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Mountain West Discussion


Chinook
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Snow ratios could really be impactful this weekend. It will be interesting to see as even some of the lower qpf's of "only" 2.5" could be crippling at 12 or 15:1.  Even 10:1 for that matter.  For what it's worth, GFS seems to be factoring at 16 or 17 in spots.  When I lived in the Midwest, ratio forecasts seemed to be a bit more accurate than in Colorado. 

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0z GFS ups total... over 6" QPF from Golden north to Fort Collins. But the southern edge of the storm has been creeping north on nearly every model run. Any farther north and total will start to decrease south of Boulder. Thinking there is a decent chance this ends up being a FOCO to Cheyenne jackpot.

GFS ensemble mean is 4"+ from Boulder to Cheyenne. Which is a 1" bump from the 12z mean over the same area.

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The 00z Euro still has a very significant amount of snow for the area, particularly near and east of Cheyenne. The GFS does have much more for Larimer/Boulder Counties. As for most of Wyoming to the Black Hills, there is a first storm that is happening right now. So the snow totals reflect both storm systems.

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1 hour ago, ValpoVike said:

And the GFS this morning must have been offended by our incredulity and added even more QPF to Larimer.  Over 8" liquid.  :lol:

Yeah, a difference between 2" and 4"+ in perhaps 20 miles doesn't make a ton of sense. Would not take this verbatim.

You know, I was thinking about what might make models better that would incorporate probabilities and "time to event" that would inject some climatology into the craziness of predicting weather 168+ hours out. The NWS does this in their forecasting, ramping up POPs etc as events get closer. I do some stats in my job but have no idea about complex modeling. Does this ring a bell with anyone?

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Forecast discussion from Boulder:

Quote

The main event that has caught eye of weather enthusiasts will
begin late Friday and into Saturday. It is also when the models
begin to have larger disagreements. What models agree upon is that
thunderstorms will develop along a dryline across western Texas
to western Kansas as a surface cyclone depends over southeast
Colorado and northeast New Mexico. Meanwhile, the cut off trough
will move slowly eastward providing ample QG ascent for a long
duration of moderate to heavy snowfall across much of our forecast
area. What the models differ on is how quickly the cut off trough
progresses eastward and how strong the upslope flow is. The GFS
and Canadian are more progressive with the cut off trough and have
stronger easterly winds at 700 and 500 mb meaning there would be
better upslope flow. The ECMWF is slower with the progression of
the trough and has weaker upslope flow. To give an example of the
difference in upslope flow between the models, the 00Z GFS has an
area of almost 60 knots of due easterly 700 mb flow across the
northeastern plains of Colorado at 21Z Saturday while the 00Z ECMWF
has between 25-30 knots of southeasterly flow during that same
time. Considering the GFS has 60 knots of upslope flow screaming
towards the mountains and foothills with strong warm air advection
and QG aloft, it`s not that hard to believe the QPF output near
Boulder is around 1.75 inches between 18Z and 00Z Saturday
evening. The question is, are the 60 knot easterly winds at 700
mb in the GFS realistic? It seems unlikely. But could the upslope
component of the wind be in the 35-45 knot range for a 12-24 hour
period? That is much more likely. What peaks my interest is that
the model that has the lowest ensemble mean for QPF in Denver is
the ECMWF with a mean of 2 inches. That means at this time there
are two main solutions, with one being a more intense but quicker
moving solution (GFS and Canadian) and the other being the less
intense but slower moving solution (ECMWF). With the models
becoming more consistent with their solutions, confidence is
growing quickly that there will be a high QPF output from this
storm perhaps in the 1.5 to 3 inch range for Denver. There is high
confidence that snow ratios will be quite low from a
climatological standpoint across our forecast area. Soundings show
low to mid level lapse rates will be near the moist adiabatic
lapse rate while any isothermal layer that develops would likely
be above -10 degrees Celsius. In addition, the warm ground
temperatures and compaction will aid in lowering snow ratios. It
is possible the average snow ratio may end up in the 8 or 9 to 1
ratio with this storm. Therefore, the messaging of 1 to 3 feet of
snow for much of our forecast area is still on track. Winds will
increase late on Saturday and through Sunday with gusts up to 45
mph possible across the eastern plains near Limon.

 

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4 hours ago, mayjawintastawm said:

Yeah, a difference between 2" and 4"+ in perhaps 20 miles doesn't make a ton of sense. Would not take this verbatim.

You know, I was thinking about what might make models better that would incorporate probabilities and "time to event" that would inject some climatology into the craziness of predicting weather 168+ hours out. The NWS does this in their forecasting, ramping up POPs etc as events get closer. I do some stats in my job but have no idea about complex modeling. Does this ring a bell with anyone?

There are models that do this, but it’s a layer on top of the underlying physical modeling

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Now might be a good time to take a holistic, Zen-like approach (is that possible here?) and let the atmosphere do what it wants to do for a while before locking anything in. 

Spring Front Range storms:

1) Quite uncommonly are mostly rain

2) Almost always overperform in the foothills; Boulder County usually wins

3) Seldom leave anybody out completely.

Namaste. :snowman:

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5 minutes ago, MidlothianWX said:

QPF totals for 0z GFS:

Regular:

Boulder: 7.3"
Denver: 3.9"
FoCo:5.0"

Para:

Boulder: 5.3"
Denver: 4.3"
FoCo: 3.5"

This run seems a bit colder - I would presume this is all snow. Also, the storm keeps getting pushed back - light snow begins early Saturday, heavier stuff moves in late morning.

What am I missing with temps?  The latest tropical tidbits looks like a crazy rainstorm over the Denver area haha

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25 minutes ago, WeatherEmperor said:

What am I missing with temps?  The latest tropical tidbits looks like a crazy rainstorm over the Denver area haha

You might be right. I could be looking at the wrong data on Pivotal. I'll leave the temp discussion to those who are more well versed in local climo.

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