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Tropical Storm Ophelia Forecast Discussion Number 4

2017-10-10 04:52:23| National Hurricane Center (Atlantic)

Issued at 1100 PM AST Mon Oct 09 2017 000 WTNT42 KNHC 100252 TCDAT2 Tropical Storm Ophelia Discussion Number 4 NWS National Hurricane Center Miami FL AL172017 1100 PM AST Mon Oct 09 2017 Ophelia's center is just south of a broad curved band of deep convection. At synoptic time, the subjective Dvorak classifications from TAFB and SAB were both at 2.5, or 35 kt, while the CIMSS SATCON was at 43 kt. Given the increasingly curved structure of the bands during the last couple of hours, the initial intensity is set at 45 kt, a bit higher than the previous advisory. While the tropical storm is fighting some moderate vertical shear currently, the global models suggest that this should lighten some between now and about day 4 as Ophelia resides between the mid-latitude and subtropical westerlies. Even though the system will be traversing cool 26C SSTs, upper-tropospheric temperatures likely will also be cool, allowing for deep convection to continue. The mid-level moisture analyzed in the SHIPS guidance appears to be somewhat dry, though the total precipitable water imagery shows distinct moistening near the system during the last couple of days. The official intensity forecast shows gradual intensification through day 3, then slow weakening thereafter. This is close to a blend of the LGEM statistical guidance and the HWRF dynamical model and is slightly above that of the previous advisory. The initial position has fairly small uncertainty as the low-level center is along the southern edge of the deep convection, as seen in the GOES-16 shortwave infrared imagery. Ophelia is moving toward the northeast at about 6 kt - somewhat unexpectedly - this evening. Despite this, the model guidance suggests that a ridge will soon build in strongly north of Ophelia and force the tropical storm to the southeast and then south during the next couple of days. Around day 3, Ophelia should get kicked out toward the east-northeast by an approaching trough in the mid-latitude westerlies. The official track forecast is north of the previous forecast in the short term and east-northeast in the long term, and is in between the HFIP Corrected Consensus Approach (HCCA) and the previous forecast. The initial tropical-storm-force wind radii was adjusted outward based upon a 2137Z AMSU size analysis. The official size forecast is based upon the RVCN consensus technique. FORECAST POSITIONS AND MAX WINDS INIT 10/0300Z 32.3N 39.0W 45 KT 50 MPH 12H 10/1200Z 32.5N 38.4W 50 KT 60 MPH 24H 11/0000Z 32.0N 38.1W 55 KT 65 MPH 36H 11/1200Z 31.1N 37.9W 65 KT 75 MPH 48H 12/0000Z 30.4N 37.5W 70 KT 80 MPH 72H 13/0000Z 30.8N 35.5W 75 KT 85 MPH 96H 14/0000Z 32.5N 32.0W 70 KT 80 MPH 120H 15/0000Z 34.5N 26.5W 60 KT 70 MPH $$ Forecaster Landsea

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Tropical Storm Ophelia Forecast Discussion Number 3

2017-10-09 22:37:49| National Hurricane Center (Atlantic)

Issued at 500 PM AST Mon Oct 09 2017 000 WTNT42 KNHC 092037 TCDAT2 Tropical Storm Ophelia Discussion Number 3 NWS National Hurricane Center Miami FL AL172017 500 PM AST Mon Oct 09 2017 Ophelia's convective pattern has continued to steadily improve with the formation of strong thunderstorms with cloud tops near -80C near the center, along with the development of tighter curved banding features in the southeastern semicircle. There has also been a noticeable increase in lightning activity within 30 nmi of the center. The TAFB Dvorak intensity estimate at 1800Z was T2.5/35 kt. However, NHC objective Dvorak estimates are ranging from T3.2/49 kt using a curved band pattern to T3.5/55 kt for a shear pattern. Based on the aforementioned estimates and the continued improvement in Ophelia's convective organization since the 1800Z TAFB estimate, the initial intensity is increased to 40 kt. The initial motion estimate is 055/03 kt. There is no significant change to the previous forecast track or reasoning. Ophelia is expected to remain embedded within a weak flow regime for the next 72 hours due to the cyclone being cut off from the mid-latitude westerlies, resulting in a slow and gradual turn toward the east and southeast during that time frame. Thereafter, a broad mid- to upper-level trough to the northwest, in conjunction with a building subtropical ridge to the south, is forecast to gradually accelerate Ophelia toward the northeast at a slightly faster forward speed of near 10 kt. The new NHC forecast track is close to the northern edge of the guidance envelope out of respect for the ECMWF model, but the forward speed is slower than the ECMWF and similar to a blend of the HCCA and TVCX consensus models. Ophelia is expected to remain over marginally warm SSTs of 26.5-27C, but temperatures aloft that are 2-3 deg C colder than normal should continue to produce steep lapse rates and strong instability, which will allow for deep convection to develop for the next 96 h or so. Both the GFS and ECMWF models continue to forecast the vertical wind shear to decrease sharply after 24 h, with the ECMWF forecasting shear values to decrease to less than 10 kt from 36-96 hours. Based on the availability of strong instability and lower shear, slow but steady strengthening is expected for the next 3-4 days, with only the occasional entrainment of very dry mid-level air preventing rapid intensification from occurring, at least in the short term. After 96 h, Ophelia will be encountering increasing southwesterly wind shear of 15-20 kt and also be moving over 25C sea-surface temperatures, a combination that should cap the intensification process and possibly even induce weakening. The SHIPS and LGEM statistical models, along with the HCCA and IVCN consensus models, have come into much better agreement on their intensity forecasts. Therefore, there is a higher degree of confidence in the official intensity forecast, which basically remains unchanged from the previous advisory. Note, it is possible that Ophelia could reach a higher peak intensity than indicated between 72-96 hours when the wind shear will be at its lowest value. FORECAST POSITIONS AND MAX WINDS INIT 09/2100Z 31.5N 39.6W 40 KT 45 MPH 12H 10/0600Z 31.6N 39.1W 45 KT 50 MPH 24H 10/1800Z 31.4N 38.5W 50 KT 60 MPH 36H 11/0600Z 30.8N 37.9W 55 KT 65 MPH 48H 11/1800Z 30.1N 37.6W 60 KT 70 MPH 72H 12/1800Z 29.7N 36.1W 65 KT 75 MPH 96H 13/1800Z 31.0N 33.7W 65 KT 75 MPH 120H 14/1800Z 32.4N 30.0W 65 KT 75 MPH $$ Forecaster Stewart

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Tropical Depression Seventeen Forecast Discussion Number 1

2017-10-09 10:33:17| National Hurricane Center (Atlantic)

Issued at 500 AM AST Mon Oct 09 2017 000 WTNT42 KNHC 090833 TCDAT2 Tropical Depression Seventeen Discussion Number 1 NWS National Hurricane Center Miami FL AL172017 500 AM AST Mon Oct 09 2017 The well-defined low pressure system over the eastern Atlantic several hundred miles southwest of the Azores has maintained an area of deep convection near and to the east of the center for the past several hours. Therefore, the system now meets the criteria for a tropical depression. The initial intensity is estimated to be 30 kt based on ASCAT data from around 0000 UTC and a 2.0/30 kt Dvorak classification from TAFB. The depression is moving slowly northward at 3 kt, and a continued slow north to north-northeast motion is expected today. A turn to the east and then southeast is forecast to begin tonight and continue through Wednesday while the cyclone is steered by the flow on the east side of a subtropical ridge. Thereafter, a mid- to upper-level trough is expected to approach the depression, and that should cause the system to move a little faster to the east and east-northeast. The NHC track forecast lies near the middle of the guidance envelope, in agreement with various consensus aids, and keeps the system far from any land areas. The depression is currently over marginally warm 27 deg C SSTs and in an environment of moderate to strong westerly shear. The models suggest that the shear will likely let up a little during the next couple of days, which could allow the depression to gradually strengthen while it remains over relatively warm waters. After that time, the shear could increase again, but the amount of shear depends on the exact track of the tropical cyclone. There is a fair amount of spread in the models at the longer range with the regional models (HWRF, HMON, and COAMPS-TC) bringing the system to hurricane strength while the statistical SHIPS and LGEM models show a much weaker system. The NHC intensity forecast leans toward the more conservative side of the guidance for now, but it should be noted that confidence in the intensity forecast is low. FORECAST POSITIONS AND MAX WINDS INIT 09/0900Z 31.1N 39.9W 30 KT 35 MPH 12H 09/1800Z 31.5N 39.8W 35 KT 40 MPH 24H 10/0600Z 31.7N 39.0W 35 KT 40 MPH 36H 10/1800Z 31.5N 38.4W 40 KT 45 MPH 48H 11/0600Z 31.0N 37.8W 45 KT 50 MPH 72H 12/0600Z 29.7N 37.0W 50 KT 60 MPH 96H 13/0600Z 29.7N 35.5W 50 KT 60 MPH 120H 14/0600Z 31.0N 32.0W 50 KT 60 MPH $$ Forecaster Cangialosi

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Tropical Depression Nate Forecast Discussion Number 17

2017-10-08 16:50:13| National Hurricane Center (Atlantic)

Issued at 1000 AM CDT Sun Oct 08 2017 000 WTNT41 KNHC 081450 TCDAT1 Tropical Depression Nate Discussion Number 17 NWS National Hurricane Center Miami FL AL162017 1000 AM CDT Sun Oct 08 2017 The center of Nate continues to move quickly north-northeastward over central Alabama. Moderate westerly shear has caused most of the deep convection and heavy rainfall to be displaced to the east and northeast of the center, and surface observations show that Nate has continued to rapidly weaken. Wind gusts to tropical-storm strength are still occurring over portions of Alabama and the Florida Panhandle, but there are no recent reports of sustained tropical-storm-force winds. Therefore, Nate is now a 30-kt tropical depression. Nate is expected to become a remnant low on Monday, and extratropical by Tuesday before it is absorbed by frontal system. The intensity foreast keeps the wind speed around 30 kt during the next 48 hours, since winds are expected to increase along the mid-Atlantic coast and southern New England coast on Monday when the post-tropical low approaches that area. Nate should continue to move quickly north-northeastward to northeastward within the mid-latitude westerlies during the next 24 to 36 hours. The cyclone is forecast to turn east-northeastward by Tuesday before it merges with the frontal system. This is the last NHC advisory on Nate. Heavy rainfall associated with Nate is expected to spread over the Tennessee Valley, the southern and central Appalachians, and the Ohio Valley during the next day or so. Future information on Nate system can be found in Public Advisories issued by the Weather Prediction Center beginning at 5 PM EDT, under AWIPS header TCPAT1, WMO header WTNT31 KWNH, and on the web at http://www.wpc.ncep.noaa.gov. These advisories will continue as long as Nate poses a flooding threat to the U.S. KEY MESSAGES: 1. Nate will bring heavy rainfall of 3 to 6 inches with isolated totals of 10 inches east of the Mississippi River from the central Gulf Coast into the Deep South, eastern Tennessee Valley, and southern Appalachians through Monday, resulting in the potential for flash flooding in these areas. 2. Moisture from Nate interacting with a frontal zone will also bring 2 to 5 inches of rain with isolated totals of 7 inches across the Ohio Valley and central Appalachians Sunday and Monday, which will increase the risk for flash flooding across these locations. 3. Wind gusts to tropical storm force are expected over portions of the Florida Panhandle, Alabama, and Georgia through this afternoon. 4. Persistent onshore flow will keep water levels elevated along portions of the northern Gulf coast through today. See products issues by your local National Weather Service Forecast Office for additional information. FORECAST POSITIONS AND MAX WINDS INIT 08/1500Z 33.1N 87.3W 30 KT 35 MPH...INLAND 12H 09/0000Z 36.2N 85.0W 30 KT 35 MPH...INLAND 24H 09/1200Z 40.1N 79.5W 30 KT 35 MPH...INLAND 36H 10/0000Z 43.1N 72.8W 30 KT 35 MPH...POST-TROP/REMNT LOW 48H 10/1200Z 45.0N 65.0W 30 KT 35 MPH...POST-TROP/EXTRATROP 72H 11/1200Z...ABSORBED BY FRONT $$ Forecaster Brown

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Hurricane Nate Forecast Discussion Number 15

2017-10-08 04:52:38| National Hurricane Center (Atlantic)

Issued at 1000 PM CDT Sat Oct 07 2017 000 WTNT41 KNHC 080252 TCDAT1 Hurricane Nate Discussion Number 15 NWS National Hurricane Center Miami FL AL162017 1000 PM CDT Sat Oct 07 2017 A ragged central dense overcast persists over Nate's center, but radar trends have shown all of the deep convection migrating to the north and northeast of the center, likely due to increasing south-southwesterly shear. Still, the northern eyewall, which will be moving onshore the Mississippi coast soon, remains quite vigorous. An Air Force Hurricane Hunter aircraft recently measured a maximum 850-mb flight-level wind of 89 kt and some SFMR values around 70 kt. Although the SFMR could be affected by wave shoaling in this case, the data as a whole appear to support an initial intensity of 75 kt. Nate's central pressure has been rising slowly, with the latest report from the plane being 984 mb. Nate has turned northward and slowed down a bit with an initial motion of 360/17 kt. On this course, the hurricane is expected to make a second landfall along the Mississippi coast within the next hour or two. After landfall, Nate should turn gradually toward the northeast and accelerate again during the next day or two while it moves between a large mid-level high off the southeastern U.S. coast and a large trough digging into the central U.S. This steering pattern will take Nate across the Deep South, Tennessee Valley, and central Appalachian Mountains during the next couple of days. The new NHC track forecast is unchanged from the previous one. With landfall imminent, no changes in intensity are expected before that time. After landfall, land and increasing shear should contribute to fast weakening, and Nate is expected to become a tropical storm in 6-12 hours and then weaken to a tropical depression by 36 hours. Nate is likely to become a remnant low by 48 hours, and the global models suggest that the cyclone will be absorbed by another low or cold front by 72 hours. That scenario is now reflected in the NHC forecast. KEY MESSAGES: 1. Nate is expected to bring life-threatening storm surge flooding near and well east of where the center makes landfall, and a storm surge warning is in effect from the mouth of the Mississippi River to the Okaloosa/Walton county line in Florida. Maximum flooding of 7 to 11 feet above ground level is expected along the Mississippi coast within the next several hours. 2. Hurricane conditions are spreading onshore along the coasts of Mississippi and Alabama, where a hurricane warning is in effect. The strongest winds are expected to occur primarily to the east of the track of the center. 3. Nate's fast forward speed after landfall will bring tropical-storm-force winds well inland across portions of the southeastern U.S. Tropical storm watches and warnings are in effect for portions of southeastern Mississippi, much of Alabama, and western Georgia. 4. Nate will bring heavy rainfall of 3 to 6 inches with isolated totals of 10 inches east of the Mississippi River from the central Gulf Coast into the Deep South, eastern Tennessee Valley, and southern Appalachians through Monday, resulting in the potential for flash flooding in these areas. 5. Moisture from Nate interacting with a frontal zone will also bring 2 to 5 inches of rain with isolated totals of 7 inches across the Ohio Valley and central Appalachians Sunday and Monday, which will increase the risk for flash flooding across these locations. FORECAST POSITIONS AND MAX WINDS INIT 08/0300Z 29.9N 89.1W 75 KT 85 MPH 12H 08/1200Z 32.1N 88.2W 50 KT 60 MPH...INLAND 24H 09/0000Z 35.7N 85.3W 35 KT 40 MPH...INLAND 36H 09/1200Z 39.3N 80.2W 25 KT 30 MPH...INLAND 48H 10/0000Z 42.1N 73.7W 25 KT 30 MPH...POST-TROP/REMNT LOW 72H 11/0000Z...DISSIPATED $$ Forecaster Berg

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