Area Forecast Discussion
Issued by NWS Portland, ME

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469 FXUS61 KGYX 281538 AFDGYX Area Forecast Discussion National Weather Service Gray ME 1138 AM EDT SAT MAY 28 2016 .SYNOPSIS... A hot day is in store for much of New England today...as high pressure just off the south coast allows for warmth to build north. Some humidity will even build into parts of New Hampshire and make it feel a few degrees warmer than what the thermometer reads. If heat is not your thing...relief is not far away. On Sunday high pressure builds into the Maritimes...and turns winds northeast and off the water. That will send a cold front through eastern parts of the forecast area...dropping temperatures some 15 to 20 degrees from today. That front will hang around the area and provide the focus for increasing showers and thunderstorms into Sunday night and Monday as tropical moisture streams northward into the area. && .NEAR TERM /TODAY/... 1130AM UPDATE... Temperatures have spiked into the mid 80s across a large part of the area already this morning. Have adjusted the forecast to show a better temperature trend for the remainder of the day and increased high temperatures just a few degrees over southern parts of the area. Dewpoint values are also significantly higher than expected especially over the coastal plain. Have adjusted the dewpoint forecast which yielded higher heat index values this afternoon. Heat index values of 95 to 100 degrees are possible in southern New Hampshire and southwest Maine. A Special Weather Statement has been issued for the area of highest heat and humidity to highlight the threats posed by this first truly hot day of the year. Winds are light or calm and with strong heating of inland areas today, a seabreeze can be expected. This will cool areas along the immediate coastline soon and the hourly temperature forecast attempts to show this happening. 1015AM UPDATE... Made some adjustments to the next few hours of the forecast based on observed temperatures and sky cover. Should see the cumulus start to build in the mountains and over southern parts of the area over the next few hours. Morning upper air observation shows potential instability exists once daytime heating gets going in full force. Although mid/upper level support for thunderstorms does not appear to exist, there will be other features which could aid in the formation of thunderstorms today. Forcing from differential heating of the terrain will be one source of potential thunderstorm initiation. In addition, the frontal boundary which has dropped southward into western Maine could provide another focus for updraft initiation, particularly where this front meets the seabreeze. Several models do show convective activity forming this afternoon in the warm/moist air mass south of the front and along the seabreeze. If storms form, there is moderate westerly winds in the mid levels that could help to get storms more organized. Decided to include small hail in the forecast wording for today though would not be surprised if a severe thunderstorm warning might at some point be needed. ORIGINAL DISCUSSION... Marine fog and stratus will mainly hang tough near and N of PWM this morning...gradually lifting with daytime heating. Then strong ridging aloft will build thru the day...and allow skies to clear. A very warm air mass in place...strong heating...and Wly flow across the interior will mean afternoon highs reaching the low 90s for many. With dewpoints in the 60s...some parts of the lower CT River Valley and ASH-MHT corridor will see heat index values near 95. That will be the main forecast concern today...though there will be a chance for afternoon showers or storms near the coast. Some of the higher res model guidance continues to show QPF developing during peak heating. Given the rising heights...forcing will be weak and anything more than widely scattered convection seems unlikely. && .SHORT TERM /TONIGHT AND SUNDAY/... Tonight into Sunday...a S/WV trof crossing the Maritimes will see high pressure build in its wake. As high centers E of Nova Scotia...the pres gradient will reverse across the region and turn winds NEly and onshore. A backdoor cold front is expected to move thru at least eastern zones Sunday. Temps could be some 20 degrees cooler vs today. Air mass aloft will remain quite warm still...so to the W of the front it could be another very warm day. This may include much of the CT River Valley. At the same time that warmth aloft may strengthen the inversion and make it all the more difficult to rid the marine influence near the coast Sunday. Precip chances will revolve around scattered convection in the very warm air mass to the W...and any low level rainfall or drizzle in the marine layer...but overall forcing remains weak again. && .LONG TERM /SUNDAY NIGHT THROUGH FRIDAY/... High impact weather: heavy rainfall Sunday night into Monday may cause localized flash flooding. A broad ridge of high pressure aloft continues to be the main large scale feature over the eastern United States through the next week. This will keep generally mild temperatures in our region under southerly flow. A weak shortwave will help to initiate showers to start the week. Our weather for Sunday and Monday will depend largely on the strength of a tropical system. While Tropical Depression Two will remain well to our south and is expected to move into South Carolina, this tropical moisture will extend into Northern New England. The strong Bermuda high will couple with an approaching front to force a plume of very moist air northwards. Precipitable water will approach 2 inches /near record levels/. Extensive southerly flow around the Bermuda high will also bring warm air allowing the freezing level to rise to around 12,000ft. The tropical moisture and high freezing level will thus set the stage for heavy rainfall. The moist airmass will yield convective showers and thunderstorms. However since storm motion vectors remain very weak /less than 10kts/ there is a risk for localized flash flooding as any storms which form will have ample moisture and not move very fast. The font will push through Monday afternoon allowing for high pressure to build back in from the Ohio River Valley. The ridge remains in place through Tuesday and Wednesday bringing seasonable temperatures with highs in the upper 70s and lows in the 50s. The upper level pattern begins to shift by the end of the week as a trough digs into the great lakes at 500mb. At the surface a low pressure system moves through southern Canada dragging another cold front through the forecast area for the end of the week. && .AVIATION /14Z SATURDAY THROUGH WEDNESDAY/... Short Term...Fog will likely hang just off the coast during the day. Otherwise VFR conditions prevail. This afternoon could see widely scattered SHRA or TSRA develop near the coast...but coverage and location remains uncertain. Long Term... Scattered showers and thunderstorms will impact all terminals on Sunday afternoon into Monday. Brief periods of IFR rain will occur across inland sites. For coastal sites expect ceilings to drop to IFR with fog overnight Sunday into Monday. After Monday afternoon conditions will improve to VFR through the end of the week. && .MARINE... Short Term...Winds and seas are expected to remain below SCA thresholds for the weekend. Cold front pushing SWwd across the waters Sunday may allow for a period of seas approaching 5 ft outside of the bays. Long Term... Large scale high pressure will remain over the region keeping winds and seas below small craft criteria. && .FIRE WEATHER... The pattern will remain warmer and humid into early next week. As a result relative humidity values will remain moderate to high. Winds will also remain on the lighter side...with high pressure generally in control. && .GYX WATCHES/WARNINGS/ADVISORIES... ME...None. NH...None. MARINE...None. && $$ NEAR TERM UPDATE...Kimble SHORT TERM...Legro LONG TERM...Curtis

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