textproduct: Gray - Portland

WHAT HAS CHANGED

No significant updates needed with this forecast package as the latest deterministic and ensemble guidance remains in reasonable agreement into the weekend.

KEY MESSAGES

1. Increasing humidity is going to pose at least a moderate heat risk in most locations through Friday.

2. Afternoon showers and thunderstorms may pose an excessive rainfall risk due to the elevated moisture and weak upper level flow today and Thursday.

3. Other than a mostly dry Sunday, daily chances of showers return into early next week, but confidence is low. Temperatures trend closer to normal, perhaps a few degrees above.

DISCUSSION

KEY MESSAGE 1 DESCRIPTION...

The increase in humidity will be noticble today as a moist air mass advects into the region. It will only begin to gradually dry out from north to south on Thursday but much of the forecast area will see heat indices in the 85 to 95 degree range with the hottest readings being in the Merrimack Valley of NH. That would be the best candidate for a heat advisory on Thursday, but at this time it looks to be marginal.

KEY MESSAGE 2 DESCRIPTION... Increasing heat and humidity is going to lead to daily chances for afternoon showers and thunderstorms through Thursday. The risk for strong to severe storms is fairly low despite the hot weather. This is due to the paltry deep layer wind shear and weak mid level lapse rates. However, a strong to marginally severe pulse storm can't be ruled out this afternoon and evening as well as Thursday afternoon with a 25-30 K theta-E differential between the SFC and mid levels at many locations, especially Thursday across southern zones. The thunderstorm threat may continue through this evening due to continued height falls in association with a weak short wave trough moving eastward just to our north. The activity should quickly wane late in the evening.

One thing we will have is very high precipitable water values today and Thursday (nearing 2 inches in some spots) with fairly light mid and upper level winds. This could lead to a few slow moving or training storms which could pose a threat for localized flash flooding with such high atmospheric moisture content.

KEY MESSAGE 3 DESCRIPTION... By Friday night, a 500mb ridge flattens as an upper-level low moves eastward across northern Canada. This will allow for a more zonal pattern across the northeastern US this weekend, with a cold front moving through on Friday. The frontal passage will being thunderstorm chances to the area Friday evening. Wraparound showers are likely to persist through the weekend as the attached surface low slowly moves into the Canadian Maritimes. Ensembles show a large spread of solutions for what happens beyond Saturday, though unsettled weather could potentially remain in the forecast for Sunday and early next week.

AVIATION /00Z WEDNESDAY THROUGH SUNDAY/

Through Thursday...VFR conditions look prevalent through the day today with the exception being localized lower conditions in sct showers and tstms. The atmosphere further moistens tonight with the development of IFR or lower conditions in lower cigs and fog quite possible. These will linger into Thursday morning before lifting while widely sct showers and tstms develop during the afternoon hours.

Outlook... Thursday night and Friday: Generally VFR with TEMPO MVFR possible in scattered showers and thunderstorms Friday. Fog is likely Thursday night.

Saturday: SHRA moves along a cold front pushing east through the day. This could bring MVFR cigs, improving to VFR south to north.

Saturday Night: VFR likely, but valley fog possible late.

Sunday: Uncertainty in coverage of showers and possible thunderstorms, otherwise VFR towards the coast.

MARINE

Generally quiet weather is expected on the waters through Friday with afternoon seabreezes as high pressure remains in the vicinity of the Gulf of Maine. Increasing moisture will likely lead to fog development tonight into Friday.

A cold front will cross the waters Saturday afternoon, but current forecasts call for winds and seas remaining below SCA criteria into early next week.

GYX WATCHES/WARNINGS/ADVISORIES

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