textproduct: Gray - Portland

WHAT HAS CHANGED

Updated the Aviation section for the 12Z TAFs. No changes otherwise.

KEY MESSAGES

1. Much above normal temperatures return and largely dry through Wednesday afternoon.

2. Summer warmth continues, with increasing humidity.

3. Daily chances for downpours/thunderstorms through Saturday ahead of a cold front.

DISCUSSION

KEY MESSAGE 1 DESCRIPTION...

As high pressure slips south of the region return flow will set up warm advection across the forecast area. Thru sunrise that will mean a strengthening inversion and favorable radiational cooling. I will continue to go a few degrees cooler than NBM with the help of MOS guidance. It will also mean that with similar mixing today, we will be tapping mid level temps around 5C warmer than yesterday. This looks pretty good for mid to upper 80s across much of the area and in line with what the NBM is putting out. Subsidence inversion remains in place aloft, so while fair weather cu is possible any vertical extent should be limited.

Moving into Wed mid level temps warm, but increasing humidity is going to make mixing a little more shallow. The conflicting signals will likely lead to temps more or less ending up near high temps today. There will also be the need to watch the remnants of an elevated mixed layer approaching the Northeast Wed. Steep lapse rates aloft will likely miss much of the forecast area, but even marginal lapse rates brushing southwestern NH will introduce some CAPE and thunderstorm potential. KEY MESSAGE 2 DESCRIPTION...

A strong ridge keeps its grip on the region through Saturday before being pushed offshore as a trough builds in from the Great Lakes. As such, it will be quite a warm airmass. Ensembles show 850 mb temperatures in the +16-17C range to end the week. If we can achieve strong mixing with surface heating, this would support widespread 80s, with an outside shot a few of the warm valley locations can touch 90 degrees, especially on Thursday and Friday. Also worth noting, this will be the first sequence of high humidity this year, as the Bermuda high establishes itself for a few days. Ensembles suggest dew points of 65-70 both Thursday and Friday, before decreasing by next weekend. Combined with the high temperatures, there will definitely be a humid summer feel to the air.

KEY MESSAGE 3 DESCRIPTION...

Late in the week the resulting troughing over the Midwest is going to try to push a cold front through New England. Timing uncertainty still remains, but guidance is beginning to zero in on Saturday as the favored day for a frontal passage, with dew points sharply falling behind it as flow turns to northwesterly and ushers in drier Canadian air. There may be a secondary weaker front on Sunday, an emerging signal in some model output.

Ahead of the cold front will present daily opportunities for downpours/thunderstorms. Activity looks scattered rather than widespread each day, however there may be a brief more widespread concentration immediately along the front with added forcing. Despite some bullish severe odds on CIPS/CSU, weak shear will likely negate any significant severe threat. In addition, the front will be rather detached from the main surface low, so forcing/dynamics will not be on the strong side either.

AVIATION /12Z TUESDAY THROUGH SATURDAY/

Through 12Z Wednesday...VFR today with west winds occasionally gusting to around 15 kt, but coastal sites may see a wind shift with a possible seabreeze. Mainly VFR tonight, except valley fog possible at HIE and LEB.

Outlook... Wednesday: VFR conditions expected to prevail. Local MVFR or lower conditions possible in showers/thunderstorms during the afternoon across western areas.

Wednesday Night: MVFR to IFR (or lower) possible due to fog and stratus. Scattered showers also possible.

Thursday - Saturday: VFR conditions likely prevail, but increasing moisture will increase the threat for marine stratus/fog and IFR or lower conditions near the coast. Afternoon showers/thunderstorms may lead to TEMPO MVFR or lower conditions.

Sunday: Local MVFR or lower possible with showers/thunderstorms along a cold front, returning to VFR behind it.

MARINE

High pressure in control will lead to light winds and small seas. As return flow persists into the late week, increasing moisture may lead to areas of low cloud and fog forming on the coastal waters.

GYX WATCHES/WARNINGS/ADVISORIES

ME...None. NH...None. MARINE...None.