textproduct: Gray - Portland

WHAT HAS CHANGED

A coating to an inch of snow is possible in time for the Tuesday morning commute resulting in minor travel impacts.

KEY MESSAGES

1. A coating to an inch of snow is possible in time for the Tuesday morning commute resulting in minor travel impacts. Temperatures rise above freezing late Tuesday morning with highs in the upper 30s to low 40s during the afternoon.

2. There are two distinct systems mid and late week to watch for the next round of widespread precipitation. The first could bring accumulating snow to at least the southern half of the forecast area, with uncertainty on northern extent.

DISCUSSION

KEY MESSAGE 1 DESCRIPTION... High pressure keeps the forecast area mostly dry through today. A short wave and attendant surface low will track north of the area tonight through Tuesday with a warm front crossing the area around the time of the Tuesday morning commute. The latest NBM continues to keep PoPs below 15 percent across much of the area while latest available Hi Res guidance suggests that at least a coating to an inch of snow will be possible as the warm front impinges on the area. Have utilized hi res guidance into the latest forecast to highlight the potential for minor travel impacts Tuesday morning. As the warm front pushes through Tuesday morning model soundings show drying aloft while low level moisture remains. This may allow for snow to briefly change to freezing drizzle while timing of this drying aloft looks to coincide with surface temperatures rising above freezing. Therefore, have stuck with rain or snow wording, but will have to monitor trends today for the potential of any freezing drizzle.

KEY MESSAGE 2 DESCRIPTION... Prominent upper and mid level jet spans across the CONUS mid week which will hasten transport of disturbances eastward. The first disturbance is and will continue to have a higher degree of uncertainty for northern New England due to its origin and how its manipulated eastward.

Parent low will be in an eddy across the Northern Plains Tuesday night with the main jet to the south. This pocket of relatively slower flow will tend to shear the system into two as the jet is quick to accelerate the southern half. This piece has been modeled to track across the Great Lakes and towards New England Wednesday and Wednesday night.

Intensification of this part is likely as it quickly traverses east. It will have decent jet dynamics to take advantage of synoptic lift, resulting in a sizable swath of precipitation. Moderate IVT across the TN Valley should also aid in the magnitude of precip, with 24 hr NBM mean of around 0.75" into the Northeast. With the storm being progressive, locations will need to maximize duration under the band of precip to see bulk of accumulation, and positioning of this remains uncertain.

As this system is pushing east, northern New England will still be in cool and dry NW flow. This can quickly whittle away weakening moisture transport into the region, limited the northward extent of the precip shield. GFS trends over the past 24 hours have shown this in combination with a further south trajectory keeping the CWA mostly dry in the Wed/Wed night period. ECMWF and Canadian guidance has held on to a more northerly track with QPF values around 0.10" to 0.40". In cluster analysis, snowier solutions CWA-wide result from greater ridging across southern Canada that displaces and weakens prevailing NW flow into NNE. There is a minority of cluster weighting that suggests stronger troughing to supply dry air to the region. This proves a wide spread of over half an inch liquid equivalent for most of the southern CWA per the NBM.

The going forecast calls for whats most probable at the time: a greater portion of precip overspreading the southern forecast area Wednesday afternoon vs. the north. Temps through the column should be cool enough for snow, but sfc temps may be well in the mid to upper 30s for portions of the area. This could lead to a round of rain/snow mix before shifting to snow overnight.

Late week, additional low pressure pushes into the Ohio Valley and towards the Northeast. How this low interacts with another rapidly moving off the Mid-Atlantic will determine longevity of the next round of widespread precip as well as intensity. For now there are a lot of possible outcomes resulting in low confidence, but consensus is wintry precip could bring slick travel to the region.

AVIATION /12Z MONDAY THROUGH FRIDAY/

Through 12Z Tuesday...VFR prevails through 00Z Tuesday, but ceilings will trend to BKN/OVC later today as a cold front approaches. Cigs thicken and lower with MVFR likely by around 06Z. Light snow between 06Z and 15Z may briefly bring about localized IFR conditions. Temperatures rise above freezing south of the mountains leading to any snow showers briefly changing to rain showers. Ceilings will improve by midday on Tuesday.

Outlook...

Tuesday night: Conditions improve Tuesday night while MVFR cigs and -SHSN continue to impact KHIE.

Wednesday and Wed Night: MVFR/IFR possible into the latter part of Wed as SN moves in from the west. There remains uncertainty how far north SN makes it in the forecast area, with IFR vis initially possible across southern NH terminals.

Thursday and Thurs night: SN decreases in coverage with potential trend to VFR into Thursday night.

Friday: Active pattern continues with chances for snow while confidence at this time range is low.

MARINE

Winds generally stay below 25 kts through much of the week as week systems cross the Northeast. Seas build to around 5 feet Tuesday night into Thursday morning. Winds will increase again by Friday night as another weak storm passes to the south bringing increased northeast winds and possible SCA conditions.

GYX WATCHES/WARNINGS/ADVISORIES

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