Snowflake "Maker" to Improve Forecasts

Take a look at this intriguing article from New Scientist.

Incidentially, it will turn sharply colder Friday night into through the weekend, with a little light snow (less than 1" accumulation) possible early Saturday morning.

Temperatures Saturday won't make it out of the teens, with lows Saturday night as low as -10. Highs Sunday will, again, only be in the teens, but Monday should warm up to near freezing, with a chance of another batch of light snow.

Quick Burst of Snow in Denver Metro

A nice band of snow associated with a frontal feature has allowed snow to develop NW of Denver. A brief period of moderate to heavy snow may bring a quick coating to an inch of snow in some spots.

Temperatures are below freezing, so careful on the evening rush!

Bad Forecast, So I'm Eating Crow

I was on a pretty good forecasting streak until today, but I was off quite a bit on last night's/this morning's supposed snowfall as well as high temperatures today -- along with quite a few other forecasting outlets, all relying on the same computer models. Both forecasts turned out to be bad because of the same reason: westerly, downsloping winds, also known as a Chinook event.

While computer models predicted that winds last night and today would be from the NW, keeping cold air filtering into the region and enough moisture east of foothills to produce snow, instead, the winds were straight out of the west. These westerly winds, which creates upslope flow on the western facing slopes (producing snow) and downslope flow on the eastern facing slopes completely dried out the atmosphere east of The Foothills -- quashing all snowfall.

In addition, downsloping causes something called compressional warming to take place, so my temperature forecast for highs in the upper teens was off by 15+ degrees as most locations saw much milder highs in the mid to upper 30s!

Better luck next time, no?

For the next several days, the weather looks pretty benign, with only slightly below normal temperatures and generally dry weather expected Wednesday through Friday. Afternoon highs each day will be in the 30s, with overnight lows from ten to the mid to upper teens. However, another very strong shot of re-inforcing cold air pushes through the region late Saturday, bringing temperatures back to levels as cold as they were this past Sunday/Monday and providing a good chance for (at least light) snow on Sunday.

Light Snow, Then Cold Eases A Bit

Though I hardly need tell anyone who went outside today, the weather was brutally cold. (Cold enough to freeze, and later burst, the pipes in my 19th century house.) In fact, after a record breaking overnight low of -19 this morning, we didn't make it past +2 degrees today! That broke the previous record-low high temperature for this date of 19 -- by a staggering 17 degrees. It's rare that records are broken by more than several degrees; to break a record by 17 degrees is almost unheard of.

We've got one more bitterly cold night tonight and morning tomorrow -- accompanied by some light snow -- before this Arctic cold eases somewhat. It looks like light snowfall early Tuesday morning may fall just in time to hamper the morning rush hour; luckily, snowfall totals will be rather light. After tomorrow's highs in the upper teens in most places, we "warm" to afternoon highs near 30 Wednesday and Thursday before another reinforcing cold shot drops temperatures at least another 10 degrees for the weekend, along with a chance for a little more light snow.

Detailed Forecast

Monday night:

• Mostly cloudy and bitterly cold
• Light snow develops after 2am
• Overnight lows -10 to zero
• Total accumulation by morning ~ 1"

Tuesday:

• Mostly cloudy with AM snow showers
• "Milder" high temperatures in the upper teens
• Total additional accumulation <1"

(Total accumulation 2am Tuesday through 4pm Tuesday = 1-2")

Overall, this looks like a pretty minor snowfall event, but it may fall at an inopportune time -- to negatively affect Tuesday AM's rush hour; be safe out there and stay warm!

Record Breaking Cold & More Snow Ahead

Temperatures colder than -10 have frozen the pipes at this particular prognosticator's house; others are probably in the same boat (if you are, please leave me a commiseratory/empathetic comment). You may remember that I warned people on this blog (on Saturday) that pipe-freezing was a distinct possibility with an air mass this cold.

Speaking of cold, Denver International Airport (DIA) was down to -19 this morning! That broke an over 100 year record by a significant 5 degrees. That's only 10 degrees short of the ALL TIME record low at DIA of -29. Phew. If you didn't catch this weekend's snow totals, you can view them after today & tomorrow's forecast (below).

Monday 12/15:

• Lots of sun during the first half of the day
• Dangerously cold highs of -2 to +6
  
• Some increasing clouds by late afternooon

Tonight 12/15:

• Mostly cloudy w/ snow developing before midnight
• Light snow will accumulate 1-2" before dawn
• Overnight lows will be near ZERO in most locations

Tuesday 12/16:

• Mostly cloudy and "milder"
• Snow showers during the first half of the day
• Less than 1" new accumulation
• Highs will be near 20

We may make it close to the freezing mark Tuesday & Wednesday, but then a re-enforcing shot of Arctic air sends temperatures south once again by late Thursday into the weekend. I have very little doubt that temperatures will stay at below (or much below) normal levels through the rest of the month of December -- so get used to this cold

Now, re-capping this past weekend's snow totals:

• Boulder - 3.6"
• Arvada - 4"
• Longmont - 3"
• Broomfield - 3.9"
• Denver - 2.3"

Stay warm out there!

Update: Dangerous Cold & Snowfall Report

Last night's Arctic cold front swept in even faster and more ferociously than expected. Temperatures dropped from the low 40s to the mid 20s in about 10 minutes around 11pm Saturday night. By midnight, it was snowing fairly heavily, with temperatures area-wide into the teens.

Believe it or not, I actually subtracted 5-7 degrees from the computer models, thinking they weren't picking up on the magnitude of the extreme cold, but I should have subtracted more like 10 or 15 degrees, because that's about how far off (too warm) they were.

By dawn Sunday most locations were near ZERO degrees, some 10+ degrees colder than the computer models thought it would be. And temperatures are holding steady or even dropping during the afternoon Sunday, meaning that today's daylight highs will be within a few degrees of zero in most locations -- a whopping 60 degrees colder than this same time yesterday.

Last night, I predicted 2-5" of snowfall; here are some official area snowfall totals:

• Boulder - 3.6"
• Arvada - 4"
• Longmont - 3"
• Broomfield - 3.9"
• Denver - 2.3"
  

As for the forecast for the next 24-48 hours: DANGEROUSLY COLD.

Sunday night:

• Becoming mostly clear with calm winds
• Lows will range from -5 to -15

Monday:

• Bitterly cold at wake up, with temperatures around -10
• Mostly sunny skies will "warm" afternoon highs to around 15

Monday night:

• Becoming mostly cloudy; 50/50 shot light snow (1-2"?)
• Overnight lows from ZERO to 5 above

Tuesday:

• Mostly cloudy with slight chance of snow showers
• Not as bitterly cold, with highs ~ 25
  

Dangerous Blast of Cold Brings Snow Wind Chills to 15 Below Zero at Times!

(scroll to the bottom to see the cut-to-the-chase forecast without commentary)

In what may be one of the most dramatic 12-24 hour turn-arounds of the winter, if not the entire year, an Arctic cold front will come barreling through Colorado late Saturday into early Sunday. Behind it's passage, a truly polar air mass, originating from northern Canada -- near the Arctic Circle -- will drop temperatures some 50 degrees, bring life-threatening wind chills and some (though not a lot) of snowfall with it.

Though Sunday will be the coldest day of the coming week, Monday may not make it out of the 20s, and we may stay below freezing for highs through the remainder of the week. In addition to the light snow that will fall Saturday night and Sunday, more light snow is possible late Monday into Tuesday and then again on Thursday. Note that western mountains, especially those in the SW quadrant of the state, will see a whopping 1-3 feet of snow in the next 48 hours.

To give you a taste of the cold to come, and how the dramatic the change will be, most places (east of the Foothills) reached close to 60 degrees on Saturday; by 8am Sunday, temperatures may be more than 50 degrees colder than that -- in the single digits! And afternoon highs on Sunday probably won't make it out of the lower teens. Lows Sunday night, too, will near records and generally range from -10 to zero east of the Foothills and from -10 to -20 over the higher terrain.

To make matters worse, north winds gusting to 30mph late Saturday night into Sunday could push wind chill temperatures could push wind chills to 15 below zero at times. Exposed skin can acquire frost bite in a matter of minutes in these conditions. Pipes may also freeze at these temperatures. I cannot stress enough that this cold blast will be not only dramatic but dangerous.

DETAILED FORECAST:

Saturday Night:

• cold front to push through greater Denver metro area sometime between 11pm Saturday and 3am Sunday; dropping temperatures as much as 25 degrees in a matter of minutes
• light snow will develop shortly after this time
• light snow with falling temperatures and gusty winds to 30mph at times may produce wind chills to 15 below zero at times
• by dawn, temperatures will be in the single digits with 1-2" of snow on the ground

Sunday:

• Light snow with temperatures in the single digits when you wake up
• Light snow continues through afternoon
• Afternoon "highs" probably only in the low teens; snow tapers off by early afternoon
  
• Temperatures will drop back to the single digits by 5pm
• north winds up to 30mph may produce wind chills to ten below at times
• Additional accumulation 1-3"

TOTAL EXPECTED ACCUMULATION

(for Boulder/Denver Metro Area)

TWO to FIVE INCHES*

*lower end of the range as you head east towards Denver; higher end of the range as you head west towards Boulder/the foothills

Sunday night:

• clearing skies as winds die down
• bitterly cold: overnight lows from -10 to zero

I'll update the site on Sunday to give you an idea what the first few days of the work week look like -- in a word, continued very cold. Bundle up!

Coldest Weather of the Winter Next Week?

**Update 12/12 @ 6:30MST**

• Cold still on track
• 1st call snowfall (east of Foothills) = 2-4"
• 1st call snowfall (western mountains) = 1-3 feet
• Full post to follow later today....
  

As I've been alluding to for a couple weeks now, it looks like the middle of December is going to be abnormally cold for Colorado (as well as much of the western half of the nation). While the exact magnitude of the cold is still somewhat unclear, I can tell you that we may have several days (or more) of temperatures 25+ degrees below average.

If the cold gets as bad as I think it very well could, we're probably talking about several days during which high temperatures will only make it to the upper single digits or low teens (!) and overnight lows could drop to -15 -- which would probably be record-breaking.

My gut feeling is that the 7 day period from December 14 through at least December 21 may be one of the coldest stretches this entire winter, if not, quite possibly, the coldest stretch.

While this doesn't look like a pattern that is going to produce a lot of snow east of the Front Range (though light snow is likely this Sunday), the mountains may see quite a lot during this period.

I'll keep you posted....

3-6+" of Snow and Frigid Temperatures

**Updated 9:30pm: This storm is over-performing, which I alluded to might happen below. I measure 4" and counting in Boulder at the moment; if snow continues at this rate for several more hours, then 6"+ is certainly in sight, especially in some isolated locations. Original post below.**
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For once, I'm actually in agreement with the official National Weather Service (NWS) forecast for 2-5." While earlier I thought totals might be on the lower end of the range, it now looks like they're likely to be higher. At 7:30pm in Boulder we already have nearly 2" of snow.

If this forecast busts, I expect it to bust "high" as opposed to "low." Meaning isolated totals of 6 to 8 inches, while unlikely, are not impossible.

You'll wake up to temperatures in the upper teens on Tuesday with highs right around the freezing mark, with some sun possible during the latter part of the day.

Report your snow totals in the comments section!

As I've alluded to recently, I think Dec 10-20 will be cold and snowy here in Colorado and another major snow event may be possible this weekend. Perhaps severe cold to follow it.

Much Colder with Snow Developing

Though temperatures nearly made it to 70 on Sunday, a complex storm system and a powerful cold front will join forces to dramatically change the weather for our area over the next 24 hours.

Morning high temperatures near 50 on Monday will quickly give way to a rain/snow mix by afternoon, with temperatures falling through the 30s.

Much colder temperatures in the low 20s will allow for accumulating snow this evening into the overnight hours. At least a couple inches of snow is likely -- with the potential for more.

The NWS is calling for 2-5" at the moment; I think accumulations may be closer to the lower end of that range, but I'll issue a full forecast later today.

Stay tuned...

Frigid with Periods of Snow Next 24 Hours Generally, Two to Five Inches Expected

**Updated 8am, Thursday December 4**

We're in the midst of the coldest air mass of this winter so far; afternoon highs on Tuesday struggling to make it out of the mid 20s, and we're sitting at 16F at 8am Thursday, as I update this. A weak area of low pressure to our south and west coupled with a sprawling area of Canadian high pressure to our north -- together -- will continue to create decent, if not great, upslope snowfall conditions through early Friday morning.

At the moment, current area snowfall totals range from under 1" (generally south & east of Denver) to nearly 3" (to the north & west of Denver in areas like Boulder). Light to moderate snow is likely to continue, with occasional breaks, through at least late afternoon Thursday. The heaviest snow will likely fall from 8am to 3pm today.

The very cold temperatures at the surface (may well not break the 20 degree mark again until Friday afternoon!!) as well as aloft will create a dry snow with high liquid to snow ratios. One last point: snowfall amounts will vary by location according to where localized bands of heavier snow develop; generally, the further north and west you head from downtown Denver, the better the chance of higher snow totals. I'm fairly confident now that Boulder does better than Denver in terms of snow totals.

DETAILED FORECAST:

Thursday:

• Overcast with periods of light to moderate snow
• Heaviest snow from ~ 8am to 3pm
• Bitter cold afternoon highs only in the teens!
  
• Accumulation: ~ 1-2"

Thursday night:

• Overcast with occasional snow flurries
• Overnight lows 8-12 degrees; coldest of season by 10+ degrees!
  
• Accumulation: under 1/2"

TOTAL EXPECTED ACCUMULATION

(for Boulder/Denver Metro Area)

TWO to FIVE INCHES*

*High country/ski areas in the foothills and further west have the potential to see up to 5-10" total by Friday AM

Note: image above courtesy Accuweather.

Why Weather is So Hard to Predict ...And Why I Love Predicting It

Meteorologists rely heavily on computer models and atmospheric simulations to predict future weather conditions. However, these predictions are only as good as the models' understanding of the underlying physics/thermodynamics (and to a lesser extent, chemistry). And since atmospheric processes are so complicated, and depend on an overwhelming number of separate factors/influences, these models have definite limitations.

While the computer models meteorologists use have improved vastly over the past 20 years, as a 5-day forecast is now as about accurate as a 3-day forecast was in the 1970s, there is still a good deal of error in these forecasts.

Case in point: one of the 2 major US computer models (there are many other national and global models) makes forecasts out 384 hours every 6 hours. This evening's "run," 5 hours ago, predicted that the temperature at 11pm would be 37 degrees. Instead, we're exactly 20 degrees warmer than that: currently it's 57.

Moral of the story? If a five hour forecast is off a whopping 20 degrees, which is an enormous bust, how do we expect these models to accurate portray what's going to happen in 24 or 48 hours, let alone 16 days from now -- which is how far out this model forecasts? That's why a not insignificant basis of my weather forecasts is from the gut/intuition as much it is from computer models.

I would be remiss if I didn't add that forecasting weather in the Boulder/Denver region is especially tricky. In fact, it's probably one of the most difficult areas in the country to forecast for because of (among other regional factors) the huge effect the Rockies have on weather patterns. "Mountain waves," "Chinook winds," and"lee troughs," and many other terms describe some of the many complexities associated with mountain weather prediction.

But all of these complexities and challenges are what make me so enamored with weather and meteorology: I, personally, see weather forecasting a bit like assembling a jigsaw puzzle -- putting lots of individual pieces of information together to form one coherent whole; but one missing piece can seriously alter the picture.

Luckily, as I've heard several times before: what other job position allows you to be wrong 50% of the time and keep your job? Now, I take exception to that "50%," and believe that my forecasts are far better than tossing a coin; that being said, I'll be the first to admit that they are far from perfect. And they never will be. Thank you, chaos theory.

Much Colder Weather on the Way

Don't let Tuesday's mild temperatures in the 60s fool you -- it's just warm air rushing into the area ahead of a powerful cold front which will drop temperatures more than 30 degrees late Tuesday into Wednesday.

In fact, we may see the coldest temperatures of the season yet so far on Wednesday and Thursday when high temperatures probably won't make it above freezing with lows in the teens.

Is this cold going to stick around, though? And what about snow chances? I'll try to address that later today.