Area Forecast Discussion
Issued by NWS Portland, ME
Issued by NWS Portland, ME
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853 FXUS61 KGYX 092248 AFDGYX Area Forecast Discussion National Weather Service Gray ME 648 PM EDT Tue Jul 9 2024 .SYNOPSIS... High pressure offshore will maintain very warm and humid conditions for much of the week. A stationary front wavering over the Northeast will provide a focus for showers and storms into this evening. A trough will approach from the west Wednesday transporting remnant moisture from Beryl into the region Wednesday into early Thursday. Thunderstorms are again possible Friday as a stalled remains over the region. A few more scattered showers and storms are expected over the weekend. && .NEAR TERM /UNTIL 6 AM WEDNESDAY MORNING/... Update... Have updated the forecast this evening as heavy showers and thunderstorms continue along and near the coastline as of 2242Z per latest radar imagery. The latest HRRR continues to push this precipitation off the coast early this evening. Have made adjustments to the near term portion of the forecast for temperatures, dew points and winds. Heat Advisories will end by 00Z for southern New Hampshire. Prev Disc... A short wave rotating through Quebec will continue to spread modest forcing for ascent atop a buoyant airmass over the forecast area. This will continue the threat for convection through sunset. Currently, MU CAPE has built to 2000+ J/kg with deep layer shear around 40 to 50 kts. This will continue to support strong to severe storms with mainly multi cell clusters posing a damaging wind threat while a transient supercell or two will bring an isolated hail threat. After rounds of broken cloud cover across southern NH, the latest satellite shows partly to mostly sunny skies here into SW Maine. While scattered strong to severe storms will remain possible across much of the area through sunset... portions of south-central NH into SW Maine are worth highlighting for severe storms into this evening as recent CAM runs show MU CAPE building in excess of 3000 J/kg. CAMs and recent runs of the HRRR suggest convection will be pushing off the coast around 8 PM with little to no shower or thunderstorm activity anticipated thereafter tonight. It will remain warm and very humid tonight with temperatures and dewpoints in the upper 60s to low 70s from north to south. This humid airmass will also likely lead to patchy fog across much of the area tonight. && .SHORT TERM /6 AM WEDNESDAY MORNING THROUGH WEDNESDAY NIGHT/... Weak high pressure will be over New England to start the day Wednesday while an upper trough and the remnants of Tropical System Beryl approach from the west. This system will draw an anomalous plume of deep moisture into the area Wednesday through Wednesday night. The remnant surface low will lift a warm front into the region Wednesday morning into the afternoon providing a focus for showers and thunderstorms. The surface low will then track towards the St Lawrence Valley through Wednesday night for additional rounds of showers and thunderstorms. As this system moves through the Northeast the warm front will bring the potential for severe thunderstorms late Wednesday morning into the evening across southern NH and SW Maine and the deep moisture in place will bring the risk for flooding across much of the interior where a Flood Watch has been issued. For more details regarding the flood threat please see the Hydrology Section below. The 12Z CAM suite is in decent agreement that filtered sunshine will allow the atmosphere to destabilize across central and southern NH into SW Maine by late Wednesday morning. All the while, a warm front lifting through the region will lead to curved and elongated hodographs. CAMs suggest that storms could initiate near the warm front by early afternoon with the 12Z HREF highlighting south central NH into SW Maine where storms could sustain strong rotating updrafts. SPC has placed much of this area into a Marginal Risk with a Slight Risk over portions of SW NH. The concern with these storm will be damaging winds early Wednesday afternoon into the evening and curved hodographs will also bring the risk for a tornado. Cloud cover will help keep air temperatures lower than today while it will still be very warm with highs in the mid to upper 80s. Dewpoints will remain oppressive in the low 70s bringing heat indices into the low 90s with the potential for a Heat Advisory needed near the lower Merrimack Valley. Given the cloud cover forecast have held off on a Heat Advisory for this package. The threat for strong to severe storms will diminish into Wednesday night while the heavy rain threat will continue across the northern half of the area into Thursday morning. It will remain warm and muggy with patchy fog likely. && .LONG TERM /THURSDAY THROUGH TUESDAY/... The remnants of Beryl exits the region Thursday morning, not before potentially leaving additional heavy rainfall over northeastern portions of our forecast area. PWAT values will continue to be extremely high with deep tropical moisture still in place. Areas river will still be on the rise during this period. Surface dew points will be in the 70s across much of the region. It will be on the very warm to hot side once again as at least partial sunshine develops during the day. Thereafter, 12Z operational models and ensemble solutions suggest persistent warmth and humidity will continue throughout the long range portion of the forecast. Short waves crossing through the region will allow for scattered showers and thunderstorms most days, especially during the afternoon hours and away from the immediate shoreline. The exception may be on Sunday when some ridging will occur aloft over the region. Overnight fog may be problematic, especially over the coastal waters with such high dew points over the Gulf of Maine. Have included patchy fog most overnight and early morning periods. A pattern change may occur next week as a trough enters southeastern Canada. Cooler and drier air may reach New England during that period. && .AVIATION /00Z WEDNESDAY THROUGH SUNDAY/... Short Term...Scattered TS will continue to be a threat through 00Z this evening. Storms diminish after 00Z with VFR likely prevailing for much of the night, although patchy fog cannot be ruled out. KRKD and KAUG will have the greatest chance for flight restrictions tonight in fog. VFR likely prevails for all TAF sites Wednesday morning. Showers and thunderstorms develop late morning into the afternoon and will bring the threat of flight restrictions mainly along a line from KLEB to KCON to KPWM and points northward into Wednesday night. Patchy fog will bring the threat for flight restrictions across much of the area. Long Term...Mainly VFR during the daytime hours through much of the extended. However, there will be some scattered shower and thunderstorm activity especially during the afternoon and evening hours leading to brief periods of IFR conditions. LIFR and IFR conditions may occur during the nighttime periods. && .MARINE... Short Term...Steady southerly flow will prevail over the waters through Wednesday while gusts remain at or below 20 kts. Seas will also stay below SCA thresholds. Storms forming over the land will have the potential to track into the waters this evening before diminishing after sunset. Humid air over the waters will continue the threat for marine fog tonight through Wednesday night. Long Term...A persistent southerly flow will continue over the coastal waters through the period. However, for the most part this will be below SCA levels. Patchy fog may be problematic with the high levels of moisture over the region leading to low visibilities at times. && .HYDROLOGY... Tropical-like downpours possible Wed-Wed night, Flood Watch in effect. WPC has issued a rare Moderate Risk for the White Mountains region of NH, with a slight risk for the remainder of the CWA. Have left S ME and NH out of the watch based on dry antecedent conditions and lesser risk of heavier rains. The remnants of TD Beryl will interact with an incoming upper level trough over the Great Lakes tonight. The low will track eastward into New England, where it will pull tropical-like moisture into the region on Wednesday. PWATs are likely to reach 2-2.25, which is 2-3 standard deviations above normal, and warm cloud depths 14.5-15k ft. This will support the warm rain collision-coalescence process and highly efficient rainfall rates. Discrete cells in the early afternoon should be progressive enough to limit flooding, but there is the possibility that training and backbuilding will occur. A northward progressing warm front will stall/occlude by Wednesday night, becoming the focus for backbuilding/training and the primary flood risk area. The CAMs and mesoscale models favor a more southern track with this boundary than the synoptic scale runs, with the consensus being around the Lakes/Mountains/international boundary regions. Current antecedent conditions do not favor widespread flooding as soils have the capacity to absorb the initial forecasted rainfall and streamflows are low which is near normal for this time of year. That being said, the rainfall rates are expected to be extreme which will overwhelm ground infiltration initially, so rapid runoff is a concern for all areas particularly in steep terrain. && .GYX WATCHES/WARNINGS/ADVISORIES... ME...Flood Watch from Wednesday afternoon through Thursday morning for MEZ007>009-012>014-020>022-033. NH...Flood Watch from Wednesday afternoon through Thursday morning for NHZ001>009. Heat Advisory until 8 PM EDT this evening for NHZ010-012-013. MARINE...None. && $$ Cannon