A train of tropical moisture will chug along tracks setup over the Eastern US by the mutual flows of an impenetrable Bermuda High over the western Atlantic Ocean and a potent upper-level low over the Ohio Valley and Mid-South this week. The resurgence of the tropical pattern will bring days of flooding rain just as the one that already happened last week. The upper-level low will drag an axis of torrential rain eastward until crashing into the Bermuda High midweek. Thereafter it will stall and bring days of heavy rains to the Atlantic Seaboard in a pattern identical to that which recently inundated parts of the Northeast with major flooding.
The upper-level low will plow southeastwards toward the East Coast early in the week. A corridor of widespread torrential rain will develop over the Ohio Valley, western New York and western Pennsylvania southward to the Gulf Coast between Monday and Tuesday. Scattered thunderstorms will develop east of the corridor of widespread heavy rains behind a warm front in the South and the Mid-Atlantic from Florida to the Jersey Shore.
The airmass beneath the upper-low’s associated trough is impeccably cool. High temperatures Sunday through Tuesday in the Midwest and Southern Plains will only reach the upper 70s and low 80s. Normal temperatures this time of year in the Southern Plains are in the low to mid 90s. The dichotomy between this cool, dry Canadian airmass and the hot, moist airmass driven into the East by the Bermuda High will result in an intensifying jet streak. Jet streaks are locally fast regions in the jet stream that typically develop over strong temperature gradients. Enhanced lift and precipitation caused by them reduces the temperature differences between two airmasses to restore balance in the atmosphere.
Significant differences in model guidance exist regarding the peak intensity of the jet streak. The GFS has a narrow corridor of a modestly strong upper-level winds whereas the ECMWF and the NAM have a broader jet stream and more robust jet streak. The latter are favored for the upcoming pattern due to their improved capacity over the GFS to resolve thunderstorms.
Wednesday through Friday, the upper-level low will stall as it encounters the steadfast Bermuda High. The jet streak and therefore the corridor of heaviest rain will shift further east, from the Appalachians to the I-95 corridor. The torrential rain will douse the same areas that experienced major flooding last week, including cities along and near the recently flooded Susquehanna River like Harrisburg, Hershey, Wilkes-Barre and Binghamton. Most of the rain will fall during the afternoon through the early overnight hours. Before the rains arrive in the afternoons, temperatures will climb into the mid 80s to lower 90s along the I-95 corridor, with slightly cooler temperatures further inland. Dew points in the 70s will make it feel much more uncomfortable than these temperatures would suggest.
Wednesday and Thursday will likely feature the most widespread rain, with rain chances Friday being slightly lower. The Bermuda High will finally nudge westward Friday, slightly increasing pressure over the coast. The upper-level low will slightly retrograde west Saturday in tandem and gradually dissipate, only keeping slight chances for downpours, mainly east of the Appalachian Mountains.
By Saturday, 2-4″ of additional rain is possible in the Ohio Valley, the Deep South, and the I-81 corridor in the northern Mid-Atlantic. Higher amounts are possible where rain continuously redevelops. Lower amounts of 1-3″ are more likely further east and west. Major flooding is likely to revisit areas along creeks, rivers and low-lying areas that already received excessive rainfall last week. With a stagnant upper-air flow, rain may continuously redevelop over the same areas while sparing others, resulting in most of the flooding being localized in nature.
The Bermuda High is expected to have shifted west enough by Sunday to curb the development of more thunderstorms. The cost of the reprieve could be another heatwave that could last through at least early next week. When the upper-level low finally dissipates, the void will be filed by hot circulations from the Bermuda High and a ridge bringing record heat to the Southwest. If the heat does return, at least the floods will quickly subside. The complete details of the heavy rain and the eventual reprieve are still uncertain. Be sure to check back for updates as we know more, especially residents of the Northeast who have already received substantial rain.