*******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 JANUARY 18 2023 4:50 PM EDT...
The following is a special report on a rare-for-January northwest Atlantic cyclonic storm with tropical characteristics which occurred offshore of the northeastern United States and Atlantic Canada on Monday January 16. The following satellite image of the storm was taken on 1300Z Monday when it was at its pinnacle as far as tropical characteristics (tropical core of thunderstorms with eye-type feature pointed to with yellow arrow):
The meteorological history leading up to the formation of the northwest Atlantic storm is illustrated by the charts below which highlight the upper-level and surface setup of the atmosphere. Prior to January 10 California was experiencing a series of vigorous eastern Pacific frontal cyclones which resulted in saturated grounds and widespread rainfall flooding. The origins of the January 16th northwest Atlantic storm can be traced to a small frontal cyclone and associated supporting upper trough that quickly pushed eastward into California on January 10 whose heavy rainfall exacerbated the flooding problems in California (https://www.cnn.com/2023/01/10/weather/california-flooding-atmospheric-river-tuesday/index.html#:~:text=Rescue%20crews%20assist%20stranded%20residents,%2C%20on%20January%2010%2C%202023.&text=Around%2010%20to%2015%20homes,and%20a%20sinkhole%20that%20developed).
While the upper trough/surface frontal system made landfall across California and quickly shifted eastward across the mountainous region of the western United States... an intense offshore frontal cyclone proceeded to dominate the northeastern Pacific from January 11 to 13. Warm surface southerly flow ahead of the offshore cyclone caused warm core upper ridging to amplify over Nevada and Alberta by January 11th... which in turn caused the amplification of the upper trough from California while it passed over the 4-corners states and Wyoming on the 11th in addition to causing the formation of a northern upper trough over Saskatchewan. The eastern divergence zone of the southern upper trough continued to support the frontal system that had quickly come in from California which had its lowest surface pressure over Colorado by 1200Z January 11 and a heavy field of precipitation to the north of the lowest pressure. The eastern divergence zone of the northern upper trough meanwhile was dropping surface pressures over the north-central United States (vicinity of Minnesota and Iowa). Due to warm core upper ridging continuing to amplify across the western United States on the 12th and 13th... the northern and southern upper troughs proceeded to merge and amplify across the central and eastern United States... and the field of low surface pressures produced by the divergence zones of both upper troughs merged into a north-south elongated frontal system whose surface pressures continued to drop to vigorous levels due to the increasing divergence on the east side of the amplifying merged upper trough. The vigorous frontal system gained most of its notoriety on January 12 while producing severe thunderstorms and tornadoes across the southeastern United States. Because the merged upper trough was not yet as amplified on the 12th as it later became... it contributed to vertical wind shear while having upper flow on its east side more westerly relative to the more southerly surface flow ahead of the surface frontal system. The mix of warm moist Gulf of Mexico surface air brought in by the southerly flow and the cold upper air temps of the upper trough aided in instability needed for thunderstorms... which combined with the vertical wind shear aided in the development of tornadoes. The most notable tornado of the outbreak passed through the populated area of Selma Alabama (https://apnews.com/article/tornadoes-disaster-planning-and-response-selma-alabama-climate-environment-7f9d8545a4ba04e3d398579b052d36dd)
The vigorous eastern United States frontal system subsequently proceeded into offshore western Atlantic waters by January 14. Because the supporting upper trough continued to be elongated with an equally elongated eastern divergence zone... the surface structure of the frontal system continued to be elongated with a belt of lowest surface pressure from the waters just offshore of North Carolina to the waters offshore of Massachusetts. Meanwhile the western United States warm core upper ridge which had contributed to the initial amplification of the frontal system's upper trough subsequently de-amplified and eroded as the cold core upper trough energy associated with the January 11th to 13th northeast Pacific cyclone began to produce new frontal systems over the western and eventually central US and to the north of the upper ridge... with the surface flow on the northwest side of these new frontal systems pulling in the cold air associated with the upper trough energy and hence de-amplifying the warm core upper ridge. Despite the loss of the adjacent amplified western US upper ridge... the upper trough of the frontal system offshore of the eastern US continued to amplify as the surface frontal system was strong enough to pull cold air associated with the upper trough southward. By January 15 the upper trough had amplified into a more circular upper vortex... and the surface frontal system equally became more circular while the area of low surface pressure offshore of North Carolina proceeded to become the dominant surface center while moving east-northeast into the northwest Atlantic waters offshore of the northeastern United States. Also noteworthy was the surface frontal system was also vigorous enough to build an impressive warm core deep-layer ridge over the northeast Atlantic... in the vicnity of the Azores... thanks to mass northward warm air transport on the east side of its large outer circulation. This deep-layer ridge blocked eastward progress of the upper vortex.
On the 15th and 16th... the surface frontal cyclonic system offshore of the eastern United States was in its classical post-mature phase... with the now well-defined surface low pressure center having whirled into a central position beneath its upper vortex where a lack of upper divergence began the surface low pressure's peak intensity and subsequent gradual decay. Because the surface frontal system had pulled cold air associated with the parent upper trough/vortex southward prior to becoming stacked beneath it... upper air temperatures had dropped to bring the 200 mb heights of the upper vortex down to approximately 1170 dekameters which is quiet cold. Meanwhile sea surface temperatures in the northwest Atlantic were running up to 2 deg C above the climatological average while the Gulf stream had a narrow ribbon of 22 deg C. It is worth noting the surface center of the frontal cyclonic storm was positioned directly over this ribbon... and combined with cold upper air temps (200 mb heights well below 1200 dekameters) the potential for instability and thunderstorm activity existed despite water temps being below the typical 26 deg C needed for tropical development. As the surface center of the frontal cyclonic storm was becoming better defined on January 15 0000Z through 1800Z... an area of showers and thunderstorms developed just off to the west and offshore of Virginia which wrapped into the center but subsequently weakened in intensity. However by 0600Z January 16 a small ring of stronger shower and thunderstorm activity began to develop around the center... leaving a more cloud-free eye-type feature at the center. The persistence of this activity through 1500Z (10 AM EDT) prompted the National Hurricane Center (NHC) to upgrade this system to Invest 90L... and issue an off-season special tropical weather outlook which stated that name assignment and advisories on a subtropical or tropical cyclone were not being planned as this system was forecast to soon depart the narrow Gulf stream warmth and into much cooler waters while accelerating north in the southerly flow between the Azores blocking ridge and new central US frontal system mentioned in the prior paragraph... which should cause a quick loss in the thunderstorm activity such that the persistence criteria needed for the NHC to declare a subtropical/tropical cyclone was not expected to be met. With this outlook I had also avoided issuing regularly scheduled birdseye view posts due to this system... instead opting to conclude this special report and release this special update once the tropical characteristics of this system subsided.
The anticipated quick northward acceleration of this system commenced early on January 17th which created the spectacle of observing a small tropical system moving across rather cold 10 deg C and single-digit deg C water temps and toward the eastern tip of Nova Scotia through 0600Z... the system probably had not had enough time yet to lose thunderstorm activity during its abrupt northward acceleration. The core of thunderstorm activity and tropical characteristics notably degraded on/after 0950Z... with the small system subsequently crossing the eastern tip of Nova Scotia shortly thereafter and reaching the eastern edge of the Gulf of Saint Lawrence/western Newfoundland for the remainder of the 17th. As of this writing... the small system has lost its identity within the larger-scale outer circulation of the seedling frontal system... with the seedling frontal system now merging with the additional frontal system that has come in from the central US to become an innocuous... typical... and broad-scale winter-time frontal system over the northeastern US and Atlantic Canada with no well-defined low pressure center. This broad-scale frontal system is expected to continue into the far north Atlantic while moving around the north side of the ongoing Azores deep-layer ridge. With no additional tropical development anticipated in the Atlantic basin in the near-term... I plan no additional regularly-scheduled birdseye view posts on the Atlantic tropics. Will resume such daily posts at the start of the next hurricane season on June 1 2023... or unless the potential for subtropical or tropical development returns to the Atlantic basin before then.
The following chart outlines the formation of tropical disturbance Invest 90-L in the northwest Atlantic... tracing its origins back to a vigorous surface frontal system/upper trough that moved into California on January 10:
(1) Archived colorized infrared satellite imagery to make this chart is available at the following link: https://www.star.nesdis.noaa.gov/goes/fulldisk.php?sat=G16
(2) Overlaid in red on each satellite image is a simplified version of the archived Ocean Prediction Center (OPC) surface analysis available at the following link: https://ocean.weather.gov/unified_analysis.php. As such red Ls mark surface low pressures/cyclones... red Hs mark surface high pressures/ridges/anticyclones... red solid lines mark the locations of fronts... and red dashed lines mark the locations of surface troughs of low pressure.
(3) Overlaid in orange and blue on each satellite image is upper-level analysis... with orange symbols indicating warmer upper air temps and blue indicating colder upper air temperatures. Arrows indicate the direction of upper-level flow... with "divergence" marking where upper flow splits and promotes the formation of low surface pressures/cyclones and "convergence" marking where upper flow collides and promotes the formation of high surface pressures/anticyclones. Dashed lines indicate the location of upper troughs of low pressure while zig-zag lines indicate the location of upper ridges of high pressure. Blue or orange Ls indicate the location of upper-level vortices/low pressure centers... blue or orange Hs indicate the location of upper-level high pressure/ridge centers. Evolution of the upper air pattern based on archived northern hemisphere GFS model runs when viewed with 500 mb heights and mean sea-level pressure (MSLP) at tropicaltidbits.com.
The following are colorized infrared satellite images showing the evolution of northwest Atlantic tropical disturbance Invest 90-L from January 15 to 17... green... yellow... orange... and red colors indicate thunderstorm intensity usually associated with tropical systems... as such the core of Invest 90-L (marked with a yellow circle in each image) had a small tropical ring of activity with at least green colors from January 16 0600Z to January 17 0600Z. Although there are green pockets that redevelop after that time (for example January 17 1800Z)... they are not in a ring shape around the center which indicates the system is no longer tropical. Also included in each satellite image is a zoomed-in uncolorized infrared or zoomed-in visible image of the core of Invest 90-L.
Sea-surface temperature map for January 16th 2023 with track of Invest 90-L's surface center overlaid. Note that this system acquired tropical characteristics on January 16 while located over the narrow ribbon of 22 deg C waters associated with the Gulf Stream. Sea surface temperature data is available at this link: https://www.nhc.noaa.gov/sst/: