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Tropical Storm IDA Forecast Discussion Number 10

2015-09-20 22:45:43| National Hurricane Center (Atlantic)

Issued at 500 PM AST SUN SEP 20 2015 000 WTNT45 KNHC 202045 TCDAT5 TROPICAL STORM IDA DISCUSSION NUMBER 10 NWS NATIONAL HURRICANE CENTER MIAMI FL AL102015 500 PM AST SUN SEP 20 2015 Deep convection associated with Ida has decreased in coverage and become somewhat less organized today. The tropical cyclone remains sheared with a small band of convection located well east of the exposed center. Although Dvorak satellite T-numbers have decreased this afternoon, the initial intensity remains 40 kt, which is based on the earlier ASCAT data. Ida is moving northwestward or 305 degrees at 15 kt, a little faster than this morning. Despite the recent increase in forward speed, Ida is expected to slow down during the next 24 hours, and become nearly stationary by Tuesday when a mid-level ridge builds to the north of the system. Ida is then forecast to meander for a couple of days, before the ridge slides eastward and allows the tropical cyclone to resume a northwestward motion. The track guidance is in good agreement during the first few days of the forecast, but again becomes quite divergent by the end of the forecast period. The ECMWF, which moves Ida more eastward during the next few days, is along the eastern edge of the guidance envelope. Meanwhile the UKMET and GFS show less of an eastern motion while the steering currents are weak, and take Ida more westward by day 5. The updated NHC track is between these scenarios and remains close to the multi-model consensus at 96 and 120 h. The westerly shear is still expected to decrease during the next day or so, but given the current organization of the system, Ida may not be able to take full advantage of the potentially more favorable upper-air pattern. Therefore, only a slight increase in intensity is predicted. After 48 hours, upper-level westerly winds associated with a trough are forecast to cause an increase in shear over the cyclone, which would likely cause weakening. The global models, however, generally deepen the cyclone between 72 and 120 h when Ida interacts with an upper-level trough. The official forecast shows little change in strength late in the period and lies between these differing possibilities. FORECAST POSITIONS AND MAX WINDS INIT 20/2100Z 18.4N 46.3W 40 KT 45 MPH 12H 21/0600Z 19.6N 47.5W 40 KT 45 MPH 24H 21/1800Z 20.9N 48.5W 45 KT 50 MPH 36H 22/0600Z 21.5N 48.7W 45 KT 50 MPH 48H 22/1800Z 21.5N 48.5W 45 KT 50 MPH 72H 23/1800Z 21.0N 48.2W 45 KT 50 MPH 96H 24/1800Z 20.9N 48.5W 45 KT 50 MPH 120H 25/1800Z 22.5N 49.5W 45 KT 50 MPH $$ Forecaster Brown

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Tropical Depression SIXTEEN-E Forecast Discussion Number 1

2015-09-20 22:36:14| National Hurricane Center (East Pacific)

Issued at 300 PM MDT SUN SEP 20 2015 000 WTPZ41 KNHC 202036 TCDEP1 TROPICAL DEPRESSION SIXTEEN-E DISCUSSION NUMBER 1 NWS NATIONAL HURRICANE CENTER MIAMI FL EP162015 300 PM MDT SUN SEP 20 2015 Recent ASCAT data indicate that the low pressure area off the west coast of the Baja California peninsula has a low-level center located within a 30-45 nm wide area of light winds. This area is small enough to meet the criterion of a well-defined center, and the low is therefore being classified as a tropical depression. The ASCAT data and a recent ship report indicate that the intensity is 30 kt. The depression is located between a mid-level ridge centered over the northwestern Gulf of Mexico and a mid-/upper-level cut-off low located well west of the Baja California peninsula. This pattern is steering the depression quickly north-northwestward with an initial motion of 345/12 kt. The depression is expected to turn northward soon and continue that heading through 36 hours. The track guidance is tightly clustered, and the NHC official track forecast is very close to a consensus of the GFS and ECMWF. The depression will be moving across the Baja California peninsula by tonight, and southwesterly shear is expected to increase to near 25 kt in 12 hours and 35-40 kt by 24 hours. Therefore, strengthening is not expected, and the system should remain as a tropical depression while it moves across the Baja California peninsula and the Gulf of California. The associated deep convection is likely to be sheared away from the center by 36 hours, making the system a remnant low when it moves into southern Arizona. Dissipation is expected by 48 hours. Moisture associated with the depression is expected to cause heavy rainfall across portions of the Baja California peninsula, northwestern mainland Mexico, southern California, and Arizona during the next several days. These rains could cause life-threatening flash floods and mudslides, especially in areas of mountainous terrain. FORECAST POSITIONS AND MAX WINDS INIT 20/2100Z 25.0N 113.7W 30 KT 35 MPH 12H 21/0600Z 27.1N 113.5W 30 KT 35 MPH...INLAND 24H 21/1800Z 30.3N 112.8W 30 KT 35 MPH...OVER WATER 36H 22/0600Z 32.8N 112.3W 20 KT 25 MPH...POST-TROP/REMNT LOW 48H 22/1800Z...DISSIPATED $$ Forecaster Berg

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Tropical Storm IDA Forecast Discussion Number 9

2015-09-20 16:52:41| National Hurricane Center (Atlantic)

Issued at 1100 AM AST SUN SEP 20 2015 000 WTNT45 KNHC 201452 TCDAT5 TROPICAL STORM IDA DISCUSSION NUMBER 9 NWS NATIONAL HURRICANE CENTER MIAMI FL AL102015 1100 AM AST SUN SEP 20 2015 The low-level center of Ida remains exposed to the west of the deep convection, due to moderate westerly shear that has been affecting the tropical cyclone during the past couple of days. A recent ASCAT overpass revealed winds of 35 to 40 kt over the eastern portion of the circulation so the initial intensity has been increased to 40 kt. The ASCAT data also indicate that the area of tropical-storm-force winds is larger than previous estimated, and the wind radii have been adjusted accordingly. Ida continues to move west-northwestward or 300 degrees at 13 kt. The forward motion of the tropical cyclone is forecast to slow down during the next day or two, as a blocking ridge strengthens to the north of the system over the central Atlantic. All of the dynamical models show Ida becoming stationary or meandering over the central Atlantic between 48 and 96 hours. The NHC forecast follows this scenario and shows the tropical cyclone stationary for a couple of days. Late in the forecast period, the ridge is expected to weaken, which should allow a northwestward or northward motion to begin, however, the track guidance is quite divergent at that time. Given the large spread in the guidance late in the period, the NHC forecast remains near the multi-model consensus at days 4 and 5. The westerly shear that has been affecting Ida is expected to decrease during the next day or so. This should allow for some strengthening, and the NHC forecast is near the IVCN consensus model and the previous advisory. After 48 hours, the global models are suggesting that an upper-level trough to the north of Ida may cause an increase in westerly shear. As a result, the updated official forecast shows no change in intensity late in the period. FORECAST POSITIONS AND MAX WINDS INIT 20/1500Z 17.3N 44.8W 40 KT 45 MPH 12H 21/0000Z 18.2N 46.3W 40 KT 45 MPH 24H 21/1200Z 19.5N 47.7W 45 KT 50 MPH 36H 22/0000Z 20.4N 48.5W 50 KT 60 MPH 48H 22/1200Z 20.8N 48.7W 50 KT 60 MPH 72H 23/1200Z 20.8N 48.7W 50 KT 60 MPH 96H 24/1200Z 20.8N 48.7W 50 KT 60 MPH 120H 25/1200Z 21.2N 49.0W 50 KT 60 MPH $$ Forecaster Brown

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Tropical Storm IDA Forecast Discussion Number 8

2015-09-20 10:46:11| National Hurricane Center (Atlantic)

Issued at 500 AM AST SUN SEP 20 2015 000 WTNT45 KNHC 200846 TCDAT5 TROPICAL STORM IDA DISCUSSION NUMBER 8 NWS NATIONAL HURRICANE CENTER MIAMI FL AL102015 500 AM AST SUN SEP 20 2015 Ida remains a sheared tropical cyclone, but there are some indications that the mid-level and deep-layer shear is beginning to abate somewhat. A burst of deep convection has developed during the past few hours near the center and in the eastern quadrant, which so.me cloud tops near -80C. The initial intensity is being held at 35 kt based on consensus Dvorak current-intensity estimates of T2.5/35 kt from TAFB and SAB, but this could be a little conservative based on earlier partial ASCAT overpass that contained slightly higher wind speeds in the eastern quadrant. The initial motion estimate is 295/13 kt. Ida is expected to move west-northwestward and gradually slow down over the next several days, possibly even stalling by days 4 and 5. Steering currents are forecast to weaken considerably after 72 hours as a strong mid- to upper-level trough digs southeastward across the central subtropical Atlantic and significantly erodes the ridge to the north of the cyclone. The global models are in good agreement on the developing synoptic flow scenario through day 3, but then diverge significantly after that with the ECMWF, UKMET, and GFDL models taking a deeper and stronger tropical cyclone more northward, whereas the GFS and HWRF models take a weaker and more shallow system westward. Given the uncertainty in the expected strength and vertical depth of Ida on days 3-5, the official forecast track lies down the middle of the guidance envelope, and is close to a blend of the consensus models TVCA and GFEX. The moderate to strong westerly vertical wind shear that Ida has been experiencing the past day or so is forecast by the GFS and ECMWF models to gradually subside and become less than 10 kt by 24 hours through 72 hours. With such a robust low-level circulation and the mid-level humidity values expected to be near 70 percent, convection should steadily increase and become better organized, producing at least some slow intensification through the next 48 hours. The leveling off of the intensity on days 3-5 is due to the uncertainty on whether or Ida moves beneath a small upper-level col region with weak winds as indicated by the ECMWF model, which would support more strengthening than what the official forecast is indicating. The NHC intensity forecast lies essentially midway between the two intensity extremes of the weaker GFS-HRWF and the stronger ECMWF-GFDL model solutions, and is supported by the intensity consensus model IVCN. FORECAST POSITIONS AND MAX WINDS INIT 20/0900Z 16.7N 43.6W 35 KT 40 MPH 12H 20/1800Z 17.5N 45.2W 35 KT 40 MPH 24H 21/0600Z 18.8N 47.0W 40 KT 45 MPH 36H 21/1800Z 19.8N 48.1W 45 KT 50 MPH 48H 22/0600Z 20.5N 48.6W 50 KT 60 MPH 72H 23/0600Z 20.6N 48.7W 50 KT 60 MPH 96H 24/0600Z 20.7N 48.7W 50 KT 60 MPH 120H 25/0600Z 21.0N 48.7W 50 KT 60 MPH $$ Forecaster Stewart

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Tropical Storm IDA Forecast Discussion Number 7

2015-09-20 04:54:42| National Hurricane Center (Atlantic)

Issued at 1100 PM AST SAT SEP 19 2015 000 WTNT45 KNHC 200254 TCDAT5 TROPICAL STORM IDA DISCUSSION NUMBER 7 NWS NATIONAL HURRICANE CENTER MIAMI FL AL102015 1100 PM AST SAT SEP 19 2015 Ida is badly sheared tropical cyclone. The center, a well-defined swirl of low clouds, remains well removed from the nearest deep convection to east. This cloud pattern is consistent with UW-CIMSS shear analyses indicating about 20 kt of westerly vertical shear over the cyclone. A partial ASCAT overpass at 0016 UTC showed believable 37-kt wind well east of the center, and the initial intensity estimate is kept at 35 kt. Global models show strong westerly shear persisting over Ida for the next day or so. With so much shear likely, little to no intensification is expected to occur. A respite in the shear is forecast by 36 hours when a piece of an upper-level trough pinches off near the Greater Antilles and drifts westward, which may allow Ida an opportunity to intensify some. The lower-shear environment should not last long, however, since a large mid- to upper-level trough is forecast to dig southwestward into the central Atlantic on top of Ida in 2 to 3 days, with the ECMWF model showing the potential for a complex trough interaction to take place. Assuming that Ida survives, the environment looks less than ideal, with confluent and strong northwesterly winds aloft and the possibility of enhanced oceanic upwelling underneath the cyclone. Weakening is therefore considered the most likely scenario, and it would not be surprising to see Ida become a remnant low during this time frame. The new intensity forecast is lower than the previous one, closest to the HWRF model. An alternate but less likely possibility is that Ida could restrengthen due to baroclinic forcing as shown in the ECMWF solution. Ida has been moving faster and more westerly because of its more shallow nature, and the initial motion estimate is 290/13. A subtropical ridge migrating westward with the cyclone should keep it on a general west-northwestward track for another day or so. Steering currents are expected to collapse in a couple of days in response to the large mid- to upper-level trough amplifying southwestward in the vicinity of Ida. The lack of steering should cause Ida to come to a halt in about 3 days, with the track forecast after that dependent on the depth of the storm. A deeper cyclone would move faster toward the north or northeast as shown in the ECMWF, while a weaker and shallower cyclone would move westward ad depicted in the GFS. The official forecast is between these two extremes but leans more toward the GFS, given the current intensity forecast, and shows a slow northerly motion consistent with the previous forecast. FORECAST POSITIONS AND MAX WINDS INIT 20/0300Z 15.8N 42.4W 35 KT 40 MPH 12H 20/1200Z 16.7N 44.1W 35 KT 40 MPH 24H 21/0000Z 17.8N 46.1W 40 KT 45 MPH 36H 21/1200Z 18.9N 47.6W 45 KT 50 MPH 48H 22/0000Z 19.8N 48.4W 50 KT 60 MPH 72H 23/0000Z 19.9N 48.5W 45 KT 50 MPH 96H 24/0000Z 19.9N 48.5W 40 KT 45 MPH 120H 25/0000Z 20.3N 48.4W 40 KT 45 MPH $$ Forecaster Kimberlain

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