18z GFS less widespread heavy snow than 24 hours priorLittle change to the going forecast this evening. Early Sunday morning the National Weather Service issued a Winter Storm Watch for all of northeast Colorado for the potential of 9 - 13" inches of snow Monday night through Tuesday. As of 7:30pm tonight that watch remains in effect, and has not yet been upgraded to a Winter Storm Warning.

Our forecast remains much inline with the NWS on this storm. While again Josh and Greg are the more bullish among us, the Brendan's Weather forecast as of Sunday night calls for 7 - 14" of snow for Denver, with locally higher amounts possible. This storm will bring with it some very cold temperatures for April, and a good dose of strong winds as well.

If you live across far eastern portions of Colorado, please stay tuned to local media and NWS outlets Monday, as the SPC has issued a slight risk for severe storms across the plains tomorrow associated with this storm: http://www.spc.noaa.gov/

You may have also noticed the new "Bust Index". Currently a six, this chart shows the potential for our current forecast to bust high, with lower actual accumulations from this storm. The reason for the uncertainty remains twofold:

1) Still some run to run differences in the exact track of this storm. Consensus is pretty good, but we all know what a few miles difference in the track of our surface low can do to snow totals around here.

2) As Matt has pointed out, there is concern that thunderstorms Monday afternoon could actually "steal" potential moisture from our snow machine, thus limiting totals in the end. In fact, this is one reason the NWS has yet to upgrade our watch to a warning.

Snowfall potential chart prepared by Josh More models coming in over the next few hours, and with our system not due in until later in the day Monday we will still have some time to iron out any last details tomorrow. Please stay tuned as our forecast totals may change, and you will want to stay on top of this potentially significant winter storm.

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18z GFS show >10" for Denver If you are enjoying these springtime temperatures, but aren't quite ready to say goodbye to winter, then today's 12z/18z model suite should be encouraging! There is decent consensus between the models that we will see a cold, wet, and snowy storm across northeast Colorado Monday night and Tuesday.

As Matt mentioned in yesterday's post, there are several things we continue to watch as we get closer to the storm. The eventual track of the surface low and temperatures being the two biggest players at this time.

Today's model runs give me more confidence with regard to our temperatures. It appears as though we will start as rain Monday night, and change over to all snow early Tuesday morning. Temperatures should be cold enough to keep us all snow through the remainder of the storm. In fact, the EURO model suggests snow ratios may be (far?) greater than 10:1 during the height of the storm, something not all that common in April. Despite the potential of these higher ratios, it is important to consider what time of year it is, and how the recent warm weather (Monday we could still hit 60°), and that surface temperature are much warmer than last month.

The track of the surface low has been wobbling run to run and between the different models. Over the last 24 - 36 hours there has certainly been a northerly bias by the models, at times concerningly so. The 18z GFS and NAM both moved a bit more south which is promising. Should today's forecast hold, northern Colorado would do better with this storm than Denver, with snow totals falling off dramatically south of Castle Rock. We will continue to monitor this, as a shift in the surface low track could greatly effect snow totals between neighboring counties. Snowfall probability chart as of Sat April, 6

Not ready to make any bold predictions given we are still several days out, but especially if the higher snow ratios verify, double digit snow totals are certainly not out of the question with this storm. For now hedging on the lower side, and hoping for better confidence one way or another as we head into Sunday. In any case, this storm is great news for our parched state!

We will continue to monitor this developing storm and bring you the very latest through the weekend. Many more model runs to come, so stay tuned!

12z GFS 6 Hour Precip We will become increasingly unsettled the next several days. Lower chances for thundershowers Saturday and Sunday, but then a better shot at rain Monday. Tuesday remains the "Million Dollar Question".

Why? Because it could be very messy, rain and snow statewide, and especially so in the metro area.

Referring to this image, a snapshot of the GFS model for Monday night at 6pm. The ground level part of the system (black L) is just into Kansas. The green shades represent the amounts/intensity of moisture (either rain or snow), and the red lines represent critical locations of temperatures. Here is why those three things are critical to Tuesday:

1) The location of the ground level system, L, will dictate our upslope vs downslope. In this image the L is too far east to give Denver and the Front Range the upslope needed for a heavy system. However, that location changes as this particular model updates (every 6 hours) and is in different locations on different computer models. Some favor A LOT OF UPSLOPE... and heavy snow/rain. The position will change in my further posts and will point out how those changes affect our forecast.

2) In future posts you will see the green shades differ greatly. In this image there is not widespread green (moisture) over the state, just patches here and there. Different models have less, some have A LOT MORE. It is worth pointing out that this afternoon's (18z) model run of the GFS looks quite a bit better than the 12z run. Models will continue to differ run to run as we head into the weekend.

3) Temperatures will be critical in giving the area rain or snow or both. Remember that weekend snow about a month ago that was so warm the snow melted as fast as it fell? Well, this scenario could play out like that one. Rain/Thunderstorm is most likely Monday, then a colder rain or snow for Monday night and early Tuesday.

These three things will be the key to some potentially messy travel early this coming workweek. Since the two most reliable models (ours and the European's) differ greatly on this storm, it is clearly too early to say... just have this system on your "radar" this weekend.

We will continue to post updates as the weekend progresses.

The 1974 Super Outbreak was largest and most violent tornado outbreak of the 20th century, and ranks just second behind the late April outbreak of 2011. In all, there were 30 F4 and F5 tornadoes reported, of 148 that touched down in the 18 hour period. According to the National Weather Service storm report, the outbreak caused 315 fatalities, 6,142 storm-related injuries, and a damage estimate of $600,000,000.

Today marks the anniversary of the April 3-4, 1974 outbreak. I have put together a visualization showing the paths of all 148 tornadoes over these two days. The circles are sized by and styled by the strength of each tornado. You can find the interactive version on my GitHub page, or click the image below.