textproduct: Gray - Portland

WHAT HAS CHANGED

Another Coastal Flood Statement has been issued for Portland and points south due to high astronomical tides late tonight/early Thursday morning.

Confidence is also increasing that we could see a few strong to severe thunderstorm over southern New Hampshire Thursday afternoon so severe wording was added to the gridded forecast here. The main threats will be damaging winds and a conditional threat for a tornado or two.

KEY MESSAGES

1. Astronomical tides are trending down but may approach minor flood stage for Portland and points south one more time late tonight.

2. An unseasonably strong storm is forecast to cross the region Thursday, leading to a chance of both severe thunderstorms and heavy rainfall.

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

4. Low pressure could bring more widespread rainfall early next week.

DISCUSSION

KEY MESSAGE 1 DESCRIPTION...

Astronomical tides will continue to decrease in magnitude over the coming days, but water levels may come close to minor flood stage (12 feet) again at Portland late tonight/early Thursday morning. Residual surge continues to hover between 0.25 and 0.5 feet. Will go ahead and issue another Coastal Flood Statement for Portland and points south for late tonight/early Thursday morning.

KEY MESSAGE 2 DESCRIPTION...

Surface high pressure continues to move well out to sea early this Wednesday as a weak surface low lifts into upstate New York. A modest upper level wave has led to some early afternoon showers and a few stray lightning strikes over the mountains. This activity should continue with minimal impacts until we start to lose daytime heating late this afternoon or early evening.

An anomalously strong system for this time of year then starts to move into the region overnight into Thursday, which should lead to widespread high precipitation chances (greater than 80 percent) across pretty much all of New Hampshire and western Maine. Isolated to scattered thunderstorms will also be likely as this system moves into the region and a surface low deepens over southeastern Ontario/southern Quebec with a cold front trailing to the south southwest and a warm front extending to the east southeast. Strong forcing, very strong shear, and weak instability could lead to some early to late afternoon strong to severe storms. The best environment will likely be over the southern half of New Hampshire, where NAM soundings suggest we could see dewpoints into the mid to upper 60s, MLCAPE values around 500 J/kg, and 0-3 km CAPE up to 125 J/kg. Additionally, shear profiles look more like something we would usually see from a cool season setup with 0-1 km values up to 40 knots and effective layer values around 60 knots. The magnitude of these shear values lend some confidence that we will see some strong to severe storms, even with the meager instability that models are forecasting. With models suggesting a low equilibrium level near 400 mb, the decent low-level instability and very strong shear could lead to some low-topped fat rotating updrafts. Strong forcing and line parallel shear suggest that a linear storm mode will generally be favored along the approaching cold front, but a few CAMs are implying we could see a couple of discrete or semi-discrete cells ahead of the cold front in the warm sector. The main threat with any of these storms will damaging winds and possibly some small hail, but with 0-1 km SRH near 200 m2/s2 and 0-3 km SRH around 400 m2/s2, there is also a conditional tornado threat. The main concern will be how early day warm advection precipitation/cloud cover evolves. If we can get some breaks in the clouds to increase instability, the threat could become a little bit more substantial and if clouds keep southern New Hampshire socked in, most storms will likely remain sub- severe. Regarding timing, CAMs suggest it will most likely be an early day window for the strongest storms, generally around 1 PM to 6 PM. Locally heavy rainfall will also be possible with any of the convection across New Hampshire and western Maine with the highest totals likely in the 1.5 to 2 inch range. That being said, we will have to watch the area in and around the White Mountains for some localized flooding potential given the antecedent ground moisture from previous heavy rainfall. Finally, winds will become fairly gusty out of the south on Thursday, especially over the higher terrain where we could see some gusts up to 45 mph or so. Some consideration was given to issuing a Wind Advisory for Coos and Grafton counties in New Hampshire but will hold off for the time being given uncertainty about how much velocity can mix down. Similarly, will keep the Gale Watch going as is for now.

KEY MESSAGE 3 DESCRIPTION...

An upper-level low remains more or less over the Canadian Maritimes Friday through Sunday. Broad cyclonic flow and waves rotating the upper low will support diurnally driven scattered showers and isolated storms as daytime instability builds in the afternoon each day. Highest chances will be in the mountains due to the terrain, but some activity is still possible downwind. Forecast soundings do support the potential for small hail and gusty winds in some storms, but severe weather is not expected.

Besides the showers and storms, the pressure gradient will be tight and mixing will be good with steep lapse rates on Friday and Saturday, supporting breezy conditions both days. Based on forecast soundings, wind gusts of 25-35 mph appear likely, possibly up to 40 mph on occasion during the day on Friday.

KEY MESSAGE 4 DESCRIPTION...

Last global models are continuing to depict a 500mb trough moving across the Great Lakes region early next week with a surface low moving offshore in the vicinity of the Mid-Atlantic region. Being this far out, there are of course some discrepancies such as timing and track of the low, but there is a pretty strong signal within the ensembles for this system to bring the next chance of more widespread rainfall, mostly in the Sunday night through Monday night timeframe. Once this system exits there should be a drying trend toward the middle of next week, but there still may be a few lingering showers depending if the 500mb trough is yet to cross.

AVIATION /18Z WEDNESDAY THROUGH MONDAY/

VFR conditions should generally continue through the rest of today and into the evening. The one exception could be over the mountains (potentially impacting HIE) where some isolated to scattered showers and thunderstorms have been observed. This activity should diminish towards evening with the loss of daytime heating.

A strong system then approaches overnight into Thursday which will lead to deteriorating conditions and widespread IFR or lower categories through most of the day on Thursday in widespread rain and thunderstorms. A few strong to severe thunderstorms will be possible, especially over southern New Hampshire. Damaging winds will be the most likely threat. Gusty southerly winds will be possible by Thursday afternoon across all terminals ahead of an approaching cold front that should cross from west to east. Winds will shift to out of the west Thursday night as winds gradually diminish.

Outlook:

Friday-Saturday: Mainly VFR, but scattered afternoon/early evening showers and isolated storms could produce brief MVFR restrictions. W/WNW winds could gust 25-30 kt both days during the daytime hours, possibly more in the 30-35 kt range on Friday.

Sunday: Mainly VFR and less breezy. Similar to Friday and Saturday, afternoon/early evening showers and isolated storms could produce brief MVFR restrictions.

Sunday night and/or Monday: Low pressure may bring more widespread IFR and rain showers.

MARINE

Winds and seas expected to remain below SCA thresholds through tonight. Then southeast winds begin to increase in response to the strong storm moving out of the Great Lakes. A strong low level jet is forecast to develop in response to this storm, and while mixing will not be strong it does appear that a period of gale force wind gusts, especially outside of the bays, is possible Thursday into Thursday evening.

Friday-Wednesday...Low pressure will be centered near the Canadian Maritimes Fri-Sat, keeping a south to southwest flow that may gust to SCA levels at times. Conditions improve on Sunday, but low pressure passing near or south of the waters may bring at SCA conditions early next week, and possibly gales depending on the track. After that, conditions improve toward the middle of next week.

GYX WATCHES/WARNINGS/ADVISORIES

ME...Air Quality Alert until 11 PM EDT this evening for MEZ022-028. NH...None. MARINE...Gale Watch from Thursday morning through Thursday evening for ANZ150-152>154-180-182-184.