textproduct: Gray - Portland

WHAT HAS CHANGED

No major changes were needed for this forecast package.

KEY MESSAGES

1.Showers and thunderstorms develop Sunday night through Monday, with locally heavy rainfall possible. An isolated strong to severe storm is possible over southern New Hampshire on Monday.

2.Humid conditions persist through the remainder of the week with late week chances for scattered thunderstorms

DISCUSSION

KEY MESSAGE 1 DESCRIPTION...

Showers and thunderstorms with localized downpours arrive Sunday afternoon into Monday, but antecedent drought conditions will significantly limit the overall flood threat. A slow- moving cold front approaching from southern Ontario will interact with a closing upper-level low to pull a plume of moisture rich air into the region. PWATs will climb to near 2 inches by Monday. Strong low level moisture convergence will support developing showers and thunderstorms, though the lack of stronger instability will largely limit any organized storms with potential to reach severe thresholds. Showers and a few thunderstorms could lingering Monday night depending on the speed of the cold frontal passage.

There is a potential for storms to move parallel to the slow-moving front, leading to localized training, supporting locally high rainfall totals. The probabilistic models show the highest rainfall in the mountains, with <15% chance of 2 or more across the area. This seems optimistic since rainfall rates and training could easily produce amounts 2-3. However, the majority of the area can expect less than 0.5. The antecedent dry conditions will be a significant buffer against high impact flooding. Ensemble guidance shows a notable absence of small stream and river responses. Outside of sensitive urban centers, regional river basins will benefit from the rainfall.

Regarding thunderstorm potential, there does seem to be at least some isolated severe storm potential along and ahead of the front on Monday. 12z NAM Nest soundings suggest that we could see dewpoints in the upper 60s and lower 70s across southern New Hampshire, yielding 1000 to 1500 J/kg of MLCAPE. This instability will be collocated with effective shear around 40 knots and strong low level shear as well. The NAM Nest is currently the most bullish with it's parameter space but other longer range CAMs do suggest the potential for at least a few strong storms. Much will depend on how early day convection evolves and whether we get warm enough ahead of the front for updrafts to become established.

KEY MESSAGE 2 DESCRIPTION...

Beyond Monday, a classic summer Bermuda High setup will take hold offshore for the remainder of the week. Persistent southwesterly flow will keep humid air locked in, with dew points hanging in the 60s to low 70s and high temperatures in the 80s. This pattern will favor diurnally driven chances for scattered downpours and weak afternoon thunderstorms particularly later in the week. However most days will still feature extended stretches of dry & humid conditions. Coastal areas can expect reprieves from the heat as onshore flow and/or sea breezes are likely most days, though it may be weak in nature.

AVIATION /00Z SUNDAY THROUGH THURSDAY/

Through Sunday: Loss of daytime heating will allow any remaining showers to gradually dissipate this evening. Then VFR conditions widespread tonight. Patchy valley fog will be possible once again, especially at LEB and HIE. However high cloud cover may delay the onset of fog. Along the Midcoast some marine fog/stratus will be possible after midnight. AUG and RKD I have both going down to IFR and LIFR respectively. Some lingering low cloud is possible around Penobscot Bay Sun, but a return to VFR is expected for all terminals.

Outlook:

Sunday and Sunday night: Increasing clouds. MVFR to IFR CIGs possible Sun night. Showers or thunderstorms possible across NH late in the period.

Monday and Monday night: Showers and thunderstorms become more widespread. MVFR and IFR likely, especially near the coast, until front crosses the area.

Tuesday - Wednesday: VFR conditions expected. Some showers possible in the afternoon with local MVFR or lower.

Thursday: MVFR or lower restrictions possible under showers and thunderstorms.

MARINE

Winds and seas are generally expected to remain below SCA thresholds thru the extended. As a front approaches Sun, southerly winds will increase and a few gusts near 25 kt are possible well outside of the bays. Guidance does not point towards a high chance of SCA conditions however. It is also possible that increasing moisture ahead of the front may lead to areas of fog developing Sun night. Waves may briefly approach SCA criteria Monday night outside of the bays.

GYX WATCHES/WARNINGS/ADVISORIES

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