textproduct: Gray - Portland

WHAT HAS CHANGED

Increased precipitation chances this afternoon to highlight an isolated convective threat across southwestern New Hampshire. A more showery Monday appears likely as a system brings light showers to northern New England. Hot weather remains in the forecast on Tuesday and Wednesday, with strong thunderstorms likely in the afternoon on both days.

KEY MESSAGES

1. A mostly tranquil evening is expected to wrap up the day today. However, some isolated thunderstorms this afternoon across southern NH can not be ruled out. If these storms come to fruition, they may be strong, with at least some embedded gusty wind potential.

2. A hot first half of the week is expected, with temperatures in the 80s. Some 90F readings look to be possible on Tuesday and Wedensday. Instability on both Tuesday and Wednesday will allow for strong thunderstorms to develop. The cold front moves through Wednesday night, with cooler temperatures and dry conditions through the rest of the week.

DISCUSSION

KEY MESSAGE 1 DESCRIPTION... Dry and mostly clear weather has started the day today. The warm temperatures and mostly clear skies will allow for an unstable and mixed environment this afternoon. The mixed environment will allow for breezy westerly winds through the day. These winds may carry some more moist and energy-ripe air from upstate New York, into southern New Hampshire. CAPE within this airmass looks to be near 500-1000J, and very high effective shear. The advection of this warmer, more unstable airmass may allow for the development of an isolated strong thunderstorm this afternoon, especially towards Keene and Cheshire county locations.

Monday will start cloudy, with scattered showers moving in by daybreak. Light onshore flow and an increased prevalence of dense clouds from the aforementioned low will allow for temperatures to stay cooler tomorrow across Maine, but warmer as one moves further to the west. Western NH is looking at high temperatures in the mid- 80s, with highs in most of Maine struggling to reach 70F. Most of the showers should be scattered and out of the area by the mid- afternoon on Monday.

KEY MESSAGE 2 DESCRIPTION... Heading into the middle of the week, more unsettled, hot and stormy weather looks to be in the forecast. 850mb temperatures around 15- 17C are likely to mix down into the upper 80s and lower 90s across southern NH on both Tuesday and Wednesday. Southwesterly winds are likely to keep any sea breeze confined to coastal Midcoast areas. It is certainly possible that we could see 90F readings at the Portland Jetport on Tuesday. Wednesday is only looking a little cooler for these areas. Across the interior, highs are likely to be in the upper 80s for Tuesday. However, a cold front will move through the area on Wednesday, getting past much of the interior before peak heating kicks in. This will allow for areas north of the mountains to have a much cooler Wednesday than areas south of them.

The efficient heating experienced on Tuesday and Wednesday will allow for the development of thunderstorms on both days. For Tuesday, dewpoints near 60F will allow for ample moisture to develop storms. In addition, CAPE between 1000-2000J and at least 30kts of mean shear will create a favorable environment for organized thunderstorms to develop. Storms will likely be scattered in nature and could contain some small hail and damaging winds if they develop.

It will likely be a similar setup on Wednesday, but with a more defined lifting mechanism as a cold front moves southeastward through the day. The frontal passage timing will probably prevent storms from developing north of the mountains, and with the cold front, its more likely we'll see a line of storms moving along the cold front. From this far out, it does seem like Wednesday's storms are a little less likely to become strong, but some gusty winds can't be ruled out with a developing outflow that forms with the line of storms that develop. There is likely to be much less CAPE to work with, and slightly cooler high temperatures will also hinder the workable energy these storms will have. In addition, the timeframe of convective storms will most likely be small. Noticeably cooler temperatures are expected by Wednesday evening after the cold front passes.

Cooler and drier weather is expected through the remainder of the week.

AVIATION /18Z SUNDAY THROUGH FRIDAY/

Through 18z Monday... Daytime cumulus and winds subside this evening with VFR continuing through Monday. Winds become SE to S Monday morning with gusts 15 to 20 kts through the afternoon. SHRA remnants from the eastern Great Lakes may rake across the western portion of the forecast area through the late morning and early afternoon, mainly for HIE and terminals near the US/CAD border.

Outlook:

Monday Night: VFR as winds become light with surface inversion developing. Marine airmass may invade or approach the Midcoast. This could result in IFR to LIFR cigs towards RKD with vis reduction due to fog.

Tuesday: Afternoon SHRA/TS chances near the US/CAN border. There is uncertainty how far east SHRA/TS develop in the afternoon.

Tuesday Night: Coverage of SHRA decreases. VFR forecast.

Wednesday: A cold front passes through the region with SHRA/TS west to east through the day. MVFR possible w/ frontal passage and in showers/storms.

Wednesday Night: Winds shift NW with VFR behind exiting cold front.

Thursday and Thursday Night. VFR expected with no sig wx.

MARINE

Weak cold front crosses the waters this evening with high pressure moving overhead into Monday. Expect conditions to fall below SCA through Tuesday morning. Winds may approach SCA criteria on the coastal waters Tuesday and Wednesday as southerly fetch builds. Winds then become offshore behind a stronger cold front Wed evening. High pressure builds in from the west mid to late week.

GYX WATCHES/WARNINGS/ADVISORIES

ME...Beach Hazards Statement until 8 PM EDT this evening for MEZ023>028. NH...Beach Hazards Statement until 8 PM EDT this evening for NHZ014. MARINE...None.