At 1545 UTC 9 Sep, the remnants of Gaston (PGI38L) were located near Jamaica (image 2) . The low-level vorticity structure continues to be very disorganized relative to previous days. The ECMWF and GFS forecasts from 0000 UTC 9 Sep are no longer able to track a pouch for Gaston after 1200 UTC 12 Sep and 0000 UTC 11 Sep, respectively. A few models (e.g., the NCEP/NAM and the NCAR/WRF) forecast that Gaston will get its "second wind" during the 72-96 h time period and approach the Yucatan peninsula as a TC in the 120-144 h forecasts from 0000 UTC 9 Sep.
At 1200 UTC 8 Sep, PGI44L was embedded in a broken area of convection that extended from13°N and 63°W to 7°N and 52°W (images 1 and 2). This convection marked the ITCZ boundary that separated westerly from easterly flow. Very moist air north of the ITCZ boundary continued to edge poleward as observed in the 72 h MIMIC-TPW loop (not shown) and as evidenced by the TPW value of 60.2 mm reported from the Barbados (78954, TBPB) sounding for 1200 UTC 9 Sep (image 4).
Air sources on either side of the aforementioned ITCZ boundary were computed from a backward trajectory analysis from 0600 UTC using the NOAA HYSPLIT trajectory model. The trajectory analysis indicates that air parcels north of the ITCZ boundary at 12°N and 61°W arrive from the east and ascend sharply after 1800 UTC 7 Sep to reach the 1000, 2000, and 3000 m levels (image 5). South of the ITCZ boundary, air parcels arrive at 8°N and 60°W from the southwest and ascend sharply after 0000 UTC 8 Sep to reach the 1000, 2000, and 3000 m levels (image 6). The computed northeastward turn of the air parcels arriving at 8°N and 60°W reflects the westward development of an upper-level anticyclone near the northeast coast of South America in the 48 h ending 0000 9 Sep (not shown). These trajectory analyses also collectively indicate that the ITCZ boundary near the location of PGI44L is characterized by converging moist air streams that ascend sharply below 700 hPa. A satellite water vapor image with superimposed satellite-derived low-level winds, the corresponding 850 hPa vorticity field, and surface land, marine, and buoy observations for 1745 UTC 9 Sep for PGI44L shows that the ITCZ region in the vicinity of PGI44L is very moist and is characterized by higher vorticity than adjacent regions to the north and south (image 7).
At 1200 UTC 9 Sep PGI38L (Gaston) was located just southeast of Jamaica near 17N 75W and was moving west at 10 knots. Most of the global models weaken the system however the NAM shows some modest intensification to a TD prior to landfall in Central America in 84 hours. This solution is being rejected at this time.
PGI39L: This system has appeared to have undergone a split (as suggested by the ECMWF 2 days ago). A primary center is located near 20N 40W. Convection has decreased in general over the last 24 hours and is confined to the north of the apparent low-level center. Shear of 25-35 knots from the west is impacting the system. 200 mb winds are relatively light at 10-20 knots however the low-level steering is to the west-northwest at 15-20 knots thus much of the shear is being driven by the low-level flow. A second center is located 17N 41W. Pouch tracking products show a position in between these two positions indicating some uncertainty in the models as to the exact location (image 8). While the more southern center is located in a lower shear environment substantial mid-level dry air has overspread the system and TPW products show that dry air in the lower levels surrounds the center (image 9).
PGI41L (Igor) was located just south of the Cape Verde Islands moving northwest at 6 knots. Over the last 24 hours PGI41L has absorbed PGI42L which was located to it’s north. The environment around PGI41L continues to be characterized by unusually high TPW. However easterly shear has resulted in the low-level center becoming exposed to the east of the convection. Convection is now located about 60 nm to the west of the center. CIMSS cloud motion vectors indicate a large system with winds in the low levels (800-950 mb) of 30 knots (image 10).
PGI43L: Center of the pouch is estimated near 10N 5W over Africa. Satellite loops show pulsing convection with this system and upper level feature track winds hint at some outflow. A pouch is identifiable in both the ECMWF and GFS, though the ECMWF depiction is the stronger of the two.
DAY 1 (Next 24 hours) Outlook:
PGI44L: The pouch analysis derived from the ECMWF forecast from 0000 UTC 9 Sep shows that PGI44L will move slowly to the northwest and be situated near 13.0°N and 63.0°W at 1200 UTC 10 Sep (image 11). The 700 hPa relative vorticity and OW parameter are forecast to increase significantly by 1200 UTC 10 Sep (image 12).
PGI39L: The northern system will continue moving west-northwest and slowly weaken under continued shear and dry air entrainment. The southern split will move nearly due west with little change intensity.
PGI41L: Igor is forecast to continue northwest in the short term then turn back to the west-northwest during the next 24 hours. ECMWF pouch forecasts show several OW centers which do not form a monopole until after 24 hours. Also the 700 mb vorticity remains steady during this time suggesting a nearly steady state for the next 24 hours. GFS on the other hand shows steady intensification in agreement with most of the other statistical and dynamic models. The exceptions are the GFDN and NOGAPS which also show a steady state for 24 hours.
(image 12) http://euler.atmos.colostate.edu/~vigh/guidance/northatlantic/ar chive/aal11_2010090912_intensity_early.png
Igor: According to the NHC at 1500 UTC 9 Sep 2010:
FORECAST POSITIONS AND MAX WINDS INITIAL 09/1500Z 14.7N 24.8W 35 KT 12HR VT 10/0000Z 15.0N 26.3W 35 KT 24HR VT 10/1200Z 15.8N 29.0W 40 KT 36HR VT 11/0000Z 16.4N 32.0W 50 KT 48HR VT 11/1200Z 16.7N 35.0W 60 KT 72HR VT 12/1200Z 17.3N 40.4W 70 KT 96HR VT 13/1200Z 18.5N 44.5W 80 KT 120HR VT 14/1200Z 20.0N 48.5W 85 KT
PGI43L: System will remain over land during this forecast period and significant intensification is not expected.
DAY 2 (24 - 48 hours) Outlook:
PGI44l: The pouch analysis from the 0000 UTC 9 Sep ECMWF 60-h forecast from 0000 UTC 8 Sep suggests that PGI44L will be located near 14.5°N and 68.5°W at 1200 UTC 11 Sep (image 13). Little further increase in the magnitude of the 700 hPa vorticity and OW parameter is indicated between 36-60 h in this pouch analysis (image 13). Considerable model forecast uncertainty in the location and strength of PGI44L is apparent in the 60 h ECMWF and GFS forecasts verifying 1200 UTC 11 (not shown). This forecast uncertainty is conveyed through Sharan Majumdar's 60 h ECMWF ensemble analysis of 700 hPa vorticity and the OW parameter overlaid on the PGI44L forecast pouch track from 0000 UTC 9 Sep (image 14). Ryan Torn's 36-h 96-member ensemble forecast verifying 0000 UTC 11 Sep also shows considerable uncertainty in the forecast position and intensity of PGI44L with some ensemble members indicating that PGI44L with be a TC by that time (image 15).
PGI39L: ECMWF loses the system at the end of the forecast period while GFS maintains a weak system which turns to the southwest. However GFS is not initializing the secondary (southern) center and it is this feature which eventually will move southwest. The northern circulation will likely move west-northwest and dissipate.
PGI41L: Igor is expected to recover from the current wind shear and strengthen steadily during this period as the system turns more toward the west-northwest. By 48 hours the center will have reached 35W.
PGI43L: The system will reach the West coast of Africa at the end of the forecast period.
PGI44L: The ECMWF, GFS, and UKMET models all continue to move PGI44L westward with varying intensities and propagation speeds. The ECMWF 120 h forecast verifying 0000 UTC 14 Sep brings PGI44L to near 17°N and 79°W. The GFS 120 h forecast verifying 0000 UTC 14 Sep places PGI44L near 15°N and 74°W, much slower that the ECMWF, and also weaker. The UKMET model 120 h forecast, while slower with PGI44L forecast to be located near 16°N and 61°W, is the most bullish of the three models with its intensity forecast. The corresponding Majumdar 120-h ensemble forecasts likewise indicate considerable variability in the location of PGI44L and the magnitudes of the 700 hPa vorticity and OW parameter (not shown).
PGI39L: System will most likely be dissipated at this time. The southern center may need to be watched however eventual spin down and dissipation is likely as well unless convection can develop in the dry air that surrounds the system.
PGI41L (Igor): The ECMWF ensemble 84 hour forecast shows reasonably high confidence that Igor will reach 40W at 1200 UTC 12 Sep and may be a hurricane (image 16). Intensification is shown by all guidance including SHIPS which maintains a moist environment and very favorable deep layer shear. There is potential for a period of rapid intensification as the system reaches deeper warm water in a favorable environment.
PGI43L: The system will reach the coast around 1200 UCT 11 Sep. As with just about every wave reach the coast during the last few weeks the model are bullish on almost immediate intensification.