textproduct: Gray - Portland

WHAT HAS CHANGED

Forecast remains on track with just some minor adjustments made to QPF/snowfall, with forecast amounts lowered some in most locations.

Precipitation onset continues to slow down a bit for the rain storm early next week.

KEY MESSAGES

1. Snow likely tonight into Saturday with light to moderate accumulations. After the low exits the region, westerly winds pick up Saturday afternoon.

2. A stronger low pressure system appears likely to bring some locally heavy rain and gusty winds to most of the forecast area early next week. Some flooding is possible as ice jams loosen and the remaining snowpack melts.

DISCUSSION

KEY MESSAGE 1 DESCRIPTION... A strong clipper system moves across the northeastern US tonight, with the snow moving through the region in two big pushes. The first wave of snow moves across the area overnight tonight. Low-level frontogenesis within the wave of snow may allow for periods of heavy snow to develop across the interior. Snow will be heavier and wet further to the south and a little drier in the north. The aforementioned wave of snow moves through quickly, with skies trying to clear out by Saturday morning. A brief dry period follows and then another hit of snow moves across the mountains Saturday afternoon. Snow covered roads are likely Saturday morning and mountain passes like Franconia Notch and the Kancamangus Pass could be slick.

For snow totals, 3-6 inches are forecast across the interior, with lesser amounts along the coast and southern New Hampshire. Upslope enhancement will allow for higher snow totals in the higher elevations in the White Mountains and the Rangeley Lakes region.

Right behind the aforementioned system, a pressure gradient will pick up and will allow for winds to strengthen. Westerly winds could be gusty across southern and western New Hampshire, as the best mixing appears to be across these regions. High pressure moving overhead Saturday evening will allow for winds to weaken overnight.

KEY MESSAGE 2 DESCRIPTION... Modest upper-level ridging will cross the forecast area on Sunday, but we will transition back into southwest flow aloft by Sunday night as the next longwave trough starts to approach. Highs on Sunday will mainly be in the low to mid 30s north, to the low to mid 40s south.

Strong low-level warm air advection and a long southeasterly fetch will pump plenty of moisture into the region ahead of a deepening low as a warm front lifts north. Compared with yesterday's guidance, the system has slowed a little bit and the widespread precipitation is now expected to begin Monday morning. The one exception could be some light snow along the international border Sunday night. Elsewhere, precipitation could begin as a wintry mix before changing over to all rain through Monday night. As of now, it appears that the heaviest rain will fall Monday evening and night. The NAEFS and ECMWF ensemble continue to suggest that PWATs, IVT, and low-level specific humidity will all be near the climatological max for this time of year (95th percentile or higher), so the threat for locally heavy rain remains a concern. The progressive nature of this system should keep the totals mitigated somewhat even with the anamolous moisture in the forecast but we will likely see general totals in the one to two inch range with some locally higher amounts. Rain should quickly diminish Tuesday morning, likely vacating the forecast area completely by early to mid afternoon, although there could be some lingering light snow showers in and around the mountains. For more on the flooding and hydrology threat, please see the hydrology section below.

Strong southeasterly winds also remain a concern for the early week system with an anamolous low-level jet expected to ramp up Monday afternoon through Tuesday morning. Forecast wind speeds at the 700mb and 850mb levels continue to lean towards the 99th percentile or greater over much of the area in the NAEFS and ECMWF ensemble. The question will be how much of this air can mix down to the surface, especially after it takes the long journey over the cold ocean waters. Widespread gusts in the 40 to 45 mph range seem reasonable but a few readings up to 50 mph aren't out of the question. Thus, there could be at least be some power outage threat.

The cold front moves through on Tuesday morning, which will lead to some cooler high temperatures on Wednesday with values struggling to get out of the 20s north or the 30s south. After Wednesday, highs should then rebound towards seasonal averages for the rest of the work week. A weak wave towards the end of the week could bring the next chance of light precipitation to the area starting Thursday afternoon.

AVIATION /00Z SATURDAY THROUGH WEDNESDAY/

Through 00Z Sunday: Conditions will deteriorate overnight to IFR as SN overspreads the TAF sites. Ceilings and visibility will then begin to improve between 09Z-13Z Saturday with VFR conditions returning. The exception may be at KHIE where clouds and periods of -RASN will likely persist at times. A few widely scattered rain/snow showers are possible during the afternoon elsewhere, potentially bringing brief restrictions. W winds will prevail with gusts up to 25 kts.

Outlook:

Saturday night: VFR likely returns to all terminals, except HIE which may hold MVFR ceilings in upslope flow.

Sunday: VFR, no sig wx. Clouds thicken and lower in afternoon.

Sunday night and Monday: MVFR to IFR cigs with any wintry mix transitioning to rain Monday morning. Rain and gusty southeast winds likely Monday.

Monday night: IFR likely continues with periods of rain and breezy conditions.

Tuesday: Conditions gradually improve to MVFR/VFR through the day.

Tuesday night and Wednesday: VFR expected.

MARINE

Southeasterly winds will pick up to SCA levels overnight. SCA winds continue on Saturday but will shift to westerlies at daybreak. Westerly winds continue through the remainder of Saturday evening, with a few gale force gusts possible over the open waters.

High pressure crosses the waters Saturday night and Sunday. Southeasterly gales are then likely by late Sunday night and Monday as a strong low pressure system tracks through the Great Lakes. Winds and seas will remain elevated through at least midweek.

HYDROLOGY

River ice across the region will need to be monitored over the coming week. Ice weakened by the recent thaw could still exhibit some breaking and moving over the weekend despite temperatures generally below freezing. However the main focus will be early next week when an anomalously moist system bring periods of moderate to heavy rainfall to the region. PWATs are looking around 1.25", putting it in the 95th percentile or possibly a new max. The event total rainfall is likely be around 1-2", but can't rule out locally higher amounts to 3 inches if the system drags out any longer. Fortunately the window for snowmelt in the mountains is expected to be brief, but any lingering snow in the valleys and points southward will likely melt-out. The overall snowmelt contribution will be ~0.5- 1". Given frozen ground, much of this will be pure runoff around 1 to 3 inches. Despite recent snowmelt, much of the overland storage remains available due to the drought. The total runoff reaching the rivers will be sufficient to raise many streams and river to near/at bankfull. Minor flooding is possible if rainfall exceeds apprx 2 inches. Remaining river ice will be the wildcard for significant flooding. The river rises are expected to be fast late Monday into Monday night, which will increase the chance for ice jam flooding. Ice jams are well known spring flood triggers, and are generally unpredictable on when or where they will develop. Reports from officials indicate a lot of ice rotted or at least weakened significantly following the recent thaw, reducing the ice jam risk for many rivers. However, it only takes ice 4" thick to support a strong jam. Interests along vulnerable river locations should be on guard and use this time to prepare and monitor conditions. It is expected that most of the areas river ice will blow out with this event.

GYX WATCHES/WARNINGS/ADVISORIES

ME...Winter Weather Advisory until 2 PM EDT Saturday for MEZ007>009- 012>014-033. Winter Weather Advisory until 11 AM EDT Saturday for MEZ020>022-026>028. NH...Winter Weather Advisory until 2 PM EDT Saturday for NHZ001>004. Winter Weather Advisory until 11 AM EDT Saturday for NHZ005- 006. MARINE...Small Craft Advisory until 2 AM EDT Sunday for ANZ150-151-153. Small Craft Advisory until 8 PM EDT Saturday for ANZ152-154. Gale Watch from Saturday evening through late Saturday night for ANZ152-154.