Professional Forecaster experience since 1977, concentrating in Aviation, Tropical and Long Range forecasting.
By: Steve Gregory , 4:42 PM GMT on September 04, 2014
INVEST 90L OFF AFRICAN COAST HEADS WESTWARD
NHC has just designated the large tropical wave emerging off the West African coast as INVEST 90L which continues moving westward at 20Kts. The designation has only just now been picked up by the various tracking groups, and apparently came too late for the 12Z Early model runs. Nonetheless, with marginal SST’s (which are ‘peaking’ on a seasonal basis during the next 2 weeks), a low wind shear environment AND the absence of significant effects from the Saharan Air Layer (SAL) should allow some very slow development during the next several days as the disturbance moves westward. In addition, many of the global models do show the system developing to some degree during the next few days – though all but the (00Z) GFS dissipate the system next week. However, the 00Z GFS model run moves the ‘weak’ cyclone westward towards the NE CARIB later next week before turning it out to sea. But the 06Z GFS run (the poorest performing of all 4 GFS cycles) shows the storm reaching the SE US in 12-15 days – and also develops a storm in the NW CARIB/GOM region at that time. BUT – again, the 06Z GFS cycle can be notoriously bad, and if it wasn’t for the seasonal peak in cyclone development, would not even be worth mentioning.
TROPICAL WAVES OVER THE ATLANTIC BASIN APPEAR BENIGN AS ANOTHER STRONG WAVE OVER AFRICA SHOWS SOME POTENTIAL
The Tropical waves we had been tracking a couple days back continue to move westward, with 3 of them now over the CARIB and the large wave now over the central Atlantic. However, very high wind shear over most of the CARIB is preventing any development of these disturbances, while stable air, partially related to the SAL, has diminished along with most all shower activity that had been accompanying the large scale wave over the central ATL. However, moisture as shown in the total precipitable water graphic clearly identifies the location and size of this once large and strong wave.
Aside from INVEST 90L off the African coast, there are 2 other significant Tropical Waves over the interior of Africa. The one in central Africa appears to be the most well developed, and is forecast by the GFS to reach the African coast in 3-4 days, and is also predicted to show some development as it moves westward over the eastern Atlantic later next week.
MOISTURE FROM ‘DOLLY’ HEADING FOR WEST TX AND NM AS HURRICANE NORBERT HEADS TOWARDS SOUTHERN CALIFORNIA
As mentioned Tuesday, higher level moisture from the remnants of DOLLY is moving north/northwestward and will soon be reaching into west TX and south central New Mexico – enhancing the risk for isolated T-storms, and especially increasing the fire potential due to an increase in lightning strikes. Meanwhile, Hurricane NORBERT, now located about 150NM SW of the southern tip of Baja, is moving Northwest, essentially parallel to the Baja coast. Extremely Warm SST’s of 30°C (86°F) will continue to support Hurricane Intensity for the next 24Hrs, with a slow but steady drop in SST’s along the projected track of NORBERT over the next few days; falling below 26°C (typically considered the ‘threshold’ for maintaining a tropical storm) over the weekend. However, moisture from NORBERT will also be streaming northward into the Desert SW starting tonight and Friday, while deeper layer moisture is likely to reach southern California by Monday. In general, the risk of T-storms, flash floods, and even localized dust storm conditions in parts of AZ will increase, while showers may impact the LAX Basin by TUE as the remnants of NORBERT approach San Diego. There is even the low probability that NORBERT could still be of Tropical Depression intensity when it approaches extreme southern California. (NOTE: some models are predicting NORBERT to turn east/southeast into Northern Baja late MON/TUE, preventing much of the moisture from significantly impacting southern CA.)
So, while there are some ‘interesting’ things going on in the Tropics – we are far from having a seasonally ‘active’ season.
Fig 1: Morning VIS image of Invest 90L place the centroid of lower level rotation to the SE of the Cape Verdes, with some curvature displayed in a couple bands of showers and isolated T-storms around the wave axis. Also note the very clear skies to the north of the Wave/Invest – indicating the lack of any dusty, SAL air.
Fig 2: The Saharan Air Layer (SAL) analysis shows the seasonal weakening of the SAL flow across the tropical Atlantic, with the overall flow of air around Invest 90L actually surging towards the NW, literally ‘pushing’ the back edge of the SAL away from the circulation field around 90L. Also note that the very large tropical wave approaching 50W has also caused the SAL to shift northward and dissipate as moist air from the deep tropics dilutes the SAL to the north of the wave.
Fig 3: SST’s along the projected path of INVEST 90L are now at their seasonal ‘peak’, with Temps varying between 25°C and 27°C . While only marginally supportive of development, they may finally be warm enough to create a bit more instability and some continuing shower activity (possibly explaining why some models show some development of the system).
Fig 4: The Total Precipitable Water (TPW) analysis chart helps identify the location of Tropical Waves even when there is little cloudiness or shower activity present.
Fig 5: Using both the enhance (colorized) IR imagery and the TPW analysis (Fig 4), we find 3, TW’s crossing the CARIB, but high wind shear up to 40Kts, especially over the eastern 2/3rds of the CARIB, is preventing any development. The large central Atlantic wave continues westward, but has lost all indications of rotation. It is, however, bringing a large moisture surge with it that is helping to dissipate the effects of the SAL. And of course, INVEST 90L can be seen emerging off the West African coast. Finally, note the convective activity along the entire eastern coastal region of Mexico associated with the remnants of former Tropical Storm DOLLY, and the moisture associated with Hurricane NORBERT moving towards NW Mexico.
Fig 6: Besides INVEST 90L near the African coast, there are 2 significant Tropical Waves over the interior of Africa, with the eastern most one over central Africa, and appears to be the system picked up by the global models as the next wave to have significant development potential when it reaches the Atlantic by early next week.
Fig 7: The IR image from early this AM shows both the moisture surge from the remnants of DOLLY and the large envelope of activity from Hurricane NORBERT moving towards NW Mexico .
Fig 8: : SST’s near NORBERT are very high (over 86°F) which will continue to support hurricane intensity for the next 24 hrs before it runs into cooler waters going northward.
Fig 9: The 12Z Early Model run forecast tracks are in general agreement that NORBERT will track parallel to Baja. However, there is some significant variation in how far west of Baja the storm will track – with a lot of variation after 120 hours (not shown on this chart) when some model projections turn the remnants of NORBERT to the EAST and SE towards Baja – and not towards southern CA.
Fig 10: The latest GFS 850mb (~5,000’) shows abundant moisture flowing into West Texas and parts of NM during the next 24hrs from the remnants of DOLLY, as moisture from NORBERT heads for the Desert Southwest.
The next FULL Tropical Update will be on SUNDAY, SEP 7. HOWEVER – if conditions warrant, I’ll have another brief update either tomorrow or Saturday by around noon time (CDT)
NOTE: I'll be issuing Weather Updates 3-Days per week (Sundays, Tuesdays and Thursdays) – EXCEPT when we have or expect active Tropical Cyclones in the Atlantic Basin or major winter events during the cool season – in which case Updates will be issued as needed. In addition, if a strong cyclone is expected to impact the US mainland – I will be posting my own detailed forecast charts.
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