06/03/05 FCST: Plains

[Jayson Prentice]

I've focused this down into June 3rd specifically... This looks to be the day as a system will move across the northern plains.

The models are becoming consistant in moving a system across the plains on Friday. A strong low pressure system looks to focus just north of the ND/Canadian border. This will have a cold front that looks to move across the northern plains with aid from a shortwave...

Precip looks to stay developed across NE/SD/KS during Friday morning, a possible MCS over ND and another along the NE/KS border during the morning hours. These look to slowly weaken but then regain strength and become more oriented along the cold front during the afternoon hours. By this time the front should be in western MN southwestward into southcentral NE. This should convene into a possible MCS once again, strongest precip core looks to be in the southern NE/IA range.

As for support for these storms... Looks to be well as of this model run. Starting at the surface, winds ahead of the front look to run around 25 kts from the southwest while just behind the front winds quickly switch to northwest at 10-15 kts.
At the 700 mb range, good moisture looks to be retianed ahead of the front. flow at this range looks consistant across the plains at 25-30 kts from the westsouthwest. At mid levels, winds are stronger around 40 kts just ahead and then are running at least 50 kts along the front from the westsouthwest... A nice vort max looks to be coincided with this front/low pressure, especially strong in SD/MN over the early evening hours as it is wrapped around the low.
Upper levels have a very strong southwesterly flow on the regards of 60 kts ahead of the front while running 75-90 kts along the front in SD/MN. This should support storms quite well across much of the northern plains.

Questions will of course be the timing as is any forecast that is on the range of 150-156 hrs out. Moisture will be a question with any system this year, but Td's are forecasted in the in at least the mid 60s for southern NE/IA while Td's in the upper 50s and low 60s are quite as impressive in MN/SD. PWAT's ahead of the front look to run well above 1 inch mark, even some 1.8 inches in the southern IA areas during the afternoon hours of Friday. This should be another good indication of southerly moist flow into this system.

Plenty of time for change to good or bad with this system, but lately I believe the models are strengthening this system and looks to have good support...

 

[Scott Lincoln]

Time will tell with this one. I dont trust much model placement more than 3 days out, so this may end up being delayed quite a bit. That has been the trend with many systems this year.

 

[Jayson Prentice]

Still looks like areas of the northern plains will see a shot at some severe weather. Td's are shown well into the upper 60's across the area, with even some low 70's... Given that the GFS could be overdoing this...

Sioux Falls AFD already mentions that some of the thunderstorms thursday/friday could be severe...

But, of course the models look to be delaying it. So the discussion on the 4th may end up being a good day as well... Tough to say this far out.

I'm lookin' for a chase day!!

 

[Scott Lincoln]

Yeah... the GFS sure likes to add more moisture and rainfall into the forecast picture than other models.

 

[Gabe Garfield]

Originally posted in the wrong thread.

Judging from the latest ETA progs (6z), Friday looks pretty sweet. The run doesn't go out all the way to 00z, but shows a pretty potent setup by 18z. CAPE values are forecast to be over 3000 j/kg in a broad swath of the southern and central plains with a max of >4000 j/kg in NE KS/SE NE.

As far as wind shear is concerned, it looks awesome. Basically, it looks like everything you'd want in a late season storm system (strong longwave trough, strong WSW jet max, strong low-level jet, etc). I don't really see any caveats at this point, except maybe a strong cap along the dryline (though convection is almost a guarantee near the sfc low).

Gabe

 

[Jeff Snyder]

I think it looks pretty good, but there are a couple of points to consider. First off, the dryline, at 18z (on the 84hr forecast from 6z NAM) is located in wester Kansas, while the 850mb jet axis is shifted 100-150 miles east, yielding much weaker shear profiles immediately ahead of the dryline. More importantly, however, is the meridional flow (southerly flow) at 250mb north of I70. 500mb flow could be a little stronger as well, but doable given >= 70 Tds and resultant 3000-5000 CAPE. I'd be somewhat concerned about the cap as well. Regardless, I would likely be chasing... oh wait, I forgot, this falls in one of only two periods this spring during which I cannot chase! Ugh! The first was the 5-12 - 5-14 time frame, the second is 6-1 - 6-5... :roll:

 

[Jayson Prentice]

The 12z ETA even strengthens what you have said about the 6z run Gabe.

CAPE is >2500 j/kg all the way from Eastern South Dakota southward through NE and into KS/MO. In northeast Nebraska, (~Norfolk) has a bullseye of CAPE around 4500 j/kg. By 00z it shows that the cap has already eroded away in eastern NE/KS.

SRH values during the morning hours has it progged above 500, while the evening hours it is a little less, only on the range of 300 along the MO RIver.

Td's at this time are forecasted to be in the 70's along the MO River by evening hours... With highs in the 80s in northern NE that would create a nice T/Td ratio in that area.

One thing that is lacking is vorticity at that time... But, that will of course have the possibility of changing as this system is still 4 days out. 250 mb winds look to have a jet streak that is progged to affect areas of NE/SD. 500 mb winds also have this streak across central NE. One thing that isn't great is the directional shear, most of the winds are from the southwest. 850 winds are better, as they are quite strong from the southeast which should create some directional shear, but not as much as I'm sure a lot of you would like to see.

I'm still going with an area of NE right now.. But, don't have a partner as of yet. Anybody interested give me a PM.. Thanks

 

[Gabe Garfield]

Originally posted by Jeff Snyder
More importantly, however, is the meridional flow (southerly flow) at 250mb north of I70.

This was definitely true in the 6z run. The 12z run, however, has veered the 500 mb flow quite a bit. See http://www.nco.ncep.noaa.gov/pmb/nwprod/an...2/fp0_084.shtml

Gabe

 

[Ryan McGinnis]

Man, but look at that cap -- 14C at the 700mb. Hope the models get more optimistic about that.

 

[Nic Wilson]

The 84 hr NAM definitely looks great for the east NE/SD border, but everyone should definitely be aware of the significant shortcomings the 84 hr NAM has had during 2005 (e.g. 84 hr. model verification has been few and far between for the NAM). I'll take a bit more pessimistic view and provide a peek of what the GFS and ECMWF are currently hinting at for Friday.

The GFS is significantly further south with the surface low and fails to bring favorable 500 mb winds further east, instead setting up a marginal severe weather set-up in central NE. The ECMWF predicts a weak 1002 mb sfc low in ND at Fri 12z, and only deepens it to 999 mb for Sat 12z. I would tend to side with the ECMWF as it has been blowing away the GFS performance wise in the month of May (and all season for that matter).

http://wwwt.emc.ncep.noaa.gov/gmb/STATS/html/acz5.html

My forecast is for no HIGH risk on Friday (likely MODERATE) and the most active severe weather day won't be until late on Saturday when upper-level support (e.g. ageostrophic imbalance, significant >40 kt 500 mb winds) can come together in the SD/NE/MN/IA region -- still much too early to pick out any specifics, but I'm not expecting the super-outbreak apparent to others, definitely not a HIGH risk by any stretch of the imagination.

 

[Mike Johnston]

Tonight's model runs continue to point toward severe setup shaping up Friday, and possibly continuing through the weekend for central and northern plains. Advection of gulf moisture is apparent with pretty strong 850mb southerly flow over the next few days, bringing 65+ tds by Thursday and 70+ tds by Friday in a wide swath through KS, western MO and on up to E NE and W IA.

On Friday, as upper level winds are forecast to be fairly uni-directional out of the southwest, probably key to isolated severe supercells in the warm sector will be position and vigor of approaching surface low. By 00z Friday, NAM and GFS are fairly consistent with 995-997mb surface low over E CO / W KS, with backing surface winds over S NE and N KS. Forecast dewpoint gradient hints at negative tilt to surface convergence line. Instability looks strongest over N central and NE KS and S central NE, while lower LCL's and strong SR helicity oriented a bit further east in N MO and S IA. Still alot of possible outcomes and geographic uncertainty this far out, but overall factors indicate Friday bears close watching.

 

[Mike Johnston]

Friday continues to hold significant potential for severe weather over central plains. While too soon for specific targeting, at this point the strongest possibility appears to be the eastern 1/3 of KS. By 00z, latest NAM is forecasting SB CAPE values of 6,000 j/kg in an area just to the east of Wichita.

Advection of gulf moisture appears to be shaping up as expected, with 30-40kt southerly flow at 850mb underway even now over western high plains. By 06z Friday, this flow is forecast at 30-50kts over central KS, with dewpoints building to >65td over broad swath of central and southern plains by 12z Friday, and reaching as high as 76td over SE KS in late afternoon. Wind crossover with height appears supportive, with 40kt from SW at 500mb, 20-25kt from S at 850mb, and surface winds from the SE, although minimal, across SE KS.

Seems like key to this setup will be deepening and eastward movement of surface low now over SE CO. By Friday, attendant warm front draped NW to SE over eastern KS is actually forecast to be quite sharp, with surface temp 94d ~Wichita and 78d ~ KC. Convergence of surface moisture pronounced along this boundary. Although surface helicity not forecast strong, 0-3km SRH values of 200-250 m2s2 indicated across the area. With very strong instability and surface convergence, cap should not be an issue and lid strength index drops off to zero in E KS, with LCL heights falling from 1,500m in Wichita vicinity to < 600m in eastern KS. If there is an obvious lacking ingredient, it is probably relatively weak deep layer shear; 0-6km shear of only ~25 kts. However, upper level SR winds of 30-50 kts may be sufficient to hold off HP mode for several hours.

If this were tomorrow, I would target Yates Center KS. However, we have several more runs to sweat through first, and consideration of boundaries/cloud cover from expected MCS(s) tomorrow night.

 

[Mike Johnston]

Although subsequent NAM runs have backed off considerably from previous off-the-chart forecast instability, and surface boundary not as sharply defined, I'm still considering SE KS as potential target for a Friday chase - as strong southerly flow continues to advect gulf moisture up through the plains. Overhead, 45-50kt SW flow at 500mb should be crossing this area at peak of diurnal cycle. Dewpoints of 70td+ should result in plenty of instability. Although cap will still be an issue, should be more workable than Thursday's target areas, as 850mb temps forecast <20 C, with negative lid strength indexes.

Most eye-catching on tonight's forecast maps is t/td spreads of 78/72, 76/70 etc. over SE KS. Wichita forecast office does seem to attribute part of surface temperature forecast to expected cloud cover from overnight MCS, which at this point is very far removed from the area - barring any new convection firing overnight, say across OK or Central KS on strength of LLJ. At any rate, if forecast holds, we could see LCL heights as low as 500m. With upper level storm-relative winds at 40 kts, decent chance for good venting to support classic mode.

Will adjust target w/ look at tomorrow am data, but still like Yates Center as good as anywhere for Friday chase.

 

[Mike Peregrine]

Well our original 'northern plains' topic may need to shift the focus a bit to the south, looks like. Here's my current 'forecast' (ha) primarily using 0z ETA, though I know for a fact none of this is trustworthy.

0-3 km SRH - values in the 200-240 range along KS/OK border and again to the south along the TX/OK border, with focus to the west along the panhandle. These areas additionally show SWEAT values of around 450.

CAPE - looks like another buoyant CAPE day, with values well over 4000 J/kg along KS/OK state line, and approaching 5000 J/kg in pocket near Wichita Falls, TX area.

0-1 km EHI - 2.8 near Pratt, KS ... and 4.4 in SW Oklahoma.

Boundary Layer - 6 km shear - 40 kt in both areas of interest.

Lid strength - above 2 in all areas, with average in the 3-5 range ... another strongly capped day, meaning we will have to see direct sunlight and high temps to break this one.

I don't know if I'm reading this wrong, but LFC and LCL heights tomorrow look like big numbers to me so far ... anyone else seeing differently? - I'm seeing 1800-2000 on both of these values across the board pretty much, but it's late and I'm tired.

I'm as confused as usual about picking a target tomorrow ... not sure yet why so much of Kansas is outlined in 5% risk area, since CAPE is bullseyed in south-central KS right now ... but this will no doubt change between now and tomorrow quite a bit so I'm going to do something different and get some sleep for a change.

 

[Ryan McGinnis]

It should be noted that now, not only does the GFS disagree with the NAM, the RUC does too. RUC's 12Z and 15Z progs look nothing at all like what the NAM has in store for north central Kansas. In fact, it takes the surface trough all the way up into southcentral Nebraska.