535
FXUS64 KMRX 171800 AAC
AFDMRX

Area Forecast Discussion...UPDATED
National Weather Service Morristown TN
200 PM EDT Wed Jul 17 2024

...New AVIATION...

.UPDATE...
Issued at 1009 AM EDT Wed Jul 17 2024

Regional radar and satellite imagery shows there`s very little in
the way of convection and shower activity across the area this
morning. Most all of the ongoing activity is situated in the
southern Tennessee mountains, towards the Tellico Plains area.
Updated PoPs to reflect this current activity, blending into the
previous forecast for the afternoon hours. Still think that part
looks ok, with numerous showers and thunderstorms expected later
today. Other notable change I made was to up expected afternoon
highs today. Plenty of sunshine has been seen this morning. The
last three HRRR runs, along with NAMNest and other guidance was a
little higher than the previous forecast for afternoon highs.
Bumped those up a couple of degrees.

&&

.SHORT TERM...
(Today and tonight)
Issued at 338 AM EDT Wed Jul 17 2024

Key Messages:

1. Patchy fog possible through the early morning hours, focused in
locations that observed rainfall this past afternoon.

2. Multiple rounds of showers and storms are possible this afternoon
through tonight. A few strong storms with locally gusty winds and
heavy downpours are possible.

3. Ambient temperatures will be cooler but heat indices will once
again approach the mid 90s to lower 100s thanks to increasing
dewpoints.

Discussion:

Currently an upper level impulse is traversing mean flow across the
lower Great Lakes region where a quasi-stationary boundary is draped
southwest to northeast. Decaying convective activity well ahead of
the front can be observed on regional radar over northern portions
of Kentucky. Lightning activity over the last 2 hours has decreased
significantly as the convection surges ahead of the primary forcing
along with the loss of daytime heating. Satellite and surface obs
show some clearing has occurred over our CWA as well. This will
allow for patchy fog into the early morning hours, primarily
focused in locations that experienced afternoon precipitation
saturating soils and lower levels of the PBL.

Best chance for early-late morning precipitation will generally be
north of I-40 as the aforementioned activity continues to progress
southeastward. However, as this will be moving through during most
stable hours minimal impacts are expected. Additional energy aloft
and increasing frontal forcing will allow for multiple periods of
showers and thunderstorms this afternoon and tonight. Latest RAP
model derived soundings suggest MLCAPE near 2000J/kg with a highly
saturated vertical profile and PWAT encroaching on 2 inches. Bulk
effective shear may even approach 20kts at times as shortwave energy
translates aloft. If shortwave timing coincides with peak heating,
stronger and more intense updrafts could lead to precip loading
bringing gusty winds and locally heavy downpours with the strongest
storms. Additionally, increasing dewpoints will allow for continued
heat indices in the mid 90s to near 100 across central and southern
portions of the East Tennessee valley.

&&

.LONG TERM...
(Thursday through Tuesday)
Issued at 338 AM EDT Wed Jul 17 2024

Key Messages:

1. A frontal boundary will remain across the area on Thursday,
allowing showers and storms to continue.

2. A break from the heat is expected behind the front, with slightly
below normal temperatures throughout the long term.

3. POPs remain elevated throughout the long term due to higher than
normal moisture content across the area.

Discussion:

Models seem to be in better agreement that the cold front will
remain across our area early on Thursday. The front looks like it
should push south of the area sometime Thursday afternoon/evening.
With the front in place, we will see continued chances for showers
and storms. The higher chances will remain south of I-40, closer to
the location of the front, with lesser chances north of I-40. By
Thursday evening we should be mostly dry as the front should be to
our south by that point.

NBM keeps slight chance to chance POPs in for most places on Friday.
While I do think we will still see some isolated showers around, not
sold on the coverage of the NBM. However, without knowing the exact
location of the front on Friday will leave NBM POPs unchanged.
Overall, drier conditions in place north of I-40. Better chances for
rainfall across the southern TN valley and southwest NC, which will
be closer to the front.

Chances for rain and storms increase again over the weekend. This is
as a shortwave to our southwest transitions the old frontal boundary
into a warm front and lifts it north through our area. Thereafter,
elevated POPs continue to remain in place through the remainder of
the long term. This is due to a Bermuda high in place across the
Atlantic and an approaching broad, longwave, trough to our west.
This will allow Gulf moisture to freely flow into the area and keep
moisture content higher than normal for this time of year. GFS and
ECMWF ens show PWATs ranging from the 75th to the 90th percentile
through this timeframe.

&&

.AVIATION...
(18Z TAFS)
Issued at 141 PM EDT Wed Jul 17 2024

The next 24 hours will see two, or possibly three, rounds of
SHRA/TSRA across eastern Tennessee as a cold front slowly
approaches and moves through the region. Expect the first round to
die down around 01z-02z, with another a few hours before 12z, and
then another tomorrow during the day. Coverage this afternoon
should be greatest so have prevailing TSRA in at all sites to
account for this. Will carry VCSH for the late night round and
leave tomorrow`s convection to later TAF issuances as confidence
in timing is less. May see some MVFR flight categories later
tonight due to the rain, but not confident enough in their
occurence to go with anything greater than a SCT025 base at all
sites for the time being.

&&

.PRELIMINARY POINT TEMPS/POPS...
Chattanooga Airport, TN             93  73  89  70 /  80  50  70  30
Knoxville McGhee Tyson Airport, TN  92  71  85  68 /  90  60  60  20
Oak Ridge, TN                       91  70  84  66 /  80  60  60  20
Tri Cities Airport, TN              89  69  80  64 /  90  60  50  20

&&

.MRX WATCHES/WARNINGS/ADVISORIES...
NC...NONE.
TN...NONE.
VA...NONE.
&&

$$

SHORT TERM...CD
AVIATION...CD