textproduct: Gray - Portland

WHAT HAS CHANGED

No significant changes with this forecast update.

KEY MESSAGES

1. Above normal temperatures persist through today.

2. A slow moving front brings scattered thunderstorms both days this weekend with lowering temperatures. Southern areas likely see continued warmth Saturday.

3. A cooler and drier airmass arrives for Monday. A warming trend follows with above normal temperatures through the middle of next week.

DISCUSSION

KEY MESSAGE 1 DESCRIPTION...

Deep layer ridging will hold along the East Coast through today while a front across southeast Canada slips south into northern Maine. Highs will again be well into the 80s and pushing to around 90 degrees across interior southern New Hampshire. There will likely be more in the way of clouds than yesterday while there will still be periods of sunshine. Dewpoints will be in the 40s and 50s which will keep apparent temperatures the same as ambient temperatures. The front sinking into northern Maine combined with some instability will bring chances for showers and perhaps a thunderstorms mainly to Somerset County. A sea breeze likely develops by early afternoon bringing some relief to coastal areas. Lows tonight will range from the mid 50s to low 60s.

KEY MESSAGE 2 DESCRIPTION...

After a stretch of pleasant weather, it will turn unsettled for a few days (fittingly, falling on the weekend). A compact but organized shortwave trough will nose into the region Saturday- Sunday. At the surface, a surface low deepens to around 1000 mb and will move through Quebec, dragging a cold front into the region on Saturday. However, the flow slows on Sunday, keeping the pressure trough in the vicinity for a second day.

There will be conditional chances for strong to severe thunderstorms both days. On Saturday, models are in agreement on a precipitation shield affecting northern NH and Maine associated with a surface low. Uncertainty exists if destabilization to the south will be enough for thunderstorm development over NH/southwest Maine but an area of warm air advection within a westerly low-lvl jet could allow for a few bowing segments or discrete cell development. SPC has placed us in a day 2 marginal risk (level 1/5) for severe weather with a slight risk (level 2/5) in portions of southwestern NH. Overall instability looks quite marginal owing to poor lapse rates, and shear remains questionable but may be enhanced further north near the surface low. By Sunday, the surface low will be exiting the region, and will drag down a pocket of cold air aloft from Canada on its backside. Lapse rates should increase and represent more of a cold core thermal profile, so despite wonky shear profiles behind the front, there may be enough fuel for another day of scattered strong to severe thunderstorms.

Finally a word on temperatures. At a minimum we should see a downward trend down from the prior week owing to increased cloud and precipitation coverage. A tricky forecast on Saturday with one or more fronts overhead. Some guidance has picked up on a backdoor cold front developing as flow turns onshore ahead of the surface low. This could bring temperatures down to the 60s especially across Maine, but it remains to be seen who can squeeze out one more day in the warm sector (best odds across southern NH). Highs across central NH and interior far western Maine will likely be in the 80s with low 90s across southern NH. Sunday should bring cooler temperatures regionwide as flow turns northerly, with highs largely in the 60s-70s.

KEY MESSAGE 3 DESCRIPTION...

A cooler a drier airmass will move into the region Sunday night into Monday behind a departing cold front. High pressure will build in from southern Canada and and crest over the area Monday before sinking south through mid week. Highs on Monday will mainly be in the 70s while NE winds turning easterly will keep coastal SW Maine and SE NH cooler. As surface high pressure sinks south ridging aloft will build into New England allowing for a warming trend and temperatures rising above normal through the balance of next week.

AVIATION /01Z FRIDAY THROUGH TUESDAY/

Through 06z Saturday...VFR with no sig wx. Coastal terminals will see onshore afternoon seabreeze today, with inland terminals mainly SW...becoming light and variable at night. A few SHRA possible towards central ME Friday afternoon, dissipating in the evening.

Outlook:

Saturday: Showers and storms spread from northwest to the coast from mid morning through the late afternoon, bringing restrictions at times.

Saturday night - Sunday: Showers and a few thunderstorms with MVFR to IFR.

Sunday night - Wednesday: Mainly VFR prevails, but some nighttime valley fog will be possible at HIE and LEB each night.

MARINE

Winds and seas remain below SCA thresholds today into Saturday morning. A cold front will slowly sink south across northern New England Saturday through Sunday. Southerly winds will increase through the day Saturday ahead of the front with gusts approaching 25 kts. The front crosses the waters late Sunday with N to NE winds gusting 20-25 kts through Sunday night night. High pressure builds in Monday and settles south of the waters through the middle of next week.

GYX WATCHES/WARNINGS/ADVISORIES

ME...Beach Hazards Statement from 8 AM EDT Friday through Friday evening for MEZ023>028. NH...Beach Hazards Statement from 8 AM EDT Friday through Friday evening for NHZ014. MARINE...None.