textproduct: Gray - Portland

WHAT HAS CHANGED

715 PM Update...Scattered showers and thunderstorms with heavy downpours continue this evening, especially over NH. As modest forcing for ascent increases some this evening the activity should become a little more widespread as it spreads eastward toward the coastal plain. Rumbles of thunder may last through midnight or 1 am along with heavy downpours.

Previously...

No major changes were needed to the forecast as hot and humid weather settles in for the next couple days with little change in model trends.

KEY MESSAGES

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

2. Showers and thunderstorms may pose an excessive rainfall risk due to the elevated moisture and weak upper level flow this evening 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... Dewpoints are going to remain elevated through Friday which will begin to introduce a heat risk as temperatures peak in the upper 80s and low 90s Thursday and Friday. Any major heat risk (apparent temperatures >= 95F) continues to look like it would be isolated to the southern tier of counties in New Hampshire as the warmest temperatures are confined to the valleys. Much of the rest of the area should expect at least a moderate risk as apparent temperatures near 90F. Model spread has decreased yesterday in regards to ambient temperatures meaning there is better agreement on all this. If you dont like the heat you'll want to plan a trip to the mountains or the coast. Southeasterly low level flow is going to ramp up which will continue to keep the immediate coastline on the cooler side, and by Friday much of the coastal plain could see some relief (in the form of temperatures only climbing into the 70s) as a result of this. The remainder of the area will have to wait for the cold frontal passage for relief (see Key Message Discussion 3 for more details on that).

KEY MESSAGE 2 DESCRIPTION... Mid and low level flow have increased in model soundings since yesterday, now showing more in the way of 15-20 kts, however with PWATs also increasing this does not change the risk for localized flash flooding. Any of the showers and thunderstorms that form out ahead of the warm front this afternoon and early evening look more isolated to scattered in regards to coverage. The convective parameter space isn't all that impressive, but a strong thunderstorm isn't out of the question as a low level lapse rates could support downburst winds. The primary risk remains localized flash flooding with the showers along the warm front, coinciding with the moisture slug, that come in later. The 12Z CAMs have come into agreement on later timing of the arrival of this larger area of showers (around 8 PM), so while thunder is a possibility it will be more isolated and elevated. This area of showers slowly moves eastward with PWATs expected to be nearing 2 inches as it does. Localized Probability Match Mean from the HREF is showing a large area of 2 to 3 inches of QPF centered over the White Mountains, with embedded bullseyes of 3 to 5 inches. This increases confidence that localized flash flooding could be an issue here and WPC has maintained it's Marginal Risk. They have also expanded it to include much of the rest of New Hampshire and into western Maine. I like to use the HREF max values as a proxy for where the ceiling is, and there are areas of 2 to 3 inches with bullseyes of 4 inches outside of the mountains. This is approaching flash flood guidance values here, but as of now I would say the higher probability for any localized flash flooding is in and around the terrain.

Downpours are likely again Thursday with anything that develops, but the latest CAMs suggest coverage of showers and storms is going to be much lower and in the areas with higher flash flood tolerance. The risk is non-zero with PWATs still 1.5 to near 2 inches tomorrow afternoon, but the lesser coverage and increasing flow aloft reduces it greatly.

KEY MESSAGE 3 DESCRIPTION... Mid level ridge axis will be centered west of New England Friday morning and will slowly translate east as a cold front approaches from the west. A hot and humid air mass will be in place, while a back door front moving through the Gulf of Maine will send cool, stable air into Maine. The combination of ridging aloft and stable air invading from the east is making for a tricky convection forecast Friday. CAMs are in decent agreement that an axis of CAPE >1500 J/kg will exist across NH into far western Maine while the placement of effective shear and source of lift is more variable. There is agreement that the cold front approaching from the west along with the best shear will arrive well past peak heating into Friday night. There is a signal for storms to form along an area of convergence Tuesday afternoon across New Hampshire and far western Maine while CAMs are quite bearish on coverage. SPC maintains a Marginal Risk for severe storms across NH and far western Maine which seems reasonable given the potential buoyancy and an approaching cold front.

The cold front will cross Friday night bringing the best chances for showers while the threat for thunderstorms will be diminishing. The cold front will move offshore Saturday morning ushering in drier airmass allowing dewpoints to fall back into the 50s. It will remain warm Saturday into Sunday with highs in 80s to low 90s in southern NH.

The next cold front is progged to cross Sunday evening and will have better upper level support as a trough axis swing across northern Maine. While the air mass will not be as humid as Friday's the timing of the front will allow for sufficient instability for storms Sunday afternoon through Sunday evening. Cooler and drier air will move in behind the front with highs in the 70s and 80s Monday and mostly in the 70s on Tuesday.

AVIATION /00Z THURSDAY THROUGH MONDAY/

Through 18Z Thursday...Scattered showers and thunderstorms remain possible this afternoon, but generally VFR conditions prevail. Shortly after 00Z Thursday conditions likely begin to deteriorate as a larger area of showers enter the area which may also include thunder. This moves across the area with generally MVFR conditions and TEMPO IFR conditions in heavier rain. IFR than looks to become more likely at all terminals as fog develops in the wake of showers. Conditions look to begin to improve back to VFR by 12Z Thursday at most terminals.

Outlook... Thursday afternoon: Generally VFR conditions are expected to continue, but there remains a chance for isolated showers and thunderstorms that could bring about TEMPO MVFR conditions.

Thursday night and Friday: Patchy fog Thursday night will bring potential for restrictions along with low cigs near the coast. Generally VFR Friday with TEMPO restrictions Friday afternoon and evening in showers and thunderstorms, mainly across NH.

Friday night: Restrictions are more likely in showers, low cigs, and patchy fog.

Saturday: Conditions improve to VFR.

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

Sunday-Monday: VFR to start Sunday. A front brings showers and storms late Sunday and potential for restrictions with improving conditions Monday.

MARINE

Generally quiet conditions are expected on the waters through Thursday with afternoon seabreezes as high pressure remains in the vicinity of the Gulf of Maine. Increasing moisture will likely lead to fog development tonight and Thursday night.

A cold front crosses the waters Friday night while winds and seas remain below SCA thresholds ahead and behind the front into Sunday. A stronger cold front crosses late Sunday with winds approaching 20 kts and seas approaching 4 feet around this frontal passage.

GYX WATCHES/WARNINGS/ADVISORIES

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