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Tropical Storm IDA Forecast Discussion Number 15

2015-09-22 04:50:21| National Hurricane Center (Atlantic)

Issued at 1100 PM AST MON SEP 21 2015 000 WTNT45 KNHC 220250 TCDAT5 TROPICAL STORM IDA DISCUSSION NUMBER 15 NWS NATIONAL HURRICANE CENTER MIAMI FL AL102015 1100 PM AST MON SEP 21 2015 Ida's cloud pattern has become increasingly disorganized. Satellite data indicate that the large separation between the cyclone's low- to mid-level centers persists as a result of a strong west- northwesterly shear. The nearest deep convection, coincident with the mid-level center, remains well removed to the southeast. The initial wind speed is conservatively lowered to 40 kt and is above the latest Dvorak intensity estimates. The tail of a large mid- to upper-level trough over the eastern Atlantic is forecast to sweep southward over Ida during the next 24 hours. Even stronger deep-layer northwesterly shear and confluence aloft associated with this feature should adversely affect the storm during the next few days, and cause weakening. One could envision Ida not surviving such a greatly inhospitable environment. However, global models depict Ida remaining a coherent feature and eventually re-strengthening, though Ida's forecast intensification occurs in different ways. The ECMWF has consistently shown Ida re- strengthening, seemingly as a consequence of baroclinic forcing from the trough interaction. Such an interaction could potentially result in Ida's taking on the characteristics of a hybrid cyclone. The GFS shows Ida becoming disentangled from the trough, and re-intensifying after the trough lifts out. Regardless of which scenario occurs, any re-strengthening of Ida as a tropical cyclone would likely be tempered by substantially drier air on the backside of the trough being entrained by the cyclone. The new intensity forecast is lower than the previous one in the short term due to the overall harsh environment affecting the storm. A modest increase in strength is shown at the end of the forecast, similar to the previous forecast. The definition of Ida's center has been deteriorating, and it has thus become harder to track. It appears though the the cyclone's forward speed has been decreasing, and the initial motion estimate is an uncertain 340/04. The deep layer west-northwesterly to northwesterly flow, associated with the trough overtaking Ida, could impart a slow eastward or east-southeastward motion for a couple of days. After that, the bulk of the track guidance shows Ida caught between two mid-level ridges, which should induce a north- northwestward and then northward motion at a gradually increasing forward speed. Only the GFS shows Ida encountering a blocking ridge with a track much farther to the west, a solution very different than its ensemble mean. The new track has been adjusted somewhat to the east overall through 72 hours and is a little faster than the previous one, in good agreement with a model consensus without the GFS. FORECAST POSITIONS AND MAX WINDS INIT 22/0300Z 21.7N 49.5W 40 KT 45 MPH 12H 22/1200Z 21.9N 49.3W 40 KT 45 MPH 24H 23/0000Z 21.5N 48.4W 35 KT 40 MPH 36H 23/1200Z 20.9N 47.5W 35 KT 40 MPH 48H 24/0000Z 20.9N 46.9W 35 KT 40 MPH 72H 25/0000Z 22.3N 47.1W 40 KT 45 MPH 96H 26/0000Z 24.4N 48.2W 45 KT 50 MPH 120H 27/0000Z 26.2N 48.9W 50 KT 60 MPH $$ Forecaster Kimberlain

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Remnants of SIXTEEN-E Forecast Discussion Number 5

2015-09-21 22:39:29| National Hurricane Center (East Pacific)

Issued at 300 PM MDT MON SEP 21 2015 000 WTPZ41 KNHC 212039 TCDEP1 REMNANTS OF SIXTEEN-E DISCUSSION NUMBER 5 NWS NATIONAL HURRICANE CENTER MIAMI FL EP162015 300 PM MDT MON SEP 21 2015 Surface observations and conventional satellite imagery indicate that the surface circulation of Sixteen-E has dissipated. Therefore, this will be the last advisory on this system from the National Hurricane Center. Although the depression has dissipated, dynamical guidance show the system's remnants moving north-northeastward into the southwestern U.S. during the next day or so and will continue to be a heavy rainfall and flash flooding event. Because the remnants of Sixteen-E continue to represent a significant rainfall threat, the NOAA Weather Prediction Center will issue public advisories while that threat exists, beginning at 8 p.m. PDT under AWIPS header TCPEP1 and WMO header WTPZ31 KWNH and on the web at http://www.hpc.ncep.noaa.gov/tropical/. FORECAST POSITIONS AND MAX WINDS INIT 21/2100Z 30.7N 111.7W 25 KT 30 MPH...REMNANTS OF SIXTEEN-E 12H 22/0600Z...DISSIPATED $$ Forecaster Roberts

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Tropical Storm IDA Forecast Discussion Number 14

2015-09-21 22:34:48| National Hurricane Center (Atlantic)

Issued at 500 PM AST MON SEP 21 2015 000 WTNT45 KNHC 212034 TCDAT5 TROPICAL STORM IDA DISCUSSION NUMBER 14 NWS NATIONAL HURRICANE CENTER MIAMI FL AL102015 500 PM AST MON SEP 21 2015 Satellite data indicate that the low- and mid-level centers of Ida have become separated, with the low-level center exposed well to the northwest of the deep convection. This appears to be the result of some unanticipated mid-level shear that is occurring below the typical 200 mb outflow layer. The latest TAFB Dvorak classification and objective ADT estimates from UW/CIMSS support an initial intensity of 40-45 kt, and based on the earlier ASCAT data the advisory wind speed is held at 45 kt, although this could be generous. The forecast upper-level wind pattern over the next couple of days, which features continued mid-level shear along with an increase in upper-level northwesterly winds, should prevent strengthening. In fact, Ida could weaken during the next 2-3 days, and perhaps not survive as a tropical cyclone. The NHC forecast assumes that Ida will maintain tropical cyclone status, and that upper-level winds will become a little more conducive for strengthening by late in the period. The updated NHC intensity forecast is lower than the previous one, but still shows some increase in strength at 96 and 120 h. Ida continues to move north-northwestward, but recent visible satellite imagery suggest that the forward speed is decreasing. The track forecast reasoning remains unchanged from the previous advisory. The steering currents around the tropical storm are expected to weaken very soon, and Ida is expected to meander eastward or east-southeastward over the next couple of days. After that time, Ida should begin a northwestward or north-northwestward motion as a mid- to upper-level trough lifts out to the northeast. The track guidance has become quite divergent this cycle with very large spread between a faster and more eastward ECMWF solution, and a slower and more westward GFS track. For now, the official forecast is near the middle of the guidance envelope, closest to the GFS ensemble mean and FSU Superensemble. FORECAST POSITIONS AND MAX WINDS INIT 21/2100Z 21.2N 48.8W 45 KT 50 MPH 12H 22/0600Z 21.6N 48.7W 45 KT 50 MPH 24H 22/1800Z 21.6N 48.1W 45 KT 50 MPH 36H 23/0600Z 21.1N 47.3W 45 KT 50 MPH 48H 23/1800Z 20.8N 46.7W 45 KT 50 MPH 72H 24/1800Z 21.5N 46.8W 45 KT 50 MPH 96H 25/1800Z 23.0N 47.8W 50 KT 60 MPH 120H 26/1800Z 25.0N 48.5W 55 KT 65 MPH $$ Forecaster Brown

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Tropical Storm IDA Forecast Discussion Number 13

2015-09-21 16:39:20| National Hurricane Center (Atlantic)

Issued at 1100 AM AST MON SEP 21 2015 000 WTNT45 KNHC 211439 TCDAT5 TROPICAL STORM IDA DISCUSSION NUMBER 13 NWS NATIONAL HURRICANE CENTER MIAMI FL AL102015 1100 AM AST MON SEP 21 2015 Ida has been producing a large cluster of very deep convection overnight and this morning. The tropical cyclone, however, is still being affected by shear as the low-level center can be seen in visible satellite imagery just northwest of the convection, and this was confirmed by a recent scatterometer pass. The initial wind speed is maintained at 45 kt, which is based on the latest TAFB Dvorak T-number and the recent ASCAT data that revealed winds of around 40 kt. The upper-level wind pattern is forecast to become a little more conducive for strengthening during the next 12 to 24 hours and the NHC forecast calls for some slight intensification. After that time, northwesterly shear is expected to increase as an upper-level trough to the northeast of Ida retrogrades westward. This could cause some weakening, but most of the intensity guidance shows little change in strength during this time, and the NHC forecast maintains an intensity of 55 kt from 24 to 72 hours. Later in the forecast period, the global models take the upper-level trough northeastward and develop a more favorable upper-air pattern over the tropical cyclone, which should allow for strengthening by days 4 and 5. The new NHC intensity forecast is similar to the previous advisory, and is close to the SHIPS and LGEM models. The forward motion of Ida continues to decrease and is now 330/6 kt. The steering currents around the tropical cyclone are forecast to further weaken during the next day or so as the mid- to upper-level trough retrogrades westward. Ida should become nearly stationary tonight, then meander eastward or east- southeastward on Tuesday and Wednesday as the mid- to upper-level flow becomes northwestward. The dynamical models have come into better agreement on this scenario, including the latest GFS run that shifted eastward and is now close to the ECMWF. After 72 hours, when the trough moves northeastward, Ida should turn northwestward at a faster forward speed. The updated NHC track has been adjusted eastward and is close to a blend of the ECMWF, GFS, and GFS ensemble mean. FORECAST POSITIONS AND MAX WINDS INIT 21/1500Z 20.6N 48.0W 45 KT 50 MPH 12H 22/0000Z 21.1N 48.1W 50 KT 60 MPH 24H 22/1200Z 21.3N 47.7W 55 KT 65 MPH 36H 23/0000Z 21.2N 47.3W 55 KT 65 MPH 48H 23/1200Z 20.7N 46.7W 55 KT 65 MPH 72H 24/1200Z 20.8N 46.1W 55 KT 65 MPH 96H 25/1200Z 22.0N 46.8W 60 KT 70 MPH 120H 26/1200Z 24.0N 48.0W 65 KT 75 MPH $$ Forecaster Brown

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Tropical Depression SIXTEEN-E Forecast Discussion Number 4

2015-09-21 16:37:22| National Hurricane Center (East Pacific)

Issued at 900 AM MDT MON SEP 21 2015 000 WTPZ41 KNHC 211437 TCDEP1 TROPICAL DEPRESSION SIXTEEN-E DISCUSSION NUMBER 4 NWS NATIONAL HURRICANE CENTER MIAMI FL EP162015 900 AM MDT MON SEP 21 2015 GOES-15 shortwave I/R and first visible images show that the depression made landfall just to the northwest of Bahia Kino in the Mexican state of Sonora. The initial intensity is held at 30 kt based on the subjective intensity estimates and surface observations along the coast of the Gulf of California. Weakening is forecast as the cyclone moves further inland, and the depression is expected to degenerate into a remnant low pressure area, or possibly dissipate, later today. The depression is moving north-northeastward and a little faster now, about 16 kt, and this general motion should continue until dissipation occurs. The official forecast is a little faster than the previous advisory and is nudged toward the multi-model consensus TVCX. The main hazard from the depression continues to be heavy rainfall produced by the deep convection displaced well to the northeast of the surface circulation. The depression is expected to produce rainfall amounts of 3 to 5 inches across the northern Baja California peninsula, the northwestern Mexican state of Sonora, and Arizona and New Mexico. Isolated maximum amounts of 12 inches are possible in Baja California and Sonora, Mexico. Isolated maximum amounts of 8 inches are possible in Arizona and New Mexico. 1 to 2 inches of rain is expected over portions of southern California through Tuesday. This rainfall could cause life-threatening flash floods and mudslides, especially in mountainous areas. FORECAST POSITIONS AND MAX WINDS INIT 21/1500Z 29.2N 112.1W 30 KT 35 MPH...INLAND 12H 22/0000Z 31.7N 111.2W 25 KT 30 MPH...POST-TROP/REMNT LOW 24H 22/1200Z 34.6N 109.7W 20 KT 25 MPH...POST-TROP/REMNT LOW 36H 23/0000Z...DISSIPATED $$ Forecaster Roberts

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