The next several days are going to be unseasonably cool and active across the Front Range. We discuss the outlook for large hail and tornadoes Friday afternoon, accumulating snow in the Mountains, the potential for flooding, and the gloomy weather sticking around for the weekend. Yes…all of that!
O
ur focus for today’s forecast discussion is an unseasonably cold and strong trough of low pressure which has dropped southward out of Canada into the northern Rockies. There are actually two distinct lows in play right now… the main storm across central Montana, and a secondary system in Oregon. This second storm will gain strength and slowly rotate southeastward into Colorado Saturday night. This will make for a rather chilly and soggy next few days across the Front Range.
GFS 500 mb height anomaly forecast for Friday morning. A potent trough is impacting the northern Rockies
Our immediate concern is the likelihood of severe thunderstorms developing near and just east of our area Friday afternoon. If you hadn’t noticed, a weak cold front moved through last night with winds turning northeasterly across the Plains at the surface. The frontal boundary will act to focus storm development Friday afternoon and evening from Denver to the Kansas state-line.
NAM CAPE forecast for Friday afternoon. A narrow band of concentrated CAPE exists along the cold front
Moderate vertical shear is definitely in-place today thanks to the brisk southwest flow aloft from the trough and the upslope winds at the surface. Our only hesitation is whether cloud cover that is currently blanketing the area will dissipate enough to realize the generous amounts of CAPE models are indicating. As of 10 AM, Boulder is currently socked into the clouds pretty good, while areas just east are basking in sunshine.
As a result, we think that areas east of I-25 and south of I-70 will stand the best chance of seeing strong to severe storms develop by early to middle afternoon on Friday. The HRRR model is showing numerous discrete supercells forming between Denver and the Kansas border, with the primary threat being golf ball sized hail, but it’s possible a few weak tornadoes will drop as well. It’s a decent day to storm chase, especially considering the close proximity of development to I-70.
HRRR model simulated radar reflectivity for Friday at 5:00 PM.
Due to the cloud cover and drier air, further west around Boulder (and possibly even Denver), storms will probably not reach severe levels, but definitely it will be quite a rainy Friday afternoon, evening and night. We usually don’t have to worry about the jet stream in late June, but alas, here we are. Wrapping around the base of the deep trough is the polar jet stream.
GFS 250 mb wind forecast. Is that the polar jet stream surging southward into Colorado in June?
The embedded jet streaks will rotate northeastward across the state Friday and Friday night. This will help to concentrate lift over northern and northwestern Colorado into Saturday morning. Banded precipitation is expected as a result in these areas. In fact, one look at the current radar animation and it’s clear lift from the jet is already dumping on northwestern Colorado.
The trough is not really moving at all, so these jet streaks will move over the same areas for an extended period of time producing widespread precipitation across the Mountains and the lower elevations north and west of Denver. Considering the remaining snowpack and potential for moderate to heavy precipitation in the next 24 hours, it’s a little surprising that the National Weather Service has not posted any highlights for the flood potential in the High Country. We definitely see this being an issue today and tonight.
As you can see below, the GFS model is predicting more than 1″ of precipitation by Saturday morning across a large portion of northwest Colorado. Higher-resolution models bring some of this heavier precipitation further south and east to include Boulder, Broomfield and Longmont. In definitely seems Boulder stands a good chance to be very soggy today and tonight, with possibly more than 1″ of rainfall.
GFS total accumulated precipitation forecast through Saturday morning
Notice that we are careful to say PRECIPITATION for the Mountains, and not RAINFALL. Yes, the cold air associated with the trough is bringing SNOW levels down quite low for late June. A BoulderCAST reader from Bozeman (Montana) sent us photos of accumulating snow yesterday morning in the city when snow levels dropped to near 5000 feet elevation. Bozeman is similar to Boulder in many ways (geographically, meteorologically, and demographically), but it won’t be getting cold enough in our neck of the woods for snow in Boulder this time!
NAM mode-derived snow level forecast for Friday night
Those jet-forced precipitation bands will be all snow across the highest elevations! Both the GFS and NAM models have snow levels falling down to near or slightly below 9000 feet. Mountain peaks above treeline could see anywhere from 4 to 10″ of new snow, with locally higher amounts possible due to the banded nature of the precipitation.
HRRR total snowfall accumulation through Saturday morning. The “snow” being shown across the Plains is actually hail.
Is that 5″ of fresh snow on the morning of the summer solstice in Steamboat? You bet!
There will likely be a brief period of drying across our region Saturday morning into early afternoon before yet another round of widespread showers and storms impacts the area again. The severe threat will be reduced on Saturday, but not completely gone. With a similar shear setup in the atmosphere, any areas that see peaks of sunshine could see storms turn severe. This is most likely to happen across southeastern Colorado, with cloud cover holding over the Denver area on Saturday. Showers and a few storms will continue into Saturday night or early Sunday morning.
High temperatures Saturday will be around 60 degrees in what can only be considered a wash-out. Sunday, on the other hand, will be drier if recent model trends pan out with temperatures warmer in the 60’s, but still well below normal. Make those outdoor plans accordingly.
Finally, if you missed our post from yesterday, we discussed our projection for the annual monsoon arriving late to the party this year, and the implications this will have on extending severe weather season for the Front Range:
Be safe, protect your garden from the hail, and enjoy the first weekend of summer…it certainly won’t feel like it!
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