textproduct: Gray - Portland

WHAT HAS CHANGED

No significant changes were made to the forecast.

KEY MESSAGES

1. Hot and humid again today with a couple of showers or storms possible in the afternoon and evening.

2. A mid level disturbance will bring a more robust chance for showers and thunderstorms Saturday.

3. Looking ahead to next week...the greatest threat for shower/storm activity will be Tuesday with a gradual cooling trend expected through the week.

DISCUSSION

KEY MESSAGE 1 DESCRIPTION... Guidance is a bit mixed on fog coverage through early this morning, but given the high dewpoints and light winds, it still remains a possibility for much of the area. River valleys are the most likely and also along the coast...but it's also possible the marine fog/stratus seen on satellite could largely remain offshore as the low-level wind direction is not very favorable for bringing it ashore, except maybe the Midcoast and around Penobscot Bay.

Fog should quickly clear up by mid-morning with another hot and humid day ahead. 850mb temperatures of +17C to +19C support highs reaching the upper 80s to lower 90s south of the mountains, and combining these with dewpoints in the upper 60s to low 70s will push heat indices into the 90s for much of northern New England. The peak of the heat is expected to be across southeastern NH extending into interior western Maine and the western ME coast, where a Heat Advisory remains in effect for heat indices expected to be in the 95F-100F range. We should see some increase in diurnal cu in the afternoon, but the 500mb pattern indicates weak shortwave ridging building in, which should add some subsidence to keep any convection very isolated much of the day. This weak ridge does begin to shift east toward this evening, so we'll see if any convection develops in NH or moves in from the west, similar to last evening.

For tonight, temperatures remain mild ranging from the mid 60s to around 70. Latest guidance is in reasonably good agreement in shortwave energy approaching from the west overnight, which will result in a chance of showers along with an increase in clouds. Valley fog is again possible, but the clouds may end up hindering this potential.

KEY MESSAGE 2 DESCRIPTION...

A warm and humid airmass remains in place for Saturday with higher chances for showers and storms with better defined shortwaves crossing northern New England. 00Z guidance is coming into better agreement with scattered showers possible in the morning with the same wave that approaches the region Friday night. A quicker exit to the east may bring a break and a better chance for some degrees of clearing and instability to build going into the afternoon hours for showers and thunderstorms to develop with the arrival of the next shortwave. The latest NAMnest, RAP, and HRRR are generally supportive of this thinking, although the amount of instability/clear is still a bit uncertain (and to that extent high temperatures). If enough instability is gained, a few strong to severe storms will be possible given 30-35 kt of deep layer shear. PWATs are still expected to be around 1.75" so rain rates could again be quite efficient, and localized flooding is possible in areas that receive persistent or multiple rounds of convection. Back to high temperatures...model consensus is mostly in the 80s for most areas, although parts of NH could still approach 90. Once showers and storms some to an end Saturday evening, fog may develop Saturday night with lows mainly in the 60s.

KEY MESSAGE 3 DESCRIPTION...

After mainly dry days on Sunday and Monday, shower and thunderstorm chances will increase Monday night into Tuesday as a wave of low pressure and associated frontal boundary crosses from the north. The timing of this disturbance as well as the one preceding it will play a large role in determining if there is any potential for severe storms/flash flooding but as of now, atmospheric ingredients are not appearing to be overly supportive. High temperatures on Monday and Tuesday will generally be into the 70s across the north with 80s south.

Temperatures will trend a little cooler through the remainder of the week, although not far from seasonable averages. A few additional weak disturbances will bring the potential for scattered showers at times, although no day currently looks to be a washout. Given the ongoing dry conditions in much of the region, rainfall would be largely beneficial.

AVIATION /06Z FRIDAY THROUGH TUESDAY/

Through 06Z Saturday night...VFR for most terminals, with a handful seeing vis restriction due to FG/BR early this morning. This is mainly for LEB/HIE and RKD. Marine stratus/fog persists today, but should stay over the waters until later this evening with MVFR/IFR possible for coastal terminals. Scattered showers/storms develop towards AUG/RKD early this afternoon. Unstable airmass means the chance for an isolated shower into the evening, but not enough to mention in a single TAF for now.

Outlook:

Saturday:MVFR to IFR possible in the morning with SHRA. IFR to MVFR again possible in the afternoon with SHRA/TSRA.

Saturday night: VFR initially, but fog develop overnight.

Sunday: Mainly VFR, but isolated showers or storms could bring localized MVFR or IFR restrictions in the afternoon.

Monday/Monday Night: Mainly VFR conditions during the daytime before SHRA/TSRA arrives overnight, likely bringing MVFR or lower restrictions.

Tuesday/Tuesday Night: SHRA/TSRA along with lower ceilings will likely bring MVFR-IFR restrictions at times.

MARINE

Through tonight...Winds and seas are expected to remain below SCA levels. Winds will be mainly out of the south to southwest at high pressure remains centered offshore of the Carolinas.

Outlook: Winds and waves are expected to remain below SCA thresholds for Saturday through the middle of next week. Localized gusty winds due to thunderstorms will be possible on Saturday and again on Tuesday of next week.

GYX WATCHES/WARNINGS/ADVISORIES

ME...Heat Advisory from 11 AM to 8 PM EDT Friday for MEZ014-018>028- 033. NH...Heat Advisory from 11 AM to 8 PM EDT Friday for NHZ010-012>014. MARINE...None.