A happy Wednesday afternoon to you! Relatively calm and stable weather has settled into the Northeast states over the past few days, despite the presence of a cold trough lingering over the region. Higher pressures have remained in control, with only a few weak and dissipating disturbances meandering nearby. This has kept the weather quiet, albeit cooler than normal, and created a bit of a facade in front of how the atmosphere is truly behaving as this week goes on.
In truth, the hemispheric height pattern is in quite a disturbed state as we speak this afternoon. A potent, anomalous high latitude block has developed from Greenland into Central Canada, disrupting the wave pattern and height field throughout the Northern hemisphere. This has led to the development of one of the strongest -NAO’s on record for the month of May, and is the impetus behind the colder than normal air which has settled into the region this week.
A blocking ridge over the high latitudes can do many thing to the hemispheric weather pattern, but most notably it slows down the overall progression of systems, and dislodges cold air away from where it typically resides. We are seeing both unfold this week, and the culmination of the block will come as the weekend approaches. The blocking ridge will continue its retrograde westward into Central Canada before it gradually dissipates. Meanwhile, the adjusted atmospheric height field over North America will aid in the interaction between two disturbances over the Eastern United States.
Forced south and slowed down by the presence of the mentioned blocking ridge, a disturbance over Southeast Canada is expected to phase and interact with a second disturbance meandering over the Ohio Valley. While the exact details of the interaction between the two remain uncertain (and likely won’t be pinned down for another day or two), their interaction alone will aid in the development of a low pressure system off the East Coast which is expected to gradually strengthen.
Favorably positioned jet dynamics in the atmosphere will support intense lift to the northwest of the low pressure area, and the advection of anomalous moisture is expected to juxtapose with this lift to lead to the potential for very heavy rain. Forecast models, including all global operational models, have come into better agreement on this occurring this weekend, and the potential for torrential rain seems heightened particularly over the Northern Mid Atlantic and Southern New England as the surface low tracks off the Delmarva and south of Long Island.
Ensemble guidance currently provides added confidence in the storm systems development over the mentioned regions, with steadily increasing support for both a deepening low pressure area and heavy precipitation. Probabilities for over 2″ of rainfall have increased on almost all global ensemble guidance over the past 24 to 48 hours, particularly over the mentioned regions of the Northern Mid Atlantic to Southern New England.
Depending on the storms exact track and intensity, a myriad of weather hazards may evolve this weekend. The first and most obvious will be the potential for torrential, heavy rain and the attendant threat for flooding in the regions that receive persistent heavy rainfall. But the additional presence of impressive jet dynamics could lead to threats for both strong, gusty winds and coastal flooding/beach erosion troubles along the area shores.
These details will have to be ironed out as the storm draws closer. Nevertheless, confidence in impacts from the storm system have increased over the past few days.
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