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Tropical Storm Marie Forecast Discussion Number 6

2020-09-30 16:43:41| National Hurricane Center (East Pacific)

Issued at 900 AM MDT Wed Sep 30 2020 000 WTPZ43 KNHC 301443 TCDEP3 Tropical Storm Marie Discussion Number 6 NWS National Hurricane Center Miami FL EP182020 900 AM MDT Wed Sep 30 2020 Marie's satellite presentation has been gradually improving. The center is embedded beneath a central dense overcast feature, and the convective band within the western semicircle has become a little more pronounced and continuous. In addition, a mid-level eye has begun to form, as observed in a 1200 UTC SSMIS microwave pass. Dvorak intensity estimates range from 45-55 kt from TAFB and SAB, and the most recent SATCON estimate was 53 kt (at around 0900 UTC). Based on these numbers and the continued improvement of Marie's structure, the initial intensity is set at 55 kt. The stage appears set for Marie to rapidly intensify during the next couple of days. Water vapor imagery indicates that the easterly shear over the cyclone has continued to decrease and should be generally low for the next 3 days, and upper-level divergence will also be in place during that period to help ventilate the storm. The thermodynamics are also favorable for fast strengthening, highlighted by sea surface temperatures of 28-29 degrees Celsius and plenty of moisture in the surrounding environment. Due to these conditions, the NHC forecast explicitly shows rapid intensification during the next couple of days, with a peak intensity likely occuring sometime between 48 and 60 hours. The peak intensity shown in the official forecast has been nudged upward slightly from the previous prediction, following the trends in the intensity guidance, however it's noteworthy that even this forecast is 5-10 kt lower than the solutions provided by the HCCA consensus aid and the COAMPS-TC model. In about 3 days, cooler waters and then increasing shear (especially on days 4 and 5) should induce a gradual weakening trend. Marie is moving westward (275/14 kt) to the south of a strong mid-tropospheric high centered near the U.S./Mexico border. This ridge is expected to maintain Marie on a westward or west-northwestward trajectory for the next 2-3 days. Even though a mid- to upper-level low near 130W longitude has caused a break in the ridge, the models have trended toward this feature weakening during the next few days. This has in turn caused many of the models to trend westward with Marie's track after day 3, as was noted in the previous advisory, but there is also greater spread than there was yesterday. The NHC track forecast remains closest to the TVCE and HCCA consensus aids, near the middle of the guidance envelope, which is bracketed on the right side by the GFS (and its ensemble mean) and the left side by the ECMWF (and its ensemble mean) by the end of the forecast period. The 12-ft sea radii remain larger than would be expected for a tropical storm of Marie's size due to a large fetch of southerly swell originating from the Southern Hemisphere. FORECAST POSITIONS AND MAX WINDS INIT 30/1500Z 14.2N 113.8W 55 KT 65 MPH 12H 01/0000Z 14.3N 116.1W 65 KT 75 MPH 24H 01/1200Z 14.5N 118.9W 85 KT 100 MPH 36H 02/0000Z 15.1N 121.5W 100 KT 115 MPH 48H 02/1200Z 15.9N 123.9W 110 KT 125 MPH 60H 03/0000Z 16.9N 125.9W 115 KT 130 MPH 72H 03/1200Z 18.0N 127.6W 100 KT 115 MPH 96H 04/1200Z 19.8N 130.7W 75 KT 85 MPH 120H 05/1200Z 21.1N 133.4W 55 KT 65 MPH $$ Forecaster Berg

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Tropical Storm Marie Forecast Discussion Number 5

2020-09-30 10:37:34| National Hurricane Center (East Pacific)

Issued at 300 AM MDT Wed Sep 30 2020 000 WTPZ43 KNHC 300837 TCDEP3 Tropical Storm Marie Discussion Number 5 NWS National Hurricane Center Miami FL EP182020 300 AM MDT Wed Sep 30 2020 Geostationary satellite imagery shows that Marie continues to become better organized with a curved band of convection over the western semicircle and a small central dense overcast feature. An ASCAT-B overpass from around 0515 UTC was very helpful in pinpointing the center, and it also revealed that Marie's tropical-storm-force wind field is quite small. Subjective satellite intensity estimates of T3.0 and the scatterometer data yielded an intensity of 45 kt at 0600 UTC, but given the continued improvement in organization, the advisory intensity has been set at 50 kt. Although Marie's outflow is somewhat restricted over the northeastern portion of the storm due to some northeasterly shear, it has been expanding over the past few hours indicating that the shear is decreasing as anticipated. Marie is forecast to remain over warm waters and in a low shear environment during the next couple of days. These very conducive conditions favor strengthening, and the NHC intensity forecast calls for steady to rapid intensification over the next 48 hours. The official forecast is in good agreement with the SHIPS, LGEM, and HFIP corrected consensus models which all predict periods of rapid strengthening over the next 48 to 60 hours. The updated NHC wind speed forecast shows a faster rate of strengthening and a slightly higher peak intensity than in the previous advisory. After 72 hours, cooler waters and a more stable environment should result in steady to rapid weakening. Maria is moving westward at about 14 kt, a little faster than before. There has been no change to the track forecast philosophy as Marie is expected to move westward to west-northwestward during the next few days while it remains to the south of a deep-layer ridge. After that time, a slower northwestward motion is predicted when Marie approaches a weakness in the ridge. The latest runs of the dynamical models have trended toward a more westward track during the first 60 hours or so, and the official forecast has been adjusted accordingly. Thereafter, the NHC track is fairly similar to the previous advisory, and it again lies near the middle of the guidance envelope. Marie's 12-ft sea radii remain larger than would be expected for a small tropical storm due to a large fetch of southerly swell originating from the Southern Hemisphere. FORECAST POSITIONS AND MAX WINDS INIT 30/0900Z 14.1N 112.5W 50 KT 60 MPH 12H 30/1800Z 14.4N 114.8W 60 KT 70 MPH 24H 01/0600Z 14.5N 117.7W 75 KT 85 MPH 36H 01/1800Z 14.9N 120.5W 90 KT 105 MPH 48H 02/0600Z 15.7N 123.1W 105 KT 120 MPH 60H 02/1800Z 16.8N 125.3W 110 KT 125 MPH 72H 03/0600Z 18.0N 127.2W 100 KT 115 MPH 96H 04/0600Z 20.0N 130.2W 75 KT 85 MPH 120H 05/0600Z 21.5N 133.0W 55 KT 65 MPH $$ Forecaster Brown

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Tropical Storm Marie Forecast Discussion Number 4

2020-09-30 04:35:27| National Hurricane Center (East Pacific)

Issued at 900 PM MDT Tue Sep 29 2020 000 WTPZ43 KNHC 300235 TCDEP3 Tropical Storm Marie Discussion Number 4 NWS National Hurricane Center Miami FL EP182020 900 PM MDT Tue Sep 29 2020 Infrared and passive microwave imagery indicate that Marie has continued to become better organized despite moderate northeasterly vertical wind shear of about 15 kt. The deep convection is now primarily confined to a single tightly curved band that wraps around the west side of the circulation. A late-arriving 29/1718Z ASCAT-C scatterometer pass revealed that Marie has a very tight inner-core circulation with a radius of maximum winds (RMW) of only 10-15 nmi and a small 34-kt wind radii extending outward to only about 20 nmi The scatterometer data also showed some 34-38 kt surface wind vectors in all quadrants. A blend of the scatterometer wind data and subjective Dvorak intensity estimates of of T2.5/35 kt and T3.0/45 kt from SAB and TAFB, respectively, the intensity has been increased to 40 kt. The initial motion estimate is 275/11 kt. As mentioned in the previous discussion, Marie's forecast track remains fairly straightforward. The cyclone is expected to move generally westward to west-northwestward for the next few days as it skirts the southern periphery of a deep-layer ridge to the north. Thereafter, a slower motion toward the northwest is forecast as Marie moves into a weakness in the ridge. The new NHC track forecast is similar to but a little to the left of the previous advisory track, and lies down the middle of the tightly packed consensus track model guidance envelope. Although the GFS- and ECMWF-based SHIPS intensity models have backed off significantly on predicting rapid intensification (RI) on this cycle, several environmental and cyclone structural parameters are still favorable for RI to occur. For about the next 72 hours, sea-surface temperatures (SST) are forecast to remain above 28C, alone with mid-level humidity values above 70 prevent and low wind shear values around 5 kt. Those environmental conditions coupled with Marie's small RMW should result in RI beginning in about 24 hours and continuing until the 60-h time period. As result, Marie is forecast to strengthen into a hurricane in about 24 hours and possibly become a major hurricane in 60-72 hours. Rapid weakening is expected on days 4 and 5 due to SSTs less than 26C, likely significant cold upwelling owing to Marie's expected large and strong wind field, and increasing southwesterly vertical shear of 20-25 kt. The NHC official intensity forecast is similar to the previous advisory, and closely follows a blend of the Decay-SHIPS and LGEM statistical models and the corrected-consensus HCCA model. Marie's 12-ft sea radii remain larger than would be expected for a small tropical storm due to a large fetch of southerly swell originating from the Southern Hemisphere. FORECAST POSITIONS AND MAX WINDS INIT 30/0300Z 13.8N 110.4W 40 KT 45 MPH 12H 30/1200Z 14.1N 112.9W 50 KT 60 MPH 24H 01/0000Z 14.6N 115.9W 60 KT 70 MPH 36H 01/1200Z 15.0N 118.8W 75 KT 85 MPH 48H 02/0000Z 15.5N 121.4W 90 KT 105 MPH 60H 02/1200Z 16.5N 123.8W 105 KT 120 MPH 72H 03/0000Z 17.6N 125.9W 100 KT 115 MPH 96H 04/0000Z 19.8N 129.3W 80 KT 90 MPH 120H 05/0000Z 21.4N 132.1W 60 KT 70 MPH $$ Forecaster Stewart

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Tropical Storm Marie Forecast Discussion Number 3

2020-09-29 22:33:58| National Hurricane Center (East Pacific)

Issued at 300 PM MDT Tue Sep 29 2020 000 WTPZ43 KNHC 292033 TCDEP3 Tropical Storm Marie Discussion Number 3 NWS National Hurricane Center Miami FL EP182020 300 PM MDT Tue Sep 29 2020 A WindSat microwave image from earlier this morning indicated that the tropical cyclone's low-level circulation was becoming better defined, with perhaps the formative stage of a cyan ring seen in the 37-GHz channel. Visible images also show the center tucked just beneath recent bursts of deep convection, and Dvorak estimates have risen to T2.0 and T2.5 from TAFB and SAB, respectively. These data, along with ambiguity analyses of recent ASCAT-B and -C scatterometer passes, indicate the depression has strengthened to Tropical Storm Marie with maximum winds of 35 kt. Marie's future track is probably the most straightforward part of the forecast. A mid-tropospheric high anchored over the southwestern United States is steering Marie westward with an initial motion of 275/10 kt. This high will remain the main driver, forcing the cyclone westward or west-northwestward for the next 3 days. By days 4 and 5, Marie is likely to reach a break in the ridge and should slow down a bit and turn toward the northwest. There are no notable outliers among the track guidance, and the small spread among the models yields higher-than-normal confidence in the track forecast. The new NHC forecast is very similar to the previous prediction, and is close to the TVCE and HCCA consensus aids. The intensity forecast is a little more challenging, but mostly because a good proportion of the guidance suggests that Marie will intensify significantly during the next few days. Relatively low deep-layer shear, warm ocean waters, and favorable upper-level divergence all favor strengthening, and several of the various SHIPS Rapid Intensification thresholds are several times higher than their climatological means. In addition, all of the dynamical models, the consensus aids, and the GFS-based SHIPS model bring Marie to hurricane strength within 24 hours. Given these signals, the NHC intensity forecast has been raised from the previous one and lies near or just below the intensity consensus in order to maintain some continuity. But given what is shown by some of the better-performing intensity models, I would not be surprised if subsequent forecasts show a faster rate of intensification or a higher peak intensity. Weakening is expected by days 4 and 5 due to cooler waters and increasing southwesterly shear. Marie's 12-ft sea radii are larger than would be expected for a small, just-developing tropical storm due to a large fetch of southerly swell originating from the Southern Hemisphere. FORECAST POSITIONS AND MAX WINDS INIT 29/2100Z 13.6N 109.5W 35 KT 40 MPH 12H 30/0600Z 13.9N 111.6W 45 KT 50 MPH 24H 30/1800Z 14.6N 114.5W 60 KT 70 MPH 36H 01/0600Z 14.9N 117.5W 75 KT 85 MPH 48H 01/1800Z 15.3N 120.3W 90 KT 105 MPH 60H 02/0600Z 16.0N 122.7W 100 KT 115 MPH 72H 02/1800Z 17.2N 124.9W 100 KT 115 MPH 96H 03/1800Z 19.6N 128.5W 85 KT 100 MPH 120H 04/1800Z 21.4N 131.6W 60 KT 70 MPH $$ Forecaster Berg

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Tropical Depression Eighteen-E Forecast Discussion Number 1

2020-09-29 10:43:25| National Hurricane Center (East Pacific)

Issued at 300 AM MDT Tue Sep 29 2020 000 WTPZ43 KNHC 290843 TCDEP3 Tropical Depression Eighteen-E Discussion Number 1 NWS National Hurricane Center Miami FL EP182020 300 AM MDT Tue Sep 29 2020 A burst of deep convection that developed near the center of the area of low pressure well southwest of the southwestern coast of Mexico during the evening has persisted overnight. Microwave imagery and a 0444 UTC ASCAT-C overpass indicated that the center of the low re-formed within the area of deep convection and with the overall increase in the organization of the system, advisories are being initiated on the eighteen tropical depression of the 2020 eastern Pacific hurricane season. The scatterometer data revealed peak winds of around 30 kt, and that is used as the advisory intensity. The depression is located over warm water and within an area of light to moderate northeasterly shear, and these conditions are expected to allow for steady strengthening over the next few days. Although the SHIPS model guidance only shows a peak intensity of 65-70 kt in 72-84 hours, the SHIPS rapid intensification index indicates a 50 percent chance of a 65 kt increase in intensity over the next 72 hours. In addition, the global models significantly deepen the system over the next few days. On the basis of the SHIP RI information and the global models, the NHC intensity forecast is above the IVCN intensity consensus close to the more aggressive HWRF and HCCA models. Late in the period, decreasing SSTs and increasing vertical wind shear should result in weakening. Given the recent re-formation of the center, the initial motion estimate is a highly uncertain 275/9 kt. A deep-layer ridge centered over the southwestern United States is expected to steer the cyclone westward to west-northwestward during the next several days. By the end of the forecast period, the cyclone will be approaching the southwestern portion of the ridge allowing it to turn northwesterly. Although there some model differences in the predicted forward speed of the depression, the models are in good agreement on the overall track forecast scenario. To account for these forward speed differences, the NHC forecast is near the middle of the guidance envelope close to the various consensus aids. FORECAST POSITIONS AND MAX WINDS INIT 29/0900Z 13.2N 107.6W 30 KT 35 MPH 12H 29/1800Z 13.5N 109.0W 35 KT 40 MPH 24H 30/0600Z 14.2N 111.2W 45 KT 50 MPH 36H 30/1800Z 14.6N 114.3W 55 KT 65 MPH 48H 01/0600Z 14.8N 117.3W 65 KT 75 MPH 60H 01/1800Z 15.3N 119.9W 75 KT 85 MPH 72H 02/0600Z 16.2N 122.3W 85 KT 100 MPH 96H 03/0600Z 18.3N 126.2W 80 KT 90 MPH 120H 04/0600Z 20.5N 129.2W 60 KT 70 MPH $$ Forecaster Brown

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