textproduct: Gray - Portland

WHAT HAS CHANGED

Further increased wind gusts late this morning through the afternoon for portions of southern NH into the ME/NH Lakes Region.

With consistency to the strong to severe thunderstorm threat this afternoon, have added gusty wind wording within the SPC Marginal Risk area.

KEY MESSAGES

1. Marginal risk for severe storms across northern Maine and New Hampshire this afternoon. Strong to damaging winds is the primary risk.

2. Winds, not associated with thunderstorms, may gust up to 45 mph during the day for locations inland from the ME coast and the Lower Kennebec Valley. These winds will subside in the evening.

3. A widespread soaking rainfall is expected Wednesday into Thursday morning as a cold front slowly pushes through the region.

DISCUSSION

KEY MESSAGE 1 DESCRIPTION...

The surge of warm temperatures out ahead of an approaching cold front will be the setup for afternoon thunderstorms, some of which could be on the strong side. Latest model guidance is in agreement on convection developing by early afternoon, and a few separate rounds riding along a SW-NE oriented cold front through northern New England. This is expected to be concentrated roughly in a 2-9 PM window, although sub-severe storms may continue for a few more hours into the night. Based on current modeled convective coverage, it is most likely that storms stay along and north of a Concord-Portland line, but cannot rule out an isolated (10-20%) chance all the way down to the Massachusetts border.

This is very much looking like a high-shear, low-CAPE environment, even by our standards. CAPE looks meager with guidance generally suggesting the ballpark of 500 J/Kg surface- based. Deep mixing will allow for high LCLs and modest dew points generally in the upper 40s- mid 50s depending on the model of choice. This suggests a high-based nature to storms, which means effective wind transport, but also a hostile environment for low-level storm rotation. Given the above, combined with weak updrafts owing to modest CAPE, expecting primarily a damaging wind risk with any storms that develop. This is further supported by robust wind fields; both 850 mb and 500 mb, winds are expected to be in the 40-50 kt range. It will not take much to mix down this robust wind field even in shallow convection. The higher shear values, combined with impressive low-level directional shear, also suggest some storm organization is possible - some guidance is hinting at a few bowing segments developing. However there is also a chance such strong shear will shred apart updrafts given the weak instability, so this will be a delicate balance to monitor into tomorrow. Overall the Marginal Risk states a severe risk that is on the low side, but still one worth monitoring tomorrow afternoon.

KEY MESSAGE 2 DESCRIPTION...

Daytime mixing today begins under a stronger LLJ than Monday. Deep mixing is expected into the afternoon, potentially to 700mb across much of southern NH and into far southern ME. The responsible jet is vast, with SW flow from the southern Plains into northern New England. Mixing height across multiple deterministic models suggests heights up to 800 to 700mb are possible, which would contain 30 to 45 kt winds.

Southwest winds increase areawide this morning, 20 to 30 mph through noontime. From here, there are roughly three regions in the forecast area with differing wind forecasts:

The strongest gusts are forecast from southern NH through the ME/NH Lakes Region and the far southern ME coast. This is where mixing will be most reliable through the afternoon when stronger winds aloft arrive. Gusts 35 to 45 mph are likely, with a few gusts up to 50 mph possible. Decided against a Wind Advisory at this time as frequency of the strongest gusts could be limited.

A more narrow time window exists for points west across the mountains and foothills. Here, increasing clouds will limit the deepest mixing potential, but strong convective gusts will still be a threat (see Key Message 1).

The last, and more complicated, includes the ME coast north of Portland and inland towards the Lower Kennebec Valley. Here, winds become more southerly, advecting a more stable marine airmass into the low levels. Resulting gust factor won't be as strong here as a low level inversion develops in the afternoon and overnight.

As daytime heating wanes in the late afternoon and evening, mixing depth decreases and gust factors decrease. It will remain breezy overnight, but only in the 10 to 15 mph range areawide.

KEY MESSAGE 3 DESCRIPTION...

There hasn't been a large shift in consensus since the previous forecast, but there remains some model differences for timing the front across the forecast area. Of slightly better agreement is when the best net lift crosses the region, Wednesday into Wednesday evening, that will likely result in the greatest rainfall rates.

The slow moving front will pass through after steady SW moisture advection has primed the area with PWATs around 1 to 1.25 inches, which will be +1 to +2 STD above normal for this time of year. These PWATs combined with some elevated instability will allow for moderate bouts of rainfall and some embedded thunder. This will be a beneficial rainfall event with amounts ranging from 0.5 to 1 inch south of the mountains and 1+ inches across the mountains and north before steady precipitation comes to an end early Thursday morning.

While the passing system will offer a cooldown from Tuesday, high temps into the latter half of the week settle in the upper 50s to around 60.

AVIATION /02Z TUESDAY THROUGH SATURDAY/

Through 06Z Wednesday...VFR overnight. LLWS is likely between 04Z-13Z at all terminals as a 40kt jet moves across the forecast area between 1-2kft. These winds will be focused above a strengthening surface inversion overnight. Its possible RKD sees a longer period of LLWS straight into this evening, with only slight mixing during the daytime.

Shower chances increase this morning, but these will be mainly in the vicinity of LEB and HIE. Otherwise VFR with wind gusts out of the SW 25-30kt. A few gusts 35-40 kt possible, especially during the afternoon and towards southern NH terminals. Expect an onshore wind north of Casco Bay (towards IWI/RKD) with gusts not as strong.

Showers become likely in the afternoon to early evening with thunderstorms also possible. These will will be capable of producing IFR to MVFR restrictions as well as strong wind gusts with the highest potential for SHRA/TSRA will be across northern NH and the western ME mountains. A trend towards MVFR ceilings is then expected in the north later this evening. Will also have to watch for marine stratus/fog that could produce IFR to LIFR conditions at RKD and PWM. If this occurs, IFR ceilings may make it to AUG. LLWS is again likely tonight.

Outlook:

Wednesday-Wednesday night: CIGs lower through the day Wednesday with rain Wednesday afternoon through Wednesday night. MVFR likely with periods of IFR overnight.

Thursday-Thursday night: Drying trend Thursday brings improving conditions with VFR likely.

Friday-Saturday: Mainly VFR. Low chances for -SHRA could bring brief restrictions at times.

MARINE

SCA conditions persist over the coastal waters through at least Tuesday night as southwest winds generally remain above 25 to 30 kt with seas continuing to build. For the Bays, seas may reach SCA levels today, but winds look marginal. For the outer waters, winds gusts approach gale force this afternoon into tonight, but low level stability may limit Gale frequency.

Persistent SSW flow continues Wednesday into Wednesday night with SCAs likely needed and wave heights 6-10 feet. Winds shift offshore Thursday and will drop below 25 kts while seas will be close to 5 feet. Winds generally stay below 25 kts Friday into Saturday with seas 3-5 feet.

GYX WATCHES/WARNINGS/ADVISORIES

ME...None. NH...None. MARINE...Small Craft Advisory until 6 AM EDT Wednesday for ANZ150-152- 154. Gale Warning from 2 PM Tuesday to 2 AM EDT Wednesday for ANZ180-182.