SUNDAY/

Continued clearing tonight with winds becoming light and variable. Widespread VFR conditions expected, but patchy valley fog might be more likely tonight than last night. If it occurs it will be most likely in the northern valleys, around HIE and LEB. VFR conditions continue Wed, with only local MVFR or lower expected in afternoon showers/thunderstorms.

Outlook:

Thursday: Widespread IFR to LIFR restrictions are likely in the morning with rain, drizzle, fog, and low stratus all on the table. These may last much of the day in western ME with improvement to MVFR or even VFR is possible in southern NH during the afternoon, but thunderstorms (some possibly strong) may bring additional IFR restrictions.

Thursday night: Can't rule out patchy fog if winds are light enough, but in general conditions should return to VFR behind a cold front.

Friday: Mainly VFR with a few afternoon showers or storms could produce MVFR restrictions. Westerly wind gusts of 25-30kt possible.

Saturday: Mainly VFR with a few afternoon showers or storms that could produce MVFR restrictions. Westerly wind gusts of 25kt possible.

Sunday: VFR and not as breezy. Scattered showers possible in the afternoon that could produce MVFR restrictions.

MARINE

Through Wednesday...Winds and seas expected to remain below SCA thresholds into Wed.

Thursday through early next week...S/SE winds will likely exceed SCA levels across the nearshore coastal waters and bays on Thursday as strong low pressure moves along the St Lawrence River Valley and a cold front approaches. Gusts may also approach gale force over the nearshore and outer waters on Thursday. Increasing moisture will probably result in fog over portions of the coastal waters until the front passes. The front crosses the waters Thursday evening and Thursday night with winds and seas possibly staying above SCA levels into Friday. Flow remains generally out of the west over the weekend as low pressure remains over the Canadian Maritimes. Another low pressure may pass near or just south of the waters early next week, possibly bringing another increase in winds and seas.

GYX WATCHES/WARNINGS/ADVISORIES

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