Area forecast discussion
National Weather Service Jackson Kentucky
1026 am EDT Tuesday may 21 2013
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 12z tafs through 12z Wednesday morning)
issued at 759 am EDT Tuesday may 21 2013
Any lingering valley fog will dissipate through 1330z. An outflow
boundary will work into eastern Kentucky during the first two hours of the
period...with isolated to scattered showers and thunderstorms to
affect loz and sme by 15z...and possibly further east at jkl around
16z. Confidence in convective evolution through the period is not
extremely high...but convection cannot be ruled out during much of
the period and thus thunderstorms in the vicinity and cumulonimbus was used for most of the period.
&&
Jkl watches/warnings/advisories...
none.
&&
$$
Update...jj
short term...jp
long term...geogerian
aviation...jp
