Right For The Wrong Reasons

I like to rehash a storm after it departs, compare it to past storms and try to learn something every time something does not go according to plan. By that I mean the models were wrong, I was wrong, the NWS was wrong. But what I am seeing more and more of this year is the errors have been on timing on the heaviest precipitation more than the total amount of precipitation (QPF).

I noticed a couple of times last year and also in 16-17. Throughout the week my concern was that based on the models’ forecast that the bulk of the precip would fall Thursday afternoon until 8 am Friday. Then we would have a swift frontal passage and little more to finish on Friday. The exception was Wolf Creek where it was supposed to be more evenly distributed between the two periods.

I also talked a lot the last couple of days about energy transferring from one storm to another and yesterday talked about the weather party on the Plains. Unfortunately, it is impossible to see this as it is happening (at least for me it is). The surface maps from the NWS are always a few hours old when they post them-you would think there is a better way, but that is what we have to work with. The good news is they are all archived so I can go back to any date and check them out.

Here is 8 am Friday, at this point, we knew the first phase of the storm never materialized as forecasted, the jet streak and moisture never made it far enough north, before moving east to make an impact.

cap15z

Let me set the scene, what you see above you is the cold front and low pressure in Utah approaching Colorado, it is leaning forward, which is called a positive tilting trough. Positively tilted troughs are weakening and increasing in speed. This is opposed to a negatively tilted trough, which is deepening, slowing down and gain strength. Negatively tilted troughs look like they are leaning backward.

In NE Wyoming there is a very strong closed low-pressure system which is attached to a developing low pressure in SE Colorado, you will see the dotted line that connects the two, that is a shortwave trough. Looking back up at the NE Wyoming low you will see a circle around the low, which means it is a closed low. Then you can see outside of that circulation another closed loop of circulation, follow that out one more layer and you will see a third closed loop of circulation, notice that all three lows are enveloped in the third loop? That is the energy being absorbed from one storm to the others.

The next explanation won’t be so long now that you understand the setup. Here is 11 am.

cap18z

The SE Colorado shortwave has become a full-fledged low-pressure system, absorbing the energy from the Utah system headed towards Colorado, but wait it didn’t kill it, it left just a few leftover pieces of energy. Notice the dotted blue line in Utah? That refers to cyclogenesis which means it is trying to become its own storm. Ultimately by around 2 pm it would give up, but while it was trying to make a comeback it was sending those snow squalls our way, a few of them can really bump totals in a hurry.

Here is the 2pm surface map.

cap21z

Now we have a massive system with a double-barrel low already affect the Plains.

Jump forward to right now and you have a massive system affecting the Plains and headed into the Great Lakes.

caplive

In the end, Wolf Creek got 12 inches Purg got 7″, Telluride had 5″ and most of the mid-elevations that I heard from got 3-5″ and 1-2″ with the lower elevations, which was pretty close to what most of us were forecasting, but for the wrong reasons…

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