textproduct: Gray - Portland

WHAT HAS CHANGED

QPF amounts have increased slightly across the area. This may lead to slight river rises, but there remains no flooding concerns. Ice amounts have also increased slightly in amount and extent. A Winter Weather advisory has been issued for Northern Franklin and Central Somerset Counties where there may be travel impacts.

KEY MESSAGES

1. Low pressure rides a frontal boundary into the area Tuesday bringing mostly rain to the area through the day Wednesday. The Western Maine mountains have the best chance of seeing light snow and wintry mix beginning overnight tonight through Tuesday night that may lead to locally slippery travel.

2. Prolonged period of precipitation chances expected through the weekend.

DISCUSSION

KEY MESSAGE 1 DESCRIPTION... A cold front begins to sink into the forecast area tonight and it may be accompanied by some light rain showers as it crosses over the mountains. Models suggest it makes it just south of the mountains before stalling. Low pressure rides this boundary into the northeast Tuesday with the main area of precipitation beginning to move in early Tuesday morning. This low track will put most of the forecast area into the warm sector with cold rain being the dominant precipitation. This is one of those situations where you don't want the high temperatures to fool you as they most likely occur just after midnight. Temperatures will fall through the night and day with upper 30s and low 40s being what we see in the afternoon. North of the front precipitation type is going to be trickier. Early tuesday morning should be straight forward, cold air moves in behind the front and light snow is the dominant precipitation type. This is quick hitting and likely results in light accumulations ranging from a coating to around an inch, just enough to realize some slippery travel for the morning commute. Cold air damming sets up along the front which may allow for some sleet to mix in the foothills briefly, but should not be overly impactful. As warmer air continues to push in the majority of the area will be plain rain by Tuesday afternoon, however it doesn't look like it is going to make it to the surface over the western Maine Mountains resulting in this area transitioning from snow to a wintry mix for the afternoon. For most of the northern area, this mostly occurs at the higher peaks which will limit travel impacts. I could see some light icing leading to some travel impacts in Central Somerset and Franklin counties however, as this area transitions through a wintry mix into light freezing rain Tuesday night with the cold air layer becoming shallower. Have gone ahead and issued a Winter Weather Advisory for this period as a result. Icing amounts look to end up in the ballpark of a tenth to two tenths, enough to warrant the Advisory for slick travel, but not enough to be impactful beyond that. When all is said and done QPF amounts have come up slightly, but CAMs suggest a more showery/drizzly/foggy afternoon Tuesday rather than steady rain through the day. Similarly, the second bout of steady precipitation Tuesday night transitions to showers early Wednesday before it comes to an end from north to south late Wednesday morning. QPF totals look like they will end up 0.50 to 0.75" south of the mountains, with amounts 0.75 to an inch" in the north. Higher elevations could see locally higher amounts, but it should be isolated there. This runoff could lead to some slight rises in the more sensitive rivers, but no flooding is anticipated.

KEY MESSAGE 2 DESCRIPTION...

Once precip begins Tue it will be difficult to find a clear break, though maybe the best of them will be late Wed. Cold front looks to have pushed thru the forecast area but will then return north again as a warm front. Showers will ride along the boundary and drizzle and dreary conditions will remain possible south. The mid level warm front is forecast to lift north of the forecast area Thu and much of Fri, but I am less convinced that the surface front makes much progress. Ensemble guidance is forecasting a seasonably strong high pressure centered northeast of the region. That is a recipe for cold air damming and temps coming in under forecast. The NBM is already hinting at this, with a sharp boundary across southern portions of the forecast area. The main shortwave and strongest moisture convergence arrives Fri into Fri night. The focus of this will be along the mid level front, which is largely over and north of the forecast area. However to the south I still think there is a reasonable chance for fog, low clouds, and maybe drizzle to continue. Another cold front crosses the region Fri night into Sat. Once again that boundary stalls and returns north and yet another cold front crosses late in the weekend. Overall I think daytime surface temps remain stuck on the normal/cool side versus above normal temps. Overnight temps likely remain mild for this time of year and could push the daily temp anomalies into the above normal range. The best chance for some wintry precip will be across the higher terrain and northern zones. But the strength of the cold air dam will determine the southward extent.

AVIATION /18Z MONDAY THROUGH SATURDAY/

Through 18Z Tuesday...VFR prevails through the remainder of the day with ceilings thickening and lowering tonight with widespread MVFR likely after 06Z Tuesday. Precipitation begins to move in around 10Z Tuesday with the expectation that all terminals see rain. This will still likely bring about IFR visibilities with IFR ceilings following closer to 18Z Tuesday as drizzle and fog persist through the afternoon.

Outlook:

Tuesday night: Mainly IFR conditions continue as rain moves through the area again.

Wednesday: Conditions will begin to improve as rain comes to an end during the day.

Wednesday Night: Some improvement to VFR possible from north to south. MVFR CIGs may linger north of the mtns.

Thursday: IFR conditions possible again as warm front lifts north. Confidence: low.

Thursday Night: IFR conditions possible with low CIGs and maybe drizzle. Confidence: low.

Friday: Areas of IFR possible. Confidence: low.

Friday Night: Areas of IFR possible. Confidence: low.

MARINE

SCA conditions continue through tonight. It looks like we may get a period of sub SCA conditions during the day Tuesday, but wind gusts and wave heights will build again Tuesday night. Winds will begin to relax again during the day Wednesday, but wave heights will remain elevated. Winds flip around from southwesterly to northeasterly early Tuesday morning, before flipping back Tuesday night, and finally flipping again as a front crosses the waters on Wednesday.

A prolonged period of at least marginal SCA conditions is likely outside of the bays. Northeast winds Wed night turn gradually southerly by Fri. Sat may see those winds become more southwesterly before a backdoor front shift the direction back to easterly. Gusts near 25 kt and seas at or above 5 feet may be possible for the majority of that period. It will also be an extended window where areas of fog will be possible.

GYX WATCHES/WARNINGS/ADVISORIES

ME...Winter Weather Advisory from 6 PM Tuesday to 8 AM EDT Wednesday for MEZ008-009. NH...None. MARINE...Small Craft Advisory until 2 AM EDT Tuesday for ANZ150-152-154.