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FXUS65 KBOU 270532 AFDBOU

Area Forecast Discussion National Weather Service Denver/Boulder CO 1132 PM MDT Fri Apr 26 2024

KEY MESSAGES

- Cooler on today, with scattered showers continuing.

- Significant snowfall for the mountains and foothills tonight through Saturday night.

- Snow level 6000 to 7500 feet MSL.

- Rain Saturday/Saturday night lower foothills, Urban Corridor, and eastern plains with 1 to 3 inches of rainfall.

UPDATE
Issued at 826 PM MDT Fri Apr 26 2024

A couple severe storms popped up earlier this evening with one inch hail reported in the Denver area. These storms have weaken and precipitation is becoming more stratiform. There may be a brief lull in the rain overnight, but by early Saturday morning widespread moderate to heavy rain and snow is expected across the Front Range. Models continue to trend cooler, so we lowered snow levels a little more, 6000-7500 feet by late Saturday morning.
This should lead to at least 4 to 8 inches of snow over the Palmer Divide and perhaps 5 to 10 for South Park where we just hoisted Winter Weather Advisories. Observations and web cameras are already showing snow along U.S. Highway 285 through South Park.
Even with warm road temperatures, snow is expected to be falling hard enough to overcome the melting rate and produce slippery conditions due to snow and slush on the roadways.

SHORT TERM /Through Saturday night/
Issued at 149 PM MDT Fri Apr 26 2024

A busy day or two lie ahead...

Before discussing the main course, let's focus on the aperitif. We cleared up nicely today east of the Front Range, nudged between the departing surface low over Nebraska, and the incoming trough to our southwest. Relatively warm surface conditions have helped destabilize us enough to tap into a stream of mid-level moisture, which is producing a few lines of high-based showers and even a weak thunderstorm or two this afternoon.

That moisture aloft will see a steady deepening this evening as the upper-level trough begins to approach, with showers and a few thunderstorms becoming more numerous. Some of these could be rather productive in the foothills and urban corridor.

An increase in QG ascent and isentropic lift becomes evident overnight, combining with a steady moistening of the column. This will translate into increasing coverage of mountain/foothill snow, and lower elevation rain along the I-25 corridor.

As the low pressure system shifts northeast across southern Colorado early Saturday, low to mid-level flow will start to carry a more substantial easterly component, cranking up the upslope flow machine and pushing precipitable water values to over 0.60" along the urban corridor and lower foothills per forecast soundings - above the 90th percentile for the date per SPC climatology. Persistent moderate to locally heavy precipitation will result, with a focus on the lee of the Front Range mountains (including foothills) and the urban corridor. The latest guidance has shown some continued cooling trends and, anticipating very efficient evaporative cooling, we'll have no trouble closing in on our wet bulb temperatures. Many of the higher resolution models have shown potential for wet-bulb zero heights between 6,500 and 7,000 ft MSL. With all this in mind, have continued to force the forecast temperatures and snow levels downward. Suspect these may need to be tweaked farther.

The peak intensity of the rain/snow looks on track for Saturday morning, with fairly steady but slightly less intense precipitation continuing throughout the day. Throughout Saturday morning, consistent snowfall rates of ~2"/hr for 6+ hours looks well within the realm of possibilities for the upper foothills and eastern mountains. As such, have upgraded all of the Winter Storm Watches to a Warning, with some minor adjustments to start and end times. Areas above 8,500 ft will see the heaviest accumulations, but impactful accumulations will likely occur below that as well.

An additional aspect of concern will be the potential for impactful accumulations over the Palmer Divide. If temperatures trend even 1-2 degrees cooler for tonight into Saturday, it could certainly have an impact on I-25 travel between Denver and Colorado Springs. Will hold off on highlights for now given lack of confidence, but this will be a case where observational trends will need to be monitored closely. Finally, some flakes could definitely mix in across other portions of the Denver metro and urban corridor, especially later in the day, although we'll be more hard-pressed to get any accumulations.

The upslope flow machine will begin to break down tomorrow evening and particularly by Saturday night, as drier downslope flow begins to fill back is as the trough pushes east. A widespread 2"+ looks quite reasonable for a majority of the I-25 corridor, and 2-3" liquid equivalent for a wide swatch of our foothills. There are still a handful of solutions that depict potential for localized higher totals. Regardless, this is a hefty late April storm, and one that will add a considerable few steps to our already impressive year-to-date precipitation across the Front Range.
Although the event will more closely resemble steady (albeit moderate) stratiform precipitation, there could be some embedded convective elements at times on Saturday, so some minor street flooding/ponding and creek rises will be possible.

LONG TERM /Sunday through Friday/
Issued at 149 PM MDT Fri Apr 26 2024

As the upper level low ejects out of Colorado on Sunday, enough moisture will remain to allow for some lingering afternoon mountain snow showers and rain showers across the plains. There should be enough instability present as models indicate MLCAPE values of 100 J/kg - 800 J/kg present across the forecast area, that thunderstorms can't be ruled out through the evening. Accumulations are expected to be light.

For Monday, weak ridging moves in behind the exiting low bringing drier and warmer conditions. Cross sections indicate a mountain wave signal developing early Monday morning with westerly flow aloft.
Temperatures will be a few degrees above normal reaching the 70s across the plains, high 60s for the foothills, and 50s for the mountains. A shortwave passing to the north will bring some weak PVA into northern Colorado as well as a strengthening jet just to our north. This could bring a chance for some light snow showers and thunderstorms across the northern mountains and a slight chance for thunderstorms to develop across portions of the plains. 700 mb temperatures indicate a cold front will pass through the forecast area Monday night, however, temperatures are still expected to make it back up to above normal highs for Tuesday. Two more disturbances will pass to our north that may bring some weak forcings to the northern portion of the state Tuesday through Thursday. Light snow showers will be possible for the northern mountains late Tuesday into Wednesday and rain showers/thunderstorms across the plains on Wednesday. A second cold front will pass through sometime Wednesday that will drop temperatures back down to more seasonal conditions for Wednesday and Thursday.

Ridging will begin building in over the area Friday through next weekend.

AVIATION /06Z TAFS through 06Z Sunday/
Issued at 1122 PM MDT Fri Apr 26 2024

Persistent rainfall and low cloud decks through the period. MFVR conditions early in the period will deteriorate to IFR shortly (after 08z) and continue through the period. There is a low probability of a rain/snow mix at APA or BJC, which could lower the ceilings/vsbys further. Surface winds will transition from NNW to NE after 08z and continue that way through the rest of the period. Sustained winds in the 8-12kt range can be expected. Once the precipitation settles down by the end of the period, fog may also need to be addressed in the outlook period for DEN (06-12Z Sunday).

BOU WATCHES/WARNINGS/ADVISORIES
Winter Storm Warning until midnight MDT Saturday night for COZ033-034.

Winter Storm Warning from 3 AM Saturday to 6 AM MDT Sunday for COZ035-036.

Winter Weather Advisory until midnight MDT Saturday night for COZ037.

Winter Weather Advisory from 6 AM Saturday to midnight MDT Saturday night for COZ041.

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