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Area Forecast Discussion
Issued by NWS Portland, ME
Issued by NWS Portland, ME
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829 FXUS61 KGYX 151424 AFDGYX Area Forecast Discussion National Weather Service Gray ME 1024 AM EDT Thu Aug 15 2024 .SYNOPSIS... Strong thunderstorms are expected this afternoon. The greatest threat will be across most of New Hampshire and adjacent western Maine. Showers will be less widespread Friday, sticking mainly in the mountains. This weekend the rain chances will become more organized and possibly linger into early next week. Hurricane Ernesto passes offshore late this weekend and Monday, bringing an increasing swell and dangerous rip currents by Saturday through midweek next week. && .NEAR TERM /UNTIL 7 PM THIS EVENING/... 10:20 AM Update...Some upper-level jet divergence and steep lapse rates coinciding with an upper-level low are continuing to prolong thunderstorms in central Maine. Instability will increase through the day as we continue to destabilize. As for this afternoon, the Storm Prediction Center has placed NH and a lot of western ME in a Marginal Risk for Severe Storms. Setup looks to be similar to yesterday`s with an upper-level low forming an east to west jet streak over the Canadian Maritimes. This jet streak has shifted west over the past 12-24 hours, shifting the right exit region of this jet streak farther to the west. This means that storms this afternoon will also develop farther to the west, most likely across central New Hampshire. Storms will likely move southward through the evening, with a few strong gusty winds and quarter-size hail possible. Update...Convection continues to percolate this morning in an area of jet divergence. I have updated PoP to continue this slowly backing southwest thru the morning...though I could see the need for additional updates...especially as we start to heat up. Previous discussion...Steep lapse rates persist along the western periphery of an upper low pinwheeling over the Canadian Maritimes. Thunderstorms have been percolating along that gradient thru much of the night. Models have had a poor handle on these storms...but as we head into the diurnal min for convection we should see these continue to slowly diminish. However with daytime heating and another S/WV trof moving thru the flow...the afternoon with feature another increase in the coverage of showers and storms. Much like yesterday steep lapse rates around 7 C/km will support ample CAPE for storms. Shear will remain light...but today looks to be a little higher than yesterday. As a result the threat of severe weather is just a little bit higher. I have included gusty winds and small hail...roughly corresponding with SPC marginal risk area. Storms should be pulse type...with collapsing cores and merging cold pools leading to gusty winds later in the life cycles. Finally near surface smoke looks similar to yesterday as well. There is some suggestion from the HRRR that concentrations of near surface smoke increase mainly across southern NH during the afternoon. I have added haze to those areas using the latest smoke model forecast. && .SHORT TERM /7 PM THIS EVENING THROUGH 6 PM FRIDAY/... After showers and storms dissipate tonight areas of fog and stratus will develop. It looks most likely around Penobscot Bay...but will be a threat all along the western ME coast. Fri will be another day with afternoon showers and storms...but this time coverage looks to be more widely scattered to isolated. In addition they look to remain confined mainly to the higher terrain into the CT River Valley. && .LONG TERM /FRIDAY NIGHT THROUGH WEDNESDAY/... Overview... A series of troughs affect New England this weekend and into early next week. Generally cool and showery conditions are expected, with some brighter periods in between. Hurricane Ernesto passes offshore late this weekend and early next week, bringing building surf and increasing rip currents. Details... By Saturday, the ridge passes and a trough begins to deepen through the Great Lakes, with low pressure developing across the Great Lakes as well. The low is very slow to make any eastward progress over the following few days as the ridge stalls and builds across Atlantic Canada through the weekend. With this pattern, showers and scattered thunderstorms likely begin to move into western areas late in the day Saturday, and make slow progress eastward overnight Saturday. The progress mostly stalls by Sunday, with western areas seeing a greater chance for showers than eastern areas. There remains some question as to just how far east it will progress, but at this point the coast stands the best chance to see the driest conditions. By Monday, a shortwave dives in and deepens the trough, while also causing it to become more amplified and help the pattern begin moving along again. With this, the greatest chance for rain and showers in the forecast arrive late Monday as the low progresses east. The trough continues to transit through on Tuesday and possibly into Wednesday, keeping the chance for scattered showers around. While all this is going on, Hurricane Ernesto is expected to pass near Bermuda on Saturday, and making it`s closest pass to Nova Scotia on Monday. Ernesto helps to amplify the ridge across Atlantic Canada, aiding in the slow down of the trough progression, while also bringing it`s own effects. Building surf and increasing rip currents remain the biggest concerns from Ernesto, with a southeasterly swell building by Saturday. The swell builds through Monday, and then will be slow to subside through Wednesday. Rip currents become increasingly more dangerous by Sunday, and continue through at least Tuesday. High astronomical tides are also occuring early next week, so some splashover is also possible. The track is favorable overall for a long duration swell event, with the storm tracking mostly northward for two days around Bermuda. && .AVIATION /14Z THURSDAY THROUGH MONDAY/... Short Term...VFR conditions start day after fog lifts...and then SHRA/TSRA develop. This will mainly occur over the western half of the forecast area with local MVFR or lower conditions. Some of those storms may be strong/severe and hail and gusty winds are a threat. Fog/stratus is possible again tonight...especially across western ME coastline. Long Term...VFR conditions prevail more often than not Saturday through early next week, but periods of MVFR ceilings and showers are likely at times through early next week, especially across interior terminals during the afternoon and evening. Nighttime valley fog is also likely at LEB and HIE each night. && .MARINE... Short Term...Winds and seas will remain below SCA thresholds. Sea breezes are expected each afternoon. Increasing humidity will also introduce the threat of marine fog and stratus. This morning convective outflow is disrupting the formation of that fog...and this is possible again tonight. Long Term...Weak pressure patterns prevail as broad high pressure builds across Atlantic Canada into this weekend. Hurricane Ernesto passes east of the Gulf of Maine late this weekend and early next week, bringing an increasing southeasterly swell starting on Saturday. The swell builds to greater than 5ft by Sunday, and continues through midweek next week. && .GYX WATCHES/WARNINGS/ADVISORIES... ME...None. NH...None. MARINE...None. && $$ SYNOPSIS... NEAR TERM...Clair/Palmer SHORT TERM...Legro LONG TERM... AVIATION... MARINE...