Sunday, March 1, 2015

March comes in like a lion, or maybe a snow leopard

Welcome to the first day of meteorological spring! We in the weather business compile seasonal weather and climate stats according to "meteorological seasons." March-May is considered "spring." Unfortunately, the first week of March looks more like a continuation of winter (with a one day exception) than spring.

Fe"brrrr"-ary recap

First, a brief recap of the month since the groundhog poked his head out of the burrow. February seemed to be when winter STARTED this year, particularly the last 2 weeks. The tables below showed that the month was preliminarily the coldest since 1978 and the 7th coldest since 1875 in Memphis. That was driven mainly by the last two weeks of the month (when normal highs are in the upper half of the 50s), which was the coldest last half of February on record, and it wasn't even close, shattering the previous coldest by nearly 5.5°F!



So I know we all want winter to end after the last two weeks. Unfortunately, except Tuesday, the first week of March looks to continue the recent cold. The month will be coming in like a lion (or maybe a snow leopard) this year!


Early this week

An inclement few days is ahead as a frontal system wavers through the area to start the week. Showers will occur today as a cold front draws near. That front moves through tonight with rain expected. Fortunately, temperatures  should stay just above freezing tonight so no icing is expected in the metro, but wind chills will be in the 20s Monday morning. A few showers will also occur Monday with a north wind  holding temperatures in the mid 40s (normal is near 60).

As low pressure develops in the central U.S., the front will pull back north overnight Monday night with temperatures rising and more showers expected. By Tuesday morning, we'll be above 50 with a south wind kicking up. Tuesday, although wet, will be the warmest day of the week with highs well into the 60s. Thunderstorms will be likely though Tuesday night as yet another cold front (this one with Arctic air behind it) moves through the region. (At this point, they do not look severe.) The Arctic reinforcement arrives Wednesday morning.

The NWS indicates a cold front moves through the region Tuesday night, followed by a reinforcing Arctic front Wednesday morning as shown above.

Wednesday - too early to call for sure

Wednesday's high will occur at midnight and temps are expected to fall all day. We should see the mercury in the 40s Wednesday morning as we wake up, but look for temps to fall to freezing sometime during the day Wednesday, with precipitation continuing.


(Yeah I've used that animated GIF in the past couple weeks already, but is there a more appropriate one?)

So this is where it gets tricky! It's too far out to know exactly when that happens, how fast the low to mid levels cool down (which is critical in precipitation type forecasting), as well as how much precipitation falls into cold air and when it ends. There are model opinions that literally run the gamut, from cold rain to Snowmaggedon (hey, why hide the truth). But these models literally change multiple times a day, and your national weather apps that run on them do too. We will not forecast ice and snow amounts and timing at this time - it's simply too early to know with any certainty! Whatever we would say, we'd be lying.

What we should ALL prepare for, though, is the possibility, perhaps even the likelihood, that another round of winter weather will affect the entire region Wednesday, maybe into Wednesday night. Snow and/or sleet/ice accumulation COULD be significant, but we won't make a call on that yet. In this situation, it's a must to follow trusted sources for weather info and prepare accordingly. Check them regularly. A forecast from Sunday or Monday will most certainly not be right come Wednesday, but trends are important.

End of the week

Looking beyond Wednesday, Arctic air pushs in once again for a few days with lows probably back in the teens and highs in the 30s to end the week. Hopefully this is Old Man Winter's LAST GASP! I know you're over it... so are we!

We'll be keeping a very close eye on it, and when we feel comfortable doing so (maybe late tomorrow) will make a first call on what to expect. Follow us on Facebook or Twitter for the latest info and you can always check our latest forecast on MemphisWeather.net, our mobile apps, or mobile website.

Erik Proseus
MWN Meteorologist

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Tuesday, February 24, 2015

UPDATED -- Our call on Wednesday's snow potential

UPDATE - 9:00pm Tuesday

The event looks to be pretty much on track, so please see the post below from this afternoon for details left out here. The models have come into decent agreement with the "outliers" a little more in line. Confidence is increasing in the forecast made below, which includes about an inch of snow for Memphis and Shelby County, more south, less north.

A few additional notes:

  1. Temperatures should recover into the 36-37° range during the morning hours. I wouldn't be surprised to see the precipitation onset as rain or a rain/snow mix for a couple of hours. Rain should arrive from the south by about noon in the I-40 corridor (earlier south). During this time, no accumulation
  2. That means that we should be all snow by 2pm or so with temps at 32-33°. The best time for snow that could accumulate will be 2-6pm for just about everybody. Flurries could last into the evening. Forecast accumulation remains as indicated in the map below (1" in the I-40 corridor).
  3. There will be "haves" and "have-not's" as a tight gradient in the snowfall accumulation is expected. As an EXAMPLE, we might see very little accumulation in northern Shelby County and 2-3" in southern DeSoto Co. (I'm not good enough to know where this gradient will set up.)
  4. Roads should be fine through mid-afternoon, but after 2pm, elevated roadways and exposed surfaces could start seeing accumulation and/or become slick, especially in north MS where snow could come down a little heavier. 
  5. The winners from tomorrow's snow will be south of the metro, somewhere in an area from Clarksdale to Oxford to Tupelo where 4-6" is expected.

We'll keep you updated tomorrow. I have meetings at work all day, but Mississippi State has cancelled classes, so the social media posts will continue unabated! Thanks #TeamMWN! Get our latest forecast on the MWN mobile apps or mobile web.


A Winter Storm Warning has been hoisted for all of north MS and a Winter Weather Advisory for southern west TN and east-central AR as yet another (I think this makes 5th in under two weeks? I've lost count...) winter storm moves through the Mid-South on Wednesday. This system once again moves along the southern stream - with the arctic jet stream to our south. In fact, of all the weather systems the past few weeks, this one seems to have the most "classic" setup for southern snow. In this case, it appears the I-40 corridor will be on the northern edge of this one!


Event setup

Low pressure will move along the Gulf coast with Arctic air in place on its north side. As moisture rides up and around the low into the southern U.S., it will fall into cold air which has seeped as far south as the I-20 corridor in MS and AL. North of that area, snow will fall. The greatest combination of moisture and dynamics, with sufficient cold air, will reside across southern AR into northern MS and AL, where the heaviest snow is expected.

Wednesday's surface map shows low pressure in the Gulf and snow (and some freezing rain) across a wide area of the interior southeast and Mid-Atlantic.

This map from NOAA shows the probabilities of an area receiving 2" of snow Wednesday. Note the best chance of significant snow will be across north MS and AL with decreasing chances north.

Details for the metro

Temperatures will rise into the mid 30s by noon Wednesday as precip approaches from the southwest. Precipitation will likely start as a rain/snow mix but quickly transition to all snow as temperatures fall back towards the low 30s after precip starts. The snow should taper off by early evening, ending as flurries during the evening hours. We are not expecting any ice accumulation with this event. Snowfall totals by Wednesday evening should look something like we have shown below.


Bottom line

What: About an inch of snow across far southern west TN to 2-3" across the southern metro (north MS)
When: Starting after 9am in Tunica/Tate Co and after 12pm in the I-40 corridor. Ending by 6-8pm.
Impacts: Roads coated with snow in north MS and possibly parts of the metro. Slick areas expected on elevated roadways in the I-40 corridor (including Memphis/Shelby County).
Bust potential: Moderate. One typically believable model (and its offshoots) have up to 4"+ along I-40. Almost everything else says about 1", or even a bit less, in the same area. For now, it's an outlier, but it makes the 1" forecast a little more plausible. On the other hand, the northern edge of the snow may not make it north to I-40, in which case we'll just root for spring!

After this system, another couple of cold days are expected, as you can see in our MWN Forecast, but then it appears we have a warm-up for the weekend and a RAIN event to start March! That's a good thing because it's much easier on me (and you!) and the roads and vehicles could use a good washing down. Plus north MS will be ready for the snow to disappear!

We'll keep you updated as the even unfolds.

Erik Proseus
MWN Meteorologist

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Monday, February 23, 2015

Weather Mythbusters: Disproving urban myths related to today's "disappearing" snow

Many of you were diligently watching radar this afternoon as snow moved towards the area from the west. You were excited by it's approach. You were disappointed by it "disappearing" from the radar loop as it got into Crittenden County, to our west. Of course, this visual didn't help to dispel the urban myths of the powers of the Pyramid and Bluff.

So let me take a moment to explain what you were actually seeing using the example loops taken from this afternoon, a radar schematic, and a cross-section of the radar returns.

First, here's one example of what you were likely seeing (click play and watch it a few times). Whoa! The snow coming in from Arkansas vanished as it got closer to Memphis! And if you looks close, it re-appears to our east! Must be the bluffs.... or the Pyramid... or the snow dome... or, or... maybe it just needs scientific explanation!

video

Here's a loop taken just a bit earlier, as seen from StormView Radar, which many of you diligently use (thank you!). It seems there's a wall or giant snow-eater on the western side of Crittenden County.


So what was really going on?  Let me explain.

Below is a similar scenario from a cross-sectional point of view (pretend the rain drops are snowflakes - and this is not exactly to scale). Precipitation fell into a very dry layer of air beneath the clouds. As it did, the precipitation evaporated before reaching the ground. It's very common when dry air is in place in the lowest few thousand feet of the atmosphere. The falling precip that evaporates is called virga (with a hard G).


The radar scans upward slightly so the radar beam samples dry air (the red dashed line) out to about 20 miles in our scenario. When it gets past 20 miles, it "sees" the falling precipitation (blue dashed line) that had evaporated by the time it got to 3000 feet and so it paints precipitation on the radar display 20+ miles away from the radar. HOWEVER, as it turns out, that precipitation we see on radar is not actually reaching the ground!

In the loop from StormView Radar above, the "snow" west of Memphis (say at Wynne) was actually well above ground, but not making it to the ground, a.k.a. virga. As you get closer to the radar (in Millington), the precip appears to vanish because the radar beam is closer to the ground and the precipitation has evaporated at that lower level. Thus, it appears the precipitation is dissipating as it moves closer to Memphis when really none of the precip to the west was reaching the ground in the first place!

So how do we know it's not reaching the ground? A couple of ways - 1) your reports, or 2) cross-sections of the radar returns through the atmosphere (see below).  #1 is why we VALUE your reports! Except right near the radar, it doesn't actually see what's going on at the ground level. We need your eyes and reports to let us know (especially when we know there is a lot of dry low-level air) if the precip is reaching the ground and what type it is.  Note: it's also why we need reports of tornadoes, because the radar is typically looking above where where tornadoes form.  They can't "see" tornadoes, though they can infer them.

As for #2, let's take a look at a cross-section of virga from this evening. The first image below shows an area of precipitation east of Memphis at 7:30. There were no reports of precipitation in this case, but the radar sure looks like it's doing something!


The red dashed line above represents the line along which the cross-section was taken below.  In the image below, we are looking sideways along that dashed line with the ground at the bottom and the top of the image at 20,000'.  The 5,000', 7,500', and 10,000' lines are also marked.


Precipitation appears from about 5,000' to about 15,000' (on average). Based on nearby observations, the base of the clouds was at about 7,500' (red horizontal line). So, the precipitation being detected is in the clouds from 7,500'-15,000'. The precipitation was falling from the clouds down to about 5,000' before dissipating due to the dry air it was falling into. So, while the top image shows precipitation over Hardeman, Chester, and McNairy Counties, a closer look actually shows this precipitation wasn't reaching the ground.

Bottom line: The bluffs, Pyramid, snow dome, and snow-eaters had NOTHING to do with the precipitation dissipating as it moved into Memphis. It was snow aloft that was evaporating before reaching the ground and the radar happened to "see" that precipitation where it was occurring - in the clouds and just below. When you get closer to the radar, the radar beam was below the evaporating precipitation and not seeing it. The fact that it showed back up on the east side of Memphis in the first loop above means that the same thing was happening over the radar as well - it just didn't see it due to the angle of the radar beam.

(By the way, if the radar were in Wynne and you lived there, you'd see nothing over Wynne and precipitation over Memphis and think that the snow was going around you there too! There aren't any bluffs in Wynne...)


Thanks for reading, and let me know if this helped you better understand what you were seeing on radar today!

Weather 102 - A Step Further: The radar product being used above and on most web and app radars only shows what the lowest angle of the radar sees. Radar actually scan at many different "elevations" above ground. There are radar products that take data from all these levels and paint the radar image based on all of them. They are called Composite Reflectivity or Vertically Integrated Liquid Water (VIL). 

Some radar programs or web displays show these products. They don't necessarily show the "donut hole" around the radar site like the above loops do, but they do show all of the virga and tend to overestimate the actual intensity of the precipitation at the ground. That's a topic for another day!

Erik Proseus
MWN Meteorologist

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Saturday, February 21, 2015

The last week of February brings... more snow and cold!?!

Wow, what a week!  It's like winter showed up as soon as the rodent popped his head up out of his hole on February 2! Somebody whack that guy!


It's been a cold winter, but we've been dry. That changed this week with multiple icy systems that nearly provided most students in the area with another week-long "winter break" just a few weeks before spring break!

Today

Many people were surprised by the cold temperatures during the day today after highs were predicted in the mid 50s. Technically the forecast wasn't wrong - it was 58 around 8am today. It just started dropping a few hours faster than we thought! Most of the day was spent in the upper 30s to lower 40s with periods of heavy rain. In fact, over 2" fell at MWN since midnight last night, plus the nearly 1/2" we got yesterday.

Now that the end-of-week system is behind us, it's time to look ahead - to more cold and another shot of winter weather! Yeah, seriously...


Sunday cold

A strong north wind and abundant cloud cover will make Sunday feel very cold. Temperatures only bottom out near freezing tonight but they don't climb much tomorrow. Highs will be in the upper 30s with north wind gusting at 25-30 mph, keeping wind chills well down into the 20s. You'll definitely notice it as you head out tomorrow.

Sunday night snow

Sunday night is the next chance of winter weather as a disturbance in the force moves across the Mid-South. With all the focus on the Friday event, this one snuck up on most of us. However it's looking fairly likely that light snow will fall Sunday late evening (after 8pm) into the overnight hours, ending by dawn Monday. How much?  The NAM4 and GFS (shown below) are very similar indicating that the heaviest will be across west-central AR, but that we could see around 1/2".

Sunday night forecast snowfall amounts per the mid-day GFS computer model. Graphic courtesy WxBell.

Sunday night forecast snowfall amounts per the mid-day NAM computer model. Graphic courtesy WxBell
With temperatures falling through the 20s overnight and a cold ground surface in place, there is definite concern that the snow will stick, possibly to roads.  The factors that might prevent that are that it will be a very dry fluffy snow and wind will still be howling at 20-30 mph. It might just blow around rather than sticking to roads. In fact, with the wind and snow, we could have an all-out "flizzard" (flurry-blizzard) during the night Sunday. [UPDATE Sunday, 7:15am -- There is some evidence to suggest the precipitation will start as a wintry mix (perhaps sleet/snow) with a small amount of ice possible at onset. This would most likely occur between 7pm-midnight before precipitation becomes all snow.]

Monday #StupidCold

As for those conditions Monday morning, it's a given that it will be #StupidCold again. Lows will be near 20 with a north wind huffing and puffing will mean wind chills at the bus stop (predicated on "winter break" not being extended yet another day) will be in the single digits. Monday's highs, meanwhile, don't get above 28 with wind chills remaining in the teens. [UPDATE Sunday, 7:15am -- A brief period of snow flurries is possible again late Monday afternoon.]


Rest of the week

For the rest of the week, the southern stream remains active as the arctic jet stream remains to our south. That will mean cold air continues to blanket the region to close out the month. With the jet to our south, any disturbances that come through could mean additional chances of light snow. Right now, we haven't pinpointed any particular time that could happen past Sunday night, but there are a few coming - we just don't have a handle on exact timing yet. Let's hope that March bring better weather than February did! The good thing is sun angles are getting higher, daylight is getting longer, and we'll eventually head towards a more spring-like pattern. It can't come soon enough! Thunderstorms are better than thunder freezing rain!


Stay warm,

Erik Proseus
MWN Meteorologist

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