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-- Discussion containing changed information from previous version are highlighted. --
000
FXUS64 KMRX 191129
AFDMRX

AREA FORECAST DISCUSSION
National Weather Service Morristown TN
729 AM EDT Mon Apr 19 2021

.UPDATE...
FOR 12Z AVIATION.

&&

.SHORT TERM...(Today and tonight)...

A mostly dry short term period with slightly below normal
temperatures.

Any lingering showers across the north associated with the exiting
mid/upper level short wave will quickly end this morning. There will
be more clouds north than south to start, with a gradual clearing
trend from southwest to northeast during the day as drier air moves
in. We will see quasi-zonal flow aloft tonight with surface high
pressure over the region. The sky will be mostly clear for much of
the night, although some increase in high clouds will be noted.
Temperatures both today and tonight will generally run just a bit
below seasonal normals.

LW

.LONG TERM....LONG TERM...(Tuesday through Sunday)...

Key Messages:

1. Warm and dry conditions Tues give way to a cold blast Tues night
and Wed with scattered showers mixing with a bit of snow in the
mountains of E TN and SW VA.

2. Widespread frost/freeze is likely Wed night and Thurs night.

3. Temps 10-15 degrees below normal continue Thurs moderating to a
few degrees below normal Fri through the weekend.

4. Next chance of rain is Fri night into Sun morning, but
exact amounts remain low confidence.

Discussion:

Tuesday through Wednesday...

A big change will arrive during this time as a strong mid/upper
trough dives into the Midwest and western Great Lakes Tues and tilts
slightly negative late in the day. This will set up a tight
isothermal gradient over the upper OH Valley where a surface low
will develop along a strong cold front Tues evening and deepen to
about 998 mb as it lifts into NY and New England Wed. This will push
the strong cold front rapidly southeastward across the TN Valley and
S Appalachians Tues night. Ahead of the front, deep southwesterly
flow will boost 850 mb temps into the 7-9 C range across our region
Tues, and with mostly sunny skies and decent mixing anticipated in
the warm sector, raised highs several degrees above the NBM yielding
mid/upper 70`s. The deterministic guidance shows slight differences
in the timing of the frontal passage Tues night with the GFS, ECMWF,
and CMC slightly faster than the NAM which does not have the front
clearing the area until after 12Z Wed. However, the 00Z runs are
coming into better overall agreement, so expect a fropa in the 09-
12Z timeframe. The best upper jet support will be in the upper OH
Valley and Great Lakes near the developing surface low leading to
weaker frontogenetic forcing across the S Appalachians Tues night,
so expect precip to be a scattered/broken band of light showers with
QPF under 0.10 inches in most places Tues night through Wed morning.
Strong CAA Wed morning will tank 850 mb temps to -3 to -6 C.
Moisture will be exiting quickly, but a brief transition to a
rain/snow mix is expected across the mountain peaks of E TN and SW
VA where a trace to 0.3 inches could accumulate Wed morning. Dry air
advection will allow all precip to end Wed afternoon. Highs Wed will
be limited to the low/mid 50`s as the mid/upper trough axis digs
into the OH and TN Valleys with continued strong CAA. This is 15-20
degrees below normal.

Wednesday Night through Friday...

Quieter but chilly weather for this period as the deep trough aloft
gradually shifts into the NE CONUS and a large surface high drops
into the OH and TN Valleys. This will maintain a winter-like NW flow
pattern. Strong radiational cooling beneath the building surface
high Wed night will yield lows in the low/mid 30`s with a few upper
20`s in SW VA and far NE TN leading to widespread frost/freeze
headlines. Highs Thurs will only reach the mid/upper 50`s in most
areas except for a few low 60`s in the far S Valley. This is still
10-15 degrees below normal. Strong radiational cooling again Thurs
night will drop lows into the low/mid 30`s except a bit milder in
the far S Valley, so a second night of frost/freeze headlines is
likely. Heights begin to increase ahead of a shortwave trough moving
through the 4 corners region Fri, and as the surface high shifts
into the Carolinas, expect highs to rebound into the low/mid 60`s
which is still 5-7 degrees below normal.

Friday Night through Sunday...

The aforementioned mid/upper shortwave trough over the 4 corners
will eject into the S Plains by Sat as a separate northern stream
trough progresses toward the Great Lakes. The deterministic ECMWF
and CMC are still faster to phase these two features compared to the
GFS which holds off on phasing until later in the weekend. This
continues the trend of the ECMWF and CMC being wetter Fri night
through Sat night than the GFS which keeps the greatest rainfall
axis to our S. Nevertheless, confidence is increasing that this will
be a fairly fast moving system which will keep rainfall amounts
manageable no matter which solution verifies. Followed the
GEFS/Euro/Canadian ensemble means which are very similar to the NBM
giving chance PoPs Fri night expanding to likely Sat, then
decreasing to chance/slight chance Sat night. A few spotty showers
could linger Sun morning, but a mainly dry day is expected as a
surface high builds in. Temps will continue to average a few degrees
below normal next weekend.

Garuckas


&&

.AVIATION...
12Z TAF DISCUSSION.

Some light showers will be around early in the period mainly
TRI, so will include a brief TEMPO group there. Clouds will
gradually decrease, with VFR conditions expected all sites for
the period. Winds will generally be less than 10kts during the
day from the north and west, then will be light overnight.

&&

.PRELIMINARY POINT TEMPS/POPS...
Chattanooga Airport, TN             70  47  77  45  58 /  10   0   0  10   0
Knoxville McGhee Tyson Airport, TN  67  44  75  44  54 /  10   0   0  20  10
Oak Ridge, TN                       68  43  75  43  54 /  10   0  10  30  10
Tri Cities Airport, TN              64  39  72  42  53 /  20   0  10  20  30

&&

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

$$