Premium: Severe Weather Threat for the Interior Northeast Friday into Saturday

Today will be another very warm and humid summery day for the Northeast with more widely scattered showers and thunderstorms popping up this afternoon and evening. Due to the stronger mid-level ridging, very weak wind fields and less instability with warmer mid-level temperatures, these storms will likely be less intense and fewer and father in between than on Wednesday. But will still be very slow-moving and capable of producing some heavy rainfall and flash flooding in some spots.

However, a shortwave energy embedded in a larger upper-level trough will be amplifying over the Great Lakes Friday and Saturday. This will cause low pressure to deepen and track northeast through the Great Lakes into Southeast Canada on Friday and Saturday. As it does so, it will drag a frontal boundary will be moving be slowly through the region, as ridging from the Western Atlantic gradually breaks down. Large scale ascent and lowering heights will support more numerous showers and thunderstorms over the Interior Northeast again.

Unlike the past few days, stronger wind fields juxtaposed with  surface-based instability build ahead of the front, will likely result in a more organized severe weather threat, first for Western and Central parts of New York, Pennsylvania, as well parts of Eastern Ohio Valley on Friday. This is where the Storm Prediction Center has slight risk for severe thunderstorms. Then perhaps again on over Eastern New York and Western New England on during the day on Saturday. Where current the Storm Prediction Center has just a marginal risk.

For Friday, models suggest strong shear with southerly jet streak, may support squall or bow line of thunderstorms with gusty or damaging winds. Winds also veering with on in lower levels may support some isolated supercells that could produce some large hail and few tornadoes, as well. Although there are some indications, veer-back-veer winds, that may mitigate threat for strong tornadoes. A higher-end severe threat overall will also mitigated by only weak to modest mid-level lapse rates, as well or any convective debris from the storms over the Midwest/Ohio Valley dampening instability on Friday.

NAM sounding tomorrow over Northwest PA, showing instability and winds veering with height, possibly support an isolated tornado risk

NAM sounding tomorrow over Northwest PA, showing ample instability with CAPES near 3000 joules and winds veering with height, possibly support an isolated tornado risk. But veer-back-veer winds between 500-700mb may prevent a more heightened tornado risk.

Precipitable water values increasing to around 1.50” will also support some thunderstorms will locally heavy rainfall and torrential downpours. However, the flooding threat will be lower then the past few days with a stronger  flow in lower to mid-levels of atmosphere, forcing showers and thunderstorms to move much faster over areas. Also not as much sub-tropical moisture from the Gulf of Mexico will be feeding into this system, as some of past system have done this summer. This is due to most of the shortwave and jet streak energy, tracking northward into Canada, along with more ridging over remaining over the Southeast US.

For the coastal plain, including much of the I-95 corridor from Boston to Philadelphia, some scattered showers and thunderstorms may occur sometime Friday night into Saturday, as the frontal boundary moves through region. But the best shear and forcing will passing with more energy further northwest. Instability will also be reduced somewhat by an onshore south-southeast flow. So for these reasons we don’t anticipated organized severe weather and widespread heavy rainfall for these areas. The amount of shower and thunderstorms may also be reduced in coverage and intensity, if frontal timing is late Friday night or early Saturday, with typically lower diurnal instability.

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NAM model showing high MLCAPE 2000-3000 joulues,, juxtaposed with 30-35kts of 500mb bulk shear

Some parts of Eastern New York and Western New England could still have a threat for more organized severe weather on Saturday. But this will largely depend on timing of the frontal boundary and convective debris from Friday’s storms. If the  frontal boundary slows down and enough sunshine breaks during the afternoon, this may allow more for more instability to build over these regions. Otherwise the main threat may be heavy rainfall and frequent lightning for these areas, mostly from better forcing, height falls, and elevated instability.

After this system moves through a cooler, drier airmass form Canada will start moving into by Saturday night and Sunday. Temperatures will be a few or several degrees below normal highs mostly in upper 70s over the Interior Northeast to middle 80s near the I-95 corridor with northwest downsloping winds. on Sunday. But it will still be a very pleasant, warm day with lower humidity for the summer.

However, the next storm will be arriving be arriving Monday with more showers and thunderstorms likely over the Northeast. But weaker ridging from the Western will allow shortwave energy with this system will digging further south, will primary low pressure tracking from out of the Upper Ohio Valley and a secondary low possibly form over the Northern Mid-Atlantic region. This solution is much of the model guidance over past few days.

ECMWF mdoel digging shortwave energy toward Mid-Atlnatic coast Monday. If coore,t this will support a secondary low over the Northern Mid-Atlantic region.

ECMWF model showing shortwave energy further south, than with Friday system. If this correct, it support a secondary low with more rainfall over Northern Mid-Atlantic region

More sub-tropical moisture from the Gulf of Mexico and Atlantic will also be able to feed into this system, with higher precipitable values near 2.00” or . Lifting especially from moisture convergence will also be enhanced by low pressure and stronger southerly low-level jet over the region. Thus with storm evolution,  there is more potential more heavy rainfall and localized flooding impacts from this storm system closer to the I-95 corridor and thecoastal plain. More details will likely become clearer over next few days.

More tranquil returns for few during during the middle of next week, as high pressure from Canada builds in again. But as we discussed yesterday in our long range update the overall pattern looks continues looks to be active and cooler than this week.  So there will be more rainfall threats for the Northeast later in the week. Temperatures will also likely run near to below average, with lower heights and storms systems over the Northeast. So stay tuned for more premium updates each day, as upcoming severe weather and/or heavy rainfall threats for the Northeast gradually become clearer.