Storm Alert - Southwest Colorado Under the Gun This Weekend

Storm Alert - Southwest Colorado Under the Gun This Weekend

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Storm Alert - Southwest Colorado Under the Gun This Weekend

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Colorado Snow Forecast

Welcome to The Barlometer Experience!

Light snow and generally unsettled conditions persist across the high country today, as our region transitions into the next phase of active weather. Snow accumulations over the past 48hrs have been spotty, but fruitful for many. Eagle County was favored (Vail, Beaver Creek), making the best out of some weak passing energy and residual moisture last night. Snow has also been good along and east of the divide (Monarch, Eldora), with some weak low-level forcing (uplsope) and moisture advection out of the southwest.

We’re about 12hrs away from our next period of heavy snowfall, which gives us ample time to analyze model output and zone-in on yet another complex weather scenario for Colorado. The peaks are white and the sun is shining (…sort of), time to set the table.

Short-term Analysis

First and foremost, let’s put this last little bit of energy to rest before discussing the large-scale storm system progged for late this evening. In advance of a stronger system dropping out of the Pacific Northwest, a leading shortwave over Utah is ushering in a little bit of moisture and lift into western Colorado this afternoon – under a weak W/SW flow. Current RAP analysis shows a weak area of positive vorticity (vort max) upstream over Utah. While not overly impressive, this energy combined with daytime heating will drop a few more inches across the central and southern zones this afternoon, before sinking south.

First product today, I have a custom map overlaying (2) different fields. I want to diagnose positive vorticity advection (PVA) via the thermal wind at the 500mb level. This analysis is a good indication of upward vertical motion in the atmosphere – aka lift. The green contours are lines of equal 1000-500mb thickness, known as isopachs. The shaded gradient is absolute vorticity at the 500mb level. Finally, the thermal wind generally follows the flow of the contours. So, I’m looking at regions of higher vorticity and where the thermal wind will advect it to. Showers this afternoon/evening will generally follow this progression.

pva

RAP Analysis – 1000-500mb Thickness + Absolute Vorticity @ 500mb

Light snow accumulations can be expected this afternoon/early evening with the passing energy, generally south of the I-70 corridor. I’m already seeing some drier air working into northern Colorado on the water vapor satellite, which will continue to sink south. So, maybe tr-3″ by this evening, favoring areas south and west of I-70. Snowfall will increase out of the 4-corners later tonight, associated with our next storm feature.

14z HRRR (high-resolution-rapid-refresh model), capturing this afternoon’s light snow. Total snowfall by 10:00pm.

hrrr

14z HRRR – Total Snowfall by 10:00pm, courtesy of NOAA

Things start to get interesting late tonight as an upper-level low trough amplifies and digs into the desert southwest. The feature will close-off over E. California this weekend, establishing deep subtropical moisture advection off the Pacific – very similar to last weekend’s storm. During the same period, northern and southern jets will phase over the southwest, keeping Colorado under moist southwest flow into early next week.

Synoptic Analysis

Let’s have a look at the overall picture this weekend, first focusing on the larger-scale features (synoptic scale). Next map today, I have 500mb geopotential height contours overlaying the infrared satellite (IR) channel this afternoon. The setup looks pretty benign at this point on Friday, but we are very much still in the development stages – jet intensification and a source of Pacific moisture have yet to be established. I’ve highlighted the center of low pressure, currently spinning over the PNW. This feature will settle underneath the western ridge over the next 48hrs, and start to get its act together.

friday synoptic

500mb Geopotential Height + IR Satellite

Snow will intensify over the 4-corners late tonight, spreading northeast into Colorado by Saturday morning as southwest flow deepens. The center of low pressure will stall/wobble Sunday and Monday, pushing jet-driven pulses of deep subtropical moisture over our state into early next week. I don’t think snowfall will be continuous during the whole period, per say, but likely associated with 2-3 stronger pulses. The first queued to arrive Saturday night into Sunday. Though, I’m not entirely sure we’ll even see an appreciable “lull” over some of the higher terrain in the San Juan…Wolf Creek, I’m looking at you.

Moisture is the key player, with unseasonably-high precipitable water (PWAT) and specific humidity values forecast for the southwest. Temperatures will also be warmer, along with lower snow/liquid ratios within the subtropical environment. Periods of valley rain and embedded thunderstorms are not out of the question as well. Below, I’ve toggled the 12z NAM – 700mb relative-humidity analysis, showing the evolution of our storm feature this weekend. Pay attention to the end of the run, as the trough taps Pacific moisture out of Baja. This pulse Sunday evening looks very, very impressive.

700mb rh

700mb Relative Humidity (roughly mountain-top level)

Mesoscale Analysis

Concerning distribution, there is no mystery that this very dynamic system will favor the southern zones. To a lesser extent, I think portions of the central zones will also do very well. Yet once again, good snowfall for the northern mountains remains a concern. Deep southwest flow did not pan-out for the I-70 corridor last weekend, which I expected. But this time around, I’m not so sure…

I believe the initial energy will favor the San Juans – Saturday & Sunday. But this system’s last-gasp Monday and Tuesday could pack a punch that would end up favoring the northern zones as well. This would be a combination of (1) the trough ejecting east and “wrap-around” moisture from the low, and (2) a cold front dropping into Colorado from the north and phasing with southern energy. Under this perceived scenario, winds would veer west/northwest on Monday, turning-on orographics for the typically-favored areas. I’ll keep a keen eye on this one for sure.

Snowfall throughout the San Juans this weekend will be in the 2-3 foot range, which is well-agreed upon by models right now. Silverton, Purgatory, Wolf Creek and Telluride all fall into this category. Recent runs have shifted the swath of heavier snow just a little bit further south into New Mexico. Otherwise, I’d actually be calling for a little bit more. Yet some models remain bullish with higher amounts in the 3 to 5 foot range… I’m not ready to go all-in on that right now, but this storm is definitely a heater. Wolf Creek will win again, making the best of deep southwesterly flow.

Concerning the central zones, weekend snowfall should be in the 1-2 foot range. This would include the Aspen mountains and Elk Range near Crested Butte. I’m seeing (2) main pushes of moisture, Saturday and Sunday afternoons, as the jet and moisture-stream migrate north a little bit. From the Roaring Fork Valley south into the Elks, I like a repeat of last weekend’s performance. Further north towards the I-70 corridor, I also like a repeat of last weekend – a broad swath of 5-9“ through Sunday (Breckenridge, Vail). A few areas may be left out a little bit though. I don’t see much this weekend for Routt County (Steamboat) or portions of the northern divide (Winter Park, Arapahoe Basin). These locations will need to wait until early Monday for their next pulse.

Below, the very latest 12. z GEM. Total accumulated snowfall through early Monday morning. Despite its resolution handicap, I like the GEM’s interpretation right now. Though it may be a bit underdone over the San Juans. Image courtesy of Weatherbell Analytics.

gemsfriday

12z GEM – Total Accumulated Snowfall by Monday Morning

Despite being 3-4 days out, all of the models agree on the final segment focusing on the northern and central zones (while still painting leftovers across the south). This would mainly occur Monday afternoon through Tuesday, but the GFS has showers sticking to the divide well into Wednesday (03/04). The potential exists for additional totals nearing 12″ for the northern and central mountain of Colorado. The UKMET model puts quite a bit more, picking up on some jet-enhancement and a brief upslope component during the event for the divide. I think the key with this evolution will be how the southern trough phases with colder energy from the north. Indications right now favor a good dose of subsequent snowfall.

Last product before I set you free this weekend, I’ve got 500mb geopotential height analysis next Tuesday morning, courtesy of the 12z ECMWF (WMO). Our weekend-weather-maker (now reserved at this point) is shown spinning off Baja, with strong northern influence sliding down the Rockies. Image courtesy of Weatherbell Analytics.

ec

12z ECMWF (WMO) – 500mb Geopotential Height

Well that’s about enough for a Friday afternoon. Check-out my Colorado specific website for regular updates on the Centennial State, and keep a keen eye out on Unofficial Networks for future winter weather analysis for you neighborhood.

It’s going to be a great weekend across the west, go out and find what you’ve been looking for.

Cheers,

Nick Barlow
@Barlometer

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