Sunday, December 4, 2016

Active weather for the first week of December

This weekend's recap

In Friday's blog, the main topic was the wet weekend weather. We got that, though it could've been much worse for the St. Jude Memphis Marathon, or even more likely, the Christmas parades later in the day. In fact, though several entrants and (by my eye test) hundreds of spectators skipped the Bartlett parade Saturday afternoon, it wasn't too bad with "heavy sprinkles" at times but no downpours. Most of the rain fell after dark. Today felt like mid-December with cool temperatures and low overcast hanging around, but very little rain.

Monday low pressure

Another topic in Friday's blog was the upcoming active wether pattern this week. I'll provide a bit more detail on that tonight, starting with tomorrow's low pressure passage. Low pressure will develop along the western Gulf coast and begin to move northeast during the day, increasing our rain chances as it draws closer. Rain chances will ramp up from the south by early afternoon, becoming likely for the entire area by late afternoon and continuing overnight as the low will pass by just to our east. The map below shows the newest hi-res North American model (NAM) surface forecast at 3am Tuesday. I've annotated the low (red L) cold fronts (blue lines) and a warm front (red line).

The newest high-resolution NAM model shows surface low pressure just southeast of Memphis at 3am Tuesday with a cold front extending south to the Gulf and another cold front approaching from the west. The position of the low means few thunderstorms, if any, and no severe weather, but a good dose of rain. (WxBell)

With the low to our east, we won't get a good southerly wind and will avoid the "warm sector" of the system east of the low. That means thunderstorm chances will remain fairly low, though a few are possible as the low approaches, and severe weather chances should remain well to our south, along the central Gulf coast. We'll get more rain to help our drought situation, but even the rain shouldn't be heavy for prolonged periods. Most model data suggests we'll top out at no more than an inch. By the time you wake up Tuesday morning, the rain should be nearly done.

Mid-week lull

Tuesday and Wednesday will be "tweener" days - between major weather systems with weak high pressure in control. Clouds are likely to stick around, though a few breaks could bring peeks of sunshine. Temperatures will generally be seasonal with lows in the 40s and highs in the 50s. Wednesday night is when the "Big Kahuna" Arctic front arrives, as posted in Friday's blog. Scattered showers are expected with the front, but precipitation should be mostly an overnight event on Wednesday night.

Late week freeze

MUCH colder air, originating in Alaska, will dive southeast into the western U.S. Monday, cross the Rockies Tuesday, and surge across the central U.S. Wednesday, arriving in the Mid-South by Thursday morning. The coldest air of the season for the U.S. will arrive on gusty wind with temperatures falling towards freezing Thursday morning and remaining in the 30s all day with wind chills in the 20s. By Friday morning, lows will reach the lower 20s with wind chills in the teens and highs again only be in the 30s, or about 15-20° below normal.

The American GFS model temperature anomaly map (how much warmer/colder than normal temperatures will be) for Friday morning shows a large area of below normal temperatures east of the Rockies. We'll be about 15° below normal late this week. (WxBell)

Snow references, and what I think

I've seen several references, including in national, model-driven "crap apps," a national weather network forecast, and even in local weather broadcasts, to the possibility of snow Thursday morning. Yes, it will be cold enough to snow at some point Thursday morning. But the two main players in the mid-range model world - the American and European model - differ on how quickly we get to "cold enough."

In my 20 years of experience forecasting Memphis weather, these types of systems (in which a strong cold front arrives from the northwest, dropping temperatures as moisture departs) are not prolific (or even marginal) snow producers. Almost always, sufficient moisture for precipitation is gone by the time the low level thermal (temperature) profiles are cold enough to support snow. If it's going to snow in these situations (at least in this part of the country), the most common scenario is to have the sudden burst of cold air produce snow flurries or a brief snow "shower" as the last of the moisture moves out. I can't rule this out early Thursday morning. I also am not including it in the forecast at this time. If I saw that possibility occurring, I likely wouldn't include it in the forecast until about 36 hours ahead of time. There's too much uncertainty in predicting the precise temperature profile to include it in this type of atmospheric setup unless it's a virtual lock. That won't happen until we're fairly close to the event in question.

Below is a piece of evidence from the colder of the two models - the American GFS - to not include snow. It indicates the higher elevations of the Ozarks is the closest place to us that would get anything worth mentioning.

The American GFS model total snowfall through Friday evening indicates that no accumulating snow will fall in the metro. A couple inches could be possible in the Ozark Plateau however. (WxBell)

After a drop into the deep freeze Thursday and Friday, a slow moderation begins next weekend. In addition, the next weather system capable of producing precipitation arrives next Sunday. At this time, it appears temperatures will be warm enough to support rain, so that's my forecast. We'll keep an eye on timing to see oif any precipitation can occur prior to the arrival of sufficiently warm air. THIS is the type of scenario that, historically, has provided a slightly better chance of wintry precipitation. Again, let me be clear, that's not in the forecast either. And it's still technically fall...

As always, get the complete forecast, with regular updates made by an actual person, not a computer model, via our website or mobile apps. Links provided below. Pay the $0.99 for human-powered weather and avoid the hype and the crap!

Erik Proseus
MWN Meteorologist

p.s. 2017 MWN wall calendars featuring pictures of Mid-South weather taken by local photographers (and myself), as well as Mid-South weather events and records throughout, are now available. These are made primarily for your enjoyment. We price them to break even. Check them out here.

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Friday, December 2, 2016

Weekend Update - Memphis weather style

Lots of activities going on this weekend in the Memphis area, involving tens of thousands locally and, literally, from across the United States.  It's St. Jude Marathon weekend and Christmas parade season!

Unfortunately, Mother Nature is not going to be too cooperative as we see a round of precipitation move up from the southwest and envelope the area during the day Saturday, lasting into the first half of Sunday. This rain will be caused by moist air rising up into the area ahead of a low pressure system that moves along the Gulf coast.

Here's what the North American Model (NAM) thinks radar will look like beginning at midnight tonight and ending Sunday morning at 6am:

NAM model forecast radar from midnight tonight through Sunday morning at 6am
Note that the greens are very light rain with the yellows being steadier, but not particularly heavy, rain. We're not talking about a deluge and no thunder will occur either. However it does look like we could start to see sprinkles early Saturday morning with basically increasing rain chances as we go through the day Saturday. With any luck, most of the marathoners will be done before rain gets very steady and it will be just a nuisance.

Here are a few key times pulled from the loop above. Note that these are not exactly what will be happening, but give us an idea of what to expect.




And here's our #StJudeRun Memphis Marathon forecast:

As you can see, it's also going to be a chilly day with most of it spent in the 40s. Once rain gets going a little more steadily in the afternoon, it'll drop back down into the 40s after peaking near 50. Fortunately, wind is not much of an issue, though there could be a bit more (maybe 10 mph) in the evening. For late afternoon/early evening parades, the news is not good unfortunately. Steady light rain could possibly be moderate at times. Rain chances by late afternoon are above 50%. By Sunday when the rain moves out, rainfall totals will likely be in the 1/2"-1" range.

Next week

The pattern remains active the first half of next week. Another stronger, wetter system moves across the Mid-South Monday. Low pressure will move very close to Memphis Monday evening and a few thunderstorms are possible. Currently severe weather is not expected.

Finally, after dry hours Tuesday and the first half of Wednesday, another front moves through Wednesday night. This is the "Big Kahuna" Arctic front you've probably been hearing about. More showers and a few thunderstorms will be possible with this front before temperatures nose dive heading into the end of next week. We'll have more details on both of these systems in the coming days, but be prepared for the likelihood of temperatures remaining in the THIRTIES (really!) most of the day Thursday and maybe Friday too! We'll all see a hard freeze Thursday and Friday nights.

Finally a note of caution

Crap apps and nationwide weather outlets that use computer model forecasts may (already be) showing you snowflakes next Thursday. YES, it could be cold enough. However, we have no good indications that precipitation will be lingering by the time it gets cold enough to snow. We'll of course let you know if that changes. In the meantime...

Erik Proseus
MWN Meteorologist

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Sunday, November 27, 2016

Cyber Monday Storm details - video forecast update

Cyber Monday will bring the threat of very heavy rain and potential severe weather to the area. In this video forecast discussion, we break down the timing and threats and then get into some detail on the meteorology of the event, concluding with the week-ahead forecast. You can get the gist of the necessary information in the first 6 minutes, but stick around for the "nitty gritty!"

If the embedded video doesn't work, you can watch on YouTube here.

Here are a couple of the graphics used during the video for those that want a brief overview.

NWS WPC forecast rainfall amounts through 6pm Monday. The metro can expect 2" of rain with some areas seeing up to 3".

The severe weather outlook from the Storm Prediction Center shows a Slight Risk (category 2/5) from Memphis south and a Marginal Risk (category 1 threat) for west TN and north AR.

Be sure to download our mobile app for the latest information, including MWN StormView Radar, our human-powered forecast, current conditions, social media feeds (continuously updated and linked below) and optional StormWatch+ severe weather notifications for your specific location(s) of interest!

Erik Proseus
MWN Meteorologist

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Saturday, November 26, 2016

Cyber Monday storm to bring heavy rain, strong wind

I hope everyone has had a great Thanksgiving weekend! It's been a cool, but not atypical, weather weekend with cloudy skies on Thursday and most of Friday and sun re-appearing today. Temperatures have peaked in the mid 50s the past couple days.

Sunday will see a return of high cloud cover ahead of the next storm system that is the main subject of this blog. Temperatures Sunday will start chilly - in the 30s - but climb to the mid 60s as south wind brings in warmer air.

On Sunday night, we'll begin to see the early stages of a major weather system that will traverse the Mid-South on Monday. Clouds thicken and lower and south wind increases further overnight with scattered showers moving into the metro before dawn Monday. The morning rush hour will likely be wet and breezy with temperatures in the mid 50s. This pattern continues through the morning hours with temperatures rusing into the low to mid 60s as dewpoints (humidity) continues to increase as Gulf moisture moves into the region on gusty south wind.

Forecast radar valid Monday at 6am showing showers over the region with heavier rainfall entering western AR, which will serve as the "main course" later in the day. All graphics courtesy
Monday afternoon and early evening will be the timeframe of greatest impact. A couple of factors will be key: 1) wind in all levels of the atmosphere will increase markedly as the day goes on, and 2) atmospheric moisture content will be very high (likely in the 90th percentile for this time of year). These factors will contribute to torrential rainfall rates and strong, gusty wind above 30 mph, especially after lunch into the evening rush hour. Though some thunder is possible, atmospheric instability is actually very low with this system, despite impressive atmospheric dynamics. High rainfall rates could actually help drag some of the wind from a screaming low level jet stream (up to 65-75 knots [75-85 mph] at 5,000 feet) down to the surface, which could result in some severe level gusts without thunderstorms actually occurring. In addition to the strong surface and low level wind, a very strong upper level jet stream of 150 kts (>170 mph) will be over the area, helping to increase the overall strength of the system. From an atmospheric moisture perspective, precipitable water values (a measure of the amount of liquid water in a column of air) near 1.50", or near record territory for this time of year in the Mid-South, will contribute to high rain rates and precipitation totals that could easily exceed 2" in most areas of the metro from early Monday morning through evening.

At 3pm Monday, the NAM (North American) model shows weak low pressure in eastern AR with surface wind in the 25-30 mph range (pinks) on its eastern side over the metro.

A little higher up, 3pm wind at 850mb (about 4,300') is forecast to be approaching 75 knots, or 85 mph, which are extreme values. Some of that wind could be "drug" to the surface by intense rainfall, resulting in severe wind gusts. 

Higher yet, at the jetstream level (200 mb or about 39,000'), Monday afternoon wind is forecast to be near 175 mph. This is about as strong as the jet stream wind gets in the winter time in this part of the country and will aid the overall system strength by creating divergent wind at this level, which acts like a vacuum cleaner, "sucking" the lower level air into the upper levels. 

Precipitable water (PW) values near 1.50" are in the 90th percentile for this time of year. PW represents the amount of  moisture in a column of air and values this high are supportive of high rainfall rates.

The Global Forecast System GFS/American) model forecasts 2.80" of total rainfall on Monday with a swath of very heavy rain across the Mid-South. Though 6-9" of rain are needed to completely alleviate the severe drought conditions in place, this would certainly help the situation!

The National Weather Service Weather Prediction Center forecast calls for over 2" of rain through Monday over the metro. (WxBell)

 To recap, the key takeaways for Monday are:

  1. Expect increasing rain intensity and wind values peak in the late afternoon hours.
  2. A few wind gusts to severe strength (60 mph) are possible due to very strong wind fields aloft.
  3. Total rainfall of 2"+ is expected with some areas of ponding likely, especially around afternoon rush hour. Flash flooding is possible, despite recent drought, due to rainfall intensity.
  4. Hail and tornado probabilities are low, but not zero.
  5. Plan for a wet commute in the morning and possibly hazardous weather for the afternoon commute.
By late Monday evening, we expect the system to pass, wind to let up a fair amount, and rain to stop. Exact timing can still change and will be updated as necessary. Stay tuned to the MWN mobile app and our social media feeds for the latest information.

Erik Proseus
MWN Meteorologist

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Visit on the web or on your mobile phone.
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MWN is a NOAA Weather Ready Nation Ambassador Meteorologist Erik Proseus is an NWA Digital Seal Holder