textproduct: Gray - Portland

WHAT HAS CHANGED

No significant changes to the forecast with this update. Confidence is increasing in potentially soaking rainfall late this week.

KEY MESSAGES

1. Outside of some shower activity in the mountains and foothills, conditions will stay mostly dry through mid week with above normal temperatures.

2. The first chance in several weeks of a soaking rainfall arrives late this week.

DISCUSSION

KEY MESSAGE 1 DESCRIPTION... Light shower activity continues in the mountains tonight as westerly flow persists. The increasing cloud cover and rising dewpoints are going to make for a comparatively mild night with temperatures only falling into the low to mid-40s.

Shower activity increases again beginning Tuesday morning as a weak cold front sinks southward into our area. The trend has been for the front and its accompanying precipitation to only make it as far south as the foothills with southern Maine and southern New Hampshire standing the best chance of remaining dry. This activity starts widely scattered and light, but a more surprising trend has been an increase in QPF Tuesday night as a warm air advection regime takes hold and the front begins to lift back north. The 02.12Z REFS mean suggests the mountains could end up seeing 0.25 to 0.50 inches with isolated higher amounts in central Somerset County. The foothills may see a tenth to a quarter of an inch, with a few hundredths possible down to a line extending from Lebanon, New Hampshire to Portland, Maine. Shower activity comes to an end Wednesday morning as the front pushes back out of our area. Skies clear as well and allow for our warming trend to peak as temperatures are able to mix into the low to mid 70s across much of the area.

KEY MESSAGE 2 DESCRIPTION...

Latest NWP model guidance continues to indicate that a frontal boundary will arrive from the west mid-week before stalling near New England late Thursday through at least Friday. During this time period, multiple weak waves of low pressure are progged to ride along this boundary, bringing our best potential for a widespread soaking rainfall in several weeks. That being said, there is significant uncertainty in where this frontal boundary will ultimately set-up, which will play a vital role in determining where the heaviest rainfall occurs.

Some model runs and ensemble suites keep the main focus of rainfall offshore and/or east of our area while other guidance brings widespread soaking rainfall to the entire region. Latest ensemble probabilities for >=1" are low to moderate, with the highest probabilities across ME but the probabilities for >=2" are significantly lower. There are also large timing differences, with some guidance showing showers beginning late Thursday and rain persisting through Saturday while other guidance brings the onset of rainfall later into Friday. The main takeaway is that while a beneficial soaking is possible, given the trend over the last several weeks, guidance may trend lower on rainfall as we get closer in time.

Temperatures will remain above avg through late week before trending closer to avg this weekend into next week.

AVIATION /18Z MONDAY THROUGH SATURDAY/

Through 18Z Tuesday: VFR conditions prevail through tonight with showers possible around the higher terrain along the International Border. Ceilings begin lowering Tuesday but look to remain VFR through the day.

Outlook... Tuesday night: MVFR ceilings possible, especially in the mountains, as more widespread showers are expected.

Wednesday: VFR conditions expected.

Thursday-Saturday: MVFR ceilings and rain are looking more likely in this timeframe, but exact timing is still uncertain.

MARINE

Sub-SCA conditions prevail through at least Thursday as high pressure settles over the waters. SCA conditions are looking more likely Friday night through Saturday as swells from a system out to sea build in. Sub-SCA conditions then return for early next week.

GYX WATCHES/WARNINGS/ADVISORIES

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