Grand Junction, Colorado

• Flooding/flash flooding concerns increase Sunday into Monday, especially for areas that have saturated soils from last week`s rain. A Flood Watch remains in effect.
• Forecast river responses have increased, with a number of gages in southwest Colorado expected to climb into at least minor flood stage. River Flood Warnings are now in effect along the La Plata River, and the San Juan River near Pagosa Springs.
• Winter weather highlights have been issued for the San Juans and central Colorado mountains for accumulating snow Sunday morning through Monday evening.

QUIET SATURDAY

Today will see eastern Utah and western Colorado under something of a dry slot, sandwiched between yesterday`s shortwave and the approaching longer wave trough. Dry is a bit of a misnomer, though, as PWATs remain around 150% of normal today, resulting in widespread cloud cover and a few stray orographic showers through the afternoon. The upper level southwesterly jet noses in after 6PM this evening, bringing an increasing chance of showers and a few thunderstorms to areas around the Four Corners. Any activity at this time is expected to be isolated to widely scattered, and trick or treating for much of the Western Slope will be in the dry.

LOWER ELEVATION RAIN, MOUNTAIN SNOW, AND FLOODING POTENTIAL

Precipitation begins to fill in from southwest to northeast after midnight tonight, becoming widespread by daybreak Sunday. All this precipitation is being fueled by a robust AR that is taking a very favorable trajectory up through southern California and setting its sights on the Four Corners region. PWAT anomalies in both the Ensemble ECMWF and Ensemble GFS are on the order of 250-350% of normal for this time of year. This equates to between 0.75 inches and 1.00 inches of precipitable water. For comparison, normal for this time of year is 0.30-0.40 inches of precipitable water. This is an exceptionally juicy airmass, and as is often the case with airmasses of tropical origin like these, it will also be a warm one. Snow levels are expected to hover between 10k and 12k feet through all of Sunday and into Sunday evening, before finally beginning to lower to 7000-9000 feet late Sunday night into Monday. What this means, ultimately, is that a majority of locations will see this precipitation fall as rain, with snow relegated to areas above 10k feet. The San Juans are looking at 1-3 inches of QPE over a 24-36 hour period, with locally higher amounts up to 4 inches possible. Elsewhere, lower elevation valleys could see 0.50-1.00 inches, with higher terrain in the 1-2 inch range during the same period. With already very saturated soils, periods of heavy rain could result in flash flooding and debris flow issues, especially for sensitive areas. Additionally, rivers are expected respond to all of this liquid precipitation, with some of the same areas hit hard last week looking at another round of river flooding. A Flood Watch is in effect for all of southeast Utah and southwest Colorado, with several targeted River Flood Warnings in effect across the same area.

While snow is confined mainly to areas above 10k feet, amounts are nothing to sneeze at. For the San Juans, snow totals by Monday evening are on the order of 12-18 inches, with locally higher amounts up to 2 feet possible. For the central Divide mountains, the Grand Mesa, and the Flat Tops, amounts are less, in the 5-10 inch range, with locally higher amounts around 1 foot possible. As a result, a Winter Storm Warning is in effect for the San Juans, with Winter Weather Advisories in effect for the central Divide mountains, Grand Mesa, and Flat Tops. As snow levels lower late Sunday night into Monday morning, some of this snow will reach pass level. Be prepared for sloppy conditions during the Monday morning commute along US-550, US-50, and along I-70 over Vail Pass.

NEXT WEEK

Behind this system, conditions dry out for a few days as high amplitude ridging moves overhead. We`ll see a slight warm up through the mid-week period, although highs remain 5-10 degrees below normal for early November. Looking further out, a quick moving clipper system has the potential to bring some light showers to the region mid-to-late next week, although model agreement is poor.

Isolated showers and possible t-storms will continue across the region through the aftn and early evening. Current trends support the next wave of showers and thunderstorms building along the CO & UT state line around 06z...before expanding in areal coverage and spreading eastward across western Colorado early Sunday morning. VFR cigs and vsby through the early evening are expected to deteriorate toward MVFR cigs across TAF sites overnight tonight through early Sunday morning. Winds remain generally light and terrain driven through the period.

Deeper moisture develops this evening into early Sunday. This is expected to produce widespread rainfall with stronger rain rates across southeast Utah and southwest Colorado Sunday and Monday, mostly below 10,000 feet. A growing concern lies in the potential for runoff around steep terrain in the region. This could produce rock, mud, and debris flows onto roadways, as well as flash flood-prone burn scars, washes, slot canyons, and arroyos. Current guidance supports 1 to 3 inches of rainfall for far southeast Utah and portions of southwest Colorado, with locally higher amounts close to 4 inches possible. As a result, a Flood Watch has been issued for 5 AM Sunday through 11 AM Monday. Latest forecast guidance has a number of rivers in southwest Colorado climbing above minor flood stage, with the gage at Pagosa Springs now expected to hit Moderate to Major flood stage. As a result, targeted River Flood Warnings along the La Plata and San Juan rivers are in effect.

Outdoor enthusiasts, motorists, and residents of areas recently impacted by flooding are encouraged to remain tuned into the forecast in the coming days.

CO

Winter Weather Advisory from noon Sunday to 6 PM MDT Monday for COZ009-010-012-013.

Winter Storm Warning from 6 AM Sunday to noon MDT Monday for COZ018-019.

Flood Watch from late tonight through Monday morning for COZ019-021>023.

UT

Flood Watch from late tonight through Monday morning for UTZ022-028-029.