textproduct: Gray - Portland

WHAT HAS CHANGED

No significant changes with this update.

KEY MESSAGES

1. Slick travel is expected as snow moves into the region late tonight and Saturday morning. 1 to 3 inches of snow is expected across interior Maine and into central/southern New Hampshire by Saturday evening. Plan for added travel time if transiting interior locations, especially to/from the mountains.

2. We are still monitoring the Sun/Mon timeframe for a potential coastal winter storm. Confidence continues to be lower than normal due to a large portion of NWP members being out to sea.

3. The middle to latter part of the work week temperature forecast remains below normal. Several nights of sub-zero ambient or wind chill temperatures are possible.

DISCUSSION

KEY MESSAGE 1 DESCRIPTION... Low pressure over the Great Lakes Friday night will slowly pivot east, but remain well west of New England. Southwest flow through the column will advect moisture into the region on a strengthening mid (100-110kt) and upper level (130-145kt) jet. Initial overunning jet dynamics should help precipitation break out over the forecast area, SW to NE, early Saturday morning. Once the column has been saturated via net lift, precip continues through the day Saturday.

Primary trough associated with this disturbance remains positively tilted across the Great Lakes, thus expect the associated moisture plume to be progressive across the forecast area. Guidance output for QPF paints a stripe of greatest amounts through interior ME and southwest/central NH. Lighter amounts should flank this corridor, i.e the mountains and immediate coast.

High confidence in precip type for this event given the absence of warm nose in the low levels. This should be a rain/snow event, with much of the area seeing flakes fall much of the day. Rain/snow line will be trickier as above freezing sfc temps line the coast and towards the interior zones. Thinking is constant precip rates will promote cooling in low levels to maintain snow falling. However, surface accumulations may be hampered along the coast due to the warmer ambient. After an initial flurry in the early morning hours, the Seacoast of NH through eastern Hillsborough county should remain mostly rain for the rest of the day.

For accumulations and resulting impacts, 1 to 3 inches is forecast for the aforementioned stripe of greatest QPF (Interior/Foothills and central to SW NH). Issued a Winter Weather Advisory for Cheshire Co, NH anticipating maximized QPF and elevation for ptype to be mostly snow, around 4 inches total. Amounts taper to a dusting to 2 inches on either side: lack of QPF for the mountains, and lower snow ratio for the coast. Main concern will be slick travel to and from the mountains, especially for trips that rise and fall in elevation in central NH.

KEY MESSAGE 2 DESCRIPTION... The NWP signal for a coastal storm Sun into Mon remains strong. At this time the consensus forecast is for it to stay largely offshore. Using both the ensemble consensus and NBM guidance you need to reach the 90th or greater percentile to bring advisory/warning criteria snowfall amounts to the forecast area. Similarly the ECMWF EFI does not have a signal for significant or extreme snowfall for this time of year as of the 16.00z run. This leaves something like a 20 percent or less chance of a moderate or greater snowfall event locally. The most likely scenario is a near miss, with up to a couple of inches along the coast. One thing to note is that both the AI versions of the GFS and ECMWF show more amplified systems, with higher QPF locally. It is not uncommon for ensemble guidance to trend more amplified in the medium range only to drift back in the near/short term. With ensemble systems that are underdispersive we have to be aware that we may not be capturing all the possible outcomes. What I can say right now is that cluster analysis indicates that the higher QPF scenarios feature a weaker shortwave that the ensemble mean. Given that this shortwave in question is already being sampled by the upper air network I would be somewhat surprised to see large changes going forward, but as I mentioned earlier there is a 1 in 10 to 1 in 5 chance of something worth paying close attention to.

KEY MESSAGE 3 DESCRIPTION... Strong ensemble support for below normal temps starting Mon night. While Mon night will be gusty as cold advection increases mechanical mixing, the remainder of the week looks to feature a series of high pressure system building towards the region. The timing of each surface ridge axis will determine the degree of radiational cooling potential, but overall I anticipate that several nights thru the middle to latter half of the week will see ambient air temps or wind chills falling below zero.

AVIATION /00Z SATURDAY THROUGH WEDNESDAY/

Through 00z Saturday...West winds continue to subside with VFR through at least 09Z. Ceilings thicken around midnight local, lowering into daybreak. MVFR cigs begin for NH terminals 9 to 12z, with SHSN or light SN reducing vis. Vis may continue to lower to IFR as snow moves across central/northern NH terminals and into ME by mid morning. SE NH terminals (MHT/PSM) may transition to RASN or RA for better vis. Can't rule out some RA also mixing in at PWM/RKD and other terminals along immediate coast.

Outlook:

Saturday Night: Gradual improvement from IFR to MVFR overnight as precipitation wraps up and ceilings begin to lift.

Sunday: VFR north to MVFR near the coast.

Sunday Night: VFR north to MVFR near the coast. Potential IFR if storm tracks farther west. Low confidence.

Monday: VFR north to MVFR near the coast. Potential IFR if storm tracks farther west. Low confidence.

Monday Night: VFR prevails at all terminals. West wind gusts 25 kt possible.

Tuesday: VFR prevails at all terminals.

MARINE

SCA conditions continue due to elevated wave heights. Low pressure lifts north of the Great Lakes through Saturday, bringing a southerly wind shift and precipitation to the coastal waters. SCA conditions likely continue through Saturday night with wave heights on the coastal waters 4 to 6 ft.

Regardless of storm track northerly winds are expected to increase thru Sun. There will be the threat for snow over the coastal waters and visibility may drop below 1 mile at times Sun night into Mon. Winds will gradually shift around to westerly as storm pulls away and cold advection begins. By late Mon and into Mon night gale force wind gusts are possible. SCA conditions will likely linger through midweek.

GYX WATCHES/WARNINGS/ADVISORIES

ME...None. NH...Winter Weather Advisory from 3 AM to 5 PM EST Saturday for NHZ011. MARINE...Small Craft Advisory until 7 PM EST Saturday for ANZ150-152- 154.