MRX AFD:
Discussion:
Bottom line up front: No change will be made to the Winter Storm
Watch at this time, due to the lowered confidence in p-types and
snow amounts. To issue a Warning beyond 24 hours, confidence must be
high that Warning criteria will be met and that impacts will be
significant.
Model trends and differences:
The NAM has thrown a wrench in the forecast with its depiction of a
warm nose spreading well northward into our area, due to its strong
southerly low level jet. This potential for mixed p-types adds much
uncertainty to the forecast snow amounts in southern sections and in
the lee of the mountains. The main difference in the NAM is that it
develops a secondary surface low in northern AL on Friday afternoon,
which tracks NE along the Cumberland Plateau to WV through the
evening. This creates a strong pressure gradient across the TN
mountains, and a 40-50 kt LLJ. However, it is noteworthy that this
is not a typical mountain wave/downslope enhancement pattern, which
is usually characterized by a 850 mb jet max to our NW, so perhaps
the NAM is overdone with the downslope warming. Also, the NAM is an
outlier in its solution, as the GFS and ECMWF do not develop this
secondary low and LLJ, and keep warm advection to our south. The
HRRR seems to be a compromise solution, and is not as aggressive
with the warm nose and LLJ as the NAM.
Precipitation types:
At onset Friday morning, temperatures profiles will support all snow
as top-down moistening evaporatively cools a dry low level layer.
HREF p-type probs show a snow/sleet/fzra mix entering southern
sections between 1 PM and 4 PM EST. The northward extent of this mix
is a big uncertainty, but for this forecast, the HREF probs look
reasonable, and keep the mix mainly to the south of I-40. The
strength of the downslope wind will affect the p-type in the
foothills north of I-40 and the Tri-Cities, and the potential for a
brief period of mixed p-types may result in lowered snow amounts
there.
Snow amounts and confidence:
The area of highest confidence in the snow amount forecast is north
of I-40 and west of I-81, in the northern Cumberland Plateau area
and KY border counties, where the p-type is highly likely to remain
all snow. In this area, amounts are expected to be in the 4-6"
range. As we go south and east from there, confidence lowers due to
the above mention p-type issues. In the southern Valley and SW NC, 1-
3 inches of snow is expected, mainly during Friday morning and early
afternoon before the mix takes over, with the potential for up to a
tenth of an inch of ice accumulation in the mid/late afternoon and
evening. Another factor in the snow amounts will be the loss of
moisture in the dendritic growth zone from the dry slot Friday
evening, which may shut off accumulating snowfall sooner than
previously forecast. The chance of accumulating precip drops rapidly
between 7 PM and midnight EST as the dry slot moves overhead.
Freezing drizzle may be a concern during this period.
Timing:
Not much has changed from previous forecasts regarding the timing of
snow onset Friday morning. Expect that the morning commute in the
Chattanooga area could be impacted by snow, potentially starting as
early as 7 AM EST but accumulating mainly in the 9-11 AM time frame
as snowfall rates increase. In Knoxville, 10 AM to noon appears to
be the likely time frame for snow to start accumulating, with Tri-
Cities being in the noon to 2 PM window. Peak precip intensity
appears to be during the afternoon, from around 1 PM EST to 7 PM
EST. Precip will begin to end near or after 7 PM from SW to NE as
the dry slot moves across the area and brings drying in the dendrite
growth zone. This end time is faster than previously forecast, but
the end time of the Winter Storm Watch will be kept at 7 AM EST
Saturday. We transition to a NW flow pattern Friday night, which may
result in some additional light snow accumulation in SW VA and the
TN mountains into Saturday morning.
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