textproduct: Gray - Portland

WHAT HAS CHANGED

No major changes needed with this morning update. Just incorporated the latest observations and freshened up the aviation section to reflect the 12Z TAF package.

The forecast remains on track. but it is worth noting that some guidance trended the axis of heavier precipitation further southeastward in their overnight runs.

KEY MESSAGES

1. Soaking rainfall expected this afternoon through tonight. Some of this rain may be heavy and localized flash flooding is possible.

2. Temperatures rebound Wednesday and Thursday, but weak disturbances keep showers possible. A cold front nears the forecast area Friday.

DISCUSSION

KEY MESSAGE 1 DESCRIPTION...

First half of the day is looking okay, but things start downhill by mid afternoon. Rain will move into the area from the southwest as a broad area of warm advection precip. Now to varying degrees CAM guidance is washing this out a bit as it moves north. There is also some bouncing around of the highest QPF axis, though you could argue for a subtle southward shift.

What we do know is that this low pressure will be seasonably strong, something on the order of once a year in early summer. However the moisture transport is strong, more like once every 5 to 10 years. ECMWF EFI has also highlighted the southern half of the forecast area for anomalous rainfall totals for this time of year. That all suggests to me that a widespread soaking rain is likely, but with higher confidence for areas south of the mtns. That in and of itself is not a problem, we need the rain for long term deficits. The hazard will be recent rainfall coupled with heavy rainfall rates.

21.12z HREF was showing max QPF in the 4 to 5 inch range. The latest runs of the HRRR have been mimicking those totals, however bouncing around different points in the forecast area. Local research has shown that the max QPF totals from CAM guidance is usually a good guide for what may fall, it is the location that struggles. So I am on the lookout for those kinds of high QPF amounts, and if they occur on already saturated ground from previous day's rainfall, flash flooding becomes more likely. Biggest areas of concern there will be the mtns, especially across western Maine, and into the lower Kennebec River Valley. There will likely be some thunder with this rainfall but the threat of severe weather looks to remain southwest of the forecast area.

Rainfall will clear out pretty quickly Tue morning from west to east. There may be isolated afternoon showers, but will be fairly inconsequential compared to the evening and overnight rains. Given the recent wet weather and high pressure pushing in Tue night, I expect more valley fog likely.

KEY MESSAGE 2 DESCRIPTION...

Upper trough will be swinging through the Northeast come Wednesday, with broad cyclonic low level flow overhead. NW flow remains over the forecast area through the day as this airmass translates east. It will bring some dry air with it, but think cloud cover may be a little underdone at this range. Would at least expect upslope clouds to remain NW of the mountains. Daytime highs should rebound from a cool start to the week, with a tight spread on expected highs along the coast and interior. More variability exists for points NW of the foothills due to greater cloud cover probability.

Mid level zonal flow continues into late week, and this keeps the door open for embedded waves to bring short term unsettled conditions. This may come in the form of diurnal showers Thursday, before guidance brings a cold front near the region towards Friday. Current timing isn't favorable for stronger storms, but extended machine learning guidance maintains a signal of possibility for the day.

AVIATION /12Z MONDAY THROUGH FRIDAY/

Through 12Z Tuesday...Conditions are returning to VFR as fog begins to dissipate, and VFR should prevail through at least 18Z. Conditions are expected to deteriorate again this evening with areas of MVFR or lower ceilings and/or visibilities as widespread rainfall moves into the region. Conditions should be able to quickly improve around 12Z Tuesday as the rain begins to taper off across the area.

Outlook...

Tuesday: Conditions improving to VFR by midday. Isolated shower possible across the northern half.

Tuesday Night: VFR conditions expected. Patchy valley fog possible with local IFR or lower.

Wednesday: Mainly VFR with more clouds than sun for the mountains.

Thursday-Friday: Diurnal showers bring potential for restrictions Thursday, with a cold front potentially bringing TS on Friday.

MARINE

South southeast winds will increase today ahead of approaching low pressure. Some marginal SCA conditions are possible across the waters, especially north of Cape Elizabeth. Winds and seas will diminish into Tue. The waters remain under cyclonic flow Wednesday as low pressure pushes into the Canadian Maritimes. This should result in conditions below SCA criteria through Friday morning. A cold front will approach the region Friday.

GYX WATCHES/WARNINGS/ADVISORIES

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