FINAL FORECAST: May 11-12 (!!) Snow

Keeping it short in the interest of time. This storm system is a dynamic one, with I would imagine still some tricks up its sleve, but here's how I see things unfolding. Note, as little as 500ft in altitude can make the difference in a storm like this from no snow to 1" of snow, or 2" of snow vs 4" of snow, etc. So these predictions are only rough guesses (with relatively low confidence) and your totals will vary accordingly -- possibly substantially

• downtown Denver: coating to an 1"
• Boulder/Longmont: 2-4"
• ~6,000-7,000 ft: 4-10"
• over 7,000ft: 12-24"

The vast majority of the precipitation will have fallen by dawn for the majority of the metro area, and I think there will be little or no additional accumulation during the day for locations under 6500ft.

Finally, my gut feeling is -- were this forecast to bust -- my numbers for Denver/Boulder would be too low as opposed to too high.

UPDATE: Upcoming Precipitation Event a Major Forecast Challenge

This is turning out to be a quite challenging forecast for the upcoming heavy precipitation event from late Tuesday into mid-to-late Wednesday. Why??

1. (Late) spring storms (especially during a rapid weakening of an El Nino period) are notoriously volatile and difficult to predict
2. The upper level energy associated with this storm is still offshore, and until it comes ashore, models will have a hard time pinpointing its characteristics
3. A 50 mile difference in the track of the area of low pressure could make all the difference in the world in terms of precipitation type and amount
4. This could be a case where only 500ft of elevation change means the difference between all/mostly rain & no snow accumulations, and several inches or more of snow accumulation.

In all honesty, color this particular prognosticator a bit flummoxed at this time.

I can say that I believe most locations above ~6,000 to 6,500 ft stand a good chance of seeing at least several inches of snow, and perhaps (much?) more. But downtown Boulder and Denver and points in between below 6,000 ft...anyone's guess.

Scenarios in order of likelihood for these lower elevation locations: 1) a mix of snow and rain with up to several inches of snow accumulation on grassy surfaces overnight early Wednesday; 2) mostly or all rain with little or no snow accumulation; 3) mostly snow with significant accumulations (this last one seems highly unlikely at this point).

For what it's worth -- probably not much -- the official NWS forecast is purposefully vague (an indication of their low confidence as well), calling for "up to 4" of snow, mostly over southern and western suburbs" Tuesday night and the same thing on Wednesday, "up to 4" of snow, mostly over southern and western suburbs."

More to follow...

URGENT: Possibly Historic Late Spring Snowstorm

Evidence is increasing that a -- possibly historic?? -- late spring snow storm is poised to strike central Colorado by late Tuesday of this week (i.e., at the time of writing...about 48 hours out). Storm systems like this are a challenge to forecast given the time of the year and a rapidly weakening El Nino, but I continue to have above average confidence that snow is quite possible even into downtown Denver, and nearly a done deal in locations >6500 ft.

Questions? Email me: jlarson05@gmail.com

*Note: image above shows one of the computer models predicting a deep trough (dip) in the jet stream and the accompanying storm system which is likely to deliver snow to central Colorado.

One Last Snowstorm?!?

Whenever I think we've seen the last snow this winter (1" on the ground on Thursday in Boulder), another potentiality pops up. Such is "spring" in Colorado, especially in an El Nino period. I have above average confidence of the potential for (a possibly major) late-spring snow by Wednesday of this coming week.

It's not a done deal, for sure, but the writing is certainly on the wall.

More to follow...

Another Chance for Snow

It appears one last blast of winter may affect our area from Thursday into the weekend. Though confidence is low at this point, a damaging freeze is possible both Thursday and Friday night's, with a slight (~20%) chance of wet snow early Thursday, and a better (~40-50%) chance of wet snow sometime during the late Friday into early Sunday period.

Ahhh, "spring" in Colorado ;)

I'll have an update with more specificity and higher confidence on Wednesday...

Major Spring Snowstorm to Dump 9-18"

I know it's a bit late to be issuing a final forecast (now that most places already are at least several inches or more into the event), but I've been far too busy to until now.

Now, the past several model runs have printed out the liquid equivalent of well over 10" of snow; however, I'd been very cautious of these amounts even up until this afternoon, because I believed that several hours of heavy snow would fall, but not stick due to plenty of good reasons: it's late March and the sun angle is high(er) and it was in the 60s yesterday so ground temperatures are warm. Also, it made it well into the 40s most places today. Thus, I believed that we would "loose" several inches or more of snow due to it not sticking at the outset; I didn't believe that snow would stick (even to grassy locations) until after dark.

Boy, was I wrong! The snow came on so heavily that it counteracted all of the above possible detriments to it sticking, and started piling up at a ferocious rate by late afternoon. Thus, I now believe that the forecast models suggesting 10-20" may be on to something, though I think totals at the upper end of that range will be isolated.

I have therefore adjusted my forecast up significantly. I now believe

9 to 18 INCHES

Is probable for the Denver metro area. My gut feeling says that the places that received the lowest snowfall accumulations with the last storm (downtown Denver and points south and east), may actually be the ones picking up the most this time around. I'd rate my confidence level on this storm as 6 or 7 on a 1-10 scale.

Note that due to the high water content in with this storm, and the heaviness of the snow, scattered power outages are certainly a possibility. Also, I believe that 2/3 to 3/4 of the entire event's snow will have fallen by sunrise tomorrow, with much lighter snow during the day Wednesday. It wouldn't surprise me if by noon the snow has stopped in many locations.

Be safe out there, and avoid being on the roads at all cost. I traveled from Denver to Boulder from 5:30 to 7:15pm tonight, and saw numerous stranded cars and accidents (3 of the latter in less than a 1/16 mile stretch in between McCaslin Blvd & Table Mesa!) and only one snow plow.

P.S. I hesitate to narrow the range to something this small, since spring snowstorms in CO are so unpredictable, but if I were a betting man, I'd guess that most totals in the area will range from the 10-14" zone.

*Note: image above courtesy Accuweather.com

Final Major Snowstorm of the Season?7-14" of Heavy, Wet Snow Likely

I believe that Denver metro will see 7-14" of wet snow out of this system. As with the last, snow will have a very tough time sticking on paved surfaces, but since the heaviest of this system will fall overnight (as opposed to during the day for the previous event), I certainly wouldn't rule that out.

More details to follow...