*******Note that forecast 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 18 2023 7:25 PM EDT...
For the mid-latitudes of the Atlantic... Tropical Depression Don has strengthened into a tropical storm and is expected to move across the open central and northwestern Atlantic over the next five days... see Don section below for more information. Further west... a surface front over the eastern United States has been slowly edging into offshore western Atlantic waters while pushed by the current frontal low pressure over eastern Canada... and the front was recently downgraded to a surface trough of low pressure as airmass contrasts have reduced across it. Meanwhile the offshore waters are covered by a narrow warm core upper ridge axis assocaited with warm air ahead of the former front... with offshore thunderstorm activity thriving due to the outflow of the upper ridge axis and in turn the latent heat release of the thunderstorms re-enforcing the upper ridge and associated upper outflow regime. Therefore watching for surface pressure falls and possible tropical development due to the upper outflow regime... however satellite imagery and the CIMSS 850 mb vorticity product (https://tropic.ssec.wisc.edu/real-time/windmain.php?basin=atlantic&sat=wg8&prod=vor&zoom=&time=) shows that a low-level rotation that was offshore of North Carolina yesterday has since become quiet elongated and therefore a well-defined center needed for tropical development is not in play at this time. Therefore NOT adding an area of interest for tropical development offshore of the eastern US as of this update.
For the tropical belt of the Atlantic... tropical activity remains suppressed by widespread dry Saharan air and upper vorticity covering the Caribbean and central tropical Atlantic.
TROPICAL STORM DON... Don continues to wander around in the open central Atlantic. Overnight the renewed thunderstorm activity in Don's circulation helped it to strengthen from a tropical depression to a minimal tropical storm with 40 mph maximum sustained winds. The tropical storm and overhead upper trough have been moving largely southeastward and now more recently south-southeastward today due to a re-building warm core deep-layer ridge over the northwest Atlantic being supported by northward warm air transport ahead of a broad surface frontal system over eastern Canada. It has lost much of its renewed thunderstorm activity since sunrise. This is likely an artifact of the lukewarm 24 deg C sea surface temperature environment... typically 26+ deg C supports tropical development. In addition the overhead upper trough has moved faster to the southeast than Don... with Don now decoupled from the supportive divergence zone ahead of the trough axis and instead Don could now be suffering from a supperssive upper convergence regime behind (norhtwest of) the upper trough axis and ahead of (southeast of) the deep-layer ridge. Albeit recently Don has seen a recovering small patch of shower and thunderstorm activity just north of its center as upper-level wind maps suggest a small area of boosting split flow upper divergence is now developing over Don and in between the back side of the upper trough and southeast side of the deep-layer ridge.
Going forward Don and the upper trough in Don's environment will continue to be influenced by the deep-layered ridge as the ridge slowly shifts east as a typical amplified mid-latitude feature. While the northeastern part of the upper trough is forecast to also drift eastward... the southwestern part is forecast to move westward around the south side of the deep-layer ridge as a cut-off upper vortex and eventually become streteched north-south once it moves around the southwest side of the ridge. Don itself will soon turn southwest...west... northwest... and then northeast while rounding the south and then western sides of the passing deep-layer ridge. Due to Don's current position my updated forecast track points are nudged eastward. The forecast track is also nudged northward as the CMC and NAVGEM have trended toward the more northern GFS solution. This means Don is likely to stay aligned with the supportive split flow upper divergence zone mentioned in the prior paragraph for 24 more hours as the tropical storm moves toward slightly warmer 25 deg C waters... and I show some strengthening of Don during this timeframe. Between 24 and 48 hours the forecast track below then moves Don westward and away from the split flow upper divergence and into a neutral (no divergence or convergence) upper flow in the north side of the cut-off upper vortex and south side of the deep-layer ridge... and I forecast Don to then weaken for a short time in this less conducive upper air pattern. By 72 hours the forecast track then puts Don in supportive split flow upper divergence between the north side of the cut-off upper vortex and southwest side of the deep-layer ridge... and by 96+ hours Don is forecast to be near or over more favorable 26 deg C waters and underneath the western side of the deep-layer ridge where low shear and upper outflow may help Don further in a tropical manner. This is why I show strengthening again during this timeframe... but for now keep the strengthening rate conservative to see if the current track model consensus holds at its current position to keep Don under the favorable upper winds. If it indeed does... I plan to increase the strengthening rate further in my next update. To finish off the 5-day forecast... I project Don to gradually weaken in the low shear environment and also lose tropical characteristics between 96 and 120 hours as Don moves northeastward into markedly cooler waters east of Newfoundland. If Don retains sufficient strength as it passes by Newfoundland... surf induced by its circulation could reach the eastern shores of Newfoundland in the day 5 timeframe.
******Infohurricanes.com forecast. Visit hurricanes.gov (hurricanes dot gov) for official forecast***********
0 Hr Position (1800Z Jul 18)... 40 mph maximum sustained wind tropical storm centered at 34.9N-39.2W
IOH 24 Hr Forecast (1800Z Jul 19)... 45 mph maximum sustained wind tropical storm centered at 33N-40W
IOH 48 Hr Forecast (1800Z Jul 20)... 40 mph maximum sustained wind tropical storm centered at 33N-43W
IOH 72 Hr Forecast (1800Z Jul 21)... 45 mph maximum sustained wind tropical storm centered at 35N-47W
IOH 96 Hr Forecast (1800Z Jul 22)... 50 mph maximum sustained wind tropical storm centered at 39N-51W
IOH 120 Hr Forecast (1800Z Jul 23)... Non-tropical remnant cyclone centered east-southeast of Newfoundland at 46N-49W
******National Hurricane Center (hurricanes.gov) official forecast as of 5 PM EDT***************************
Peak Strength (1800Z Jul 22)... 50 mph maximum sustained wind tropical storm centered at 40.5N-49.5W
Loss of tropical cyclone status (1800Z Jul 23)... 45 mph maximum sustained wind non-tropical remnant cyclone centered east-southeast of Newfoundland at 46N-47.5W
...COMPUTER MODEL SUMMARY...
Source...Florida State University Experimental Forecast Tropical Cyclone Genesis Potential Fields (http://moe.met.fsu.edu/tcgengifs/)
1200Z (Jul 18) CMC Model Run...
**For Tropical Storm Don... curves south to 33.5N-40W through 30 hours... subsequently curves west and then northwest to 39.8N-48W through 90 hours while gradually weakening... continues north into cooler waters through 120 hours where it weakens to a remnant low near 47N-46W
**Compact tropical low develops in the northwest Atlantic near 39.5N-64.5W by 30 hours from current thunderstorm activity offshore of the eastern US... weakens as a less tropical surface low while moving northeastward across cooler waters and into the south-central coast of Newfoundland through 60 hours.
1200Z (Jul 18) ECMWF Model Run...
**Not available at above-mentioned source
1200Z (Jul 18) GFS Model Run...
**For Tropical Storm Don... curves south to 34N-40W through 24 hours... subsequently curves west to 35N-46W through 66 hours while strengthening into a high-end compact tropical storm... while maintaining strength accelerates northwest to 40N-52.5W through 99 hours... passes just southeast of Newfoundland through 120 hours as a compact tropical storm gradually losing tropical characteristics over cooler waters
**Compact and short-lived tropical cyclone develops in the northwest Atlantic near 40.2N-61W by 45 hours from current thunderstorm activity offshore of the eastern US... weakens as a less tropical surface low while moving northeastward across cooler waters and into the southwestern coast of Newfoundland through 75 hours.
1200Z (Jul 18) NAVGEM Model Run...
**For Tropical Storm Don... curves south to 33.8N-39.8W through 30 hours... subsequently curves west and then northwest to 39N-50W through 96 hours while weakening to a remnant low... remnant low continues northeast to 43N-48W through 120 hours.