Area Forecast Discussion
Issued by NWS Portland, ME

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000 FXUS61 KGYX 112239 AFDGYX Area Forecast Discussion National Weather Service Gray ME 639 PM EDT Fri Aug 11 2017 .SYNOPSIS... Low pressure over the Great Lakes will move north of the region tonight through Saturday night...with a cold front moving through New England Saturday night. This will bring a few rounds of showers and thunderstorms...before drier air works back into the region on Sunday. Quiet and mild weather is expected for Monday and Tuesday of next week with high pressure centered south of the region. && .NEAR TERM /UNTIL 6 AM SATURDAY MORNING/...
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640 pm Update: Previous forecast on track with no changes needed. Input latest mesonet data. Most of the convection over the mountains and foothills of Maine now lifting north of the area as forecast for this evening. Previous Discussion: High Impact Weather Potential: Minimal. Current Pattern: Southwesterly flow aloft continues as of this writing with a somewhat broad and slow moving trough centered over the Great Lakes gradually moving east. This feature has slowly deepened over the past 24 hours...and will continue to do so through tonight. Weaker shortwaves have been rotating around this feature providing some impetus for shower/storm activity over our area. This will continue tonight...with a vorticity center now moving into the eastern Great Lakes rotating north and east tonight...which will be the focus for convection overnight and thus the primary focus for this forecast period. Through this evening...Showers and thunderstorms across the mountains of NH and western ME will continue to lift north as deepening southwesterly flow takes deeper moisture plume north towards /and north of/ the international border...with somewhat drier air /see afternoon dewpoints over southeastern NH and southwestern ME/ being advected north. Poor H7-5 lapse rates suggest little support for severe convection...but locally heavy rain due to slow storm motion is likely. Temperatures will fall back through the 70s /and into the 60s in the mountains/ through 8pm. Tonight...Aforementioned shortwave will drive through the region tonight with strengthening southwesterly jet at H8 impinging on western New England and beginning to spread over my northern and western zones overnight. There is a weakly coupled jet signature associated with this wave as well...but the center of the implied forcing for ascent again is primarily west of the region...perhaps brushing New Hampshire. A lot will depend on afternoon/evening convective evolution...but upstream trough will be a bit closer than yesterday...and we were able to see convection across northern and western areas last night. Thus...with +1-2 sigma PWAT air in place...see no reason that at least scattered shower/embedded thunderstorm activity will again impact these areas...with chances again diminishing further east. With weak flow in place and a similar airmass...would expect a repeat of the fog we saw last night as well...with perhaps a bit more given some expected rainfall. Similarly...with little in the way of airmass change...expect similar low temperatures...generally upper 50s to lower 60s.
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&& .SHORT TERM /6 AM SATURDAY MORNING THROUGH SATURDAY NIGHT/... High Impact Weather Potential: There is a low risk for a few strong to severe thunderstorms late Saturday afternoon and Saturday evening primarily near the CT Valley. Patchy dense fog will be possible Saturday night. Pattern Summary: Great Lakes trough will continue to march east...with Friday night shortwave passing north of the region on Saturday...with the larger scale shortwave making it/s closest approach Saturday Night as attendant surface low passes well north of the region...with a cold front moving through northern New England Saturday evening. Primary concerns center around the timing of this cold front and it/s potential to generate more widespread shower and thunderstorm activity late Saturday. Saturday: With overnight shortwave departing at daybreak...forcing for convection will wane through the morning hours as we enter a region of implied subsidence associated with anticyclonic vorticity advection. In addition...expect a good deal of cloudiness at daybreak which will put a hold on daytime heating through at least the morning hours. Southwesterly flow aloft continues however... and while forcing will initially be weak/non-existent...approaching Great Lakes trough should allow for modest forcing for ascent to overspread the region /particularly western areas/ by afternoon. These areas also look to have the best llevel instability as the sun will likely break out first here. With the approaching mid level wave H7-5 lapse rates also begin to improve as the mid levels cool...which has been a limiting factor to convection over the past 3 days. Thus...expect convection to develop along and ahead of the approaching cold front which will still be well west of the region...with some potential for a few shower/storms to pop up over the terrain in NH. Then...expect activity to make a run at my western zones by late in the day...and given enough heating this convection could be robust. The updated SWODY2 from SPC brings the marginal risk into my western zones and this is in good agreement with best instability potential and will highlight the risk for severe weather in the afternoon Hazardous Weather Outlook. High temperatures will be tricky with morning cloud cover and only partial clearing for a time before convective debris moves in from the west. Temperatures aloft will be similar to today...and would not be surprised if some locations /particularly over NH/ make it back into the 80s...but confidence is lower than normal given cloud cover and precipitation concerns...so will remain closer to guidance consensus. Saturday Night: Ensemble/deterministic guidance consensus favors cold frontal passage within a few hours of 6Z Saturday night/Sunday Morning. Clearly this will result in minimal available instability as the front moves through...but energy associated with arriving Great Lakes trough does take on some semblance of a negative tilt overnight with 120 kt 1.5 PVU jet streak overhead. Overall... dynamics don/t quite come together to bring widespread rain...but with what looks like good mid level support for ascent and +1-2 sigma PWATs ahead of the boundary...expect most locations will see some showers during the overnight with embedded thunder likely as well. Surface winds will shift westerly towards daybreak Sunday...and without good dry advection setting in until after this...low temperatures will be mild...around or above 60 in most locations. Given the moist southerly flow ahead of the boundary and dewpoints climbing into the low-mid 60s ahead of the front...expect fog development. && .LONG TERM /SUNDAY THROUGH FRIDAY/... The most interesting period of the extended is looking to be right at the outset on Sun. Most model guidance is showing the cold front will have yet to clear the forecast area. The result is a narrow window near midday Sun...when clearing will allow for some rapid heating ahead of the frontal boundary. There should be enough lingering moisture to provide 500-1000 J/kg CAPE in the warm sector. Based on model forecasts it is not out of the question that 0-6 km layer shear approaches 40 kts ahead of the front. So there will be a small period of time when a strong storm or two is possible prior to frontal passage. This matches the marginal risk from SPC and the SWODY3 well. Beyond Sun there is little change to the long term. The region will remain more or less under the influence of broad trofing and cyclonic flow aloft. That will continue to pose a risk for widely scattered convection...especially if any weak fronts can ripple thru the flow. Ensemble guidance is showing lower than normal PWAT anomalies...so widespread significant precip looks unlikely. In addition...without significant advection one way or another temps should stay pretty consistent and mirror recent days only a few degrees cooler. && .AVIATION /00Z SATURDAY THROUGH WEDNESDAY/... Short Term... Summary: Southwesterly flow continues across the region through Saturday ahead of an approaching cold front. After scattered activity this afternoon...a mid level disturbance in this flow will bring more widespread showers/storms to the region tonight. The approaching cold front will move through the region Saturday night with additional shower/storm chances. Restrictions: VFR all sites at the moment...but deteriorating conditions are expected tonight as a band of scattered showers and potentially embedded thunderstorms moves through the region. At the same time...moistening low level airmass will allow for fog and low stratus development with LIFR/IFR conditions likely at all terminals for a time starting this evening and continuing through daybreak Saturday. Expect gradual improvement back to VFR on Saturday...with another round of showers and potential thunderstorms Saturday evening...with a repeat of fog/low stratus likely Saturday night. Winds: South and southeasterly winds 10kts or less will continue through Saturday with a wind shift to southwesterly late Saturday night as the cold front moves through the region. LLWS: No LLWS expected through Saturday Night. Thunder: Through this evening...any thunder threat will likely be restricted to HIE/AUG...but only isolated thunder is expected and thus will not include in the TAFs through this evening. Tonight... some embedded thunderstorms are possible in the area of showers expected to move through the region. Thunder will again be possible particularly over our NH terminals by Saturday afternoon. Long Term...VFR conditions are expected to prevail thru early next week. Cold front will cross the forecast area around midday Sun...with scattered SHRA/TSRA possible. Local MVFR or lower conditions are possible with any convection...and if it were to impact any terminals AUG would be most likely. Otherwise we remain in general cyclonic flow with no significant high pressure overhead. Regardless some valley fog may be possible at LEB and HIE most nights...with typical LIFR conditions. && .MARINE... Short Term...Winds/waves will remain below SCA levels through Saturday night with primary potential hazard being areas of fog tonight through Saturday. Long Term...Winds and seas are expected to remain below SCA thresholds thru the first half of next week. The one exception may be Sun as winds shift off shore behind the cold front. While winds should remain below 25 kts...the change in direction may allow seas to build to near 5 ft E of Jeffreys Ledge. && .EQUIPMENT... UPDATE: The start date for the downtime for the GYX radar has been adjusted to Sunday, August 13th. The weather radar at Gray (GYX) will be down for maintenance for approximately two to three days, beginning August 13th. During that time technicians will be installing important upgrades. This work has been scheduled to minimize any potential impacts to office operations. During the outage, radar coverage is available from adjacent radar sites including Albany-New York (KENX), Burlington-Vermont (KCXX), Taunton-Massachusetts (KBOX), and Hodgdon-Maine (KCBW). && .GYX WATCHES/WARNINGS/ADVISORIES... ME...None. NH...None. MARINE...None. && $$

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