textproduct: Caribou

WHAT HAS CHANGED

- Have hoisted a Winter Storm Watch for Northwest and Northeast Aroostook County from Thursday night to Friday midday. Combination of snow, sleet and freezing rain to bring the potential of significant impacts.

- Increased snow, sleet and freezing rain totals areawide for Thursday night into Friday with increasingly good agreement in the ensembles.

- Increased winds and wind gusts on Friday with increased pressure gradient.

KEY MESSAGES

1) Widespread wintry precipitation late Thursday through Friday evening, which could greatly impact Thursday evening, Friday morning commutes.

2) Another low pressure system moves through region this weekend, bringing potential for another round of snow/rain/freezing rain, that could impact travel.

DISCUSSION

KEY MESSAGE 1...Widespread wintry precipitation late Thursday through Friday evening, which could greatly impact Thursday evening, Friday morning commutes.

KEY MESSAGE 1 DESCRIPTION... Setup...A surface low-pressure system, associated with 500mb shortwave energy, will track into the Great Lakes. Meanwhile, surface high pressure developing north of the Gaspe Peninsula will establish a cold air damming (CAD) pattern at the surface with light east winds. Thursday evening, a warm front will lift north into Maine, riding up and over the cold air as temperatures aloft at 750mb rise above freezing. PWATs will increase to 1.5-2 sigma overnight as moisture advects northeastward. Expect precipitation to break out across the area from southwest to northeast. Surface winds will shift to the S-SE into Friday morning, beginning to erode the boundary layer CAD from south to north, aided by the increasingly warm April sun angle. A cold front will cross the area Friday evening, bringing an end to the precipitation.

PTypes and Timing...As mentioned, PWATs will increase due to warm air advection aloft, reaching 1.5-2 sigma overnight and 2-3 sigma on Friday. Thursday evening temperatures will be in the mid to upper 20s north, 28-31F for the Central Highlands and Bangor Region, and 30-32F along the Downeast coast. Precipitation will start around 9 PM to midnight across much of the CWA, reaching far Downeast by 2 AM Friday. Initially, a mix of snow, sleet, and freezing rain is expected along the Downeast coast northward to The Airline (Route 9) and the Bangor Region, which may create slippery travel overnight. North of there, precipitation will initially develop as snow (with a few inches possible) before mixing.

The 750mb warm nose will rapidly increase over a deep cold pool. Hi-Res CAM soundings are in fairly good agreement regarding the sleet and freezing rain transitions. The initially shallow 750mb warm layer will gradually deepen, leading to a transition from sleet to freezing rain. Toward daybreak, winds will shift to the SE as the CAD erodes. This will change the Downeast coast and Bangor area to plain rain, while freezing rain continues along Route 6 from Greenville to Topsfield. North of there, a prolonged period of sleet is expected from the Baxter Region to the St. John Valley. By mid-morning, continued erosion of the CAD will bring all rain to the Bangor Region, Dover-Foxcroft, and Lincoln, encroaching on Topsfield to Houlton.

The combination of the warm April sun angle and strengthening SE winds (gusting 15-25 mph) will erode the remaining CAD, turning the entire CWA to rain by mid-afternoon. Rain will continue into the evening until a dry slot works north ahead of the cold frontal passage, as the low tracks to our north in Quebec. Precipitation may briefly switch back to snow in the St. John Valley, perhaps leaving an additional dusting. This could lead to a flash freeze across northern zones as temperatures fall quickly back below freezing.

Accumulations...Snow will be mainly confined to the north, with 2-4 inches (potentially up to 5 inches) expected in the St. John Valley, Route 11 north from Oxbow to Portage and the Caribou/Presque Isle areas. The Baxter Region to Houlton and the Moosehead Region can expect 12 inches, while areas further south will likely see only a minor dusting. There is potential for significant sleet accumulations (0.2-0.5 inches) from the Central Highlands into Northern Maine.

The main uncertainty lies in freezing rain totals, which depend heavily on the strength of the CAD and the increasing sun angle by the time precipitation falls as freezing rain in Northern Maine on Friday. Up to a glaze of ice is possible from just inland of the Downeast shoreline northward to Southern Aroostook County. Up to a tenth of an inch of ice is possible across Northern Maine, with up to a quarter-inch possible in the Greenville area and North Woods.

The greatest chance of heavy mixed precipitation reaching warning criteria exists in Northwest and Northeast Aroostook County, so a Winter Storm Watch has been issued there from Thursday evening to midday Friday. South of this watch, additional Winter Weather Headlines will likely be needed for travel impacts, but we are holding off for now due to lingering uncertainties.

Impacts...Travel will become slippery nearly areawide late Thursday evening due to snow, sleet, and freezing rain. Untreated surfaces will cool down significantly Thursday night, leading to slick conditions. As temperatures warm overnight in Downeast areas, travel will gradually improve as surfaces become slushy, and eventually wet. Friday morning, conditions will improve from the Bangor Region northward, reaching Northern Maine by midday. The increasingly warm April sun angle will allow surfaces to heat up and turn wet. The greatest concern for the Friday morning commute will be from the Bangor and Calais regions northward to the St. John Valley. Drivers should anticipate slippery roads, leave extra braking distance, and use caution when walking on untreated surfaces.

KEY MESSAGE 2...Another low pressure system moves through region this weekend, bringing potential for another round of snow/rain/freezing rain, that could impact travel.

KEY MESSAGE 2 DESCRIPTION... A large upper level trof will develop a large surface low Saturday night through Sunday. Both ensemble models and deterministic have come into very good agreement with the track and timing of the low moving through the region. Precipitation starts as snow in the north, and rain in the Central Highlands, southwards. As warm air aloft advects in, chance for some freezing rain/sleet, mainly in the Central Highlands, northwards. As sun rises and temperatures increase, all precipitation transitions over to rain. All models are consistent in both anticipating a round of precipitation to occur in the north Saturday night into Sunday, and another round of precipitation throughout the state Sunday into Sunday night ahead of moving in cold front. Continue to monitor the forecast for updates, as precipitation types could heavily depend on pressure system tracks.

AVIATION /18Z WEDNESDAY THROUGH MONDAY/

MVFR/IFR conditions this evening will improve to VFR by early tonight. In the north, after midnight, MVFR/IFR cigs/vsby in patchy fog will move into the area. Light and variable winds.

Thursday...MVFR in the morning for the north and VFR in the south. Once the patchy fog clears in the north, VFR for all terminals. Light and variable winds.

Thur night...MVFR/IFR for all terminals in SN/FZRA/IP. SE winds 5-15 kts.

Friday...IFR/LIFR in the morning. SN/FZRA/IP expected from BGR northward with -RA at BHB. IFR/LIFR at northern terms due to FZRA/IP changing to -RA with -RA at BHB and BGR. Late day improvements to MVFR at southern terms. SE winds 10-15kt gusting up to 25kt shifting S in the afternoon. LLWS possible.

Friday night...IFR north becoming MVFR with MVFR becoming VFR south. VCSH/-SHSN possible at northern terms. Wind shift expected between 05-08Z. Winds SW 5-15kt gusting up to 20kt shifting to the N-NNW 10-15kt.

Saturday...VFR becoming MVFR late day. NE winds 5-15kt.

Saturday night...IFR/LIFR with SN/IP/FZRA at northern terms with SN/RA at BGR and RA at BHB. SE winds 10-20kt gusting up to 30kt. LLWS possible.

Sunday...MVFR/IFR with RA likely. S winds 10-15kt gusting up to 30kt. LLWS possible.

Sunday night...MVFR/IFR and RA early becoming VFR late night. Winds shift W 5-15kt.

Monday...VFR. Low chance of MVFR due to SHSN at northern terms. W-NW winds 10-15kt.

MARINE

A Small Craft Advisory continues through tonight and into Thursday. Seas 4-7 ft. Winds and seas calm by Thursday afternoon and into early Thursday night. A SCA will be needed for later Thursday night as wind gusts increase to 25 kts.

Small Craft Advisory conditions expected Thursday night into Friday evening with gusty S winds and seas around 5ft. Winds shift Friday night and expecting SCA conditions into Sun night. Sat winds from NW shifting NE. Winds S on Sun and W on Sun night. Low possibility of brief gusts to Gales Sunday on the outer waters 15-25nm offshore. SCA may need to continue Monday due to swells. Looking at precip/vsby...expect rain to reduce vsby at times Thurs Night into Friday. Rain again Sat night into Sun night.

CAR WATCHES/WARNINGS/ADVISORIES

ME...Winter Storm Watch from Thursday evening through Friday afternoon for MEZ001-002. MARINE...Small Craft Advisory until 2 PM EDT Thursday for ANZ050-051. Small Craft Advisory until 2 AM EDT Thursday for ANZ052.