Tropics Quiet Down as Heat Builds Across the Eastern Half of the Nation
WEDNESDAY, 02SEP15 / 3:55 PM CDT
NEXT REGULAR WX UPDATE ON FRIDAY, SEP 4TH
NOTE: ABBREVIATED UPDATE TODAY DUE TO ‘COOL SEASON’ SOFTWARE CONVERSION WHICH I HOPE TO HAVE COMPLETED BY NEXT WEEK
TROPICS – FRED WEAKENS TO DEPRESSION AS IT HEADS OUT TO SEA 
Within a day after Hurricane FRED pummeled the Cape Verde Islands (though no major damage was reported) FRED began pulling in the Dry / Dusty air associated with the SAL – and with cooler SST’s – has led to a far more stable environment, reducing most of the storm's convection. Increased wind shear overnight also led to a decoupling of the lower level vortex with the upper level circulation field. In short – FRED is on the verge of becoming a remnant Low, and will likely dissipate completely by tomorrow as it moves towards the northwest out to sea.
MID-SUMMER HEAT FOR THROUGH LABOR DAY
There’s been little change in the projected weather pattern for the next 6-8 days, with broad high pressure developing across the nation from the Rockies eastward, As upper level heights rise – Temps will follow – with Temp anomalies expected to average 4˚ to 12˚(F) above normal east of the Rockies, while a deep TROF in the PAC NW southwestward to off the CA coast will continue to keep Temps below normal.
For Week 2, however, things get ‘murky’. The operational model runs have been swinging back and forth on the development of a deep TROF from central Canada southward into the central US and a more ‘zonal-like’ flow pattern developing over the northern half of the nation. To complicate things – the Ensembles have been less than helpful, with many concurrent model runs looking more like the ‘opposite’ solution of the related operational(OPNL) run. (In other words – a colder OPNL run has a ‘warmer’ ensemble run – while the warmer OPNL runs have a cooler ensemble solution). And just to further complicate the outlook for Week 2 – and quite possibly at the root of the large swings in solutions – is the forecast for Hurricane IGNACIO to move northward to near Alaska by Labor Day. EX Typhoons in the Pacific have a tendency to wreak havoc on the overall hemispheric pattern which the models have a difficult time in handling correctly.
Given the uncertainties for Week 2 – I’ve opted to go with the very latest operational (OPNL) 12Z GFS model run – but tempered the actual Temps closer to normal levels as shown in the Ensemble runs – and supported by the latest available EURO (ECMWF) model which has generally had warmer solutions.
It’s also important to keep in mind that ‘Normal’ Temps are now falling briskly – so even when Temp anomalies for Week 2 are shown above normal – actual Temps may really be significantly cooler than Normal Temps are for week 1.
ALASKA WEATHER
A strong high pressure ridge extends N-S from the north central EPAC across the sate – with Temps averaging near to above normal. A strong TROF should move from rapidly eastward from Siberia – and will extend across the state and deepen further south next week as heights rise rapidly over Siberia. By mid-month, most models agree on a large upper level Low should be positioned across the state, with a deep and amplifying TROF extending south/southwest into the north-central EPAC. This should result in much colder conditions developing over the state during Week 2 – and may continue beyond mid-month.
EL NIÑO /MJO
The broad atmospheric pattern continues to be strongly influenced by the ongoing, very strong El Niño event. Lower level westerly wind anomalies extends across all but the far eastern EPAC – and will continue to maintain or even intensify SST anomalies further during the next 2 weeks. Several multi-variate indices that are used to measure the over strength of El Niño have set record highs – and in some categories – are now on par with the Super El Niño events of 1982 and 1997.
The MJO remains weak and effectively incoherent – with El Niño likely making it difficult for a strong signal to develop – or at the very least – making it quite difficult to ascertain if there is a signal! Still, there is general tendency for stronger MJO cycles to develop during the cool season, and some of the Ensemble model members have hinted at the development of a more coherent signal by mid-month.
**
CLICK IMAGE To Open FULL Size image in a new window
Fig 1: Overview of Tropical Atlantic BasinIf you think this graphic looks a lot like the ones from JULY - you’d be right. Dry, stable air dominates much of the tropical ATL basin, with Tropical cyclone FRED the lone survivor of the past 2 weeks ‘flurry’ of activity. And it (FRED) won’t last long either. A tropical Wave near 47˚W longitude has little moisture associated with it, and really just represents the leading edge of another SAL surge (though to be sure, the SAL is not nearly as strong as it was earlier this summer). Another TW/TROF has entered the eastern CARIB and also represents the leading edge of much drier air, while shower activity in the far western CARIB is really just the very weak remnant TROF from ERIKA. The somewhat stronger TW approaching the west African coast (and was mentioned 2 days ago) – has been moving very slowly westward (less than 5Kts). While worth watching, none of the models show much development of this system.
Fig 2: The TPW (Total Precipitable Water) Loop (Top) and latest image frame (bottom) with ‘V’ wind vector component analysis) highlights areas with deep layered moisture. ‘Bluish’ colors represent dryer air while the darker, ‘orange’ tones highlight areas of deep moisture in the column of air above the surface. The 72 hour looped imagery clearly shows the strong circulation of moisture around FRED, with only a faint hint of the TW in the central ATL. The most interesting feature is the ‘wave’ that just entered the eastern CARIB, with strong mid-level rotation. But all that’s rotating around this system is dry air!
Fig 3: Satellite derived Upper level wind analysis from CIMSS across the central and western ATL basin. The rather chaotic wind pattern of a couple days ago has developed into a very organized pattern with a strong and still developing upper level High with its east-west ridge extending from the SE US eastward to the central Atlantic - while a large, upper level Low has developed over the western CARIB. Wind shear that finally declined last week over the CARIB is back – with 40-50Kt shears across the entire southern half of the CARIB – assuring no system can develop in this region anytime soon.
Fig 4: LOW level Wind analysis across Africa Aside from FRED that will soon be out of the picture, the Tropical Wave / Low over western Africa has a well-developed low level circulation; and while similar in location to where FRED was last week, it is not as strong - with considerably less convection. Also of note is the general latitude these upstream waves are moving along – much further south of where they normally would be at the peak of hurricane season (which is right now). Most ‘classic’ Cape Verde storms in SEP come off the coast much closer to the Cape Verde Islands, and then move westward. That’s far from the case this year.
Fig 5: VISIBLE Image of FRED . While some convection remains, most of it is has collapsed during the past day, with almost none present near the Low Level Circulation Center (LLCC) – sheared away by 20-30Kt wind shears.
Fig 6: Tropical Cyclone Track and Intensity Forecast chart for FRED from the US NAVY. The official forecast (which is identical to the NHC forecast since the NAVY follows NHC forecasts EXACTLY in the ATL and EPAC basins) has Tropical Depression FRED weakening to a remnant Low over the next couple days – and will probably dissipate completely before week’s end. Unless there's a sudden change to the outlook – this will be my last update on FRED.
Fig 7: VISIBLE image of IGNACIO. While I said I would no longer provide updates on IGNACIO – because it's now expected to head for Alaska by the weekend – potentially impacting the hemispheric flow pattern – I decided it was worth taking one more look at what is now a ‘borderline’ Tropical Storm/Hurricane. The storm circulation field has grown (as the storm weakened) and is starting to take on non-tropical characteristics as it heads northwestward around the western periphery of the sub-tropical ridge to its east.
Fig 8: Tropical Cyclone Track and Intensity Forecast chart for IGNACIO from the US NAVY. The track and intensity forecasts call for the storm to gradually recurve North and then Northeast at an ever increasing forward speed. As the week progresses, the storm will continue to weaken through the week – but as it approaches Alaska next weekend, it could intensify into a large and strong extra-tropical storm system as the deep TROF now over Siberia moves across Alaska with strong baroclinic forcing meeting up with the warm & moist air from the remnants of IGNACIO. While this type of ‘event’ is not unusual – it’s typically associated with tropical cyclones from the WPAC – not from EPAC storms!
Fig 9 MJO Analysis and Forecast charts The MJO signal remains weak and incoherent – and will project little influence on the tropical weather during the next week. The ECMF (EURO) Ensemble model forecast doesn’t deviate much from this outlook – but some of its ensemble members are showing ‘attempts’ at developing a somewhat stronger or more coherent signal as we head towards mid-month.
Fig 10: Temperature ANOMALY forecast for Week 1 is based STRICTLY on the MOS data forecasts from the todays 12Z operational GFS model run with only minor adjustments towards the raw model data points for Days 6 and 7. An unseasonably warm week ahead for all but the western US (where ‘cooler than normal’ is still mighty warm!). Confidence in the forecast anomaly Pattern and Magnitude is near average for this time of year, with readings of ‘3’ for both the anomaly Pattern and the Magnitude on a Scale from 1 to 5.
Fig 11: Running Precipitation totals during the next 10-days (and only for totals > 0.75”) based STRICTLY on the GFS. Remnant moisture from ERIKA and typical late summer weather will generate significant PRECIP in the SE US. As a weak short-wave TROF being ejected from the deep TROF in the PAC NW moves into the central US next week, an enhanced area of Precip associated with a weak surface front will bring some higher Precip totals from the Great Lakes southwestward to the north Texas. In Alaska - the strong upper level TROF and associated surface storm – combined with the remnants from IGNACIO – may bring some very heavy Precip to the southern portion of the state.
Fig 12: The Week 2 Temperature ANOMALY forecast is based on the 12Z run of the GFS (80%) integrated with the 12Z GFS Ensemble (10%) and the 00Z EURO model Ensemble (10%) 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. While the above outlook isn’t too different from my Monday Forecast 2 days ago – there was a lot of ‘back and forth’ during the intervening period! (The ‘cooler’ than normal values from the lower Plains to Ohio Valley is due to above normal frontal related Precip that will keep daytime Temps below normal.) T0 help put the forecast for ‘Above Normal’ readings during Week 2 into perspective – I’ve highlight the expected weekly total Cooling Degree Days (CDD's). From the Week 1 forecast of 70 Cooling Days – Week 2 will total near 48 CDDs - MUCH cooler actual Temps than during Week 1. (I begin ‘transitioning from CDD’s to HDDs (Heating Degree Days) later this month. Overall Confidence in the forecast anomaly Pattern and Magnitude is below average for Week 2, with readings of ‘2’ for both the anomaly Pattern and Magnitude on a Scale of 1 to 5.
✭ The Next WX Update is Scheduled for FRIDAY, SEP 4 ✭
Want to be alerted when I post an Update? Then follow me on Twitter!
Steve
NOTES:
1. A GENERAL Glossary of Weather Terms can be found HERE
2. Another Glossary of weather terms is available HERE
WEDNESDAY, 02SEP15 / 3:55 PM CDT
NEXT REGULAR WX UPDATE ON FRIDAY, SEP 4TH
NOTE: ABBREVIATED UPDATE TODAY DUE TO ‘COOL SEASON’ SOFTWARE CONVERSION WHICH I HOPE TO HAVE COMPLETED BY NEXT WEEK
TROPICS – FRED WEAKENS TO DEPRESSION AS IT HEADS OUT TO SEA Within a day after Hurricane FRED pummeled the Cape Verde Islands (though no major damage was reported) FRED began pulling in the Dry / Dusty air associated with the SAL – and with cooler SST’s – has led to a far more stable environment, reducing most of the storm's convection. Increased wind shear overnight also led to a decoupling of the lower level vortex with the upper level circulation field. In short – FRED is on the verge of becoming a remnant Low, and will likely dissipate completely by tomorrow as it moves towards the northwest out to sea.
MID-SUMMER HEAT FOR THROUGH LABOR DAYThere’s been little change in the projected weather pattern for the next 6-8 days, with broad high pressure developing across the nation from the Rockies eastward, As upper level heights rise – Temps will follow – with Temp anomalies expected to average 4˚ to 12˚(F) above normal east of the Rockies, while a deep TROF in the PAC NW southwestward to off the CA coast will continue to keep Temps below normal.
For Week 2, however, things get ‘murky’. The operational model runs have been swinging back and forth on the development of a deep TROF from central Canada southward into the central US and a more ‘zonal-like’ flow pattern developing over the northern half of the nation. To complicate things – the Ensembles have been less than helpful, with many concurrent model runs looking more like the ‘opposite’ solution of the related operational(OPNL) run. (In other words – a colder OPNL run has a ‘warmer’ ensemble run – while the warmer OPNL runs have a cooler ensemble solution). And just to further complicate the outlook for Week 2 – and quite possibly at the root of the large swings in solutions – is the forecast for Hurricane IGNACIO to move northward to near Alaska by Labor Day. EX Typhoons in the Pacific have a tendency to wreak havoc on the overall hemispheric pattern which the models have a difficult time in handling correctly.
Given the uncertainties for Week 2 – I’ve opted to go with the very latest operational (OPNL) 12Z GFS model run – but tempered the actual Temps closer to normal levels as shown in the Ensemble runs – and supported by the latest available EURO (ECMWF) model which has generally had warmer solutions.
It’s also important to keep in mind that ‘Normal’ Temps are now falling briskly – so even when Temp anomalies for Week 2 are shown above normal – actual Temps may really be significantly cooler than Normal Temps are for week 1.
ALASKA WEATHER A strong high pressure ridge extends N-S from the north central EPAC across the sate – with Temps averaging near to above normal. A strong TROF should move from rapidly eastward from Siberia – and will extend across the state and deepen further south next week as heights rise rapidly over Siberia. By mid-month, most models agree on a large upper level Low should be positioned across the state, with a deep and amplifying TROF extending south/southwest into the north-central EPAC. This should result in much colder conditions developing over the state during Week 2 – and may continue beyond mid-month.
EL NIÑO /MJO The broad atmospheric pattern continues to be strongly influenced by the ongoing, very strong El Niño event. Lower level westerly wind anomalies extends across all but the far eastern EPAC – and will continue to maintain or even intensify SST anomalies further during the next 2 weeks. Several multi-variate indices that are used to measure the over strength of El Niño have set record highs – and in some categories – are now on par with the Super El Niño events of 1982 and 1997.
The MJO remains weak and effectively incoherent – with El Niño likely making it difficult for a strong signal to develop – or at the very least – making it quite difficult to ascertain if there is a signal! Still, there is general tendency for stronger MJO cycles to develop during the cool season, and some of the Ensemble model members have hinted at the development of a more coherent signal by mid-month.
**
CLICK IMAGE To Open FULL Size image in a new window
Fig 1: Overview of Tropical Atlantic BasinIf you think this graphic looks a lot like the ones from JULY - you’d be right. Dry, stable air dominates much of the tropical ATL basin, with Tropical cyclone FRED the lone survivor of the past 2 weeks ‘flurry’ of activity. And it (FRED) won’t last long either. A tropical Wave near 47˚W longitude has little moisture associated with it, and really just represents the leading edge of another SAL surge (though to be sure, the SAL is not nearly as strong as it was earlier this summer). Another TW/TROF has entered the eastern CARIB and also represents the leading edge of much drier air, while shower activity in the far western CARIB is really just the very weak remnant TROF from ERIKA. The somewhat stronger TW approaching the west African coast (and was mentioned 2 days ago) – has been moving very slowly westward (less than 5Kts). While worth watching, none of the models show much development of this system.
Fig 2: The TPW (Total Precipitable Water) Loop (Top) and latest image frame (bottom) with ‘V’ wind vector component analysis) highlights areas with deep layered moisture. ‘Bluish’ colors represent dryer air while the darker, ‘orange’ tones highlight areas of deep moisture in the column of air above the surface. The 72 hour looped imagery clearly shows the strong circulation of moisture around FRED, with only a faint hint of the TW in the central ATL. The most interesting feature is the ‘wave’ that just entered the eastern CARIB, with strong mid-level rotation. But all that’s rotating around this system is dry air!
Fig 3: Satellite derived Upper level wind analysis from CIMSS across the central and western ATL basin. The rather chaotic wind pattern of a couple days ago has developed into a very organized pattern with a strong and still developing upper level High with its east-west ridge extending from the SE US eastward to the central Atlantic - while a large, upper level Low has developed over the western CARIB. Wind shear that finally declined last week over the CARIB is back – with 40-50Kt shears across the entire southern half of the CARIB – assuring no system can develop in this region anytime soon.
Fig 4: LOW level Wind analysis across Africa Aside from FRED that will soon be out of the picture, the Tropical Wave / Low over western Africa has a well-developed low level circulation; and while similar in location to where FRED was last week, it is not as strong - with considerably less convection. Also of note is the general latitude these upstream waves are moving along – much further south of where they normally would be at the peak of hurricane season (which is right now). Most ‘classic’ Cape Verde storms in SEP come off the coast much closer to the Cape Verde Islands, and then move westward. That’s far from the case this year.
Fig 5: VISIBLE Image of FRED . While some convection remains, most of it is has collapsed during the past day, with almost none present near the Low Level Circulation Center (LLCC) – sheared away by 20-30Kt wind shears.
Fig 6: Tropical Cyclone Track and Intensity Forecast chart for FRED from the US NAVY. The official forecast (which is identical to the NHC forecast since the NAVY follows NHC forecasts EXACTLY in the ATL and EPAC basins) has Tropical Depression FRED weakening to a remnant Low over the next couple days – and will probably dissipate completely before week’s end. Unless there's a sudden change to the outlook – this will be my last update on FRED.
Fig 7: VISIBLE image of IGNACIO. While I said I would no longer provide updates on IGNACIO – because it's now expected to head for Alaska by the weekend – potentially impacting the hemispheric flow pattern – I decided it was worth taking one more look at what is now a ‘borderline’ Tropical Storm/Hurricane. The storm circulation field has grown (as the storm weakened) and is starting to take on non-tropical characteristics as it heads northwestward around the western periphery of the sub-tropical ridge to its east.
Fig 8: Tropical Cyclone Track and Intensity Forecast chart for IGNACIO from the US NAVY. The track and intensity forecasts call for the storm to gradually recurve North and then Northeast at an ever increasing forward speed. As the week progresses, the storm will continue to weaken through the week – but as it approaches Alaska next weekend, it could intensify into a large and strong extra-tropical storm system as the deep TROF now over Siberia moves across Alaska with strong baroclinic forcing meeting up with the warm & moist air from the remnants of IGNACIO. While this type of ‘event’ is not unusual – it’s typically associated with tropical cyclones from the WPAC – not from EPAC storms!
Fig 9 MJO Analysis and Forecast charts The MJO signal remains weak and incoherent – and will project little influence on the tropical weather during the next week. The ECMF (EURO) Ensemble model forecast doesn’t deviate much from this outlook – but some of its ensemble members are showing ‘attempts’ at developing a somewhat stronger or more coherent signal as we head towards mid-month.
Fig 10: Temperature ANOMALY forecast for Week 1 is based STRICTLY on the MOS data forecasts from the todays 12Z operational GFS model run with only minor adjustments towards the raw model data points for Days 6 and 7. An unseasonably warm week ahead for all but the western US (where ‘cooler than normal’ is still mighty warm!). Confidence in the forecast anomaly Pattern and Magnitude is near average for this time of year, with readings of ‘3’ for both the anomaly Pattern and the Magnitude on a Scale from 1 to 5.
Fig 11: Running Precipitation totals during the next 10-days (and only for totals > 0.75”) based STRICTLY on the GFS. Remnant moisture from ERIKA and typical late summer weather will generate significant PRECIP in the SE US. As a weak short-wave TROF being ejected from the deep TROF in the PAC NW moves into the central US next week, an enhanced area of Precip associated with a weak surface front will bring some higher Precip totals from the Great Lakes southwestward to the north Texas. In Alaska - the strong upper level TROF and associated surface storm – combined with the remnants from IGNACIO – may bring some very heavy Precip to the southern portion of the state.
Fig 12: The Week 2 Temperature ANOMALY forecast is based on the 12Z run of the GFS (80%) integrated with the 12Z GFS Ensemble (10%) and the 00Z EURO model Ensemble (10%) 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. While the above outlook isn’t too different from my Monday Forecast 2 days ago – there was a lot of ‘back and forth’ during the intervening period! (The ‘cooler’ than normal values from the lower Plains to Ohio Valley is due to above normal frontal related Precip that will keep daytime Temps below normal.) T0 help put the forecast for ‘Above Normal’ readings during Week 2 into perspective – I’ve highlight the expected weekly total Cooling Degree Days (CDD's). From the Week 1 forecast of 70 Cooling Days – Week 2 will total near 48 CDDs - MUCH cooler actual Temps than during Week 1. (I begin ‘transitioning from CDD’s to HDDs (Heating Degree Days) later this month. Overall Confidence in the forecast anomaly Pattern and Magnitude is below average for Week 2, with readings of ‘2’ for both the anomaly Pattern and Magnitude on a Scale of 1 to 5.
✭ The Next WX Update is Scheduled for FRIDAY, SEP 4 ✭
Want to be alerted when I post an Update? Then follow me on Twitter!
Steve
NOTES:
1. A GENERAL Glossary of Weather Terms can be found HERE
2. Another Glossary of weather terms is available HERE
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