Tropical Storm ERIKA Heads for the Northeast CARIB - and Possibly Florida

By: Steve Gregory , 9:14PM,GMT on August 26,2015

WEDNESDAY, 26AUG15 / 4:15 PM CDT
A DEDICATED UPDATE ON TROPICAL STORM ERIKA WILL BE ISSUED TOMORROW

TROPICAL STORM ERIKA POTENTIAL THREAT TO FLORIDA

Minimal Tropical Storm ERIKA - located 250NM east of Antiqua – is moving WNW at 14Kts, with surface winds (just barely) near 40Kts based on a morning RECON (though a ship report very near the center only reported sustained winds of 20Kts!).

ERIKA is in a generally poor environment for intensification with moderately strong Northwesterly wind shear over 20Kts along with dry air to the north and west of the storm. Countering these negatives are very warm SST’s and a developing high level outflow pattern. It appears that wind shear will remain relatively high for the next 2 days as the storm moves WNW towards the Bahamas by the weekend.

IF (a Big IF) ERIKA survives as a tropical cyclone through the weekend, environmental conditions are expected to become considerably more favorable for intensification – significantly increasing the risk to Florida.

NEAR TO BELOW NORMAL TEMPS IN THE EAST TO WARM DRAMATICALLY NEXT WEEK

The unseasonably high amplitude TROF in the eastern US will begin weakening and lifting out to the northeast during the next few days as a strong TROF develops over the eastern Pacific - forcing upper level heights to rise over the central US next week. By late next week, a broad, upper level ridge will develop over the eastern half of the nation, with a southwesterly flow of much warmer air overspreading the central and northern Plains eastward into the Midwest - and by Week 2 - into the northeastern US with much above normal Temp anomalies. ERIKA may bring very heavy rainfall to Florida next week – but even if it doesn’t intensify, the moisture associated with the system is still likely to bring significant Precip to the region.

EL NIÑO / MJO

SST anomalies in the Niño 3.4 region are near +2.0˚C, with readings over +2.5˚ in the eastern portion of the ‘region’ and near +1.0˚C in the far western portion area. Some model forecasts, responding to the recent, larger than expected increase in the SST anomaly – have increased their forecast for OCT to near +3.0˚ - but several ensemble member forecasts remain closer to a +2.0˚C anomaly. With the continuing low level westerly wind anomalies, persistent very high OHC (Ocean Heat Content) and the possible development of a more significant WWB in SEP - SST anomalies in the Niño 3.4 region are likely to increase to +2.5˚C by OCT – putting this event close to a ‘Super El Nino’ intensity.

At the same time – the very warm SST’s in the GOA continues to slowly decrease, with the maximum anomaly having shifted to the southwest. However, the latest PDO reading has increased for the 2nd month in a row – with above normal SST’s now covering most of the eastern Pacific east of the Dateline (a very unusual event in and of itself).

The MJO remains quite weak, and virtually every model, including their respective ensemble members, call for a continuation of a weak and generally incoherent signal for the next 10 days. (Based on the MJO cycle ‘length’ over the past 6 months, another, more significant MJO may occur in mid or late SEP). Nonetheless, there have been 2 westerly wind ‘impulses’ over the past few weeks that generated weak but significant Kelvin Waves that have moved across the equatorial EPAC, and were primarily responsible for the most recent increases in SST anomalies.

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Fig 1: Visible images of ERIKA from this morning (top) and this afternoon (bottom). Despite a strong burst of convection this morning, ERIKA continues to struggle to maintain itself as moderately strong Northwesterly wind shear and dry air is preventing the storm from getting better organized – let alone intensifying. The location of the surface center this morning was made possible by the micro-wave image (below, Fig 2) which clearly shows the Low Level Circulation Center (LLCC) on the NW side of the convection. (This location was then confirmed by RECON which also reported MAX winds 40 Kts.) By this afternoon, the continuing Northwesterly wind shear and dry air led to the LLCC being completely exposed to the NW of the main convection. In fact, several low level vortices may also be developing under the convection – and it’s quite possible ERIKA no longer has tropical storm force winds at the surface.


Fig 2: Microwave image of ERIKA early this morning highlighted the large-scale cyclonic flow around the LLCC – with the strong convection (bright red color tones) to the southeast of the low level circulation field. This 'tilt' in the structure of the storm (from NW to SE) is preventing the system from intensifying.


Fig 3: Deep level Wind Shear analysis. Wind shear over 20Kts is impacting the storm – and is likely to continue for the next 2 to 3 days - and could prove to be too strong for ERIKA to survive (as it was for DANNY). In addition, interaction of the storm’s circulation field with the mountainous terrain of Puerto Rico and Hispaniola could also lead to dissipation. However, most models track the storm further north of the Islands – and this may allow the storm to maintain itself long enough to reach the Bahamas on Saturday - at which time shear is expected to fall below to 15 Kts or less - and upper level winds are expected to become more favorable for intensification.


Fig 4: Satellite derived winds above 350mb (~24,000 ft) over the eastern US and southwest Atlantic An unusually deep TROF for this time of year is over the eastern US, with a leading TROF extending southward into the western CARIB where a quasi-stationary closed Low is located just south of Cuba. The TROF is expected to weaken over the next few days as the upper level high centered near 30˚N and 65˚W begins to build back into the southeast US - with a well-defined upper level High developing over the western Bahamas and Florida. If ERIKA manages to survive as it heads towards the western Bahamas - this upper level anti-cyclone could lead to strong intensification of the system as shown by some of the specialized hurricane intensity forecast models.


Fig 5: Steering Level winds in the 700-850mb (~5,000’ to 10,000’) layer. The deep TROF over the eastern US has resulted in a ‘break’ in the east-west orientated sub-tropical ridge over the Atlantic, with a second center over Texas.The east/southeast wind flow to the south of the ridge is steering ERIKA to the W/NW. Since the TROF is expected to weaken by MON, ERIKA should maintain it’s generally WNW track through then. If the TROF doesn’t weaken as fast as expected, the break in the ridge could allow the storm to recurve to the north before it reaches Florida. OTH – if the ridge builds back in and is stronger than forecast, ERIKA could take a more westerly course into the GOM (Gulf of Mexico).


Fig 6: SST’s along the projected Track for ERIKA SST’s are over ~28˚C and increase to near 30˚C north of the CARIB, along with a substantial increase in the OHC.


Fig 7: Tropical Cyclone model forecasts for ERIKAThe forecast tracks continue to call for ERIKA to maintain a W/NW course towards south Florida. Most intensity forecasts show little change in intensification until Sunday when the storm should be in the Bahamas. Again – assuming ERIKA survives the shear and dry air – it will find a much more favorable environment for intensification, and ERIKA could reach CAT 2 intensity by MON.


Fig 8: Tropical Cyclone Track and Intensity Forecast chart for ERIKA from the US NAVY The forecast track for the next 5 days has followed the model guidance and is close to the model consensus for the track - and on the lower end of the intensity model consensus, bringing the storm to near the south Florida coast as a CAT 1 hurricane. (The next 48 hours will be critical to whether the storm survives as it heads into the Bahamas.)


Fig 9: Global SST anomaly chart (top) and SSTA’s in the equatorial EPAC (bottom) Well above normal SST’s dominate the entire eastern Pacific – an extremely unusual event. Notably, anomalies have continued to drop off in the GOA as the MAX anomaly has shifted towards the Southwest, Anomalies in the Niño 3.4 region are now near +2.0˚ - making this El Niño a ‘Very Strong’ event. Also noteworthy is the below normal SST’s in the equatorial west Pacific (WPAC) – a leading indicator for lower SST’s in the GOA. Meanwhile in the Atlantic basin – SST’s have climbed back to near normal in most of the equatorial Atlantic, while remaining well below normal in the high latitude region in the NORATL.


Fig 10: Upper Ocean Heat Anomalies in the Equatorial Pacific for the past year This chart highlights the series of Westerly Wind Bursts that have led to the strongest El Niño event (so far) in nearly 20 years. One of the most remarkable aspects shown by this Hovmöller chart is the seemingly non-stop down-welling phase of all the Kelvin waves since March when a record breaking MJO and Equatorial Rossby Wave (ERC) first ‘triggered’ the current El Niño event. While there clearly have been up-welling phases to each Kelvin wave – their impact was quite limited, and really only 'appeared' in the sub-surface Temp anomalies. The most recent Kelvin Wave wasn’t especially strong, but in concert with the ongoing, low level westerly wind anomalies in the central/eastern Pacific, managed to cause a strong surge in the heat anomaly (‘brown’ shaded) between 120˚S and 140˚W – the ‘key’ region for El Niño.


Fig 11: Low Level (850mb (~5,000’) wind anomalies Westerly wind anomaly magnitudes increased over the past week – partially due to tropical cyclones - further increasing the SST anomalies in the EPAC – while easterly wind anomalies in the far eastern equatorial Pacific was also related to tropical cyclone activity in the eastern Pacific..


Fig 12: 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. Cool Temps this week in the East will begin rising over the weekend, with above normal Temps nationwide by early next week. Confidence in the forecast anomaly Pattern and magnitude is still above average.


Fig 13: Running Precipitation totals for US during the next 10-days (and only for totals > 0.75”) based STRICTLY on the GFS Potentially very heavy Precip over Florida northward into the Carolinas if ERIKA tracks into that region – with heavier Precip expected in the PAC NW as far south as Northern California – and across the inter-mountain west as the summer Monsoon interacts with a cold front moving into the west from a relatively strong set of storms in the EPAC. Again - note how far south over the EPAC the area of heavy Precip is shown over the EPAC - fairly unusual even for the first week in SEP ... a sure ‘sign’ of El Nino’s influence.


Fig 14: 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 above normal Temps should overspread the nation east of the Rockies while a deep TROF in the eastern Pacific will bring much cooler readings to the west of the Continental Divide. Overall Confidence in the forecast anomaly Pattern and magnitude is about average now for the anomaly pattern – but still a bit below average for the anomaly magnitudes in both the northeastern and southwestern regions.

✭ A DEDICATED UPDATE ON TROPICAL STORM ERIKA WILL BE ISSUED TOMORROW✭

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Steve