Area Forecast Discussion
Issued by NWS Knoxville/Tri Cities, TN

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FXUS64 KMRX 210648

Area Forecast Discussion
National Weather Service Morristown TN
248 AM EDT Fri Jul 21 2017

.SHORT TERM...(Today and tonight)...A hot day in store today as mid
level ridge remains over the region, centered just to our west.
Model sounding continue to look on the dry side and weak capping may
limit convection especially in valley areas.  Will continue with the
slight chance PoPs for most valley areas with low chance in the
mountains for this afternoon and evening, although would not be
suprised if even that may be overdone. Temperatures will be in the
90s for the valley areas, and heat index values expected to be
around 100 in the southern valley and should approach 100 up into
the central valley for a few hours this afternoon. However, should
stay below heat advisory levels.

.LONG TERM...(Saturday through Thursday)...At the beginning of this
period (Saturday morning), the upper ridge is still holding on while
at the surface our weak moist flow from the Gulf of Mexico
continues.  An upper level shortwave is tracking through the
northern plains. It looks like there will be enough instability to
induce some isolated to scattered diurnal showers and storms on

By Sunday, this shortwave moves southeastward into the Great Lakes
and the southern end of this shortwave starts carving pretty heavily
into our ridge.  The front associated with this system will still be
to our north on Sunday, but models agree that a weak wave of lift
will be coming into the area that will increase the chances of
showers and storms. By Monday as the front approaches, a leeside
trough also develops in the Carolinas, that splits the organization
of this system--with some organization along the W-E oriented front
as it sags into our area, but also there is some organization along
the NNE-SSW oriented lee trough.  For Tuesday through Thursday,
models start to diverge on the placement of the front as well as
whether we stay under the influence of the Great Lakes trough
(keeping better chances of rain in the area )or if the ridge from
the Plains builds back over our area (that would tend to dry us out
again). In general, there is enough agreement that the front will
probably sag south of the area and take the better rain chances
south, so our higher POPs will be in our southern areas.

With the aforementioned pattern changes, the hot and steamy thermal
conditions over the past several days will be subsiding as the upper
ridge fades.  Saturday will still be very steamy, but staying below
heat advisory levels, Sunday not quite as hot, and then for Monday
through the rest of the forecast, the Heat Index will be closer to
seasonal normals.


Chattanooga Airport, TN             96  75  93  75 /  20  20  30  20
Knoxville McGhee Tyson Airport, TN  94  73  92  74 /  20  20  30  20
Oak Ridge, TN                       95  73  92  75 /  20  20  30  20
Tri Cities Airport, TN              93  70  91  71 /  20  20  30  20




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