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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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