Denver, Colorado

First Measurable Snowfall Prediction Contest 2014

Posted by @brendansweather

It’s already snowed once, now it’s time to predict when it will snow for real!
I’m not asleep at the wheel per se, but busy, really busy. With that, I narrowly escaped missing our annual first measurable snowfall forecast contest. Not to worry; had last week’s early season cold shot given us our first measurable snow, we would have figured out another way to challenge your forecasting skills!

The contest
This year’s first place prize will be 2 lift tickets to Arapahoe Basin ski area. Awesome! A huge thank you to A-Basin for getting involved and contributing these tickets.

Second place isn’t too shabby either, a $25 gift card to Sports Authority. Third place will be a very cool, and of course very geeky
Weather5280 t-shirt. Not sure what all those symbols mean? Basically, the perfect snowstorm for Denver. You can check out what all the symbols mean here.

The challenge is to forecast which day Denver (DIA) will receive its first
measurable snowfall of the season. This means that 0.1 inches of snow or greater must be recorded at DIA.

The contest will run of 10 days: September 17th to September 27th (weather permitting). Each contestant will be limited to one entry. We’ve included some helpful hints in creating your forecast on the
contest page, but two things to keep in mind: Denver’s average first snowfall is October 19, and we’ve been forecasting a cooler and wetter October than normal.

Contest page:
Predict Denver’s First Measurable Snowfall, 2014

Odds and ends
We’ve never asked you to jump through any hoops to enter in the contest, and the same will be true again this year. That said, please help us get the word out by sharing this contest on Twitter, Facebook, and elsewhere. We sincerely appreciate all of our readers, and we see this as a fun opportunity to get everyone excited for our fast-approaching snow season in Colorado and to say thank you for being awesome!

Good luck and happy forecasting!


Comments on the Southwest: Odile to Bring More Heavy Rain to Arizona, New Mexico

Posted by @brendansweather

A week after Phoenix saw it’s wettest day on record, the southwest is gearing up for another round of heavy rain and flooding from now Hurricane Odile, spinning into the Gulf of California. Current modeling takes the heaviest precipitation southeast of the Phoenix area, but flash flooding concerns across much of Arizona and New Mexico will be heightened for the second half of the week.

Hurricane Odile:
Odile forecast track
Forecast track of hurricane Odile (Weather5280)

Here’s the rainfall forecast from the GFS through 00z Saturday (EURO looks similar), a swath of greatest totals across southeast Arizona and western New Mexico.

GFS forecast OdileSource: WeatherBell Analytics

Is this amount of moisture for the region normal for this time of year? No, but also not unheard of when the eastern Pacific is active. We’ve discussed at several opportunities this summer our expectations for
enhanced eastern Pacific hurricane activity and sure enough, that is panning out. Take a look at the PWAT anomalies forecast for the second half of the week across the southwest, well above normal.

Odile PWAT anomalies

Southern Colorado will also try to pick up some beneficial moisture over the next week. At the present, it appears areas south of the Palmer Divide will have the greatest chance at seeing increased shower activity through the weekend -- but we’ll keep an eye on things across northern Colorado as well. For more, check out this week’s State of the Atmosphere.

The eastern Pacific stays active for the foreseeable future. Behind Odile is another disturbance with a 60% chance for formation in the next 48 hours. Today’s 12z GFS tracks it further west than Odile, but will certainly need watching as it develops.

NHC Pacific ForecastSource: NHC

The State of the Atmosphere: Sunday, September 14, 2014

Posted by @MattMakens247wx

You’ve no doubt felt this weekend’s warm-up. It would be very hard not to notice, coming from that frigid end of the week we had.

Friday and Saturday morning were both within 2-degrees of setting record lows at DIA.  The trace of snow Friday morning did not qualify as a “first snowfall” so officially Denver hasn’t had its first snow yet.

Short term Outlook:
Look at us now, back to average temperatures this weekend. Hold the presses though, we have another shot of colder air to move through tomorrow. A weaker cold front has moved into Wyoming, and will push over northern Colorado the next 24 hours or so. This one will bring only a minor dip to the temperatures, but still a noticeable one. Highs in the 60s and low 70s to start off the week.

Monday temperature forecast DenverSource: WeatherBell Analytics

This system doesn’t pack nearly as much punch as late last week’s cold blast did, and doesn’t have the moisture to work with either. Spotty storms, and some cloud cover are expected with the weak upslope flow on Monday. By Tuesday, this system will have moved out and we are back to another warm-up.

All global and climate models indicate warmer than normal temperatures for us this mid-week.  That is directly in-line with the 500mb height pattern indicating some ridging and warming temperatures. Indicated 500mb heights usually put the Front Range in the middle 80s, and luckily that’s right where ensembles keep the highs for Denver through Thursday. Good agreement is always fantastic.

GEFS Temperature Forecast

Oh, but wait. That good agreement tends to fall apart a bit later this week and the weekend.  By Friday another front will move through, relatively weak again, with a decrease in highs and improved rain chances. The disagreement between a cooler GFS, and a warmer ECM is a minor difference of 5 or so degrees. Regardless, I fall back to my 500mb heights and that does drop our highs back into the 70s for sure. This time around, though, there will be more humidity around for the trough to work with to foster rainfall.  o, rain chances appear, at least now, to be in the 30-40% range to end the workweek and start the weekend.

Long-term Outlook
These next two systems are relatively minor. However, we have our eyes set on the end of September and first two weeks of October for something more significant.

We’ve mentioned that timeframe before… two significant troughs will be moving through the region.  As depicted in the European, the 500mb pattern is anomalously shallow. That is a strong indication that these troughs will drag significantly colder air over the surface and down over Colorado. There is a slight deviation to the strength of both if you look at the CFS projections for the same events.

Source: WeatherBell Analytics

The climate model prefers a less significant cooldown. Although both situations are plausible.  Both do show that we will see colder than average temperatures in the first week of October and then again the second week. The difference is amount of cold air. I’m leaning toward the ECM based on past performance, however we can confirm that with better data as time gets closer.  We’ll keep an eye on the teleconnections to see if the AO, PNA, EPO and WPO give us indications as to the strength of the cold. We’ll have to wait though for that as those forecasts a reliable starting 14 days prior to an event.

Forecast teleconnections

In the meantime, enjoy the relatively warmer than normal pattern the rest of September (aside from some brief cooldowns along the way). This is quite a shift from the early half of the month, which has Denver 5 degrees colder than average to date.

Fall colors update
Last week we wrote about a few favorite places to view fall colors in Colorado. Here’s a collection of photos, dates, and locations to help show you the best areas of Autumn colors:

Interactive Map

Update on Fast-Approaching Cold Front

Posted by @coloradowx

It’s nice to look back on a forecast we put out Sunday, and note that 72 hours later little has changed in our thinking regarding our upcoming sharp, albeit short-lived, cold shot. Forecast confidence has gone up now that the GFS has finally (multiple days later) come into good alignment with the ECMWF and GEM models that were the basis of the ideas we put out in Sunday’s “State of the Atmosphere.” What we do have now, however, is a better idea of timing and also a more nuanced view of possible impacts.

Since there’s a lot to talk about, let’s jump right in. As of press time our Canadian cold front is stretched across Montana, northern Wyoming, South Dakota. It is slowly but surely pressing southward. While today’s highs were well into the 70s, that’s still below normal for this time of year, though it’s much warmer than
most of us will be either Thursday or Friday. I say most, because the timing of the initial push of cold air Thursday is a little uncertain; since the front is pressing southward, northern areas of the Denver metro area will see the coolest highs on Thursday (probably the 50s to low 60s), whereas areas further south may hit the mid-60s, or perhaps even 70°, before the front starts rolling through.

Temperature departure from normal United StatesSource: TWC

At any rate, expect mostly sunny skies to start our Thursday, with temperatures in the upper 40s in most spots. As the day wears on you’ll notice a steady increase in cloud cover, as well as the possibility of scattered thundershowers developing by mid-to-late afternoon. (The best chance for thunderstorms will be be over the east-central and southeast Plains, where a couple storms might briefly reach severe criteria.) In addition, winds will shift from out of the southwest to the northeast, with breezy conditions, especially by early evening. Gusts of 25-30mph may be commonplace as the front rolls through, with a few spots possibly even seeing gusts to 40mph. Our best guess on timing is that front will move through midday for northern Colorado and reach Denver and points south Thursday afternoon and evening; (keep your eye on Twitter for updates.) During this time, temperatures will begin to drop steadily. Though many spots may still be in the 50s by evening rush, by mid-evening, temperatures will be falling through the 40s into the 30s by midnight.

The key period we’re focusing on in terms of possible tangible impacts for Denver metro is after midnight Thursday into the early morning hours Friday. During this time, an upslope flow will develop behind the front, ushering in continued VERY cold air from the north, and supporting scattered showers that may transition from rain to wet snow by dawn Friday. Models are clustered in the 32-36° range for lows across Denver metro by Friday morning, so frost might be an issue in some locales, though cloud cover may limit impacts. Note that by late evening Thursday into Friday morning, northeast winds of 15-25mph will be commonplace, with occasional higher gusts; this will produce calendar-challenging wind chills in the mid-to-upper 20s during sustained winds, with values potentially even cooler than that during gusts. Bundle up!


GFS wind chill forecast ColoradoSource: WeatherBell Analytics

As most of you want to know about the “S” word, I’ll reiterate that our thoughts on that haven’t changed much from a few days ago either. As mentioned above, as temperatures drop into the mid-30s by the overnight period Thursday into Friday, scattered showers may mix with or change to wet snow by dawn. Pockets of drizzle, or potentially even isolated pockets of freezing drizzle, may also appear in spots. Snow is not a sure bet for Denver, though we feel fairly confident some folks in the Denver metro area will be reporting flurries and wet snow showers by early Friday morning. We agree with what NWS Boulder put in their most recent forecast discussion: “If snow does fall in the [Denver] metro area, accumulations will likely be very light and melt quickly.” If you live over 6,000-6,500ft, you probably have the best chance at seeing flakes. Those hoping for accumulating snow versus conversational flakes may have to wait a few more weeks...or possibly longer.

Denver snowfall forecast September 12 2014Source: WeatherBell Analytics

Scattered showers of rain or wet snow may linger in spots into the mid-to-late morning Friday, as temperatures hover in the mid-to-upper 30s in most areas. (For point of reference, the average high/low for Denver on Friday is 80°/50°.) By about noon Friday most precipitation will have come to an end. Decreasing clouds will probably bump afternoon highs into the mid-to-upper 40s for most areas; we are highly skeptical that the mid-50s indicated by models will come to pass as we’re under the influence of a strong area of high pressure continuing to displace near-record cold air across the area. (Below: GFS shows temperatures 30-40° normal across the metro area by late morning Friday.)

GFS two meter temperature anomaly Denver, ColoradoSource: WeatherBell Analytics

By Friday evening winds will have diminished considerably, and we may even see some partial clearing. Temperatures will drop to the mid-to-upper 30s across most spots by early Saturday morning, and these slightly warmer values than Friday morning mean most places will escape frost this time around.

As we indicated in the
weekend’s “State of the Atmosphere,” because of the overall pattern aloft, the cold we see late Thursday into Friday will be very transient in nature. That means that with partly to mostly sunny skies, temperatures on Saturday could rebound well into the 70s! Overnight lows Saturday night will drop to near-normal seasonal levels -- around 50° for most folks.

Sunday looks like a true winner of a day, with mostly sunny skies and temperatures in the mid-to-upper 70s. We would not be surprised if some spots hit 80°! The outlook for early next week is for temperatures staying at slightly below-normal levels for this time of the year, with highs mostly in the mid-to-upper 70s, and overnight lows primarily in the 50s.