textproduct: Gray - Portland

WHAT HAS CHANGED

Have cancelled the Tornado Watch and the Flood Watch as the threat has ended. A line of showers with embedded thunder will continue east through central Maine over the next couple of hours with skies already clearing to the west behind this line.

KEY MESSAGES

1. An unseasonably strong storm will cross the region today, with increasing chances for severe thunderstorms and localized flash flooding this afternoon and early evening.

2. Unsettled pattern continues over the weekend into early next week.

DISCUSSION

KEY MESSAGE 1 DESCRIPTION...

Overall expectations remain largely unchanged for the remainder of this afternoon into early this evening as we are currently watching a squall line quickly approaching the Connective River from the west. The environment out ahead of this line is packing plenty of wind shear with SPC mesoanalysis having 50-60kt of effective shear across NH and parts of far western ME and 0-1km SRH exceeding 300 m^2/s^2. This level of speed and directional shear is more than sufficient for severe weather in areas where there is enough instability with the primary hazard being damaging wind gusts along with the threat of tornadoes. The tornado threat exists both within any discrete cells than can develop out ahead of the line as well as within the line.

Looping back to the instability, the highest risk for severe storms will be in NH, especially along the CT River Valley and central and southern NH where model consensus brings in roughly 500 J/kg of surface-based CAPE. There may also even be instances of damaging winds within cells with little to no lightning as motion will be fast. The threat extends east into far western ME, but SBCAPE then drops off pretty readily with eastward extent with onshore flow in place. That said, it won't take much to mix down the strong winds from aloft, but it's possible storms become more elevated in nature as they progress through western ME (although still enough elevated instability for thunderstorms and torrential downpours).

Last, but certainly not least, is the threat of flash flooding, especially in and around the White Mountains extending into parts of the western ME mountains and foothills. These areas have seen the most rain over the past week, and torrential downpours combined with rain from this morning will pose a risk of flash flooding. Some area river rises are also expected with the Pemi at Woodstock and the Swift River at Roxbury possibly reaching minor flood stage.

A few showers may stick around through this evening behind the primary line of convection, but drier air and subsidence moving in behind the front keep things dry the rest of tonight, with the exception of a few upslope rain showers in the mountains. Winds could get a bit breezy overnight with low temps in the 50s.

The upper-level low will slowly move east toward the Canadian Maritimes during the day Friday. Broad cyclonic flow and embedded waves stemming from this low will provide enough lift to get showers and a few storms going, mainly in the afternoon to early evening, as daytime instability builds. This will be mainly in and around the mountains, and a few storms may contain small hail.

Otherwise, it will be a breezy and partly sunny day, although there may be periods of mostly cloudy skies with steep low-level lapse rates and enough instability aloft. The steep lapse rates and good mixing will bring down stronger winds from aloft resulting in a breezy day. Based on forecast soundings, wind gusts of 25-35 mph are likely with up to 40 mph possible on the higher end. Highs will be mainly in the 70s to low 80s south of the mountains, except a bit cooler in the mountains.

Winds likely remain somewhat breezy into Friday evening but are expected to continue to ease overnight. A few showers remain possible in the mountains with upslope flow, but whatever activity develops during the day is expected to fade around sunset. Overnight lows will be mostly in the 50s, except some upper 40s are possible in the mountains.

KEY MESSAGE 2 DESCRIPTION...

One upper low will be lifting northeast away from the area Saturday into Sunday while another upper low approaches from SE Canada late Sunday into Monday.

Saturday will be breezy with NW winds 15 to 20 mph gusting up to 30 mph. Upslope near blocked flow will lead to plenty of clouds and showers along and north of the mountains while shower chances decrease south of the mountains with more in the way of sunshine. Highs will range from the low 60s north to near 80F near the coast with the aid of downsloping winds. The gradient will slacken over the area Sunday allowing for light winds turning onshore in the afternoon. A surface trough over the area combined with SB CAPE building to 1000 J/kg will allow for diurnal showers and thunderstorms to develop across much of the area Sunday. The lack of shear will likely keep any storms sub-severe while steep mid level lapse rates and low freezing levels could allow for some small hail.

Short wave ridging briefly builds in Sunday night into Monday morning ahead of the next upper low for a drying trend. As the upper low over SE Canada approaches another short wave will be tracking through the Ohio Valley. Models diverge on whether the short wave and upper low can phase. A phased solution would lead to an organized area of low pressure bringing a soaking rain late Monday into Tuesday morning while an unphased system would bring more in the way of showers. The latest NBM brings PoPs into the 60-70 percent range Monday afternoon through Monday night, which seems reasonable given the time range. While there will likely be some dry periods through the middle of next week the overall pattern will support active weather with persistent troughing near the Great Lakes through the Northeast.

AVIATION /21Z THURSDAY THROUGH TUESDAY/

Through 00Z Saturday...IFR to MVFR restrictions remain likely this afternoon into early evening with ongoing SHRA and TEMPO TSRA that could also produce strong winds gusts. Conditions likely improve fairly quickly to VFR from west to east this evening and tonight behind a cold front with the last sites to improve being RKD and AUG. However, HIE may see a period of MVFR ceilings. Southwest winds may also gust 20-25 kt. LLWS is also forecast through tonight.

For Friday, primarily VFR, but scattered showers and isolated storms could produce brief MVFR restrictions with the higher potential will be at HIE. Otherwise westerly wind gusts 25-30 kt are likely with some gusts possibly exceeding 35 kt.

Outlook:

Friday night: VFR expected. Winds may remain breezy early on but are expected to ease overnight.

Saturday: Upslope clouds and showers will likely bring at least MVFR to HIE while mainly VFR prevails elsewhere. NW winds 15 kts will gust to around 25 kts during the morning into the afternoon and will diminish Saturday evening.

Sunday: Mainly VFR with light winds turning onshore in the afternoon. Late morning through afternoon showers and thunderstorms will bring brief restrictions.

Monday-Tuesday: Low pressure may bring more widespread IFR and rain showers.

MARINE

Gales remain likely this afternoon and early evening over the waters with potential for even stronger gusts in offshore-moving thunderstorms, especially from Casco Bay southward. Conditions then turn to SCA levels tonight and Friday as winds turn west to southwesterly behind a cold front. Winds are expected to stay below SCA levels Friday night, but seas may remain above 5 ft for portions of the waters.

WNW winds will gust to 20 kts Saturday with wave heights running 3-5 feet. Winds and seas diminish Saturday night and Sunday. An area of low pressure may develop near coastal New England Monday bringing the potential for at least SCAs.

GYX WATCHES/WARNINGS/ADVISORIES

ME...None. NH...None. MARINE...Gale Warning until 8 PM EDT this evening for ANZ150-152>154- 180-182-184. Small Craft Advisory until 8 AM EDT Friday for ANZ151.