Wednesday, December 17, 2014

First look ahead to Christmas weather in the Mid-South

Everybody and his cousin  (who lives in the Memphis area) are inquiring minds and want to know: "Are we going to get a White Christmas??"  I'll answer that 2 ways. First, read this excellent blog post MWN intern Kevin put together a couple of days ago. Historically speaking, it's highly unlikely. But that's not the answer you want a week out. So here is our first "official" look at holiday weather, using actual forecast data.

We first have to get through the next several days, which bring us a couple of rounds of precipitation - the first one overnight tonight and another Friday afternoon and night. Sunday and Monday look pleasant with increasing temperatures (50 Sunday, mid 50s Monday), then the next weather system arrives Tuesday the 23rd with a good chance of rain.

Tuesday's system should clear out by Wednesday morning, so Christmas Eve looks to be  windy chilly but likely dry. Right now we're calling for temperatures in the 40s with partly sunny skies and gusty north wind. In fact, to our north, a huge low pressure system is being predicted by multiple long-term models to be over the Great Lakes region with  impacts across a wide expanse from the Northeast to the Tennessee and Mid/Upper Mississippi Valleys (see GFS model solution below). This system will be responsible for our wind and could get a few snow showers as close as middle TN on Christmas Eve.  If you have travel plans, particularly by air, north and east of Memphis on Christmas Eve, this system could cause all kinds of delays.

GFS forecast surface map and precip type late Christmas Eve. A "mega-low" over the Great Lakes region would have ea large sphere of influence over the eastern half of the country. Graphic courtesy WeatherBell Analytics.

As for Christmas Day, as the large low pressure system to our north moves slowly into southeast Canada, it's influence on the Mid-South will lessen and high pressure will build in. Therefore, as of this minute, I expect an "average" Christmas Day with cool and dry conditions. Of course, things can still change rapidly between now and then and details are yet to be ironed out.

Experimental GFS modeled temps and precip for the next 10 days. This is NOT a forecast, just one possible solution. Days are listed near the bottom from left to right, temps are the red/black lines and rain amounts are the blue bars. Graphic courtesy WeatherBell Analytics.

So while we may not have the White Christmas you would like, hope is not lost! The same models showing the mega-low are showing a rapid warm-up to end Christmas week, but beginning the last weekend of the year in to early January, signs are pointing towards a general cool-down. Perhaps we can get some moisture to work its way into below average temperatures and bring some snow for early 2015! No bets on that this far out though!

We'll continue to keep you updated on our latest thinking on our social media channels below and will update the blog again as needed.

Erik Proseus
MWN Meteorologist

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Monday, December 15, 2014

Christmas Day weather in Memphis – What does history tell us?

Undoubtedly one of the most common questions meteorologists get during the holiday season: “Any chance of a White Christmas this year?” That is no less true here in the Mid-South where a significant snowfall at any point in the winter season can be a unique (and disruptive!) event. Fortunately, thanks to a lengthy modern weather record available for Memphis that dates back to 1875, we have a pretty good idea just how likely a White Christmas is, at least statistically speaking. As you'll see below, and probably as you were already expecting, the odds are not in snow lovers’ favor!

Before jumping ahead to our chances of a White Christmas, here are some temperature statistics for Christmas Day in Memphis, thanks to data compiled by NWS Memphis and the National Climatic Data Center. The coldest Christmas Day in 138 years of records was in 1983 as a major Arctic air outbreak was gripping much of the nation. The high in Memphis that day was a mere 16 degrees after a bone-chilling morning low of 0! No snow that day, though it was plenty cold enough to support it!

High/low temperature map Christmas morning 1983. Brutal cold dominated the nation along/east of the Rockies except south Florida.
On the flip-side, the warmest Christmas Day in the records was in 1889 where the temperature reached a balmy 76 degrees after a mild low of just 63, a month that ended up by far the warmest December in Memphis history. When you average it out, a seasonable Christmas Day isn't too bad - with a high temperature around 50 and a low near or just above the freezing mark, and skies that average partly cloudy to partly sunny at worst.

In terms of any type of precipitation, measurable amounts (0.01” or more) have fallen on Christmas Day 36 times. The vast majority of those years featured just liquid rainfall in mainly light amounts, but one major soaker occurred in 1987 when 4.24” of rainfall was reported at Memphis, following an equally soggy Christmas Eve. Some Mid-South residents spent that Christmas dealing with flooding, the hardest hit of those in West Memphis, AR where cleanup was still underway after a destructive F3 tornado just 11 days earlier on December 14 (link). In terms of wintry precipitation not in the form of snow, sleet or freezing rain has been reported on Christmas Day a handful of those times, but just trace or light amounts.

Now the statistic you most want to know. Just how likely (or unlikely!) is a White Christmas in Memphis? The answer definitely lies on the “unlikely” side of the equation; in fact it’s more appropriate to label a White Christmas in Memphis as exceptionally rare. Only once in 138 years of records has measurable snowfall occurred. That was in 1913 when 3.5” of the white stuff made for the sole exception to the rule we still consider today. However, there have been several other years where snowflakes have been seen in the Memphis skies on Christmas Day but with no accumulation. The most recent was in 2012, a year where Memphis just barely missed an all-out blizzard that hit parts of Arkansas on Christmas Night, with around an inch of snow falling the morning after Christmas in Memphis.

Listing of all recorded snowfalls on Christmas Day at Memphis International Airport

Officially, the National Climatic Data Center considers any city to have a “White Christmas” if at least one inch of snowfall is on the ground on Christmas morning, no matter when that snowfall occurred. Using this definition, Memphis does have a few additional years it can add to that list. Those include 1962, 1963, 1998, and 2004, in addition to the 1913 snowfall that occurred on Christmas Day. 2004 is likely the most remembered as the city had around 2” of a sleet and snow mixture covering the ground following a winter storm on December 22. Long-timers of the Memphis area may even better remember the historic Memphis snow of December 22, 1963. 10” of that 14.3” snowfall total still lay on the ground that Christmas morning!
Official "White Christmases" in Memphis, which had at least 1" of snow on the ground.

NCDC's "White Christmas" probabilities for the US (greater than 1" of snow on the ground Christmas morning)
When considering the NCDC criteria, the odds of a White Christmas in Memphis - with at least 1" of snowfall on the ground - end up at around 4.1%. Slightly better than the 0.8% odds if you only consider measurable snowfall on Christmas Day itself! Either way, clearly the odds don't fall on your side if you're looking for a White Christmas and want to spend the holiday in Memphis. Perhaps one Christmas soon we'll be able to make a new exception to the rule! Until then, average expectations for any given year would indicate a cool, but not cold, Christmas Day and at best modest chances of precipitation, which almost always falls in liquid form.

Kevin Terry, MemphisWeather.Net

Top Image is "Graceland Christmas" by Thomas Kinkade, borrowed from artofthesouth.com.

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A warm start to a chilly week with periods of wet weather

Yesterday was a great December day with temps in the mid 60s and lots of sunshine. Despite off and on rain, it's also a mild day today as the rain out-raced the cold front, so warm Gulf air continues to flow into the region as the cold front lingers to our west. The front will move through late this afternoon however, bringing an end to the 60s for the foreseeable future. This was just the first in a series of weather systems to affect the region this week.

Tuesday and Wednesday will be "tweener" days as high pressure briefly takes over following today's front and our next weather system affects the region Wednesday night. Colder air will filter in behind tonight's front bringing highs Tuesday back down to near normal in the 50s. Lows will drop to near freezing Wednesday morning and clouds will increase during the day, holding temperatures down in the 40s for the daytime hours.

Forecast mid-level pressure (500mb or ~18,000') Wednesday morning indicating a flat ridge over the southern U.S. Graphic courtesy WeatherBell Analytics.

The pattern shift that persists through the end of the week begins Wednesday night as moisture rides up into the area on west-southwesterly wind flow, bringing a period of light precipitation - the second weather system of the week. Right now, it appears temperatures will remain above freezing all night with lows in the mid 30s. This scenario bears watching though in case surface temps trend any colder. The most likely precipitation type if NOT rain will be light freezing rain. Again, ice is not currently in the forecast.

GFS forecast precipitation amounts from midnight through 6am Thursday as moisture and precip arrive from the west. Amounts range from 0.25-0.50". Graphic courtesy WeatherBell Analytics.

GFS forecast temps at 6am Thursday, indicating mid 30s across the metro. Graphic courtesy WeatherBell Analytics.

Very light precipitation is possible again on Thursday and Thursday night with cloudy skies and cool temperatures. The last weather system of the week arrives early Friday as low pressure develops over Texas and moves east. Odds are that this low will bring a fair amount of rain Friday into early Saturday as it travels by to our south. Amounts of 1-2" are possible. There are signs from one medium-range computer model (the European) that there could be enough cold air filtering in on the backside of the low Friday night to POSSIBLY change rain over to light snow. However, none of the other models depict this same scenario as of now. In fact, the European model became much less bullish with snow in this morning's data, so confidence increases in a relatively warmer solution (but still cold for those who will be out in the elements!).


As always, there are many factors that will affect the eventual outcome, including strength and track of the low and resultant amount of cold air that moves in and how quickly it does so. We are carrying RAIN in the forecast but continue to monitor. We're hopeful for a mainly dry, but chilly, weekend as the season officially changes from autumn to winter Sunday evening. Also, looking ahead, it currently looks dry and cool for the Autozone Liberty Bowl next Monday (40s).

Farther out (too far if it were any day but Christmas), long-range models are pointing to one more southern stream system around Tuesday/Wednesday similar to this Friday's, then a chilly Christmas Day that looks dry. We'll update again in a few days with the latest thinking and be sure to follow us on social media for any updates. Also, be looking for a blog later tonight on the history of Christmas Day weather in Memphis! Hint: it doesn't bode well climatologically for a white holiday.

Erik Proseus
MWN Meteorologist

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Monday, December 8, 2014

November 2014 Climate Data and Forecast Accuracy

November Recap

November was a cold and dry month across the Mid-South. In fact, the month tied for the 7th coldest on record. The cold also led to some of the earliest snowfall on record with two measurable events - the 4th and 7th earliest on record. Though it was a cold month, for the months of September-November (defined as meteorological fall), the average temperature was just 1.2 degrees below normal due to slightly above normal temps in September and October. Precipitation for the same three-month period was 1.25" below average at 11.31". There was no severe weather during November.



Memphis International Airport, Memphis, TN

Temperature
Average temperature: 47.4 degrees (5.8 degrees below average)
Average high temperature: 56.5 degrees
Average low temperature: 38.2 degrees
Warmest temperature: 75 degrees (4th)
Coolest temperature: 20 degrees (18th)
Records set or tied: Record warmest low temperature - 61 on the 30th
Comments: November 2014 ranked as the 7th coldest November on record (since 1875). Ten days saw temperatures fall below 32, which is 7 above normal. For the year, the average temperature at Memphis is 62.6 degrees, which is 2.1 degrees below average.

Precipitation
Monthly total: 2.56" (2.93" below average)
Days with measurable precipitation: 8
Wettest 24-hour period: 1.46" (4th-5th)
Total Snowfall: 0.2" (0.2" above average)
Records set or tied: None
Comments: Measurable snow fell on two days in November (0.1" each), which marked the 4th and 7th earliest measurable snowfalls on record. Only two days had precipitation above 0.50". As of the end of November, the yearly precipitation has been 55.03", which is 7.09" above (or 115% of) average.

Miscellaneous
Peak wind: South/43 mph (30th)
Average wind: 8.5 mph
Average relative humidity: 61%
Average sky cover: 50%

Click here for a daily statistical recap for Memphis International Airport.

Cirrus Weather Solutions / MemphisWeather.net, Bartlett, TN

Temperature
Average temperature: 45.9 degrees
Average high temperature: 56.1 degrees
Average low temperature: 35.5 degrees
Warmest temperature: 74.6 degrees (4th)
Coolest temperature: 19.1 degrees (18th)
Comments: None

Precipitation
Monthly total: 2.81" (automated rain gauge), 2.86" (manual CoCoRaHS rain gauge)
Days with measurable precipitation: 11
Wettest date: 1.13" (5th) (via automated gauge)
Comments: None

Miscellaneous
Peak wind: Southwest/30 mph (29th)
Average relative humidity: 73%

Click here for a daily statistical recap for Bartlett, TN.

MWN Forecast Accuracy

MWN average temperature error: 1.66 degrees
MWN forecast temperatures within 2 degrees of actual: 77%
MWN average dewpoint error: 2.89 degrees
MWN forecast dewpoints within 2 degrees of actual: 58%

MWN's forecasts extend out five periods (2.5 days, or roughly 60 hours). Historical accuracy statistics can be found here.

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