Teenager. Weather aficionado. Soccer fan. Realist. Posts subject to sarcasm. Goal: National Hurricane Center.
By: TropicalAnalystwx13 , 5:17 AM GMT on August 02, 2013
Gil, the seventh named storm and fifth hurricane of the 2013 Pacific hurricane season, continues its west-northwest track across the eastern Pacific basin this morning. As of the latest advisory from the National Hurricane Center, Gil was positioned within 20 nautical miles of 14.4°N 126.4°W, about 1230 miles west-southwest of the southern tip of Baja California. Maximum sustained winds were up to 85 mph, the minimum barometric pressure was down to 985 millibars, and the system was tracking in the aforementioned direction at 14 mph. Despite the increase in intensity, Gil has changed very little in organization since this time yesterday. The cyclone continues to display a central dense overcast, though convection in the eastern semicircle of the storm seems to have been eroded somewhat by stable air. Outflow is fair in all four quadrants, except to the north and south where it is excellent. A series of microwave passes reveal that the inner core of Gil has changed a bit over the subsequent 24 hours, most notably a substantial increase in the size of the storm's eye. This feature has been visible intermittently throughout the day, but was recently obscured by a large burst of convection directly atop the center. The satellite intensity estimate from UW-CIMSS-ADT continues to be conservative with the strength of Gil, currently reading T3.4/53 kt. The 00z intensity estimates from SAB and TAFB were T4.5/77 kt and T5.0/90 kt, respectively. A blend of these data would typically give an intensity of 85 knots; however, given the cyclone's ragged appearance on satellite imagery, the NHC has opted to be conservative for the time being. As evidenced by an afternoon ASCAT pass, Gil is a very small system. Tropical storm-force winds extend outward up to 60 miles from the center, and hurricane-force winds extend outward up to 25 miles.
Figure 1. Infrared satellite imagery of Hurricane Gil.
Forecast for Gil
The intensity forecast for Gil remains a bit complicated. The rate of intensification observed after formation and into yesterday indeed slowed as stated it may. Despite seemingly favorable atmospheric conditions, the storm has failed to intensify steadily as originally predicted. Wind shear as analyzed by SHIPS is a whopping 1 knot and sea surface temperatures are near 28°C. However, mid-level relative humidity values have dropped to the upper 50s (%). It is possible that the system is occasionally ingesting dry air, which would be supported by a lack of convection in one quadrant of the storm. Fortunately for Gil, it is expected to pass through a region where values rise to the mid-60s over the next 72-96 hours. All things considered, the intensity forecast has been lowered from yesterday's prediction but continues to show steady intensification over the next 48 hours at least. A significant shift in the amount of time Gil spends in a favorable wind shear environment has occurred during the past few model suites today; as a result, both the SHIPS and LGEM show the system becoming a Category 2 hurricane at peak. I have no reason to disagree. Between 48 and 72 hours, the hurricane is expected to cross the sub-26°C isotherm and steadily weaken instead. My forecast follows a steady weakening trend throughout the forecast period, though it should be noted that a few reliable models weaken the storm much slower than depicted.
Gil is moving towards the west-northwest at 14 mph. There has been little change to the forecast track of the system, and there hardly ever is when dealing with East Pacific tropical cyclones during the hurricane season. The storm is being steered in the aforementioned direction as a result of a deep-layer ridge to the systems north-northeast. This motion is expected to continue over the next three days as the pattern remains locked in place. By days 4 and 5, Gil is expected to turn towards the west-southwest as the system becomes embedded within the traditional east-to-west low-level trade wind flow across the East Pacific. This is supported by all reliable global model guidance. Due to this motion, Gil is not expected to be a threat to the Hawaiian islands like its predecessor, and should remain hundreds of miles from any landmasses throughout the entire forecast period.
...FORECAST POSITIONS/MAX WINDS...
INIT 02/0300Z 14.2N 121.8W 75 KT 85 MPH
12H 02/1200Z 14.7N 123.4W 75 KT 85 MPH
24H 03/0000Z 15.0N 125.4W 80 KT 90 MPH
36H 03/1200Z 15.3N 127.1W 85 KT 100 MPH
48H 04/0000Z 15.6N 128.9W 80 KT 90 MPH
72H 05/0000Z 15.8N 132.3W 65 KT 75 MPH
96H 06/0000Z 15.7N 135.5W 55 KT 65 MPH
120H 07/0000Z 15.6N 138.6W 40 KT 45 MPH
Invests 90E and 91E of no concern
The East Pacific is a hotbed of activity lately as a result of the upward pulse of the MJO. In addition to Hurricane Gil we find invests 90E and 91E.
As of the 0z ATCF update, Invest 90E was located at 12.7°N 116.8°W. Maximum sustained winds were 30 mph and the minimum barometric pressure was 1009 millibars. This disturbance has been distinguishable for days, forming within the Intertropical Convergence Zone shortly after Gil was initially classified as a tropical depression. Satellite loops reveal 90E isn't particularly well organized. A vigorous low-level circulation is observable but the disturbance lacks deep, widespread, and organized shower and thunderstorm activity. Wind shear over the disturbance as analyzed by the SHIPS was less than 10 knots, the disturbance was in a region of decent atmospheric moisture, and sea surface temperatures were just below 29°C. All of these factors favor at least slow development of the system. However, due to its proximity to Hurricane Gil, conditions may become less favorable in the long term as wind shear in the form of outflow impedes on the system. 90E is expected to cross the 26°C isotherm in roughly 96 hours, at which time any development will cease. The National Hurricane Center is currently giving 90E a medium chance, 40%, of developing over the next 48 hours and a medium chance, 50%, of developing over the next 48 hours. I agree with these percentages. Regardless of development, 90E is not a threat to land.
Figure 2. Infrared satellite imagery of Invest 90E.
On the other side of Hurricane Gil lies recently-declared Invest 91E. As of the latest ATCF update, the disturbance was located at 12.0°N 136.1°W. Maximum sustained winds were 30 mph and the minimum barometric pressure was 1009 millibars. Dissimilar to 90E, the environment is marginal at best for development. Wind shear is less than 10 knots and sea surface temperatures currently lie at or above 28°C, but the former is expected to increase and the latter are expected to decrease rapidly over subsequent days. The disturbance is within a region of dry, stable air; mid-level relative humidity values were analyzed in the lower 50s. The system has a 48-hour window before conditions become even more unfavorable for development. The NHC is currently giving 91E a low chance, 10%, of becoming a tropical cyclone over the next 48 hours and a low chance, 20%, of becoming a tropical cyclone over the next 120 hours. I put these chances at Near 0% and 10%, respectively. Regardless of development, 91E is not a threat to land.
Watching the Atlantic
The Atlantic as a whole this evening remains pretty quiet. The Gulf of Mexico is dominated by difluence and moderate wind shear, the Caribbean is dominated by high wind shear, the subtropical Atlantic is dominated by stable, sinking air, and the central and eastern Atlantic continue to be engulfed in a large and potent region of Saharan Air Layer. However, in this small area southeast of Florida, in the Bahamas, lies what is once again Invest 91L, a.k.a the remnants of never-ending Dorian. Over the past few days, the remains of the system have been interacting with an upper-level low north of the Greater Antilles. Just recently, however, has this low backed southwest into the northwestern Caribbean, allowing for a relaxation in wind shear. As evidenced by radar out of Miami, Florida and satellite imagery, 91L has a decent amount of vorticity. Surface pressures have been falling near Andros Island over the past few hours as well. Convection had decreased in coverage and intensity until most recently, when a respectable burst of shower and thunderstorm has occurred over the suspected center. This may just be a result of diurnal maximum - when instability is the greatest as a result of the maximized temperature difference between the atmosphere and surface - however...which would not be unexpected given the system's performance since degeneration.
Invest 91L is located in a region of weak steering. A ridge lies to the system's northeast and a stalled-out frontal boundary attached to a trough lies over the central Gulf of Mexico. A majority of the model guidance indicates a slow west-northwest to northwest motion over the next day or so, before accelerating northeast out to sea as whatever the system becomes is absorbed into the aforementioned trough. Wind shear is low, sea surface temperatures are warm, and the environment is moist. One glaring problem is a lack of convergence - air piling up and rising at the surface - however. Surface convergence is required to establish a well-defined surface low which is in return able to sustain organized shower and thunderstorm activity. This is the exact same issue Ex-Dorian had several days ago near the northeast Caribbean Sea. Development chances are low unless this is established. The NHC is currently giving 91L a 30% chance of developing in both the 2-day and 5-day periods, and I agree with these chances.
We are likely to see a break in meaningful disturbances over the next few days after 91L. There are indications that we may have to watch the Caribbean in the day 10-15 period, however. The MJO is currently located in the East Pacific, and assuming it remains uniform as forecast by the global models, should enter the basin in a little over a week. In addition, the TUTT - an upper-level trough that produces strong wind shear - is expected to retrograde westward, providing a favorable upper-air environment across the west and southwest Caribbean Sea. As a result, development chances are likely to be increased. Both the 12z and 18z GFS runs developed a tropical cyclone in the western Caribbean Sea by day 15, though the latter run did not show nearly as much development as the former run due to prolonged interaction with Central America. The CMC is indicating an area of low pressure in the southwest Caribbean at the end of the period; it will be interesting to see if subsequent runs show further development.
CSU's updated hurricane forecast
As a reminder, William Gray and Phil Klotzbach of Colorado State University will be releasing their updated forecast for seasonal activity in the Atlantic during the 2013 hurricane season. In several news articles, Phil has expressed concerns about a slightly drier-than-average environment across the East Atlantic. In addition, concerns have been expressed about cool sea surface temperatures - which have rebounded in a big way over the past two weeks - across the East Atlantic, which may act to hamper the intensity of storms. I expect CSU will lower their numbers by one to two storms in each category (tropical storm, hurricane, major hurricane). Their previous forecast called for 18 named storms, 9 hurricanes, and 4 major hurricanes.
I may have a new blog tomorrow,
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