Forecast for 1800 UTC 8/13/2010:
PGI25L returned to identification today after being discontinued yesterday (2). This system is at the northern extent of a wave located near 15N/62W (1). Little to no convection is present in the vicinity although it is embedded in a region of relatively high total precipitable water (3). Global models do not persist the pouch much beyond 36-48 hours as it moves further north into much less favorable environmental conditions (10). Based on observations and model forecasts, no development of this system is expected.
PGI27L is identified by a mid-level trough located at 20N/40W is currently associated with a weak shift in the winds but is not convectively active. As the disturbance continues to move eastward both the GFS and ECMWF do continue representing weak vorticity maxima. Convection and moisture are forecast to increase beyond 24 hr although none of the model guidance significantly intensifies this system. The disturbance became well defined on 8/9, 0000 UTC near 17.5W (14). It has been contending with dry air for the duration of its life. At 8/9 0000 UTC a pouch was identified at 12N, 31W. Currently, there is some cloud cover but no deep convection associated with the system (15) which as of 1200 UTC was located at 40W/20N. A weak shift in the winds is visible in the satellite derived winds. With a more robust shift and trough axis visible in the GFS analyses. Although, the system is very dry currentlyboth the GFS and ECMWF are tracking a weak vorticity maxima westward so monitoring of the system should continue (14). In addition, the GFS and ECMWF are predicting a slow increase in convection and moisture over the next 72 hr (16, 17). By 72h (8/16 1200 UTC) a weak trough axis is forecast by the GFS to be at 60W/20N and experiencing between 10-20 kts of shear.
PGI-28L is currently located at 15N/12W. There is intense convection associated with this disturbance (4); however, the disturbance is forecast to move northeastward over the next 72 hr associated with a weakness in the east Atlantic mid-level ridge. Beyond 72 hr some part of the disturbance may continue moving westward as the ridge builds back in but should not develop due to the high shear and dry air surrounding the system. The disturbance became well defined on 8/9, 0000 UTC near 15E. Over the last few days the African easterly wave has developed a strongly rotating center of deep convection currently located at 10W/ 17N (15). A weakness in the East Atlantic mid-level ridge near 12W/30N is causing all of the models to predict a northwestward track of the system over the next 72 hr. In addition, moisture is forecast to decrease as the disturbance encounters subsidence associated with the upper-level trough producing a weakness in the ridge. Beyond that time the GFS is predicting a rebuilding of the mid-level ridge. Although the majority of the vorticity associated with this disturbance appears to recurve northward in the guidance (18) there is some westward propagation of vorticity in the GFS which may be partly associated with this disturbance (14). However, all guidance is suggesting very dry air and high shear beyond 72 hr for the portion of this disturbance that does continue westward into the Atlantic.
PGI-29L/Ex-TD05 is located on the Gulf coast of Mississippi near 88.4W/30.7N and has widespread deep convective showers associated with it, and although a well-defined low-level circulation is no longer evident, there is a broad, E-W elongated low- to mid-level trough/vorticity maximum with an upper-level anticyclone centered over the southeast U.S (5). Therefore, vertical shear is fairly low over the main convective region of the disturbance. The model consensus is for the pouch to track northward over land before re-curving southward towards the Gulf (6,7). A trough currently over the northwest U.S. will lift as it tracks towards the east coast, leaving 29L in generally weak anticyclonic flow in the south central U.S. The consensus is for some model intensification (albeit weak) once the disturbance re-emerges into the Gulf at 84-96 hours. The consensus track is as follows: 14/0200 UTC: 86.9W/31.6N; 14/1400 UTC: 86W/32.3N; 15/0200 UTC: 85.2W/32.3N; 15/1400 UTC: 84.8W/32.3N; 16/0200 UTC: 84.8W/31.2N; 16/1400 UTC: 85.7W/30.3N; 17/0200 UTC: 87.2W/29.2N; 17/1400 UTC: 88.9W/29.5N. In terms of mission interest, a Global Hawk test flight on Tuesday/Wednesday in the Gulf may coincide with the re-emergence of the disturbance in the Gulf.
At 1200 UTC on 8/13/2010, low total Precipitable water values were present in most of the tropical east Atlantic north of 10N from 50W to near the African west coast (8). A trailing 6-hour composite of AMSU TPW shows values below 40 mm through the majority of this region (8). The GOES water vapor channel suggests that dry air is present at the upper levels as well. The dry east Atlantic air mass is largely dust-free except for a very dusty plume exiting West Africa north of 20N and a less optically thick aerosol optical thickness maxima extending from 20N/50W southeast to 10N/30W. Part of this second feature includes the location of PGI-27L. (11). It is possible that the dry air is currently suppressing convection in PGI-27L, however the vorticity structure of PGI-27L is fairly unimpressive and the dynamics could be contributing to the dearth of convection as well. Because there is little convection or even cloud cover in PGI-27L, dust microphysics are not currently an important factor in this system. Further to the east, the environment over Africa is currently very moist. METEOSAT water vapor imagery shows impressive upper level moisture near the southern and northern easterly wave tracks. (13). This includes the environment of PGI-28L. Unfortunately AMSU and MODIS products are not available over this area, but PGI-28L is entering the Sahara desert and is likely picking up high levels of dust from the surface. A 1200 UTC sounding from Dakar, Senegal shows a fairly moist (dew point temperatures > 20 C) column below 600 hPa (9). Regardless of the actual dust content in PGI-28L, convection remains vigorous.
Dry Air is expected to remain a dominant feature over the east Atlantic for the next several days while moving eastward into the central Atlantic. Analysis of the NCEP global model ensemble shows a probability of less than 10% that the relative humidity will reach 85% or greater at 850 hPa over the east/central Atlantic by 8/15 at 0000 UTC, and a similar situation exists for 8/16 at 0000UTC (10). The GEOS-5 dust forecast (12) the emerging dust plume from West Africa, and advects it into the east Atlantic over the next several days. This dusty air mass is forecast to dissipate by the time it reaches the lesser Antilles, however areas in the central Atlantic north of 15N should experience high dust levels between now and the morning of 8/18 (12). The combination of high dust and low relative humidity values will likely act to suppress convective activity in PGI-27L.
PGI-28L Is forecast to take a Northwest track as it exits the African continent. As the system crosses the Sahara desert, dust will continue to be a factor in the convective activity within PGI-28L. Ensemble forecasts from NCEP predict that 700 hPa RH values should remain above 70% at the predicted pouch location until 8/15 at 0000 UTC (10). However, the likelihood of RH remaining above this threshold after the pouch exits the coast drops significantly. The GEOS-5 forecast for dust AOT suggests that this area will be highly dust loaded at the point where PGI-28L leaves the coast as well (12). As the convection within PGI-28L tries to remain organized over the ocean, dust may play a role in the microphysics and the mesoscale convective organization.
Forecasters: Inglish, Zawislak, Janiga, Martin
For static images associated with this forecast, please see the attached PDF document.
