Excellent environment for Florence off East Coast Next Week
Discussion: Operational forecast models are becoming more consistent projecting a westerly track for Hurricane Florence that brings the storm into or near the East Coast later next week (Fig. 1-2). The concern of such a track is the improving environment (warmer ocean surface) in the northwest North Atlantic basin which would allow Florence to gain and maintain major hurricane classification status.
Fig. 1-2: The 6-10 day precipitation total forecast by the European model (left) and American model (right) identifies the range of possible tracks of Hurricane Florence next week. The European model takes Florence into the Mid-Atlantic States while the American model loops Florence north-northeast just missing Cape Cod.
Another somewhat unique circumstance is presence of a strong easterly phase of the quasi-biennial oscillation (-QBO). In JUN/JUL 2018 the –QBO was strongest on record (Fig. 3) although weakening slightly during AUG but still intense. A strong –QBO is present when easterly flow in the upper atmosphere across the equatorial region is stronger than normal and acts to lower ventilation required for significant tropical cyclones to form and flourish. When strong –QBO is present generation of hurricanes in the deep tropics is difficult to produce. The best “analog” for a strong –QBO regime occurred in 2005 when a record 28 tropical cyclones occurred. Notice most of the activity occurred in the subtropics rather than the deep tropics (Fig. 4).
HOWEVER, in response to the strong easterly shear in the upper atmosphere across the equatorial region the subtropics (north of 20N) have diminished upper shear. In the upper weather atmosphere (troposphere) there tends to be (relatively) high latitude easterly steering currents. These conditions, low shear environment and easterly steering in the subtropics provides ideal conditions for hurricanes. This condition is present.
ADDITIONALLY, the surface water off the East U.S. Coast and particularly east of the Northeast U.S. Corridor is possibly the warmest on record (Fig. 5). The threshold (81F) to sustain a hurricane extends well north over the western North Atlantic basin (Fig. 6).
Interestingly, Florence became a major hurricane over waters slightly cooler than 81F (yesterday). NOAA/NHC did not forecast this sudden intensification. The likely cause of the unexpected intensification was a channel of ventilation in the upper atmosphere in the SUBTROPICS related to the strong –QBO regime.
Adding fuel to the fire for the mid-to-late 2018 North Atlantic tropical cyclone season is the failure of El Nino to emerge (Fig. 7). The inhibiting westerly shear associated with El Nino has not developed. The upper shear pattern present early-to-middle season (so far) should ease. September and October are likely to be at least normally active (or more active than normal) for hurricanes especially in the subtropics.
Regarding Florence…models increasingly favor a western track toward or reaching the East U.S. Coast mid-to-late next week (Fig. 8). The environment to allow a major hurricane to flourish in the western portion of the North Atlantic basin subtropics is excellent due to very warm ocean surface and enhancing residual effects provided by the deep tropics –QBO regime. Florence will be an extremely dangerous major hurricane tracking west of Bermuda next week.
Fig. 3: In JUN/JUL 2018 the –QBO was the strongest on record. In 2005 when a record 28 tropical cyclones occurred the –QBO was also very intense causing most storms to flourish in the subtropics.
Fig. 4: During strong –QBO diminished upper atmospheric ventilation suppresses deep tropics hurricane activity. In the subtropics the opposite condition occurs, upper ventilation is enhanced. The best example of the influence of QBO was during 2005 when a record number of systems occurred most flourishing in the subtropics.
Fig. 5: Daily SSTA analysis identifies extremely warm waters east of the U.S. Northeast Corridor.
Fig. 6: The 81F threshold to sustain hurricanes has shifted well north in the western North Atlantic basin.
Fig. 7: The 12-week history of the Nino SSTA regions in the central and eastern equatorial Pacific Ocean indicate a failure of El Nino to develop. No El Nino implies mid-to-late 2018 tropical cyclone season could turn active.
Fig. 8: ECMWF model 10-day forecast track of Hurricane Florence.