Area Forecast Discussion
Issued by NWS Portland, ME

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000 FXUS61 KGYX 271527 AFDGYX Area Forecast Discussion National Weather Service Gray ME 1127 AM EDT Thu Jul 27 2017 .SYNOPSIS... A cold front moves across the area today bringing scattered showers in the morning and possible thunderstorms in the afternoon. High pressure will nose into the region Friday and into the weekend as a low pressure slips south of the area. This high will remain established over the Northeast through early next week with an extended period of pleasant weather. && .NEAR TERM /TODAY/... 1130AM UPDATE... Decided to take another stab at the hourly PoP forecast to try to refine the timing of the showers and possible thunderstorms this afternoon. Rain beginning to move into southwest New Hampshire from Vermont at this hour with more scattered showers further north along the front currently arriving in the Lake Champlain area. This front will be the focus for any thunderstorms that would develop this afternoon in our area. HRRR model continues to portray decent convective development over the northern part of the area with this front, though we remain a little skeptical, favoring more isolated thunderstorms. 930AM UPDATE... Have updated the forecast primarily to refine the timing of the next batch of rain seen on regional radar headed toward southern New Hampshire. Have also reassessed the threat for thunderstorms today based on early morning observations. Considering the degree of cloud cover observed this morning, it`s hard to get excited for thunderstorm chances today. However, it is worth noting that much of the instability which arrives today is due primarily to low level (surface to 850MB) moist advection from the southwest combined with cooling aloft (700MB to 500MB) with the arrival of a trough. Even without significant heating today, there could be a few thunderstorms which form along the leading edge of the trough. Wind shear is generally too weak to support supercell thunderstorms, but some small hail or gusty winds would be possible with any storms that develop. Most likely timing would be between 2PM and 8PM primarily over the northwestern half of the forecast area where the best cold advection aloft will be occurring. Have attempted to portray this thinking in the gridded forecast. No other significant changes were needed to the forecast at this time. 730AM UPDATE... First area of showers is moving out of the eastern zones at this hour. A second batch of showers is moving E out of NY...and looks to track across southern zones mainly. I have adjusted PoP up and temps down for the next several hours as a result. This will also delay any possible destabilization for afternoon thunderstorms...so some enhanced wording was removed. ORIGINAL DISCUSSION... A more uncertain forecast than usual in the near term this morning...as it looks like we have a couple rounds of precip in store. The first will continue crossing the region this morning. A lead s/wv trof is forcing an area of mainly light showers thru the Northeast. It is likely many locations will see rain falling...but less likely that they will see measurable precip. This shower activity will linger into the mid morning especially farther south where the front will begin to lag as flow becomes more and more parallel to it. This all complicates the forecast...because the second round of potential precip is heavily dependent on daytime heating. A secondary s/wv trof is forecast to spark scattered thunderstorms after the morning clouds/showers move off. The more sunshine we see...I expect the better chance for and greater coverage of thunderstorms. At this time it looks like significant cloud cover may linger into the afternoon...so I shied away from likely PoP and this reflects the uncertainty. That being said...with a decent jet streak overhead sufficient shear will be in place for storm organization. If we can get some pockets of sun...a stronger storm or two is possible. With the uncertainty about that sun...SPC has pulled the remainder of the forecast area from the marginal risk. I have also trimmed the enhanced wording back to mainly southern NH. If storms do form...gusty winds and small hail are possible...mainly in the 18 to 22 or 23z window. && .SHORT TERM /TONIGHT AND FRIDAY/... Trof axis swings across the area tonight...bringing an end to the showers and storms as it shoves the cold front off the coast. Farther south the flow is more or less parallel to the front...so the tail end should stall in the vicinity of southern New England. This may keep some clouds and possibly a stray shower around southern NH. Surface ridging tries to nose into the region...but we remain in cyclonic flow aloft into Friday. This should keep the boundary layer fairly mixed overnight...and allow for diurnal cloud cover increases Friday afternoon. && .LONG TERM /FRIDAY NIGHT THROUGH WEDNESDAY/... A coastal low develops over DELMARVA Friday night then moves east northeast along the southern New England coast Saturday into Saturday night. Models continue in good agreement of the track and in keeping the northern fringe of the precipitation shield just south of the ME/NH coastline. An overcast day expected Saturday along southern and coastal areas while further north partly to mostly sunny skies expected. A light northeast breeze will keep temps mostly in the 70s Saturday. By Sunday the coastal low will have exited well to the east allowing high pressure to build over the area allowing mostly sunny skies and seasonable temps mostly in the 75-80 degree range. A more zonal west to southwest flow aloft develops early to mid week allowing warmer temps to spread across the region. Generally dry conditions expected early to midweek except for maybe a few air mass afternoon showers or thunderstorms mainly in the mountains and foothills. && .AVIATION /14Z THURSDAY THROUGH MONDAY/... Short Term...VFR conditions are expected to prevail thru Fri. An area of SHRA will move thru the forecast area this morning...the heaviest of which may bring a brief period of MVFR conditions. Ahead of the cold front this afternoon scattered TSRA are possible. These may bring MVFR or lower conditions...but coverage is uncertain enough to preclude mention in the TAFs at this time. Tonight into Fri high pressure tries to nose back into the region...and widespread VFR conditions return. Long Term...VFR conditions through the weekend and into next week. && .MARINE... Short Term...Winds and seas are expected to remain below SCA thresholds. The exception may be near 25 nm where a few 5 ft seas are possible late today in southerly flow ahead of the front. Long Term...SCA conditions may develop over the outer waters Saturday into Saturday evening as the coastal low passes southeast of the area, otherwise winds and seas to remain below SCA criteria. && .GYX WATCHES/WARNINGS/ADVISORIES... ME...None. NH...None. MARINE...None. && $$ NEAR TERM UPDATE...Kimble SHORT TERM...Legro LONG TERM...Marine

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