Scientific Forecaster Discussion
Area forecast discussion
National Weather Service Nashville Tennessee
546 am CST sun Nov 23 2014
Update...for 12z aviation forecasts.
Aviation...looks like a rather messy time for flying during this
forecast period, especially in regard to low ceilings and strong and
gusty low level winds. Intensifying surface low, moving from the
Southern Plains to the Great Lakes area, will produce a broad band
of rain showers and isolated thunderstorms across the Tennessee Valley
today and this evening. Rain intensity decreases after 02z, but
low ceilings remain. The only VFR conditions are expected to be early
in the forecast period, up through about 15z. After that, look
for a mix of MVFR and IFR conditions for the remainder of the
South-to-southwest surface winds will become gusty today and
linger past the end of the forecast period. Vertical wind profile
also shows increasing speeds with height. 850mb jet increases from
30-45kts early today, to 40-60kts by this afternoon, with surface
winds increasing to 15-20kts during the same period.
Previous discussion... /issued 335 am CST sun Nov 23 2014/
As advertised...an intense upper level shortwave has developed over
the southern U.S. Tonight. Ongoing showers and thunderstorms near
the Gulf Coast have resulted...and most of that activity is what
will move into the southeast U.S. Over the next 18-24 hours. The
question is how intense will the activity be across middle Tennessee.
There are now indications that the main upper level impulse will
track further to the southeast than originally thought. With that energy
not phasing up well with the surface low...the overall punch of this
system may very well remain south of the mid-state. That said...dew
points are still expected to climb throughout the morning hours and
by this afternoon...should be into the middle to upper 50s. That's
a far cry from the dry lower levels currently being experienced
across the state but with this modest increase in low-level moisture
and strong deep layer shear that will develop later this morning...
widespread rain and scattered thunderstorms will no doubt move
through the middle-state today. In regards to the potential for severe
weather threat here at home...I would like to see more instability.
On top of that...forecast soundings are showing a very saturated
atmosphere. All of these things make ME lean towards no severe
weather. But...with a 65 knots low level jet expected to develop by 18z
today...any convection that is realized will carry the potential for
those winds to reach the ground. For this reason...I am going to
hold onto a small chance for isolated damaging wind gusts this
afternoon in the hazardous weather outlook. Rain totals should be
fairly moderate...with a maximum of 1 to 1.25 inches expected.
What may end up being the headline for this weather event is the
pressure gradient winds. As the rains move out of the area this
evening...sustained winds of 20-25 miles per hour are expected to develop...
along with the likelihood for gusts in excess of 35 miles per hour. This puts
US on the verge of a Wind Advisory late Sunday night into the
morning hours on Monday. Have decided to cover this with an Special Weather Statement
at this time...but an advisory will likely be needed.
While most of the trees across the area have had their foliage
thinned out due to the autumn fall...a saturated ground and gusts in
excess of 40 miles per hour will likely cause plenty of small branches to
break...but keep in mind that weak trees could also succumb to the
strong winds. Light weight...unsecured items...such as trash cans
could also blow over...so take the necessary precautions to secure
these items before this evening.
Some light rain may be seen through Monday afternoon but beyond
that...it could be next weekend before we see anymore
precipitation. While it won't be too cold the rest of the
week...but temperatures will fall back below normal by Tuesday
and remain there through the remainder of the week.
Aviation...06z taf discussion.
Upper level low over Texas is fueling a moisture feed from the Gulf
of Mexico northward. This upper low will also develop a surface low
overnight across the plains. This surface low will push towards the
Great Lakes by the end of this taf period...and will spread warm-
advection showers and persistent rain across middle Tennessee on Sunday.
Short-range models seem to indicate MVFR ceilings and scattered rain
showers through early afternoon Sunday. Then a more persistent
rain will set up...with a few thunderstorms and rain being possible as well.
Conditions at that point would most likely drop to IFR at all
terminals...perhaps sooner at csv. And a few instances of LIFR
conditions will be possible mainly after 20-21z.
As the low moves towards US and onto the north...southeast winds will
become gusty and veer around to the south-southwest by the end of this taf
period. We could start to dry out after 00z as the system pulls
northward quickly...but held onto IFR ceilings for Sunday night.