Forecast for 1600 UTC 8/27/2010:
Synoptic Overview:
Five systems are of interest today—Hurricane Frank south of Baja California, intense
convection over the Gulf of Mexico, Hurricane Danielle, TS Earl, and PGI36/AL97L, all in the Atlantic
(1). Important flow features are anticyclonic flow over the Gulf of Mexico, a middle latitude trough
between the U.S. East Coast and Danielle, and another middle latitude trough between Danielle and
Earl. Water vapor imagery shows large values of PW over much of the Gulf, Caribbean, and southern
Atlantic. The exceptions are dry air from Africa that is streaming westward and partially surrounding
the Atlantic TCs. Considerable dry air also is located over the northern Atlantic. This dry air plunges
southward behind two middle wave cyclones. The MODIS aerosol product shows relatively high
aerosol optical depth associated with the dry air from Africa. Thus, the dry air in the vicinity of the TCs
also is aerosol laden. The aerosol region dissipates as one heads west into the Caribbean.
Hurricane Frank
The 11 AM EDT NHC fix on Frank consists of the following:
8:00 AM PDT Fri Aug 27
Location: 18.8°N 112.0°W
Max sustained: 75 mph
Moving: WNW at 6 mph
Min pressure: 987 mb
Frank has begun to weaken and further weakening is expected (25). Although the vertical shear
remains approximately constant during the forecast period, the TC is beginning to move over
considerably colder water. The current SST is ~ 27 C, but when the Global Hawk (GW) flies near it on
Saturday, the SSTs are expected to be ~ 3 C colder. The model consensus forecast for the intercept
period (~ 26 h into the 1200 UTC forecast) calls for winds to be reduced to 45-50 kt. The storm is
projected to move slowly northward.
Gulf of Mexico Convection
A stationary frontal system extends from the Louisiana coast southwestward over the Gulf of
Mexico, A large area of intense convection formed over this area last night and is persisting throughout
the morning (26). Storm tops actually are higher in this region than in Frank. NHC has not mentioned
this area in their discussion, and development seems unlikely due to proximity to land and other
factors. A smaller, weaker area of deep convection is located just offshore of Brownsville. This region
is categorized as a PGI. No development is expected.
Tropical Storm Danielle
The 11 AM EDT NHC fix on Danielle contains the following:
11:00 AM AST Fri Aug 27
Location: 26.9°N 59.8°W
Max sustained: 135 mph
Moving: NW at 12 mph
Min pressure: 946 mb
Danielle has a well defined eye, and is producing well defined upper level outflow (4).
Although the storm is encompassed by dry, aerosol laden Saharan air (10), it does not seem to have
entered the core region and impacted the storm. The models show that Danielle is expected to begin
weakening as she moves northward and then eastward over the northern Atlantic, encountering stronger
wind shear and colder SSTs (11, 12).
PGI 36/ AL 97
PGI 36 moved off the African coast yesterday and produced a large area of convection during
the evening hours—including a large squall line that propagated toward the southwest (18). The
system has no closed circulation; however it already has considerable vorticity at 850 mb. The system
is expected to develop further in the next 24 hours—following the same basic path as Danielle and Earl
(20). The ECMWF is considerably more aggressive in producing development than is GFS (not
shown).
PGI 36 is the latest in a train of waves exciting Africa. These waves are following very similar
paths and are being influenced by the Low Level Jet. Long range models indicate that there a may be a
several day gap before the next system exits Africa. These are long range progs, and may have little
skill.
Features of Interest:
Dust and Dry Air in the Atlantic Basin:
As of 1200 UTC, most of the Tropical east Atlantic was dust-laden above climatology oceanic
values (7). The dusty area covered the area bounded by 50W between 10N and 20N. The exceptions
being inside the cloud shields of TC Earl and PGI-36L/AL97L. North of 15N and west of 50W the
basin is also very dry (6). This area is under the influence of the mid-Atlantic ridge and ridging patterns
aloft, and contributed to the baroclinic zone supporting an enhanced African Easterly Jet. Of note is the
fact that MODIS and GEOS-5 suggest that dust and dry air are wrapping around TC Earl. GEOS-5
expects the track of TC Earl and its cyclonic flow to bring low levels of dust over the central Caribbean
starting on Saturday 8/28 near 0000 UTC. The dust is forecast to spread as far west as the East coast of
Nicaragua by 0600 UTC on 9/1, but will soon after dissipate as Earl turns northward, and the dust
source region is cut off.
Hurricane Danielle/PGI-31L:
Hurricane Danielle strengthened to a major hurricane overnight and is now a Category 4 hurricane with
a well-defined eye and surrounding ring of cold cloud tops (8). Danielle is currently in a moderately
low shear environment (5-10 m s-1) (9) and, as opposed to yesterday, there appears to be entrainment of
some dry air at low to mid-levels in the outer bands (10).
The models are in good agreement that Hurricane Danielle will continue to move NW for the next 24
hours, then turn north, and finally be absorbed by a trough exiting the eastern United States at 48 hours
from latest runs initialized at 1200 UTC 27 August 2010. This will accelerate the motion of Danielle to
the NE, with no threat of impact to Maine or coastal regions of Canada (11). SHIPS has shear
continuing to decrease for the next 24 hours, however at 48 hours, shear increases significantly to near
30 m s-1 with interaction of the trough. In terms of intensity, less than half of the models keep Danielle
as a Category 4 hurricane out to 36 hours, but most have the hurricane weakening starting at 24 hours.
Any further intensification of Danielle is limited due to the possibility of further entrainment of dry air
and the eventual travel into to an environment with increased wind shear (12).
The increased proximity of the trough will increase shear and absorption of Danielle by 72 hours will
usher in the beginning of extratropical transition. In addition, after 72 hours Hurricane Danielle will
transition into an environment with SSTs colder than 26°C, which will contribute to additional
weakening.
Hurricane Earl/PGI-34L:
As of the 5am advisory, the NHC has Tropical Storm Earl located near 15.9N/41.6W, minimum
sea level pressure of 1003hPa, maximum sustained winds of 40kt, and moving west at 15kt. As of the
11am advisory, the NHC has Tropical Storm Earl located near 15.7N/43.6W, with maximum sustained
winds of 40kt (13). The convective activity was more concentric this morning, unlike yesterday when
the convection was seemingly elongated north/south (14, 15). The convection is very much cyclic and
not deep in all quadrants (14, 15). As of 1345UTC, the main convection was to the west with mostly
anvil in other quadrants (14, 15). The tropical storm is very much disorganized. Overall, dust does not
seem to be as present, but dry air (perhaps from subsidence) is prevalent. There are a number of upperlevel
features near Earl; Danielle to the northwest, an upper-level dry cyclonic circulation immediately
to the west and an upper-level trough to north (4). The upper-level trough to the north is the main
ventilation flow for Danielle as an impressive stream of high water vapor is emerging from the
anticyclonic outflow, and entering the western side of the trough (3). For Earl, a stream of moisture is
emerging from the northern outflow of Earl, and flowing into the downstream portion of the trough (4);
the flow looks almost diffluent over Earl. The deep vertical wind shear (850-200 hPa) is weak (5-10kt)
over Earl (5). All signs point to gradual intensification this weekend as it gets farther west from the
upper-level trough, maintains relatively favorable upper-level winds and increasing SSTs. The model
spread of intensification is shown in (16). The consensus (from 1200UTC today) is for hurricane status
sometime Sunday morning, a CAT1 hurricane through Monday with intensification to CAT2 by
0000UTC Tuesday, and finally perhaps a major hurricane Wednesday 0000UTC. Looking forward, Earl
may be in range from Florida late Wednesday/early Thursday as it maintains a northwestward track.
The consensus track for 0600UTC is as follows:
28/0600UTC: 47W/17N; 29/0600UTC: 55W/18N; 30/0600UTC: 58W/19N; 31/0600UTC:
63W/21N; 01/0600UTC: 66W/24N
The consensus track for 1200UTC is as follows (17):
28/1200UTC: 49.5W/17N; 29/1200UTC: 55.5W/17.5N; 30/1200UTC: 60W/18.5N;
31/1200UTC: 63W/20.5N; 01/1200UTC: 66W/25N
The GFS 0600UTC initialization forecast for MSLP is as follows:
27/1200UTC: 42W/16N; 27/1800UTC: 44W/17N; 28/0000UTC: 46W/17N; 28/0600UTC:
50W/17N; 28/1200UTC: 51W/17N; 28/1800UTC: 53W/17N; 29/0000UTC: 55W/17N;
29/0600UTC: 56W/17N; 29/1200UTC: 57W/17N; 29/1800UTC: 58W/17N; 30/0000UTC:
59W/18N; 30/0600UTC: 60W/18N; 30/1200UTC: 61W/18N; 30/1800UTC: 63W/19N;
31/0000UTC: 64W/20N; 31/0600UTC: 64W/21N; 31/1200UTC: 65W/21N; 31/1800UTC:
66W/22N; 01/0000UTC: 66W/23N; 01/0600UTC: 67W/23N; 01/1200UTC: 68W/24N;
01/1800UTC: 70W/25N; 02/0000UTC: 71W/26N; 02/0600UTC: 72W/27N; 02/1200UTC:
73W/28N; 02/1800UTC: 73W/29N; 03/0000UTC: 73W/30N; 03/0600UTC: 70W/33N
The ECMWF 0000UTC initialization forecast for MSLP is as follows:
27/0000UTC: 40W/15N; 28/0000UTC: 47W/17N; 29/0000UTC: 53W/18N; 30/0000UTC:
59W/19N; 31/0000UTC: 61W/21N; 01/0000UTC: 63W/26N; 02/0000UTC: 65W/33N
HWRF has a CAT2 by 0600UTC Monday and CAT3 by 1800UTC Monday so perhaps a potential for
rapid intensification on Monday in the HWRF. The track is similar to consensus. GFDL is more
aggressive; at hurricane by 1800UTC Saturday and a CAT2 by 18Z Sunday.
Invest AL97L/PGI-36L:
On the morning of 8/27 PGI-36L was named Invest 97L by the National Hurricane Center. The
easterly wave which moved off the African coast near 0000 UTC on 8/26 now contains an analyzed
surface low pressure center located at 22W/13N. It is unclear from ASCAT surface winds whether there
is a closed circulation at this time. Infrared imagery from METEOSAT shows two distinct areas of
convective activity. The primary cluster has formed a squall-line over the last 12 hours from 30W/13N
extending southeast to 25W/8N. Cloud tops in this line were near -65 C this morning. The second
convective center was near 26W/13N. Both of these are West of the cyclonic vorticity center at 850 mb
according to CIMSS analysis. (18). TPW is high in the near storm environment with values between 60
and 65 mm according to AMSU. The Saharan Air Layer is currently located more than 5 degrees to the
north of the system according to TPW and MODIS aerosol optical depths. GEOS-5 model forecasts for
dust particle mass suggest that the dusty airmass should stay well to the north of this system as a
depression tries to form. The convection is located over a fairly warm ocean surface with temperatures
just over 28 degrees C. the system should enter warmer sea surface temperatures as it follows the
forecast track guidance. Deep layer shear over AL97L is northeasterly with moderate values near 10-12
kt. The wave is just south of a strong upper layer ridge. This wave is part of a very active African
monsoon flow. The African Easterly Jet axis is located just to the north of AL 97L and extends well to it
east, reinforcing cyclonic vorticity within Tropical Storm Earl as well (19). The above listed factors add
up to a very favorable environment for development of this system. GFS and ECMWF both develop
the Okubo-Weiss vorticity at 700 mb within this system beyond the threshold critical for development
(2 x 10-9 s-2). GFS waits 84 hours for significant development. ECMWF is more bullish and the OW
threshold is met by today at 1200 UTC, after this point the vorticity and OW quickly increase off the
provided scale (20). The ECMWF seems to have a more accurate handle on the environmental
conditions as its initial shear values are closer to what is currently analyzed. Model guidance for
intensity suggests that AL97L will eventually become a hurricane, but that this will slowly happen over
the next 72-96 hours (21).
Hurricane Frank:
Frank is still a minimal category one hurricane with winds of 75 mph as of 1500 UTC August
27, 2010. At 15 UTC, Frank was located at 18.8N/112.0W and moving to the WNW at 6 mph.
Satellite imagery indicates that the deep convection associated with Frank has decreased over the past
12-24 hours (22). Frank is currently located in a region with SSTs around 27°C and relatively low
wind shear. However, the storm’s forecast track will take Frank into much cooler waters over the next
couple days (23). In fact, there is good model consensus that Frank’s winds will decrease to tropical
storm force within the next 24 hours.
The mid-latitude trough currently off the U.S. Pacific coast is expected to deepen and force Frank to
begin curving northward in the next 24-36 hours (24). At that time, Frank will move into a region with
SSTs cooler than 26°C. Therefore, the models agree that Frank will continually weaken over the next
several days (25). Currently, the Global Hawk is scheduled to depart for Frank on Saturday at 1300
UTC. When the Global Hawk reaches the storm around 1600 UTC Saturday, Frank will most likely be
a tropical storm with winds of 45-55 knots. Frank is expected to dissipate by Tuesday, and the remnant
low will continue drifting slowly northward towards Baja California.
The NHC forecast track for MSLP within Frank is as follows:
28/0000UTC: 19.3N/112.7W; 28/1200UTC: 20.0N/113.6W; 29/0000UTC: 20.8N/114.0W;
29/1200UTC: 21.2N/114.0W; 30/1200UTC: 22.0N/114.0W; 31/1200UTC: 22.5N/114.0W (remnant
low)
For images associated with this forecast, please see the attached PDF document (below).
Forecasters: Martin, Zawislak, Fuelberg, Reynolds, Reinhart
