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    First warmer then colder then... rinse repeat the roller coaster ride. Dramatic temperature drop over the weekend as first of multiple Polar air masses come down from Canada this month and next. Coldest air of the season to date by Monday. In the transition another big snowstorm in the Midwest to New England. Could Atlanta see a few snow flakes? Yes not out of the question but I would not hold your breath. As of now at least, it looks like even if we did it would be brief and not last or matter with no accumulation outside of NE mountains and most of us won’t see any snow. The sharp change to sharply colder is the real story. As I’ve said repeatedly in past blogs since Fall, the prospect for snow or ice looks alive before we warm up in April. As in the rest of life, the meeting of cold and moisture is all about the timing. The pattern looks to turn ripe but we have to wait till we have an actual system to watch.  FORECAST SURFACE CHARTS FRIDAY THROUGH NEXT TUESDAY: KEY JET STREAM FEATURES FORECAST BY END OF MONTH: SUNDAY ECMWF SURFACE WEATHER FORECAST: The GFS and Canadian Models show no snow whatsoever.  Cold is the real story.  SUNDAY AM WIND CHILL: WIND CHILL END OF DAY SUNDAY: ECMWF FORECAST WIND CHILL FACTOR MONDAY MORNING: ECMWF SUNDAY EARLY MORNING TEMPERATURES: ECMWF END OF DAY SUNDAY SURFACE TEMPERATURE ESTIMATE: By Monday morning the freeze line is as far South as the Mouse House near Orlando down to Tampa Bay area: ECMWF ENSEMBLE FORECAST LOWS MONDAY MORNING: ECMWF MODEL FORECAST HIGHS MONDAY: ECMWF ENSEMBLE AND GFS ENSEMBLE TEMPERATURE PROJECTIONS: No actual snow or ice storms to track for Atlanta, yet.  For more Follow me on Twitter @MellishMeterWSB.
  • Many months ago I wrote how I thought the thermometer roller coaster ride would continue into early Spring, well it’s sure looks like it’s continuing. Another warm-up to above-normal temperatures is coming Wednesday through Friday. But the CHART ABOVE shows a predicted response to the Sudden Stratospheric Warming (SSW) event and the related disruption of the Stratospheric Polar Vortex propagating down toward the troposphere where weather occurs with a -AO (Arctic Oscillation.  GFS ENSEMBLE FORECAST 35-40,000 feet in altitude response to SSW: The PNA (Pacific North America) Index shows a weaker signal for a return to cold but is still somewhat supportive (negative= warm positive+cold): The EPO is very supportive of the pattern shift to colder suggesting a Pacific Jet Stream retraction and split flow full of short-wave energy as Pacific convection (MJO) Madden Julian Oscillation activates (EPO+warm/-cold): A “Blocking” pattern is forecast in the North Atlantic which is also supportive of East Coast storms and a colder turn as indicated by a -NAO: The thermometer roller coaster response: GFS ENSEMBLE FORECAST TEMP ANOMALIES:  The CIPS Analog Guidance is also supportive: There is remarkable support from all the numerical model equations and their ensemble variants showing help from the Pacific and Hudson Bay vortex for a sustained pattern change as discussed in blog weeks ago: It does not yet look “frigid” but cold enough for Southerners and back below-normal.  However, models have been trending a PIECE of the Tropospheric Polar Vortex (TPV) closer to the U.S. and IF that model adjustment trend were to continue in the weeks ahead then just maybe we end up looking at something akin to 2014-15 by February.  These sharp changes always involve a big swing in the jet stream pattern and thus cyclogenesis can be expected with a low pressure storm track from Texas to Virginia and off the coast as a Nor’easter providing snow from the Plains and Midwest to parts of New England. It should bring us mostly rain and maybe thunder. The colder air follows and we’ll have to monitor potential for snow afterwards: SURFACE PRESSURE PATTERN NEXT SUNDAY: However, as seen in the “spaghetti plots” there’s plenty of uncertainty on any specifics on low pressure location and precipitation amount or type:   ACCUMULATING SNOW RISK IN GREEN AREAS FRIDAY THROUGH NEXT MONDAY: FORECAST 500MB JET STREAM PATTERN EVOLUTION: ECMWF MODEL 10-DAY ENSEMBLE MEAN SLEET/SNOW: It looks like below-normal temperatures will have some staying power and that should OPEN THE WINDOW OF OPPORTUNITY for one or more sleet/snow risks January 20-February 8, possibly beyond.  I’d like to show you some analog research results but unfortunately the government shutdown has that data source down. Non-model methods such as the Bearing Strait Rule are also supportive of the long-range pattern shift: There are signals that the pattern shift lasts into March in weaker form i.e. no early Spring: REMINDER for newbies: this does not mean every day or every week is cold the next 3-months. Meanwhile, our viewing weather looks very IFFY for this as of now: Maybe we get lucky and the timing changes, always possible when looking that far ahead in weather.  Forecasts beyond 5 days are speculation and are not worth your time or mine. General patterns we are good at up to 4 weeks but specific weather details no. For more Follow me on Twitter @MellishMeterWSB.
  • The above surface weather chart is for today and tonight into early Sunday morning. Spotty drizzle at times this afternoon will transition to scattered light showers late afternoon and early evening then become widespread rain as the night goes on diminishing to spotty light showers or drizzle on Sunday. Any real problems from ice accumulation should remain in the far Northeast corner of Georgia.  As far South as Northern Hall County and adjacent areas there may be a brief period of sleet and freezing rain tonight or early Sunday but only insignificant ice accretion is possible mainly on objects above the ground with no real problems expected before it changes to just rain there as well. Just rain for the majority of the Metro Atlanta area.  However, as explained in blogs all week and in the past with “the wedge” you never say never to the chance of a last minute surprise.  The odds of that in this scenario look low but we’ll keep our eyes on it just in case when the bulk of the precipitation moves in tonight with rainfall amounts averaging half an inch to one inch.  WINTER WEATHER ADVISORY IN PURPLE: WEATHER PREDICTION CENTER GUIDANCE: ESTIMATED RAINFALL TOTALS NEXT 48 HOURS: For more Follow me on Twitter @MellishMeterWSB. 
  • Long time follows have the knowledge of “Miler A” and “Miller B” type winter storm systems that affect the Southern and Eastern U.S. Others can google the web machine :) The system this weekend is a “Miller B”, with the low pressure storm tracks seen in the map above.  It includes “the wedge” pattern or “CAD event” which I’ve explained hundreds of times over the years, google if you’re unfamiliar.  The NORTHEAST CORNER of the state has the highest risk of significant ice accumulation from a wintry mix tomorrow evening that changes to just rain on Sunday. But... In the far North and East suburbs of Metro Atlanta some ice accumulation is possible especially on elevated surfaces not in contact with the ground. The greatest odds as of now look to be Northern Hall County and adjacent areas. Some slippery spots possible on roads mainly bridges and overpasses.  For the majority of the Atlanta area just cold rain showers. However, as I’ve explained in prior blogs all week long and in past years “the wedge” is notoriously tricky to forecast and most models lack the resolution to handle them well, we often find ourselves near or on the line between rain and ice or snow with a variance of just a degree or two making all the difference in the world, and the normal and expected margin of error in forecasts can NOT cover for that fact. We will keep our eyes on ALL the lines but especially the “tweener” zone between green and orange.  Those lines are not written in stone and are not magic walls in the sky, but are a transition zone at least 30 miles North or South and can and will shift either direction in future forecast updates so stay tuned and check back. SURFACE WEATHER CHART FIRST HALF OF SATURDAY: SURFACE WEATHER CHART SATURDAY LATE AFTERNOON: SURFACE WEATHER CHART SATURDAY NIGHT-EARLY SUNDAY MORNING: SURFACE WEATHER CHART SUNDAY: WEATHER PREDICTION CENTER FREEZING RAIN GUIDANCE PROBABILITIES: NATIONAL WEATHER SERVICE ADVISORY (I would include counties just East of this as well): For more follow me on Twitter @MellishMeterWSB.
  • The all too familiar “wedge” pattern or Cold Air Damming (CAD) develops this weekend as we get the squeeze play between high pressure to our Northeast and low pressure moving in from the west in the active sub-tropical jet stream. The above surface weather chart is for the first half of Saturday. The bulk of the precipitation will hold off until late Saturday afternoon into early Sunday morning with more intermittent scattered light rain or drizzle Sunday afternoon into early Monday.  As long-time followers know you never let your guard down on a wedge. They are too capable of last minute surprises on temperature and precipitation-type. As of now I think Metro Atlanta gets a cold rain... any brief SLEET OR FREEZING RAIN far North and East suburbs (Pickens-Hall County and adjacent areas) would not last or matter if it happens at all.  For the Far Northern and Northeast Georgia mountains a dusting of snow and a tenth inch of ice before changing over to just rain Sunday afternoon and evening.  But monitor my forecast on the weekend in case things change! FORECAST SURFACE WEATHER CHART END OF DAY SATURDAY AND SATURDAY NIGHT: SUNDAY MORNING SURFACE CHART: See the PivotalWeather Forecast Skew-T thermodynamic diagram sounding profile Pickens to Lumpkin to North Gwinnett Counties Saturday night (riding the line): What I think at this point: So for now I think any sleet if any at all would not last or matter for the Metro, but will keep an eye on the area near and north of dashed green line just in case, and monitor to see if those lines shift.  Please remember the dead horse I’ve been beating for 30 years, the lines we show you are not magic walls in the sky but a transition zone of about 30 miles North or South.  For more follow me on Twitter @MellishMeterWSB and download the WSB RADIO APP. WPC WINTER GUIDANCE: NGM Model is an outlier and is discarded for now but can not be dismissed out of hand and shows why we must monitor data for changes:
  • Well we’ve been warning about the cold snap since Monday and here it is with Canadian High pressure parked right over the Southeastern U.S. as seen in the surface weather chart above. The air mass is also very dry with dew points in the teens, so when that air is heated in your home the relative humidity will drop and may cause dry skin and lips or even static electricity. Temperatures will bottom out tonight-Friday morning with a hard freeze in the mid to upper-20s and remain chilly into Monday with a narrow up and down range.  Take precautions for vehicles, plants, pets and pipes.  The next weather system includes another “wedge” or CAD event this weekend, the low pressure system in-between a “Miller A and Miller B” type, closer to a B. The best odds of any significant ice in the NE Georgia mountains, although a nuisance amount is briefly possible in the FAR North and East suburbs. But wedges are notoriously tricky to forecast and models struggle with them so check back for updates in case things change.  SURFACE WEATHER CHART FIRST HALF OF SATURDAY: SURFACE WEATHER CHART SUNDAY MORNING: FORECAST FREEZING RAIN POTENTIAL SATURDAY NIGHT-SUNDAY MORNING: Some minor ice is possible anywhere in the green (.01) but I think the best odds of a light amount is near and north of the orange line as it looks now, obviously subject to change.  MODEL BLEND FORECAST HIGH AND LOW TEMPERATURES NEXT 5 DAYS: For more follow me on Twitter @MellishMeterWSB and download the WSB RADIO APP.
  • The surface weather chart above this morning shows the first cold front moving into South Georgia and another near the Ohio River coming our way tomorrow to re-enforce the cold air mass replacing the April-like weather we had with a few days in the mid to upper 60s in Atlanta.  A hard freeze comes tonight and again Thursday night. Certainly a Wardrobe Change Alert with NW winds gusting 15-30 mph producing a nasty Wind Chill Factor: An uncomfortable temperature change the next 3-7 days more in line with the calendar.  REGIONAL FORECAST MAX TEMPERATURES NEXT 3 DAYS: FORECAST MINIMUM TEMPERATURES NEXT 5 DAYS: Next it still looks like we get yet another El Nino sub-tropical jet stream (STJ) low pressure system more or less a “Miller A” type with a wedge pattern on the weekend.  ECMWF MODEL ENSEMBLE FORECAST SURFACE PRESSURE SATURDAY EVENING: (PivotalWeather) NOAA SURFACE WEATHER MAP FORECAST SUNDAY MORNING: ECMWF ENSEMBLE MODEL FORECAST SURFACE PRESSURE SUNDAY EVENING: (PivotalWeather) WPC FORECAST ODDS OF AT LEAST AN INCH OF SNOW SATURDAY-SUNDAY MORNING: ESTIMATED RAINFALL TOTALS SATURDAY AFTERNOON THROUGH SUNDAY NIGHT: As always “the wedge” (CAD event) is always one of the most tricky weather patterns to forecast and temperatures and precipitation amounts and types are subject to significant change. So check back for updates and follow me on Twitter @MellishMeterWSB for more and download the WSB RADIO APP. 
  • We had below normal temperatures in October and November, then above-normal December into the start of January.  So far we have seen a lack of response from the atmosphere in the U.S. to large-scale drivers such as the El Nino, Madden Julian Oscillation (MJO) and Sudden Stratospheric Warming (SSW) above the North Pole. Put another way the earth system has not yet coupled together. It is a fallacy that all SSW events automatically disrupt the Polar Vortex and bring it toward North America, it can and does but sometimes the PV is directed away from the U.S. even though a split occurs. Also the SSW and Polar Vortex are in the stratosphere, weather occurs in the troposphere. The feedback from one to the other often takes many weeks to unfold.  Most cold waves occur WITHOUT the involvement of the PV or a SSW event.  STRAT WARM EVENT HAS NOW OCCURRED PV DISRUPTION: To simplify things the warm air above the North Pole near outer space has to work down to near the surface to displace the arctic air and force it to other regions.  Meanwhile the MJO keeps getting stuck in warmer phases despite models showing it moving into colder ones in their long-range projections.  GFS ENSEMBLE MJO PROJECTION: The reason may be that the Modoki El Nino is barely a Modoki and the El Nino in general has been not far from neutral and is showing signs of weakening.  Regardless of the Modoki Index, there is basin-wide anomalous warmth in most of the Pacific Ocean. In addition thanks to the global ocean warmth of the climate there is a lack of temperature gradient north of the El Nino Pacific zones reflected in the Southern Oscillation Index (SOI) suggesting the jet stream has not yet coupled to the oceans or vice versa if you prefer.  WIDESPREAD WARMTH IN THE PACIFIC OCEAN: Thus we have not seen the weather response normally associated with the large-scale drivers. It has been more like a La Nina base state in the Northern Hemisphere with more of a strong zonal jet stream with embedded transient troughs and closed upper level lows. The nearly complete circuit of the jet around the globe is both rare and weird. Hence the reason the winter forecast analogs may fail this year.   But it’s worth noting an “early winter” was not the forecast made back in October, so if the back end pattern changes we’ll still be in business, only time will tell.  The models are trending in that direction, but it’s worth noting that they have been flip-flopping for a month now like a fish out of water: (WxBell graphics) They are suggesting a return to a positive Pacific North America (+PNA) pattern which is supportive of the transport of colder air from North to South by the jet stream so a change is expected going forward the rest of this month and February in one step forward and one step back fashion.  SOME models are even suggesting a snow threat for the Mountains of Tennessee and adjacent states next weekend. The bottom line is I have not canceled winter (not that it’s up to me) and with a chill in the short term following the unseasonably warm spell, the pattern change is still expected to come before the end of the month and into February. And the split-flow pattern is also expected with the active Sub-tropical jet stream providing moisture and storm track.  The halfway point of the snow season since 1981 for the lower 48 is January 19th, half of snow falls before that date and half after that date. See map below for your area: For more follow me on Twitter @MellishMeterWSB.
  • The widespread and occasionally heavy rain of the overnight and early morning hours quickly transitions to just spotty light showers and drizzle for the rest of the day and early evening with dry periods in-between.  The FLASH FLOOD WATCH continues for the entire area until 7pm as impacts on rivers, creeks and streams will linger even after the rain largely ends late this morning.  AND, as forecast all week long, it still looks DRY FOR THE ENTIRE WEEKEND with the bonus of above-normal temperatures. This will be the first 100% dry weekend for Atlanta since mid November! So enjoy the first weekend of 2019.  RAP MODEL SIMULATED RADAR MID TO LATE FRIDAY MORNING: RAP SIMULATED RADAR LATE AFTERNOON FRIDAY: HRR MODEL SIMULATED RADAR 5PM FRIDAY: Sunshine returns for the weekend, don’t be afraid of the bright thing in the sky. For more follow me on Twitter @MellishMeterWSB.
  • I hope you had a chance to read my blog of December 16th explaining how you can get rain with very cold temperatures and snow with fairly mild temperatures. I wonder how many took the challenge of the forecast quiz in the blog.  Here is the link to that blog post.  As promised I have the answers to the quiz below: Example A is a SLEET sounding: Freezing or colder at the surface and higher up in the atmosphere, but with a small warm nose in-between to partially melt snow which refreezes to ice pellets on its way down to earth. Example B is a SNOW sounding: As seen above in the Skew-T the entire atmosphere is below-freezing, NO warm layers or warm nose. Example C is a FREEZING RAIN sounding: In this case any snow aloft melts to rain in the large warm nose and falls to the surface where temperatures are below 0C so rain freezes on contact. The Skew-T sounding profile below was not on the quiz, it is a COLD RAIN AND ONLY RAIN SOUNDING: Temperatures are above freezing from the surface to high enough aloft to melt any snow or sleet to rain before it reaches the ground.  For more follow me on Twitter @MellishMeterWSB.