Area forecast discussion National Weather Service Jackson Kentucky 135 PM EDT Tuesday may 21 2013 Update... issued at 135 am EDT Tuesday may 21 2013 The air is extremely unstable and the Storm Prediction Center just updated their outlook and put eastern Kentucky in an area of slight risk for severe thunderstorms. The main threat is for strong winds...however hail can not be totally ruled out and will likely see small hail in the area today. With the potentially severe storms in the area...the possibility of gust fronts well out ahead of the thunderstorms is possible and could create some unexpected strong winds. Confidence in the timing of convective evolution is not high...and was based on a combination of the NAM and high resolution rapid refresh model. Depending on how much rain occurs...there may be a time of Post frontal mist which would last a few hours and then improve. Due to low confidence on timing and extent of the storms...this was not added to the tafs. Also expecting to see some mist at the taf stations and valley fog in the valleys and near areas that get a lot of precipitation today. Update... issued at 1026 am EDT Tuesday may 21 2013 The forecast soundings are forecasting extreme instability today. The NAM buffer sounding for 4 PM at Jackson is calling for total totals of 51 and a lifted index of -7.6. The cape is forecast to be 3479 joules. The wind is not favorable for severe thunderstorms as there is very little shear. There is also very little dry air aloft. The main issue today will be a trigger to get the storms going. There is an old outflow boundary currently bisecting eastern Kentucky from north to south. Because it is so early in the day...the low level instability is currently not favorable and most likely will not be favorable until this outflow boundary moves into West Virginia. It may fire up near the West Virginia border around 11 am. Will have to wait and see. Currently do not have a good feel on exactly where and when the thunderstorms are going to initiate. Used the output of the high resolution model to try and time some impulses through the area...however confidence is fairly low until convective initiation actually occurs. With all the convection that has occurred out to the west...there are bound to be some old boundaries that move through this afternoon. Also...with the valley fog that occurred this morning...some differential heating boundaries may be able to generate a storm. Once the storms are generated...any outflows they produce should propagate across the entire area. Usually with this type of sounding over the area...the thunderstorms produced will be pulse type storms. A few of these storms may become severe and produce wind gusts up to 60 miles per hour and 1 inch hail. Due to the lack of wind shear and lack of any strong boundaries...tornadoes are very unlikely today and would probably need some intersecting outflow boundaries to even have a prayer of forming. Update issued at 759 am EDT Tuesday may 21 2013 Grids were freshened based on recent observation and to account for the outflow boundary and convection working east across central Kentucky that likely will not completely dissipate before reaching eastern Kentucky. This led to some changes in probability of precipitation...generally to raise them in the west during the am...and slightly in the afternoon across the remainder of the area. && Short term...(today through wednesday) issued at 324 am EDT Tuesday may 21 2013 A closed upper level low was centered over the Dakotas early this morning with a shortwave trough working toward the MS valley region and another shortwave working from the southern rockies toward the Southern Plains. At the surface...an area of low pressure was located near the South Dakota/Minnesota border with a warm front extending east across the Great Lakes into New England and a cold front trailing south and then southwest into the Southern Plains. Prefrontal convection has weakened but extends from the western Great Lakes into he lower Ohio Valley region...north of the ob and pah area and then continues south generally along the MS river and then west into east central Arkansas. It does appear that this convection has outflowed with an outflow boundary from central in down to near the evv vicinity. Locally across eastern Kentucky 11-3.9 imagery indicates a bit of low clouds near Black Mountain and in SW Virginia and valley fog...especially in the Big Sandy region. Some debris cirrus si also passing overhead. Early this morning...valley fog will likely continue and become dense in some areas before dawn...despite some passing cirrus. Coverage of this should be a bit more widespread over the southeast that experienced convection over the past 24 horus. A pre first period will be used to cover this. The 4z hrrr and 0z NAM generally seem to bring an outflow boundary from convection to our north and west across the commonwealth this morning and into eastern Kentucky in the afternoon. Any convection along with will probably be isolated to scattered in nature. The 0z NAM and 0z GFS differ considerably on instability from midday into the afternoon. The 0z NAM forecasts a considerable amount of midlevel dry air and steeper lapse rates aloft as compared to the 0z GFS while the GFS is more tame. The 0z NAM likely moistens up the boundary layer a tad too much leading to dewpoints nearing 70 and resulting in total totals over 50...cape over 50 and Li of -9c or lower. Reality would probably be somewhere in between...more on the order of cape in the 2000 to 3000 j/kg range and Li -4c to -6c or so and 21z sref has high probabilities of cape greater than 2000 j/kg and Li less than -4c. This would still be supportive of some stronger storms and possibly a locally severe storm with heavy rain...especially if there were to be any cell mergers. A one or two hour difference in cumulus development and convective development will determine maximum T today. However...it still appears that maximum T will be close to breaking the record for today at jkl and possibly come close at loz. Loss of daytime heating will lead to less activity after 0z as the strengthening low level jet and approaching cold front/middle level shortwave should focus convection to our west and northwest for much of tonight. If clearing takes place...valley fog should develop and possibly become dense...especially where any convection occurs later today. At this time...we plan not mention dense fog just patchy fog to areas of fog. On Wednesday...the cold front and middle level wave will approach the area...though the low level jet core with the stronger winds aloft generally to our north and northwest. Some degree of debris clouds may work across the area on Wednesday...but moderate 0z models and 21z forecast moderate instability and more in the way of shear as compared to today. This should lead to strengthening of convection as it moves into eastern Kentucky and probably an uptick in coverage as well. One or more lines of storms...possibly strong to locally severe should work across the region. Gusty winds would appear to be the primary threat with storms on Wednesday...though some small to marginally severe hail could occur as well as locally heavy rain. Likely probability of precipitation were continued for Wednesday. Once again timing of arrival of debris clouds and convective development will determine maximum T on Wednesday...but eastern locations should see this occur the latest and should average warmer than more central and western locations. Long term...(wednesday night through monday) issued at 415 am EDT Tuesday may 21 2013 The models remain in good agreement with a blocky upper level pattern to take shape across the Continental U.S. Into early next week...although details become more murky by this weekend...with some disagreement with the smaller scale features. As such...generally relied on a blend of the guidance. A deep upper level trough will swing through the Ohio Valley Wednesday night into Thursday...keeping good chance to likely probability of precipitation threatening eastern Kentucky. There has been a trend of a quicker exit with this feature. Instability will wane quickly by late in the day on Thursday...so removed thunder chances for Thursday night. Cool high pressure will then bring dry conditions to conclude the work week. This weekend...the European model (ecmwf) and GFS differ quite a bit with the timing and evolution of the next system to affect the area. The European model (ecmwf) is pretty transient with the high pressure...and allows a warm front to move in quickly by early Saturday. This boundary then stalls across the southern half of the area...with occasional short wave troughs traversing the northwest flow aloft. The GFS is much slower here...and keeps the surface boundary shunted more to the north. Given the uncertainty...did allow for a return of slight to chance probability of precipitation...although not as aggressive as the European model (ecmwf). Temperatures will generally average a little below normal through the period...with Friday being the coolest day...as highs will retreat to the 60s. && Aviation...(for the 18z tafs through 18z Wednesday afternoon) issued at 135 PM EDT Tuesday may 21 2013 The air is extremely unstable and the Storm Prediction Center just updated their outlook and put eastern Kentucky in an area of slight risk for severe thunderstorms. The main threat is for strong winds...however hail can not be totally ruled out and will likely see small hail in the area today. With the potentially severe storms in the area...the possibility of gust fronts well out ahead of the thunderstorms is possible and could create some strong low level wind shear. Confidence in the timing of convective evolution is not high...and was based on a combination of the NAM and high resolution rapid refresh model. Depending on how much rain occurs...there may be a time of Post frontal mist which would last a few hours and then improve. Due to low confidence on timing and extent of the storms...this was not added to the tafs. Also expecting to see some mist at the taf stations and valley fog in the valleys and near areas that get a lot of precipitation today. && Jkl watches/warnings/advisories... none. && $$ Update...jj short term...jp long term...geogerian aviation...jj