FTB 05-31-2016: Scattered Heavy Rainfall Likely In The Southeast Plains

Issue Date: Tuesday, May 31st, 2016
Issue Time: 10:40AM MDT

— A LOW flood threat has been issued for the Urban Corridor, Palmer Ridge, Southeast Plains and Southeast Mountains

This morning’s water vapor image, below, shows many players across the Rocky Mountain states and the central US. Of particular important is the surface low-pressure system located over North Dakota. This is associated with an attendant trough that extends southward into central Colorado. Also associated with this low is a moist cool front that has entered Colorado from the northeast. Meanwhile, a cut-off low continues to drift along in southern Arizona. This feature looks like it is less and less likely to impact our state.

wv_markup

With the above features in mind, we considered the following factors for assessing today’s flood threat:

  • Moisture levels continue to be fairly limited for heavy rainfall with Precipitable Water (PW) values running close to seasonal normal in the 0.5 – 0.7 inch range east of the Divide and lower to the west; PW is expected to stay steady or increase, especially in the Southeast Plains as a weak low-pressure circulation may develop over Southeast Colorado later today. PW’s may reach 1.0-1.2 inches from about La Junta southeastward.
  • The cool front has increased moisture content, but also brought in a stable layer above the surface. Thus, instability will be focused immediately south of the cool front boundary. Roughly along and south of the Palmer Ridge.
  • Dynamics are neutral to slightly favorable for heavy rainfall. The presence of the trough will provide weak positive vorticity advection east of the Divide. However, most of the fuel for heavy rainfall will likely be provided by solar heating.
  • Storm steering winds will be 25mph+ today and storm training is not expected. It will likely be a “one-and-done” type day with heavy rainfall potential. The only exception is in the far southeast portion of the state where a nocturnal low-level jet may continue to fuel storm activity after sunset.
  • Dry air entrainment is expected over southern parts of the Raton Ridge which will likely limit rainfall rates.

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.
FTB_20160531

Zone-Specific Forecasts:

Front Range, Urban Corridor, Northeast Plains, Palmer Ridge, Southeast Plains, Southeast Mountains, Raton Ridge:
Partly sunny early with scattered to widespread showers and thunderstorms developing by early afternoon and lasting into the late evening and early overnight hours in the far southeast. Maximum 1 hour rainfall rates are shown below:

Front Range/Urban Corridor/Northeast Plains: 1.3 inches per hour
Palmer Ridge/Southeast Plains/Southeast Mountains/Raton Ridge: 1.9 inches per hour

Primetime: 1pm to 8pm in northern sections and 1pm to midnight in the Southeast Plains

A Low flood threat has been issued for isolated field and roadway flooding. Mud flows and debris slides are possible over steeper terrain, including fire scar areas.

Central Mountains, Northern Mountains, San Juan Mountains, San Luis Valley, Grand Valley, Northwest Slope, Southwest Slope:
Mostly sunny early then turning partly cloudy. An isolated showers and perhaps a weak thunderstorm is possible over the higher terrain, with dry conditions elsewhere. Max hourly rain rates up to 0.2 inches. Gusty winds up to 50 mph will be possible under the thunderstorm updrafts. Flooding is not expected today.

STP 05-31-2016: Scattered Strong Storms, But Fairly Limited Rainfall

Issue Date: Tuesday, May 31st, 2016
Issue Time: 9:20AM MDT

Memorial Day turned out pleasant for most of the state with ample sunshine and warm, but not hot temperatures. Just like Sunday, a round of thunderstorms developed by early afternoon off the higher terrain east of the Divide and spread into the eastern Plains by later in the afternoon. Closer to the mountains, storms dealt with limited instability and very dry air above the surface. The result was limited rainfall amounts, up to about 0.20 inches, and gusty winds after storm passage. However, as storms progressed farther east, deeper moisture meant more instability and fuel for heavy rainfall and other severe weather. Rainfall amounts up to 1 inch per hour were observed in northeast Colorado, mostly notably Sedgwick, Logan and Baca counties. In addition:

• hail up to 1.5 inches was reported in Kiowa County,
• gusty downdraft winds up to 60mph were reported in Kit Carson, Cheyenne, Logan and Yuma counties,
• a 13-minute long tornado was reported in Logan County, near Peetz, around 2:15PM.

In all, 1 tornado and 5 severe thunderstorm warnings were issued across the Northeast and Southeast Plains. Fortunately, all rainfall fell in an orderly fashion and no flooding reports were received yesterday.

For specific rainfall estimates in your area, please check our STP map below.

STP_20160531

FTO 05-30-2016: Elevated Flood Threat Tuesday, Followed By Prolonged Warm and Dry Period

Issue Date: Monday, May 30th, 2016
Issue Time: 12:45PM MDT

threat_timeline

May has been a cool month for most of Colorado (see our Storm Total Precipitation discussion for 5/30/2016). As the water vapor image shows, below, an amplified pattern with at least two distinct ridge-trough couplets is seen across the vast North Pacific Ocean. At face value, this may seem to suggest that unsettled weather will continue as these features move east in the jet stream’s steering winds. But given that we are about to enter the first week of June, this is never guaranteed! Indeed, it appears that after tomorrow’s passage of a large-scale trough (Event #1) that will deliver widespread showers and thunderstorms, the overall pattern will calm down significantly.

watervapor_20160530

The reason for a calmer pattern will be the development of a stout early summer ridge that is currently in its infancy off the southwest US coast. The map below shows the GFS ensembles’ depiction of the upper atmosphere’s pattern for next Sunday 6/5. Note that by this time, there is essentially unanimous support for a ridge that covers the entire western United States. The implications of this are a prolonged warm and mainly dry period for Colorado starting mid-week last through at least mid-week next week. This does not imply that rainfall will not be found, but simply that it will be generally light and likely favoring high-elevation locations in accordance with Colorado’s summer climatology.

gfs_500mb

Thereafter, guidance is no longer as consistent, with some suggestions of the ridge quickly breaking down and storminess returning east of the Divide. There is enough loose consensus for us to bring in Event #2 starting late next week. However, there is not enough consensus to be able to estimate rainfall amounts, yet.

Event #1: Tuesday, May 31st

Elevated Flood Threat for Southeast Colorado

A cut-off low will propagate eastward, providing favorable dynamics for a widespread rain event for the Palmer Ridge, Southeast Mountains, Raton Ridge, Southeast Plains and parts of the Urban Corridor. A cool front passage will limit instability the farther north one goes, thus lighter rainfall rates will result north of Colorado Springs. In areas to the south, rainfall amounts up to 2 inches will be possible as at least one round of thunderstorms moves through the area during the afternoon and evening hours. An elevated flood threat is warranted for this region. Check out tomorrow’s Flood Threat Bulletin for the most up-to-date discussion and rainfall estimates.


Legend

Event #2: Saturday, June 11th through Monday, June 13th

No Apparent Flood Threat as timing of ridge weakening is uncertain

After a prolonged warm and mainly dry period, there are indications that the upper-level ridge will break down. This will open up the door for northwest flow and disturbances that could generate heavy rainfall mainly east of the Continental Divide. At this time, there is not enough guidance to pinpoint rainfall amounts. However, climatologically speaking, the northeast part of the state, including the Front Range and Urban Corridor, is the most likely target for the threat.

FTB 05-30-2016: Strong storms possible in the east, but pleasant elsewhere

Issue Date: Monday, May 30, 2016
Issue Time: 10:20AM MDT

— A Low flood threat has been issued for the Northeast Plains, Palmer Ridge and Southeast Plains

This morning’s weather map is a bit busier than past days. As the water vapor image below shows, yesterday’s transient ridge is a thing of the past. Today, it is replaced by an elongated, but broken trough stretching from the Canadian border south-southwest into the Baja of California. A closer inspection reveals this trough is actually comprised of two different centers of action: a main one from about Utah northward and a secondary one in the form of a weak cut-off low in southwest Arizona. Several days ago, it appeared these two features would move as one and could provide for a widespread rain across CO. This is no longer the case. Instead, the northern feature will propagate rather swiftly eastward, while the southern feature will more-or-less lose touch with steering winds and may not move much.

wv_markup

The northern feature is the main player today. It will trek eastward accompanied by a surface low centered over western South Dakota and a weak cold front stretching into Colorado’s Northeast Plains. With precipitable water values between 0.8 and 1.0 inch expected in the region by this afternoon,this will be the focal point for a few stronger thunderstorms capable of heavy rainfall, hail up to 1.5 inches and perhaps a tornado along the Kansas border. A Low flood threat has been issued for eastern areas.

For everyone else, expect a similar day to Sunday with isolated to scattered showers and thunderstorms from early afternoon into the evening hours. Just like yesterday, the bigger impact of this action will be gusty winds as a dry lower-atmosphere will help evaporate most of the rainfall before it reaches the ground.

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.
FTB_20160530
Zone-Specific Forecasts:

Northeast Plains, Southeast Plains, Palmer Divide:

Mostly sunny early with scattered thunderstorms developing by early afternoon and lasting into the early evening hours. A few of the strongest storms will be capable of produce heavy, short-term rainfall, hail up to 1.5 inches and wind gusts up to 60mph. An isolated tornado is also possible. Max 1 hour rainfall rates up to 1.7 inches will lead to isolated field and road flooding. A Low flood threat has been posted.

Primetime: 1pm to 8pm

Front Range, Urban Corridor, Raton Ridge:

Sunny early with isolated to scattered showers and a weak thunderstorm developing by early afternoon and lasting into the early evening hours. Hourly rainfall rates up to 0.4 inches are possible, along with gusty winds up to 45 mph. Flooding is not expected.

Primetime: 12pm to 7pm

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

Mostly sunny early then turning partly cloudy. Isolated showers and perhaps a weak thunderstorm are possible mainly over the higher terrain. Max hourly rain rates up to 0.25 inches. Gusty winds up to 50 mph will be possible under the thunderstorm updrafts. Flooding is not expected today.