NWMSGuy
Member
AFD from Memphis this afternoon. Going against the NAM sounds like:
The main headline for the forecast period is the expected winter
storm Friday. There is exceptional confidence in the progression
of the upper pattern into Friday at this range. A cutoff low over
the southwest CONUS will phase with a belt of strong upper flow to
its north tomorrow. These waves will then shear out, becoming an
amplified and positively tilted trough extending from Canada with
a base in northern Mexico. Flow on the lee side of the trough
will be strong (> 70 kt) and heavily diffluent and will aid in the
development of a surface low that will travel along the Gulf
Coast into this weekend. The high pressure currently sitting over
the Midsouth will hold cold air in place to the north of this
system. As the surface low approaches, moisture being drawn out
from over the gulf will isentropically lift over this cold air
mass and produce precipitation with support from the upper
diffluence starting Thursday night into Friday morning.
There still exists some ambiguity with respect to how
precipitation types will evolve during this event. The warm nose
has been consistently between 925 and 700 mb the past few days,
isothermal, and +/- 1 F around 32 F across much of the region.
Today, much of the guidance bifurcated with respect to the
strength and location of any frozen/liquid precipitation. Most
long-range guidance (ECMWF/GFS/GDPS) and HRRR/RAP/HREF keeps this
profile below freezing for much of the Friday as far south as
Clarksdale and Tupelo. However, NAM (12/3km respectively) bring
the rain/snow line up to the TN/MS border. As such, NBM brought a
mix up with the NAM as the higher resolution members don`t go out
as far. Opted to keep snow further south in line with medium/hi-
res guidance. Regardless, the depth of the near-surface cold
layer still appears to be shallow, which still allows for decent
confidence that this will be only a rain/snow event.
In terms of accumulations, the above uncertainties are reflected
in LREF/NBM probabilities. LREF brings a large swath of > 60%
chance of seeing 4 or more inches from a line between Clarksdale
and Tupelo up to the TN/KY border. This is in line with a
HRRR/RAP-like solution. On the other hand, the NBM has the same
chances only from the TN/MS border to TN/KY. However, the
probability of seeing 2 or more inches jumps to > 60% to across
northern MS, lending enough confidence that we think the entire
CWA will experience warning criteria with a widespread area of 4-6
inches across a large portion of the Midsouth. Some convective
mesoscale banding could also provide locally higher snowfall
amounts. The timing for accumulation is expected to start between
00z and 06z Friday, lasting until 00z Saturday in the eastern
portions of the region.
The system leaves by Saturday morning with drying weather behind
it. The highs/lows should remain cold, especially in areas that
see lasting snowpack after the storm ends. The current forecast
calls for lows to drop below 20 for multiple days into next week
across northern portions of the region and in the 20s across
Mississippi. A few showers or flurries are possible Monday with a
cold front, but otherwise high pressure appears likely to keep the
remainder of the forecast dry.
The main headline for the forecast period is the expected winter
storm Friday. There is exceptional confidence in the progression
of the upper pattern into Friday at this range. A cutoff low over
the southwest CONUS will phase with a belt of strong upper flow to
its north tomorrow. These waves will then shear out, becoming an
amplified and positively tilted trough extending from Canada with
a base in northern Mexico. Flow on the lee side of the trough
will be strong (> 70 kt) and heavily diffluent and will aid in the
development of a surface low that will travel along the Gulf
Coast into this weekend. The high pressure currently sitting over
the Midsouth will hold cold air in place to the north of this
system. As the surface low approaches, moisture being drawn out
from over the gulf will isentropically lift over this cold air
mass and produce precipitation with support from the upper
diffluence starting Thursday night into Friday morning.
There still exists some ambiguity with respect to how
precipitation types will evolve during this event. The warm nose
has been consistently between 925 and 700 mb the past few days,
isothermal, and +/- 1 F around 32 F across much of the region.
Today, much of the guidance bifurcated with respect to the
strength and location of any frozen/liquid precipitation. Most
long-range guidance (ECMWF/GFS/GDPS) and HRRR/RAP/HREF keeps this
profile below freezing for much of the Friday as far south as
Clarksdale and Tupelo. However, NAM (12/3km respectively) bring
the rain/snow line up to the TN/MS border. As such, NBM brought a
mix up with the NAM as the higher resolution members don`t go out
as far. Opted to keep snow further south in line with medium/hi-
res guidance. Regardless, the depth of the near-surface cold
layer still appears to be shallow, which still allows for decent
confidence that this will be only a rain/snow event.
In terms of accumulations, the above uncertainties are reflected
in LREF/NBM probabilities. LREF brings a large swath of > 60%
chance of seeing 4 or more inches from a line between Clarksdale
and Tupelo up to the TN/KY border. This is in line with a
HRRR/RAP-like solution. On the other hand, the NBM has the same
chances only from the TN/MS border to TN/KY. However, the
probability of seeing 2 or more inches jumps to > 60% to across
northern MS, lending enough confidence that we think the entire
CWA will experience warning criteria with a widespread area of 4-6
inches across a large portion of the Midsouth. Some convective
mesoscale banding could also provide locally higher snowfall
amounts. The timing for accumulation is expected to start between
00z and 06z Friday, lasting until 00z Saturday in the eastern
portions of the region.
The system leaves by Saturday morning with drying weather behind
it. The highs/lows should remain cold, especially in areas that
see lasting snowpack after the storm ends. The current forecast
calls for lows to drop below 20 for multiple days into next week
across northern portions of the region and in the 20s across
Mississippi. A few showers or flurries are possible Monday with a
cold front, but otherwise high pressure appears likely to keep the
remainder of the forecast dry.
️
