Scientific Forecaster Discussion
Area forecast discussion...updated
National Weather Service Jackson Kentucky
801 am EDT Tuesday Apr 21 2015
issued at 745 am EDT Tuesday Apr 21 2015
Hourly grids have been freshened up based on recent observation
and satellite trends. Much of the middle level cloud cover has moved
off to the east and southeast of the area. Gusty winds from late
morning into the afternoon is the main weather concern for the
Short term...(today through wednesday)
issued at 335 am EDT Tuesday Apr 21 2015
Early this morning...a ridge ws in place across much of the
western Continental U.S....with a broad trough from the plains into the
eastern Continental U.S.. within this trough was a rather potent closed low
over the western Great Lakes region. Shortwaves continue to rotate
around this closed low and through the trough. At the
surface...relatively weak surface high pressure over the lower MS valley
region had nosed into the the region. One of these shortwaves
continues to depart to the north and east of the area...and as it
departs...the current middle level cloudiness should scatter out.
During the near term period...the axis of the western Continental U.S. Ridge
will move east into The Rockies while a system moves into the
Pacific northwest. Further downstream the closed upper level
system should meander to the northeast and be centered near the
Ontario/Quebec border to the south of James Bay on Wednesday
evening. As this occurs...a couple of additional shortwaves will
rotate into the lower Ohio Valley region along with a surface cold
front. These will have limited moisture to work with...but a
gradual saturation of the lower levels should occur from this
evening into early on Wednesday and some isolated to scattered
showers are anticipated from this evening into the day on
The pressure gradient and winds through the column to the south
of this closed low should be rather strong today...as a cold front
begins to approach the lower Ohio Valley region. Some gusty winds
are expected once again from late this morning into the early
evening hours. The 25 to 35 miles per hour range for these gusts looks good
as deep mixing should allow momentum Transfer to the surface along
with rather widespread sustained winds of 10 to 15 miles per hour from late
morning into the afternoon. The airmass today will also be rather
dry as drier air advects in from the west and mixes down today.
Dewpoints should drop into or through the 30s. This should allow
for some relative humidity down into the 25 to 30 percent range for locations
generally south of Interstate 64. RAWS observations indicate that
10 hour fuels are rather moist at this time. However...some drying
of these fuels should occur today with the lower relative humidity and gusty
winds. The fwf will be highlighted for the much drier relative humidity today
along with the gusty winds. At least the gusty winds will also be
continue to be highlighted in the severe weather potential statement.
As already noted...the first of the two shortwaves should bring
some showers to at least the northern part of the area during the
late evening into the start of the overnight hours. As the cold
front moves into the area...there may be another peak in coverage
of showers as the second shortwave rotates across the Ohio Valley
late Tuesday night into Wednesday.
Under the influence of the upper level trough temperatures will
average at least 5 degrees below late April normals today and on
Wednesday. The anticipated cloud cover and winds on Tuesday night
should lead to a limited diurnal range.
Long term...(wednesday night through monday)
issued at 312 am EDT Tuesday Apr 21 2015
The models are in good agreement aloft through the remainder of the
work week before they start to diverge. They all depict the deep
upper low...initially spinning over the Great Lakes...moving to
eastern Canada by Friday morning. As this happens...heights will
rise over eastern Kentucky while sustained northwest middle level flow
shifts east taking the bulk of the northern stream energy along with
it. Flat ridging will then move into the deep south and Tennessee
Valley ahead of an open trough over the Southern Plains. Plenty of
energy from this trough will slip east over Tennessee and Kentucky
by Saturday morning with the core of the trough passing through the
Bluegrass state later Saturday or Saturday night. The European model (ecmwf) is most
distinct of the models with this transition. It has this trough
crossing the state during Saturday night while the Gem and GFS move
it through Kentucky earlier during the day Saturday...about 12 hours
sooner. The pattern continues to be muddled through Sunday and into
Monday as general northwest flow brings more energy into the area...
in the European model (ecmwf)/S case in conjunction with the main northern stream
trough retrograding back to just north of the Great Lakes.
Meanwhile...the GFS holds its best energy back to the northwest
through Monday owing to its core lower heights remaining more
contained to the east. Given the model agreement early on followed
by increasing uncertainty with time will favor a general blend
solution with flagging confidence late in the forecast.
Sensible weather will feature a chilly and unsettled pattern for
much of the extended forecast. At the start...low pressure will
dominate the northeast portion of the nation with a cold front
settling just south of Kentucky. Weak high pressure may manage to
keep Thursday and Thursday night dry before the front starts to lift
back north on Friday. As a result...showers will be a good bet to
return by Friday night...continuing into the day Saturday...when surface
low pressure moves out of the Southern Plains and through the area.
The European model (ecmwf) is more robust with this due to its slower and stronger
surface low not really passing through east Kentucky until Saturday
night. The GFS...and Gem...take this surface wave through about 12 hours
earlier and with less quantitative precipitation forecast and lower probability of precipitation. In the wake of all this...
Sunday should at least end dry as another weak area of high pressure
moves into...and through...the Ohio Valley. However...by Monday
proper...additional showers will be possible as another area of low
pressure moves into the Ohio Valley. Thursday night looks to be the
coolest of the mornings with some middle and upper 30s possible...
though Saturday night may turn out to be equally chilly should the
GFS/Gem solution end up being the more correct one. Also Wednesday
night could be quite cool...especially our northern counties.
Agricultural concerns will need to monitor these nights and be on
the lookout for a potential of frost.
The CR grid blend came in reasonably well given the preferred model
mix with the bulk of the adjustments made to probability of precipitation over the weekend
given the crux of the model disagreements at these time steps. Also
made some minor terrain based adjustments to temperatures each night...but
particularly Thursday and Saturday nights.
Aviation...(for the 12z tafs through 12z Wednesday morning)
issued at 755 am EDT Tuesday Apr 21 2015
Another middle level disturbance and associated cold front will
begin to approach the region during the first 12 hours of the
period. Although VFR ceilings and visible are anticipated...gusty west or
west southwest winds with sustained speeds in most locations over
12 knots with gusts to around 25kt are expected to develop around 14z and
continue through around 23z or so. Winds should then slacken to
10kt or less through the end of the period. As the front moves
closer to the Ohio River during the last 12 to 14 hours of the
period...isolated to scattered showers are anticipated and a
thunderstorm cannot be ruled out. However...probabilities are too
low to include any visible or ceilings lowering into the MVFR range and
thunderstorm chances are too low to include just yet.