Complex Storm System to Bring Snow to CO But A Challenging, Low Confidence Forecast

In what is turning out to be a challenging and low-confidence forecast, a complex storm system will push through Colorado over the next 24 to 36 hours. Unfortunately, computer models are in relatively poor agreement regarding the exact evolution of events and, as a result, potential snowfall accumulations. Complicating the matter is that there is likely to be a very tight gradient in snowfall accumulations -- and this gradient may lie directly over Boulder County.

Basically, the heaviest snowfall amounts will be centered over the western third to half of the state, and will be somewhat elevation dependent, meaning that the heaviest snowfall accumulations will be over the western mountains, with much lighter amounts expected as one goes east towards Boulder and Denver. This may be an event where 15 miles makes the difference between almost no snow and several inches -- and where another 15 miles makes the difference between 3" and 8". In other words, quite a challenge to forecast!

TOTAL ESTIMATED ACCUMULATIONS:

• downtown Boulder/Denver: dusting to 2"
• western Boulder County: 2-4"
• western mountains/ski slopes: 6-12"+

ROUGH GUESS BOULDER TIMELINE:

• 11am - 5pm: Cold and dry
  
• 5pm - 10pm: Scattered snow showers
  
• 10pm - 4am: Light snow at times (?)
• 4am - 10am Sat: stray flurry possible
  
• 10am - 4pm Sat: shower possible; milder.

DETAILED FORECAST:

Today (11/30)

Expect increasing clouds today with cold temperatures climbing from near 20 at 11am to around freezing by mid-afternoon. A stray flurry cannot be ruled out during the late afternoon hours, but most of the flakes should wait until well after sunset.

Tonight (11/30)

We'll look for overcast skies this evening with scattered light snow or flurries developing during the evening hours. Expect nearly steady temperatures around 30. During the overnight hours a few periods of light snow may bring a dusting to 2" to the area; highest amounts to the west. Temperatures will rise to near freezing.

Saturday (12/1)

Expect overcast skies during the morning hours with a few flurries or sprinkles possible; temperatures will rise to the mid 30s. Moderate snow will continue in the higher elevations and locations to the west of Boulder. Mostly cloudy during the afternoon hours with a shower possible and highs to near 40 degrees.

Pictured below: one computer model's forecast accumulation map. Boulder denoted with black square; note tight west-east gradient to expected snowfall amounts.

Turning Much Colder on Wednesday Cold Weather to Linger Thereafter

Don't let Tuesday's mild high temperatures near 60 fool you into thinking we'll be granted mild weather for the remainder of the week; quite the contrary, in fact: high temperatures may not surpass the freezing mark on Wednesday, with bitterly cold wind chills in the teens. And, for the rest of the week through the weekend high/low temperatures primarily in the 30s/teens will be considerably below-normal for this time of year.

Wednesday

A strong cold front and associated upper-level trough will push through early in the day, accompanied by gusty northwesterly winds of 20-40+mph. The result will be high temperatures only around freezing with wind chill temperatures in the teens much of the day - Brrrrr! Clear and bitterly cold Wednesday night, with overnight lows dropping to the low to mid teens in many places.

Thursday

Temperatures won't feel nearly as frigid on Thursday as they will be on Wednesday (especially due to lighter winds), but they still will be cooler-than-average. I expect mostly sunny skies with highs in the low to mid 40s and overnight lows in the mid to upper teens.

Friday

Turning cloudy and colder again on Friday as a weak area of low pressure approaches from the north and west. The result will be high temperatures only in the low to mid 30s and a slight chance of a few snow showers; overnight lows will fall into the teens.

Weekend Outlook

The outlook for Saturday and Sunday calls for partly to mostly cloudy skies and cold high temperatures in the mid 30s. A few stray snow showers cannot be ruled out either day, though most locations -- especially Boulder Valley -- will likely remain dry.

Snowstorm Wrap Up And A Peek Ahead

I'm happy to say that the strong cold front and associated snowstorm which affected Boulder Tuesday into Wednesday behaved largely as expected. Though temperatures did not get nearly as mild on Tuesday before the cold front's (early) passage, and despite the (welcome) fact that temperatures did not get quite as cold Tuesday night during the snow as I anticipated, my snowfall prediction of 2-5" was right on.

I measured somewhere between 3.5-4" of snow in the backyard of "madKast manor" on Wednesday morning. Somewhat unusually for Boulder, we received almost as much snow as any locale in the region. Snowfall totals were less as one headed south and east from Boulder: Denver International Airport (DIA) only reported about 1.5".



Pictured: the season's first real accumulating snow in Boulder, as seen from the backyard of "madKast manor" on Wednesday morning.

A Look Ahead

For those of you remaining in Boulder, the forecast is for considerably colder-than-normal weather through the remainder of the week into the first half of the weekend.

• Thanksgiving will feature mostly sunny skies with highs near freezing and lows in the teens.
• Friday will feature mostly cloudy skies and a chance of a few snow showers possible; any accumulations will be quite light, with highs ahead near freezing and lows in the low 20s.
• A bit milder on Saturday, with mostly sunny skies and highs near 40.
  
• Milder still on Sunday, with continued mostly sunny skies and highs near the average of 50.

Cold Front Has Pushed Through

The strong cold front I talked about in the previous post has pushed through the area several hours earlier than expected. As such, it is possible that we won't make it much above 40 -- not nearly as mild as I had previously suggested. Temperatures will begin to drop further during the early afternoon hours, accompanied by breezy conditions.

Check out the 24-hr temperature change map; at this time yesterday, we were 40 degrees warmer!


Pictured: 24-hr temperature change map; courtesy Weather.com

Snowfall of 2-5" is still on track.
Estimated start time now between 8-11pm this evening.

Say Hello to Winter...and Some Snow!

Often on the day prior to a cold front's passage strong southerly winds transport warm air into the region "ahead" of the front. That's precisely what happened today, when gusty southwesterly winds to 40mph allowed temperatures to climb to 74 in downtown Boulder, some 20 degrees above normal.

And while high temperatures in the low to mid 50s early in the day Tuesday will reach typical levels for this time of year, a very potent cold front will come barreling through shortly after noon. I expect that temperatures near 55 at noon will drop to the low 40s by late afternoon, low 30s by 6pm, and into the mid 20s by mid-evening! To add insult to injury, gusty ENE winds 20-40mph during the afternoon and early evening will allow for wind chills (perceived temperatures) in the low 20s by 5pm.



Pictured above: forecast temperatures (blue) and wind chills (pink) on Tuesday.

A fairly weak wave of low pressure will ride along the cold front, likely bringing snow showers and perhaps some heavier snow squalls to the region by mid-evening Tuesday -- best guess start time is 7-10pm. Periodic snow showers will continue during the overnight period, with temperatures dropping into the upper teens and wind chills near zero at times.

I expect that by sunrise Wednesday morning we'll be looking at 1-3" of snowfall in most places. Snow showers will become more scattered in nature during the day Wednesday with perhaps an additional inch or two possible; highs will likely not make it out of the mid 20s.

**MOST LIKELY TOTAL SNOWFALL ACCUMULATION = 2-5"**

Pictured: estimated snowfall accumulations for Colorado -- Boulder County in red box; image courtesy Accuweather.com

As a caveat, I should say first that it's known to be very difficult to accurately forecast snowfall totals in Boulder (due to local orographic variations) and also that there is some disagreement between the two major computer models: one is predicting slightly less (1-3"), while the other is suggesting considerably more (5-7"). I think our final totals will end up somewhere in the middle of these two ranges.

Will twitter updates as needed Tuesday. (I'm "joshlarson" on twitter.) Any specific questions, shoot me an e-mail at josh AT madkast DOT com.

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Next Week's Snowstorm Still on Track

I think I have good news both for snow lovers as well as those flying out of Denver for Thanksgiving next week; (I, for one, happen to fit in both of those categories.) While snow is still likely, the storm system will not strengthen to its full potential and will be rather fast-moving. Most likely snowfall totals at this point appear to be in the 3-6" range.

I think that any snow will probably hold off until late Tuesday -- it will most likely begin after dark -- and will like continue during the overnight hours into Wednesday morning. The period of heaviest snow looks to be from about 4am to noon Wednesday (though the timing may change somewhat as we get closer to the event).

Here's to hoping that 3-6" of snow does not cause serious travel delays on Wednesday, though even that (relatively small) amount of snow is likely to cause some travel disruptions. Below is a tentative snowfall accumulation map. Boulder and Denver are both in the 3-6" range.


As I mentioned in the previous post, don't let the rather mild weather today through Monday deceive you: though we may eclipse the 70 degree mark on Monday, Tuesday's highs will drop to the mid 40s and Wednesday's highs may not make it out of the low 20s! And it is possible that after the snow stops, Wednesday night's lows may drop to the single digits. Is winter finally almost upon us?

Snowstorm Potential Next Week

It's looking more and more likely that we'll see a moderate (to perhaps major) accumulating snowstorm in the two days preceding Thanksgiving -- that would be Tuesday, Nov 20 - Wednesday, Nov 21. Confidence is pretty high given that we're talking about a storm system 5-6 days away.

Though since we're still almost a week out, the timing could change by 12-18 hours in either direction, the most likely time period for snowfall would be from mid-day Tuesday through mid-day Wednesday. If snowfall rates (and accumulations) are high enough, I forsee the potential for travel-related delays at DIA.

At the moment, it appears that the most likely first-guess accumulation amounts may be in the 3-6" or 4-8" range; however, I should add that the potential certainly exists for over a foot or more of snow if the storm system really gets cranking.

One other thing that is a near-certainty is how cold it will be during this snowstorm. By far the coldest air of the season thus far will accompany any snow that falls. I expect temperatures to be in the upper 20s when snow starts falling on Tuesday, dropping into the teens during the overnight and then likely not making it out of the low 20s on Wednesday. It's possible that overnight lows Wednesday will be in the upper single digits to low teens.

Here is one of the more accurate forecast models' snowfall prediction for the period of the storm:



More updates as necessary, and enjoy the very mild weather in the mid 60s Friday through Sunday -- definitely the calm before the storm.