Afternoon Thunderstorm Forecast and Monitoring 6/18


Updates (see original forecast below)-

Thunderstorms have developed along the foothills to the west of Boulder and Longmont. Storms look to be intensifying quickly and one report of pea-sized hail has already been made from the storm northwest of Nederland.

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Thunderstorm Position and Intensity at 4:45pm


THIS AFTERNOON’S THUNDERSTORM FORECAST-

A few thunderstorms are in the forecast for this afternoon and evening!

Denver will see a few thunderstorms, but severe storm development should stay far to the east/northeast of the metro area. Thunderstorms should begin to build around town over the next couple hours and will move to the northeast fairly quickly.

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Expected Storm Position and Intensity at 8pm from 20Z HRRR Model Run

Severe Storm Risk- Current models are showing severe storms developing east of DIA and E-470 (picture) with storms continuing to develop and eventually tracking along I-76. Threats from these storms will be large hail, damaging winds, and possibly a tornado.

As always, feel free to ask any questions that you might have.

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Severe Storms Possible Along Front Range on Friday

understanding_categoriesSevere storms with damaging hail, wind, and a slight possibility of a small tornado are in the forecast! These storms will be the beginning of what will be a soggy and cool weekend that will have Denver’s high temperatures dipping into the mid-50s. While these storms will have the potential to cause damage, current forecasts suggest that the danger should be minimal. The Storm Prediction Center currently has the front range in a Marginal Thunderstorm Risk and areas to the east, such as Aurora, in a Slight Thunderstorm Risk.

Forecast

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NAMNEST Simulated Reflectivity (estimated storm position and intensity) at 4pm Local Denver Time

 

A small lee-side low pressure will develop tomorrow in along the Colorado/New Mexico border that will allow for increased transport of moisture and instability to the front range. All models are currently showing thunderstorm activity around the Denver metro area tomorrow that will start in the early to mid-afternoon. The positioning of the storms (as usual) is fairly hard to predict, but the models are agreeing on the overall intensity of the storms. The GFS has things pushed a bit further east and has the primary wave of storms over eastern Denver and Aurora, whereas the NAM and 3k NAM have the storms developing right along the foothills. I’m inclined to agree with the NAM models at this time as orographic forcing and the boundaries created by the mountains always play an important role in storm development.

Timing

Storms will begin developing in early to mid-afternoon (~2-3pm) and will begin moving eastward. This means that there is a high likelihood that there will be storms occurring during rush hour and that it may be a better idea to wait to commute if a storm is nearby. Storm intensities should begin decreasing in the evening and areas of light to moderate rain could occur overnight. The times with the most intense storms should be from 3-7pm.

Threats

microburst_schematic_nubilt.jpg

Primary threats from these storms will be hail and strong winds. Current vertical profiles in the area show a very wet atmosphere high and a drier atmosphere at the surface. This has the potential to create damaging wind events as the rain aloft falls into the drier air beneath it, absorbs heat and evaporates, and then the cooler air around it accelerates towards the ground (cold air is heavy and sinks! Heat rises!). These may or may not be seen as microbursts. While a small tornado is not out of the question, the needed wind shear and helicity don’t look to provide a favorable environment for development.

As always, thanks for reading and let me know if you have any questions! I’ll update when new information is available.

Cheers!

Andrew

Upslope Storm to Bring 4-8″ of Snowfall to Denver Metro Area

Update: Models have diverged slightly in maximum snowfall totals with
some predicting a bit less and one predicting a bit more. I'm staying 
in the center at the original 4-8", which I think is realistic. The
Short Range Ensemble Forecast (SREF) model has upped its totals, but
it tends to get a bit crazy with its accumulations as we approach
major events. So, we'll stick with the others that are generally 
agreeing on the position of the system and the precipitation that we
can expect. With that being said, focal points for this storm such as
the foothills, Palmer Divide, and higher mountain terrain could see
amounts a bit higher in the 6-12" range with localized amounts even 
a bit higher.

Greetings! After what seems like years, we finally have an exciting winter storm that will be coming through the Denver and Front Range regions!

ermahgerdWith very low seasonal snowfall accumulations leading into this weekend, we need to embrace our wonderfully overdue moisture-brining storm! However, this storm has a bit more in store for us than just snowfall. We’ll see a decent drop in temperature with this system and that could contribute to freezing drizzle before the snow starts. So, this could definitely end up as a a fairly impactful and, at times, dangerous storm.

We’ll break down the forecast like this:

  • National Weather Service Winter Storm Watch
  • Storm Timing and Development
  • Expected Accumulations and Threats

National Weather Service Winter Storm Watch

...WINTER STORM WARNING IN EFFECT FROM MIDNIGHT TONIGHT TO 5PM MST
SUNDAY AFTERNOON THAT INCLUDES FORT COLLINS, BOULDER, DENVER, AND 
GREELEY. HEAVY SNOW POSSIBLE. PLAN ON DIFFICULT TRAVEL CONDITIONS 
REDUCTIONS IN VISIBILITY ARE POSSIBLE DUE TO BLOWING SNOW. TRAVEL 
WILL BE IMPACTED SUNDAY MORNING AND INTO THE AFTERNOON HOURS. 
PRECAUTIONARY/PREPAREDNESS ACTIONS...

A Winter Storm Warning means significant amounts of snow, sleet
and ice are expected. This will make travel very hazardous or
impossible. The latest road conditions for the state you are
calling from can be obtained by calling 5 1 1.

Storm Timing and Development

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GOES Infrared Satellite Imagery at 5:22 am MST

Currently, clouds related to the current system can be seen in the image above draped over northeastern Colorado into Utah and Nevada and down into Arizona. Cloud cover will continue to increase and will move eastward during the day today and we should start seeing heavy coverage around the area in mid- to late-afternoon.

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Forecasted Mean Sea Level Pressure and Temperature at the surface at 5pm Mountain Time on Saturday

The low pressure system that is currently sitting in eastern Colorado will continue to strengthen and will center itself over southeastern Colorado near the Oklahoma panhandle late Saturday night into Sunday morning before moving eastward Sunday afternoon. As it progresses, we will notice temperatures that will cool off beginning at 2pm when winds should shift from easterly/northeasterly to predominantly northerly. Temperatures will fall from the upper 40s on Saturday until daybreak on Sunday when a slight increase during the day will occur. Lows over night are expected to be in the low 20s and highs for Sunday will be in the mid- to upper 20s.

Precipitation is expected to start in the region around midnight on Saturday/early Sunday morning. Currently, the precipitation is expected to start off as freezing drizzle or freezing rain because of the relatively warm temperatures that will exist until late Saturday night. The freezing drizzle should turn to snow quickly and snow will remain dominant until the storm system moves out on Sunday afternoon.

You’ll notice that the low pressure system in the image above (black lines) looks particularly circular and has a good position for creating snowfall for Denver. But, there are a couple ingredients missing for that blizzard that everyone wants. First, this is a relatively dry system with minimal moisture input which will limit the amount of snow that we get. Another reason that we won’t see feet of snow is that the storm system will move out of the area too quickly. We really need a blocking pattern (holds weather pattern still-ish for a while) to really give us those storms that last days and give us heavy dumps of snow. We’re just not going to get that out of this one, unfortunately.

Expected Accumulations and Threats

The Global Forecast System (GFS), North American Mesoscale(NAM), Short Range Ensemble Forecast (SREF) models all agree on the Denver metro area getting between 4-8″ of accumulation for this system. As this is an upslope event, meaning that the winds will be coming from the east/northeast and will travel “up-slope” when they hit the mountains, I expect the largest accumulations to be along the western foothills and south along the Palmer Divide in areas like Castle Rock and Highlands Ranch. It will be important to monitor the latest model output as the High Resolution Rapid Refresh (HRRR) model comes into range and begins to create snowfall predictions as well.

The biggest concerns with this storm will be the freezing drizzle falling early and occasionally heavy snow. If freezing drizzle builds up on the ground and a layer of snow is combined with it, roads will be very icy and dangerous on Sunday morning. However, current thoughts are that the impact from freezing drizzle will be minimum due to the short period that it will fall. Driving conditions will be hazardous when the icy and snow-packed roads are combined with reduced visibility from heavy and blowing snow. Avoiding travel is a good idea if it is non-essential.

I will update this post as new model runs come out and new information becomes available.

Thanks for reading!

-Andrew

Snow to Return to Denver Tonight and Tomorrow

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After a long absence, snow will arrive overnight tonight in the Denver area and along the front range. For some, this is a welcome departure from the hot summer temperatures that we have experienced. Others aren’t so excited about the arrival of the cold and fluffy white stuff. Regardless of your feelings about it, snow will be here and we will actually be receiving quite a bit of it!

As always, the forecast will be broken down into a few different parts and they are as follows-

  1. National Weather Service Winter Storm Warning
  2. Storm Timing
  3. Accumulation Totals

 

National Weather Service Winter Storm Warning

...WINTER STORM WARNING REMAINS IN EFFECT FROM MIDNIGHT TONIGHT
TO 3 PM MDT MONDAY...

* WHAT...Heavy snow expected. Some secondary roads and bridges
  and overpasses may become snow or slush covered. Plan on slow
  and hazardous travel conditions, including during the morning
  commute on Monday.
* WHERE...Fort Collins, Boulder, Denver and Castle Rock.
* WHEN...Midnight to 3 PM Monday.
* ADDITIONAL DETAILS...Scattered power outages may occur due to
  heavy wet snow accumulating on trees still bearing their
  leaves.

Storm Timing

Cold air is now funneling into Colorado following the passage of a recent cold front and rain will be arriving in the metro area late this evening. Current winds are from the northeast and are helping to bring some moisture into the area, but will transition to northerly winds as time passes. Rain will continue until shortly after midnight when it will transition to snow and will remain as snow through Monday afternoon. The most intense snowfall should occur immediately after the transition to snow and mid-morning from approximately 7-10am. So, give yourself some additional time for your morning commute because it is looking to be a sloppy one!

Accumulation Totals

hrrr_sner

HRRR Forecasted Snow Totals Through Monday at 3pm

This is shaping up to be a weak upslope storm which means that accumulation totals will be highest in the western parts of the metro area. Accumulations will be between 4-8″ with the slightly higher amounts falling near Boulder and east of the Denver area. While 4-8″ is a fairly substantial amount of snow, the soil and ground temperature is warm enough to melt most of it once it hits the ground. However, neighborhoods and side roads will probably have some light snow accumulation and slush. This will be a moisture laden and heavy snow that will have the possibility of breaking tree branches that have leaves.

As always, let me know if you have any questions!

Thank you for reading!

-Andrew

Tornadoes are Absolutely Severe Weather

A meteorologist from a local TV station made a statement on his Facebook today that “tornadoes are not severe weather” and that ruffled the feathers of a lot of meteorologists that knew this was blatantly false. In fact, tornadoes are specifically listed as a type of severe weather by the National Weather Service. Now, this could have been an innocent mistake, but it points to a more concerning fact that the weather community has to grapple with every day; meteorologists struggle to find credibility.

Most people’s interactions with “meteorologists” happen through TV weather weatherman_JPG_1414881307872_9409764_ver1.0_640_480personalities talking about current or future weather. (Pro-tip: look for an “AMS” emblem at the beginning of the weather segment. If they don’t have one, they aren’t an American Meteorological Society certified meteorologist and haven’t gone through many physics, mathematics, and even chemistry courses needed for the designation.) We turn on our TV look at what the weather will be and go about our day. However, this is hardly foolproof! We’ve all been in that situation where the weather turns out much different than expected, which adds credibility to the old adage “Being a meteorologist is the only job where you can get paid to be wrong all the time”. The truth is that weather is incredibly hard to predict due to several issues, some of which are outlined in this post about Denver’s forecast for Autumn and Winter in 2016. The difficulty in modeling weather can lead to wrong forecasts and a lack in credibility, but how does that relate to this instance?

With public perception of meteorologists being less than favorable, we don’t want to create further problems to overcome. We need to be reliable on facts and weather definitions so that, although our forecasts may not always be accurate, we can still maintain credibility.

Cue our incident from today. A “meteorologist” (in quotes because I don’t know their credentials), who is a self proclaimed severe weather authority put out blatantly false information about the fundamentals of severe weather. That was concerning because, as a community, we have to strive for credibility and any wrong information that then has to be disputed makes us look less than reliable. That is why there was such a strong reaction against what this person said and a lot of upset people posting in the comments of this post.

So, what’s the point of all this? My point is simply, please be aware of who you are watching/listening to/reading and their credentials. Everyone makes mistakes, but if something seems particularly odd to you, don’t hesitate to challenge the expert on their knowledge. If they’re really an expert, they should be able to backup their position with evidence and references. If you don’t want to be confrontational, do some of your own research and look at information from peer reviewed sources. Don’t believe that a Volkswagen is a vegetable? Go through literature that defines the characteristics of vegetables. Think that the Earth is flat? Conduct an experiment by finding the edge and jumping off it… Whoa, that got dark fast.

Anyway, don’t believe everything someone tells you just because they seem like they’re an authority. Look at their credentials, background, and if what they say makes sense when compared to peer reviewed fact and then make a decision for yourself.

BONUS! Letter to news station’s weather department-

Hi [redacted] Weather Team,

I’m writing to you to voice concern over the spread of misinformation by one of your meteorologists on his Facebook account. [Redacted] stated today that a tornado is “not severe weather” because ” Only hail larger than 1 inch, and wind gusts greater than 58 mph, are considered severe weather.” This is blatantly false based on the National Weather Service definition of severe weather, which is “A severe thunderstorm refers to a thunderstorm producing hail that is at least 1 inch in diameter or larger, and/or wind gusts of 58 mph or greater, and/or a tornado.”\

The largest concern is that we (yes we, I am a meteorologist as well who worked as a Associate Scientist at NCAR and is now working on a PhD) have a responsibility to the community to educate them correctly on weather. This is especially true in the broadcast field where most of the populous gets their meteorological knowledge. It’s concerning to to think that people who see this information may be less likely to act correctly in the case of a tornado emergency because they were told that the phenomena is not considered “severe weather”.

Now, I understand that mix ups happen and that we can all have problems recalling knowledge from time to time, but this is a pretty significant mix up coming from someone who deems themselves to be an authority on severe weather. I am not writing this to head hunt and do not want to get anyone in trouble. Instead, I’m asking that you make sure that your department have correct information before they potentially misinform the public. It’s hard enough to be considered credible as meteorologists when it can be hard, if not impossible, to nail down forecasts. Not understanding basic weather definitions does not help our credibility as a community.

Thank you for your attention to this issue.

Snow to Impact Denver with Most Areas Seeing 3-6″ and a Few Seeing 6-12″

Just a quick forecast today.

Storm Timing (snowfall amounts below)-
Rain showers are expected to develop around the Denver metro area around noon and will continue through the afternoon. They are expected to increase in coverage between 5-6pm and change over to snow in the early evening. Snow will continue to fall until early afternoon Saturday.

Snowfall Amounts-
Most areas around Denver and the surrounding suburbs will see 3-6″ of snow by the time everything is done. Lower amounts will be on the north side of town and higher amounts will be to the south and west. Northerly wind will sling moisture onto the Palmer Divide and that will result in the possibility of snow amounts 4-8″ for Highlands Ranch, southern Aurora, and parts of southern Littleton. 6-12″ (possibly higher in small areas) in possible in Castle Rock, Monument, parts of Parker.

I’ll update as new information comes out! Have a great Friday!

 

UPDATE 1 [12:45]: The National Weather Service has issues a Winter Storm Warning effective until 6pm on Saturday.

...WINTER STORM WARNING IN EFFECT FROM 6 PM THIS EVENING TO 6 PM
MDT SATURDAY...

The National Weather Service in Denver has issued a Winter Storm
Warning, which is in effect from 6 PM this evening to 6 PM MDT
Saturday. The Winter Weather Advisory is no longer in effect.

* TIMING...Rain and snow showers late this afternoon will change
  to all snow during the early evening. Periods of heavy snow are
  likely during the night. Lighter snow will continue on Saturday
  but road conditions should be improving.

* SNOW ACCUMULATIONS...6 to 12 inches. The heaviest amounts will
  likely be in the southern part of the Denver metro area.

* WIND/VISIBILITY...North winds 15 to 25 mph east of Interstate
  25. Visibility may be reduced to a half mile or less in the
  heavier snow showers tonight.

* IMPACTS...Wet snow may accumulate on already leafed out trees,
  resulting in broken tree limbs and scattered power outages. Some
  roads will become snow and slush covered tonight during the
  heavier snow showers. Road conditions should improve during the
  day Saturday.

PRECAUTIONARY/PREPAREDNESS ACTIONS...

A Winter Storm Warning means significant amounts of snow are
expected or occurring. This could make travel hazardous. Residents
are encouraged to check road conditions before you travel and be
prepared for winter travel conditions. Also monitor snow
accumulation on trees and be prepared for possible power outages.

A Bit of Snow and Frigid Temperatures this Weekend

weird

Edit: Holy moly! Way more snow than expected! The model runs late last night did a bit better with snow totals, but they still underproduced. We’ve been dry lately so I can’t argue with some extra snow.

After a bit of moderate weather over the last few days, we’re heading into another arctic air outbreak that will bring us some snowfall along the front range and low temperatures in below zero with pretty severe wind chills. So, if you’re embarking on a journey away from your home to buy Christmas presents, go to the Broncos game, or just down the street to get groceries, be prepared for slippery conditions and very cold temperatures!

Today’s forecast will be broken into the following sections-

  1. Current Conditions
  2. Storm System Timing
  3. Expected Low Temperatures
  4. Expected Snow Totals
  5. Summary

Current Conditions

brrrr

Large drop in temperature at Marshall, Colorado at ~14:00UTC, which is 7:00am MST. A cold front passage caused the drop and the continuing decrease.

As you can see in the plot above, we experienced a powerful cold front at 7am that dropped temperatures around the metro area 20°F in the course of about thirty minutes. Along with the cold front came some intense winds that are gusting up to 25mph at the airport and are causing wind chill temperatures that hover right around zero. Currently there aren’t a lot of clouds hovering overhead and there aren’t any snowflakes to be be seen… yet.

Storm System Timing

Obviously, we’ve already started down the path of our winter storm with the cold front that passed through earlier this morning. As we continue through the day, our sunny skies will change to that familiar grey that means snow is on its way. Sunset is currently looking to be the time when cloud cover and chance for snowfall really increases. Snowfall around Denver will begin this evening and will continue through tomorrow morning. The frigid temperatures, however, will be sticking around all weekend until we return to highs in the 40s on Monday.

Expected Low Temperatures

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Expected low temperatures across the region on Saturday night

Low temperatures are expected to be between 0°F and -5°F for most of the metro area tonight with wind chills that could reach as low as -20°F. Saturday night is expected to be a few degrees colder, but winds are expected to be lighter which means wind chills should remain roughly the same. At this point in time, it looks like our low temperatures will be a few degrees warmer than the record lows for the metro area but there’s always a possibility that will change. Regardless of whether they’re record-breaking or not, please keep your pets inside, dress in layers, and have an emergency kit in your car with a blanket. It doesn’t pay to gamble with these cold temperatures!

Expected Snow Totals

This storm is shaping up to be similar to the last arctic storm that we had which means that we’re looking at 1-3″ for most areas (I know, it’s not thrilling). Denver and the immediately surrounding cities should expect 1-3″ while areas to the south and west can expect 2-4″. Contrary to popular belief, lower temperatures do not mean more snow. In fact, it’s the opposite. Cold air cannot hold as much moisture as warm air and so we get small accumulations with arctic storm systems. This is also why we get lots of snow from our spring blizzards, the warmer air can hold more moisture and dump it on us!

Regardless of how much snow we get, it could be very icy on the roads tomorrow.

Summary

Snow will start this evening and will continue through Saturday morning with accumulations for most areas around 1-4″. Temperatures will continue to fall over the next two days and many areas will see below-zero temperatures tonight and Saturday night. Wind chills will be extreme and could be as low as -20°F overnight, which means that precautions should be taken if you need to go outside. These severe low temperatures will continue through Sunday, but 40s will return on Monday.

Thanks for reading! Let me know if you have any questions!

-Andrew