Déjà-Vu of June? … And the 4 Faces of SAL

By: Steve Gregory , 9:05PM,GMT on August 7,2015

FRIDAY: 07-AUG-15
(Next Regular WX Update MONDAY – AUG 10)

NOTE - I WILL BE ANSWERING THE COMMENTS IN MY PREVIOUS BLOG DURING BY 5PM CDT

TROPICS

The Eastern Pacific continues on its tare this summer as Tropical Storm HILDA takes aim on Hawaii – while the Atlantic remains dormant.

TS Hilda some 1,200NM ESE of Hilo has estimated sustained winds of 50Kts, and is still nearly a week away from becoming a potential threat to the Islands. While official forecasts show the storm turning northward prior to reaching the Islands next week, the storm is still heading due West, slightly off the forecast track. With wide model differences in the forecast track after 72 hours, the storm does need to be monitored closely.

Meanwhile, in the Atlantic – there are no significant tropical waves as the basin continues to be impacted by either high wind shear, dry and stable air – or a combination of both. The somewhat stronger system that came off the African coast a couple days ago has already begun to fall apart as it heads NW into much drier air and cooler SST’s. There are 2 somewhat more robust disturbances over Africa – with the first likely to come off the west African coast over the weekend and the 2nd one around the middle of next week. Neither one of these systems appears likely to develop into cyclones.

While the SAL (Saharan Air Layer) flow is much weaker – periodic increases in shear and surges of the SAL appears to be too big to overcome by even these somewhat stronger tropical waves – at least for the next 7-10 days. Beyond then, the GFS does try to spin up a couple systems into bona-fide cyclones, but beyond 10 days out, there is a strong model bias towards developing systems, and for now, need to be discounted.

A WEATHER PATTERN REMINISCENT OF JUNE

Until the latest global model runs, there was excellent overall consistency and agreement among the operational and Ensemble members that near to slightly below normal Temps would prevail from the Midwest to the Northeast for the rest of the month. This outlook began to change slightly with the 06Z run, but much more so with this morning’s 12Z operational GFS run.

Until this AM, the basic theme was one with the sub-tropical ridge becoming firmly established over the intermountain west – with occasional advancement into the southern Plains before retrograding back towards the west. This ‘oscillation’ was in response to short wave TROFS moving from western Canada east/southeast towards the eastern US every 3-5 days, forcing the ridge back over the Rockies. Once the short wave reached the eastern states and lifted out to the northeast, the ridge would move back towards the Plains. Under this pattern, cooler/drier air would work its way into the Midwest, Ohio Valley and Northeast behind each short wave – with Temps rebounding a bit as the ridge moved back towards the Plains.

However, with the 06Z and especially the 12Z model run, the western ridge is shown advancing into the central US in 10 days – and holding in place thru the end of the forecast period. This implies much warmer conditions spreading into the Midwest and much of the east during Week 2. And while the latest GFS Ensembles are not as aggressive in bringing back the heat in the east, they have warmed somewhat compared to earlier runs. More interesting, is their depiction of a very broad, zonal-like flow developing across North America, with an elongated sub-tropical ridge extending across the entire nation from California to Georgia and then out over the Atlantic. Given the sharp shift in the operational runs, I opted to heavily weight Week 2 Temp Forecast towards the GFS Ensemble solution.

ALASKA – MILD BUT WET & STORMIER CONDITIONS ON THE WAY

Aside from small run-run to run differences, all global models are in agreement that upper level heights will begin falling off by next week, with considerably stronger TROFs and related storms becoming the dominant feature over the state during Week 2 (a completely ‘normal’ development by mid-August). The main storm track is likely to be from near the Kamchatka Peninsula eastward into the Bering Sea, with secondary storm tracks along the North coast and from the eastern Aleutians to northern GOA.

EL NINO / MJO

The strong El Nino event continues to drive the tropical weather pattern, with SST anomalies in the benchmark Niño 3.4 region holding steady at +1.7˚C. However, as shown below, this reading belies the anomaly differences now found from the western side of the region to the eastern. SST anomalies over +2.5˚C are found in the eastern area, but readings are slightly under +1.5˚C further to the west. In all likelihood, the cooler readings in the western portion of Niño 3.4 is due to extensive and persistent cloudiness and T-Storm activity, and not because of some other atmospheric influence, or up-welling. This kind of persistent cloud cover/Precip is completely consistent with a well-developed El Niño.

The MJO, while slightly stronger than last week, remains generally weak and/or incoherent, with most (but not all) models keeping it that way through the upcoming week. So while the MJO will not be playing a major role in the EPAC for the next 10 days, persistent westerly wind anomalies – consistent with the El Niño base state - will continue in the central Pacific.

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Fig 1: Color Enhanced IR (infrared) image of Tropical Storm HILDA some 1,200NM ESE of Hilo, Hawaii. Deep convection can be seen on the east side of the storm as shown by the bright white color enhancement which corresponds to very cold cloud tops.


Fig 2: Color Enhanced IR (infrared) image of Tropical Storm HILDA with Wind Shear OverlayHILDA is a relatively small storm, and the CIMSS analysis shows wind shear under 10kts – normally quite conducive for intensification. However, with the storm moving fairly quick towards the west, intensification has remained slow today. Still, most specialized hurricane models call for the system to become a Hurricane during the next 24-36hrs, and maintain CAT 1 intensity into early next week as it heads generally West-Northwest.


Fig 3: Forecast Track/Intensity for HILDAThe NAVY forecast calls for Hilda to head generally W/NW through MONDAY before recurving NW and then Northward later in the week - passing about 300NM east of the Big Island as a strong Tropical Storm. However, besides the typically large errors for storm tracks of 5-days or more, there is a wide variation in model forecasts beyond 3 days out, lowering the confidence in the above forecast considerably.

CLICK IMAGE to open full size image in a new window
Fig 4: Color Enhanced IR (infrared) Overview image of the Tropical Atlantic The main area of ANY real interest is the Tropical Wave now to the west of the Cape Verdes. The system was still showing mid-level rotation this morning, but little in the way of convection. This system is likely going to dissipate completely within 24 hrs. The SAL is considerably weaker now, and appears restricted to only a few hundred miles off the African coast.



Fig 5: The TPW (Total Precipitable Water) Loop (Bottom) and latest image frame (Top) 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 highlights the 3 TW’s over the Atlantic – none of which pose a risk of development. The somewhat stronger system near the Cape Verdes has lost its convection as it moves NW into the much drier / stable environment. The ITCZ is clearly seen in this depiction – but is relatively weak with little significant convection noted.


Fig 6: 96 Hour Forecast for Dust Transport. Todays forecast animation for Dust flow (that has been very accurate over the last few days) shows an area of very dense dust coming off the North African coast but then slowly dissipates, with most of the dust laden air remaining north of 15˚N latitude.


Fig 7: GFS Wind Shear Forecast for the Atlantic Basin (Still in ‘Beta’) The wind shear forecast (from last night) has been set to highlight shear values of 20Kts or higher – a level GENERALLY considered too strong for the INITIAL formation of a tropical storm. This large-scale view continues to show well above normal shears across the CARIB and at times, across much of the tropical Atlantic. Potential cyclones coming off the African coast are shown spinning up during Week 2, but a close look shows shears near the potential cyclone are actually quite low. As storms intensify, high shear values are typical well removed from the vortex itself.

CLICK IMAGE to open full size image in a new window

Fig 8: Four Satellite Derived Images of the ‘SAL’ These are the 4 images I routinely use (when available) to analyze the location and general intensity of the SAL that I outline on the ‘Atlantic Overview Chart’ (Figure 4, above).



Fig 9: Water Vapor Image of EPAC and ENSO region SST Anomalies. Tropical Storm HILDA is shown, along with the area corresponding to the slightly lower SST anomalies in the western portion of the Nino 3.4 region. Note the implied extensive cloudiness/T-Storms in the Water Vapor image that corresponds to the 'cooler' SST anomalies.


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 minor adjustments towards the raw model data points. Above normal readings across much of the nation as a bit cooler Temps hang on across the Great Lakes and Northeast. Confidence in the forecast anomaly Pattern and magnitude is solidly above average with readings of ‘5’ and ‘4’ on a Scale from 1 to 5 for the anomaly pattern and magnitude respectively.



Fig 11: Running Precipitation totals for US during Week 1 (and only for totals > 0.75”) based STRICTLY on the GFS Heaviest Precip will continue in the far Southeastern US, with a secondary region in the Midwest, primarily due to frontal passages, and not widespread or prolonged periods of Precip.


Fig 12: The Week 2 Temperature ANOMALY forecast is based on the 12Z run of the GFS (60%) integrated with the 12Z GFS Ensemble (40%) 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.A very warm period is likely across the nation – though the above normal readings in the Midwest/Great Lakes and New England are somewhat suspect at this time per today’s discussion. Confidence in the forecast anomaly Pattern and magnitude was poised to increase to ‘Above Average’ – but is now Below Average by summer standards – with reading of ‘2’ on the anomaly pattern and magnitude on a Scale from 1 to 5 due to the strong reversal of model forecasts this morning in the northeastern quarter of the nation. .



Fig 13: Today’s Temperature anomaly forecasts are based on the 12Z run of the Operational GFS using the GFS MOS data A generally mild period will continue across the southern portions of the state – and the North Slope area where southerly winds will prevail thru the period. Disturbances start showing up later next week, which is the primary reason for somewhat cooler conditions developing in the central region. A significantly stormier and somewhat cooler anomaly pattern is likely to develop during Week 2.

✭ The Next REGULAR Weather Update will be on MONDAY, AUGUST 10 ✭

‘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

Steve

NOTES:

1. A GENERAL Glossary of Weather Terms can be found HERE

2. Another Glossary of weather terms is available HERE