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Tropical Storm IDA Forecast Discussion Number 6

2015-09-19 22:34:38| National Hurricane Center (Atlantic)

Issued at 500 PM AST SAT SEP 19 2015 000 WTNT45 KNHC 192034 TCDAT5 TROPICAL STORM IDA DISCUSSION NUMBER 6 NWS NATIONAL HURRICANE CENTER MIAMI FL AL102015 500 PM AST SAT SEP 19 2015 Ida's low-level center has been exposed northwest of the deep convection all day due to about 20 kt of westerly shear, and the initial intensity remains 35 kt based on Dvorak estimates from TAFB and SAB. Vertical shear is expected to be steady or increase slightly during the next 12-24 hours, so little, if any, strengthening is anticipated in the short term. The shear could then decrease between 36-72 hours as Ida moves beneath a narrow upper-level ridge, but there is high uncertainty as to how the upper-level environment will evolve during the next few days. The intensity models continue to disagree on the future intensity. While the GFDL and HWRF bring Ida to hurricane strength in 2 to 3 days, the SHIPS and LGEM models have actually backed off from this morning's runs and show Ida getting no stronger than about 45 kt through day 5. Since the upper-level environment only seems marginally conducive for strengthening, the NHC intensity forecast remains closer to the statistical models and lower than the intensity consensus. Ida has sped up a bit, and the initial motion is 300/12 kt. Low- to mid-level ridging will continue to steer the storm west- northwestward for the next 24 hours. After that time, the ridge is expected to weaken, the steering currents will collapse, and Ida will meander between days 3 through 5. The spread in the track models increases considerably beyond 48 hours, with the ECMWF, GFDL, and HWRF showing a northeastward turn by day 5, while the GFS and UKMET continue to show a westward motion. Given the large spread in the models, very little motion is indicated in the official forecast at the end of the forecast period. This scenario is the same as that shown in previous forecasts. FORECAST POSITIONS AND MAX WINDS INIT 19/2100Z 15.6N 40.8W 35 KT 40 MPH 12H 20/0600Z 16.4N 42.5W 35 KT 40 MPH 24H 20/1800Z 17.6N 44.5W 40 KT 45 MPH 36H 21/0600Z 18.9N 46.1W 45 KT 50 MPH 48H 21/1800Z 19.9N 47.2W 50 KT 60 MPH 72H 22/1800Z 20.5N 48.0W 55 KT 65 MPH 96H 23/1800Z 20.0N 48.0W 55 KT 65 MPH 120H 24/1800Z 20.5N 47.5W 50 KT 60 MPH $$ Forecaster Berg

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Remnants of NINE Forecast Discussion Number 14

2015-09-19 22:31:41| National Hurricane Center (Atlantic)

Issued at 500 PM AST SAT SEP 19 2015 000 WTNT44 KNHC 192031 TCDAT4 REMNANTS OF NINE DISCUSSION NUMBER 14 NWS NATIONAL HURRICANE CENTER MIAMI FL AL092015 500 PM AST SAT SEP 19 2015 Organized deep convection has been absent from the depression for more than 12 hours, with Dvorak classifications of too weak to classify at 12Z and 18Z. In addition, visible satellite imagery shows that the center of the depression has continued to become elongated today and is no longer well defined. As a result, the system is no longer a tropical cyclone and this will be the final advisory. The remnants could continue to produce some intermittent bursts of convection during the next couple of days. The motion has been around 290/04 for the past few hours, and the remnants of the depression should continue to move generally west-northwestward for the next couple of days. Additional information on this system can be found in High Seas Forecasts issued by the National Weather Service, under AWIPS header NFDHSFAT1 and WMO header FZNT01 KWBC. FORECAST POSITIONS AND MAX WINDS INIT 19/2100Z 18.8N 49.9W 25 KT 30 MPH...REMNANTS OF NINE 12H 20/0600Z...DISSIPATED $$ Forecaster Brennan

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Tropical Storm IDA Forecast Discussion Number 5

2015-09-19 16:34:34| National Hurricane Center (Atlantic)

Issued at 1100 AM AST SAT SEP 19 2015 000 WTNT45 KNHC 191434 TCDAT5 TROPICAL STORM IDA DISCUSSION NUMBER 5 NWS NATIONAL HURRICANE CENTER MIAMI FL AL102015 1100 AM AST SAT SEP 19 2015 As was noted yesterday, Ida will have an obstacle course to navigate during the next several days, and the storm already appears to have encountered its first hurdle. Westerly shear, on the order of 20 kt according to UW-CIMSS analyses, has begun to affect the cyclone, and the low-level center is now exposed to the northwest of the deep convection. Dvorak CI numbers remain 2.5, and the initial intensity therefore remains 35 kt. The SHIPS guidance indicates that the vertical shear should be steady or even increase slightly during the next 24 hours, so only slow strengthening is anticipated during that time. The best window for more significant intensification appears to be between 36 and 72 hours when the shear is forecast to decrease. Another round of increased shear on days 4 and 5 should limit strengthening or induce weakening. The intensity models are split on Ida's future intensity, with the SHIPS and LGEM models only modestly strengthening the cyclone through day 5 while the GFDL and HWRF models bring Ida to hurricane strength by day 3. Due to the uncertainty of how Ida will interact with the complex environment around it, the NHC intensity forecast is a bit below the intensity consensus and is very similar to the previous forecast. Low- to mid-level ridging to the north and northeast of Ida is inducing a slightly faster west-northwestward motion, or 295/10 kt. The ridge is expected to weaken within the next 48 hours, leaving Ida between two deep-layer lows, one located over the eastern Atlantic and the other northeast of the Leeward Islands. This should cause Ida to become nearly stationary or meander by days 3 through 5. With the exception of the UKMET model, the other track models seem to be trending toward a solution in which the eastern Atlantic trough has at least some influence on Ida's motion, and the updated NHC track forecast now shows a slow eastward drift on days 4 and 5. Overall though, the NHC track forecast is very similar to the previous one and is close to the model consensus aids. FORECAST POSITIONS AND MAX WINDS INIT 19/1500Z 14.8N 39.7W 35 KT 40 MPH 12H 20/0000Z 15.7N 41.4W 40 KT 45 MPH 24H 20/1200Z 16.9N 43.4W 45 KT 50 MPH 36H 21/0000Z 18.2N 45.1W 50 KT 60 MPH 48H 21/1200Z 19.3N 46.5W 55 KT 65 MPH 72H 22/1200Z 20.5N 47.5W 60 KT 70 MPH 96H 23/1200Z 20.5N 47.0W 60 KT 70 MPH 120H 24/1200Z 20.5N 46.5W 55 KT 65 MPH $$ Forecaster Berg

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Tropical Depression NINE Forecast Discussion Number 13

2015-09-19 16:33:37| National Hurricane Center (Atlantic)

Issued at 1100 AM AST SAT SEP 19 2015 000 WTNT44 KNHC 191433 TCDAT4 TROPICAL DEPRESSION NINE DISCUSSION NUMBER 13 NWS NATIONAL HURRICANE CENTER MIAMI FL AL092015 1100 AM AST SAT SEP 19 2015 Visible satellite imagery shows that the center of the depression is losing definition and becoming elongated. In addition, the system has been without organized convection since around 0700 UTC. Given the continued influence of strong shear and dry air, the system is expected to become a remnant low within 12 hours or could dissipate as soon as later today if the center continues to lose definition. The initial motion has been slower over the past few hours, and the current estimate is 285/05. The low- to mid-level subtropical ridge north of the cyclone should steer it or its remnants toward the west-northwest until the system dissipates completely in the next 24 to 36 hours. The new NHC track is close to the previous one adjusted for the initial position and motion. FORECAST POSITIONS AND MAX WINDS INIT 19/1500Z 18.4N 49.8W 25 KT 30 MPH 12H 20/0000Z 18.8N 50.9W 20 KT 25 MPH...POST-TROP/REMNT LOW 24H 20/1200Z 19.4N 52.5W 20 KT 25 MPH...POST-TROP/REMNT LOW 36H 21/0000Z...DISSIPATED $$ Forecaster Brennan

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Tropical Storm IDA Forecast Discussion Number 4

2015-09-19 10:38:03| National Hurricane Center (Atlantic)

Issued at 500 AM AST SAT SEP 19 2015 000 WTNT45 KNHC 190837 TCDAT5 TROPICAL STORM IDA DISCUSSION NUMBER 4 NWS NATIONAL HURRICANE CENTER MIAMI FL AL102015 500 AM AST SAT SEP 19 2015 The overall organization of Ida's cloud pattern continues to increase. Banding features are becoming more prominent in the southern part of the circulation, with a new band forming on the west side. In addition, a burst of convection has been ongoing during the past few hours near the apparent center. Satellite classifications still support an intensity of 35 kt, and this value is used as the initial wind speed. Satellite fixes suggest that the northwestward motion of Ida has sped up to 9 kt this morning. A mid-level ridge is expected to remain the dominant steering mechanism for the next couple of days, causing the storm to move to the west-northwest or northwest during that time. Models are in good agreement and only a slight northward adjustment was made to the official forecast then. Beyond 2 days, there is considerable uncertainty on how an amplifying trough over the east-central Atlantic will affect Ida. The cyclone seems likely to slow down due to steering currents collapsing, but there is poor agreement on whether the trough will pick up Ida, sending the storm east-northeastward, or leave the storm behind. For example, the normally reliable ECMWF and GFS models are going in opposite directions at day 5, with the ECMWF favoring a stronger trough solution while the GFS has Ida missing the trough and drifting westward. With so much uncertainty, I have elected to show a stationary Ida after day 3, preferring not to commit to either solution at this time. This forecast blends the previous forecast with the model consensus, although leans more heavily on the former. While the shear near Ida is low for now, most of the global models show an increase during the next day or so. This is expected to somewhat temper future strengthening, although since most of the other environmental factors are still favorable, a gradual intensification is forecast. The long-range intensity prediction, however, is very complex due to the potential interaction of the trough, which yields very different shear profiles, along with dry air aloft and possible upwelling of cooler water as Ida moves slowly. With the track more uncertain than average and these challenging factors, it seems prudent to remain conservative with the wind speed forecast. The official intensity forecast is close to the previous one, which ends up on the low side of the guidance at days 4/5. It almost goes without saying that these conditions are leading me to have a rather low confidence in the intensity forecast at 72 hours and beyond. FORECAST POSITIONS AND MAX WINDS INIT 19/0900Z 14.4N 38.5W 35 KT 40 MPH 12H 19/1800Z 15.3N 39.9W 40 KT 45 MPH 24H 20/0600Z 16.5N 42.2W 45 KT 50 MPH 36H 20/1800Z 17.6N 44.0W 50 KT 60 MPH 48H 21/0600Z 18.9N 45.6W 55 KT 65 MPH 72H 22/0600Z 20.5N 47.5W 60 KT 70 MPH 96H 23/0600Z 20.5N 47.5W 55 KT 65 MPH 120H 24/0600Z 20.5N 47.5W 55 KT 65 MPH $$ Forecaster Blake

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