Premium Weekly Outlook: A cut-off low brings unsettled weather and rainfall

So far this month, while there have been cool shots, we’ve seen warmer than normal temperatures dominate for a longer period. It has also been quite dry. But more seasonable temperatures and unsettled weather is looking likely for this weekly period overall. So much-needed rainfall may also be on the way, especially for Long Island which is currently under a severe drought.

Today has gotten of to a chilly start with temperatures in the 40s over most of the region. Some interior locations and the Pine Barrens dropped down into the middle to upper 30s with some pockets of frost. Sunshine will dominate much of the day and it will be pleasant Autumn afternoon overall. But some clouds will begin increasing later this afternoon, as a frontal boundary associated with a closed upper-level over Great Lakes begins approaching the region. Temperatures will still reach the upper 60s to lower 70s across most of the region.

It will turn mostly cloudy during the evening hours. Then some showers ahead of a frontal boundary will start moving into the region around midnight, as lifting increases from frontogenesis and moisture advection off the Atlantic. These showers will arrive over parts of Western NJ and Lower Hudson Valley during late evening hours, likely after 10pm, then into NYC metro and eastern areas around or sometime after midnight. Low temperatures tonight will be much warmer with cloud cover over the region — in the upper 50s over the interior to lower to middle 60s closer to NYC.

Last night's GFS model valid for Tuesday morning shows a band of showers and embedded heavy rainfall associated with a strong upper-level low and jet streak.

Last night’s GFS model valid for Tuesday morning shows a band of showers and embedded heavy rainfall associated with a strong upper-level low and jet streak.

Showers will likely continue into Tuesday morning. Some heavy downpours and isolated thunderstorms are possible. Lowering heights and divergence from an upper-level jet streak will provide ample lift and ascent. Abundant moisture will be streaming northward with this front. Rainfall totals around 0.50” to 1.00” appear to be likely over much of the region. Some elevated instability could also result in some isolated thunderstorms with localized heavier rainfall that produces some flooding on area roadways. This frontal boundary will quickly move east of the region later tomorrow afternoon, which may prevent widespread significant rainfall totals. Showers will taper off during the early afternoon hours and then skies will clear for more sunshine later in the afternoon behind this front as more subsidence builds into the region. Sunshine will help temperatures reach the upper 60s to lower 70s again by late afternoon.

Last night's GFS model valid for Thursday morning shows the powerful upper-level low cutting off and meandering in the Ohio Valley. This will help propel plenty of moisture our way and could keep weather unsettled for a few days.

Last night’s GFS model valid for Thursday morning shows the powerful upper-level low cutting off and meandering in the Ohio Valley. This will help propel plenty of moisture our way and could keep weather unsettled for a few days.

However, an upper-level low associated with frontal boundary will still be west of the region. It will then close or cut-off over the Ohio Valley, allowing it to meander in that general area for a few days. There is some uncertainty on how much this upper-level low will directly influence our weather by bringing more rainfall in the region. But some more unsettled weather, with at least some showers appears to be likely by Wednesday night or Thursday. A couple of disturbances rotating around the upper-level low may induce a couple waves of low pressure along the frontal boundary, which will be stalling not too far offshore. Strong mid-level southerly flow, a strong thermal gradient, and low-level convergence will keep some lift over the region. Abundant low-level moisture from the Atlantic will still be feeding into this entire system. So the main concern is for more heavy rainfall later this week.

Last night's GFS model valid for Thursday shows very strong low-level flow from the Atlantic pushing into our area. This will help to bring plenty of Atlantic moisture into the region. The way the onshore flow is very fast and then hits a brick wall over land also leads to a classic convergence boundary, yielding even more lift for rain. If this verifies, heavy downpours and flash flooding would be likely.

Last night’s GFS model valid for Thursday shows very strong low-level flow from the Atlantic pushing into our area. This will help to bring plenty of Atlantic moisture into the region. The way the onshore flow is very fast and then hits a brick wall over land also leads to a classic convergence boundary, yielding even more lift for rain. If this verifies, heavy downpours and flash flooding would be likely.

There are some model differences in handling shortwave energy associated with the cut-off low. Some show the upper-level low pushing further south over the lower Mid-Atlantic and Southeast regions. This leads to surface low pressure development further offshore again. Other models area showing the upper-level low deeper and further north, with the surface low pressure being tucked underneath and tracking northwestward further north into the Delmarva region. Regardless, the cut-off low should linger into Friday and Saturday. Some improving weather may follow by next Sunday, with cut-off low weakening, opening up, and tracking into the Canadian Maritimes.

Temperatures later in the week and this weekend will depend on the amount of  cloud cover and rainfall. But the airmass will likely support temperatures more seasonable for this time of year — mainly lower 70s for highs and middle 50s for lows. More sunshine on any given day could perhaps support temperatures slightly above normal. Stay tuned for more updates throughout the week, as forecasts will likely be fine-tuned.