Thursday, January 22, 2015

9:50pm update on snow potential for Friday - as challenging as it gets

Will keep this brief, as it's getting late, but after earlier today backing off the potential for snow during the day on Friday and calling for just a chance of a rain/snow mix as precip ends Friday evening, models (and the atmosphere) are again leaning a bit more towards a potentially cooler solution for the early morning hours Friday.

Any snow that can mix in will likely be caused by an atmospheric process called "dynamic cooling," which is too complicated to explain here but is basically a lowering of air pressure due to rising motion. The lower pressure causes temps (above the surface) to cool. Even 1-2 degrees could be enough to mix snow with rain in the early to mid-morning hours, even though we'll be in the mid 30s at the surface. The challenge is that this process is very difficult to forecast until it occurs.  We can see that the potential is there, but we don't know if it will be enough to push us over the edge for snow.  What we do know is that 1000' up, precipitation will be mostly snow! Whether it melts prior to reaching the surface is the big question.

We also know this - even if some snow mixes with rain during rush hour through mid-morning, the ground and air temperatures will be ABOVE freezing. Any accumulation (should there be enough snow to cause it) would be on grassy surfaces or bare ground and would be minimal and quickly washed off by rain. Roads are expected to be wet, not icy.

We'll see what we wake up to. In the meantime, plan on schools being in session, roads being wet, and temperatures being cold! Get the latest official MWN forecast on our website or apps, linked below.

Erik Proseus
MWN Meteorologist

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Update on Friday's snow threat - the news isn't good

This blog post has been superceded

A more recent update to expected conditions can be found here.



Original post: 11:20am Thursday

After raising the flag yesterday on the possibility of a rain/snow mix Friday, unfortunately it appears we're going to have to lower it to half mast, in remembrance of years gone by when, according to all of you, big snow happened multiple times every year.


While I'm not ruling out ALL snow tomorrow, the latest data has trended a tad warmer during the day. As mentioned in yesterday's blog post, a couple degrees either way could make a big difference. In this case, the couple degrees was on the warm end, starting with morning lows closer to 37-38 rather than 35. Though temps won't rise more than a couple of degrees Friday with rain falling, there is a big difference between 40 and 36 when it comes to snow. So go ahead... I know you want to...

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If you're still reading, it's probably because I said "not ruling out ALL snow." Colder air does move in after dark Friday and some lingering precip could still be around. Therefore, I've kept a slight chance of light snow in the evening forecast. Don't get your hopes up and don't change plans. Roads will be fine due to previous warmth. Just look forward to another decent weekend with highs back to 50+. The solace comes in the fact that our "biggest snows" - by official records, not people's selective memories - tend to come in the last half of winter and very early spring. Winter's not over!

Erik Proseus
MWN Meteorologist

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Wednesday, January 21, 2015

MWN Winter Weather Discussion for Friday, January 23

Despite the Mid-South being just past the coldest period of the year according to climate records, our current stretch of 55°+ days, which stands at 6 and includes a few mid 60s, is the longest such streak since Nov. 19-24. That is about to end though, as a cold dome of high pressure over the Rockies shifts east across the northern tier of the U.S. Today will be the last day in the 50s  for several days.

South of the high pressure that moves by to our north, a low pressure system over south Texas will get its act together and also move east along the Gulf coast. The Mid-South ends up in a squeeze play with cold air from the north moving in as moisture wraps around the low to our south. This is one of our classic winter weather patterns, but it always depends on the details, which are never the same from one storm to the next. How cold will it get and how much moisture will be present. As usual, the intersection of the "just cold enough" air and available moisture will be right over the region. There is still disagreement in the models on the amount of precipitation and degree of cold air. Yesterday, most models were still in the "no precip" camp, but they are coming around.

NWS forecast position of the low pressure responsible for potential Mid-South snow. The "Gulf low" track is a favorable one for the production of snow in the winter in Memphis.
Light rain looks to move into the area during the day Thursday, though most precipitation will remain over MS, so only light amounts are expected with temperatures in the 40s. By Thursday night, air temperatures cool into the mid 30s as light rain becomes more likely with the low over coastal Louisiana. Temperatures will probably fall to the mid 30s by dawn Friday morning. It is from this point on that we consider the potential for a rain/snow mix - through the day Friday as the low moves along the northeast Gulf coast and a trailing upper level trough moves through the region. Temperatures at the surface will likely stay in the mid 30s all day Friday with precipitation falling, then fall to near 30 for a low Saturday morning with precip ending Friday evening.

So, tell us straight - how much are we talking and schools be cancelled?
A big factor on any snow accumulation is the fact that we have been warm recently, so streets and even the ground have warmed up a bit from our early January deep freeze. For now, my early expectation is that some areas in the metro might see up to in inch of snow, mainly on grassy or exposed surfaces. Most major and secondary roads would be fine in this scenario, though slick spots could develop in those areas that get slick first - bridges and overpasses. (Note that brine pre-treatment, if applied, will wash off prior to any snow falling. It won't do any good.)

I have NO IDEA about schools, and they aren't making a decision this early anyway. The scenario could completely change by Friday morning. A couple of degrees either way could result in snow amounts ranging from nothing to a few inches based on the amount of precipitation expected. For now, plan ahead for all possibilities and let those paid to make those decisions Friday morning do their job, which always involves the safety of the children over anyone's convenience.

From the NOAA/NWS Weather Prediction Center - probability of 1" or more snow on Friday.
A low chance of 1" doesn't mean no snow however.
Finally, a word about content you might see on social media. The graphic from NWS-Norman (Oklahoma City) below sums it up very well. "Beware the Share" - don't share stuff that doesn't pass the eye test or is from a source you don't know and trust. Anyone can draw a map of the worst case scenario and post it as gospel.



We'll update again as new information arrives or our opinion changes, but no later than Thursday afternoon. Any intermediate updates will be posted on our Facebook and Twitter feeds.

Erik Proseus
MWN Meteorologist

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Friday, January 16, 2015

A reprieve from the January cold!

We've had our share of cold weather to start 2015 with an average temperature for the first half of the month that is over 6 degrees below normal! That will make the next 5 days or so even more enjoyable as warmer air overtakes the region producing multiple days with highs in the 50s and lows above freezing.

Even though this past week was climatologically the coldest week of the year, the Mid-South has been in "outperform" mode. Through January 15, the average high temp has been just under 43 degrees, which is 7 degrees below normal. The average low of 27.2 is nearly 5.5 degrees below normal.
Temperature so far this month (blue bars) have generally been well below normal (brown bands). We'll have more blue bars in the red area (above normal) by this time next week!
It's also been fairly dry to start the year with precipitation about an inch below normal already. The dry spell continues, but now with more pleasant temperatures that should result in more folks outside this weekend, soaking in some welcome sunshine and shedding the winter coats! Even a weak cool front Saturday night will do little to suppress temps as highs both Saturday and Sunday reach the upper 50s. No rain is expected with this front as dry air throughout the atmosphere will mean just a few clouds Saturday night and a brief wind shift.

Output from the "new" GFS model (see below) for Saturday night as a weak cold front moves through. The closest precip looks to stay along and north of the Ohio River as dry air won't support rain this far south.
Monday will see highs approach 60 and Tuesday would be the same except for a few more clouds with another cold front that is also expected to pass throughout precipitation. We're forecasting another day of highs in the upper 50s. (Our normal high this time of year is near 50.)


The tide appears to turn as we head into the end of next week as cooler air seeps in behind Tuesday's front with a general downward trend in temperatures. In addition, a "southern stream" weather system, which rides the subtropical jet stream positioned in the northern Gulf of Mexico, will throw abundant cloud cover over the area late next week. Being a week out, precipitation forecasts could change significantly, but for now most of the precip looks to stay just to our south. However, with colder air back in place, we'll have to monitor this system closely for the potential of a wintry mix. Too soon to make any calls on that though!

Output from the "new" GFS model (see below) for next Thursday night as low pressure moves through the northern Gulf in the "southern stream." Temps will be cold; the question is where the northern edge of the precipitation will be. Graphic courtesy WeatherBell Analytics.
For now, enjoy the beautiful January weekend that continues into early next week! These are what we call "bonus days" in a typically cold, often cloudy, time of year.

Side note - for weather geeks

For you weather weenies out there, we have referenced and posted graphics on this blog in the past few months from a "parallel" or "upgraded" GFS computer model that was being tested alongside the operational GFS. The GFS is the American workhorse model - with global output and long-range capabilities. As of Wednesday morning, January 14, that "upgrade" became the "new" operational GFS. There are several improvements within the model, but perhaps the biggest is an increase in horizontal resolution by nearly four-fold in the ten day forecast and more than three-fold out to 16 days.

An increase in horizontal and vertical resolution was also accomplished on the ensemble modelling system that the GFS is a part of (the GEFS). This increase in resolution was enabled by a recent tripling in capacity of the supercomputers NOAA runs the models on, mostly funded by the Hurricane Sandy supplement that directed tens of millions of federal dollars into a lagging NWS computing infrastructure.

NOAA supercomputers "Tide" and "Gyer" process NWS model data used globally for atmospheric prediction.
Other valuable improvements in various computer models are forthcoming this year, but this month's upgrade was certainly welcome and follows other recent upgrades, including the operational launch of the HRRR high-resolution rapid refresh model nowcasters rely heavily on and the upgrade of the high-resolution hurricane model, known as HWRF. All of the model upgrades, collectively, will only help to improve the accuracy of forecasts provided by your local weather folks. As you know - garbage in, garbage out.

Erik Proseus
MWN Meteorologist

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