Stormy Pattern As Arctic Cold Develops - Hints of Pattern Change

By: Steve Gregory , 9:28PM,GMT on November 6,2015

FRIDAY: 06-NOV-15 / 3:30 PM CDT
(NEXT FULL UPDATE MONDAY)

POSTING SCHEDULE: ‘FULL’ UPDATES ON MONDAYS & FRIDAYS … TEMP FORECAST CHARTS ON WEDNESDAYS AND WHEN WARRANTED

UNSEASONABLE WARMTH TO TAKE A BREAK – STRONG STORM TO DEVELOP NEXT WEEK

The much above normal Temps with near and record setting high Temps will be ending over the weekend as a cold front from the Great Lakes to Gulf coast moves eastward off the east coast on Saturday. Still, overall, Temps will still average above normal during the next 10-days before cooling down to near normal levels. While still quite uncertain – the last 3 GFS model runs have begun forecasting a pattern shift during Week 2 that could lead to a push of arctic air across the eastern half of the nation by the end of Week 2 – though I believe if the pattern shift occurs, it will only last for a week or so.

While the cold front now moving into the east will bring widespread rain along the entire east coast – locally heavy rain may occur across the Gulf coast - partially a result of tropical moisture flowing northward ahead of the front in association with a tropical disturbance now moving into the SW GOM (this system is NOT expected to develop into a tropical cyclone).

The main WX event next week will be a strong storm formation in the central US accompanied by strong, gusty winds and heavy Precip from Texas northward to the Great Lakes around mid-week before spreading across the east coast by the end of next week. A second and similar storm system is then likely to develop during Week 2 – but that system will be followed by much colder air. At the same time, Temps will rebound to above normal levels with drier conditions in the west - assuming the latest model runs verify!

ARCTIC COLD BUILDING IN ALASKA & NW CANADA

While Temps have been well above normal for months on end in Alaska, the large scale but ‘subtle’ pattern change forecast since last week is still expected to bring a huge reversal in Temps across much of Alaska next week as a fairly deep, arctic air mass develops across the center of state and expands into the Yukon in NW Canada. But with the GFS (and most ensemble members) now forecasting a more dramatic pattern change during Week 2, the developing artic air mass is expected to nose into the northern Rockies and then much of the eastern half of the nation during the latter half of Week 2.

Please note that the Week 2 Temp forecast chart is the AVERAGE Temp anomaly for all 7-days of Week 2, and can be a bit misleading in that above normal Temps during the first portion of the Week are offsetting the much colder readings during the end of the week when Temps may average 5 to 15 degrees below normal across much of the Midwest and east – including the deep south.

STRONG MJO SIGNAL SUPPORTS ARCTIC AIR PUSH BEFORE BEING ‘OVERWHELMED’ BY EL NIÑO

To some degree, the new outlook for much colder conditions can be traced to the strong MJO now moving slowly across the Indian Ocean – which teleconnects with colder weather in the central / eastern US. In addition, the GFS ensemble forecasts for the primary teleconnections are in excellent agreement supporting a strong cold air outbreak starting in 10 days. (See below Figures.)

However, all global models are still forecasting the strong MJO to weaken rapidly during Week 2 - before it reaches the Western Pacific - most likely due to the extremely strong El Niño which will effectively ‘block’ the advancement of the atmospheric oscillation – or certainly mute its impact on the northern hemispheric flow pattern.

Therefore, between the still quite progressive flow supported by the strong El Nino event and the rapid weakening of the MJO, I am still quite confident that by the end of the month and/or opening week in DEC, Temps will return to above normal in the east and below normal out west where much stormier conditions are likely to develop – along with a much greater chance for significant rainfall in SOCAL.




Fig 1: Water Vapor (WV) Image Loop The WV image loop highlights the upper air Low moving over the SW GOM with deep, tropical moisture flowing into the system from the CARIB where a broad area of Low pressure is about to cross the Yucatan as it moves slowly NW. (This image loop will automatically update over the weekend.)


Fig 2: GFS 10-Day Forecast for the Primary Teleconnections The overnight GFS ensemble forecasts for the AO, NAO and PNA Teleconnections are all pointing to a warmer than normal Temp pattern over the eastern US until mid-month when the AO and NAO both turn negative – implying much colder conditions in the east. OTH – the PNA is expected to remain in a ‘neutral’ phase – as it’s being strongly influenced by the flow pattern over the eastern half of the Pacific.


Fig 3: MJO Analysis and Global Model Forecasts The very strong MJO in the central portion of the Indian Ocean is moving eastward, and is partially responsible for the rare tropical cyclones in the Arabian Sea . While there are timing differences between the various global models as to when the MJO will begin weakening – they are all calling for a dramatic weakening before it reaches the WPAC. While it’s still possible the signal will be ‘hidden’ from analytic view by the anomalous atmospheric flow pattern associated with El Niño (meaning it could in fact move across the Pacific – eventually influencing SST’s and convective activity), the very good agreement between all the models and the ECMWF 4 week ensemble forecast (not shown), strongly suggests the signal will not propagate over the Pacific, and will in fact retrograde WESTWARD before starting to re-intensify at the end of the month. Either way – there is NO DOUBT the MJO will play a significant role in the US WX pattern during the second half of NOV and into DEC.


Fig 4: MJO Signal Correlation with Temp anomalies over the US While the correlation between the MJO and US Temps is not very strong – the strong signal in the Indian Ocean (referred to as ‘Phase 2’) strongly resembles the Week 2 Temp anomaly forecast shown below.



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 pattern and location of TROF’s and Ridges. The Polar Jetstream (which on average is typically found around the 250mb level during the winter) continues to intensify – and we are now seeing a far more persistent and stronger sub-tropical jet stream that is always located on the ‘warm side’ of the polar jet (i.e. to the east and south of the polar jet) – with the CORE of the jet at significantly higher altitudes (close to 150mb, or ~ 45,000 Ft). Note that TROF’s are forecast to continue moving eastward (a progressive’ pattern) through the forecast period - including the development of a strong N/S orientated jet ‘core’ over the Pacific near 160˚W longitude north of Hawaii, with a developing sub-tropical jet stream near Hawaii that extends to the east. This forecast pattern suggests the eventual development of a deep TROF near the West coast during the last week of NOV, which would lead to a strong storm formation in the far west before the end of the month..


Fig 6: GFS 15-Day Precipitation Pattern Loop (for mounts greater than 0.10” only) The animation shows total Precip forecasts in 24 hour time steps – with locally heavy rains this weekend across the Gulf coast region before a major storm develops in the lower Plains/Mississippi Valley around the middle of next week. A second storm formation during Week 2 will be followed by a much colder air mass later during that week. Typical for late autumn, periodic and heavy rainfall is expected to continue in the far PAC NW.


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. Above normal Temps east of the Rockies and below normal in the west will continue through the upcoming week – though not nearly as mild as the prior 2 weeks. Confidence in the forecast anomaly Pattern and Magnitude remains above average for this time of year due to very good model continuity, 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 forecast has been followed closely – resulting in below average confidence in the forecast due to the changing pattern, and some growing disagreements with the EURO models which still has a somewhat warmer solution compared to the GFS. Confidence in the anomaly Pattern and magnitude is now below average – even for this time of year - which is always somewhat uncertain – with readings of ‘2’ for both the anomaly pattern and magnitude - on a scale of 1 to 5.


Fig 9: Temperature Anomaly Forecast for Alaska during Week 1 Well above normal Temps will finally come to an end next week across moist of the state – with very cold conditions expected across the central and southern portions of the state during Week 2. Temps will remain above normal along the north arctic coast due to a flow over the still ‘open’ arctic ocean waters.

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Steve