Cold Snap Will Be Short-Lived as Tropical Flow Takes Aim on US

By: Steve Gregory , 8:29PM,GMT on October 16,2015

FRIDAY, 16-OCT-15 / 3:30 PM CDT
NEXT UPDATE: MONDAY - OCT 19

FLASH FLOOD POTENTIAL IN THE SW WITH COLDER WX IN THE EAST

The deepening TROF over the eastern US is bringing the first significant cold air outbreak of the season to the eastern US as western ridging continues to keep Temps well above normal over the western half of the nation. At the same time, the upper Low near southern California that brought a flash flood and major mud slide to an area just north of LAX yesterday, continues to drift very slowly eastward while tapping into tropical moisture from the EPAC. As this system moves slowly eastward, the potential for additional flash flooding in parts of the southwest will continue through the weekend - with the heaviest Precip moving from near Arizona to New Mexico and possibly west Texas - during the weekend.

By early next week, the eastern US TROF will weaken rapidly as it lifts out to the northeast and high pressure begins to form across the eastern half of the nation while a strong and amplifying TROF in the NORPAC/GOA region moves towards the west coast and then inland over the Rockies during Week 2. This pattern evolution will eventually bring much warmer conditions to areas east of the Rockies during Week 2 while below normal Temps - and wetter conditions (including high elevation snows) - to the western US. With somewhat better run-to-run model consistency(finally!) over the past couple days – in addition to continuing very good agreement between the GFS and ECMWF operational models (and their respective ensemble members) for Week 2 – confidence in the Week 2 anomaly pattern and magnitude has improved, though it remains somewhat below average.

MAJOR STORM SYSTEMS DEVELOPMENTS AHEAD

There is a growing consensus among all models on the development of a tropical disturbance over the southwest GOM next week and into the start of Week 2. With the possible development of a Tropical Low or weak cyclone in the western Gulf next week along with a flow of deep tropical moisture northward towards the Texas coast – combined with the approach of the upper Low and then larger scale upper TROF from the west – there is growing potential for torrential rains and possible flash flooding across portions of Texas northward into the southern Plains.

In addition, there is a low but significant chance that a second tropical disturbance may form near the Bahamas on the south side of the upper air High that is expected to develop over the eastern US during Week 2. This possible 2nd disturbance may then drift westward – bringing heavy rains to Florida at the end of Week 1 and/or during Week 2.

FORECAST DEVELOPMENT OF STRONG MJO COULD LEAD TO MAJOR IMPACT ON EL NIÑO AND US WEATHER

Despite the MJO still being quite weak, the major global models continue to forecast a major MJO development next week in the western Indian Ocean, with both the GFS and ECMWF models calling for a moderate to strong MJO moving eastward across the Indian Ocean by the end of the month.

If this forecast verifies – including the 30-Day extended outlook from the ECMWF model – a strong signal could be moving across the western Pacific region by mid-November. The region ahead of a strong and well defined MJO typically leads to an increase in easterly trade winds, and would logically be expected to interact Destructively with the base state (El Niño). Likewise, the trailing region behind the MJO typically bring enhanced westerly wind anomalies, constructively interacting with the base (El Niño) state. Keeping in mind that forecasts beyond ~ 10 days are not known for their accuracy, the MJO evolution described would first lead to cooling of SST’s in the EPAC during late NOV before warming them in December. In addition, strong MJO's during the cold season tends to influence the overall weather pattern in the northern hemisphere - regardless of any ongoing ENSO event. (More on this if/when it develops.)

ALASKA WEATHER
Above to much above normal Temps statewide and wet weather across the south is likely to continue as little change in the overall upper air pattern is seen through next week.

The upper air ridge over western Canada northwestward into central Alaska and a sharp, upper TROF from near Siberia southeastward across the Aleutians will continue to bring mild but wet weather to much of the mainland. Early next week, a strong storm is likely to form just south of the Aleutians which is forecast to move into the GOA late next week before weakening – but this storm will bring gale to near storm force winds to portions of the Bering Sea and Aleutians, along with heavy Precip along the Gulf coast and Kodiak Island around mid-week before the storm begins to fill (weaken). A 5-7 day break in the stormy conditions is likely during Week 2 along with somewhat colder conditions across the state , though they will still average above normal. With little change in the overall flow pattern, very cold arctic air now developing over Siberia is unlikely to move into the state any time soon.




Fig 1: 500mb (≈ 18,000 Ft) Height Forecast Loop for the next 2 Weeks The above loop shows the expected evolution of the upper air 500mb pattern during the next 2 weeks. (NOTE: The Model run Date shown on the first time step image is incorrectly shown as SEP 16 instead of OCT 16. HOWEVER – the model run graphics from the overnight GFS model run is CORRECT!). Winds flow parallel to the (solid) height lines, with lower heights associated with Low pressure / TROFs (and colder Temps) and higher heights associated with high pressure systems (and warmer Temps aloft). Shading relates to height anomalies – with ‘bluish’ colors highlighting below normal heights (and Temps) and ‘reddish’ shading associated with above normal heights (and Temps). Note the upper Low now near California should drift very slowly eastward before dissipating next week. The deep TROF now over the eastern US is quickly replaced by high pressure next week, while deep TROFs/Storm systems in the NORPAC move across the Gulf of Alaska (GOA) and then turn southeastward towards the western US - and then eastward toward the central US during the second half of Week 2. It’s worth noting that the ECMWF model is very similar to the GFS forecast solution, but is calling for a somewhat stronger TROF in the western US and a better defined tropical Low in the western Gulf of Mexico...


Fig 2: GFS 10-Day Forecast for the Primary Teleconnections While the AO, NAO and PNA are not especially strong at this time, the forecasts arguably exhibit a typical ‘El Niño pattern’ with relatively large excursions every few days – typical of a 'progressive' pattern. In addition, the 'spread' in solutions from the various Ensemble members (red lines) leads us to a below average confidence level in the forecast for major hemispheric TROFs and Ridges at the more extended time periods.


Fig 3: MJO Analysis and Global Model Forecasts Even though the MJO remains weak – both the GFS and ECMWF operational and ensemble models continue to forecast the development of a moderate to strong signal in the Indian Ocean region starting next week. Once a week, the medium range ECMWF model is run to generate a 30-Day MJO forecast (bottom right panel above). While there is a large spread of Ensemble Member forecasts, they all generally call for a Moderate to Strong MJO moving eastward across the Indian Ocean during the first half November.


Fig 4: 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. Below normal Temps will be spreading from the Midwest to most of the eastern US this weekend before Temps rebound strongly in the Midwest starting early next week. Much above normal Temps will continue in the west but will fall off to near normal in the far west by the end of next week. Confidence in the forecast anomaly Pattern and Magnitude is near average for this time of year, with readings of ‘3’ for both the anomaly Pattern and actual Magnitudes on a Scale from 1 to 5.


Fig 5: 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. With better continuity between model runs – especially since yesterday morning - Confidence in the anomaly Pattern and Magnitudes has edged up a bit, but remains below average for this time of year, with readings of ‘2’ on a Scale of 1 to 5. (The lowest confidence continues to be in the actual Temp anomaly magnitudes compared to the overall anomaly pattern.)


Fig 6: GFS Total Precipitation during the Next 10-Days (for mounts greater than 0.5” only) Heavy Precip is likely across the GOM at times, and possible tropical disturbances – especially over the western Gulf - may bring especially heavy to extreme rainfall to portions of Texas northward into the southern Plains while another tropical disturbance MAY bring heavy Precip to south Florida. The upper Low now near southern CA will drift very slowly eastward and could bring additional flash flooding events to portions of the southwest before dissipating late next week.


Fig 7: Temperature Anomaly Forecast for Alaska Well above normal Temps will persist across mainland Alaska for the next 8-10 Days, along with stormy weather across the Aleutians, Kodiak Island and southern coastal areas. During Week 2, the warm Temp anomalies will begin to fall off across the state – but will remain mostly above normal levels.

✭ The Next Update will be on MONDAY OCTOBER 19 ✭

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Steve
NOTES:

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

2. Another Glossary of weather terms is available HERE