textproduct: Gray - Portland

WHAT HAS CHANGED

Update to the Aviation section for the 12Z TAFs. No other changes were needed.

KEY MESSAGES

1. A coastal storm brings rain and snow to portions of the forecast area today into tomorrow. Winter conditions remain possible tonight, with the highest confidence of accumulating snow at elevations over 1000 feet.

DISCUSSION

KEY MESSAGE 1 DESCRIPTION... The current regional MRMS mosaic shows the system taking shape to our southwest. It will continue to move eastward overnight and the initial warm air advection push as it nears may bring some light rain to southern New Hampshire. Otherwise, the main system pushes into southern New Hampshire after sunrise and will initially be all rain (except for maybe in the highest elevations of the Monadnocks and White Mountains). Rain reaches into the southern Maine coast in the afternoon as the initial low pushes off the coast and out to sea. Again, high temperatures climb into the 40s area wide, so snow will only be present at high elevations through today.

The story changes a bit as we see the secondary low develop and follow in the wake of the first one. Temperatures are expected to bottom out near or below freezing by Friday morning which means tonight is the best chance for snow flakes to make it the surface. However, accumulation is still a big question and there are a few factors that are going to affect it. The first is marginal temperatures; while everyone has a chance to see flakes tonight they may not last long after hitting the "warm" ground, or accumulations may only occur on elevated or grassy surfaces. The second factor is dry air being pulled in from the north as both lows move east of the area. This is going to dictate the northern extent of the precipitation shield and the hi-res models are beginning to start resolving that. I certainly think the 12.00Z HRRR is overdoing it, but I could see a situation where northern New Hampshire and the Western Maine Mountains see little or nothing. I have moderate confidence in precipitation at least making it up into the foothills. Here is the breakdown:

QPF and Snow - Guidance continues to struggle with the exact track and speed of the low, and a closer pass and longer residence time would increase totals. However, the consensus seems to be on the further east track with the low being progressive; the result is light QPF and thusly snow totals. The REFS mean shows about a half inch to an inch of QPF in southern New Hampshire and far southern Maine, with only a quarter to a tenth up into the New Hampshire and Maine foothills. Most of this is going to fall as rain before the change over tonight, which is why I think outside of the mountains a dusting to a half inch of snow accumulations are all this system is going to be able to muster, especially after factoring in surface temperatures. The caveat here is that the WPC snowband probabilities are non-zero in southern New Hampshire and should a band develop, elevated rates would lead to better accumulation, but with the marginal surface temperatures my gut says even this scenario likely only amounts to an inch or two. Elevations over 1000 feet stand the best chance of seeing several inches.

Winds and Coastal Flooding - Both a non-issue in this case. The further east track only brings 20-25 mph winds to the coastal plain with lighter winds inland. Tides are not particularly high either right now, so even with a foot or two of surge we remain comfortably below flood thresholds.

Early morning commuters will want to be wary of slick spots Friday, but overall the system will be exiting and precipitation tapers off through the morning. The upper low hangs around through the weekend so showers continue as well, mainly in the mountains, but nothing of significance. High pressure than builds in for at least the first half of next week bringing drier weather and trending temperatures back to seasonable normals.

AVIATION /12Z THURSDAY THROUGH MONDAY/

Through 12Z Friday: VFR continues ths morning with ceilings lowering to MVFR for southern terminals as early as this morning but especially this afternoon and evening with rain overspreading the area. MVFR, and possibly IFR, conditions prevail through tonight as rain changes over to or mixes with snow across the interior.

Outlook...

Friday: MVFR/IFR to start, but precipitation tapers off through the morning. Ceilings may remain MVFR. Moderate confidence.

Saturday-Sunday: Improving to VFR at most terminals. HIE may stay MVFR or near MVFR. Moderate confidence.

Monday: VFR conditions expected. Moderate confidence.

MARINE

Northeasterly gales are possible tonight as low pressure passes just east of the Gulf of Maine. 25 to 30+ knot winds likely continue through Friday night along with 5-7 foot seas. Sub-SCA conditions are then anticipated through the weekend and into at least the middle of next week as high pressure builds into the region.

GYX WATCHES/WARNINGS/ADVISORIES

ME...None. NH...None. MARINE...Gale Watch from this evening through Friday afternoon for ANZ150-152-154-180-182-184.