Monday, September 15th 2014
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.
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.
Source: 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.
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.
Sunday, September 14th 2014
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.
Source: 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.
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.
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.
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:
Wednesday, September 10th 2014
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.
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!
Source: 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.
Source: 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.)
Source: 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.
Sunday, September 7th 2014
Warm Start to the Week; (Near-) Record Cold Finish?
While all eyes are on the late-week cold shot that Weather5280 team members have been discussing (primarily on Twitter) for the better part of the last week, let’s take a look at the more benign weather expected to start off the week before tackling the late-week cold.
Monday offers us a blend of clouds and sun with a slight chance of scattered late day thunderstorms, though I think most areas will remain dry. High temperatures will be near- or even a couple degrees above-normal for the date -- mostly in the mid-80s. If you’re a warm weather person, bask in Monday’s temperatures as it will all be downhill from there. Expect mostly cloud skies with a stray thundershower Monday night, with comparatively mild overnight lows in the upper 50s.
Tuesday will start to be a day of transition, as mostly westerly flow aloft is expected ahead of the first push of cold air from the north. Count on more clouds than sun with widely scattered thundershowers; like Monday, most spots are liable to remain dry. It will be cooler, too, with highs probably in the mid-to-upper 70s.
The first push of cold air from our aforementioned incoming cold blast will likely come through late Tuesday into early Wednesday for the Denver metro region. This will lead to plenty of cloud cover and a better chance for showers on Wednesday -- especially during the first half of the day -- than either of the two prior days. Models currently have highs in the mid-70s, but I have reason to believe we’ll fall short of this, and probably not escape the 60s for the duration of the day.
Let’s put all of our attention now on the cold shot that’s poised to deliver sharply cooler temperatures to the region by late Wednesday into Thursday -- lasting into the first part of the weekend. As long as a week ago, certain models (chiefly the ECMWF) suggested a deep trough of low pressure aloft would form over the western US, allowing unseasonably cold air from Canada to press into much of the western and central US by late week. Several days later, other models followed suit. As of press time, the ECMWF, Canadian GEM, and GFS/CFS have finally started to get into reasonably good agreement regarding this cold spell.
The CFSv2 forecasts the following temperature departures for the 5-day period of Sept 9-14. Note that the values are in degrees Celsius.
In addition, forecast tools that rely on analogs (or previous patterns that have resembled the one we’re about to enter) suggest that by late Thursday the air on top of us (at 850mb) may be more than 2 standard deviations below normal. Translation into lay-people-speak? Near-record to record cold temperatures are possible by Thursday into Friday as the air aloft is dramatically colder than normal for this time of year.
In short, we’ve got a heck of a cold shot on our hands, though the overall pattern aloft will make it quite transient in nature. The coldest period appears to be late Wednesday into early Saturday, with Thursday and Friday easily the coldest two days of the week.
While it’s a bit too far out to say with confidence, several models also keep a lot of cloud cover and light precipitation lurking around the region during the day Thursday and Friday; (I’ll come back to this in a moment). This will help to keep temperatures even lower than they might otherwise be (especially during the day time).
While models (particularly the hapless GFS) are still trying to get a handle on exactly how cold it will be Thursday and Friday, it appears highly unlikely that temperatures would make it out of the low 50s for highs at this point -- versus average highs for this period near 80 degrees! Should some of the colder model solutions pan out, it is not out of the realm of possibility that Denver metro could be mired in the upper 30s to low 40s for highs either day -- nearly 40 degrees below normal for this time of year.
At this point, I cannot rule out the possibility of frost by Friday morning across Denver metro, though cloud cover, among other things, will be the key determinant in just how cold we get. One of the cooler model solutions (GEM) seen below shows air temperatures in the low-to-mid 30s by Friday morning across Denver metro!
The exact impacts of this upcoming cold shot on growing things, and the agricultural community in general, probably won’t be clear until about mid-week. But at this point, given current modeling, I would say there’s probably ~60% chance of frost across parts of the Denver metro area by Friday morning, with a ~30% chance of a light freeze.
Remember what I said earlier about some models showing light precipitation lurking around the region both Thursday and Friday? Depending on just how much cold air is in place by late Thursday into Friday, it is not entirely out of the question that some locations could see the first wet snowflakes of the season -- particularly >7000ft in elevation. I am not currently calling for snow in Denver proper, but certain nearby locales may have a shot at some flakes during this time period. One model brings light snow awfully close to Denver! We’ll obviously have a much better handle on these chances by mid-week.
Bottom line: Thursday and Friday each offer the potential for near-record or record cold temperatures particularly during the days, but possibly also at night. For Denver, the record-low maxes for Thursday and Friday -- 41 and 40, respectively -- might be in jeopardy, though I think we’ll probably end up a bit milder than those values. The record overnight lows these two days are 32 Thursday and 31 Friday; again, should some of the coldest current model solutions pan out, those values could also be in jeopardy.
For people groaning while reading about the potential for record cold temperatures and perhaps even the season’s first snowflakes for some, particularly at higher elevations, take a deep breath: this cold shot will be very transient in nature; temperatures rebound very quickly by Saturday afternoon -- with highs well into the 60s (if not possibly even the 70s) likely. Sunday appears milder, still, with highs likely well into the 70s.
For people grinning while reading about the potential for record cold and possible snowflakes, while unfortunately for you this cold shot will be transient, we continue to believe that there is higher than normal potential for below average temperatures overall during the September through November period for central Colorado, with moisture during much of this time frame also appearing to be near- or above-normal. In short, we think this late week cold shot may be just a taste of things to come this fall...