textproduct: Gray - Portland

WHAT HAS CHANGED

The risk for severe storms on Thursday has increased.

KEY MESSAGES

1. Astronomical tides are trending down but remain high for at least the next 24 hours.

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

3. The weather pattern remains unsettled Friday into early next week with daily chances for afternoon showers and a few storms.

DISCUSSION

KEY MESSAGE 1 DESCRIPTION...

See Tides/Coastal Flooding section below.

KEY MESSAGE 2 DESCRIPTION...

If anything model trends have been towards a stronger storm Thu. Ensemble mean is now 992 mb tracking thru southern Quebec. Mind you this is a mean, so there are plenty of members stronger than that. The response to this kind of out of season storm will be to induce a strong low level jet across the area. Initially southeast winds will help to force a large area of rainfall along the warm front along with gusty winds. Rainfall forecast have ticked up along with the low pressure strength, especially in and around the White Mtns. This is where we have had our most recent flash flooding, so there is already a fairly low threshold for additional flooding. I would not be surprised to see QPF continue to creep up as more mesoscale guidance is added to the mixer (e.g. 00z HREF which is rolling out at this time). 00z HRRR is already showing max QPF in the 3 to 4 inch range, which is reasonable given the expected synoptic forcing combined with warm season instability and moisture.

Once rain moves northeast parts of the forecast area will break into the warm sector. As we start to mix out, we will be able to tap the stronger winds aloft and southwesterly wind gusts may push 35 to 40 mph at times. NBM seems to be underplaying this threat given what the forecast soundings look like. So I have increased both wind speed and gusts during the daytime Thu.

Then the focus turns to severe weather Thu afternoon. It is always iffy trying to rely on rapid destabilization afternoon morning clouds and rain around these parts, but given the kinematics in place it will not take much instability for severe weather to be a possibility. There are some long range CAMs already showing a broken line initiating over Vermont and tracking into NH before weakening. SPC has increased the risk for severe storms to slight across New Hampshire and Western Maine, and guidance like CSU machine learning have both a wind and tornado risk. Considering the forecast for linear storms and 0-3 km shear in excess of 50 kt, there is a definite threat for mesocyclogenesis with any localized bowing segments or breaks in the line. Even though forecasts are for relatively shallow convection, equilibrium levels less than 30,000 ft, there is more than enough wind in the low levels for a severe threat. That severe threat should diminish fairly quickly once we lose daytime heating, but gusty synoptic winds may linger thru the night.

KEY MESSAGE 3 DESCRIPTION...

Broad cyclonic flow dominates the forecast period from Friday into early next week, with a pair of upper lows allowing shortwaves to rotate through the broader trough. This will make for an extended period of unsettled weather with daily chances for showers and storms. Although we will be dodging raindrops at times, seasonable temperatures and dewpoints should make for pleasant conditions to be outdoors if that is in your plans.

Vertically stacked low pressure slowly moves north of the area on Friday through the weekend. With the resulting persistent northwesterly flow I would expect to see mostly cloudy conditions north of the mountains as clouds get banked up to the terrain. South of the mountains should see clearer conditions, but clearer conditions are going to lead to bubbling cumulus fields and airmass thunderstorms during the afternoon with cooler air moving in aloft. In fact with 700 mb temperatures dropping to the freezing mark during this time, it wouldn't be out of the question for some of these storms to contain small hail. The Thursday system takes the extreme moisture values with it, but PWATs look to still end up in the 0.75 to 1 inch range so brief downpours are likely with any flooding risk less likely.

Global models are in reasonably good agreement on a second upper low digging in early next week, generating a surface cyclone at the base of the trough. There are unsurprisingly some timing differences here, and different ideas about the evolution of the aforementioned surface low, but it is fair to say that the signal for continued unsettled weather is strong.

AVIATION /06Z WEDNESDAY THROUGH SUNDAY/

Widespread VFR conditions expected thru morning though some patchy valley is expected to develop by sunrise. This looks like a more favorable set up than last night, and if it occurs it will be most likely in the northern valleys, including around HIE and LEB. VFR conditions continue during the day, with only local MVFR or lower expected in afternoon showers/thunderstorms returning to widespread VFR after sunset.

Outlook...

Thursday: Areas of IFR or lower early in the day with rain moving across the area. Breaks are expected into the afternoon to VFR/MVFR especially across southern half of the area. Additional local MVFR or lower possible with showers/thunderstorms in the afternoon.

Thursday Night: Conditions improving to VFR overnight.

Friday-Sunday: Generally VFR conditions with afternoon showers and thunderstorms potentially bringing TEMPO MVFR conditions to some terminals.

MARINE

Winds and seas expected to remain below SCA thresholds thru tonight. Then southeast winds begin to increase in response to a 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 Thu into Thu evening.

Elevated wave heights may still be present Friday and part of Saturday as low pressure slowly departs the area. Will also have to watch for the potential for a stronger distant storm that some models show in this timeframe that may result in elevated swell reaching the Gulf of Maine. There should be some window of sub-SCA conditions late in the weekend, but will have to monitor early next week for elevated wind gusts and seas with another low pressure system modeled to be in the vicinity of the Gulf of Maine.

TIDES/COASTAL FLOODING

Astronomical tides have peaked and continue to drop in magnitude over the coming days, but after water levels reach around 12 ft at Portland early this morning we will have to continue to watch surge trends again tonight. Residual surge continues to hover between 0.25 and 0.5 ft and with a forecast of 11.4 ft just before 2 am tonight, I anticipate water levels once again between 11.5 and 12 ft. That may be enough for another Coastal Flood Statement.

GYX WATCHES/WARNINGS/ADVISORIES

ME...None. NH...None. MARINE...Gale Watch from Thursday morning through Thursday evening for ANZ150-152>154-180-182-184.