Big Picture: The large-scale flow conditions in the middle and upper troposphere across the CONUS, Gulf of Mexico, Caribbean Sea, and North Atlantic at 1215 UTC 7 Sep are summarized by the high-resolution water vapor winds (image 1) and Atlantic scale IR image for 1215 UTC 7 Sep (image 2). Features of interest include TC Hermine located over southern Texas with an anticyclonically curving outflow band extending toward the lower Great Lakes, the remnants of "quasi-comatose" Gaston (PGI38L) on "life-support" and located south of St. Croix, a weakening upper-level cold low situated to the west of PGI38L to the south of eastern Cuba, an area of convection in the ITCZ near 10°N and 61°W, a huge area of dry air in the middle and upper troposphere over the central North Atlantic, and a trio of disturbances (PGI39L, PGI41L, and PGI42L) over the extreme eastern Atlantic and western Africa (image 3). Hermine made landfall over extreme northeastern Mexico late last evening with peak wind gusts of 60-65 kt. Had Hermine not made landfall it likely would have continued to intensify in a weak shear environment over SSTs of 29-30°C.
A base reflectivity image for 1359 UTC 7 Sep (image 4) shows that Hermine was still a tropical cyclone based upon the presence of a partial eyewall and cyclonically curved convective banding. A later base reflectivity image for 1854 UTC 7 Sep shows that Hermine still possess attributes of a tropical cyclone (image 5). Note the strong feeder bands of heavy precipitation in the southeasterly onshore flow to the east of Hermine (image 5). These feeder bands are sustained by high TPW values (69.1 mm at CRP from the 1200 UTC 6 Sep sounding, (image 6). A band of heavy rain that extends from eastern Oklahoma east-northeast to northern Arkansas (image 5) likely marks the onset of a predecessor rain event (PRE) discussed yesterday. This PRE is developing near the poleward edge of Hermine's moisture shield and near a weak surface baroclinic zone (image 7).
At 1200 UTC 7 Sep, NHC had downgraded the potential for tropical cyclogenesis in the "quasi-comatose" Gaston (PGI38L) within the next 48-h to 20%...down from 70% yesterday and 80% on Sunday. Convection had decreased within the center of circulation, and become more elongated along the northwest flank of the disturbance (image 8). The more enhanced southerly shear on the western side of PGI38L was associated with the upper-level southerly flow east (west) of an upper-level cold low (subtropical anticyclone) that we have been monitoring for several days (image 1). Note that mid and upper-level dry air is still apparent on the east and northeast flank of PGI38L, but the system appears to be moving into a relatively more moisture rich environment over the northern Caribbean (image 8). A zoomed in view of PGI38L nicely shows the locations of dry air, deep moisture, and upper-level flow (image 9). Note how convection stretches northwestward into the confluent zone similar to its behavior yesterday. Its remains to be determined how the southerly deep-layer vertical wind shear on the western flank of PGI38L affects its evolution in the coming hours and beyond. The pouch sweet spots are progged to move westward in the coming days, with the UKMET and ECMWF taking the pouch on a westward track, while the GFS (NOGAPS) have a more southerly (northerly) track.
At ~1830 UTC 7 Sep, the base reflectivity imagery from the San Juan, PR, WSR-88D shows a region of convection passing south of STX in association with PGI38L (image 10). After a temporary intensification of the convection - and the appearance that PGI38L was getting its act together - it appears that cloud tops are beginning to warm yet again.
At 1200 UTC 7 Sep, PGI39L was located near 20°N 33°W, PGI41L near 15°N and 22 W, and PGI42L near 14°N 5°E. Unlike yesterday morning, PGI39L was not associated with any well-organized outbreak of convection (image 11), while PGI41L was associated with a mature organized outbreak of convection. PGI42L was associated with a strong westward-propagating linear convective system over the African continent. Further comments on the evolution of PGI42L can be found in the extended range section. Comments on potential development in the eastern Caribbean within the next couple of days will also be commented on at the end of the extended range discussion.
DAY 1 (Next 24 hours) Outlook:
Hermine: The NHC forecast follows:
FORECAST POSITIONS AND MAX WINDS INITIAL 07/1500Z 28.3N 98.2W 35 KT 12HR VT 08/0000Z 30.1N 99.2W 25 KT...INLAND 24HR VT 08/1200Z 32.3N 99.9W 20 KT...POST-TROP/REMNT LOW 36HR VT 09/0000Z 34.9N 99.3W 20 KT...POST-TROP/REMNT LOW 48HR VT 09/1200Z 37.6N 97.7W 20 KT...POST-TROP/REMNT LOW 72HR VT 10/1200Z...DISSIPATED $$ FORECASTER PASCH/CANGIALOSI
The biggest threat posed by Hermine's slow northward movement will be from heavy rain-induced flooding along and to the east of the storm track in the region where the feeder bands in deep southeasterly flow are located and along the developing PRE to the northeast of the center of Hermine.
ITCZ Tropical Disturbance: The ECMWF 24 h forecast verifying 1200 UTC 8 Sep generates a 700 hPa trough and an embedded cyclonic disturbance marked by a weak layer-mean 925-850 hPa vorticity maximum along the ITCZ near 11°N and 59°W (not shown). This vorticity maximum is embedded in a region of high TPW (57-60 mm). The corresponding GFS forecast produces a disturbance with a somewhat weaker vorticity maximum farther to the east (11°N and 58°W), but in a slightly drier environment (TPW ~51-54 mm; not shown). The important difference between the two models is that the developing ITCZ disturbance in the ECMWF model is in a ~5 mm higher TPW environment.
In the next 24-h, PGI38L is progged to continue to move westward through the northern Caribbean. The ECMWF ensemble forecast shows some variability in north-south position, but in general the disturbance position is progged to be located near 68°W (image 12). The deterministic GFS takes PGI38L on a more southern track (not shown), and keeps it weak. Note the shallow nature of the system continues, with the pouch and OW parameter being best defined at 925 hPa - rapidly weaking upward (image 13).
In both the GFS and ECMWF, PGI39L is forecast to track northwestward and slowly dissipate, while PGI41L will move to near 22°W and intensify (image 12).
DAY 2 (24 - 48 hours) Outlook:
Hermine: The storm and associated PRE will likely continue to produce inland flooding in parts of the Plains and Midwest as it moves slowly northward and then north-northeastwards.
ITCZ Tropical: The GFS forecast from 1200 UTC 7 Sep loses this disturbance whereas the corresponding ECMWF forecast shows a north-south elongated disturbance defined by a layer-mean 925-850 hPa vorticity maximum that extends from 10°N and 62°W to 15°N and 60°W (image 9). This vorticity maximum remains embedded in a high TPW region (~60 mm; image 14).
In the day 2 forecast, PGI38L is progged to move acress the northern Caribbean to near 71°W in the ECMWF ensemble (image 15) . Conversely, the deterministic GFS takes PGI38L on a more southerly track, and many of the statistical models (e.g., SHIPS) intensify the system - differing from the dynamical model solutions that depict continued weakening (not shown).
By 1200 UTC 9 Sep, the GFS and ECMWF have weakened PGI39L to a remnant low-level vorticity center near 19°N 40 W, while PGI41L moves to near 14°N and 28°W and intensifies to a TC (image 15). It is extremely likely that PGI41L will undergo genesis well before reaching the PREDICT domain (40°W).
Hermine: The ECMWF and GFS models from 1200 UTC 7 Sep continue to forecast that the remnant layer-mean 925-850 hPa Hermine vorticity maximum track along a broad anticyclonic path from Texas to the Ohio Valley. Numerous mesoscale convective systems and very heavy rains are likely to occur along the track of the remnant Hermine vorticity maximum as it moves clockwise around a subtropical anticyclone located over the Southeast. Diabatically driven divergent outflow from the remnants of Hermine will help to advect lower values of potential vorticity northeastward and strengthen a jet in northwesterly behind a deepening trough over the northeastern US and the western Atlantic between 48-72 h.
ITCZ Disturbance: By 1200 UTC 10 Sep, the ECMWF is developing a slow-moving weak tropical disturbance/depression over the central Leeward Islands near 14°N and 62°W (image 16). By 1200 UTC 12 Sep, The ECMWF has intensified the aforementioned disturbance into a tropical cyclone that is located near 16°N and 70°W (not shown). No such tropical disturbance is apparent in the corresponding GFS forecasts. A possible significant difference between the two model runs is that the GFS is generally lower TPW values (by 5-10 mm) in the ITCZ where the disturbance forms. To help assess the forecast discrepancy between the operational ECMWF and GFS models with regard to the aforementioned ITCZ disturbance, note that quite a few of the ECMWF ensembles show this ITCZ disturbance in the 700 hPa vorticity and OW fields as evidenced by the 96 h ensemble forecast verifying 1200 UTC 11 Sep (not shown).
PGI38L: In the long range, the ensemble forecasts continue to take PGI38L on a westerly track into the western Caribbean (not shown) where Kerry Emanuel's maximum potential intensity is a maximum (not shown). The intensity forecast from the ensemble members range from a weak low pressure center to a strong Cat 4 storm by 120-h (not shown) - representing a substantial divergence in the ensemble member forecasts. The pouch products indicate that PGI38L will continue southwestward - rather than westward in the ensemble - and weaken the system significantly by 120-h. Beyond 120-h in the 1200 UTC 6 Sep GFS and ECMWF (not shown), the disturbance completely dissipates over the western Caribbean.
In the extended range, Ryan Torn's HWRF/EnKF 96-member ensemble shows that PGI38L continues on a westward course reaching near 77-79°W by 72-h (image 17). The ensemble indicates that while most of the solutions have a weak system - consistent with the global models - the southern most members (over and south of Jamaica) appear to redevelop PGI38L into a weak to moderate tropical storm. The ECMWF ensemble is consistent with the relative uncertainty in forecast position through 72-h (not shown).
The HWRF/EnKF ensemble for PGI39L (image 18) shows dissipation by 72-h - consistent with the GFS and ECMWF deterministic model runs. Meanwhile, the HWRF/EnKF ensemble forecast for PGI41L (image 19) shows some uncertainly in east-west position at 72-h, with a fairly large spread in intensity - ranging from a weak low to a minimal hurricane. The key point for PGI41L is that tropical cyclogenesis appears to occur well before the system reaches the PREDICT domain (40°W).
The ECMWF deterministic run moves PGI42L off the African coast by 1200 UTC 11 Sep as an intense easterly wave (not shown). It appears that tropical cyclogenesis is forecasted in the immediate vicinity of the west African coastline - much like PGI41L in that genesis is forecasted to occur well before the system reaches 40°W.