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Hurricane ALEX Forecast Discussion Number 6

2016-01-15 03:43:14| National Hurricane Center (Atlantic)

Issued at 1100 PM AST THU JAN 14 2016 000 WTNT41 KNHC 150243 TCDAT1 HURRICANE ALEX DISCUSSION NUMBER 6 NWS NATIONAL HURRICANE CENTER MIAMI FL AL012016 1100 PM AST THU JAN 14 2016 Satellite imagery indicates that the convective structure of Alex has decayed during the past 6 hours, with the eyewall breaking open and the banding dissipating in the southeastern semicircle. Based on this, the initial intensity is lowered to 70 kt. The hurricane is moving over colder sea surface temperatures of less than 20C. However, the cyclone should begin extratropical transition in the next few hours, and the global models suggest there will be enough baroclinic forcing to maintain hurricane-force winds through and after the transition despite the cold water. Based on this and input from the Ocean Prediction Center, the new intensity forecast shows little change in strength until Alex is absorbed by a large extratropical low in about 72 hours. The initial motion is 010/19. There are no changes to the forecast philosophy from the previous advisory and only minor tweaks to the forecast track. Alex continues to be steered by the flow of a shortwave mid-tropospheric trough that is rotating around the developing large baroclinic low to its west and northwest. This evolution should cause the cyclone to turn gradually to the left with increasing forward speed over the next couple of days. The dynamical track guidance models remain in excellent agreement for the first 36 hours of the forecast, and the official forecast is near the model consensus. Alex's wind field is likely to expand as it undergoes extratropical transition. The wind radii forecasts are based primarily on guidance from the NOAA Ocean Prediction Center. FORECAST POSITIONS AND MAX WINDS INIT 15/0300Z 34.7N 27.6W 70 KT 80 MPH 12H 15/1200Z 38.4N 27.8W 65 KT 75 MPH 24H 16/0000Z 45.0N 29.1W 65 KT 75 MPH...POST-TROP/EXTRATROP 36H 16/1200Z 52.7N 32.1W 65 KT 75 MPH...POST-TROP/EXTRATROP 48H 17/0000Z 59.0N 37.0W 65 KT 75 MPH...POST-TROP/EXTRATROP 72H 18/0000Z...ABSORBED BY EXTRATROPICAL LOW $$ Forecaster Beven

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Hurricane ALEX Forecast Discussion Number 5

2016-01-14 21:36:06| National Hurricane Center (Atlantic)

Issued at 500 PM AST THU JAN 14 2016 000 WTNT41 KNHC 142036 TCDAT1 HURRICANE ALEX DISCUSSION NUMBER 5 NWS NATIONAL HURRICANE CENTER MIAMI FL AL012016 500 PM AST THU JAN 14 2016 Alex has been maintaining a fairly impressive appearance on satellite imagery, with a well-defined 15-20 n mi diameter eye embedded within cold cloud tops. Recent images do suggest some warming/erosion of the tops over the southwest quadrant. Dvorak T-numbers remain near 4.5 so the current intensity is held at 75 kt. Sea surface temperatures are now below 20 deg C and should continue to cool along the path of Alex. This, along with a little increase in southwesterly shear, should result in gradual weakening. However, Alex is expected to maintain hurricane strength while passing near or over the Azores. In 24 hours or so, the global models show a distinct warm front over the northeast portion of the circulation. This suggests extratropical transition, and the official forecast reflects this. Later in the forecast period, the global models show the system merging with another extratropical cyclone over the northern Atlantic. The initial motion is slightly east of due northward or 015/19. There is essentially no change to the track forecast reasoning. Alex continues to be steered by the flow of a shortwave mid-tropospheric trough that has been rotating around a broader trough to its northwest. This evolution should cause the cyclone to turn gradually to the left with increasing forward speed over the next couple of days. The dynamical track guidance models are in excellent agreement for the first 36 hours of the forecast, and the official forecast is near the consensus of these models. This is basically an update of the previous NHC track prediction. Alex's wind field is likely to expand as it nears and makes the extratropical transition. The wind radii forecasts are based primarily on guidance from the NOAA Ocean Prediction Center. FORECAST POSITIONS AND MAX WINDS INIT 14/2100Z 33.6N 27.8W 75 KT 85 MPH 12H 15/0600Z 36.4N 27.5W 75 KT 85 MPH 24H 15/1800Z 41.8N 27.9W 70 KT 80 MPH...POST-TROP/EXTRATROP 36H 16/0600Z 49.5N 29.7W 65 KT 75 MPH...POST-TROP/EXTRATROP 48H 16/1800Z 56.5N 34.0W 60 KT 70 MPH...POST-TROP/EXTRATROP 72H 17/1800Z...MERGED WITH ANOTHER EXTRATROPICAL CYCLONE $$ Forecaster Pasch

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Hurricane ALEX Forecast Discussion Number 4

2016-01-14 15:34:43| National Hurricane Center (Atlantic)

Issued at 1100 AM AST THU JAN 14 2016 000 WTNT41 KNHC 141434 TCDAT1 HURRICANE ALEX DISCUSSION NUMBER 4 NWS NATIONAL HURRICANE CENTER MIAMI FL AL012016 1100 AM AST THU JAN 14 2016 Remarkably, Alex has undergone the transformation into a hurricane. A distinct eye is present, embedded within a fairly symmetric mass of deep convection. Water vapor imagery shows that the upper-level trough is now west of the cyclone, with divergent flow over the center - indicative of a tropical transition. It is very unusual to have a hurricane over waters that are near 20 deg C, but the upper-tropospheric temperatures are estimated to be around -60 deg C, which is significantly colder than the tropical mean. The resulting instability is likely the main factor contributing to the tropical transition and intensification of Alex. With these changes, the government of the Azores has issued warnings for most of the Azores islands. The initial intensity is set to 75 kt in accordance with the analyzed Dvorak T-number of 4.5. Only slight additional intensification seems possible since the system will be passing over even colder waters during the next day or two. In 36 hours, the global models suggest that the cyclone will become extratropical as it begins to merge with a large low pressure area at high latitude. The post-tropical cyclone is then likely to lose its identity after 48 hours. The initial motion is north-northeastward or 020/17 kt. Alex is being steered by a shortwave mid-level trough that is rotating around a larger trough to the northwest. This should cause the cyclone to turn northward and north-northwestward and accelerate over the next couple of days. The official track forecast is very similar to the previous one and also quite close to the consensus of the tightly-packed dynamical model forecast tracks. Alex is the first hurricane to form in the month of January since 1938, and the first hurricane to occur in this month since Alice of 1955. FORECAST POSITIONS AND MAX WINDS INIT 14/1500Z 31.5N 28.4W 75 KT 85 MPH 12H 15/0000Z 34.3N 27.7W 80 KT 90 MPH 24H 15/1200Z 38.9N 27.7W 75 KT 85 MPH 36H 16/0000Z 45.3N 28.6W 60 KT 70 MPH...POST-TROP/EXTRATROP 48H 16/1200Z 53.0N 31.5W 60 KT 70 MPH...POST-TROP/EXTRATROP 72H 17/1200Z...DISSIPATED $$ Forecaster Pasch

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Subtropical Storm ALEX Forecast Discussion Number 3

2016-01-14 09:49:35| National Hurricane Center (Atlantic)

Issued at 500 AM AST THU JAN 14 2016 000 WTNT41 KNHC 140849 TCDAT1 SUBTROPICAL STORM ALEX DISCUSSION NUMBER 3 NWS NATIONAL HURRICANE CENTER MIAMI FL AL012016 500 AM AST THU JAN 14 2016 Since the previous advisory, the convective structure of Alex has continued to improve with a 15-nmi clear eye now embedded within a solid cloud shield of tops colder than -50C, with a ring of cloud tops near -60C surrounding the eye in the northern semicircle. The intensity of Alex is difficult to ascertain due to its subtropical characteristics, and satellite intensity estimates range from ST3.5/55 kt from TAFB to a tropical T4.5/77 kt from SAB. NHC AODT intensity estimates range from T4.0/65 kt to T4.5/77 kt using a tropical pattern and various eye scenes. Since the overall cloud pattern of Alex has improved markedly since the earlier 50-kt ASCAT-B scatterometer wind data, and a clear and distinct eye feature is now evident, the intensity is being raised to 60 kt. Alex continues to gradually turn toward the left and the initial motion is now 020/16 kt. The cyclone is forecast to turn northward during the next 12 hours or so as it is steered by deep southerly flow between a large extratropical low centered over the northwest Atlantic and a subtropical ridge over northwestern Africa. A general northward motion is expected to continue through at least 36 hours, which should bring the center of Alex through the central Azores in about 30 hours or 1200 UTC 15 January. By 48 hours, Alex should start to turn more toward the northwest and west as it moves closer to the center of a larger extratropical low forecast to be south of Greenland on days 3 and 4. The new forecast track is essentially just an update of the previous advisory track through 36 hours, and lies close to the consensus model TVCA. Global and regional models, plus geostationary and microwave satellite data, indicate that Alex remains a vertically stacked low pressure system up to the 200 mb level, suggesting that the cyclone is still primarily a subtropical storm. However, water vapor imagery during the past few hours has been hinting that some weak upper-level outflow might be trying to develop. GFS and NAM model forecast soundings indicate that the current inner-core region of Alex with Lifted Indices (LI) of -2 and CAPE values of 400-500 are only expected to decrease to LI of -1 and CAPE near 300 as the cyclone nears the Azores islands in 24-30 hours. In addition, the inner-core region is forecast to remain saturated up to the 200 mb level, along with precipitable water values around 1.60 inches. This would suggest that Alex could remain as a subtropical cyclone by the time it reaches the Azores, and some slight strengthening is even possible as 300 mb temperatures are forecast to decrease from -40C to around -42C in the inner core. The official intensity has been nudged upward and is similar to a blend of the ECMWF and HRWF model intensity forecasts. Gale- and storm-force winds, as well as locally heavy rains, are likely to affect portions of the Azores beginning tonight and continuing into early Friday. Recent communications with the Azores Meteorological Office indicate that wind gusts to a least 70 kt (130 km/h) are forecast to occur across portions of the central and eastern Azores islands. However, stronger gusts will be possible at higher elevations, especially over mountain tops and ridges. Interests in the Azores should closely monitor the progress of Alex and official forecasts issued by the Azores weather office. FORECAST POSITIONS AND MAX WINDS INIT 14/0900Z 30.1N 29.3W 60 KT 70 MPH 12H 14/1800Z 32.4N 28.3W 60 KT 70 MPH 24H 15/0600Z 36.4N 27.6W 60 KT 70 MPH 36H 15/1800Z 42.0N 28.2W 60 KT 70 MPH...POST-TROP/EXTRATROP 48H 16/0600Z 49.1N 29.7W 60 KT 70 MPH...POST-TROP/EXTRATROP 72H 17/0600Z 60.0N 40.0W 60 KT 70 MPH...POST-TROP/EXTRATROP 96H 18/0600Z...MERGED WITH EXTRATROPICAL LOW $$ Forecaster Stewart

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Subtropical Storm ALEX Forecast Discussion Number 1

2016-01-13 21:32:31| National Hurricane Center (Atlantic)

Issued at 500 PM AST WED JAN 13 2016 000 WTNT41 KNHC 132032 TCDAT1 SUBTROPICAL STORM ALEX DISCUSSION NUMBER 1 NWS NATIONAL HURRICANE CENTER MIAMI FL AL012016 500 PM AST WED JAN 13 2016 Curved bands of cloudiness and showers/thunderstorms associated with the low pressure system over the eastern subtropical Atlantic have become better defined over the past 24 hours. Although the convection is not very deep, it is likely of at least moderate intensity given the relatively shallow tropospause over the area. Given the increased organization, and the apparent dissipation of nearby frontal features, advisories are being initiated at this time. The cyclone is co-located with an upper-level low, and appears to have only a weak warm core, so it is being designated as a subtropical storm. The current intensity is set to 45 kt in agreement with an earlier scatterometer overpass. A ship traversed the northern portion of the circulation earlier today and did not observe winds of tropical storm force and this is reflected in the advisory wind radii. The cyclone has been turning toward the left as it moves in the flow on the east side of a shortwave trough, and the initial motion is northeastward or 055/12 kt. The trough is expected to continue to swing counterclockwise around a broader mid-latitude cyclonic gyre, and this should result in Alex turning northward and north-northwestward over the next several days. The official forecast track follows the dynamical model consensus. Although the shear is not forecast to become very strong over the next several days, the cyclone will be moving over progressively colder waters. Therefore no increase in strength is shown for the next day or so. In the latter part of the forecast period, some strengthening is possible due to baroclinic processes. By 96 hours, the global models show the cyclone merging or becoming absorbed by another extratropical low at high latitudes. Alex is expected to become an extratropical cyclone by the time it passes near or over the Azores, so no tropical storm warnings are being issued for those islands. However, gale force winds are likely to affect portions of the Azores beginning late on Thursday or early on Friday. Alex is the first tropical or subtropical storm to form in January since an unnamed system did so in 1978, and is only the fourth known to form in this month in the historical record that begins in 1851. FORECAST POSITIONS AND MAX WINDS INIT 13/2100Z 27.1N 30.8W 45 KT 50 MPH 12H 14/0600Z 28.9N 29.7W 45 KT 50 MPH 24H 14/1800Z 32.0N 28.2W 45 KT 50 MPH 36H 15/0600Z 36.3N 27.5W 45 KT 50 MPH 48H 15/1800Z 42.3N 28.2W 50 KT 60 MPH...POST-TROP/EXTRATROP 72H 16/1800Z 56.0N 34.0W 50 KT 60 MPH...POST-TROP/EXTRATROP 96H 17/1800Z...ABSORBED $$ Forecaster Pasch

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