textproduct: Gray - Portland

WHAT HAS CHANGED

QPF has trended up slightly in the global ensembles for early next week, but much uncertainty remains regarding how this system will unfold.

KEY MESSAGES

1. Pleasant weather continues through Sunday night under high pressure.

2. Quiet weather comes to an end with unsettled weather returning for the first half of next week with the potential for areas of heavy rain.

DISCUSSION

KEY MESSAGE 1 DESCRIPTION... It will continue to be a pleasant afternoon and evening across the region with temperatures into the lower 80s and low humidity prevailing. Diurnally driven cu will dissipate with the loss of heating this evening and this combined with light winds and low dew points will allow for another night of radiational cooling. There are some suggestions that a bit of high level cirrus may arrive late tonight but this tends to not have a significant impact on cooling. As a result, used mostly MOS guidance for forecast lows with most places falling into the upper 40s to lower 50s. Valley fog is likely once again overnight and there could also be some fog towards the Mid Coast and adjacent islands.

Surface high pressure will persist on Sunday into Sunday night, allowing for continued dry conditions with near average temperatures. High temperatures will be into the 70s to lower 80s (coolest along the coast), with low temperatures mostly into the lower 50s.

KEY MESSAGE 2 DESCRIPTION... Global models are in good agreement that an unsettled pattern develops next week as several shortwaves look to traverse to the broader 500 mb trough that has been sitting over the region for quite some time now. However, this is about all that the models agree on at the moment as the extent of precipitation we may see as a result is very much up in the air. The Euro ensemble has maintained its position of having a seasonably strong vertically stacked low pressure move through the area in the Tuesday-Wednesday timeframe. This is combined with stronger moisture transport bringing PWATs up to 1.5 to 1.75 inches south of the mountains, and as a result the ECMWF Ensemble is the most bullish on QPF with high probabilities (50-60%) of widespread amounts greater than an inch, and some moderate probabilities (30-40%) starting to show up for amounts greater than 2 inches. The GEFS and Canadian Ensemble are currently not favoring the development of an upper low, and even keep the surface low more to our south. The GEFS also has much weaker moisture transport with PWATs in the 1.25 to 1.5 range. As a result the current probabilities for more than an inch of QPF is very low (~10%). Unsurprisingly this is making the current NBM spread very large with the 25th percentile around a tenth of an inch and the 75th percentile upwards of 2.5 inches. To say there is currently uncertainty in the rainfall amounts is an understatement, but the hope is that over the next couple days the RAOB network will begin sampling this developing pattern, and can better inform the models.

Thunderstorms are possible each afternoon as instability gathers, even with daytime temperatures bordering on the seasonably cool side. At this point there isn't anything that screams severe weather, but the uncertainty in low pressure tracks isn't helping that case either. I think the best way to summarize the long term at this point is that there is a high confidence in seeing periods of showers (and some thunderstorms) which may be heavy at times, but exact amounts, the track of low pressure, and the duration low pressure lingers are very low confidence and trends are going to have to be monitored closely over the coming days.

AVIATION /18Z SATURDAY THROUGH THURSDAY/

Through 18Z Sunday... VFR conditions will prevail through Sunday night. The exceptions are at KLEB and KHIE where valley fog is once again likely, especially between 09Z-12Z Sunday. Marine fog is also possible at KRKD between roughly 08Z-13Z Sunday. SSE winds will persist through 00Z Sunday at 5-15 kts before becoming light and variable tonight and then becoming SSE again on Sunday. No LLWS is anticipated through the period.

Outlook... Monday: Generally VFR. Isolated afternoon showers and storms are possible which may bring about TEMPO MVFR restrictions.

Tuesday-Thursday: This period is quite uncertain, but MVFR conditions are likely to be the 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 Sunday night 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.