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Hurricane Hilda Forecast Discussion Number 7

2021-08-01 10:37:01| National Hurricane Center (East Pacific)

Issued at 200 AM PDT Sun Aug 01 2021 000 WTPZ43 KNHC 010836 TCDEP3 Hurricane Hilda Discussion Number 7 NWS National Hurricane Center Miami FL EP082021 200 AM PDT Sun Aug 01 2021 Hilda's strengthening last evening was short lived. Infrared and water vapor imagery show high-level clouds from a weather disturbance to the east impinging on the eastern side of Hilda's circulation, suggestive of moderate easterly shear. In fact, 85-GHz SSMIS data from 0241 UTC shows that the microwave presentation has degraded significantly, with most of the deep convection displaced to the west of the low-level center. Hilda's initial intensity is being held at 75 kt based on Dvorak CI numbers of 4.5 from TAFB and SAB, although that estimate could be generous. Hilda continues to move west-northwestward, or 285/9 kt, to the south of a subtropical ridge which is expected to build westward over the Pacific during the next few days. Despite the placement of this ridge, the dynamical models indicate that Hilda is likely to have some degree of binary interaction with the disturbance to its east and thus take on a northwestward heading on days 2 and 3. After the interaction, the ridge should then cause Hilda to turn back toward the west-northwest and then west by days 4 and 5. While there are some model outliers, the spread among the guidance has actually decreased over the past 24 hours. The updated NHC track forecast is not too different from the previous forecast, except that it is a little slower to account for recent model trends. The moderate easterly shear affecting Hilda is unlikely to abate much during the next couple of days, especially as the distance between the hurricane and the disturbance to the east decreases. Still, Hilda will remain over relatively warm waters during that period, and the models suggest that the hurricane will either maintain its current intensity or strengthen slightly through 48 hours. Hilda is then expected to move over waters colder than 26 degrees Celsius, causing a steady weakening trend through the end of the forecast period. The new NHC intensity forecast is a little lower than the previous forecast to account for the current structure of the cyclone and the latest model solutions, although the forecast intensities are not as low as what is shown by the HCCA and IVCN consensus aids. FORECAST POSITIONS AND MAX WINDS INIT 01/0900Z 14.5N 119.6W 75 KT 85 MPH 12H 01/1800Z 14.8N 120.8W 80 KT 90 MPH 24H 02/0600Z 15.3N 122.1W 80 KT 90 MPH 36H 02/1800Z 15.9N 123.3W 80 KT 90 MPH 48H 03/0600Z 16.7N 124.3W 75 KT 85 MPH 60H 03/1800Z 17.6N 125.4W 65 KT 75 MPH 72H 04/0600Z 18.7N 126.9W 55 KT 65 MPH 96H 05/0600Z 20.1N 130.9W 45 KT 50 MPH 120H 06/0600Z 20.7N 135.3W 35 KT 40 MPH $$ Forecaster Berg

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Tropical Depression Nine-E Forecast Discussion Number 6

2021-08-01 04:41:26| National Hurricane Center (East Pacific)

Issued at 500 PM HST Sat Jul 31 2021 000 WTPZ44 KNHC 010241 TCDEP4 Tropical Depression Nine-E Discussion Number 6 NWS National Hurricane Center Miami FL EP092021 500 PM HST Sat Jul 31 2021 The tropical depression's classification as a tropical cyclone is in doubt. It has not produced sustained organized deep convection for over a day and is nearly devoid of even moderate convection at this time. In addition, the surface wind field is poorly defined. A prominent swirl noted in the previous forecast package moved quickly southeastward and dissipated, leaving only a broad, elongated low centered east of previous estimates. The most recent TAFB Dvorak fix still supports an intensity of 25 kt. If organized deep convection does not redevelop soon, the system could become a remnant low or open into a trough at any time. Even if the depression is able to maintain its status as a tropical cyclone, the close proximity of rapidly intensifying Hilda to the east will likely prevent it from strengthening during the next 72 h, and this is reflected in the new NHC intensity forecast. After that time, Hilda is forecast to weaken, which could open a window for intensification (or re-formation) late in the forecast period. The official intensity forecast is now below the intensity consensus at most forecast hours. It is worth noting that the operational regional hurricane models do not capture storm-to-storm interactions very well, and this is likely influencing the relatively high intensity forecast produced by the HWRF. The eastward adjustment of the initial position has necessitated a large eastward shift in the forecast track based on the new center position. Otherwise, the general reasoning behind the NHC track forecast is similar to the previous advisory. A slow, westward to west-northwestward motion is expected for the next few days. Beyond that time, differences regarding the specifics of any direct interaction with Hilda is the primary source of uncertainty in the track forecast. Confidence in the forecast, especially at that long range, remains low. The NHC forecast is based primarily on a consensus of the GFS and ECMWF global models. FORECAST POSITIONS AND MAX WINDS INIT 01/0300Z 11.5N 127.5W 25 KT 30 MPH 12H 01/1200Z 11.8N 128.4W 25 KT 30 MPH 24H 02/0000Z 12.0N 130.0W 30 KT 35 MPH 36H 02/1200Z 12.2N 131.5W 30 KT 35 MPH 48H 03/0000Z 12.5N 133.0W 30 KT 35 MPH 60H 03/1200Z 12.9N 134.2W 30 KT 35 MPH 72H 04/0000Z 13.5N 135.6W 30 KT 35 MPH 96H 05/0000Z 14.5N 138.0W 35 KT 40 MPH 120H 06/0000Z 15.5N 141.0W 40 KT 45 MPH $$ Forecaster Zelinsky

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Hurricane Hilda Forecast Discussion Number 6

2021-08-01 04:36:53| National Hurricane Center (East Pacific)

Issued at 800 PM PDT Sat Jul 31 2021 000 WTPZ43 KNHC 010236 TCDEP3 Hurricane Hilda Discussion Number 6 NWS National Hurricane Center Miami FL EP082021 800 PM PDT Sat Jul 31 2021 Hilda has resumed strengthening during the past several hours, including the short-lived appearance of an eye in the central dense overcast. Recent 37-GHz microwave imagery confirms an eye is developing, but indicates that the eyewall is not yet closed on the northeastern side of the eye. Satellite intensity estimates from TAFB and SAB are 77 kt, while the CIMSS satellite consensus is near 80 kt. Based on these data, the initial intensity is increased to a possibly conservative 75 kt. Although Hilda is experiencing some easterly vertical shear, conditions are generally conducive for strengthening during the next 24 h, and the early part of the intensity forecast has been adjusted upward based mainly on current trends. After that time, decreasing moisture and decreasing sea surface temperatures along the forecast track should cause steady weakening. The latter part of the intensity forecast has only minor changes from the previous forecast and follows the trend of the intensity guidance. The initial motion is now a little slower at 285/9. The subtropical ridge to the north should steer Hilda generally west-northwestward for the next several days, with a more northwestward motion around 96 h as the cyclone passes south of a weakness in the ridge. Overall, the guidance envelope has shifted a little northward since the previous advisory, and the new forecast track is also adjusted northward. It should be noted, though, that the global models continue to show the possibility of erratic motion due to Hilda interacting with TD-9E to the west and the weaker, but larger, low pressure area to the east. The most drastic example of this is in the Canadian model, which shows Hilda taking a significant turn to the south before resuming a northward motion. FORECAST POSITIONS AND MAX WINDS INIT 01/0300Z 14.3N 118.7W 75 KT 85 MPH 12H 01/1200Z 14.7N 120.0W 85 KT 100 MPH 24H 02/0000Z 15.2N 121.7W 90 KT 105 MPH 36H 02/1200Z 15.8N 123.1W 85 KT 100 MPH 48H 03/0000Z 16.4N 124.2W 80 KT 90 MPH 60H 03/1200Z 17.2N 125.4W 75 KT 85 MPH 72H 04/0000Z 18.3N 126.8W 65 KT 75 MPH 96H 05/0000Z 20.0N 130.5W 50 KT 60 MPH 120H 06/0000Z 20.5N 134.0W 40 KT 45 MPH $$ Forecaster Beven

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Tropical Depression Nine-E Forecast Discussion Number 5

2021-07-31 22:39:21| National Hurricane Center (East Pacific)

Issued at 1100 AM HST Sat Jul 31 2021 000 WTPZ44 KNHC 312039 TCDEP4 Tropical Depression Nine-E Discussion Number 5 NWS National Hurricane Center Miami FL EP092021 1100 AM HST Sat Jul 31 2021 Visible and scatterometer satellite data continue to show depression getting stretched further from west-to-east, with the surface circulation now elliptical-shaped about 500 nmi long and 250 nmi wide. A 1757Z ASCAT-B pass revealed an ill-defined low-level circulation center and one 26-kt wind vector 60-70 nmi west of the center. Unlike a few hours ago, deep convection has begun to wane with cloud tops now warmer than -70 deg C. The intensity has been held at 25 kt based on the ASCAT wind data and the overall poor appearance in satellite imagery. The initial motion estimate is an uncertain 270/04 kt. The aforementioned ASCAT data suggest that either the center has reformed farther to the southwest near a small burst of deep convection, or that that feature is just a smaller swirl rotating around the larger gyre envelope. Thus, the estimated center location is an average between the previous center position and the small center noted in the ASCAT data. Otherwise, there is no significant change to the previous forecast tack or reasoning. After 72 hours, the models remain is major disagreement on how much, if any, binary interaction occurs between the depression and Tropical Storm Hilda, located about 650 nmi to the east. The new 12Z GFS model remains the most extreme and takes the depression northward on days 4 and 5, with the ECMWF again the weakest with little interaction with Hilda. The remainder of the NHC track guidance lies somewhere in between these two extremes. As in the previous advisory, the best call for now is to punt by remaining close to the previous forecast track, with the new official NHC track forecast still lying inside the southern edge of the guidance envelope between the ECMWF solution to the south and the consensus models farther north. The depression's future intensity, and even its existence as a tropical cyclone, depends heavily on the track over the next 120 hours. A more westward motion as per the ECMWF would keep the cyclone over warmer water and in a more favorable upper-level pattern, whereas a sharp northward motion like the GFS is predicting would take the cyclone over cold SSTs below 25 deg C and into a strong wind shear environment. Another negative factor continues to be the west-to-east stretching of the depression's low-level wind field caused by Tropical Storm Hilda's larger and stronger circulation. As more of the southerly/southwesterly cross-equatorial low-level inflow gets drawn away from the elongated cyclone and into Hilda, most of the depression's inflow will be cooler and drier air trade wind flow coming into the northern semicircle, which would induce weakening. The previous intensity forecast is being maintained for this advisory, which continues to show little strengthening for the next 48 hours, followed by only modest intensification thereafter. However, this remains a low-confidence forecast. FORECAST POSITIONS AND MAX WINDS INIT 31/2100Z 11.6N 128.4W 25 KT 30 MPH 12H 01/0600Z 11.5N 129.1W 25 KT 30 MPH 24H 01/1800Z 11.6N 130.4W 30 KT 35 MPH 36H 02/0600Z 11.8N 131.9W 30 KT 35 MPH 48H 02/1800Z 12.2N 133.5W 35 KT 40 MPH 60H 03/0600Z 12.6N 134.7W 35 KT 40 MPH 72H 03/1800Z 13.0N 135.8W 40 KT 45 MPH 96H 04/1800Z 14.0N 138.1W 45 KT 50 MPH 120H 05/1800Z 15.4N 140.8W 45 KT 50 MPH $$ Forecaster Stewart

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Tropical Storm Hilda Forecast Discussion Number 5

2021-07-31 22:36:19| National Hurricane Center (East Pacific)

Issued at 200 PM PDT Sat Jul 31 2021 000 WTPZ43 KNHC 312036 TCDEP3 Tropical Storm Hilda Discussion Number 5 NWS National Hurricane Center Miami FL EP082021 200 PM PDT Sat Jul 31 2021 After strengthening quickly overnight and early this morning, Hilda's intensity appears to have leveled off for now. There continues to be hints of an eye feature in satellite images and deep convection is organized in curved bands around that feature. The latest Dvorak estimates are 3.5/55 kt from TAFB, 4.0/65 kt from SAB, and 4.1/67 kt from CIMSS at the University of Wisconsin. A recent ASCAT-B overpass showed a maximum wind of around 50 kt southeast of the center, but the coarse resolution of the instrument is unlikely to capture the storm's true intensity. Based on a combination of this data, the initial intensity is held at 60 kt, but this could be a little conservative and Hilda is very near hurricane strength. The aforementioned ASCAT data was also used to adjust the wind radii inward at the initial and short range forecast times. Steady strengthening seems likely during the next 24 to 36 hours as the environment remains generally favorable, consisting of relatively warm SSTs, low wind shear, and a high amount of moisture. The NHC intensity forecast during that time period is the same as before and lies at the high end of the model guidance, near HCCA. Beyond 36 hours, however, decreasing moisture and progressively cooler SSTs should end the strengthening trend and gradually cause weakening during the remainder of the forecast period. The NHC intensity forecast lies near the middle of the guidance envelope for that portion of the forecast. Hilda is moving west-northwestward at 12 kt. A subtropical ridge should keep the storm on a general west-northwest heading during the next several days, but the more skillful models like the GFS and ECMWF show some interaction with Tropical Depression Nine-E to the west and a low pressure area to the east. These interactions will likely cause some slow downs and an erratic motion at times during the forecast period. There continues to be a fair amount of spread in the models, especially in terms of forward speed, and the new forecast is a little to the north of the previous based on the latest guidance. This track prediction is slightly slower than the consensus aids, and has given more weight to the GFS and ECMWF models. FORECAST POSITIONS AND MAX WINDS INIT 31/2100Z 14.1N 117.9W 60 KT 70 MPH 12H 01/0600Z 14.5N 119.3W 70 KT 80 MPH 24H 01/1800Z 15.0N 121.0W 80 KT 90 MPH 36H 02/0600Z 15.5N 122.5W 85 KT 100 MPH 48H 02/1800Z 16.0N 123.8W 80 KT 90 MPH 60H 03/0600Z 16.8N 125.1W 75 KT 85 MPH 72H 03/1800Z 17.8N 126.7W 65 KT 75 MPH 96H 04/1800Z 19.3N 129.8W 50 KT 60 MPH 120H 05/1800Z 20.4N 133.7W 40 KT 45 MPH $$ Forecaster Cangialosi

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