TUESDAY 11-AUG-15
(Next Regular WX Update WEDNESDAY – AUG 12)
HURRICANE HILDA SLOWS ITS ADVANCE TOWARDS HAWAII - NO MAJOR IMPACT EXPECTED
After maintaining its CAT 1 Intensity through this morning, HILDA’s forward motion has slowed considerably, and it’s latest satellite signature shows a storm that is now weakening.
The small Hurricane – located near 17.5˚N / 151.4˚W, or 280NM SE of Hilo, Hawaii, has MAX sustained surface winds estimated near 70Kts. The storm is now heading NW at ~4Kts with an estimated pressure of 985mb.
The storm’s forward motion has slowed as steering level winds have weakened, but should continue on a general NW track for the next 24-36 hrs. By that time, however, very strong wind shear should have weakened the storm to tropical storm intensity – with MAX sustained winds of ~50Kts. As the wind shear ’tears’ into the eye-wall, it will weaken the strong and deep convection found within the eye-wall. It will also allow much drier air just to the west and northwest of the storm to be pulled into the vortex - leading to accelerated pace of weakening in 36-48 hrs.
These structural changes will dramatically lower the average level of HILDA's steering current winds to that of the deep easterly trade winds in the region. As the steering currents become dominated by the easterly trades – the storm will begin heading on a more westerly track towards the Islands (though the trade winds also vary in their direction). By the time the storm is in vicinity of the Big Island late THU, it should have weakened to a tropical depression with MAX winds around 30Kts - and possibly even weaker as a remnant Low.
There is actually greater confidence in the forecast Intensity than the track itself – a highly unusual situation. The latest specialized hurricane model track forecasts show a very wide spread in solutions, most of which is related to the just how fast the storm weakening process will be – in addition to overall uncertainty in the synoptic scale environments impact on the storm’s inner circulation field.
An Air Force RECON is currently enroute to the storm and will provide a much better assessment of the storm intensity and structure than can be obtained from satellite imagery alone. The new RECON data will be used to initialize the 00Z model runs this evening,
I’ll provide another Update if there’s a significant change from the current forecast. Otherwise, the next Regular Update will be posted tomorrow afternoon.
Steve
**
Fig 1: Color Enhanced IR (infrared) image of Tropical Storm HILDA now about 280NM Southeast of Hilo, Hawaii. Deep convection is still seen near the storm’s core, with cloud top Temps (the ‘white’ speckles) near -75˚C. This implies T-storm tops are near or even above 50,000 ft. However, the storm signature has now taken on an elongated appearance, a sure sign that the storm is weakening.
Fig 2: Water Vapor Imagery Loop. The 8 hour loop shows the forward motion of the storm has slowed significantly, with the system becoming elongated N-S. Very dry air in the western semi-circle will begin to penetrate into the central core circulation during the next 24–36 hours - assuring a rapid weakening during the next 48 hours.
Fig 3: Upper Level Wind Analysis highlighting HILDA’s outflow pattern at upper levels. Outflow from the top of the storm is a critical component of a tropical storm as it serves as a ‘vent’ for the strong inflow of air near the surface. The stronger and more developed the outflow pattern atop a storm – the stronger the storm can get. At this time, outflow is still fairly good to the north of the storm and fair to the south - but very poor in the western semi-circle. This outflow pattern (or lack of it) is responsible for the elongated appearance of the storm. In addition, a large upper level Low to the north of the Hawaiian Islands, with the unusually strong (for summer) sub-tropical jet between it and the Islands – is responsible for the strong wind shear environment the storm is encountering.
Fig 4: Color Enhanced Imagery of HILDA with Shear Analysis from CIMSS. Deep level wind shear is now approaching 40Kts, a value typically associated with rapid weakening of most tropical cyclones. Shears increase to 60-70Kts over the Islands, and this strong shear is not expected to change much during the rest of the week as the upper low and sub-tropical jet stream are expected to remain quasi-stationary.
Fig 5: Early 12Z Cycle run of specialized Hurricane forecast models for HILDA’s TRACK. An unusually wide spread of model solutions for the track of HILDA beyond 36 hours out. The only real option in a case like this is to use the mean of model projections - which is what the official forecast is (Track ID: OFCI). Most of the dynamic models have the more westerly solutions and with the current situation, there is really no preference for 1 model over the other. When the 12Z GFS model completes early this afternoon, it may provide better insight into the overall, large scale environment which serves as the ‘boundary’ level input for the specialized model runs later today and tonight.
Fig 6: Early 12Z Cycle run of specialized Hurricane forecast models for HILDA’s Intensity. Intensity forecasts are usually the most unreliable compared to track forecasts – but today’s model runs break that ‘rule of thumb’ somewhat. There is generally very good agreement on the weakening trend, though variations in the rate of weakening beyond 12 hours is one of the main reasons the track forecasts show such a wide spread in solutions. The models with the most rapid weakening rates tend to be the ones with a more westerly track forecast as the storm's motion becomes increasingly dominated by the trade winds.
Fig 7: Official Forecast plot from the NAVY (which religiously follows NWS forecasts).
✭ The Next REGULAR Weather Update will be on WEDNESDAY AUGUST 12 ✭
NOTE: SPECIAL UPDATE ON EL NINO Scheduled for this THURSDAY, AUG 13
‘Normal’ Summer Update Schedule is MON, WED & FRI - but can vary by a day or so
However, whenever Tropical Cyclones Potentially threaten the US, I’ll be providing Updates 1 or more times per day
You can follow me on Twitter - (I 'Tweet' whenever I've added a New WU Blog).
No comments:
Post a Comment