textproduct: Gray - Portland

WHAT HAS CHANGED

No major changes were needed for this update. Just blended in the latest observations of the sensible weather fields to the going forecast.

KEY MESSAGES

1. Pleasant weather continues through Monday under high pressure.

2. Quiet weather comes to an end with unsettled weather returning for midweek with the potential for periods of heavy rainfall and low probability of localized flash flooding.

DISCUSSION

KEY MESSAGE 1 DESCRIPTION...

Quiet weather will continue today and Monday with areas of morning valley and coastal fog. Temperatures will remain near seasonable normals. The seabreeze will keep the coast cool during the afternoons. There is a non-zero chance of a few showers or thunderstorms later Monday afternoon and evening across western NH and the western ME mountains. However it appears that the forcing for ascent associated with an approaching short wave trough will remain far enough west to keep the area mostly dry.

KEY MESSAGE 2 DESCRIPTION...

It is becoming clear next week will turn wet, especially from Wednesday onward. A broad upper-level trough will sit over Hudson Bay. Coming into focus is the likelihood of a cutoff low pressure stalling and occluding over the Northeast, bringing at minimum a chance for showers and cloud cover for much of next week, but also the potential for locally heavy rainfall coming in a few separate waves.

The GFS/GEPS has joined the Euro and its ensembles in supporting a more amplified and less progressive low pressure system. However spread still remains, with the GFS members clustered over coastal New England, while the Euro members are further north into the interior Northeast. PWATs, while not the most excessive, rise to 1.50-1.75", which combined with dew points in the 60s, as well as pretty strong dynamics (many members deepen this surface low below 1000 mb), the stage will be set for plenty of heavy downpours. It is unclear how much of this precipitation will be stratiform or convective, but noting some MUCAPE on guidance, which would be most likely during the afternoon hours each day if any surface heating can occur.

The ECMWF ensemble is the most bullish on QPF, while the other members focus the highest totals to our south. As a result, the current NBM spread is very large. The 50th percentile has 1.00-1.75" across the forecast area, however the 90th percentile indicates 2.50- 4.00". This is a robust signal at this timeframe and indicates a high "tail" to the QPF distribution. However, keep in mind this represents three to four day totals, so while locally heavy rainfall is possible, not overly concerned about short-fused flash flooding at this time. July has been anomalously dry for some with minor- moderate drought conditions still persisting, so a wet week is generally good news, as long as the rainfall is not overly excessive.

Despite the increasing humidity, daytime temperatures will likely trend on the cool side, given the increased cloud cover and lowering 500 mb heights/thickness values by midweek. This would be especially likely if a more southern low pressure track becomes probable.

AVIATION /12Z SUNDAY THROUGH THURSDAY/

Through Monday...VFR conditions will prevail through Monday. The one exception could be for the next hour or two at RKD where some fog continues. SSE winds will persist through Monday at 5-15 kts.

Outlook...

Tuesday-Thursday: This period is a bit uncertain, but MVFR conditions are likely to be most common as low pressure lingers overhead or somewhere close by. This is expected to bring widespread rain showers. Fog would likely develop overnight in the wake of rain showers bringing possible IFR restrictions.

MARINE

Winds and seas will remain below SCA criteria through tonight as high pressure remains near or over the Gulf of Maine.

Generally sub-SCA conditions should start the week, but as low pressure moves into the region toward midweek chances of 25kt+ winds gusts are going to increase. Wave heights are more likely to reach or exceed 5 feet late week as the aforementioned low pressure moves over the waters.

GYX WATCHES/WARNINGS/ADVISORIES

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