textproduct: Gray - Portland

WHAT HAS CHANGED

Refreshing the aviation discussion for the 12Z TAFs.

KEY MESSAGES

1. Hot, well above normal temperatures move into the region today and Wednesday. Widespread highs in the upper 80s to lower 90s are expected today, with warm overnight lows in the upper 60s to around 70. Another round of hot temperatures is possible Wednesday, mainly focused from the interior to the coast.

2. An approaching cold front will bring chances for thunderstorms today and chances for showers and storms Wednesday when the front crosses. Any thunderstorm that develops this afternoon will have the potential to become severe.

3. The second-half of the week will feature cooler temperatures and drier conditions. Unsettled weather is possible this weekend.

DISCUSSION

KEY MESSAGE 1 DESCRIPTION...

Not much has changed regarding the hot temperatures expected today and tomorrow. Any lingering fog over the coastal areas or interior should dissipate early this morning, giving way to a clear sky and plenty of sunshine. Rapid heating will lead to well above normal highs in the upper 80s and lower 90s across southern New Hampshire and portions of interior Maine. The Heat Advisory for Hillsborough and interior Rockingham counties looks to be in good shape with no changes anticipated. With limited moisture, heat indices will be a bit on the marginal side (approaching 95) but quite a change from what we have seen thus far in the Spring.

Temperatures should remain quite warm overnight tonight as a southwest breeze continues ahead of an approaching cold front. Lows may only dip into the upper 60s and lower 70s across the south. Wednesday will be quite warm again but not as hot as today. Southern New Hampshire and interior/coastal Maine should still see highs in the mid 80s to around 90, but locations around and north of the mountains will be a bit cooler behind the front, mainly in the lower 70s to lower 80s.

KEY MESSAGE 2 DESCRIPTION...

The severe weather setup for today has become a little bit more interesting, especially over southern New Hampshire. A mid/upper level ridge will break down slightly through the day and most of the region will see some modest height falls. A weak surface low will move across Maine, dragging a surface trough with it. This boundary may be a focus for some isolated shower/storm development in the afternoon. These storms will be very hit or miss, and most locations will remain dry with limited synoptic forcing in the area. The one exception may be southern New Hampshire where a compact shortwave will move in from Vermont and provide some better lift. A few CAMs are now suggesting that a cluster of storms may develop in the early afternoon over the higher terrain in southern Vermont before moving into southern New Hampshire mid/late afternoon. NAM Nest soundings ahead of this modeled cluster suggest an environment characterized by 1500 to 2000 J/kg MLCAPE and 30 to 35+ knots of 0-6 km bulk shear. This environment will be favorable for a few organized multicell clusters or even a transient supercell structure/bowing segment. Thus, any storms that move across southern New Hampshire could become severe capable of damaging winds up to 60 mph and some large hail. Forecast Significant Hail Parameter (SHIP) values are generally just less then 1 so the threat of very large hail seems low, but hail up to the size of ping pong balls seems plausible if we do see any transient supercell structures or sustained updrafts. The tornado threat appears to be fairly low given weak low-level shear.

For the rest of the forecast area, some strong to marginally severe storms will be possible but instability will be decreasing moving north/northeast and forcing becomes more nebulous. The shear profile will be fairly similar. Showers and storms will move out and/or diminish with the loss of heating but we could see a few showers linger across the mountains overnight. A cold front then moves across most of the forecast area early in the day on Wednesday. Thunderstorms on Wednesday will generally be limited to areas along and ahead of the front, which will likely be south of the mountains. We could see enough heating to see a stronger storm or two around the interior or extreme southeast New Hampshire before the front pushes out over the waters, but the probability for severe weather appears low at the moment. A few showers may linger behind the front.

KEY MESSAGE 3 DESCRIPTION...

High pressure moves in from the west, bringing cooler and drier conditions. This high pressure will keep more active weather to the south of the region. Over the weekend, a low may move into the area and bring meaningful rain to the area. However, there is still some uncertainty in timing as some guidance brings in the unsettled weather on Sunday while others don't have precipitation moving in until Monday. High temperatures will generally be in the 60s with lows in the lower 40s.

AVIATION /06Z TUESDAY THROUGH SATURDAY/

Through 12z Wednesday...VFR through early this afternoon before scattered showers/storms develop this afternoon into early evening. These may produce MVFR to IFR visibilities and strong winds, and TEMPO TSRA has been put in some of the TAFs where confidence is high enough. Otherwise west to south west winds will gust 15 to 25 kt today before diminishing this evening tonight. Showers/storms push offshore or dissipate this evening with mostly VFR overnight, except there is potential for MVFR ceilings and a few showers at HIE. LLWS may also need to be added for tonight in the next TAF issuance.

Outlook:

Wednesday: Scattered showers and TS may bring localized restrictions late morning early afternoon. Otherwise mainly VFR with gusts 20-25 kt out of the west.

Wednesday night: VFR with NW winds gusting up to 20 kts.

Thursday through Friday: Mainly VFR.

Friday night through Saturday: Clouds may increase and lower across southern NH and Coastal Maine. Low confidence in height and extent, but could see periods of MVFR south of the mountains.

MARINE

Onshore winds advect moisture over the coastal waters this morning which may lead to some areas of fog. Showers and storms develop inland this afternoon and may track towards the NH/ME coast by evening. SW winds continue overnight, nearing 20-25 kt, which is just shy of SCA criteria. However, seas are forecast to build to 5-6 ft over portions of the coast waters, and small craft advisory has been issued.

Southwest winds turn westerly Wednesday as a cold front approaches and crosses the waters during the afternoon. Winds may gust up to 25 kts with seas approaching 5 feet. Winds and seas are forecast to remain below SCA thresholds Thursday into Saturday as high pressure builds over the Northeast.

GYX WATCHES/WARNINGS/ADVISORIES

ME...Air Quality Alert from 10 AM this morning to 10 PM EDT this evening for MEZ023>028. NH...Air Quality Alert from 10 AM this morning to 10 PM EDT this evening for NHZ013-014. Heat Advisory from 11 AM this morning to 8 PM EDT this evening for NHZ012>015. MARINE...Small Craft Advisory from 5 PM this afternoon to 8 PM EDT Wednesday for ANZ150-152.