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Tropical Storm KARL Forecast Discussion Number 25

2016-09-20 16:36:14| National Hurricane Center (Atlantic)

Issued at 1100 AM AST TUE SEP 20 2016 000 WTNT42 KNHC 201436 TCDAT2 TROPICAL STORM KARL DISCUSSION NUMBER 25 NWS NATIONAL HURRICANE CENTER MIAMI FL AL122016 ISSUED BY THE NWS WEATHER PREDICTION CENTER COLLEGE PARK MD 1100 AM AST TUE SEP 20 2016 Karl's center remains exposed to the southwest of the deep convection, as it appears that the cyclone has been unable to escape the influence of a mid-/upper-level low to its west. Dvorak Current Intensity estimates remain 3.0 from TAFB and 2.5 from SAB, and the latest UW-CIMSS ADT estimate is T2.5. The initial intensity is therefore held at 35 kt. SHIPS and UW-CIMSS shear analyses indicate that the vertical shear that has been affecting Karl has not yet decreased, but it is now southerly at around 15 kt. The global models continue to indicate that the shear should decrease during the next few days, but this probably won't happen until Karl becomes more separated from the upper-level low. The thermodynamic environment has improved slightly since yesterday, as the cyclone appears to be associated with a large moisture envelope with mid-level relative humidities increasing into the 50 percent range. Little change in strength is likely during the next 24 hours, but after that time intensification is expected due to warm SSTs, more moisture, and lower shear. The official intensity forecast continues to show Karl as a hurricane in the 3-5 day range, but the updated forecast has been nudged downward slightly based on the latest guidance. The new forecast is very close to SHIPS guidance and is near the upper end of the guidance envelope. Karl's center has been straddling the 20th parallel, but the 12-hour motion estimate is 275/15 kt. Karl should be reaching the western extent of the subtropical high during the next 48 hours, and its motion should therefore turn northwestward by day 2 and then northward by day 4. After that time, the cyclone is expected to accelerate toward the northeast when it gets picked up by the mid-latitude westerlies ahead of a trough. The cross-track spread in the guidance has decreased since yesterday for the time period when Karl recurves, although there are significant speed differences after recurvature. Most notably, the ECMWF is significantly slower than the other models and doesn't show Karl being picked up as quickly by the mid-latitude trough. To split the difference, the updated track forecast is very similar to the previous one and is close to a consensus of the GFS and ECMWF. FORECAST POSITIONS AND MAX WINDS INIT 20/1500Z 20.0N 54.3W 35 KT 40 MPH 12H 21/0000Z 20.7N 56.0W 35 KT 40 MPH 24H 21/1200Z 21.9N 58.3W 35 KT 40 MPH 36H 22/0000Z 23.5N 60.5W 45 KT 50 MPH 48H 22/1200Z 25.1N 62.7W 55 KT 65 MPH 72H 23/1200Z 27.6N 65.2W 70 KT 80 MPH 96H 24/1200Z 31.0N 63.5W 85 KT 100 MPH 120H 25/1200Z 36.5N 54.5W 90 KT 105 MPH $$ Forecaster Berg/Krekeler

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Tropical Storm KARL Forecast Discussion Number 24

2016-09-20 10:59:42| National Hurricane Center (Atlantic)

Issued at 500 AM AST TUE SEP 20 2016 000 WTNT42 KNHC 200858 TCDAT2 TROPICAL STORM KARL DISCUSSION NUMBER 24 NWS NATIONAL HURRICANE CENTER MIAMI FL AL122016 500 AM AST TUE SEP 20 2016 Karl's satellite appearance late yesterday gave the impression that it had increased some in organization, with the low-level center partially underneath a large mass of deep convection. Since that time, the low-level center appears to have outrun the convection, becoming exposed well to the west of the remnant convective mass whose cloud top temperatures have warmed significantly. The cloud pattern is typical of a tropical cyclone undergoing significant westerly shear. Dvorak classifications have decreased to T2.5 from TAFB and SAB, and on this basis, the initial intensity estimate is lowered to 35 kt. The initial motion is a faster and a more westerly 290/15. Karl should continue to be steered generally west-northwestward around the southwestern periphery of well-defined low- to mid-level subtropical ridge over the central Atlantic during the next couple of days. The cyclone should move into weakness in the ridge along 65W in 48 to 72 hours, which should result in a gradual turn toward the north with a considerable decrease in forward speed by day 4. Around 96 hours, Karl should become susceptible to the mid-latitude westerly flow after it crosses 30N as it begins to undergo a sharp recurvature. The models vary tremendously on how quickly Karl will recurve by day 5, with the GFS-based guidance whisking the cyclone rapidly northeastward in response to a deep-layer trough moving through the northeastern United States and Atlantic Canada. The ECMWF and the majority of its ensemble members show Karl lagging way behind before being ejected northeastward by a slowly amplifying trough. The new track forecast is shifted to the left and is slightly faster through day 3, largely because of the initial re-positioning of the cyclone. After that time, a compromise between the much faster GFS-based guidance and the ECMWF results in a slower and somewhat more eastward forecast track. Karl can be seen approaching an upper-tropospheric cold low in water vapor imagery. The unfavorable juxtaposition of Karl and this feature should not result in much intensification for about the next 36 hours, since the flow over Karl is convergent and there is some deep-layer westerly shear. This is the rationale for keeping the short-term intensity forecast low. By 48 hours and beyond, the shear is forecast to have finally decreased while the cyclone reaches near 30 deg C waters, and intensification is likely. With the flow becoming increasingly divergent over Karl around the time it recurves in 3 to 4 days, the intensification could occur more rapidly than this forecast indicates, and there is some chance that Karl could become a major hurricane. Late in the period, increasing southwesterly shear could become a mitigating factor and the intensity is shown to be leveling off by day 5. The new NHC intensity forecast is a little lower than the multi-model consensus initially and about the same or a little lower at days 3-5. FORECAST POSITIONS AND MAX WINDS INIT 20/0900Z 20.2N 52.8W 35 KT 40 MPH 12H 20/1800Z 20.9N 54.6W 35 KT 40 MPH 24H 21/0600Z 21.7N 57.0W 35 KT 40 MPH 36H 21/1800Z 23.0N 59.2W 45 KT 50 MPH 48H 22/0600Z 24.7N 61.5W 55 KT 65 MPH 72H 23/0600Z 27.4N 64.7W 70 KT 80 MPH 96H 24/0600Z 30.3N 64.5W 90 KT 105 MPH 120H 25/0600Z 34.0N 57.9W 90 KT 105 MPH $$ Forecaster Kimberlain

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Tropical Storm PAINE Forecast Discussion Number 10

2016-09-20 10:51:47| Tropical Depression LIDIA

Issued at 200 AM PDT TUE SEP 20 2016

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Tropical Depression THIRTEEN Forecast Discussion Number 3

2016-09-20 10:42:16| National Hurricane Center (Atlantic)

Issued at 500 AM AST TUE SEP 20 2016 000 WTNT43 KNHC 200841 TCDAT3 TROPICAL DEPRESSION THIRTEEN DISCUSSION NUMBER 3 NWS NATIONAL HURRICANE CENTER MIAMI FL AL132016 500 AM AST TUE SEP 20 2016 The cloud pattern of the depression is only slightly better organized than yesterday, with some broken convective banding over the northeastern portion of the circulation. Overall, however, the system is elongated from southwest to northeast. The initial intensity is held at 30 kt, in agreement with Dvorak estimates from both TAFB and SAB. The tropical cyclone is expected to remain in a low shear environment for the next 48 hours or so, and therefore some strengthening is forecast during that time frame. Later in the period, the dynamical environment is likely to become hostile for strengthening since shear associated with an upper-level trough is forecast to increase to around 30 kt in 3-4 days. Given this expected change in the large-scale environment, weakening should begin in 2-3 days and the system is predicted to revert to tropical depression status by the end of the forecast period. Although the dynamical guidance shows some relaxation of the shear at the end of the period, the current thinking is that by that time the system will be too disrupted to be able to recover. The official intensity forecast is close to the LGEM guidance through 72 hours and a little below it at days 4 and 5. There is considerable spread in center fixes and the initial motion is an uncertain 295/8 kt. A weakness in the subtropical ridge along 30W-40W is likely to induce a more northwestward track during the next day or so, followed by a return to a west- northwestward motion as the ridge rebuilds a bit to the north of the cyclone. Late in the forecast period, the system is forecast to turn back toward the northwest due to another weakness in the ridge. The official track forecast is somewhat slower and to the right of the previous one but close to the new dynamical model consensus. FORECAST POSITIONS AND MAX WINDS INIT 20/0900Z 14.4N 29.6W 30 KT 35 MPH 12H 20/1800Z 15.4N 30.7W 35 KT 40 MPH 24H 21/0600Z 16.6N 32.2W 40 KT 45 MPH 36H 21/1800Z 17.5N 34.0W 45 KT 50 MPH 48H 22/0600Z 18.4N 35.8W 50 KT 60 MPH 72H 23/0600Z 20.8N 38.3W 45 KT 50 MPH 96H 24/0600Z 23.4N 40.6W 35 KT 40 MPH 120H 25/0600Z 26.5N 43.5W 30 KT 35 MPH $$ Forecaster Pasch

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Tropical Depression THIRTEEN Forecast Discussion Number 2

2016-09-20 04:45:06| National Hurricane Center (Atlantic)

Issued at 1100 PM AST MON SEP 19 2016 000 WTNT43 KNHC 200244 TCDAT3 TROPICAL DEPRESSION THIRTEEN DISCUSSION NUMBER 2 NWS NATIONAL HURRICANE CENTER MIAMI FL AL132016 1100 PM AST MON SEP 19 2016 The coverage of deep convection associated with the depression has decreased somewhat during the past several hours, but the circulation remains well established with several fragmented curved bands. Satellite intensity estimates from TAFB and SAB support maintaining the initial wind speed of 30 kt. The cyclone is moving west-northwestward at 8 kt. A weakness in the subtropical ridge over the east-central Atlantic is expected to persist for the next several days due to a series of shortwave troughs moving through the area. As a result, a continued west-northwestward to northwestward motion at about the same forward speed is expected throughout the forecast period. The models are in fair agreement, and the NHC track prediction lies on the left side of the guidance envelope, in best agreement with the ECMWF model. The environmental conditions for strengthening during the next several days are mixed. Although the shear is expected to be light and waters warm during the next couple of days, the atmosphere is not overly moist in the vicinity of the depression. These conditions should allow for slow strengthening during the next 48 hours. Beyond that time, however, the system is expected to move in less favorable conditions of stronger shear, even drier air, and more marginal SSTs. These conditions should halt strengthening and induce gradual weakening. The NHC intensity forecast lies close to the intensity model consensus for the first few days, but is lower than the consensus at days 4 and 5 given the expected hostile conditions. FORECAST POSITIONS AND MAX WINDS INIT 20/0300Z 14.2N 29.0W 30 KT 35 MPH 12H 20/1200Z 15.0N 30.4W 35 KT 40 MPH 24H 21/0000Z 16.1N 32.2W 40 KT 45 MPH 36H 21/1200Z 17.2N 34.0W 45 KT 50 MPH 48H 22/0000Z 18.1N 35.7W 50 KT 60 MPH 72H 23/0000Z 20.0N 38.9W 45 KT 50 MPH 96H 24/0000Z 22.0N 42.0W 35 KT 40 MPH 120H 25/0000Z 25.0N 44.5W 30 KT 35 MPH $$ Forecaster Cangialosi/Brown

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