textproduct: Gray - Portland

WHAT HAS CHANGED

No significant updates were needed this morning as the forecast remains on track. Just performed the routine freshening of the near term grids and the aviation discussion.

KEY MESSAGES

1. Expect similar highs as yesterday this afternoon with scattered showers and isolated storms.

2. Widespread rain moves in tonight followed by a chance of thunderstorms Friday afternoon.

3. Seasonably warm with chances of showers or thunderstorms most days beginning this weekend.

DISCUSSION

KEY MESSAGE 1 DESCRIPTION...

Most will likely see dry conditions today but a weak low will lead to some hit or miss showers and storms this afternoon. More of this activity could make it's way out of the mountains today than yesterday but it will still be isolated to scattered in nature. RAP forecast soundings suggest we could see MUCAPE values in the 1000 to 1500 J/kg range across most of the forecast area with some effective shear in the 25 to 30 knot range. Thus, a few stronger storms seem possible with some gusty winds and small hail.

Temperatures will be similar to yesterday with highs outside of the mountains mainly in the mid 70s to lower 80s. Some of the higher terrain will see cooler highs in the upper 60s and lower 70s. A seabreeze will likely develop and move onshore, so coastal areas could end up a little bit cooler than currently forecast.

KEY MESSAGE 2 DESCRIPTION...

A potent shortwave trough approaches from the west tonight with a warm front lifting north into the region. Warm air/moisture advection will lead to widespread rain spreading across the forecast area overnight and into Friday morning. Not much has changed regarding our thinking in totals with this overnight batch of rain, generally a quarter to three quarters of an inch with locally higher amounts up to two inches possible.

CAMs then continue to move this precipitation out quickly Friday morning with some clearing possible ahead of an approaching cold front. If we do clear out like some of the longer range CAMs are suggesting, we could see some decent instability. 00z NAM and HRRR forecast soundings are suggesting the potential for widespread MLCAPE in the 700 to 1200 J/kg range and effective shear around 50 knots ahead of the front Friday afternoon. Forecast hodographs are long and straight with a unidirectional shear profile aloft. Thus, we may see some splitting supercells along the frontal boundary as it moves from west to east across the forecast area Friday afternoon and into the evening hours. Deep layer shear vectors do have a perpendicular component to the front so storm mode may lean towards discrete or semi- discrete but the splitting cells may keep things messy with several storm interactions. All said, it does look like we may have some strong to severe storm potential Friday afternoon across New Hampshire and portions of western Maine. A few of the machine learning and AI models also continue to highlight Friday for potential severe weather. Large hail may end up being the main threat if storms can remain discrete. Forecast soundings are rather meager in their depiction of DCAPE, but damaging winds will be possible under any rotating updrafts.

KEY MESSAGE 3 DESCRIPTION...

There is not much to say about the extended forecast as far as daily details go because each day looks pretty similar. As a large scale ridge builds across the central CONUS heights will tend to be driven down locally. Daytime heating and at times seasonably cool temps aloft is a recipe for diurnal showers and thunderstorms to pop up. Once again NBM guidance has scattered to isolated PoP coverage most days and I do not see a problem with that.

The ridging west driving local heights down will also tend to keep the real heat at bay for a while. In addition the pattern is more conducive to a backdoor cold front than true heat. So while the NBM guidance has mid to upper 80s on the coast, my preference is for temps closer to what MOS is showing which is more in line with 10th percentile NBM guidance. That would be generally in the upper 70s with 80s inland.

Towards the middle of the week the ridge starts to try and roll over into New England and temps may start to creep up. It may also lead to some stronger winds and aloft and perhaps some more organized thunderstorm activity. Way too early to get into details, but some of the longer range machine learning guidance is pointing towards low end severe storm threat possible by Wed.

AVIATION /12Z THURSDAY THROUGH MONDAY/

Through 12Z Friday...VFR conditions should generally prevail through this afternoon. A few more isolated showers and storms could develop in the afternoon but confidence is not high enough to include mention at any specific site this far out. Winds will be light and variable this morning with some southerly gusts in the 10 to 12 knot range by the afternoon. Rain chances will then increase from west to east overnight as ceilings also lower with fog into MVFR and eventually IFR/LIFR categories.

Outlook...

Friday: MVFR or lower in showers and thunderstorms. Strong to severe storms possible in the afternoon.

Friday night: Conditions generally improve to VFR as showers and storms diminish. Some fog possible.

Saturday: VFR conditions expected. Local MVFR or lower possible in showers or thunderstorms.

Saturday Night: VFR conditions expected. Patchy valley fog possible, especially in the northern valleys.

Sunday - Tuesday: VFR conditions expected. Local MVFR or lower possible in showers or thunderstorms. Patchy valley fog possible at night.

Tuesday Night: VFR conditions expected. Patchy valley fog possible, especially in the northern valleys.

MARINE

Through Friday...Conditions are expected to remain below SCA levels through Friday. Winds become more southerly today through Friday as a cold front and low pressure approach. The waters will see increasing chances for rain showers tonight and Friday along with the possibility of fog.

Saturday through Tuesday...Winds and seas expected to remain below SCA thresholds. Winds will generally be offshore for much of the period. Showers and storms will be driven primarily by daytime heating, but a few may survive out over the coastal waters and need to be watched for gusty winds and small hail.

GYX WATCHES/WARNINGS/ADVISORIES

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