Update on Sunday 4/9

First of all, I should mention that one of our veteran members, Squall Line, was able to intercept a tornado today in West-central Iowa - nice job Squall Line. I nominate Squall Line for the 1st ever Chaser of the Month award, though he's got a bit of a home-court advantage...!

Now that I owe Squall Line a burrito because I lost our bet of under 2.5 tornado reports before dark, I want to point out something. I think severe weather forecasting has come a long way even within the past few years. For example, I was watching the weather channel today (which, if you hold an atmospheric degree, is illegal in some states), and the severe weather expert Greg Forbes, who was a grad students for the legend Ted Fujita, was pointing out how well SPC's "significant tornado parameter" coincided with the 3 or 4 supercells occuring around 8pm in Iowa and Nebraska: see image below. This location was under the gun from about 2 days out, and I would argue that it wouldn't have drawn a moderate risk based on the tools we had 5 years ago. Sure, some improvement is merely from increasing resolution, but that cannot explain all of it.

Anyways, enough with the flattery, we've got an impressive setup for severe weather tomorrow close to the good ol' Badger State. However, as always, there's uncertainty ... For tomorrow, in my mind, there are two uncertainties:

1. Evolution of nighttime convection, and it's impact (through mid & high-level cloud "debris") on surface heating tomorrow.
2. Whether it's worth chasing a potential tornado when you're dealing with the Mississippi River, hilly terrain and ~50mph storm motion.

My Go Zone forecast is below and here's the explanation for it: the surface cyclone, currently fairly broad will consolidate into one center by later tonight. The supercells and other small convective clusters look to merge into an MCS-looking blob and carry on north and east through the night. The GFS looks like it's suffering from a case of an underactive convective parameterization scheme and generates splotchy precip covering Iowa, Minn and Wisconsin for the next 18 hours straight. I think this model's out to lunch - I like the NAM a lot more.

At the surface, the cyclone's triple point will move northeast with the mean flow and will be positioned near Willmar, Mn at 18Z and Hayward, Wi at 00Z. Looks like convective initiation will be around 20Z west of the Mississippi and the storms may be discrete all the way down the cold front due to the high sfc-500mb shear and some veering even on the front itself. However, the best shot for discrete cells may be closer to the triple point, like today, which is why I'm thinking of going a bit north towards the warm front and backing surface winds.

Now it's time to figure out whether its worth braving the trees, hills and river crossings to chase...

Chasing update Saturday and Sunday

The amount of uncertainty for the severe weather threat on Saturday and Sunday seems to finally be subsiding, but there are still issues.

Let's start with Saturday, the map below shows a few key features that will dictate the Go Zone at 00Z Sunday (Saturday evening). First, let me state that we're only considering conditions during normal business hours (i.e. before sunset). The impressive trough currently in the west will crawl east so that by 00Z the best forcing is just coming off the Front range. There's also a decent upper-level ridge positioned just east of the Mississippi. The sfc low is around Sterling, CO with a diffuse occluded dryline/warm front boundary extending towards Hastings, NE where the triple point is positioned. My concerns are:

1) Forcing too far away to initiate storms before sunset (or even twilight, ahem, Squall Line).

2) Models have large discrepancy in precip output. In fact, only two models generate precip before 00Z: the GFS and the NMM. The NMM's precip is in areas with dewpoint depressions of 10 C and higher, making this a good shot for isolated, great looking supercells, but not a good tornadic threat. The GFS's precip output basically summarizes my best guess for a pre-sunset Go Zone. Initiation would be ~6pm with any storms that do go quickly becoming tornadic. However, the chance of precip, based on the GFS ensembles and SREF is <25%, making me guesstimate a pre-sunset tornado threat of 2-5%. My over/under for daytime tornadoes Saturday is 1.5.

Now moving on to Sunday afternoon: much more interesting the past few runs. All models have been slowly the progression of the low, now the position at 00Z Monday is near Albert Lea, MN. There is a high probability of elevated nighttime convection Saturday night, which may actually provide a few strong tornadoes initially as the cells are still surface-based, say 9-11pm in western Iowa? This elevated convection will very slowly crawl east because the upper-wave is moving NNE making it difficult to sustain any MCS that move towards the east. South of the residual cloud cover from the morning MCS should see ample heating, with a nicely veering wind profile, good speed shear, good BL moisture and forcing coming into the area by 00Z. Initiation of discrete supercells should be earlier than Sunday, say 4pm with my Go Zone being MSP south to Tripoli, Iowa. Because the frontal boundary will be crawling along, we may see a prolonged discrete threat. Over-under for daytime tornadoes is 15.

4/9 & 4/10 Severe Weather Update ***Updated***

Updates coming this afternoon once ARW and NMM are available...

Alright since Dima did a pretty good layout of the forecast based on the 12z runs this morning I'll focus on a few other areas we may have over looked. How and why has the entire system slowed down over the past 3 days in the models and now the evolution goes from a positively tilted trough off in Cali to a negatively tilted trough in MN?

First of all since there are no observations off the west coast the model does not know the strength or location of the subtropical jet. Once it moves on shore where radiosondes are launched a much different evolution occurs. Due to the location of the upper trough from the mid latitudes this allows for the angular momentum to be reduced allowing for the subtropical jet to be further north (Due to the angular momentum typically out flow from tropical convection struggles to make it past ~20N). This allows the unstable tropical air to get wrapped up in the upper wave producing a much higher tropopause! What are the implications of this higher tropopause? Simple really, greater instability! Lets look at some observation and forecast soundings...

First 12z from this morning

The air above 200mb is the first we see of this tropical air being included into the wave. Below is another look at it from a forecast sounding 6hrs later in the same region.

It appears the tropopause begins at 250 but then the thermal profile suggest there is something else present...
Here is what a cross section through the area would look like...

Once these two jet structures "phase together" (for lack of a better term) the tropopause is consequently higher, moving the EL closer to 150mb. All the while the subtropical jet has been destabilizing the air out ahead of the upper wave at 500mb, setting the stage for 2,500+ J/kg of CAPE, seen below.

Then, as a result of this destabilization convection occurs, transportating low PV air near the surface upward and eastward, enhancing the jet along the ridge, amplifying it, enhancing the dynamics and the entire upper wave in just 48hrs goes from this...

to this...

Pretty crazy? And now the jet structure has a greater depth, with the subtropical and polar jet vertically stacked, enhancing the upper level dynamics and forcing...Sounds like a moderate and potentially major tornadic outbreak set up to this guy.

Did I say, "Can't Wait!"?

Convergence

This is getting ridiculous, SPC storm reports

Hey folks, I was just going back to look at the monster wind outbreak from Monday across the Gulf Coast and Southern MS/OH Rivery Valleys and noticed how the total number of reports has swelled to 1444!!!! That's toally insane!!! However I also would like to point out how absolutely disgusting the duplicate reports and (should be) non-reports were. Below the map you can see just a simple taste of the last 4 tornado reports. Three of them are the exact same and the final one should not even be a report as its for a "possible funnel cloud"

"0620 9 NW EASTMAN DODGE GA 3229 8329 AT 1992 FRIENDSHIP UMC RD A DOUBLE WIDE TRAILER WAS DESTROYED. 1 OAK TREE UPROOTED. WINDS ESTIMATED AT 130MPH. 1 FATALITY AND 2 INJURIES. TORNADO CLASSIFIED AS AN EF2. (FFC)

0637 3 SE BAILEYS PARK DODGE GA 3228 8332 *** 1 FATAL, 2 INJ *** AT 1992 FRIENDSHIP UMC RD A DOUBLE WIDE TRAILER WAS DESTROYED AROUND 237 AM EDT. 1 OAK TREE UPROOTED. WINDS ESTIMATED AT 130 MPH. 1 FATALITY AND (FFC)

0640 9 NW EASTMAN DODGE GA 3229 8329 *** 1 FATAL, 2 INJ *** AT 1992 FRIENDSHIP UMC RD A DOUBLE WIDE TRAILER WAS DESTROYED. 1 OAK TREE UPROOTED. WINDS ESTIMATED AT 130 MPH. 1 FATALITY AND 2 INJURIES. TORNAD (FFC)

0720 5 NE TALLAHASSEE COML A LEON FL 3060 8431 FUNNEL CLOUD REPORTED ... NOT ON GROUND. 2000 N MERIDIAN RD (TAE)"

We all know that these storm report totals are preliminary and we also all know how bloated these will tend to be on any given event especially with sheriffnadoes ans what not, but this is just ridiculous. Getting slightly annoyed I went to search around the SPC webpage and very quickly came across this statement on the main page:

Note: On March 8, 2011, the SPC removed space/time filtering on incoming National Weather Service (NWS) Local Storm Reports (LSRs). This filtering had been used by SPC in an attempt to reduce duplicate reports and limit artificially inflated initial estimates of severe weather events when many reports arrived for the same event. Space/time filtering is no longer being applied to decoded NWS LSRs and this approach is consistent with NWS storm-based verification methods. However, identical reports should still be removed from SPC logs and should not appear on the preliminary maps and lists.

Now much more annoyed......I take a deep breath put together this post as I wait for the 12z products to come in and wonder how this data gets used for Severe Weather Storm Climatology products (Yes, it really does eventually).

Active End of the Week ***Updated***

Forecast sounding just west of Cedar Rapids 0z Sunday.

Alright, TON of uncertainty leading up to the severe weather Thursday-Sunday...

Starting with Saturday a very strong jet will be located over the southern plains with an impressive 120 knot jet. When looking long range, a good indicator for severe weather, as I have noticed for the past couple of years is where the winds change direction coming out of the trough. This has little proof, but holds some merit, blow in the highlighted region is where this wind shift occurs.  Often times this is where the upper level support is maximized with upward vertical motion through the jet axis due to the 4 quadrant jet model and curvature.

The GFS has sped up the entire pattern, which has decreased my confidence. The biggest question mark and least confidence I have is how far north the 60+ surface dewpoints reach. As was the case this past week the models way over did the surface dew points resulting in mostly a hail and wind show. Below are the sfc dew points valid at 0z Sun.

Without spitting out to many details as the models are likely to have variability in the coming days, below is my "Go Zone" for Saturday. Red arrows are 700mb flow with blue sfc. Saturday 700mb winds 45-55 knots and Sunday 50+ knots.

 

...and Sunday

To sum it all together, convection will be on going Friday night along and north of the warm front, warm sector destabilizies during the day with central Iowa as the best chance for Tors. Mostly due to the SE'erly component of the surface winds closer to the low center, better forcing aloft and the highest dew points or smallest dew point depressions. Saturday night there is on going convection with squall line feature that tracks across Iowa, WI and Ill leaving 15-21z or so for the best chance for descrete cells occuring earlier in the day Sunday.  Sunday destabilization will occur early in the day as surface dewpoints will already be in the 50s and 60s across WI and ILL.

Again, this is all based on the 12z run, I would expect the pattern to lag behind what is currently shown by the GFS and influenced where I put out Day 4 and 5 "Go Zones". The models will probably show little consistency and all this analysis will blow up in my face!!!

And a little humor with a NWS fail...

Severe Weather Outbreak potential for this weekend (4/8-4/10)

Chasers, 4/5/11 UPDATE: Wow, so 00z model runs from last night have complicated things a bit more with the GFS being a bit more progressive than the ECMWF. Even more difficult, is the fact that the ECMWF has the 300mb trough getting cutoff from the main flow for a 18-36hr period (Icky). Both models show timing issues with upper level support for earlier in the weekend and have lost the negatively tiled feature to the upper level trough, :( . However, Sunday is still looking very favorable across the lower Midwest/Mississippi river valley. Madison will not be a bad place to be starting from if these solutions hold. This setup is looking strangely similar to yesterdays system with multiple modes of severe convective activity but shifted to the West a few hundred miles. Given model discrepancies to the evlolution of the upper level system, I'll be hanging out along the dryline until further notice and holding off making a "Go Zone" (want to see SREF, 1st), lol. Come on ECMWF solution!!!! Anyone else care to take an early crack on Saturday and Sunday? 4-4-11: I've been watching the models very closely for the last 3-5 days for this upcoming weekend and it appears there could be several rounds of severe weather days with Sunday being the prime day (very good shot of a tornado outbreak) across the Mid Mississippi Valley. This evening I will post my thoughts on this coming weekend and speculate on how it could shake out complete with model output. Details to follow in a couple of hours. Dan aka Squall Line

For those who are fans of hail...

NWS Milwaukee has a short summary of some of the hail events in SE WI yesterday:

http://www.crh.noaa.gov/news/display_cmsstory.php?wfo=mkx&storyid=66161&source=0

While no tornado reports yesterday, there was 2.5 inch hail in Middleton, WI!

Helicity