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Tropical Storm Miriam Forecast Discussion Number 9

2018-08-28 10:38:42| National Hurricane Center (East Pacific)

Issued at 200 AM PDT Tue Aug 28 2018 039 WTPZ45 KNHC 280838 TCDEP5 Tropical Storm Miriam Discussion Number 9 NWS National Hurricane Center Miami FL EP152018 200 AM PDT Tue Aug 28 2018 Miriam's cloud pattern has still not improved very much, with microwave data showing the low-level center still slightly displaced from the deep convection due to about 10 kt of northwesterly shear. Dvorak estimates from TAFB and SAB, as well as the UW-CIMSS SATCON, all remain near 55 kt, which will remain Miriam's initial intensity. Miriam is expected to remain over warm waters and in a relatively low-shear environment for the next 48-72 hours, which should allow the cyclone to strengthen modestly over the next few days. At 72 hours and beyond, Miriam will encounter significantly stronger shear ahead of an amplifying mid- to upper-level trough northeast of the Hawaiian Islands, and the cyclone will also move over sub-26C waters. These conditions should cause a rather quick weakening trend on days 4 and 5, with Miriam barely hanging on as a tropical storm by the end of the forecast period. The intensity guidance has decreased somewhat on this forecast cycle, probably because Miriam has less time before it reaches stronger shear. The updated NHC intensity forecast lies between the HCCA guidance and the IVCN intensity consensus, with the forecast peak intensity just slightly below that of the previous advisory. The initial motion is due westward, or 270/10 kt, with Miriam located along the southern periphery of the subtropical ridge. The amplifying trough northeast of Hawaii is breaking down the ridge and should cause Miriam to turn sharply toward the northwest and north between days 2 and 4. There are significant speed differences among the models by days 4 and 5, related to how deep Miriam is when it interacts with the deep-layer trough. The ECMWF, which maintains a deeper, stronger vortex, shoots Miriam quickly northward, ending up more than 600-700 nm north of where the GFS and HWRF models have the cyclone by day 5. Given where the consensus aids lie between those two extremes, the updated NHC track forecast has been slowed down a bit from the previous forecast. FORECAST POSITIONS AND MAX WINDS INIT 28/0900Z 14.0N 133.5W 55 KT 65 MPH 12H 28/1800Z 14.0N 135.1W 60 KT 70 MPH 24H 29/0600Z 14.1N 137.1W 70 KT 80 MPH 36H 29/1800Z 14.4N 138.8W 75 KT 85 MPH 48H 30/0600Z 15.1N 140.1W 80 KT 90 MPH 72H 31/0600Z 17.9N 141.0W 80 KT 90 MPH 96H 01/0600Z 21.5N 141.5W 55 KT 65 MPH 120H 02/0600Z 26.0N 144.0W 35 KT 40 MPH $$ Forecaster Berg

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Tropical Storm Miriam Forecast Discussion Number 8

2018-08-28 04:35:10| National Hurricane Center (East Pacific)

Issued at 800 PM PDT Mon Aug 27 2018 917 WTPZ45 KNHC 280235 TCDEP5 Tropical Storm Miriam Discussion Number 8 NWS National Hurricane Center Miami FL EP152018 800 PM PDT Mon Aug 27 2018 Miriam hasn't changed much over the past several hours. Last-light visible imagery and a couple of microwave overpasses around 0000 UTC show that the tropical storm remains lightly sheared from the northwest. Although outflow from the deepest inner-core convection seems to have become better established, cloud tops from outer bands to the north and east are still moving toward the center of the cyclone, indicating there is still shear below the outflow layer. The initial intensity has been held at 55 kt, in agreement with nearly all of the objective and subjective satellite estimates. Miriam continues to move westward, and the forward speed is now 10 kt. The guidance remains in very good agreement on the track of Miriam for the next 36 hours, with only slight speed differences between the various models. Beyond that time, the tropical storm should begin to turn north-northwestward toward a break in the subtropical ridge created by a mid- to upper-level trough over the central Pacific. The model spread increases drastically at this point, with the ECMWF showing a quicker turn and a faster north-northwestward motion than the GFS, with most of the other guidance in between. For now, the NHC forecast has not been significantly changed, and remains near the corrected consensus, HCCA. The moderate shear currently affecting Miriam is forecast by the global models to continue for the next 24 h or so, preventing the cyclone from strengthening significantly. Between 24 and 72 h, this shear is forecast to decrease, allowing the cyclone to strengthen at a quicker rate. By the end of the forecast period, Miriam should quickly weaken as it encounters higher shear and cooler SSTs. The HWRF is a notable outlier, showing much faster intensification, but it is possible this model is not properly representing the shear currently affecting Miriam. That said, if the shear decreases sooner than expected, it is possible that Miriam could intensify much faster than currently anticipated. The new NHC intensity forecast is close to the intensity consensus, and shows a slightly slower initial intensification rate for Miriam, and a faster decay by day 5, than the previous advisory. FORECAST POSITIONS AND MAX WINDS INIT 28/0300Z 14.0N 132.5W 55 KT 65 MPH 12H 28/1200Z 14.0N 134.1W 60 KT 70 MPH 24H 29/0000Z 14.0N 136.2W 65 KT 75 MPH 36H 29/1200Z 14.2N 138.1W 75 KT 85 MPH 48H 30/0000Z 15.0N 139.5W 85 KT 100 MPH 72H 31/0000Z 17.5N 141.1W 85 KT 100 MPH 96H 01/0000Z 21.5N 142.0W 65 KT 75 MPH 120H 02/0000Z 26.0N 144.0W 40 KT 45 MPH $$ Forecaster Zelinsky

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Tropical Storm Miriam Forecast Discussion Number 7

2018-08-27 22:32:36| National Hurricane Center (East Pacific)

Issued at 200 PM PDT Mon Aug 27 2018 553 WTPZ45 KNHC 272032 TCDEP5 Tropical Storm Miriam Discussion Number 7 NWS National Hurricane Center Miami FL EP152018 200 PM PDT Mon Aug 27 2018 Miriam has become a little better organized since the last advisory, and the convective banding has become better defined around the low-level center. However, this has not yet resulted in an increase of the various satellite intensity estimates, which remain mostly near 55 kt. Based on this, the initial intensity remains a possibly conservative 55 kt. Satellite imagery continues to indicate northwesterly to northerly vertical shear affecting Miriam, with cirrus clouds from the outer band to the north blowing into the central convection. The initial motion is 270/13. A large low- to mid-level ridge to the north of Miriam should continue to steer it westward for the next 36-48 h with some decrease in forward speed. After that, a large mid- to upper-level low to the northeast of the Hawaiian Islands is expected to cause the ridge to break between 140W-150W, with Miriam turning northwestward and north-northwestward in response. The track guidance is in good agreement on this scenario, but there remain some differences in Miriam's forward speed after recurvature between the faster ECMWF/UKMET and the slower GFS. The new forecast track is similar to the previous track through 72 h, then is nudged a little to the east based on an eastward shift in the consensus models. The global models continue to suggest a gradual increase in northwesterly shear over Miriam during the next day or so, followed by decreased shear from 24-48 h. There is some divergence in the intensity guidance through 72 h, with the SHIPS/LGEM models showing less strengthening than the HWRF/HMON/corrected consensus models. This part of the new intensity forecast is little changed from the previous forecast and lies between these two model camps. After 72 h, increasing southwesterly shear and cooler waters are expected to cause Miriam to weaken. FORECAST POSITIONS AND MAX WINDS INIT 27/2100Z 14.0N 131.8W 55 KT 65 MPH 12H 28/0600Z 14.0N 133.5W 65 KT 75 MPH 24H 28/1800Z 13.9N 135.5W 70 KT 80 MPH 36H 29/0600Z 14.0N 137.5W 75 KT 85 MPH 48H 29/1800Z 14.5N 139.0W 85 KT 100 MPH 72H 30/1800Z 16.5N 141.0W 85 KT 100 MPH 96H 31/1800Z 20.0N 142.0W 75 KT 85 MPH 120H 01/1800Z 24.5N 143.5W 50 KT 60 MPH $$ Forecaster Beven

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Tropical Storm Miriam Forecast Discussion Number 6

2018-08-27 16:36:06| National Hurricane Center (East Pacific)

Issued at 800 AM PDT Mon Aug 27 2018 364 WTPZ45 KNHC 271436 TCDEP5 Tropical Storm Miriam Discussion Number 6 NWS National Hurricane Center Miami FL EP152018 800 AM PDT Mon Aug 27 2018 The convective structure of Miriam has become better organized this morning, with a continued increase in overall banding. An earlier AMSR2 microwave overpass also revealed a well-defined band over the southeastern and southern portions of the circulation but there was some evidence of northwesterly shear with the center near the northwestern edge of the primary convective band. The initial intensity has been increased to 55 kt, which is supported by a Dvorak Data T-number of 3.5 from TAFB, earlier AMSU data, and recent UW/CIMSS ADT estimates of T3.7 (59 kt). Satellite fixes indicate that Miriam is moving westward or 270 degrees at 12 kt. The cyclone should remain on a westward heading over the next 48 hours while it is steered by a large deep-layer ridge to the north. After that time, a large mid- to upper-level low to the northeast of the Hawaiian Islands is expected to produce a break in the ridge between 140W-150W. This should cause Miriam to turn northwestward, then north-northwestward between days 3 through 5. The track guidance is in good agreement on this scenario, but there are some differences in Miriam's forward speed after recurvature begins. The ECMWF and UKMET take Miriam much faster northward than the remainder of the dynamical models, and the NHC track forecast is once again close to the various consensus aids at 72 h and beyond to account for the speed differences. The global models suggest that there will be a gradual increase in northwesterly shear over Miriam during the next day or so, however, this is not expected to prevent the cyclone from becoming a hurricane within the next 24 hours. The moderate shear is forecast to relax by Tuesday night, which should allow for additional intensification. The intensity guidance is not quite as aggressive as before, and the NHC intensity forecast, which lies between the IVCN intensity consensus model and the HFIP corrected consensus, has been adjusted slightly downward. Increasingly southwesterly vertical wind shear and cooler waters are expected to cause Miriam to weaken late in the forecast period. FORECAST POSITIONS AND MAX WINDS INIT 27/1500Z 13.9N 130.5W 55 KT 65 MPH 12H 28/0000Z 13.9N 132.3W 65 KT 75 MPH 24H 28/1200Z 13.8N 134.3W 70 KT 80 MPH 36H 29/0000Z 13.9N 136.3W 75 KT 85 MPH 48H 29/1200Z 14.1N 138.1W 85 KT 100 MPH 72H 30/1200Z 15.5N 140.8W 85 KT 100 MPH 96H 31/1200Z 18.7N 141.8W 75 KT 85 MPH 120H 01/1200Z 23.3N 143.2W 55 KT 65 MPH $$ Forecaster Brown

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Tropical Storm Miriam Forecast Discussion Number 5

2018-08-27 10:36:05| National Hurricane Center (East Pacific)

Issued at 200 AM PDT Mon Aug 27 2018 996 WTPZ45 KNHC 270835 TCDEP5 Tropical Storm Miriam Discussion Number 5 NWS National Hurricane Center Miami FL EP152018 200 AM PDT Mon Aug 27 2018 Miriam's cloud pattern and overall convective organization have continued to improve, with a tight comma-cloud pattern now evident in infrared imagery. Two recent ASCAT passes around 0600Z indicate that the low-level center is now located just inside the northwest quadrant of the convective comma head, and that the radius of maximum winds (RMW) is still about 20 nmi that was ascertained from earlier passive microwave low-level ring data. The ASCAT data indicated peak winds of only 37 kt, but this is likely an underestimate of Miriam's actual intensity due to a) the data having been located near the swath edge and b) due to Miriam's small RMW. The advisory intensity of 50 kt is based on a average of subjective and objective satellite intensity estimates that range from T3.0/45 kt to T3.6/57 kt, respectively. The previous advisory's 34-kt wind radii remain unchanged based on the aforementioned ASCAT wind data. Miriam is now moving due west or 270/13 kt. There are no significant changes to the previous forecast track or reasoning. The cyclone is expected to maintain a general westward motion for the next 72 h as Miriam moves along the southern periphery of a strong ridge to its north. By 96 h and beyond, Miriam is forecast to move northward to north-northwestward through a break in the subtropical ridge near created by a strong mid- to upper-level trough/low forecast to drop southward between 140W-150W from the upper-low's current position over the central North Pacific. The latest model guidance has come into better agreement on both the timing and location of the northward turn, although noticeable speed differences still exist, with the ECMWF and UKMET being the faster models. The NHC official forecast track lies close to the previous advisory track, and is just a tad north of the HCCA, FSSE, and TVCN consensus models. Steady strengthening still appears likely for the next couple of days. Miriam's upper-level outflow has improved some and is less restricted in the northwest quadrant, a signal that the shear is beginning to relax, which would support steady or significant strengthening in the short term. By 36-48 h, however, the shear is forecast by the global models to again increase to 15-20 kt from the northwest, which should inhibit the intensification process during that time. Around 72 h, the shear is expected to decrease yet again, followed by another increase in the shear. Rather than showing roller-coaster fluctuations, the NHC intensity forecast just calls for steady strengthening through 48-72 h, followed by a gradual weakening trend, which mirrors the previous advisory trend and is close to an average of the HCCA and FSSE intensity models. FORECAST POSITIONS AND MAX WINDS INIT 27/0900Z 13.9N 129.2W 50 KT 60 MPH 12H 27/1800Z 13.9N 130.9W 60 KT 70 MPH 24H 28/0600Z 13.8N 133.0W 70 KT 80 MPH 36H 28/1800Z 13.8N 135.1W 80 KT 90 MPH 48H 29/0600Z 14.0N 137.1W 90 KT 105 MPH 72H 30/0600Z 14.7N 140.2W 90 KT 105 MPH 96H 31/0600Z 17.5N 141.6W 80 KT 90 MPH 120H 01/0600Z 22.0N 142.4W 55 KT 65 MPH $$ Forecaster Stewart

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