FTO 05-29-2017: Prolonged Stretch of Elevated Moisture But Flood Threat Uncertain; Snow Melt To Pick Up Speed

Issue Date: Monday, May 29th, 2017
Issue Time: 12:40PM MDT
Valid Dates: 6/30 – 6/13

Today’s North Pacific Ocean water vapor imagery, below, shows that two distinct jet streams (polar and subtropical) spanning all the way into North America’s west coast. Within each of these are embedded large-scale disturbances that will, at various speeds, continue their march eastward. Also of note is a strong upper-level ridge that is centered across the US Pacific Northwest. This is causing much above normal temperatures across WA, OR and ID. The ridge will never make much progress into Colorado over the next 15-days but will occasionally bulge in, causing above normal temperatures especially west of the Continental Divide. This will cause a multi-day period of enhanced snow-melt next weekend, and we have incorporated this by adding a new “Snow melt” category in our timeline above. At this time, base flows are well below flood stage for a majority of streams; thus, we foresee a generally orderly melt-off though there will undoubtedly be localized regions that will need to be monitored for low-grade flooding.

Moving back to the precipitation topic, an unusual amount of uncertainty exists for this forecast period due to the continuous interaction between the two jet streams. Essentially, it will be difficult to ascertain the degree to which disturbances will cut-off from the main flow, which would dictate their speed of travel and thus, the longevity of their precipitation threat. Nonetheless, there is significant amount of certainty that moisture will be in abundance beginning later this week, as shown in the GFS Ensemble plumes for Denver’s precipitable water (PW) forecast, below. The climatological value is about 0.5-0.55 inches for early June. However, a majority of ensembles show above average values, with several showing values greater than 1 inch, for a prolonged stretch. As PW is a strong predictor of heavy rainfall in Colorado, it is likely that at least isolated heavy rainfall will been seen east of the Continental Divide over the next 7-10 days. This chart is indicative of conditions east of the Continental Divide. West of the Continental Divide, moisture is anticipated to be closer to normal, meaning chances of flooding are not as great. Unfortunately, placing the location of heavy rainfall is more of a challenge.

We have identified 3 “Events” that will provide elevated precipitation chances for Colorado over the next 15 days. These are described below.

Event #1: Wednesday (5/31) through Thursday (6/1)

No Anticipated Flood Threat as precipitation chances increase in southwest and eastern Colorado

A shortwave disturbance will affect Colorado from the southwest beginning Wednesday and continuing into Thursday. Precipitation coverage will first increase in the San Juans and Southwest Slope on Wednesday then spread eastward over the Continental Divide on Thursday. On Wednesday, expect localized heavy rainfall with up to 0.5 inches possible in isolated locations of southwest Colorado and up to 0.75 inches in the foothills east of the Continental Divide. On Thursday, up to 1 inch of rainfall will be possible along the entire I-25 corridor, Palmer Ridge and Raton Ridge. At this time we are not providing a precipitation map because the precipitation coverage is not certain, and the heavy rainfall should be quite isolated. Stay tuned to daily Flood Threat Bulletins for more detail on each day.

Event #2: Friday (6/2) and Saturday (6/3)

No Anticipated Flood Threat as daily precipitation likely mainly east of the Continental Divide

Immediately following Event #1, another larger disturbance is expected to make a dent into the Pacific Northwest Ridge and affect Colorado. However, the ridge will continue to hold its ground, making conditions unfavorable for large-scale precipitation. While daily precipitation chances are likely statewide, and especially east of the Continental Divide, there is too much uncertainty to warrant a precipitation map. Isolated daily rainfall amounts up to 1 inch are possible given the instability and moisture. However, the precipitation coverage does not appear to be large enough to support a flood threat at this time.

Event #3: Monday (6/5) through Wednesday (6/7)

No Anticipated Flood Threat due to uncertainty in whether disturbance will make it to Colorado

Unlike Events #1 and 2, the main uncertainty with Event #3 is whether or not it will make it into Colorado. Enough of a consensus exists that supports an increase in precipitation chances first west of the Continental Divide on Monday, then spreading east on Tuesday and Wednesday. Similar to Event #2, it appears that at least a weak ridge will continue to maintain a position over Colorado. Thus, while isolated precipitation is likely, coverage is not certain at this time. The next Flood Threat Outlook (to be issued on Thursday) should provide more confidence on this event.

 

STP 05-29-2017: Seasonably Cool, Mainly Dry Sunday

Issue Date: Monday, May 29th, 2017
Issue Time: 9:50AM MDT

Summary:

After a crystal clear morning statewide, a few scattered clouds along with some showers and an isolated thunderstorms popped up in north central Colorado on Sunday. With cool temperatures and limited moisture, rainfall had a hard time reaching the ground but up to 0.2 inches did manage to fall for a few lucky locations.

For everyone else, it was a seasonably cool Sunday with temperatures maxing out 5-10 degrees below normal.

For precipitation estimates in your area, check out our Precipitation Map below. Flooding was not reported on Sunday.


Storm Total Precip Legend

FTB 05-29-2017: Scattered Afternoon Showers and Storms, But Flooding Not Expected

Issue Date: Monday, May 29th, 2017
Issue Time: 9:30AM MDT

— Flooding is NOT expected today

Happy Memorial Day!

This morning’s water vapor image, below, shows a large trough centered over the northern Great Plains providing Colorado with seasonably cool temperatures but limited moisture for thunderstorm activity. Statewide, this morning’s precipitable water values are in the 0.4 – 0.6 inch range, and a continuation of these levels is expected over the next 24 hours. As we head closer into the summer season, strong insolation can create enough instability even with meager moisture. Such will be the case today. With mainly clear skies this morning, temperatures will warm to near seasonal averages, and enough instability will be generated for scattered showers and weak thunderstorms. Coverage will be highest east of the Continental Divide, where the moisture supply is better. Most locations will see light rainfall, but a few heavier downpours will be likely. However, the duration of the downpours will be short enough to limit runoff. Thus, flooding is not expected today.

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, scroll below the map.

Zone-Specific Forecasts:

Raton Ridge, Southeast Mountains, Southeast Plains, Northeast Plains, Front Range, Urban Corridor, Palmer Ridge:

Mostly clear this morning then partly cloudy with isolated to scattered showers and thunderstorms possible. Highest coverage will be in the foothills and western plains. Max 1-hour rainfall up to 0.7 inches except up to 1.0 inch in the Southeast Plains. Small hail up to 0.5 inches could accompany the stronger cells. Flooding is not expected today.

Primetime: 1PM to 9PM

San Juan Mountains, Southwest Slope, Grand Valley, San Luis Valley, Northwest Slope, Northern Mountains, Central Mountains:

Sunny this morning then becoming partly cloudy with isolated showers and a weak thunderstorm possible mainly in the higher elevations of the Continental Divide. Max 1-hour rainfall up to 0.3 inches. Small hail (up to 0.25 inches) could accompany the stronger cells. Flooding is not expected today.

Primetime: 12PM to 7PM

STP 05-28-2017: Showers And Storms Dotted The State On Saturday

Issue Date: Sunday, May 28th, 2017
Issue Time: 9:45AM MDT

Summary:

Although more stable air settled into most of Colorado on Saturday, showers were still common as a strong disturbance traversed the state. For the most part, 24-hour rainfall accumulations of 0.25 – 0.5 inches were common, mainly east of the Continental Divide. For many, the rainfall occurred with well below normal temperatures, making us forget that June is almost here.

Instability and the potential for heavy rainfall was limited to southeast Colorado, where a lee-side circulation developed and helped advect the necessary moisture. Several rounds of showers and storms moved across the Palmer Ridge, Southeast Plains and Raton Ridge, causing isolated heavy rainfall as well as hail. The most impressive rainfall totals appear to be in Huerfano and Baca Counties where a few locations likely saw over 1.5 inches (highest rain gauge reading was 1.25 inches). Hail of 1.75 inch diameter was reported as a severe thunderstorm (that even briefly carried a tornado warning) developed over Otero County and then trekked southeastward.

For precipitation estimates in your area, check out our Precipitation Map below. Flooding was not reported on Saturday.


Storm Total Precip Legend