Teenager. Weather aficionado. Soccer fan. Realist. Posts subject to sarcasm. Goal: National Hurricane Center.
By: TropicalAnalystwx13 , 4:12 AM GMT on July 06, 2013
Tropical Storm Erick has been steadily intensifying today, and the storm is very close to attaining hurricane intensity. As of the latest advisory from the National Hurricane Center, the system was located within 30 nautical miles of 16.7N 103.5W, or about 165 miles south-southeast of Manzanillo, Mexico. Maximum sustained winds were unchanged from the previous two updates, but the minimum barometric pressure had fallen to 991 millibars. Erick was moving towards the northwest at 10 mph, farther right than its west-northwest trajectory this morning. Even satellite loops reveal a well-defined tropical cyclone, with upper-level outflow in all quadrants except the northeast, prominent spiral banding namely confined to the south and east semicircle, and deep shower and thunderstorm activity just east of the center. Convective activity has warmed relative to this morning, but this is likely attributed to diurnal minimum. During this phase, the air temperature is warmer than the ocean temperature. This creates a region of subsidence above the ground, or a cap, that limits the height – and intensity, accordingly – of thunderstorms. The peak of this phase occurred a few hours ago, however, and we should see the storm become more active convectively by morning. Several instruments have done a good job at sampling Erick this afternoon, with both ASCAT Metop-A and ASCAT Metop-B catching the entirety of the system; both showed winds of 45-50 knots. Given the low bias of the instrument, this would suggest an intensity of 50 to 55 knots, which gives credence to the NHC's set intensity at the time. Several microwave passes throughout today have indicated a developing inner core, and in fact, a 0052z F-17 91H pass indicated the presence of an eyewall and associated eye with Erick. This eye has not yet become visible on geostationary imagery, and this prevented the NHC from upgrading the system to a hurricane; however, this feature could become visible at any time, and Erick could become a hurricane at any point tonight as well. In fact, latest satellite images show the storm has taken on the '9' structure, an appearance usually indicative of a developing hurricane. Satellite intensity estimates from SAB...TAFB...and UW-CIMSS-ADT...were T3.5/55 kt...T3.5/55 kt...and T3.6/57 kt, respectively. It should be stated that the auto-generated SAB value has reached the hurricane threshold, at T4.0/65 kt.
Figure 1. Infrared satellite imagery of Tropical Storm Erick.
Forecast for Erick
The environment for Erick continues to favorable for steady intensification. An anticyclone aloft is providing low wind shear for the storm while also creating decent ventilation channels, mid-level relative humidity values remain near 80% indicating a lack of dry air, and sea surface temperatures are currently above 29C. One potential caveat to a more quick pace of intensification is the potential for land interaction over the next 24 hours or so, combined with lowering ocean heat content. The National Hurricane Center has stated concerns about the storm tracking over a cool track created by former Hurricane Dalila, but this is not depicted in the STEXT file. As just mentioned, the environment is not dry; however, if Erick passes too close to the coastline of Mexico, strong winds that move inland will be capable of pulling more stable air off the mountainous terrain of the region, subsequently disrupting the inner core of the cyclone. Due to this concern, I have lowered my peak intensity from 80 knots to 75 knots, which is ever so slightly above the SHIPS/LGEM consensus. Between 36-48 hours out, Erick is expected to cross the 26C isotherm as it pulls away from the southwestern coastline of Mexico; combined with very dry air farther west, no ocean heat content, and unimpressive divergence, the storm is expected to weaken to a tropical storm. Further exposure to these conditions should weaken Erick to a tropical depression in 72 hours, and the storm is likely to become a post-tropical cyclone in 96 hours.
Erick is currently moving northwest in response to a dying mid-level area of high pressure further inland over Mexico. This motion is expected to continue for the next two days or so before the ridge over the West United States shift southeastward and causes Erick to turn west-northwest. Until then, the concern for a landfall is gradually increasing. The 00z statistical and dynamical model suite is further east than its processor, which was farther east than its processor as well. The global models have been shifting ever so slowly east as well, and the 18z GFS namely is showing the center of Erick moving onshore near Cabo Corrientes in 24 hours. While I am not quite ready to depict such given that the model consensus is just offshore, it should be stated that the possibility of a landfall is not zero. Due to the proximity to the coastline, the government of Mexico has extended the Tropical Storm Warning to include the shore from Zihuatanejo to Cabo Corrientes, Mexico. Given the ongoing trends, it would likely be viable to issue a Hurricane Watch for the northern portion of this warned area. Remember, a Tropical Storm Warning means tropical storm conditions are expected within 36 hours. A Hurricane Watch means hurricane conditions are possible within 48 hours. The warned regions should expect winds of at least 40 mph and torrential rainfall capable of accumulating to a half foot of more. Such accumulation will likely lead to life-threatening mudslides, landslides, and flash flooding. Increased swells and rip currents are to be expected along the coastline. At the end of the forecast period, Erick is expected to track due west as its upper-level center decouples from its low-level center and the latter becomes embedded within the east-to-west flow across the Pacific.
...FORECAST POSITIONS/MAX WINDS...
INIT 06/0300Z 16.7N 103.5W 60 KT 70 MPH
12H 06/1200Z 17.5N 103.9W 65 KT 75 MPH
24H 07/0000Z 18.9N 105.4W 75 KT 85 MPH
36H 07/1200Z 20.2N 106.8W 70 KT 80 MPH
48H 08/0000Z 21.4N 108.9W 60 KT 70 MPH
72H 09/0000Z 23.0N 113.0W 35 KT 40 MPH
96H 10/0000Z 23.0N 117.5W 25 KT 30 MPH...POST-TROP/REMNT LOW
120H 11/0000Z 23.0N 122.5W 20 KT 25 MPH...POST-TROP/REMNT LOW
Dalila nearly dead
Several hundred miles west of Tropical Storm Erick lies a dying Tropical Depression Dalila. As of the latest advisory from the National Hurricane Center, Dalila was located within 15 nautical miles of 17.1N 122.6W, or about 435 miles south-southwest of the southern tip of Baja California. Maximum sustained winds were 30 mph, the minimum barometric pressure was 1006 millibars, and the storm was movi--drifting--west at 2 mph. Satellite loops show the storm is but a low-level swirl with a few high clouds above. After firing convection persistently through early this afternoon, decreasing sea surface temperatures have abruptly halted this process. If convection does not fire over the next 6 hours, the storm will no longer meet the criteria of a tropical cyclone. Regardless of status, the system is not expected to be a threat to any landmasses.
Figure 2. Infrared satellite imagery of Tropical Depression Dalila.
In the Gulf of Mexico lies the surface trough that has been mentioned over the past few days. A bit of forced convergence and help from the Bay of Campeche led to the formation of a weak but distinguishable low-level circulation this evening. Little convection exists over this feature, and what is present is being heavily sheared from the west as a result of upper-level winds around the base of the large trough north of the system and upper-level outflow from Tropical Storm Erick in the East Pacific. Satellite loops reveal that the main swirl of 94L, inherently dubbed by the National Hurricane Center, may be dissipating as a new one forms farther northeast in a region of maximized convergence. Regardless, it is unlikely this system will become a tropical cyclone anytime soon. The current steering pattern indicates 94L should drift northward into the weakness created by the aforementioned trough for the next day or two; after this time, the ridge situated off the North Carolina coastline is expected to expand westward and push the disturbance into the Texas coastline. Development chances are not zero, but likely lie between 20-40%. As the ridge off the East Coast builds westward, the trough is expected to weaken and pull northeast. The southern portion of this trough, much like what has happened over the central Atlantic the past two days, is expected to get pinched off and develop into a cut-off upper-level low that backs southwestward. While this setup should allow for upper-level ridging, and lower shear accordingly, to develop across the western Gulf of Mexico, the question remains whether or not the system is inland at this point, and if not, whether it has sufficient time to become a brief tropical cyclone. Regardless of classification, beneficial rainfall is expected along the immediate upper Texas and Louisiana coastlines. As of the 8pm Tropical Weather Outlook, the NHC gave this system a 20% chance of tropical cyclone formation within 48 hours.
In the central Atlantic north of Puerto Rico lies a well-defined, cut-off upper-level low. This feature developed from a strong trough over the central Atlantic that was subsequently pinched off by an expanding region of high pressure nearby. This same high has caused the low to begin retrograding southwestward, and this motion is expected to continue for a little while longer before a turn towards the west occurs. Satellite loops reveal this low is essentially bare of shower and thunderstorm activity, though this is not to be unexpected from a cold-core feature. Maps from the University of Wisconsin-Madison indicate the storm has been steadily working down towards the surface, and in fact, a balm of vorticity already exists at 850 millibars. However, this feature is far from meeting the criteria of a tropical cyclone, and may never do so. Sea surface temperatures are very warm, in excess of 28C. The biggest structure for this upper-level low will be the strong pressure gradient created by the high to its north; this gradient is accelerating the low level trade winds across the region the low is tracking into. A tropical cyclone requires a closed low-level circulation; if winds are moving quickly westward, it is difficult to get easterly winds on the southern side of a low-pressure area. Regardless of development, gusty winds and sporadic rainfall is likely to affect the Bahamas and Florida through the weekend and into early next week. The NHC has not highlighted this feature in their TWO, but I believe it has a 10% chance of eventually becoming a tropical cyclone.
Our last stop takes us to the eastern Atlantic, where yet another well-defined tropical wave for this time of the year is positioned. Satellite loops do indeed show a well-defined system, with an obvious low-level spin just north of an area of deep convection. Were this August, this wave would be able to become quite a tropical cyclone; however, because it is July, the Saharan Air Layer is in abundance across the region, and mid-level dry air as a result of the strengthened Bermuda high lurks nearby as well. Convective activity is being fueled solely by the monsoon trough at this time. If the feature detaches, which it needs to do in order to meet the criteria of a tropical cyclone and which it is expected to do, it is unlikely to look nearly as healthy as it does at this point. Wind shear is actually a conducive 5-15 knots across the entirety of the central and eastern Atlantic. As the wave tracks west to west-northwest, it is likely to continue to find a favorable shear pattern, though once again, moisture is scarce. None of the reliable computer models develop this wave, though many do show it impacting the Lesser and Greater Antilles with decent rainfall in roughly a week. The NHC has actually highlighted this feature in their TWO, and they give the feature a 10% chance of development within 48 hours.
Figure 3. Infrared satellite imagery of the Atlantic basin, highlighting the three areas of interest.
Tropical Storm Risk update
Tropical Storm Risk, the public consortium consisting of experts in insurance, risk management, and seasonal climate forecasting at University College London, has issued their July forecast update from the Atlantic basin. Quite surprisingly, the experts have lowered their seasonal numbers for the 2013 Atlantic hurricane season to 15 named storms, 7 hurricanes, and 3 major hurricanes, with an Accumulated Cyclone Energy index of 112 units. This reduction comes as a result of so-called cooling of the Main Development Region over the past few weeks. While I admit ocean temperatures cooled relative to the past few months in June, they have since rebounded, and are forecast to continue warming for much of this month. Their in-house model has changed from a forecast of MDR SSTs being 0.19C above average during the August-September timeframe to a forecast of MDR SSTs behind -0.1C below average during the August-September timeframe; as stated in their report, this would be the most dramatic switch in ocean temperatures since their inception. I see no reason to believe why such a forecast would pan out, however. A secondary issue brought up in their report was the concern of trade winds across the Caribbean and Atlantic during the peak of the season. This may be a valid concern due to the unusual strength of the Bermuda High the past two months; it is worth noting, however, that the high is typically strongest this month and levels off in intensity thereafter. You can view their full report here.
I'll have a new blog tomorrow,
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