FRIDAY, 21AUG15 / 7:00 CDT
A Brief Tropical Update will be posted tomorrow and Sunday with the next
‘Limited’ Full Update scheduled for MON, AUG 24
DRY AIR AND SHEAR NOW IMPACTING HURRICANE DANNY
After rapidly intensifying earlier today to (‘officially) CAT 3 Intensity – latest satellite imagery clearly shows the storm weakening as dry air works its way in towards the core – and southwesterly wind shear has begun to increase across the storm. (The storm is likely producing CAT 2 wind gusts in a small area near the surface – with sustained winds of about ~75Kts.) And while there’s still a small chance the storm could ‘rebound’ with an increase in convection during the typical diurnal uptick phase later tonight - the rate of weakening observed during the last 6 hours strongly suggests this will not happen.
In addition to dry air intrusion and continued increase in wind shear, the upper level outflow pattern – which wasn’t overly impressive at any point to begin with – has also degraded during the last 6 hours.
The global models along with most of the dynamical Hurricane specific forecast models are in excellent agreement on the projected track for DANNY, and reasonably good agreement that the storm will dissipate by Day 5 as it moves across the northern CARIB. The inherent uncertainty in track and (especially) intensity forecasts beyond 48 hours is further exacerbated due to the unusually small size of this storm and the likelihood the storm will move over or very close to Puerto Rico and the extremely mountainous terrain of Hispaniola. Nearly every storm that has moved over Hispaniola (including Major Hurricanes) has either dissipated or was tremendously weakened – and with only a couple rare exceptions – were never able to regain their former strength. In all likelihood, DANNY will be a WEAK tropical storm when it approaches Puerto Rico Monday – and will dissipate over or near Hispaniola by late Tuesday.
TROPICAL STORM KILO A REAL & SIGNIFICANT THREAT TO HAWAII
Tropical storm KILO – some 700NM south of Honolulu and moving WNW – is still poorly organized, with several Low Level Circulation Centers (LLCC) still seen on VIS imagery well east of the deep convection and higher level circulation field. During the last 8 hrs – these low-level ‘swirls’ have been slowly migrating towards the deep convection, and will either become aligned with the mid-upper level circulation field, or more likely – will reform within the convective field. This system is somewhat of a ‘mirror image' of the way DANNY developed which had an incredibly well developed low level circulation field, and then developed deep convection and its upper level circulation.
SST’s and total OHC (Ocean Heat Content) are very high and an upper level outflow pattern has already been established – but moderately strong Easterly wind shear of 15-20Kts is still impacting the storm which is responsible for the 'tilted' alignment of the storm structure. However, virtually all models call for the shear to lighten up over the weekend, allowing the storm to gradually intensify – especially starting Sunday when the large-scale weather pattern shows a weakening of the sub-tropical high to the north of the storm.
As the sub-tropical high weakens – an upper level Low and associated TROF is expected to intensify to the NW of Hawaii. As the upper air pattern changes, the forward motion of the storm will slow and the storm will begin recurving to the Northwest and then North/Northeast on Monday/Tuesday. The approach of the upper TROF should lead to increased upper level divergence over KILO - helping to intensify the cyclone.
The latest GFS is especially aggressive in intensifying KILO next week to Major (CAT 3) intensity – but it’s VERY IMPORTANT to remember than intensity forecasts are especially problematic – and extremely so beyond 48-72 hours. Nonetheless, this is a potentially significant threat to Hawaii – and needs to be monitored closely.
Low-level RECON missions are expected to begin later tonight and will continue at 12-hour intervals. However, the G-IV jet aircraft normally used to fly high level RECONs (to ascertain the upper level environmental conditions around the storm) is already tasked with investigating Hurricane DANNY, and will not be available for high level reconnaissance for KILO (at least not until DANNY is no longer a significant threat). Instead, the current plan calls for the aircraft being tasked with low-level RECONs to also sample ‘higher level’ conditions surrounding the storm once every other day.
UNSEASONABLY WARM TEMPS EXPECTED DURING THE FIRST WEEK OF SEPT
The global models are once again exhibiting large differences in the expected upper air pattern beyond 7-10 days. Seasonably mild conditions should persist for the next 2 weeks – with unseasonably warm Temps along the immediate east coast next week. However, Week 2 Temps are now expected to surge in the western US with only slightly warmer conditions in the east as the sub-tropical high builds over the inter-mountain west -with a somewhat more zonal-like flow in the east.
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Fig 1: Visible images of DANNY from earlier this AM (top) and late this afternoon (bottom) The top images were taken just as DANNY intensified to Major, CAT 3 intensity. The bright white, circular ‘disc’ with the eye in the center is called a CDO (Cold Dense Overcast) and on this visible image, depicts the cirrus clouds being generated by the intense T-storms that make up the Eye Wall. The ‘close-up’ of the CDO and eye is on the right side. The training mission RECON that by happenstance was occurring at the same time, reported the eye had a diameter of 18NM, but due to image resolution of 1KM (per pixel) – I originally calculated a diameter of half that (highlighting the limitations of satellite imagery vs. in-situ observations by manned RECON flites). The bottom image taken just 6 hours later shows the impact of both dry air intrusion and initial effects of increased southwesterly wind shear. The yellow arrows are pointing to cumulus ‘rope clouds’ emanating outward from the storm. These shallow height cumulus clouds and isolated T-storms are separated by totally clear skies due to dry air entrainment in the mid-levels. Close examination of the cloud signature close to the center highlights the now cloud-filled eye with thin bands of T-storms ringing the storm - and ‘dry’ air between these narrow bands of storms seen thru the thin cirrus cloud shield. Also noteworthy is the larger scale depiction of high level cirrus clouds that extend a relatively long distance towards the northeast, with no significant cirrus in the west and southern semi-circle. This is due to the increasing southwesterly wind shear.
Fig 2: Deep Level (850mb-200mb – 5,000’ to 40,000’) Shear analysis from CIMMS Southwesterly wind shears have increased to over 20Kts near DANNY, with the storm heading into 30Kts by tomorrow. While shears are not expected to increase much beyond ~30Kts, these shears are simply too high for DANNY to withstand for much longer. (Based on current trends, it’s entirely possible that DANNY could weaken to Tropical storm force by this time tomorrow night – though official NHC forecasts do not call for that much weakening until Sunday night.)
Fig 3: Water Vapor (WV) imagery taken around the same time as the VIS image above The Eye is still faintly visible. (When DANNY was at peak intensity and skies were clear in the eye – the ‘color’ within the eye was exactly the same as the surrounding ocean (deep blue).) The earlier circular symmetry of the storm has already become elongated (N/NW thru S/SE) - and dry air is already impinging on the western side of the storm as dry air is being ingested inward toward the storm’s inner core.
Fig 4: Hurricane DANNY Track and Intensity Forecast chart from the US NAVY The latest forecast from NHC (which the NAVY follows exactly) calls for DANNY to slow down during the next 12 hours and to reach the CARIB by Monday morning as a Tropical Storm. The storm should then move pass Puerto Rico (which desperately needs the rain) by early TUE.
Fig 5: Visible images of KILO KILO is a minimal Tropical Storm, and imagery shows the broad, low level circulation centered well east of the deep convection (due in large part to the moderate easterly wind shear aloft). During the course of the day, several low level vortices became evident (X’s) – with the larger scale low level centroid shown by the larger ‘dashed circle’). The 'icon' to its west is the ‘official’ location of the storm – and does seem to coincide with a developing circulation within the deeper convection.
Fig 6: Color enhanced IR (Infra-Red) image.Very deep convection is indicated by cloud top Temperatures of -75˚C (roughly equivalent to 50,000’ in that area of the world). Without the above visible image – we wouldn’t know that that the low-level center wasn’t even close to the deep convection! The cirrus outflow is amazingly symmetrical for such a weak storm.
Fig 7: Deep Level (850mb-200mb – 5,000’ to 40,000’) Shear analysis from CIMMS EASTERLY vertical wind shear is ~15kts over the ‘bulk’ of the convective field. These shears are expected to lessen during the weekend as the sub-tropical high to the north of the storm weakens - allowing the system to go ‘vertical’ – and begin intensifying.
Fig 8: Upper level satellite derived wind analysis. Anti-cyclonic winds continue to develop over the top of the storm system – especially in the northern semi-circle. Model forecasts call for the development of a stronger outflow pattern by Monday which will allow the storm to intensify.
Fig 9: Hurricane model forecasts Considering the kind of tracking directional changes being forecast – these tracks are actually in fairly good agreement. Several of the more reliable models are calling for KILO to reach CAT 3 Intensity when it's very close to Kauai in ~5 days. The latest GFS forecast is in excellent agreement on this intensity forecast as well.
Fig 10: Tropical Storm KILO Track and Intensity Forecast chart from the US NAVY The latest forecast from NHC (which the NAVY follows exactly) calls for KILO to slowly intensify by Sunday, reaching CAT 1 intensity as it recurves towards the north – passing close to Kauai on WED as a CAT 2 (though several models call for it to reach CAT 3 intensity by that time.)
Fig 11: Temperature ANOMALY forecast for Week 1 is based STRICTLY on the MOS data forecasts from the todays 12Z operational GFS model run with minor adjustments towards the raw model data points. Relatively cool Temps will continue in the Midwest/Ohio Valley southward across the lower Mississippi Valley during the upcoming week. Confidence in the forecast anomaly Pattern and magnitude is still above average with readings of ‘4’ on a Scale from 1 to 5 for the anomaly and magnitude forecast.
Fig 12: The Week 2 Temperature ANOMALY forecast is based on the 12Z run of the GFS (80%) integrated with the 12Z GFS Ensemble (20%) using the projected pattern along with explicit surface and 850mb (~5,000 FT) Temp forecasts. Some Temps are adjusted for known / expected anomalous thermal patterns & projected storm system passages.Much warmer Temps should dominate the nation west of the Mississippi, with near normal Temps on average across the eastern third of the nation. Overall Confidence in the forecast anomaly Pattern and magnitude is low, with a reading of ‘2’ for the anomaly pattern and only ‘1’ for the magnitude due to the return of large swings in projected Temps from one model run to the next.
✭ A Brief Tropical Update will be posted tomorrow (THU - AUG 22) ✭
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Steve
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