Area Forecast Discussion
Issued by NWS Portland, ME

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000 FXUS61 KGYX 202058 AFDGYX Area Forecast Discussion National Weather Service Gray ME 458 PM EDT Thu Oct 20 2016 .SYNOPSIS... A slow moving frontal boundary and a developing low pressure center will bring wet weather and cool onshore flow to the region through Friday night. On Saturday...the low will gradually lift north into Quebec. On the backside of this low...colder air will arrive on brisk northwest winds for the latter part of the weekend and early next week. && .NEAR TERM /UNTIL 6 AM FRIDAY MORNING/... Update...Have mainly adjusted PoP based on latest observations and radar trends. Recent HRRR runs seem to have a fair handle on precip trends tonight...and so I leaned the forecast in this direction. That would mostly have rnfl coming in waves tonight...with the first batch moving thru at this hour. The next wave comes in from the S around midnight as onshore...Sly flow increases...followed by the heaviest round Fri. The mtns should see fairly continuous PoP until Fri morning...when warm front lifts into Canada. Previous discussion...At 18z...a 1027 millibar high was centered near the Bay of Fundy. A cold front extended from western Quebec southward through the eastern Great Lakes. A second boundary extended from the mid Atlantic southward through the Mississippi valley with a series of weak waves riding along it. For tonight...the surface high will quickly exit into the maritimes. Warm air advection clouds and light precipitation was entering the forecast area from the west. Expect amounts will be light tonight with strengthening onshore flow contributing some drizzle and fog by later tonight. Rather uniform lows with onshore flow and cloud cover with readings in the upper 40s to lower 50s. && .SHORT TERM /6 AM FRIDAY MORNING THROUGH FRIDAY NIGHT/... On Friday...digging shortwave energy with the upper trough going negative tilt before closing off over the mid Atlantic states by Saturday morning. At the surface...slow-moving cold front with strengthening inflow off the Atlantic and a developing surface low tapping subtropical moisture. Rain will increase in coverage and intensity as we head through the day Friday with heaviest occurring late Friday into Friday night. Highs Friday should be mainly in the lower and mid 60s. Steadier rain should taper to showers by Saturday morning as the surface low meanders overhead. Lows Friday night will be in the upper 40s and lower 50s. && .LONG TERM /SATURDAY THROUGH THURSDAY/... Upcoming hazards: Potential for winds gusting into Wind Advisory thresholds over the weekend. Locally heavy rainfall is possible. 12Z model suite gradually coming into somewhat of a consensus for the track and evolution of upcoming strengthening storm. This complex system will involve a hybrid tropical system in some capacity, however the transfer and evolution of moisture into the developing Nor`easter is still not totally clear. In any case, models in good agreement that surface low pressure will begin to undergo cyclogenesis off the Mid Atlantic coastline tomorrow before heading northeast. Surface pressures continue to fall Friday night as a negatively tilted, dynamic upper level trough digs into the region. The question of the day is how far south can the upper level low form and how progressive is this feature? This will affect the outcome for rainfall totals across the region. QPF: As of now it appears northern areas will be situated in the best dynamics and for the longest period of time, thereby picking up the most precipitation. Up to 2.5 inches of rainfall is expected in the mountains with less to the south as surface low develops overhead, shutting off the precipitation early with a dry slot forming. There is an outlier with the Canadian model which appears to bring a stripe of heavy rainfall to southern areas, bringing up to 4 inches of rainfall to the Portland area. First scenario appears more likely with heavy rainfall in the mountains. Some of this moisture may be enhanced by the terrain with locally higher amounts. Antecedent conditions are dry so flooding is not expected at this time. Very strong wind fields develop, possibly as early as late Saturday over New Hampshire depending on the exact forecast track. The strong wind gusts may be mainly diurnally driven and will likely spread into Maine by Sunday morning. Winds may gust as high as 45+ mph, leading to the possibility of some power outages. This may be delayed until Sunday as this is dependent of the passage of the upper level low in a strong tilted surface to upper level area of low pressure. A short wave will move through the fast flow on Sunday night bringing more scattered showers to the region. Snow showers may begin to develop in the mountains as boundary layer temperatures begin to cool. Thereafter, gusty westerly winds and wet bulbing effects will allow for areas of rain and snow showers in the mountains with light accumulations of snow possible early next week. && .AVIATION /21Z THURSDAY THROUGH TUESDAY/... Short Term /through Friday night/...VFR deteriorates to IFR and LIFR tonight in rain...drizzle...and fog. We`ll likely see an extended period of instrument flight rules as slow-moving low pressure and an associated frontal system traverse the area. Long Term... IFR to LIFR conditions expected into early Saturday with the expected rain. A cold front pushes east through the area Saturday, allowing for partial clearing conditions to VFR over southern areas while in the mountains and foothills MVFR conditions in scattered showers will persist through much of the weekend and into early next week. && .MARINE... Short Term /through Friday night/...Increasing and persistent onshore flow will bring small craft conditions to the open waters and marginal small crafts to the bays tonight through Friday night. Long Term... A strengthening area of low pressure moves through central New England on Saturday with a trailing cold front in its wake. This will set the state for a strong west to northwesterly flow behind it which may gust into gales at times through the rest of the weekend. && .TIDES/COASTAL FLOODING... The Friday 4 pm astronomical tide in Portland will be 10.6 feet. A southeasterly flow ahead of the system may allow for building storm tides during the day. This will be monitored as we move through the workweek for possible advisory headlines for some splash-over. && .GYX WATCHES/WARNINGS/ADVISORIES... ME...None. NH...None. MARINE...Small Craft Advisory from 11 PM this evening to 8 AM EDT Saturday for ANZ150>154. && $$ NEAR TERM...Legro is the U.S. government's official web portal to all federal, state and local government web resources and services.