The Season Is Turning as Tropics Finally Wake Up – Kind Of…

By: Steve Gregory , 9:52PM,GMT on August 17,2015

MONDAY / 17-AUG-15
SPECIAL UPDATE ON EL NIÑO Part II - is Scheduled for tomorrow, TUESDAY, AUG 18
(Next Regular WX Update WEDNESDAY, AUG 19)

INVEST 96L CONTINUES MOVING WESTWARD – SLOW DEVELOPMENT STILL LIKELY

INVEST 96L continues moving due west at ~15Kts and remains embedded within the ITCZ, which, along with the SAL impinging on the system, has prevented significant intensification during the past 24 hours. On the ‘plus’ side (for those 'wanting' a storm...), SST’s are above 28˚C and upper air divergence associated with an ant-cyclonic (‘clockwise’) flow is helping to maintain significant convection. In addition, the Tropical Low lies within a very low shear environment which, if the system can move out from the disruptive environment of the ITCZ, should allow the system to intensify into a tropical storm within the next 24-48 hours.

The latest explicit forecast data from the SHIPS Forecast model calls for 96L to become a tropical storm within 48 hrs, and this seems reasonable. However, it also calls for the system to become a strong CAT 1 Hurricane by the end of the week – but at this point, appears too aggressive. The system really needs to gain latitude in order to remove itself from the hostile influence of the ITCZ, and to move towards warmer waters, yet stay far enough to the south to avoid significantly higher wind shears.

TRANSITION TOWARDS FALL-LIKE PATTERN LIKELY AS MODEL FORECASTS WHIP-SAW

The global models have been going through some major gyrations for time periods beyond 7-10 days, with very large differences between model run cycles and significant difference between the major models and their respective ensembles. These kind of variances are typical during seasonal transition periods, though the extent of differences now being exhibited by the models is somewhat unusual for this early in the ‘transition period’.

In the shorter term, a strong short wave TROF now in the Great Plains is moving quickly eastward with significantly cooler air in its wake. The associated cold front is triggering widespread and locally severe T-Storms from the Great Lakes to central Plains states and will pass off the east coast by the end of the week. Another strong frontal system will reach the Plains by the weekend, and will also be followed by another cool air surge by early next week. By that time, a relatively weak but high amplitude TROF will be located in the eastern US with the sub-tropical ridge over the inter-mountain west. But beyond then (~10 days) – there is very large disagreement on the upper air pattern – with some model runs calling for a very strong, deep long wave TROF in the eastern Pacific and a broad, sub-tropical high re-establishing itself over the center of the nation – while other model runs almost literally reverse this configuration – with a ridge near the west coast and a broad upper level TROF forming over central North America. For purposes of today’s Week 2 forecast, I’ve opted for the deep TROF over the EPAC and a broad ridge over the central US – but blended it with the ensembles for a bit cooler conditions in the eastern US (though still above normal).

EL NIÑO / MJO

SST anomalies have ticked up slightly to +1.9˚C as a small increase in anomalies over the eastern most portion of the Niño 3.4 region were greater than the slight lowering of anomalies in the western portion of the 3.4 region, just east of the Dateline.

The MJO remains weak and incoherent, though there are some model indications that a stronger signal may develop in the far western tropical Pacific towards the end of the month. But for now, the MJO will not be a significant factor in the weather across the tropics through next week.

NOTE: The “Special Update on El Niño – Part II” will be posted late tomorrow (TUESDAY) unless INVEST 96L intensifies faster than currently anticipated.

ALASKA – FALL ARRIVES

The seasonal transition to Fall has begun ‘right on schedule’ as a far more dynamic pattern is emerging across higher latitudes, with a series of stronger storm systems now impacting the state. The storm track during the next 10 days is likely to extend from eastern Siberia to western Alaska – and then east-southeast towards the southeastern coastal areas in the far Northeastern GOA (Gulf of Alaska). Large swings in Temperature anomalies (especially in the interior) are expected every 4-6 days in association with each storm and frontal passage.

‘CALLING ALL ALASKANS’ Because of the very large geographical size of the state, please let me know if you would prefer to see more detailed information on the ‘southern half’ of the state instead of the entire state. You can send me a Weather Underground E-mail or can use the ‘Comment section’ at the end of each blog posting.

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Fig 1: Overview of Tropical Atlantic Basin. There’s been little change in the overall pattern across the central and western tropical ATL, but a decidedly more active environment has developed during the past few days across the eastern ATL, especially as the ITCZ has become more active with a series of tropical waves/disturbances moving westward through it from Africa. At the same time, however, the SAL layer has again begun to flow further to west, though remains much weaker with far less ‘dust’ content overall. Still, the drier and more stable air is hindering the kind of cyclone formation activity that normally would be seen by this point in the season. The rather small tropical Low (Invest 96L) is still heading due west, and while it has held up longer than any other systems this year, the SAL is making it very difficult for the system to really spin up into a cyclone.


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Fig 2: Experimental 24-hour Loop Quasi-True Color for EATL / West AfricaWith the higher temporal resolution from METEOSAT in this region, the low and mid-level rotation of 965L becomes evident – along with the upstream systems that I have marked as TW’s in FIG 1 – but most of them are probably just small-scale disturbances embeded within the ITCZ.


Fig 3: ‘Close-up’ Experimental color Enhanced IR (Infra-red) and Water Vapor (WV) imagery of INVEST 96L with wind shear analysis from CIMSS. Locally deep convection continues to surround 96L, but is significantly weaker than at this time yesterday (though it’s a partially diurnal effect). However, wind shear remains quite low in the 5-10Kt range which, if it continues, should prevent the system from completely falling apart in the immediate future.


Fig 4: TPW (Total Precipitable Water) image Loop (Top) Careful analysis of the above imagery reveals a well-developed cyclonic circulation around 96L, though most of the moisture is in the low and mid-levels.


Fig 5: Shear analysis for the entire Atlantic Basin Mid level moisture is overspreading the central and eastern ATL, with the area of very dry air (the ‘orange’ tones) clearly seen to the south of the ITCZ. The individual disturbances within the ITCZ (including INVEST 96L) are clearly seen – and are revealed by the Green-Red areas of extremely deep moisture associated with strong convection (T-Storms).


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Fig 6: Upper Air wind analysis (above 30,000 ft) Winds across the central and eastern ATL are far more conducive to cyclone formation, with an upper air anti-cyclone (‘H’) located to the immediate north of INVEST 96L. This feature APPEARS to be moving westward in tandem with 96L, implying the systems are becoming physically ‘connected’. This will be a crucial factor if 96L is able to intensify into a tropical cyclone. (An upper air High Pressure system is critical to the intensification of a tropical cyclone as it serves as a way for the storm to ‘vent’ the air being pulled rapidly into the storm near the surface. In a ‘perfect’, intense hurricane – the upper level High sits vertically atop the hurricane’s eye, with 2 and sometimes 3, outflow ‘channels’ of jet stream speeds radiating out from the center and away from the surface storm.)


Fig 7: Early 12Z Cycle run of specialized Hurricane forecast models for INVEST 96L. Most of the models run earlier today are in very good agreement on the general track for 96L, but with varying rates of intensification. Discounting the ‘shallow’ system model solutions (which is what the system will likely follow if it does NOT intensify), the dynamic models call for the system to become a Tropical Storm by WED, with most of the ‘better’ models calling for it to reach CAT 1 Hurricane Intensity by the end of the week.


Fig 8: GFS Jetstream Forecast for next 2 Weeks Overall, the jet stream pattern is becoming far more dynamic as TROFs developing at higher latitudes over Siberia begin amplifying southward as they move east/southeast. The TROFs are digging into the north central US during Week 1 before moving eastward and lifting out to the NE as they approach New England. The sub-tropical high has 2 distinct centers, one in the far SW US and the other over the SE US. But during Week 2, the ridge is shown consolidating and building over the central and eastern US as a deep TROF forms over the EPAC near the west coast with a ridge further to its west.


Fig 9: 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. Much cooler weather is expected over the center of the nation - while it remains warm in the east and especially northeast – though nowhere near as warm as it’s been during the past week. Confidence in the forecast anomaly Pattern and magnitude is above average with readings of ‘5’ and ‘4’ on a Scale from 1 to 5 for the anomaly pattern and magnitude respectively.


Fig 10: Running Precipitation totals for US during the next 10-days (and only for totals > 0.75”) based STRICTLY on the GFS With stronger cold fronts moving into the central US and east – considerably more Precip is expected over the eastern half of the nation as a result of stronger and more widespread T-Storm activity.


Fig 11: 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 return to the center of the nation, with modestly above normal readings in the east – assuming the upper level high pressure system does in fact overspread the eastern half of the nation - something that is far from certain. Overall Confidence in the forecast anomaly Pattern and magnitude is very low, with readings of ‘1’ for the anomaly pattern and magnitude on a scale of 1 to 5 - due to the large variations shown between model runs.


Fig 12: Today’s Temperature anomaly forecasts are based on the 12Z run of the Operational GFS using the GFS MOS data. Stormier weather and decidedly cooler flow will bring below normal readings to much of the state during the rest of the month.

✭ The Next REGULAR, Full Weather Update will be posted on WEDNESDAY, AUGUST 19 ✭

NOTE: SPECIAL UPDATE ON EL NINO Part II is 'Scheduled' for tomorrow

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Steve