*******Note that forecasts and outlooks in this post are NOT the official forecast from the National Hurricane Center (NHC). They are my own detailed views on the Atlantic tropics based on current observations and latest computer model runs. As such do not make decisions based on my posts...consult news media...watches and warnings from your local weather office...and any evacuation orders issued by local governments to make the most informed and best decisions. Visit the NHC website hurricanes.gov (hurricanes dot gov) for the latest watches/warnings and official forecasts on active tropical cyclones.**********
...WEDNESDAY JULY 21 2021 11:40 PM EDT...
The large tropical wave of low pressure currently in the east Atlantic primarily consists of a swirl of dry Saharan air with clusters of thunderstorms to the south. Due to this dry air…tropical development here is not anticipated.
Elsewhere…see area of interest #1 section below for more details on the potential for tropical development in the western Atlantic in the coming days due to the forecast decay of a surface front and its parent upper trough.
AREA OF INTEREST #1…A portion of the current northeast Canada upper trough is forecast to become an eastern US-to-western Atlantic upper trough cut-off by the current upper ridge over western North America. The divergence zone of this cut-off upper trough is supporting a surface frontal low over the coastal northeast US…with this frontal low expected to drive a cold front into western Atlantic over the next couple of days. Meanwhile the warm sector of a strong frontal cyclone to move across Canada…also over the next couple of days…is also forecast to weaken the cold core cut-off upper trough…which would reduce wind shear induced by the trough and potentially allow for western Atlantic tropical development at one or more areas along the front. Looking at model runs over the last couple of days…the best consensus is for the formation of a frontal low at the tail end of the front and tail end of the upper divergence zone associated with the cut-off upper trough either inland or just offshore of the southeast US…with the upper trough subsequently weakening as discussed above and opening the door for possible tropical development by days 4 and 5. My outlook below is based on that scenario…with the NHC joining in and also highlighting the waters offshore of the southeast US in their tropical weather outlook over the last day. My peak odds of development over the next five days are kept at a low 10% as the latest model runs still do not explicitly forecast a tropical cyclone to form. Of note…another tropical disturbance could also form further offshore along the front and later warrant another area of interest in the coming days. In fact the GFS and ECMWF seem to agree on another more offshore low pressure forming along the front and roughly in the vicinity of 37.5N-62.5W in about 3 days…which could tropically develop if the upper trough weakens enough and before the surface low moves northeast to cooler water.
Meanwhile…it should be noted that anything tropical that does form offshore of the southeastern US is expected to be steered northeastward in the flow ahead of the forecast strong frontal cyclone to move across Canada in the 2+ day window. As such in my updated outlook below…I move the forecast disturbance northeast to the waters offshore of North Carolina by day 5. However even while staying offshore…any tropical development could cause coastal sea swells and rip currents for the southeast US coast and northwestern Bahamas.
******infohurricanes.com outlook. Visit hurricanes.gov (hurricanes dot gov) for official outlook***********
IOH 24 Hr Outlook (0000Z Jul 23)…0% chance of tropical cyclone formation (western North Carolina-South Carolina border near 35N-82W)
IOH 48 Hr Outlook (0000Z Jul 24)…0% chance of tropical cyclone formation (eastern Georgia-South Carolina border near 32.5N-81.5W)
IOH 72 Hr Outlook (0000Z Jul 25)…0% chance of tropical cyclone formation (offshore of the southeastern US near 30.5N-79.5W)
IOH 96 Hr Outlook (0000Z Jul 26)…10% chance of tropical cyclone formation (offshore of the southeastern US near 31.5N-77.5W)
IOH 120 Hr Outlook (0000Z Jul 27)…10% chance of tropical cyclone formation (southeast of Cape Hatteras North Carolina near 34.5N-75W)
...COMPUTER MODEL SUMMARY...
Source...Florida State University Experimental Forecast Tropical Cyclone Genesis Potential Fields (http://moe.met.fsu.edu/tcgengifs/)
1200Z CMC Model Run...
**For area of interest #1…weak surface frontal low forms offshore of Georgia near 30.5N-79.5W at 54 hours…drifts southwest to Florida east coast thru 78 hours…at this location weakens to a surface trough at 96 hours.
1200Z ECMWF Model Run...
**For area of interest #1…weak surface frontal low forms offshore of Georgia near 30.5N-79.5W at 72 hours…moves east-northeast and reaches 35.5N-69W at 168 hours.
**Additional surface frontal low forecast to form near 38N-61.5W at 72 hours…moves to cooler water near 40.5N-53.5W at 96 hours.
1800Z GFS Model Run...
**For area of interest #1…surface frontal low forms over land at the northern SC/GA border at 33 hours…moves southeast to the waters offshore of Georgia at 42 hours…moves west into southern Georgia coast by 54 hours…weakens to a surface trough that extends offshore by 63 hours…offshore portion of trough consolidates to surface low near 31.2N-71.5W at 78 hours…surface low opens to a trough near 35N-68W at 120 hours.
**Additional surface frontal low forecast to form near 35.5N-65W at 75 hours…moves to cooler water SE of Newfoundland near 45N-48.5W at 105 hours
1800Z NAVGEM Model Run...
**For area of interest #1…surface low forms on the southeast US coast near the SC/GA border at 36 hours…drifts south to the northeast Florida coast thru 60 hours…weakens to a surface trough over Florida by 78 hours
**Additional surface frontal low forecast to form near 37N-53W at 132 hours
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