Strong Warm-Up on the Way as El Niño Flow Pattern Becomes Firmly Entrenched

By: Steve Gregory , 7:37PM,GMT on October 19,2015

MONDAY: 19-OCT-15 / 2:40 PM CDT
NEXT UPDATE: THURSDAY - OCT 22

PARADE OF TROFs AND RIDGES TO BRING ACTIVE WX PATTERN FROM COAST TO COAST
WITH FLASH FLOODING THREAT FROM SOUTHWEST US TO TEXAS

As the long wave TROF in the eastern US moves east and weakens, it’ll be replaced by building heights and ridging by the end of the week. Temps over the eastern US will rebound strongly across the east as the week progresses – and should remain above normal through next week.

The developing TROF in the west will continue to amplify this week as it progresses slowly eastward, triggering locally heavy showers and T-Storms(!) across the southwestern US which will eventually work its way into Texas later this week. As the ridge builds in the east and the TROF moves across the Rockies into the Plains by the weekend, an influx of deep, tropical moisture emanating from the EPAC across Mexico will surge northward into Texas. This will bring the threat of torrential rains with a high potential for Flash Flooding in portions of Texas. Widespread rainfall totals of 2”-4” with local totals exceeding 10” is quite possible. While the timing for both the onset and duration of this torrential rain event over TX has varied some over the past few days – the rainfall totals forecast by the global models has remained extremely high – with 5-day totals forecast as high as 15” in portions of Texas. Later next week, a storm system formation in the Plains along with the advancement of frontal boundaries will likely bring Precip to the Midwest and much of the eastern US.

With model forecasts becoming somewhat more consistent from one run to the next - and strong forcing from the tropics due to El Niño – the large-scale forecast for a progressive and broad zonal-like flow across North America for the next 2 or more weeks - generally above normal Temps should continue into early November. Of course, there are likely to be at least 1 or 2 unforecast TROF amplifications across the eastern half of the nation. These currently unforeseen developments would lead to stronger cold air surges behind each system – but broadly speaking – Temps should average significantly above normal during the next 3 or more weeks.

In addition to the operational model forecasts calling for above normal Temps into early November – the NMME Extended Forecast model is also calling for above normal Temps across all of North America during November (see below Fig. 2 below) – with the exception of the SW US where Temps are likely to average below normal as Upper Level TROFs become increasingly stronger with above normal Precip expected to affect that region.

STRONG MJO STILL FORECAST TO DEVELOP THIS WEEK

Even though the latest MJO analysis still shows a very weak signal, ALL major global models are forecasting the development of a moderate to strong MJO signal during the week – most likely in the western Indian Ocean region.

However, the models are now forecasting the signal to be quasi-stationary, or to move very slowly eastward across the Indian Ocean by the opening week of November. (Earlier forecasts called for a more definitive movement towards the east, with the signal reaching the western Pacific during the first week in NOV). While the new forecasts may only be a transient model solution, it may also be due to the very strong impact of El Niño on the atmospheric flow pattern which is either generating a partially false MJO signal, or is actually preventing the
typical eastward propagation of the MJO into the EPAC. Only time will tell.

ALASKA WEATHER
Well above normal Temps statewide and very wet weather across the south is likely to continue through the rest of the month with little change in the overall upper air pattern.

A mean upper air ridge over western Canada northwestward into central Alaska with a strong upper level TROF from Siberia south and east across the Aleutians is continuing to bring a 'warm' air flow across the state. A storm formation will bring heavy Precip to the southern coastal region this week, with the next storm moving towards the southwestern portion of the mainland early next week - with another, possibly much stronger storm to follow in about 10 days. Even though actual Temps will fall off some during Week 2, they will remain solidly above normal.




Fig 1: MJO Analysis and Global Model Forecasts While the MJO remains weak – both the operational and ensemble models continue to forecast the development of a moderate to strong signal in the Indian Ocean region during the week ahead, with a strong MJO forecast to remain over the Indian Ocean or possibly nearing the far Western Pacific by the first early November.


Fig 2: NMME Ensemble Forecast ENSO region 3.4 The Monthly NMME model is forecasting SSTA’s in the benchmark Niño 3.4 region ST anomalies to peak during in DEC, followed by a fairly rapid decline during the Spring. These forecasts show a SST Anomaly evolution that’s somewhat slower than the CFSv2 operational model (see last Friday’s Blog post) by about a month – with the same overall magnitude of changes.


Fig 3: CFSv2 November Temp Anomaly For North America The CFSv2 Model forecast for Temp anomalies call for well above normal Temps over nearly all of North America during November – with the exception of the SW US where Temps are expected to average a bit below normal due to a very active storm track.


Fig 4: 15-Day 250mb ( ≈35,000 Ft) Wind Forecast Loop The Polar Jetstream (which typically is found around the 250mb level on average) shows a highly progressive pattern with a series of short wave TROFs embedded in the flow - with the larger scale mean TROF located in the far eastern Pacific near the West coast. By the end of the forecast period, a stronger sub-tropical jet stream can be seen developing off the California coast and just west of Baja.


Fig 5: 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. Temps will rise to well above normal levels across the nation during the Week ahead. Overall Confidence in the forecast anomaly Pattern and Magnitude is above average for this time of year, with a readings of ‘4’ for the anomaly Pattern and ‘3’ for actual Magnitudes on a Scale from 1 to 5.


Fig 6: 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.Above normal Temps are likely to persist through Week 2, but with a fairly strong TROF moving across the West into the central US – Temps may end up averaging closer to normal due to extensive cloudiness, Precip and colder air aloft. NOTE: The forecast for a Nationwide total of 65 Heating Degree Days (HDD) is only a little above the lowest value observed for the same week during the last 14 years. With better model run consistency over the past week, Confidence in the anomaly Pattern is now near average for this time of year with a reading of ‘3’ on a Scale of 1 to 5. However, with moderate to very extreme Precip expected in some areas and still some large differences in forecast Temps beyond 10 days out – Confidence in the actual Temp anomaly Magnitude remains below average, with a reading of ‘2’ on a Scale of 1 to 5.


Fig 7: GFS 10-Day Total Precipitation Loop (for mounts greater than 0.25” only) Locally heavy Precip is quite likely from the southwest US (east of California) into Texas – with the potential for extremely heavy, flash flood producing rains across these areas - especially later this week in portions of Texas as deep, tropical moisture moves northwards from the deep tropics across the western GOM on into Texas. Towards the end of the 10-Day period, a moderate Precip total is possible from the lower Plains into the Great Lakes region as a storm system and cold frontal boundary moves across the central US.


Fig 8: Temperature Anomaly Forecast for Alaska during Week 1 A nearly continuous period of above to much above normal Temps appears likely through the end of the month – but anomalies will decrease during Week 2.

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Steve