textproduct: Gray - Portland

WHAT HAS CHANGED

No major changes for this forecast package.

KEY MESSAGES

1. Well below normal temperatures continue through the work week with periodic low chances of snow showers, mainly across the mountains.

2. Model trends are being closely monitored as confidence is increasing in low pressure taking a track up the East Coast this weekend. Impacts will depend on the proximity of this system to the coast which is highly uncertain at this time.

3. There is a signal for a return of more seasonable temperatures early next week.

DISCUSSION

KEY MESSAGE 1 DESCRIPTION... A progressive upper level ridge axis will continue to propagate across the region today from west to east with a shortwave on it's heels. As we transition back into southwest flow aloft, we will see some isolated to scattered snow showers later this afternoon and into the evening, mainly across the mountains as the weak wave moves across. Most will see little to no new accumulation with this activity, but a quick tenth of an inch or two will be possible under the heavier showers. We will see a couple more weak shortwaves traverse the forecast area through the end of the work week, but most of the light snow related to these waves will mainly be confined to the mountains and foothills.

The week will also remain cold as we continue to dig out from the previous storm. NBM highs today through Thursday will generally range from the upper single digits and lower teens north, to the upper teens and lower 20s south. Friday then may be the coldest day of the week with highs forecast to range from the low to mid single digits above zero north, to the mid/upper teens south. Each night we should also see widespread lows around or below zero.

KEY MESSAGE 2 DESCRIPTION... It is hard to ignore the sharp trend eastward in the overnight runs of the Global models (and the 12Z GFS/GEFS), but some flip flopping at this time range is not uncommon. What seems the most certain is that low pressure is going to be taking a track up the East Coast, but the proximity to the coast is one of the most uncertain details at this time while also being the largest determinate of potential impacts. If models continue to trend toward a more amplified ridge out west, the trough is going to end up sweeping the system further out to sea, but a less amplified ridge would allow it closer to the coast. It seems fair to say that this low is going to be stronger than the one we just dealt with and as a result I would say that coastal areas have the best chance of seeing something. A deeper low means a more expansive precipitation shield so even a more out to sea track could bring accumulating snow to the coast. The other aspect is that this system will be passing during a time of high astronomical tides, so its proximity to the coast will make the difference between just some splash-over and minor coastal flooding. Interior locations should still keep a close eye on the forecast, but at this time it appears impactful weather is less likely. We expect changes and for more details to be able to be fleshed out in the coming days.

KEY MESSAGE 3 DESCRIPTION... Warm air advection aloft behind a departing trough combined with high pressure building in at the surface looks to put temperatures on an upward trend, back to near normal, heading into next week. Unfortunately, this is the climatological coldest part of the winter for us so the reality of this is that we are just going from very cold to cold. 850 mb temperatures modeled to rise to around -10C should be good for temperatures rising into the upper 20s to near the freezing mark south of the mountains (with low 20s to the north) depending on how quickly high pressure builds in and clears skies.

AVIATION /18Z TUESDAY THROUGH SUNDAY/

While VFR conditions are generally expected through Wednesday, isolated to scattered snow showers could lead to some local IFR/MVFR or lower visibilities this afternoon and into the early evening. The threat of snow showers ends later in the evening with mainly VFR prevailing tonight, except at KHIE where upslope flow may allow for prevailing MVFR.

Outlook:

Thursday-Saturday: VFR should prevail at all terminals with no significant weather expected late this week.

Sunday: Low confidence currently in what Sunday's conditions are going to look like as they rely heavily on the track of a coastal storm. At least widespread MVFR is likely, with widespread IFR possible in snow.

MARINE

While off-shore winds diminish through the day, SCA conditions will be in place through the first part of tonight with elevated seas. As the core of the cold air aloft moves overhead this afternoon, snow showers will become more numerous over the waters. Local visibility below 1 mile is possible. Seas then start to diminish overnight and through the day on Wednesday.

A period of wind gusts 25-30kts is likely outside of the bays Thursday night into early Friday morning as a pressure gradient tightens over the region. These ease during the day Friday, with a brief window of tranquil weather through the day Saturday as high pressure crosses the waters. Saturday night a strong coastal low pressure system approaches the region quickly ramping winds up to SCA and potentially gale force by Sunday morning. Depending on the strength of this system storm force winds would not be out of the question for Sunday. Seas rise to 8-10 ft as this system passes as well. Conditions would then ease through the day Monday potentially falling below SCA criteria by Tuesday morning.

GYX WATCHES/WARNINGS/ADVISORIES

ME...None. NH...None. MARINE...Small Craft Advisory until 10 PM EST this evening for ANZ150>152-154.