Near Record Warmth on The Way / December Outlook - Updated

By: Steve Gregory , 8:38PM,GMT on December 2,2015




 

WEDNESDAY: 02-NOV-15 / 2:40 PM CDT

NEXT FULL WX UPDATE ON FRIDAY

MAJOR TROF TO DEVELOP IN WEST WITH PROLONGED WARMTH IN THE EAST

As the upper Low over the Great Lakes weakens and moves into SE Canada, associated Precip over the eastern US should move east of the coast by early FRI. A TROF moving onshore in the PAC NW will amplify as it moves over the Plains this weekend, followed by another strong short wave early next week., These systems will effectively combine – triggering a storm formation in the SE US which will move northeastward bringing mostly rain and gusty winds to the eastern states as the week progresses.

By late next week, a subtle pattern shift will lead to the development of a highly amplified, long wave TROF over the western US with a building ridge over the eastern US in in 10-14 days. This should result in sending average Temps from above normal to much above normal in the east and north central US during Week 2 - while Temps fall to well below normal over the west. However with the lack of deep arctic air over Canada, Temps should only drop off to around 2 to 8 degrees below normal over the west, while readings of 8 to 18 degrees above normal appear likely from portions of the upper Midwest eastward into the mid and North Atlantic region by around mid-month.

DECEMBER TEMP AND PRECIP OUTLOOK – UPDATED

As it turns out, the DEC Temp anomaly pattern is likely to resemble the Week 2 Forecast (shown below) as the TROF in the GOA shifts westward and weakens, leading to a major downstream TROF formation over the western US while heights rise (and therefore, Temps) over the eastern US. The overall warmest period is likely to occur in the DEC 12-24 time period – with the potential for various record warm readings from the upper Midwest eastward into the northeastern US. Since high amplitude long wave TROFs over the western US rarely last more than 10-12 days at a time (even during strong El Niño events), the TROF is likely to move eastward across the nation during the last week of DEC, allowing Temps to cool down to more seasonal levels in the central and eastern US during late DEC (though they are expected to rebound back above normal during early JAN).

Based on the latest dynamical model solutions and trends during NOV, the primary storm track is likely to continue coming onshore in the PAC NW during the next 3 weeks, with unsettled conditions across the inter-mountain west - with a secondary storm track from near NM/TX eastward into the SE US and then northward along the coast.. The storm track that I initially anticipated would shift into CA during the first half of DEC now looks like it will hold off until the end of the month (and continuing thru JAN) when a deep TROF should redevelop back towards the EPAC.





Fig 1: Temperature and Precipitation ANOMALY forecasts for the month of DECEMBER, 2015 SIG changes compared to my previous outlook: Temps are not as cool in the Gulf coast region with more Precip shown in the Midwest/Great Lakes and NE US.



Fig 2: 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, and anomalous heights (bluish shading for below normal, and reddish shades for above normal). The strong closed Low over the Great Lakes weakens rapidly as it moves into SE Canada by FRI. By that time, a strong Short Wave TROF will be moving onshore in the PAC NW and will shift to Great Plains over the weekend as a second short wave moves quickly in behind it. Ultimately, these TROFs will coalesce into a fairly strong system in the SE US by next WED before tracking to off the mid-Atlantic coast by the end of the week. By that time, the upper level TROF in the NORPAC ill have weakened and shifted westward, allowing short wave TROFs to start carving out a long wave TROF across the western US during latter portions of Week 2.


Fig 3: 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 will continue nationwide, with the warmest readings starting by Friday in the Midwest. Overall Confidence in the forecast anomaly Pattern and Magnitude is above average with readings of ‘4’ for the pattern and anomaly magnitudes - on a Scale from 1 to 5.


Fig 4: The Week 2 Temperature ANOMALY forecast is based on the 12Z run of the GFS (80%) integrated with the 12Z GFS Ensemble (10%) (ECMWF 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 to much above normal Temps will dominate the eastern 2/3rds of the nation as a TROF deepens over the interior West – along with cooling Temps. Confidence for the anomaly pattern is typical for the season, with a reading of 3 on a scale of 1 to 5. With a strong Temp anomaly pattern developing, confidence in the actual anomaly values is below average (with the chance Temps in the east could be warmer than shown) – with a reading of ‘2’ on a scale of 1 to 5.


Fig 5: GFS 15-Day Precipitation Pattern Loop (for mounts greater than 0.10” only) The major storm in the upper Midwest will spread heavy rains across the east with a break in the widespread storminess going into early next week before the next large storm develops out west and then the central US by Thanksgiving.

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