textproduct: Gray - Portland

WHAT HAS CHANGED

No significant updates needed with this forecast package as temperature and dewpoint trends for this week remain steady.

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.

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

DISCUSSION

KEY MESSAGE 1 DESCRIPTION... Not much has changed in regards to the heat risk this week as model trends have held steady in regards to 850 mb temperatures rising to +15 to 17C Wednesday and then remaining there through Friday. This should be good for most areas to see temperatures rising into the upper 80s to 90F each day. The exceptions are going to be locations in the higher terrain and at the coast where it likely ends up a bit cooler. The dewpoints are going to be more dynamic, and that is where the difference will be felt. Monday and today have felt quite pleasant thanks to dewpoints generally in the upper 40s and 50s, but by Wednesday 60F degree dewpoints begin entering the picture. Wednesday shouldn't feel overly oppressive, but there is going to be a noticeable mugginess taking hold. Thursday looks like the peak of the humidity as dewpoints climb into the mid to upper 60s across much of the area. This makes those upper 80s and 90 degree temperatures feel more like the low to mid 90s. As has been previously mentioned, the warmest temperatures look confined to the lower New Hampshire river valleys making any major heat risk threat (apparent temperatures >=95F) quite isolated. Uncertainty also remains in just how warm we are able to get on Thursday with current 12Z NBM 25-75th percentile spread a good 5 to 7 degrees across New Hampshire and 5 to 9 degrees across western Maine. The factors being the possibility of convection leading to more cloud cover for the interior and then strength of the seabreeze for the coast. This is a similar story for Friday where spread is currently 10-15 degrees across most of the area. All this being said I don't have the confidence to definitively say if any Heat Advisories will be needed or not on these days at this point, but the best chance currently looks like it would be in southern New Hampshire.

KEY MESSAGE 2 DESCRIPTION... Increasing heat and humidity is going to lead to daily chances for afternoon showers and thunderstorms through at least Friday. The risk for strong to severe storms is diminishing as the best upper level forcing looks to remain well to our west. However, these showers and thunderstorms are still going to be forming in a very moist environment with weak flow aloft so focus may have to shift to excessive rainfall potential. In fact, latest ensemble means suggest PWATs rising into the 1.5 to near 2 inch range during the day Wednesday which on its own suggests downpours are going to be likely with anything that develops. When combined with the fact that forecast soundings show upper level flow only around 10kts, excessive rainfall due to slow moving and/or training storms becomes something that needs to be watched. WPC is currently highlighting the White Mountains in a Marginal Risk largely due to the terrain enhancement and resulting runoff. Locally 1 and 3 hour flash flood guidance is lowest here as well, suggesting that 1-2 inches in that amount of time could cause issues. Elsewhere in the CWA 1 and 3 hour flash flood guidance is on the order of 2-3 inches, so while it will take more, it is still a possibility with PWATs being as high as they are going to be. PWATs slowly decrease Thursday and Friday, but still look to be on the order of 1.25-1.5 inches with flow still light on Thursday, so a close eye is going to have to be kept on this. Flow increases to 20-30kts on Friday which is better, but depending on how the previous days evolve a risk could still exist.

KEY MESSAGE 3 DESCRIPTION...

A cold front will be passing through the region Saturday morning with showers expected to accompany it. Even though Saturday is better represented in guidance, there is some uncertainty for expected high temperatures. With showers and clouds spread along the passing front, how quickly these clear with sunny breaks will play a role in temperature recovery. This difference is magnified in IQR spread from points south (morning rain/cloud recovery) vs. north (early/mid afternoon rain/cloud recovery). For example, a max temperatures IQR in Concord, NH sees a range of about 7 degrees vs. Augusta, ME with a range of around 16 degrees.

The cold front will knock heat and humidity down into early next week, but still see good reason to have forecast highs a couple degrees above normal. This means highs in the mid to upper 70s, to around 80 in southern NH.

For precipitation, NBM displays nearly a daily chance of showers through mid-week. This could be magnified by a wide range of solutions among the ensemble, but for now the confidence in calling a rain free day for the entire CWA is low. Once guidance comes into better alignment, a dry day could be in the cards for either Sunday or Monday as high pressure tries to make its way through the Northeast. On the flip side, there is also not a lot of confidence in a single day being a complete washout.

AVIATION /18Z TUESDAY THROUGH SUNDAY/

Through 18Z Wednesday...VFR generally prevails through Wednesday morning. HIE and LEB may see some valley fog tonight but confidence is low so kept any restrictions out of the TAFs with this package.

Outlook... Wednesday afternoon/night: Scattered showers and thunderstorms begin to enter the area Wednesday afternoon which may bring about TEMPO MVFR conditions. TEMPO IFR is also possible if a heavier shower moves over a terminal. In the wake of showers and storms, a more widespread fog is possible Wednesday night with IFR conditions possible at all terminals.

Thursday-Friday: Generally VFR with TEMPO MVFR possible in scattered showers and thunderstorms. Fog is possible again 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 from midweek through 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.