Does a Harvey-Style High Pressure Block Stall Florence in Mid-Atlantic?

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Does a Harvey-Style High Pressure Block Stall Florence in Mid-Atlantic?

Implications: Long-duration catastrophic flooding event

Executive summary: Last August Major Hurricane Harvey roared inland southern Texas then stalled for a week dumping 50 in. of rain on Houston. A large high pressure ridge over north/northeast North America “blocked” Harvey from turning north and northeast like most inland-traveling tropical cyclones. The concern is that Florence, likely a major hurricane striking the North Carolina coast Thursday night also stalls in the Mid-Atlantic region blocked by an eastern Canada/Northeast U.S. super strong high pressure ridge. The result is similar, historical heavy rainfall over a large portion of the Mid-Atlantic region favoring North Carolina and Virginia as Florence stalls for several days.

Discussion: Well-documented is the very warm surface water ahead for Florence and the primary reason forecast models intensify Florence to a category 4 hurricane fairly quickly as she moves into the warm water region by tomorrow. Another aspect of this storm becoming more clearly evident in weekend model runs is the blocking high pressure ridge setting up over the Northeast U.S. this week guiding Florence into the Mid-Atlantic coast then stalling Florence for days in the Mid-Atlantic region causing a potential Harvey-like immense rainfall/flooding scenario (Fig. 1-2).

A warmer than normal ocean surface not only fuels upper ocean heat to allow hurricanes to flourish during late summer/early autumn in the North Atlantic basin but also lead to a warming atmosphere and 500 MB upper ridge patterns. Note the warmth in the west/northwest North Atlantic basin last August fueling the upper ridge that blocked Harvey (Fig. 3). Right now the western North Atlantic basin seas surface temperature anomalies (SSTA) are warmer than last year (Fig. 4). The upper ridge pattern over the Northeast U.S. is (also) forecast stronger than last August. The ridge pattern is almost certain to stall Florence in the Mid-Atlantic region for many days.

The implications are an immense amount of rain over the Mid-Atlantic region. The 120-hour rainfall forecast beginning next Thursday and into early next week indicate a number much higher than 12 in. over North Carolina, Virginia and parts of Maryland (Fig. 5). I’ve edited 20-30 in. across this chart but the Harvey analog indicates more could occur if the ECM forecast is correct.

Tropical cyclone models are in general agreement on an approach to the North Carolina Coast (Fig. 6) as a category 4 hurricane (Fig. 7) which agrees with the ECM model (and NOAA/NHC forecast). All models slow Florence in the Mid-Atlantic region, the ECM over inland areas and the GFS just offshore. The ECM inland-stall producing catastrophic rainfall and flooding is the one to prepare for.

Fig. 1-2: The 500 MB anomaly observations for Aug. 24-27, 2017 identifying the high pressure block that stalled Harvey in Texas last August (left). A similar (stronger?) blocking pattern sets up this week over the Northeast U.S./eastern Canada keeping Florence in the Carolinas (right).

Fig. 3-4: The August 2017 and current SSTA patterns in the North Atlantic basin. Warm fuel for the blocking high pressure ridge over northeast North America.

Fig. 5: ECMWF 5-day rainfall forecast beginning next Thursday into early next week. The NC/VA/MD amount goes off-the-chart. CIC conservatively estimates 20-30 in. or rainfall given the scenario forecast by the European model.

Fig. 6: Tropical cyclone model forecast tracks for Florence.

Fig. 7: Tropical cyclone model intensity forecasts for Florence.

Fig. 8: European model forecast track for Florence.