textproduct: Gray - Portland

WHAT HAS CHANGED

Minor update to reflect observed trends in temperatures and to update the Aviation discussion.

KEY MESSAGES

1. Active weather continues with light snow showers tonight, mainly in New Hampshire, and a more widespread, but low impact snowfall Monday night into the Tuesday morning commute. Accumulations are expected to be less than an inch, but enough to make for a period of slippery travel. This is followed by temperatures climbing above normal through midweek.

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... Vertically stacked low pressure continues northeastward into Atlantic Canada tonight with a trailing shortwave progged to move through our area after midnight. This will generate some snow showers, but they will be running into the high pressure that has built overhead today. The latest CAMs suggest this dries them out pretty quickly, but I think most locations in New Hampshire have a good shot at seeing some flakes with locations in Maine less likely. These will result in little to no accumulation, so even though you may see some light flakes if your on the road overnight or Monday morning, I would not anticipate travel impacts.

Snow showers look more widespread Monday night as low pressure moving through Quebec drags a frontal boundary through the northeast. Similarly, snow doesn't begin until late in the evening and with Monday being the start of our warm up (temperatures climbing into the mid- to upper 30s) it may not stick right away in most places. As a result accumulations are expected to be less than an inch, and much less in most places. 13Z NBM probabilities of measurable snow are less than 20% and are likely being skewed by the 12Z CAMs being drier and more showery in nature. I would like to continue messaging a coating to an inch, but with the understanding that many locations outside of the mountains are going to fall on the low end of the range. North of the mountains I do feel that a half inch to one inch is possible with the added upslope enhancement. The general idea is that a period of slippery travel is possible for the Tuesday morning commute, but with snow coming to an end shortly after and temperatures climbing into the upper 30s and low 40s impacts should be minimal.

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 /00Z MONDAY THROUGH FRIDAY/

Through 00Z Tuesday...VFR prevails through 00Z Tuesday, but ceilings will trend to BKN/OVC overnight and lower to around FL050 by day break Monday. There is a low chance of light snow showers at New Hampshire terminals overnight. Only included VCSH at LEB and HIE at this time. Ceilings scatter Monday afternoon Outlook...

Monday night: Cloud trend back to BKN/OVC Monday night with ceilings lowering to MVFR at most terminals. Light snow showers overnight may briefly bring about localized IFR conditions, with the greatest chance of this at LEB and HIE.

Tuesday-Tuesday night: Ceilings trend back to VFR for Tuesday, except that HIE may stay MVFR.

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.

MARINE

Sub-SCA conditions return this afternoon, with tranquil conditions lasting through the day Tuesday as high pressure moves over the waters. Strong low pressure passing east of the Gulf of Maine will build a swell into out waters Tuesday night with wave heights 3-5ft. Northerly winds today will shift easterly for Monday as a disturbance crosses the waters tonight. Winds then shift to southeasterly for Tuesday as low pressure passes out to sea, and finally to westerly by daybreak Wednesday.

A period of winds around 30 kts are possible Wed into Thurs as low pressure passes over or near southern New England. NE or onshore flow as the low approaches would result in building wave heights, particularly for the southern coastal waters.

GYX WATCHES/WARNINGS/ADVISORIES

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