Update: Saturday 6:35am

When you are in the midwest, you never like to hear about severe hail and tornadoes... but we are. Once we get through Sunday, the weather will improve considerably. It's late afternoon and evening Sunday and into the early morning hours of Monday (midnight to 3-4am) that is the concern. If you have enclosed trailers for your car, I would certainly use them on Sunday.  This is from the Storm Prediction Center this morning in their Day 2 Outlook:

Probabilistic Outlook

EARLY IN THE PERIOD...
ELEVATED THUNDERSTORMS MAY BE ONGOING ALONG THE NOSE OF LLJ (Low level Jet) ACROSS THE UPPER RED RIVER VALLEY. THIS ACTIVITY SHOULD BE ROOTED AOA (at or above) 850MB (~5000 feet) AND THE GREATEST THREAT WITH THESE STORMS SHOULD BE MARGINALLY SEVERE HAIL BEFORE THEY PROPAGATE INTO SRN MANITOBA. 

HOWEVER... 
GREATER SEVERE RISK WILL OCCUR ALONG THE COLD FRONT DURING THE LATE AFTERNOON AS BOUNDARY LAYER WARMS... FORCING SBCAPE IN EXCESS OF 3000 J/KG FROM CNTRL MN...SWWD INTO KANSAS. LATEST THINKING IS FRONTAL CONVECTION SHOULD DEVELOP AFTER 21Z (5pm) WITHIN AN INCREASINGLY SHEARED ENVIRONMENT THAT SHOULD INITIALLY SUPPORT SUPERCELL STRUCTURES. ACTIVITY ACROSS CNTRL KS MAY BE SOMEWHAT HIGH-BASED AS TEMPERATURES SOAR INTO THE 90S. HOWEVER...UPDRAFTS THAT EVOLVE ACROSS NEBRASKA INTO MN WILL BE CONSIDERABLY LOWER AND MULTIPLE HP SUPERCELLS COULD DEVELOP ALONG THE FRONT BEFORE STORM MERGERS RESULT IN AN ELONGATED MCS-TYPE COMPLEX. 

LARGE HAIL IS LIKELY WITH THIS ACTIVITY AND DAMAGING WINDS MAY ALSO BE NOTED. A FEW TORNADOES CAN NOT BE RULED OUT WITH EARLY DISCRETE SUPERCELL STRUCTURES. FRONTAL MCS SHOULD PROGRESS INTO THE MID-UPPER MS VALLEY DURING THE OVERNIGHT PERIOD.

Click image for full size This is from the Omaha NWS Area Forecast Discussion this morning:

Click for full size - Saturday Morning Water Vapor Image. Note the drier air (darker color) that will be moving into Lincoln today. Also note the blob over Montana, that is Sunday evenings weather maker.

THE MAIN SHOW WILL PROBABLY WAIT UNTIL LATE AFTERNOON OR EARLY EVENING AND BE MORE TIED TO LOW LEVEL CONVERGENCE ALONG THE APPROACHING COLD FRONT. AGREE WITH SPC DAY 2 OTLK WHICH HAS ALL OF OUR AREA IN A SLGT RISK. 


ML CAPE VALUES ARE EXPECTED TO REACH 3000 TO 4000 J/KG IN THE AFTN AND LINGER INTO THE EVENING. 0 TO 6 KM BULK SHEAR VECTORS IN COMBINATION WITH EXPECTED CAPE SHOULD BE SUFFICIENT FOR STORM ORGANIZATION. INITIALLY DISCRETE STORMS WILL LIKELY MERGE INTO ONE OR MORE CLUSTERS AND/OR A LINE. 21Z SUNDAY TO 06Z MONDAY  (5pm until 2am ) SEEMS TO BE THE MOST FAVORED TIME BASED ON FORECAST SOUNDINGS AND OTHER CURRENT MODEL DATA. IN ADDITION TO THE SEVERE THREAT...LOCALLY HEAVY RAIN IS PROBABLY GOING TO BE AN ISSUE DUE TO THE RECENT ISSUES WITH SOME STREAMS/RIVERS.

0-6 km Shear - Shear is the change in wind direction with height.

Convective Available Potential Energy. A measure of the amount of energy available for convection. CAPE is directly related to the maximum potential vertical speed within an updraft; thus, higher values indicate greater potential for severe weather. Observed values in thunderstorm environments often may exceed 1000 joules per kilogram (J/kg), and in extreme cases may exceed 5000 J/kg. Mean Layer CAPE - CAPE calculated using a parcel consisting of Mean Layer values of temperature and moisture from the lowest 100 mb above ground level.

SBCAPE (Surface-Based Convective Available Potential Energy) is a measure of instability in the troposphere. This value represents the total amount of potential energy available to a parcel of air originating at the surface and being lifted to its level of free convection (LFC). No parcel entrainment is considered. The CAPE and CIN calculations use the virtual temperature correction.

A multiple ingredient, composite index that includes effective storm-relative helicity (ESRH, based on Bunkers right supercell motion), most unstable parcel CAPE (muCAPE), and effective bulk wind difference (EBWD). Each ingredient is normalized to supercell "threshold" values, and larger values of SCP denote greater "overlap" in the three supercell ingredients. Only positive values of SCP are displayed, which correspond to environments favoring right-moving (cyclonic) supercells.

Update: Friday 6:03pm

ALTHOUGH ELEVATED CONVECTION COULD PERSIST INTO SUN AFTN AIDED BY LOW LEVEL JET...PER NAM/GFS...12Z ECMWF GENERALLY KEPT AREA DRY UNTIL 00Z MONDAY WHEN STRONG UPPER JET PUNCHES INTO REGION ON SOUTH SIDE OF THIS NEXT UPPER TROUGH. DUE TO UNCERTAINTY IN AMOUNT OF ELEVATED CONVECTION PRECEDING MAIN LATE AFTN/EVENING DEVELOPMENT LEFT IN MORNING POPS NW 2/3RDS AND DID NOT BREAK UP THE AFTN PERIOD TO ACCOUNT FOR POSSIBLE HIGHER CHANCES LATE IN THE AFTN. DID BOOST MAX TEMPS A FEW DEGREES SOUTH ON SUNDAY AS THAT AREA DID SEEM THE LESS SUSCEPTIBLE TO IMPACT OF MORNING WARM ADVECTION CLOUDS PSBL PRECIP. STRONG/SEVERE STORM CHANCES APPEAR HIGHER THAN PAST SEVERAL DAYS AS 0-6KM SHEAR IS STRONGER AND MAX TEMPS NEARING 90 SHOULD INCREASE MLCAPE VALUES. ALTHOUGH HEAVY RAIN WILL LIKELY OCCUR WITH THE ACTIVITY AS WELL...VERY HARD TO PIN DOWN WHERE AXIS OF HEAVIEST WILL OCCUR AS A CASE COULD BE MADE FOR IT SETTING UP JUST ABOUT ANYWHERE IN OR EVEN POSSIBLY JUST OUTSIDE THE FA.

Previous Post

.LONG TERM...(SATURDAY NIGHT THROUGH WEDNESDAY)
ISSUED AT 327 AM CDT THU AUG 28 2014
THERE IS A BREAK IN THE STORMY WEATHER SATURDAY NIGHT... HOWEVER THUNDERSTORM CHANCES INCREASE SUNDAY AFTERNOON AND SUNDAY NIGHT. DO MAINTAIN LIKELY POPS DURING THIS TIME WITH A SHORTWAVE TROF PUSHING THROUGH THE PLAINS AND HIGH PWATS RETURN AHEAD OF THE FRONT. HAVE A DRIER PERIOD FORECAST MONDAY THROUGH WEDNESDAY.

BOTH MODELS DEPICT THE ADVANCE OF A FAIRLY STRONG UPPER TROUGH INTO THE CENTRAL U.S. DAY 4 /SUN 8-31/...ALONG WITH AN ASSOCIATED/SHARP COLD FRONTAL PROGRESSION.

THE MOST SUBSTANTIAL SEVERE RISK THIS PERIOD REMAINS APPARENT DAY 4 FROM THE CENTRAL PLAINS NEWD TO THE UPPER MS VALLEY...AS THE UPPER TROUGH SHIFTS INTO/ACROSS THIS AREA. AS THE ASSOCIATED COLD FRONT ADVANCES INTO A MOIST/DESTABILIZING AFTERNOON AIRMASS...VIGOROUS THUNDERSTORM DEVELOPMENT IS EXPECTED TO OCCUR. WITH A BELT OF 40 TO 50 KT SWLY FLOW PROGGED TO SPREAD ACROSS THE MID MO VALLEY VICINITY THROUGH THE DAY ATOP LOW-LEVEL WARM-SECTOR SSELYS...SHEAR WILL BECOME FAVORABLE FOR ROTATING STORMS. AS A RESULT...A RISK FOR LARGE HAIL AND LOCALLY DAMAGING WINDS -- AND POSSIBLY A FEW TORNADOES -- SEEMS LIKELY TO MATERIALIZE...PARTICULARLY WITHIN A ZONE FROM NRN KS NNEWD INTO MN/WRN WI DURING THE AFTERNOON AND EARLY EVENING HOURS...AND THEN INTO IA/NWRN MO THROUGH THE EVENING HOURS.

Updated 9:17 pm, Thursday

.LONG TERM...(FRIDAY NIGHT THROUGH TUESDAY) ISSUED AT 318 AM CDT WED AUG 27 2014 A FEW SHOWERS LINGER INTO FRIDAY NIGHT...BUT SATURDAY LOOKS MAINLY DRY. THERE IS STILL SOME POCKETS OF MOISTURE AND ELEVATED INSTABILITY SO WILL NEED TO KEEP AN EYE ON THIS. LIFT INCREASES SATURDAY NIGHT THROUGH SUNDAY NIGHT AS ANOTHER SHORTWAVE TROF AFFECTS THE AREA WITH A GOOD CHANCE FOR RAIN SUNDAY NIGHT.

EXTENDED FORECAST DISCUSSION 
NWS WEATHER PREDICTION CENTER 
COLLEGE PARK MD 
1053 AM EDT MON AUG 18 2014 
VALID 12Z THU AUG 21 2014 - 12Z MON AUG 25 2014 

 ...HEAT TRIES TO BUILD ACROSS MUCH OF THE SOUTH...  DURING THE FOURTH WEEK OF AUGUST, SUMMER WILL TRY TO PRODUCE SOMETHING IT HAS BEEN SHORT ON THIS YEAR EAST OF THE ROCKIES: HEAT. A BLOCKY PATCHWORK OF UPPER HIGHS AND LOWS WILL LOCK IN VARIOUS REGIMES ACROSS THE UNITED STATES AT THE MEDIUM RANGE--WITH HOT, DRY CONDITIONS IN THE CENTRAL SOUTH, AND COOL, SHOWERY CONDITIONS ALONG THE EAST COAST AND IN THE NORTHERN HIGH PLAINS. HIGHS SHOULD TOUCH THE CENTURY MARK IN TEXAS, OKLAHOMA, KANSAS, MISSOURI, AND ARKANSAS WHERE THE STILL-CONSIDERABLE LATE AUGUST SUN HAS FEW CLOUDS OR RECENTLY-WET SOIL WITH WHICH TO COMPETE.

Heat Index Thursday 5PM

Nino3.4 Forecast

PDO (Pacific Decadal Oscillation)

+PDO anomalies

Larry's Winter Weather Statistics for Atlanta...

In his free video from this weekend (click on the link below), Joe Bastardi shows a map with a prediction from WeatherBell.com of a very cold Southeast winter. He has much of the SE US near or even colder than 4 F below normal. In all of the years of following JB, I can't recall a forecast quite that cold for the SE. However, IF there really ends up being a weak Nino fall/winter peak, a +PDO, and a -NAO, I could see this or even colder than this actually happen. Perhaps he is already thinking somewhat along those lines.

 For Atlanta (KATL), he's going for a little colder than 4 below normal. How cold is that? Well, that would be very close to a top 10 cold winter for Atlanta, with records going back to 1879-1880. The very cold winter of 2009-2010, which was the coldest by a good margin since 1977-1978, was 4.4 F below average. So, JB is going for something similar to 2009-2010 in terms of cold. He's also going for well above average snowfall including ~150% of norm at ATL/RDU (from Atlanta to Raleigh):

http://www.weatherbe...y-august-9-2014

12 Coldest (back to 1879-80) KATL Winters’ ENSO
(WEN=weak El Nino | WLN=Weak La Nina)

    Winter……DJF Temp Anom…ENSO

1) 1976-1977……….…..-7.6……WEN

2) 1977-1978…………...-6.2………WEN

3) 1904-1905……………-5.8……...WEN

4) 1962-1963……………-5.8………WLN or NN

5) 1935-1936……………-5.7……….NP

6) 1963-1964……………-5.7………*WEN or MEN

7) 1939-1940………….-5.5…………WEN

8) 1901-1902…………...-5.4……….NN

9) 2009-2010………….-4.4…….…SEN

10) 1885-1886………….-4.3………WEN

11) 1894-1885.............-4.3...........WLN

12) 1969-1970…….…..-4.1……….WEN

Note how many weak El Nino's are listed. Also, note 1963-1964, specifically. On the previous version of the NOAA ONI table, it was considered weak. However, it has been revised as moderate. Regardless of the specifics, the general idea is that weak Nino's have been associated with more very cold Atlanta (and SE) winters per instance than any other ENSO state. However, it still needs the help of a +PDO and -NAO to give it the best shot at being very cold per history. Now, 2004-2005 and 2006-2007 were mild overall. I suspect that the +NAO was a major reason. Then again, February of 2007 was quite cold and the NAO did drop to -0.47 that month.

*NOTE: Also, notice the relative absence of La Nina (only two Weak La Nina and one was later revised to Neutral N) and the strong tendency toward weak ENSO, in general (Weak El Nino, Weak La Nina, Neutral N, or NP).

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*NOTE: It isn't necessarily true that the weak Nino forcing, itself, causes the big cold. Rather, it may very well be that having a weak Nino allows for the best chance for all factors, combined, to allow for the big cold and may allow it to be amplified somehow.

 From previous 2012 post: 
To further illustrate my point, there have been 26 December/January/February's since 1899-1900 with a known +PDO/-NAO/-AO. Note these KATL stats for these +PDO/-NAO/-AO winters:

1) 6 Weak Nino's: all 6 had negative anomalies that averaged a very impressively cold -5.0 F. A whopping 5 of 6 very cold.

2) 10 moderate to strong Nino's: 8 had negative anomalies and 2 had positive anomalies. The ten averaged -1.3 F. Only 1 of 10 very cold.

3) 6 neutral ENSO: 3 had negative anomalies and 3 had positive anomalies. The 6 averaged -1.2 F. 2 of 6 very cold.

4) 4 Nina: 2 had negative anomalies and 2 had positive anomalies. The 4 averaged -1.3 F. 1 of 4 very cold.

In summary, whereas the 6 weak Nino's averaged a very cold -5.0 F and 5 of 6 were very cold including the 2 coldest on record, the 20 others averaged only a modestly cold -1.3 F or about 25% of the weak Nino's negative anomaly magnitude and only 4 of 20 were very cold. So, it is quite evident to me that the weak Nino state, itself, is associated with a strong tendency toward cold winters at Atlanta that tends to pile onto the cold that tends to already be associated with +PDO/-NAO/-AO and is unlike any other ENSO state, including stronger Nino's. 

In other words, nothing comes close to beating the weak Nino/+PDO/-NAO/-AO combo for a cold and wet winter in Atlanta.
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*NOTE: Before anyone tries to attribute west based (vs. east based NAO) as being the most important factor for cold Nino winters, note that the very cold 1976-1977 and 1969-1970 were east based while the mild 2004-2005 was west based. 1977-1978 and 2009-2010 were west based while 1963-1964 was neutral based. My point is that I don't see a clear cut correlation for cold unlike what I see for weak Nino's. On the other hand, I suspect there may be a good correlation of heavier precipitation for a west based NAO.

West based appears to be the most likely for this winter assuming there is a Nino. For a combo of heavier precipitation and cold, I suspect that west based might be best hope.

Winter Temperatures - Joe Bastardi and WeatherBell

Winter Precipitation - Joe Bastardi and WeatherBell