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Tropical Storm IDA Forecast Discussion Number 20

2015-09-23 10:45:59| National Hurricane Center (Atlantic)

Issued at 500 AM AST WED SEP 23 2015 000 WTNT45 KNHC 230845 TCDAT5 TROPICAL STORM IDA DISCUSSION NUMBER 20 NWS NATIONAL HURRICANE CENTER MIAMI FL AL102015 500 AM AST WED SEP 23 2015 Shortwave infrared satellite imagery and a 0413 UTC GCOM microwave pass suggest that multiple low-level swirls are revolving around Ida's center, which has been fixed to the west-northwest of an ongoing burst of deep convection. This pattern is the consequence of 30 kt of west-northwesterly shear, as analyzed by UW-CIMSS. Maximum winds remain 35 kt based on earlier ASCAT data, and it appears that Ida's intensity probably won't change much during the next couple of days. Vertical shear is forecast to gradually weaken, but it likely won't fall to less than 15 kt for at least another 48 hours. Some strengthening is possible beginning on day 3 once the shear has decreased. With the exception of the GFDL, the intensity models are in close agreement through day 4, with some more spread by day 5. Based on this latest guidance, no changes were made to the previous NHC intensity forecast for this advisory. Although Ida's exact center is difficult to locate, the entire circulation has seemingly been drifting southward since the previous advisory. This is probably for good reason, as Ida is embedded within northerly flow near the base of a mid-tropospheric trough which stretches across the eastern Atlantic. The estimated initial motion is 180/2 kt, but Ida will likely gain an eastward component soon while being tugged by the eastern Atlantic trough. The subtropical ridge is expected build westward over the eastern Atlantic by 48 hours, which should impart a slightly faster northward motion through the end of the forecast period. The spread in the 00 UTC track guidance decreased on this forecast cycle compared to earlier runs, with the UKMET model being the only obvious outlier. Given the tight clustering, the NHC official track forecast was again adjusted westward, mainly on days 3-5, and lies very close to the various multi-model consensus aids. FORECAST POSITIONS AND MAX WINDS INIT 23/0900Z 20.4N 47.2W 35 KT 40 MPH 12H 23/1800Z 20.4N 46.3W 35 KT 40 MPH 24H 24/0600Z 20.5N 45.8W 35 KT 40 MPH 36H 24/1800Z 21.0N 45.7W 35 KT 40 MPH 48H 25/0600Z 21.7N 45.9W 35 KT 40 MPH 72H 26/0600Z 23.0N 47.0W 40 KT 45 MPH 96H 27/0600Z 25.0N 47.5W 45 KT 50 MPH 120H 28/0600Z 26.0N 47.0W 50 KT 60 MPH $$ Forecaster Berg

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Tropical Storm IDA Forecast Discussion Number 19

2015-09-23 04:52:27| National Hurricane Center (Atlantic)

Issued at 1100 PM AST TUE SEP 22 2015 000 WTNT45 KNHC 230252 TCDAT5 TROPICAL STORM IDA DISCUSSION NUMBER 19 NWS NATIONAL HURRICANE CENTER MIAMI FL AL102015 1100 PM AST TUE SEP 22 2015 Last-light visible imagery, microwave imagery, and a timely ASCAT pass around 00Z showed that the center of Ida is located on the northeastern side of the larger gyre seen earlier today, just west of a new burst of deep convection. This structure is consistent with the 30 kt of westerly shear seen in UW-CIMSS satellite analyses. The initial intensity is set to 35 kt based on data from the ASCAT pass and in agreement with the latest Dvorak classification from TAFB. Little change in intensity is expected during the next 48 hours, with moderate to strong shear forecast to continue. By 72 hours the shear decreases a bit, which should allow for a little strengthening. The new NHC intensity forecast is close to a blend of the latest SHIPS, LGEM, and HWRF models. The initial position results in a relocation of the center to the northeast of the previous advisory with a slow east-southeastward drift as the initial motion. Ida is currently embedded in the base of a mid-latitude trough which should result in a continued slow east-southeastward motion during the next day or so followed by a gradual turn toward the northeast and then the north by 48 hours as the trough moves away. A slightly faster north-northwestward motion is forecast at days 3 through 5 as a mid-level ridge builds to the north of the tropical cyclone. During the latter portion of the forecast period much of the guidance has shifted westward, now showing a weaker Ida remaining south of the mid-latitude westerlies through day 5. The NHC track during this time has been nudged to the left of the previous one adjusted for the initial position. However, out of respect for continuity the NHC forecast remains close to the ECMWF on the eastern side of the guidance envelope and is well right of the consensus aids. Given the large shift in the guidance this cycle, confidence is quite low in the track forecast late in the period. FORECAST POSITIONS AND MAX WINDS INIT 23/0300Z 20.9N 46.8W 35 KT 40 MPH 12H 23/1200Z 20.7N 46.4W 35 KT 40 MPH 24H 24/0000Z 20.5N 45.8W 35 KT 40 MPH 36H 24/1200Z 21.0N 45.3W 35 KT 40 MPH 48H 25/0000Z 21.6N 45.1W 35 KT 40 MPH 72H 26/0000Z 23.0N 45.5W 40 KT 45 MPH 96H 27/0000Z 25.0N 46.5W 45 KT 50 MPH 120H 28/0000Z 26.0N 47.0W 50 KT 60 MPH $$ Forecaster Brennan

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Tropical Storm IDA Forecast Discussion Number 18

2015-09-22 22:36:34| National Hurricane Center (Atlantic)

Issued at 500 PM AST TUE SEP 22 2015 000 WTNT45 KNHC 222036 TCDAT5 TROPICAL STORM IDA DISCUSSION NUMBER 18 NWS NATIONAL HURRICANE CENTER MIAMI FL AL102015 500 PM AST TUE SEP 22 2015 The low-level center of Ida has been placed in the middle of a gyre defined by several small vortices. The cyclone is still sheared with the strongest convection located within a cyclonically- curved band to the south of the center. Given the lack of scatterometer data, and little change in the satellite presentation, the initial intensity is kept at 40 kt. Ida is embedded within the southern end of a mid-level trough, which is forcing the cyclone to drift east-southeastward at about 4 kt, and is also causing shear over the cyclone. The evolution of this trough will be crucial for the future of Ida. Global models indicate that Ida will drift generally eastward embedded in the trough for the next 48 hours. After that time, most of the models lift the trough out, leaving Ida south of an amplifying ridge. This long range forecast pattern should result in the cyclone turning toward the north-northwest as indicated in the NHC forecast, and also a decrease of the shear with favorable conditions for strengthening. Since the environment is currently quite hostile, no important change in intensity is indicated during the next 3 days. By the end of the forecast period, when the cyclone becomes detached from the trough, some slight increase in strength is forecast. The NHC track forecast continues to be in the middle of the guidance envelope, and is very close to the ECMWF and GFS consensus. If the global models are correct, Ida will be meandering over the Atlantic for several more days. FORECAST POSITIONS AND MAX WINDS INIT 22/2100Z 20.5N 47.8W 40 KT 45 MPH 12H 23/0600Z 20.2N 47.2W 40 KT 45 MPH 24H 23/1800Z 20.0N 46.5W 40 KT 45 MPH 36H 24/0600Z 20.0N 46.0W 40 KT 45 MPH 48H 24/1800Z 20.5N 45.5W 40 KT 45 MPH 72H 25/1800Z 22.5N 45.7W 40 KT 45 MPH 96H 26/1800Z 24.5N 47.0W 50 KT 60 MPH 120H 27/1800Z 26.5N 46.5W 55 KT 65 MPH $$ Forecaster Avila

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Tropical Storm IDA Forecast Discussion Number 17

2015-09-22 16:33:26| National Hurricane Center (Atlantic)

Issued at 1100 AM AST TUE SEP 22 2015 000 WTNT45 KNHC 221433 TCDAT5 TROPICAL STORM IDA DISCUSSION NUMBER 17 NWS NATIONAL HURRICANE CENTER MIAMI FL AL102015 1100 AM AST TUE SEP 22 2015 Ida continues to be a sheared cyclone with the low-level center located to the northwest of an area of very deep convection. The upper-level outflow in the southern half of the cyclone has become better defined since yesterday, but it is not existent elsewhere. Dvorak numbers are decreasing and do not support 40 kt at this time. However, I would wait for scatterometer or any other data, if available, to reduce the winds, if necessary. The cyclone is embedded within an uncommon flow pattern. It is located at the southern end of a mid-level trough, which is forcing the cyclone to move east-southeastward at about 7 kt, and also causing shear. The evolution of this trough will be crucial for the future of Ida. Unanimously, global models keep the cyclone drifting generally eastward for the next 2 days or so, while embedded within the trough. After that time, all models forecast the trough to lift out, leaving Ida south of an amplifying ridge. This forecast pattern should result in a decrease of the shear with favorable conditions for strengthening, and a slow motion of the cyclone toward the north-northwest. It is interesting to note the current agreement of all global models with this scenario, and at long range, the GFS and ECMWF primarily forecast a significant strengthening of Ida. Given the uncommon pattern and the currently hostile environment, the NHC forecast keeps the cyclone with the same intensity for the next day or two, and allows for some intensification once Ida becomes detached from the trough. The NHC track forecast is in the middle of the guidance envelope, and follows the trend of the ECMWF and GFS consensus. FORECAST POSITIONS AND MAX WINDS INIT 22/1500Z 20.5N 47.5W 40 KT 45 MPH 12H 23/0000Z 20.5N 47.0W 40 KT 45 MPH 24H 23/1200Z 20.5N 46.3W 40 KT 45 MPH 36H 24/0000Z 20.0N 45.5W 40 KT 45 MPH 48H 24/1200Z 20.5N 45.0W 40 KT 45 MPH 72H 25/1200Z 22.0N 45.5W 45 KT 50 MPH 96H 26/1200Z 23.5N 46.5W 50 KT 60 MPH 120H 27/1200Z 25.5N 47.0W 55 KT 65 MPH $$ Forecaster Avila

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Tropical Storm IDA Forecast Discussion Number 16

2015-09-22 10:37:55| National Hurricane Center (Atlantic)

Issued at 500 AM AST TUE SEP 22 2015 000 WTNT45 KNHC 220837 TCDAT5 TROPICAL STORM IDA DISCUSSION NUMBER 16 NWS NATIONAL HURRICANE CENTER MIAMI FL AL102015 500 AM AST TUE SEP 22 2015 Shortwave infrared satellite imagery indicates that Ida's low-level circulation center is now moving or developing east-southeastward closer to the large mass of deep convection that has been persisting in the southeastern portion of the larger circulation. The initial intensity is being maintained at 40 kt, especially now that the low-level center has moved closer to the mid- and upper-level circulations as seen in microwave and conventional satellite images. This intensity is supported by a Dvorak current intensity estimate of T3.0/45 kt from TAFB. The initial motion estimate is now 105/04 kt. The recent and much anticipated turn to the east-southeast appears to have occurred either due to actual storm motion or due to redevelopment of the low-level center closer to the strong convection. Regardless of the cause in the Ida's recent motion change, the global and regional models are in very good agreement on a continued general slow motion toward the east for the next 48 hours or so as the southern portion of a large mid- to upper-level trough moves across and captures the cyclone. By 72 hours, the trough lifts out to the northeast and releases Ida, allowing the cyclone to move slowly toward the northwest by day 4 and toward the north on day 5. As would be expected in such a weak flow regime, the model guidance is widely divergent after 72 hours with the UKMET, HWRF, and NAVGEM models taking Ida more toward the west-northwest, whereas the GFS, ECMWF, GFDL, and Canadian models take Ida more toward the northwest and north. The one thing that the all of the models do agree on, however, is that Ida is not expected move very quickly during the next 5 days. The NHC track forecast is similar to but slower than the previous advisory track, and lies close to a blend of the consensus models TCVA and GFEX. Given the uncertainty in exactly when and where the low-level center of Ida will move underneath the mid/upper-level circulation due to fluctuations in the deep-layer vertical shear profiles during the next 3 days, the intensity forecast calls for no significant changes in the strength of the cyclone during that time. By days 4 and 5, however, some gradual strengthening is expected as the vertical shear abates somewhat while Ida is moving over 29C sea-surface temperatures. The official intensity forecast is similar to the previous advisory, and closely follows the IVCN intensity consensus model. FORECAST POSITIONS AND MAX WINDS INIT 22/0900Z 21.3N 48.6W 40 KT 45 MPH 12H 22/1800Z 21.3N 48.1W 40 KT 45 MPH 24H 23/0600Z 20.9N 47.3W 40 KT 45 MPH 36H 23/1800Z 20.6N 46.7W 40 KT 45 MPH 48H 24/0600Z 20.8N 46.4W 40 KT 45 MPH 72H 25/0600Z 22.0N 46.7W 40 KT 45 MPH 96H 26/0600Z 23.0N 48.0W 45 KT 50 MPH 120H 27/0600Z 25.2N 48.5W 50 KT 60 MPH $$ Forecaster Stewart

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