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

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660
FXUS64 KMRX 230858
AFDMRX

Area Forecast Discussion
National Weather Service Morristown TN
358 AM EST Thu Feb 23 2017

.SHORT TERM...(Today and Tonight)...Dense fog has formed overnight
after skies cleared late Wednesday and moisture lingered from day
time rainfall. Winds are light area wide. Temperatures are in the
mid 40s to lower 50s. Will continue Dense Fog Advisory through
10 AM EST. High pressure was located off the Atlantic Coast with
low pressure over Florida and a cold front across the Great Lakes
extending through the Mid Mississippi Valley. The weak pressure
gradient over the Forecast Area will tighten up by late morning as
the sun warms the atmosphere with fog dissipating and light south
to southeast winds developing as cold front to the northwest
moves into the Ohio Valley. Temperatures will warm into the upper
60s to mid 70s as skies become partly to possibly mostly sunny
once the low level moisture gets mixed out. Tonight another mild
night with some patchy fog and lows in the upper 40s to mid 50s.
As deepening surface low moves from the central Plains to the Mid
Mississippi Valley. Light south winds will continue overnight.

.LONG TERM...(Friday through Wednesday)...
An active period of weather forecast over the long-term forecast.
The main focus will be the possibility of severe storms late
Friday night into early Saturday morning. At the beginning of the
period a robust upper level low will be traversing the Central
Plains into the Mid-Mississippi River Valley. A surface low will
track from near Kansas City to Chicago during the day Friday.
Strong low level southerly flow will advect in warm air and
moisture to the Southern Appalachians. Temperatures are forecast
to be at and even above record values. Please see the records
below for more information.

Chattanooga: 77 set in 1890
Knoxville:   76 set in 1890
Tri-Cities:  72 set in 1982

A cap will be in place around 700-800 mb and should keep any
convection from forming throughout the day. Winds will increase to
near advisory criteria during the timeframe with the tightening
pressure gradient from the approaching cold front.

The main focus for severe weather will come from roughly 9 pm Friday
through 4 am Saturday. There are still some timing issues to work
out but the consensus is that this will be overnight event. Models
indicate a low level jet of 45-55 kts around 02z-07z Saturday. Model
soundings indicate SBCAPE values of around 500-1000 J/Kg. These
soundings also show some unrealistic decoupling and these values
could be slightly underdone. The 0-1 KM SRH will be around 200-300
m2/s with deep layer 0-6 km shear around 50-60 kts. It looks as
though the highest threat area will be north of I-40 and west of I-
75 where the best combination of shear and instability well be.
However, can not rule out severe weather across any location in the
forecast area. The line should begin to weaken in the 2am-6am
timeframe as the low level jet lifts out to the northeast and the
instability begins to wane. For now, it appears that all modes of
severe weather will be possible, especially in the area of highest
risk mentioned above. Due to the low level shear, can not rule out
an isolated tornado and will go ahead and add this to the HWO.

Much cooler and drier air will filter in behind the front on
Saturday with noticeably cooler temperatures. Northwesterly winds
will also be near advisory criteria on Saturday as well. The GFS
indicates a few wrap around showers across SW VA and NE TN. The GFS
soundings are fairly dry with PW values dropping to around a quarter
of an inch. Therefore, lowered PoPs during this timeframe.
Additionally, this was the timeframe where some light snow
shower/flurries were mentioned but based on the latest guidance this
is looking less likely.

A widespread freeze is anticipated Sunday morning and this could
pose an issue to any sensitive vegetation that has begin to bloom due
to the recent warm weather. Mid to upper 20s will be possible across
SW VA and the higher elevations. Dry conditions will feature on
Sunday with surface high pressure shifting to the east.

Models are in fairly good agreement with widespread showers
developing on Monday ahead of an advancing warm front. Isentropic
lift ahead of this boundary will create widespread precipitation.
There is some difference about how far north the boundary
progresses. The GFS indicates that the boundary will move into
southern portions of the area with a few strong thunderstorms
possible. The ECMWF keeps the boundary to the south with just
showers. For now, will side more toward the ECMWF with just showers
mentioned in the forecast. This warm front will eventually lift
through the area by Tuesday with another system expected to move in
on Wednesday. The system on Wednesday could bring another round of
strong to severe storms to the area but this is well out in the
extended and will not mention it in the HWO at this time.

&&

.PRELIMINARY POINT TEMPS/POPS...
Chattanooga Airport, TN             74  54  77  50 /  10  10  10  70
Knoxville McGhee Tyson Airport, TN  74  53  76  53 /  10  10  10  80
Oak Ridge, TN                       74  53  76  50 /  10  10  10  80
Tri Cities Airport, TN              71  48  76  54 /  10  10  10  80

&&

.MRX WATCHES/WARNINGS/ADVISORIES...
NC...Dense Fog Advisory until 10 AM EST this morning for Cherokee-
     Clay.

TN...Dense Fog Advisory until 10 AM EST /9 AM CST/ this morning for
     Anderson-Bledsoe-Blount Smoky Mountains-Bradley-Campbell-
     Claiborne-Cocke Smoky Mountains-East Polk-Grainger-Hamblen-
     Hamilton-Hancock-Hawkins-Jefferson-Johnson-Knox-Loudon-
     Marion-McMinn-Meigs-Morgan-NW Blount-North Sevier-Northwest
     Carter-Northwest Cocke-Northwest Greene-Northwest Monroe-
     Rhea-Roane-Scott TN-Sequatchie-Sevier Smoky Mountains-
     Southeast Carter-Southeast Greene-Southeast Monroe-Sullivan-
     Unicoi-Union-Washington TN-West Polk.

VA...Dense Fog Advisory until 10 AM EST this morning for Lee-Russell-
     Scott-Washington-Wise.

&&

$$

TD/MA



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