FTO 09-28-2015: 2016 Snowpack Forecast

Issue Date: 9/28/2015
Issue Time: 4:30PM

*If you are looking for the Flood Threat Outlook, please scroll down to the next post.

Due to the extreme importance of the winter snowpack for our water supply, we decided to end the Flood Threat Bulletin season (our daily product ends on 9/30, while today is the last Flood Threat Outlook) with a special seasonal snowpack forecast for the state. Before we begin to discuss our findings, a disclaimer is in order: this is NOT an official forecast! It is simply our estimate of what we think this winter will bring.

What are we predicting?

One of the most useful variables for water supply planning is the Snow Water Equivalent. But even more specifically, the maximum Snow Water Equivalent (hereafter, SWEmax), which generally occurs from March to early June. Because the SWEmax usually varies, sometimes significantly, across Colorado, we decided to split the state up into seven snowpack “regions”. These regions were established using watershed boundaries. The table below shows these regions as well as the 2 or 3 SNOTEL snow gauges that were used to characterize each basin. We only use gauges with 35+ years of data, in order to ensure a robust signal.

regions_table

What do we use as predictors?

Our experience shows that forecasting streamflow and snowpack several months in advance requires a thorough analysis of large-scale atmospheric and oceanic conditions across the globe. These conditions are quantified through what we refer to as Hydro-Climate Indices (HCIs), which include commonly heard HCIs such as the El Nino-Southern Oscillation (ENSO) index, the Palmer Drought Severity Index (PDSI) and the Arctic Oscillation along with many others. Knowledge of the state of HCIs is important because they exert a large influence on weather patterns even far from their “home region”. For example, ENSO is defined using tropical Pacific SSTs, but can significantly influence precipitation patterns in Indonesia, Brazil and even the southeastern United States. Our experience suggests that ENSO and the PDSI are especially useful for long-range forecasting in Colorado. Thus, we will use these two indices as predictors. In addition, we will use the time rates of change, or tendencies, of these indices. For example, the ENSO tendency is currently strongly positive since El Nino conditions are developing (it would be negative if La Nina conditions are developing). Also, just to clarify, the term “ENSO” implicitly includes within it both El Nino and La Nina events.

Assembling the forecast ingredients

We began by looking at our two predictors. The figure below shows global sea-surface temperature anomalies. The region of importance is shown in the black box. Note the very strongly positive anomalies that characterize the moderate/strong El Nino event that we are currently experiencing. Through an anomalously active subtropical storm track, El Nino typically results in much higher than normal precipitation in Southern California and the southwest U.S. However, its impact on Colorado is limited to the southwest part of our state.

NOAA_OISSTNext, the chart below shows that sea-surface temperatures in the so-called NINO3.4 region (a way of measure the ENSO strength) have been steadily warming over the last 3-6 months. This is important, because unlike ENSO itself, a positive tendency in ENSO is associated with below normal snowpack across most of Colorado. Thus, the combination of a positive ENSO + positive ENSO tendency can result in a cancellation of either signal.

Nino3.4

The next predictor we looked at was PDSI, and its tendency. The maps below show PDSI values across the western United States during April and August. Due to an incredibly wet spring/early summer, most of eastern Colorado has positive PDSI values. Meanwhile, it is clear that the PDSI tendency over the past ~6 months has also been positive across most of the southwest United States and the Rocky Mountain states. For example, look at southern Utah and northern Arizona. Due to land-atmosphere feedbacks, both positive PDSI and positive PDSI tendency suggest above normal snowpack across most of Colorado.

 PDSI_april_august2016 SWEmax Forecast

After building forecast models for each of our seven snowpack regions, we now present our 2016 SWEmax forecast in the map below. We present the results as a percentage of normal.

SWE_forecast_mapThe North Platte and Gunnison basins are expected to see below normal SWEmax, while the South Platte basin is near normal. Meanwhile, above average SWEmax is expected in the Arkansas, Colorado/Yampa/White and Dolores basins.

A few other important issues:

  • Note that the Rio Grande basin does not have a forecast. We could find any significant skill here using our four predictor variables. This is not entirely unexpected given the closed-in nature of the basin, which makes it less amenable to long-range predictions using the HCI methodology.
  • Note that the North Platte and South Platte forecast numbers are italicized. This indicates that those forecasts have low confidence. However, low confidence is better than no confidence (as was the case for the Rio Grande).

FTO: Final FTO of the Season, Two Events in the First Six Days

Issue Date: 9/28/2015
Issue Time: 12:27 PM

**THIS IS THE FINAL FLOOD THREAT OUTLOOK OF THE 2015 SEASON. WE WILL POST AN ADDITIONAL SNOWPACK FORECAST WITHIN THE NEXT COUPLE OF HOURS. STAY TUNED.**

Threat_Timeline_20150928

After 43 Flood Threat Outlooks, we have finally made it to the final FTO, number 44. In what has been a summer of statewide drought-busting, the final FTO throws another “active” week at Colorado before quieting down as mid-October approaches. Both events in this FTO period occur during the first 6 days, with the final 9 days marked by dry conditions. Without further ado, let’s break down the water vapor image below and then jump right into the individual event discussions.
Event #1 is the result of a steady stream of mid-level moisture from the west and cool, upslope flow at the surface. While the mountains, especially northern and central mountains, will see isolated-to-scattered showers/weak thunderstorms, the focus will be on southeastern Colorado along and near the higher terrain. Cool, upslope flow at the surface in the wake of today’s cool front will increase moisture into the region, and a disturbance embedded in the flow aloft will promote the development of showers and thunderstorms. Tuesday will be the main show, with Wednesday beginning the pattern transition into Event #2.

WV_20150928
Event #2 brings an elevated flood threat to Colorado, but with caveats. This storm system is fairly low confidence for being 4-6 days out, a result of both its location over the Pacific (lack of observations), and the time period during which it is occurring (transition between seasons). The forecast below reflects a storm track/evolution that is favorable for a wet period (Friday through Sunday) across Colorado. By Sunday night, the upper-level low will be east of the state, with the upper-level ridge building back across the region. The high pressure ridge will likely remain in charge through the remainder of the FTO time frame, bringing about an extended dry period to much of the western US.

Event #1: Tuesday (09-29-2015) and Wednesday (09-30-2015)

No Apparent Flood Threat

A steady stream of mid-level moisture from the west and cool, upslope flow at the surface will bring about Event #1. The main focus for this event will be southern extents of the Front Range/Urban Corridor, western extent of the Southeast Plains, Raton Ridge, and the Southeast Mountains. Upslope flow behind today’s cool front will transport moisture into the region, promoting the development of showers and thunderstorms. Brief periods of moderate rainfall will bring totals near 1 inch within the area outlined on the map, while isolated-to-scattered showers/weak thunderstorms across the High Country will mainly produce light rainfall. Wednesday (and for that matter, Thursday) will be drier as transition begins into this weekend’s system.

Legend

Event #2: Friday (10-02-2015) through Sunday (10-04-2015)

Elevated Flood Threat, but Storm System is Still Coming into Focus

The upper-level low will move across the region on Friday afternoon into early Sunday, promoting a 2-to-3 day wet period for Colorado. Eastern Colorado will be the main focus for this event as a cool front surges southward, bringing upslope flow and low-level moisture into the area. Mountain peaks will likely see snow on Friday night and Saturday, as the upper-level low brings much cooler temperatures to Colorado. This forecast will need to be watched closely, as the confidence level is fairly low given the above factors.

Legend

STP 09-28-2015: Sunny and Dry with Near-Perfect Conditions to Catch the Lunar Eclipse

Issue Date: Monday, September 28th, 2015
Issue Time: 9:00 AM MDT

Summary:

Another day of sunshine and warm temperatures graced Colorado, allowing for a great end to the weekend. No precipitation fell anywhere across the state, and only a few mid- and high-level clouds were noted. If you were able to get outside between approximately 7 and 9 PM MDT last night, you were treated to quite the lunar show, as mostly clear skies provided a near-perfect look at the night sky. If you were unable to get outside and enjoy the show in real-time, take some time today to look up pictures from your favorite news source.

No flash flooding was reported yesterday.

20150928_STPImage
Storm Total Precip Legend

FTB 09-28-2015: Changes on the Way, Rain Re-Appears in the Forecast

Issue Date: 9/28/2015
Issue Time: 8:35 AM

NO FLOOD THREAT IS FORECAST.

The upper-level ridge has been/will continue to be flattened today by an upper-level disturbance working across the northern US, placing Colorado under west-northwest flow aloft. Embedded in the flow aloft is a fairly moist plume of mid-level moisture; this will increase precipitable water values slightly over those seen below in the IPW graph. The increase in moisture, combined with marginal instability, will provide more cloud cover today, as well as increase the chances for showers/weak thunderstorms over the higher terrain (especially the Northwest Slope, Northern Mountains, Front Range, and Central Mountains).

IPW_09282015

Associated with the disturbance is a cool front that will slowly work its way southward across eastern Colorado today/tonight. The front, combine with sufficient moisture at the surface and aloft, will allow for spotty showers and weak thunderstorms to develop from the Front Range/Southeast mountains eastward this afternoon through the evening, continuing into the overnight hours. Rain heavy enough to cause flash flooding is not expected. Instead, most showers/weak thunderstorms will produce light-to-moderate rainfall. For more details on rain rates and timing, please see the zone-specific forecasts below.

Today’s Flood Threat Map

For more information on today’s flood threat, see the map below (hover over threat areas for more details). For Zone-Specific forecasts, jump below the map.

Flood Threat Legend

Zone-Specific Forecasts

Northwest Slope, Northern Mountains, Front Range, Southeast Mountains, Central Mountains, and San Juan Mountains:

Isolated-to-scattered showers and weak thunderstorms are expected, with the best coverage across the Northwest Slope, Northern Mountains, Front Range, and Central Mountains, and lesser coverage across the San Juan Mountains and Southeast Mountains. Maximum rain rates will be 0.5-0.8 inches/hour, with the heaviest rainfall (relatively speaking) falling across the Front Range/Continental Divide.

Timing: Noon – 10 PM, with a few isolated showers continuing into the overnight/early morning hours.

Urban Corridor, Northeast Plains, Southeast Plains, Palmer Ridge, and Raton Ridge:

Partly-to-mostly sunny early, with clouds increasing through the afternoon and evening as the cool front works its way through the area. Isolated-to-scattered showers/weak thunderstorms are expected, developing from north-to-south in the wake of the cool front. Maximum rain rates will be 0.6-0.9 inches/hour. Extended periods of rainfall will potentially result in 1.2-1.6 inches/3-hour rain rates across the Palmer Ridge, southern Urban Corridor, Raton Ridge, and Southeast Plains.

Timing will break down as follows:

Northeast Plains: 2 PM – 1 AM, with lingering isolated showers into the morning hours tomorrow
Urban Corridor: 2 PM – 1 AM, with lingering isolated showers until 4 AM
Palmer Ridge: 4 PM – 1 AM, with lingering isolated showers into the morning hours tomorrow
Raton Ridge: 5 PM – Midnight, with a few isolated showers lingering until 4 AM
Southeast Plains: 5 PM – 1 AM, with lingering isolated showers into the morning hours tomorrow

Grand Valley, San Luis Valley, and Southwest Slope:

Mostly sunny and warm, with high temperatures similar to yesterday’s readings. A bit more mid- and high-level clouds will encroach on the area today, more so for the Grand Valley and Southwest Slope than the San Luis Valley, providing a little natural shade during the afternoon and evening hours.