04/19/05 FCST: Central Plains/Southern Plains

I'll bite for the Tuesday of next week as a potential severe weather day.

My forecast:
12Z NAM shines some interesting light on the situation for Tuesday. North central Kansas appears to be a hotspot as LI's of -8 will be in the area, along with CAPES of up to 3000 J/Kg. Dewpoints are progged to be in the lower to mid 60's, which is a good sign of severe weather happening in the area. Low pressure will be centered in southwestern Kansas near the Colorado border at 0Z Wednesday (7PM Tuesday). The only problem I could see that would hinder the development, is shear. I may be wrong, but I am taking that from the 500mb winds map.
 
Well, this is 84 hours out on the morning NAM from the 16th, but after a quick review there does appear to be at least some potential for tornadic supercells back across eastern CO to perhaps western KS if the general conditions forecast more or less verify. Based on this model forecast, lee cyclogenesis should lead to upslope flow conditions north of the low, which in this run is in extreme se CO by 00Z of the 20th, though this appears to be in response to a weak shortwave crossing the Rockies, and such small features are generally poorly forecast, but in general 25-35 knot SW flow when combined with easterly flow north of the low (should it appear) creates a very favorable shear environment, with steep mid-level lapse rates overlying a rather marginal low-level moisture field, but excess mixing bias by the NAM could mean that low 50 dewpoints could make it across the CO state line to greet convection rolling off the higher terrain. The likelihood of this scenario looks pretty low at this time - but certainly something to watch for in later forecasts.

Glen
 
Here's what I'm seeing from the 12z ETA for 84hrs out (0z Wed)... at 500mb, southwest flow aloft with good divergence over the areas east of the Rockies; winds in Eastern Colorado are in the 20-30 range. Theta-E at 850 shows a nice max in Central Kansas with a little tounge of 320s pushing into Central Colorado, arching back toward the Denver area. A LOW is forecasted to be in extreme southeast Colorado with 850 winds in Northeast Colorado out of the northeast at about 5 to 10, which gives some weak upslope over the area. SBCAPE follows the theta-E tounge into Eastern Colorado with values upwards of 1000J/kg. Again, the biggest concentration of CAPE is over Central Kansas with values over 2500J/kg. 3km helicity values pushing 150 to 200[m^2/s^2] while 1km values are hardly evident. On a final note, the ETA does break out precip over the Denver area and areas to the south and east. The biggest concern... temperatures; I overlooked it on the first glance, but cooler air may filter down and kill off convective chances in a hurry.

While this is a long way out, it does bare watching as perhaps our first chance for severe weather over the Eastern Plains. Lot of things could (and likely will) change before Tuesday actually arrives, but I'm eyeing this as the first chance to chase Colorado's Eastern Plains!
 
My target as of now would be Red Cloud, NE. Maximized CAPE, helicity (at 3km anyway), UVV spikes stacked from h85-h5, and even some precip from the ETA. Problem is winds. If those slow speeds verify, it will be a blobfest, and I don't mean tornado-inducing rainfoots. With little if any venilation going on should these wind profiles verify, seems like about a 30-60 minute window of tornadic potential before the storms rain themselves out into a big mushy slopfest. If higher CAPE values can be realized than currently forecast, I may become a bit more optimistic. However, with less than 30kts at h5, we'd need at least 3000-3500j/kg to sustain any updraft for tornado potential. If this wind profile was happening in late May or June, it'd be an automatic chase. But this time of year, and plus the fact we're about 20 or so days "behind" climo-wise, I just don't think 2500 will do much with the pathetic windfields currently being forecast. Fortunately, the ETA has been underestimating CAPE spikes all season.
 
Definately looks like some potiental exists for Tuesday. I've been watching this time period the last week and don't have a lot of confidence in the timing and location of the features right now. I think though the GFS and NAM (ETA) spells out the potiental for severe weather. At this time I would be most interested In Central/Western Kansas. However even where I live all the way up In Brookings, SD both models would seem to show surface front coming into 1500 J/KG with some decent (not great shear).
 
I still have very little confidence in this storm system. Low pressure this day is centered at the western end of the Oklahoma panhandle. CAPES are still progged to be up to 3000 J/Kg, though Hastings AFD is mentioning CAPES of up to 4000 J/Kg for the area, along with HP supercells. Winds will still be the killer for anything developing at the lower levels, only around 20 knots near the Hastings area. Not much has changed since the 12Z NAM from yesterday, still the same.
 
What happened? Geese... The latest NAM brings in >5000j/kg SBCAPE into portions of central NE all the way southward into KS. Incredible...

..Nick..
 
Nick,

I am currently looking at the NAM run from the RAP site, and the current projection is for CAPE values of 3000 to 3500. The previous run had values of 2500 to 3000. It also has LI values of -8 which I believe are the same as before. The only major changes that I see is the placement of the max CAPE, which has moved about 150 miles northward into Nebraska. The main problem is that the models are having trouble with the placement of the system.

If you are seeing different values on a different model, email me the link...or IM it to me if it suits your fancy. I would like to see it. Right now I am not seeing a major change in intensity of values... mostly just the placement. I guess time will tell.
 
CAPE values continue to be rising with each NAM run. With the new 12Z NAM run, CAPE values will be around 4000 J/Kg in a swath in south central Nebraska. CAPE values of up to 3500 J/Kg will be in an area from Omaha, Nebraska, southwestward to Wichita, Kansas. Lifted Index of -10 can be found in the area of Red Cloud, Nebraska, home to the 4000 J/Kg CAPE bullseye. SRH at the 3km level will be around 100-150. Also in Red Cloud, Nebraska, a 65-70 degree dewpoint is expected. A Low Pressure system will be centered near the meeting of the Colorado/Kansas/Nebraska borders. The Low pressure is progged to be at 999mb. There is still a lack of 500mb winds that could kill off any chances for major severe weather developing in the area, but with other severe weather parameters in the area, will still think that severe weather is possible in the area.

For my suspected area of severe weather, my graphic is available at the forecast link on my signature.
 
Nebraska panhandle still looks to have a confluence of adequate conditions for at least a chance of tornadic storms. Still evidence that convective mode might not end up very discrete - but shear looks adequate and very steep mid-level lapse rates coupled with modest moisture should support vigorous convection. With a short drive from there to where the next day looks to set up - could be a worthy two day event given how the longer range forecasts look.

Glen
 
Chase target for today, April 19

Chase target:
30 mi S of North Platte, NE.

Timing:
Storm initiation 5 PM CDT.

Storm type and intensity:
A few F0/F1 tornadoes possible, along with hail to 1.5â€￾.

Discussion:
AM analysis showed closed upper-level circulation centered in SERN OR. Meanwhile, a lead S/WV was ejecting ahead of it in ERN CO, as was depicted in 700mb and WV analysis. At the surface, surface low pressure was organizing near IML while a nearly stationary front extended NE of this feature though LBF to BBW to SUX. A second boundary – probably outflow, extended along the NE/KS border. Both the front and outflow boundary will serve as foci for storm development later this afternoon as the S/WV approaches the area. Moisture was plentiful, as indicated by ST near and along this boundary along with SFC dewpoints around 55F – these are good dew points as the elevation in this area is around 3000 ft. Finally, scattered ACCAS was noted in WCRNTRL NE, suggesting possible instability near the top of the EML. The 12Z LBF sounding showed a nearly 100mb deep moist layer.

13km RUC guidance has initialized well with regard to surface temperature and the position of the surface boundary and low pressure, while surface dewpoints were about 5F too low in WRN NE. Storms will fire by 22Z along the boundary in SRN NE. Expect best deep layer shear to exist along the WRN extent of the convection with the approach of the upper-level system. MLCAPE’s around 2000J/kG, along with 0-6km shear around 40 kts and 0-3km SRH around 300m^2/s^2 along and immediately N of the boundary will support vigorous, rotation updrafts in the stronger storms in SWRN NE.

- bill
 
OUN's current Hazardous Weather Outlook says that "THERE IS A SLIGHT RISK OF SEVERE THUNDERSTORMS TONIGHT ACROSS WESTERN OKLAHOMA AND WESTERN NORTH TEXAS."

I find that interesting as there is currently no SLGT risk area in Oklahoma for today. Perhaps they are anticipating changes in the 1630Z DAY1 update? Does this mean that us Oklahomans can loook forward to an increased severe threat in at least western portions of the state?

I wonder if the upper winds in western Oklahoma/NW TX are a little bit stronger than previously forecast?

KR
 
Day 1 is here again, boy this forecast thread hasn't seen much attention lately. Oh well. Much more complex forecast scenario today - two key model forecast deficiencies to note after looking at morning obs is the further south and east movement of the cold front in ne CO - as well as the much more robust capping inversion over the central plains - extending further into central NE than was progged by last night's models. What looks better than forecast is the amount of moisture north of the surface trough along I-70 this morning - with mid to upper 50 dewpoints potentially available to advect back into extreme sw NE. Morning convection in w NE/SD appears to be along reinforcing surface cold front boundary - with another trough extending ne from the eastward drifting sfc low near the CO/NE/KS, just north of MCK to GRI this morning. Shortwave ridge now over sw NE should keep things capped there for a while - but water vapor shows band of ascent crossing the continental divide. RUC model guidance agressive in colapsing the boundaries down to a single e-w band along the NE-KS line by 00Z, and recovering the ne CO area. Not sure how much I buy either idea. Still think this day has potential for some tornadic storms - but the shift in the surface features has potentially moved the instability out away from the better upper level shear. early target LBF with plans to change that after further review.

Glen
 
Taking a look at the latest RUC model, maybe our attention should be shifted more eastward into Iowa. If the clouds are to break up and SBCAPE can increase, the increased helicity indicated on the RUC working with the increased forcing of the cold front could bring some severe storms into that area. The latest RUC model basically just forms storms in Iowa during the evening and doesn't form much west of Omaha.
 
A strong, >40kt SW LLJ continues to advect substantial boundary layer moisture east of the dryline from IA southward into TX... The NAM has surface-based CAPE progged >4500j/kg in southeast Nebraska, with very strong low-level shear (>300m2/s2 0-3km SRH). I'd call for supercells with very large hail (given the extreme instability and low wet bulb heights) along with a few isolated tornadoes.

Latest outlook graphic:
http://midwestchase.com/wxforecasts/04_19_05-4.gif

..Nick..
 
Perhaps chasers in the high plains should think more about northeast CO late this afternoon. Cool front has backed into this area a little more strongly than earlier forecast, so surface dewpoints in the upper 40s to lower 50s are advecting toward the Denver area. The associated frontal cloud band is thin and won't be much of a hinderance to reacing convective temperature. 12Z RUC has the best forecasts sounding for severe at 20/00Z along the front from Denver east-southeastward. CAPES are forecasted to be about 2000 j/kg with helicity 200 m**2/sec**2...these are good values for this area. Also, upper flow will be approching 50 kts which is stronger than points farther east. My play would be along and just south of I70 from Denver to
Limon...scattered large hailers and a few tornadoes.
 
early target LBF with plans to change that after further review.

Just gave another look around - and think I'll venture a bit further west. happy to see the low-level clouds thinning out - and convection starting to fire in the northern front range. Cold front continues to sit just east of Sidney, with the low retrograding a bit back toward the west from it's earlier position. Expected ne cell motion even for a supercell in this area has me favoring where the boundary is more ne-sw oriented. Very strong capping inversion according to the RUC analyses seems to bound se edge to around McCook - with west bounded by cf. Retreating warm front north of low wrapping back around to near Ovid, CO, which could be the best area for CI on the next few hours. Think I'll drift my virtual chase crew west to Ogalalla - and have them plan to play the boundary intersecting through there as convection comes up from the sw later today.

Glen
 
This is becoming a class Douglas County/Elbert County setup for areas near the Front Range... Denver's latest HWO uses some pretty strong wording for areas south and southeast of the Denver Metro area. A weak cold front is resting against the front range and I would image storms will kick off from there. Terrain features of the Palmer Divide are also gonna aid with development as winds are in an upslope direction. Storms interacting with the frontal boundry will likely become severe with hail probably a very good bet across areas of the Metro area itself. Brief landspout tornadoes are also very likely in areas east of I-25. I'm gonna take my 4 hour break and shoot towards Parker to play for a bit.. storms expected to fire about 2ish oughta be enough time to play before having to go to my other job!
 
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