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FXUS65 KBOU 241821 AFDBOU

Area Forecast Discussion National Weather Service Denver/Boulder CO 1221 PM MDT Wed Apr 24 2024

KEY MESSAGES

- Slight Risk of severe thunderstorms Thursday, east of a line from Sterling to Akron, including large hail, damaging winds and isolated tornadoes. A marginal risk south and west of this area.

- Cool and unsettled Friday into the weekend, with accumulating moderate snowfall in the mountains, widespread rain showers over the plains

UPDATE
Issued at 1212 PM MDT Wed Apr 24 2024

Not much to change at this hour. Weak convection is starting to develop and should produce scattered light showers over the northern mountains. Outlook for tomorrow is a bit muddled with questions about getting warm air and moisture together somewhere.
Agree with the new SPC outlook that activity over the moisture will mostly be elevated but some severe threat late afternoon/early evening on the edge. This is in line with current thinking, but the tornado threat may not exist after the first convection.

SHORT TERM /Through tonight/
Issued at 330 AM MDT Wed Apr 24 2024

One fairly quiet weather day today before we get more active Thursday through much of the weekend. Satellite and radar shows a few showers lingering in these early morning hours over the far northeast plains, while ridging is starting to take hold with clearing skies building in from the southwest. The ridging and warm advection will bring a bump in temperatures today, with much above normal readings expected. Most of the plains and I-25 Corridor will reach the mid to upper 70s (slightly cooler far east), while mountain areas will enjoy 50s and 60s.

There is slight drying aloft moving into the forecast area, but temperatures are warming and models still show a bump in instability this afternoon. Thus, despite the upper ridging we still expect isolated showers and storms late this afternoon and evening, favoring areas in the mountains and across the plains closer to the Wyoming and Nebraska borders. We can't totally rule out isolated high based showers farther south along a surface convergence zone, but that would be quite weak and only brief sprinkles/very light rain.

For tonight, low level moisture advection begins in earnest across the northeast plains as low level flow turns south/southeast. That will draw in the current airmass in the Texas Panhandle with dewpoints rising into the mid 40s overnight. Stratus and patchy fog expected to develop, with higher ridges being more prone to advection fog.

It will also be breezy to windy at times over the Palmer Divide and eastern plains today and tonight, with gusts of 30-40 mph expected at times. Lighter winds elsewhere.

LONG TERM /Thursday through Tuesday/
Issued at 330 AM MDT Wed Apr 24 2024

On Thursday, a couple severe thunderstorms will be possible over the northeast plains. The first short wave trough will move across CO around 00z Friday, with moderate mid and upper level QG ascent ahead of the trough in the afternoon. At the surface, an west- east oriented warm front will move off the Palmer Divide, with southeast winds north of the front advecting low level moisture in the plains. Fog and/or stratus will be an issue through Thursday morning north fo the front. Model soundings indicate ML 100mb CAPE around 1000-1200 j/kg in the afternoon, with 0-3km storm relative helicities of 150-200 m2/s2. The combination of instability/shear appears to be sufficient to warrant a slight risk of severe thunderstorms, primarily east of a line from around Sterling to Akron. Large hail, damaging wind as well as a tornado or two may be possible. West and south of this line, the risk will be more on the marginal side. Timing wise, it appears that the storms will initiate along the warm front around 21z then move northeast through 03z.

By 12z Friday, the upper low will be over southwest NE, with wrap around moisture associated with northwesterly flow on the west side of the exiting trough, impacting the plains through the day.
There will be enough moisture and instability around for snow showers to in the mountains, and rain showers/isolated tstms over the plains. The best coverage for showers would appear to be to the north and east of Denver on Friday. By Friday night, a short wave ridge will move across the forecast area which may allow for a brief decrease in precipitation intensity overnight but not necessarily coverage as the impact (subsidence) from the ridge will be short lived.

On Saturday, the next trough will move out of the Great Basin and into central CO by 00z Sunday, then exiting into southwest NE/northwest KS by 12z Sunday. Moderate mid/upper level QG ascent will be over northeast CO through the day Saturday, then decrease from west to east Saturday night. In this time, the models generate anywhere from 6-14 inches of snow in the higher mountains. The main window will be from 06z Saturday to 00z Monday. If this trend continues, then either a Winter Storm Watch or Advisory may be issued. Light snow accumulations can also be expected over the Palmer Divide Saturday night into Sunday morning. Across the northeast plains, the pops will start to decrease Sunday afternoon into Sunday night. Total qpf generated by the models will be around 2 inches, with the highest amounts in the mountains, and across the northeast plains towards the WY and NE.

By early next week, the trough will have exited to the east but enough moisture will be available at least in the mountains for a chance of residual showers on Monday, then dry and warmer weather returns on Tuesday.

AVIATION /18Z TAFS through 18Z Thursday/
Issued at 1212 PM MDT Wed Apr 24 2024

VFR through tonight. Isolated showers/storms are expected to stay north of the Denver area, but there is a chance (20%) of shower outflow producing a period of north winds at 10-20 knots for a couple hours in the evening.

BOU WATCHES/WARNINGS/ADVISORIES
None.

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