textproduct: Gray - Portland

SYNOPSIS

A cold front will continue to push towards the area this evening with rain and snow showers ahead of it. We will clear out only briefly before the next storm begins to move in early Tuesday. This storm will spread snow over the area and bring the first widespread accumulation to most areas south of the mountains. Moderate to heavy snow is likely to affect the morning and evening commutes on Tuesday before snow comes to an end. The storm track may still change and shift the heaviest snow amounts around, but most locations are expected to see some accumulations. Once that storm exits the region Wednesday, the rest of the week will be cool and breezy with mountain snow showers lingering.

NEAR TERM /THROUGH TONIGHT/

Key Messages: * On and off snow will become more consistent late this afternoon for the mountains. Road conditions will fluctuate with elevation across the north as snow will tend to accumulate better on the hills and mountains.

Low pressure exiting the Great Lakes will translate northeast this afternoon, bringing an occluded front along the US/CAN border. Off of this, a brief warm sector and followup cold front will pass overhead this evening.

Expect precipitation to be most continuous mid to late afternoon as the best forcing pushes through. It has taken awhile for the atmospheric column to saturate so far today, thus keeping precip at the surface intermittent. While snow will change to rain across the interior, precip likely remains snow in the mountains and northern valleys through the event. No changes in expected accumulation through tonight here, with greater accums tied to elevation. Dry air will then filter in aloft west to east later this evening. This will end precipitation as winds shift west. NW winds in the mountains will keep mention of snow via upslope processes.

SHORT TERM /MONDAY THROUGH MONDAY NIGHT/

Breezy and dry day Monday as high pressure pushes into the Northeast and temperatures generally fall through the day. Daytime highs push into the 30s to around 40 along the coast in the morning. These then look to tumble through the afternoon as CAA rushes in on NW winds. These will make it feel cooler...like the 20s along the coast and interior, with teens and single digits in the mountains.

Winds will tend to become light in the evening, but cold air mass will just be getting overhead. Overnight lows in the single digits across the mountains with teens through the interior are expected.

LONG TERM /TUESDAY THROUGH SUNDAY/

Key Message: First significant winter storm of the season is likely Tue into early Wed. Some additional track changes are expected...but many parts of the forecast area are under threat of at least 4 to 8 inches of snow.

Impacts:

*Hazardous travel conditions are anticipated both the Tue morning and evening commutes due to falling and accumulating snow. The first inch is looking more likely by Tue morning...which when many snow related vehicle accidents occur.

*Snowfall may be heavy at times with visibility at or below one quarter mile. In addition snowfall rates of 1 inch per hour are possible.

Forecast Details: A strong and rapidly deepening cyclone will pass south of New England and skirt the edges of the Gulf of Maine on Tue. This storm will bring the forecast area its first widespread accumulating snowfall of the season.

There has not been much change in modeling since yesterday...with perhaps a slight narrowing of forecast envelope. That still leaves plenty of uncertainty in track given the thermodynamic set up.

At a high level overview...models have a strong jet streak lifting over the Northeast...placing much of the area in the right entrance region favored for lift. The global suite places that jet streak over Maine...while the more aggressive NAM is over the St. Lawrence Valley. These differences carry down thru the mid levels where the NAM has a closing H7 low over the forecast area and the GFS/ECMWF remain open waves farther south. Likewise the -4 C isotherm...a proxy for heaviest snow location...stays near the coast for a GFS/ECMWF solution but bisects the forecast area on the NAM.

One consistent feature is that model guidance still favors a laterally quasi-stationary mesoscale band set up. This is the primary reason why confidence remains lower than normal. While it will likely snow a lot underneath that band...snowfall amounts will drop off quickly to the northwest...and also southeast where marine influence will be higher. A shift of 25 miles or more at this forecast range is not unreasonable. This also means that a NAM-like scenario remains a possibility for locations under the mesoscale band. The 30.12z NAM has over 2 inch QPF in a stripe thru the center of the forecast area. I think this is overblown...but the training nature of the banding and what could be intense lift is certainly enough to make a stripe of a foot or more a possibility. However to keep snow amounts within reason...I opted for a 50/50 blend between the mean and 50th percentile QPF.

Based on ensemble guidance and associated forcing features...I am favoring southern NH and the western Maine coast for the highest totals. A winter storm watch has been issued for the entire area south of the mtns. This is a bit broad due to the uncertainty but also because it will be the first real accumulation for nearly all of those areas and occuring during busier travel times.

The snow will wind down quickly early Wed morning...with northwest winds taking over and upslope snow showers developing. That will generally continue thru the remainder of the work week...with another stronger trof arriving over the weekend into early next week. The rest of the forecast beyond Tue/Tue night is NBM due to the focus on the storm.

AVIATION /18Z SUNDAY THROUGH FRIDAY/

Short Term...Ceilings trend widespread MVFR this afternoon. Brief period of IFR slides west to east across terminals into this evening, but may arrive as SCT...becoming BKN at times. Mountain downslope helps keep HIE MVFR/VFR unless vis in snow hinders this evening. RA will be dominant precip type south and east of LEB/IZG/WVL this evening, with SN to the north. Expect general improving trend after 00z, with VFR at most terminals outside of the mtns by 9z. HIE remains in upslope snow into early Monday morning.

Long Term...Conditions rapidly deteriorate Tue morning as snow moves into the area. Widespread IFR to LIFR is expected for most of the TAF sites. Storm track remains lower confidence at this time and could dictate which TAF sites see IFR or lower conditions. Farther north and HIE would experience more IFR...but rain may allow some MVFR conditions to creep into coastal terminals like PSM...PWM...and RKD. Conversely a farther south track may keep HIE MVFR or VFR while bringing a longer period of IFR or lower to coastal sites. Once the storm pulls away and winds turn northwesterly VFR returns quickly to places south of the mtns...while HIE will likely see a continuation of upslope clouds and snow showers with local MVFR.

MARINE

Short Term...No significant changes to forecast with today/tonight's system. Window of gales will be shortlived as front passes over this evening. These slacken towards midnight, but will tend to have some gusts to Gale on the outer zones through tonight and into Mon as NW winds increase behind the system.

Long Term...Strong and deepening low pressure will pass near the entrance to the Gulf of Maine on Tue into Wed morning. Strengthening northeast winds and building seas are anticipated. Depending on the storm track gale force wind gusts are possible for the outer waters...especially Tue night into Wed. Winds will remain gusty on the waters thru the end of the work week.

GYX WATCHES/WARNINGS/ADVISORIES

ME...Winter Storm Watch from Tuesday morning through late Tuesday night for MEZ012>014-018>028-033. NH...Winter Storm Watch from Tuesday morning through late Tuesday night for NHZ004>015. MARINE...Gale Warning from 4 PM this afternoon to 10 AM EST Monday for ANZ150-152. Gale Warning from 4 PM this afternoon to 10 PM EST this evening for ANZ151. Small Craft Advisory until 7 PM EST Monday for ANZ153-154.