textproduct: Gray - Portland

WHAT HAS CHANGED

Snowfall amounts have increased slightly Friday night through Saturday, particularly across southern NH and coastal SW Maine where amounts of 2 to 3 inches are forecast.

Cheshire County has been added to the Extreme Cold Watch for Saturday night. Confidence is increasing that additional cold weather headlines will likely be needed Sunday night as well.

KEY MESSAGES

1. Some slick travel is likely across New Hampshire and southwest Maine Saturday as accumulating snow starts late Friday night and continues through Saturday afternoon. Snowfall amounts are generally 1 to 3 inches with lesser amounts across central Maine.

2. A frigid airmass will push across the Northeast this weekend, resulting in dangerous wind chill values Saturday night and Sunday night. Widespread temperatures below zero overnight are expected, with gusty winds making it feel like the teens below zero from the coast to the foothills, and 20 to 30 below zero for the western third of the forecast area.

DISCUSSION

KEY MESSAGE 1 DESCRIPTION...

Once again this winter we will be looking at accumulating snowfall from an inverted trough late Friday night through the day Saturday. Short wave energy exiting the Mid Atlantic coast will lead to low pressure developing well southeast of New England while some trailing short wave energy and cold front approach from the northwest. Flow over the Gulf of Maine will increase out of the ENE Friday night into Saturday while more northerly flow prevails over land. These converging winds will provide a source of low level lift as mid level support slides over western New England. Available 12Z guidance is in decent agreement that light snow will overspread New Hampshire Friday night and into far western Maine by Saturday morning. CAMs have been highlighting some enhanced snowfall rates offshore or near the NH Seacoast and coastal SW Maine that are common with inverted troughs. Model soundings here do show strong lift, decent moisture, and some instability withing the dendritic growth zone that would support a few inches of snowfall to fall in short period of time. The challenge at this time range is pinning down where/if these enhanced snowfall rates will occur.

The latest NBM brings 1 to 3 inches of snow across much of New Hampshire and SW Maine with little to no snowfall in Franklin/Kennebec Counties and points eastward. This seems reasonably in line with the latest round of 12Z models. Points of uncertainty at this time is the eastward extent of accumulating snowfall across Maine and whether localized higher amounts will occur along the NH Seacoast and coastal SW Maine. Accumulating snowfall is forecast to end Saturday evening.

KEY MESSAGE 2 DESCRIPTION...

Mid-level trough will begin digging south across the eastern CONUS Friday and continue to sharpen Saturday. This will initially move a very cold airmass into northern New England for the second half of the weekend. To accentuate the cold, a tightening pressure gradient from offshore low pres will increase winds over the forecast area overnight into Sunday. Additional embedded shortwaves will keep winds gusty into Monday morning.

The combination of the cold air mass and added winds will create dangerous wind chill values. Two consecutive nights of wind chill values well below zero are forecast; Saturday night and Sunday night. While exact values will continue to be refined, still believe coldest readings will be across the western ME mountains, northern New Hampshire, the Whites, and down through western NH and the Monadnocks. Here values could dip into the 20s to around 30 below zero at night. Have moved to extend the Extreme Cold Watch down through Cheshire Co, NH on Saturday night. Additional headlines will likely be needed Sunday night as well.

With the widespread gusts expected, even temperatures in the single digits above and below zero from the coast to the foothills will present wind chills in the teens to around 20 below zero. During daytime, mixing is expected to bring the stronger gusts and more persistent winds. Thus Sunday and Monday mornings will still hold onto this chill, only rising to near 0 by or after noontime.

AVIATION /18Z THURSDAY THROUGH TUESDAY/

Through 18Z Friday...Mainly VFR prevails through the day Friday. Clouds thicken and lower Friday afternoon while ceilings generally stay at VFR.

Outlook:

Friday Night: Ceilings lower to MVFR/IFR through Friday night with light snow overspreading the area.

Saturday: Areas of IFR or lower are possible in snow especially across NH and coastal SW Maine. Snow comes to an end Saturday evening. NW winds gust to 25 kts Saturday afternoon.

Saturday Night thru Sunday Night: Improvement to VFR Sat evening, with HIE still seeing MVFR cigs. NW gusts to 25 kts. No sig wx.

Monday: NW winds slacken, gusts to 20 kts. VFR. No sig wx.

Tuesday: VFR. No sig wx.

MARINE

Winds and seas remain below SCA thresholds through Saturday morning with winds generally out of the northeast. Light freezing spray is possible tonight. Northeast to north winds approach 25 kts Saturday evening.

Gale conditions begin Saturday night as northwesterlies increase due to tightening pressure gradient overhead and well offshore low pressure. A break in Gale strength wind is possible Sunday, before returning Sunday night. Should be noted that there will also be a lengthy period of moderate freezing spray Saturday night through Monday morning. Accretion rates may approach 2 cm/hr or 0.8 in/hr at times as a very cold airmass arrives.

GYX WATCHES/WARNINGS/ADVISORIES

ME...Extreme Cold Watch from Saturday evening through Sunday afternoon for MEZ007>009. NH...Extreme Cold Watch from Saturday evening through Sunday afternoon for NHZ001>003-005-007-011. MARINE...Gale Watch from Saturday afternoon through Sunday afternoon for ANZ150>154.