2011-04-27 FCST: AL,TN,MS,KY,OH,IN,WV,GA,VA

[Matthew Havin]

Let's get this party started. Wednesday is setting up for a significant severe weather outbreak for the TN/OH Valleys. SPC has a D2 MOD risk issued from Cleveland, Ohio all the way down to central MS. A rapidly strengthening low pressure system will bomb out over the Ohio Valley by Wednesday afternoon < 985mb, with the NAM hinting at a secondary low near the Memphis area.....fueled by a 100+ knot 500mb jet core over the TN Valley by the afternoon. A massive area of 200,000+ square miles are threatened by a significant....potentially historic tornado outbreak.

There are conditional concerns with what is going on w/ tonight's HIGH risk event. If things can move out by late morning and then proceed to clear out in the TN Valley during the midday....... then we are looking at a HIGH risk event with 4000+ CAPE, 250+ 0-1km SRH, and 6+ 0-1km EHI. Supercells blowing up in the early afternoon in N MS/W TN/W KY... then becoming somewhat more linear w/ embedded elements by late afternoon/early evening as the system moves over Nashville/Huntsville metros...where greatest tornado threat exists.

 

[Michael Riedl]

0z NAM does not recover airmass much further north than TN/KY state line, if this were to happen there goes the MOD and Potential HIGH for the upper OHV.

Last night's 0z NAM had a much smaller event happening tonight and the MCS was further east at the forecast time for now, which lead to the potential significant outbreak as Far north as Central Ohio, the NAM brings that back to just the SE.

CAPE looks very good, Directional shear is just ok tomorrow as it appears NAM wants to veer winds, even though there is a deepening low in the SE, and the upper dynamics are self explanitory.

I think there wil be tornadoes tomorrow, but as of now my confidence of any significant tornado outbreak (wind looks good tomorrow) is no very good.

 

[Dan Robinson]

Some comments on the northern portions of the risk area. The NAM (continued with tonight's 0z run) has been showing warm-sector wind profles and hodographs the likes of which I've never seen in this particular region: eastern Kentucky, southeast Ohio and western WV after dark tomorrow. Wind profiles improve in the hours at and after sunset. However, unlike previous runs, tonight's NAM removes the instability shown earlier. There is a catch-22 with backing surface winds just west of the Appalachians due to downsloping and drying with flow over the mountains.

Check at 27 hours (time sensistive)
http://beta.wxcaster.com/cgi-bin/parse_skewt_trace_all.cgi?model=NAM&STATIONID=kcrw

The key will be the substantial task of clearing out the current mess - if this occurs, I would expect the current Day 2 risk to maintain.

 

[Dave Gallaher]

As presumed, the High Risk for north Alabama, south central Tennessee, and extreme northeast Mississippi and northwest Georgia has been implemented. Dews are already 64-65 across north AL at 0635Z. Flash flood watch is in place here as well, fairly saturated conditions awaiting possible 4-6 additional inches by Wednesday pm. Instability should be way up the scale.

 

[Bill Hark]

Mods, please ad Virginia:

Although most chasers will be focused on the high risk today in north and central Alabama, I think there is a decent potential for tornadic storms in western Virginia. I am biased since I do live in Virginia! The trough is shifting eastward and by later today, 40 – 60 kt winds from the southwest will spread across most of Virginia with the stronger winds to the west. I also like the more southerly predicted winds at the 850 level toward western Virginia. Both the NAM and RUC are in agreement by 00Z. At the surface, winds will be from the southeast due the low shifting northeast to over southern Indiana by 00Z, again shown on both NAM and RUC. I rarely get to see nice backed surface winds in Virginia. Predicted dewpoints will be sufficient at 65 for western Virginia. CAPE will be a bit lowish at 500 – 1000 in the target area but decent for tornadic storms along with helicity values of 300-400. The 4km WRF shows precip entering western Virginia by 2300 and shifting eastward. Storm mode may be more linear though I don’t trust the 00Z run as much. Waiting for the next run. Right now, I would target the Rocky Mount area of Virginia (just south of Roanoke) and adjust. There are decent roads leading out in all directions though western travel is limited by the mountains.

Bill Hark

 

[Chip Redmond]

Definitely a tricky forecast farther north than the high risk. I have totally changed my forecast about four times in the last day (and wasted a ton of time in the process). Right now it looks like there is a window of recovery after the MCS in TN moves northeastward. I think the MCS will weaken a bit as it turns northward, away from its moisture source but the HRRR keeps its strength well through 00z. Should be interesting to see if storms can actually get going in it's wake and farther to the north. Extensive cloud cover will prohibit any good structure too if they do get going with forcing from convergence along cold front.