*******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.**********
...TUESDAY JULY 20 2021 11:43 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… the less reliable NAVGEM and CMC models have been forecasting short-term tropical development along the cold front currently draped across the western Atlantic. However current satellite imagery shows no organized thunderstorm activity along the front…and there is little time for tropical development to occur here before a frontal low forecast to form along the coastal northeast US dominates the region. Meanwhile…the coasts northeast US frontal low is forecast to drive the cold front that was over the Great Lakes region this past afternoon into the west Atlantic in the coming days…with prospects for tropical development better along this front…see area of interest #1 section below for more details.
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 forecast to support a surface frontal low over the coastal northeast US over the next 24 hours…with this frontal low driving the cold front that was over the Great Lakes region in the afternoon into the western Atlantic by 3+ days. Meanwhile the warm sector of a strong frontal system moving across Canada…also in the 3+ day timeframe…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 since yesterday…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 day 5. My outlook below is based on that scenario…albeit another tropical disturbance could also form further offshore along the front and later warrant another area of interest in the coming days. My 5-day odds of development are kept at a low 10% as today’s model runs are more shy in showing development compared to yesterday.
It should be noted that anything tropical that does form along the front will likely stay offshore of the eastern US while steered northeastward in the flow ahead of the forecast strong frontal system to move across Canada in the 3+ day window.
******infohurricanes.com outlook. Visit hurricanes.gov (hurricanes dot gov) for official outlook***********
IOH 24 Hr Outlook (0000Z Jul 22)…0% chance of tropical cyclone formation (western North Carolina-Virginia border near 36.5N-80.5W)
IOH 48 Hr Outlook (0000Z Jul 23)…0% chance of tropical cyclone formation (western North Carolina-South Carolina border near 35N-82W)
IOH 72 Hr Outlook (0000Z Jul 24)…0% chance of tropical cyclone formation (eastern Georgia-South Carolina border near 32.5N-81.5W)
IOH 96 Hr Outlook (0000Z Jul 25)…0% chance of tropical cyclone formation (offshore of the southeastern US near 30.5N-79.5W)
IOH 120 Hr Outlook (0000Z Jul 26)…10% chance of tropical cyclone formation (offshore of the southeastern US near 31.5N-77.5W)
...COMPUTER MODEL SUMMARY...
Source...Florida State University Experimental Forecast Tropical Cyclone Genesis Potential Fields (http://moe.met.fsu.edu/tcgengifs/)
1200Z CMC Model Run...
**Tropical cyclone formation suggested along current west Atlantic front near 37.5N-70W at 18 hours…weakens to a remnant low near 40.5N-60.5W at 42 hours.
**For area of interest #1…weak surface frontal low forms offshore of Georgia at 114 hours…low pressure loses definition offshore of South Carolina at 126 hours out ahead of stronger eastern US cold front trailing from large Canadian frontal cyclone.
1200Z ECMWF Model Run...
**For area of interest #1…weak surface frontal low forms on the US east coast at the FL/GA border at 96 hours…low pressure loses definition offshore of Georgia at 120 hours out ahead of stronger eastern US cold front trailing from large Canadian frontal cyclone.
**Additional surface frontal low forecast to form near 40N-52.5W at 144 hours (however this location is over waters too cool for tropical development)
1800Z GFS Model Run...
**For area of interest #1…surface frontal low forms over land at the northern SC/GA border at 72 hours…moves southeast to coastal Georgia at 90 hours…moves east and offshore as a surface trough of low pressure by 111 hours…closed circulation forms along surface trough near 34.5N-72.5W at 141 hours
**Additional surface frontal low forecast to form near 36.5N-63.5W at 96 hours…compact tropical cyclone formation suggested near 40.5N-54W at 111 hours (at northeast end of warm Gulf Stream current)…quickly transitions to a non-tropical frontal cyclone over cooler water near 44N-46W by 126 hours.
1800Z NAVGEM Model Run...
**Tropical cyclone formation suggested along current west Atlantic front near 37.5N-67.5W at 18 hours…transitions to a non-tropical frontal cyclone over cooler water southeast of Newfoundland near 44N-54W by 48 hours.
**For area of interest #1…no development shown