Radar — KBOX
Surface Analysis — Eastern U.S. (North)
Satellite
Nearest monitored road: Central Square (Secondary)
No significant changes to the going forecast this evening. Will consider a flash flood watch for portions of nh for the next forecast package.
Previously...
Forecast rainfall has increased and consolidated across southern new hampshire through thursday, with a building flash flood threat.
1. Showers and thunderstorms will produce heavy downpours through early evening across southern and central new hampshire. Lingering or training storms may produce localized instances of flash flooding.
2. A cut-off low develops over the region tuesday night and wednesday and remains in the vicinity through friday. This will bring multiple rounds of rainfall with locally heavy rainfall possible resulting in isolated to scattered flash flooding.
3. Some periods of drier weather this weekend with warmer temperatures, but daily chances for precipitation continue, especially sunday.
Key message 1 description...
Showers and thunderstorms are underway across much of new hampshire this afternoon as a moist airmass moves into daytime instability.
The environment has a number of components supporting heavy rainfall rates through early evening: high pwats, low steering flow, and ample instability. PWAT values around 1.5 to 2" are being supporting by southerly vapor transport ahead of closed low exiting the great lakes. The broad nature of the low lacks a strong llj at this time, thus low SHEAR and storm motion is expected. This overlap with daytime instability and generous dewpoint temps means showers and storms will continue to develop and wash out into early evening.
The primary threat with these storms will be lightning and heavy downpours, where repetitive training or slow storm motion could result in isolated flash flooding. Urban areas will also be susceptible due to poor drainage should these heavy storms roll through. Rain rates at area weather stations have generally been 2 to 4 inches per hour. This is expected given the environment. The good news is the lack of SHEAR has allowed most storms to rain into their updrafts, reducing their longevity. Thus while these rates are concerning, they haven't been sustained long enough to overcome otherwise high flash flood guidance in the region. Will continue to monitor the area through early evening, as repeat rounds of heavy rainfall over recently saturated ground will tend to increase this threat.
The large, dynamic system in the region will slowly pivot eastward overnight. Guidance today has ramped up QPF in new england overnight, with a pivoting band of forced rainfall occurs just west of new hampshire overnight. This will slowly near western new hampshire through the early morning hours wednesday. Across the rest of the forecast area, onshore flow increases and will continue to advect moisture inland. It is expected that stratus will fill in, with fog developing along the coast and interior overnight.
Key message 2 description...
Cutoff low pressure will move across the delmarva on wednesday morning to southern new england on thursday before slowly exiting to the east over the gulf of maine by friday. This will continue to bring periods of moderate to locally heavy rainfall to the region with the most impactful period expected to be on wednesday. Temperatures will generally be below avg through the end of the week given the onshore flow combined with clouds and rainfall.
The h85 SE llj will increase during the day on wednesday, which may allow for some backbuilding of convective showers within an expected favorable environment for heavy rainfall. The latest 12Z href ensemble mean 24-hr QPF through 00Z thursday ranges from between 1- 2" across much of south-central nh with less than an inch elsewhere. Portions of northwestern me may mot see much rainfall during the day. As a result, an isolated to scattered flash flood threat exists, especially where cells do manage to back build and fall across both the urban corridors and the mountainous terrain of nh. River flooding is unlikely given the large available storage space but rises on rivers and streams can be expected. High temperatures will be mainly into the 70S but high dew points will make it quite muggy.
Periods of showers and embedded downpours will continue on thursday, with the highest rainfall amounts expected across south-central nh and extending into southwestern me. The showers will be very slow moving with backbuilding possible, and therefore a flash flood threat will remain... Especially given that soils will likely be already saturated in some locations. Flow will become more northeasterly on thursday night, with a diminishing flash flood threat.
Total rainfall is currently expected to range from less than an inch across northwestern me and into northern coos county nh with generally between 1-3" of rain further to the south. The 12Z href lpmm does show the potential for localized amounts of up to around 5", with the areas of greatest concern for these amounts across the whites as well as across southern nh.
The cutoff low will finally exit to our east on fri, allowing for brightening skies and high temperatures of around 80 degrees.
Key message 3 description...
Saturday should feature drier conditions as the cutoff low dissipates over the gulf of me. Highs will be into the lower 80S with partly cloudy skies. Sunday into early next week looks to be potentially unsettled again at times as a series of disturbances cross down from the north, allowing for scattered showers and storms. This will help to keep temperatures near seasonable avg with no significant heat in sight as we head into the month of august.
Through 18Z wednesday...shra and ts will continue across nh terminals this afternoon and early evening. Decreasing coverage is expected after 00Z. These will bring brief IFR conditions, mainly due to very heavy downpours lowering vis. Primary overnight impacts will be broadening IFR stratus, with fog late until early morning. Lowest ceilings will be towards the interior and foothills where LIFR conditions are expected. Fog, potentially dense, develops across much of coastal and interior maine.
Ceilings tend to remain IFR wed morning, with ra increasing across nh west to east.
Outlook:
Wednesday-thursday: periods of ra/tsra combined with low ceilings will bring IFR or worse restrictions. Winds will be primarily out of the SSE at 5-15 KTS, gusts to 25 KTS. Locally stronger winds are likely within any thunderstorm.
Friday-sunday: expect conditions to slowly improve on friday as low pressure moves away from the region though scattered showers and a thunderstorm are possible. VFR conditions should dominate saturday outside of an isolated afternoon shower/storm. Winds at or below 10KTS.
Broad low pressure moves east into new england tonight. Onshore flow will result in abundant marine stratus and vis reducing fog tonight into wednesday.
Sca conditions likely wednesday evening through early fri with S-SE wind gusts outside of the bays up to 25 KTS with seas of 3-7 ft.