MONDAY: 02-NOV-15 / 3:00 PM CDT
(NEXT TEMP FORECAST CHART UPDATE ON WEDNESDAY)NEW POSTING SCHEDULE: ‘FULL’ UPDATES ON MONDAYS / FRIDAYS … TEMP FORECAST CHARTS ON WEDNESDAYS AND WHEN WARRANTED
UNSEASONABLY MILD TEMPS TO PERSIST IN THE EAST AS
BELOW NORMAL TEMPS DEVELOP OUT WEST AND ACROSS ALASKAMuch above normal Temps will prevail east of the Rockies this week while Temps fall to near and then below normal levels across much of the West as upper level heights fall off in that region. While NOT a major pattern shift, seasonal forcing will cause enough of a shift over the EPAC to allow much colder Temps to overspread Alaska starting by the upcoming weekend. Temps will fall to near zero in Anchorage with overnight lows of -20˚F to -30˚F in some of the colder, central locations across the mainland.
The highly progressive and steadily increasingly upper level flow will continue, with all global models showing a mean upper level TROF over the west and ridging in the east. Despite significant day to day differences between the GFS and ECMWF models – especially during Week 2 – overall, the models are in good agreement that on average, the mean location of the main upper level TROF with below normal Temps will persist out West, while ridging and above normal Temps will dominate much of the east at least thru mid NOV – and probably thru the rest of the month.
Rain in the mid and south Atlantic states region will be ending during the next day or so – while the western TROF brings widespread Precip to the intermountain west. As the western TROF moves out across the center of the nation by the end of the week, another heavy rain event is expected to develop over portions of Texas. Similar to the most recent storm, rain will then spread northward to the Great Lakes and Ohio Valley during the weekend, eventually reaching most of the east coast.
As the NEXT major TROF develops in the west and deepens while heading into the central US early next Week, a potentially major storm system should develop over the Plains and Mississippi Valley – bringing another round of heavy rains from Texas north to the Great Lakes and then eastward to the east coast. Behind this system, a significant cold air surge should overspread the eastern half of the nation before the subsequent upstream TROF /storm system begins to approach the PAC NW by the end of Week 2.
STRONG MJO SIGNAL TO BE OVERWHELMED BY INTENSIFYING EL NIÑOThe Westerly Wind Burst (WWB) that occurred during late OCT in the equatorial Central Pacific (CPAC) triggered another moderately strong oceanic Kelvin Wave (KW). As conjectured over 10 days ago, SST anomalies have jumped up in the benchmark ENSO 3.4 region – with the latest ONI (SSTA based) anomaly now at +2.7˚C. This is now equal to the mean peak anomaly value reached during the 1997/98 Super El Niño event (in the 3.4 region). In addition, the latest ENSO forecasts are now calling for El Niño to peak in early DEC.
The strong MJO that developed a week ago in the Indian Ocean is expected to move very slowly eastward this week before weakening rapidly next week. In a broad sense, this MJO HAD the POTENTIAL to be the ‘trigger’ for a decline in SSTA’s in the EPAC during the end of NOV or early DEC (as forecast by most models). However, with the MJO forecast to weaken rapidly while still in the Indian Ocean, I do not yet see any apparent forcing mechanism that would trigger the long term decline in SSTA’s within the next 30 days – especially since the cold ocean anomalies at depth in the WPAC has still not shown any advancement towards the east. In addition, other ENSO indicators such as the anomalous Precip and low level winds in the tropical Pacific have not changed – and operational forecasts do not yet forecast a significant change. That said - the climatological ‘peak’ for strong El Niño events occurs during DEC – and unless we get another especially strong WWB during the next month or so – the statistical odds support the peak for the current event occurring sometime in DEC. Regardless, keep in mind that even if El Niño does begin to decline next month – the atmosphere will almost certainly maintain an El Niño like flow across the northern hemisphere thru the entire winter.
Fig 1: Hovmöller Chart for 850mb (~5,000 Ft) Westerly Wind Anomalies in the Tropical Pacific The above chart highlights the Westerly Wind Bursts since the summer, with 2 strong WWB's occurring in OCT. These WWB’s are the precursor indicators for the most recent rise in SST anomalies across the EPAC (see below Fig 2).
Fig 2: The Hovmoller chart for upper ocean Heat Anomalies across the tropical Pacific The WWB’s mentioned above have caused an increase in surface level westerly wind anomalies triggering another significant oceanic Kelvin Wave that developed in OCT. Down-welling occurs just ahead of the KW ‘peak’ (shown by the black dashed lines), while general upwelling occurs behind the wave peak and near the wave trough axis. The above shows that the development of El Niño this year began in earnest with the first very strong Kelvin Wave that moved across the tropical Pacific last March, with at least 3 more subsequent KW’s since then.
Fig 3: The most recent 4-week SST anomaly analysis chart SST anomalies greater than 3˚C covered much of the EPAC, with a secondary region of warm anomalies of +2˚C or more off the SW US and Baja coast. Indeed, much of the NORPAC east of the Dateline continues to average much above normal - and has clearly led to the highly anomalous upper level wind flow that has developed over the past month across the NORPAC and North America. While there is no broad consensus yet - I tend to believe that the secondary SST anomaly MAX will lead to heavier Precip across California whenever there are major storm events. An example of this ‘enhanced’ Precip occurred over the past few days in the PAC NW with record daily rainfall totals.
Fig 4: MJO Analysis and Global Model Forecasts The new and very strong MJO cycle that began over a week ago in the Indian Ocean is projected to propagate slowly eastward this week before weakening rapidly when the signal approaches the south China Sea next week – and model forecasts are in good agreement with each other and have shown good consistency in the forecasts since last week. It is quite possible that the signal will be ‘overwhelmed’ by the very strong El Niño base state – accounting for the forecast weakening of the signal before it can reach the Pacific. HOWEVER, it's also possible that the signal is being ‘hidden’ from analytic view by the anomalous atmospheric flow pattern associated with El Niño.
Fig 5: 500mb (≈ 18,000 Ft) Height Forecast Loop for the next 2 Weeks (Top) and corresponding 250mb ( ≈35,000 Ft) Wind Forecast Loop (Bottom)The above loops show the wind patterns and location of TROF’s and Ridges. The Polar Jetstream (which on average is typically found around the 250mb level) continues to intensify, with Jetstream core speeds exceeding 180Kts at times. In general, the wind flow pattern at higher levels (like 250mb) are similar to those found at 500mb, but 250MB TROFS are typically located to the west of the TROF location at 500mb. Note how strong TROF’s first appearing to the west of Alaska (with the Polar Jet Stream ‘core’ curving around the TROF axis) are forecast to dive into the western US every few days, and then move East/Northeast towards the eastern US. IN ADDITION – we are now seeing the development and persistence of the sub-tropical jet stream that generally is shown originating near or east of Hawaii, and then tends to appear to the south of the polar jet stream core around the ‘base’ of each strong TROF as they move eastward across the US. The sub-tropical jet sometimes appear to ‘merge’ with the polar jet stream – but they are both separate from - and at a higher altitude than - the polar jet stream. The jet stream is essentially the steering mechanism for – and provides the atmospheric ‘lift’ for – associated surface storm systems. Note the especially strong and highly amplified TROFs that develop out west later this week and again next week. These TROFs will trigger the storm systems expected to bring heavy Precip first to the PAC NW and to Texas before spreading across the rest of the eastern US. Around Days 10-12, the TROF is forecast to become very strong in the central US, with a 3 to 5 day period of more seasonal Temps expected to follow over the eastern US.
Fig 6: GFS Ensemble Forecast for 500mb (~18,000 Ft) at Day 16 Completely ‘opposite’ of climatology – a mean TROF is forecast over western North America – with ridging in the east. This is an extremely unusual pattern – and rarely forecast this far in advance. A GREAT indicator of the forcing during a strong El Niño cold season.
Fig 7: 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 8. Much above normal Temps are expected this week – with a cooling trend east of the Rockies from west to east starting at the end of the week. Confidence in the forecast anomaly Pattern and Magnitude is above average for this time of year, with readings of ‘4’ on a Scale from 1 to 5.
Fig 8: 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 or expected anomalous thermal patterns & projected storm system passages.The GFS forecasts which are in generally good agreement with the European (ECMWF) model – though there are significant day-to-day differences in the location of individual TROFs (typical for such a progressive flow pattern). The cooler Temps shown across the Gulf coast region is primarily due to the above normal Precip expected in that region. Confidence in the anomaly Pattern and magnitude is about average for this time of year with readings of ‘3’ on a scale of 1 to 5.
Fig 9: GFS 15-Day Precipitation Pattern Loop (for mounts greater than 0.10” only) The new animation shows total Precip forecasts in 24 hour time steps – with 2 more VERY heavy rain events forecast for Texas and other portions of the east – especially a=over the southern tier of states. More heavy rain is expected in the PAC NW as well – with heavy rain reaching into California towards mid-month.
Fig 10: Temperature Anomaly Forecast for Alaska during Week 1 Much above normal Temps will finally come to an end later this week – with very cold conditions expected across the central and southern portions of the state during Week 2 as the upper air pattern shifts enough to allow arctic air to advect into and develop over the state.
NOTE: NEW POSTING SCHEDULE: ‘FULL’ UPDATES ON MONDAYS / FRIDAYS … TEMP FORECAST CHARTS ON WEDNESDAYS AND WHEN WARRANTED
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Steve
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