Tropical Storm IDA Spinning its Wheels as Mild WX Dominates the US

By: Steve Gregory , 9:33PM,GMT on September 21,2015

MONDAY, 20-SEP-15 / 4:00 PM CDT
NEXT WX UPDATE ON FRIDAY, SEPTEMBER 25TH

FULL UPDATES RESUME *FRIDAY* WHEN THE UPGRADE IS COMPLETED

PROGRESSIVE FLOW PATTERN TO CONTINUE

While the dominant pattern includes a TROF from the Gulf of Alaska (GOA) to the western US and broad ridging over much of the eastern half of the US – individual short waves are continuing to move rapidly through this ‘primary pattern’, and occasionally amplifying as they move across the Midwest and eastern US. This upper air pattern will bring generally above normal Temperatures to much of the nation during the next 2 weeks, with brief 2-3 day cool downs in the northern tier of states through the first 10 days of October.

The warmest and mostly dry weather will prevail over the central US, with near normal Precip in the PAC NW. Above normal Precip is likely in the southeastern US northward along the eastern coastal Plain towards New England due to a near continuous ‘tropical’ air flow (with heaviest Precip likely when mid-latitude frontal boundaries associated with amplifying short wave TROFs come into play).

While it appears extremely unlikely that tropical storm IDA in the central ATL will directly impact the eastern US – the global models are hinting at possible (weak) cyclone developments near the Bahamas and/or western CARIB during Week 2 - which could then move towards the SE US. It’s worth noting that seasonality also supports an increased level of tropical influence from these regions during OCT.


TROPICAL ATLANTIC

With the exception of weak and near stationary Tropical Storm IDA in the central Atlantic, the already subdued activity in the tropical Atlantic is expected to continue, with a marked decline in tropical wave activity from Africa.

The relatively small and weak T.S. IDA is near 21˚N / 48˚W – or about 1,000 NM ENE of the Leeward Islands. The storm is moving slowly towards the W/NW at about 5Kts - and most models call for the storm to meander around its current location for the next few days due to a very weak steering wind pattern. At the same time, moderate wind shear of ~15Kts is hindering intensification - and while shears are forecast to ease off tomorrow, only slight intensification appears likely since shears are expected to increase again starting THU, with steering currents also expected to increase as high pressure builds to the north of the storm. Initially, the storm is expected to head towards the Northwest, but the latest global model runs call for an approaching TROF to move off the North American coast, turning the storm towards the North and eventually east/northeast, away from the US, during the weekend

EL NIÑO / MJO

With low level wind anomalies across the central Pacific continuing, and an extremely weak/incoherent MJO signal, there have been no significant changes to the very strong El Niño. SST anomalies in the benchmark Niño 3.4 region remain very high at +2.4˚C, with no changes seen in sub-surface heat anomalies either.

With no significant changes to the extremely weak MJO signal now expected for the rest of the month - and no short term changes in the low level westerly wind anomalies either - the chances for additional and significant intensification of El Niño during the next 2 weeks seems unlikely. It is, however, worth mentioning that a highly unusual and large area of unusually warm water has developed off the west coast of Mexico during the last few weeks, and is totally ‘separate’ from El Niño. Additional information on this will be provided next week when I hope to have additional time and resources to review this new ‘feature’.



CLICK IMAGE To Open FULL Size image in a new window
Fig 1: Overview of Tropical Atlantic BasinTropical Storm IDA is located in the central Atlantic near 21˚N/48˚W, and despite the relatively strong convection, moderately strong wind shear has caused the vertical structure of the vortex to take on a NW to SE orientation, preventing intensification. With the exception of a weak tropical wave along the west African coast, there are no other significant tropical weather systems anywhere else in the tropical ATL basin.


Fig 2: The TPW (Total Precipitable Water) Loop (Top) highlighting 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 deep moisture associated with tropical storm IDA as it developed, and the higher moisture totals associated with the ITCZ (Inter Tropical Convergence Zone). A somewjat higher water content can be seen at the end of the loop associated with the Tropical Wave along the African coast.


Fig 3: Visible Image of Tropical Storm IDA The above image shows the high level cirrus outflow above the storm, but the Low Level Circulation Center (LLCC) is located well to the NW of the deep convection. The ‘tilted’ structure of the storm is due to the Northwesterly wind shear, and is preventing the storm from intensifying.


Fig 4: Color Enhanced image of tropical Storm IDA The above image also shows the high level cirrus outflow above the storm associated with the deep convection. Without other types of imagery (like the above VIS image ) – it would appear the storm center was located in the ‘center’ of the circular cloud field near 20˚N/ 47˚W.


Fig 5: Upper Level Wind analysis from CIMSS. Very strong westerlies are seen over the eastern US, with an upper Low near 20˚N/60˚W and an anti-cyclonic wind flow above IDA. This type of high level flow above the storm and the upper Low to the west of it – is generally quite conducive to intensification. However, moderate wind shear across the storm has led to a ‘tilted’ vertical structure and is preventing significant intensification.


Fig 6: Steering Wind analysis from CIMSS . The mean wind flow for a weak tropical storm in the 850mb to 500mb layer (~5,000ft to 18,000ft) shows a weak southeasterly flow, which is steering the storm slowly towards the NW. The upper level High well to the north of the storm will build during the next few days, forcing the storm to move somewhat faster towards the NW. As the TROF near the Great Lakes moves out over the North Atlantic and strengthens later this week, it will turn the storm towards the N/NE, and eventually out to sea.


Fig 7: Current ENSO region SST anomalies along with a similar SSTA analysis from 2 Weeks ago. Very little overall change change in SST anomalies has occurred over the last 2 weeks – with readings around +2.4˚C above normal. However, during the 2 week period, SSTA’s DECREASED by about 0.3˚C in the western portion of the 3.4 region, while increasing by about 0.4˚C in the eastern area. The ‘change’ is in response to an oceanic Kelvin Wave (KW) that moved across the EPAC during the period –with down welling (and warming) ahead of the KW, and upwelling (cooling) behind it.


Fig 8: Changes in SST ANOMALIES during the past MONTH. Additional warming is noted in the ENSO regions, while Temps also warmed in the far North Atlantic. SST’s cooled in the GOA, with sharply cooler conditions in the west central Pacific.


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 only minor adjustments towards the raw model data points for Days 6 and 7. Temps will average above normal across the entire US as an upper level ridge dominates most of the nation. Confidence in the forecast anomaly Pattern and Magnitude is above average for this time of year, with readings of ‘5’ for the anomaly Pattern and ‘4’ for the anomaly Magnitude - on a Scale from 1 to 5.


Fig 10: 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. Above normal Temps are expected to continue during Week 2 (and into early October) – though more frequent and somewhat stronger short wave TROFS moving across the nation with lower the anomalies in most areas; especially in the southeast where above normal rainfall will cap high temps,. Overall Confidence in the forecast anomaly Pattern is near average, but confidence in the anomaly Magnitude is a bit below average for Week 2 due to the rapidity of Temp changes expected ahead of and then behind each short wave - resulting in a Confidence Level reading of ‘3 for the general pattern, but ‘2’ for the anomaly magnitudes on a Scale of 1 to 5.

✭ The Next WX Update is Scheduled for FRIDAY, SEP 25 ✭

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Steve