Chinook

I don't know if this is going to accumulate. At Fort Collins, there has been 0.77" of liquid equivalent since yesterday. There is only a trace of snow right now. I guess I can deal with that. I did plenty of shoveling in February. I'm not sure I want to do more shoveling today or tomorrow.

Why not use math? The 10% tornado area is 113,928 sq mi, the 5% tornado area is 140,381 sq mi, and the 2% tornado area is 168,806 sq mi. The SPC system forecasts a probability that any one point in a certain zone will see a report within 25 miles. That means that each point in space is something of a 25 mi radius storm-report-detector. A 25-mi radius circle takes up (pi)*(25)^2 square miles of area. Aside from using more advanced methods of statistics, let's just apply simple math. The number of 25 mile circles covered by the 10% outlook is 58.02, and likewise, 71.50 circles, and 85.97 circles. Then (58.02)*(0.10)+(71.50)*(0.05)+(85.97)*(0.02) = 11.1 tornadoes. 11 seems like a possible number of tornadoes. Qualitatively, I think it's reasonable to say the 10% risk for a large section of the Ohio Valley/Mississippi may seem too much. Otherwise, using real statistical methods may calculate a different number of expected tornadoes.

Tiny tornado vortex signature (near Albany TX)

we've had two tornado reports here. The current tornado warning is not directly over the velocity couplet (unless the radar is delayed somehow)

Storm-relative velocity shows about 75kt on the right hand side, 35 kt on the left hand side. (same storm as the last post)

This storm near Maryneal, TX has ramped up its rotation in the last couple of scans.

My place is getting 1/4" hail and some lightning right now. We haven't seen hail for a really long time!

As for the 20z HRRR, there is really only one major thunderstorm with a helicity track, and that's 2-4 counties west of Fort Worth. I expected the HRRR to have more of these.

Models have been pretty inconsistent with this storm, which should be starting tonight. The 12z GFS has a QPF of 1.85" for downtown Denver, with 12.3" of snow (Kuchera ratio.) It also has this heavy snow at 33 degrees F, with 20kt winds at the surface and 40 kt winds at 1km above ground. I would imagine that yesterday's winter storm watch for 3-7" and gale-force winds, is pretty reasonable, given the uncertainties. The NWS still could upgrade/downgrade the headlines as needed. As with fall/spring storms, this one may be rain, then transitioning to snow, which brings some uncertainty with it. Blizzard warning for Weld County, east of DIA, Castle Rock --this text valid for Greeley

The SPC has increased the wind risk to 30% and tornado risk to 10% into southern Indiana and Illinois. Some of the convection-allowing models show updraft helicity tracks in IN/IL late at night, well after 00z.

we have a winter storm watch for 3-7" across the Denver CWA, low elevations, 8-16" for over 9000ft.

I guess you can still get a burger to go...

Winter storm watches have been posted for Cheyenne's CWA and portions of Nebraska.

On Tuesday evening will have a dryline near the Pecos River, but perhaps not a lot will happen. The first interesting part of this could be Wednesday evening around Lubbock to Midland/Odessa. The low-level convergence there will be a weak cold front, with 50-55kt winds at 500mb over the region. The models have upper 40's dew points into SE New Mexico, that's why it won't be a dryline.

Today's 12z ECMWF has up to 7.8" (10:1 ratio) at Fort Collins, and 27" (10:1 ratio) in the low mountains northwest of Cheyenne for the next 10 days.

A storm system will develop on Thursday with a fairly low pressure - likely resulting in heavy snow in the Rockies and possibly near Fort Collins and Cheyenne. Edit: most models show heavy snow for east/north Wyoming into South Dakota. Another possibility is severe weather for portions of the Plains. As of yet, the SPC has not shown a 15% contour on its 4-8 day outlook. Quote:

One year ago (March 13th, 2019) an incredible 968mb low formed in southeast Colorado, bringing blizzard conditions and 7.1" of snow to Denver, and 14.0" at Cheyenne. In Fort Collins and Loveland, downsloping winds caused much of the snow to become rain or very compacted snow. That was pretty weird, but Fort Collins/Loveland has had a good share of snow in the past 12 months - March 1, 2019 to today. http://www.greatlakes.salsite.com/blizzard_of_2019_surface/12.gif

There could be a TDS near Ash Flat, AR. It's kind of tough to say, with all radars being far away from that area.

Now the "weird storm" is nearly upon us, as this will be the general setup tomorrow. Not as much snow will fall in New Mexico as previous forecasts had shown. Rain/snow should be fairly light near Denver. The only winter storm watches are for the San Juan Mountains and also central Nebraska.

Here's an interesting slightly new plot type from SPC HREF- Max and Min (highly negative) updraft helicity tracks. As has been discussed, there is a range of possibilities with supercells/ elevated supercells tomorrow. The SPC may move the risk area farther north into parts of Indiana.

The 00z models are in the process of finishing up the forecasts for tomorrow. At this time, I don't think there's a need to extend the slight risk much north of Cincinnati OH. The 00z HRRR may be unrealistic about deep convection near Indianapolis. I'm not sure. Maybe elevated CAPE could produce some hail near Indianapolis.

There is some heavy rain moving into California and Arizona tonight, for lower elevations. Coming up, we will see a turn to stormy weather for the West and northern Plains, including a lot of snow for many areas of Wyoming, Montana, and possibly Nebraska. This whole system should provide some helpful drought relief for areas of California and Arizona. As for the first storm, Friday and Saturday ,the whole situation may skip by the Denver area with light snow amounts.

I am wondering if we should start a new medium-range tornado/severe weather thread, for general purposes. At the current time, there are no days with a slight risk or 15% risk on the extended SPC outlooks. This could change soon, but I'm not sure.

I did a little something different with this loop. It shows the 700mb-400mb relative humidity and SLP, so you can see the moisture streams joining up on the East Coast. http://www.greatlakes.salsite.com/Mar_6_2020_700mb_loop.html

It looks like a 500mb low will be in Arizona and New Mexico, on Friday morning, possibly tracking toward SE Colorado and SW Kansas. We will have to see how the models handle the synoptic details in the next few days.