Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
408 PM EST Fri Nov 27 2020
Valid 00Z Sat Nov 28 2020 - 00Z Tue Dec 01 2020
Days 1 to 3...
An upper low will bring light snowfall accumulations to portions
of NM and the TX Panhandle tonight into early Saturday. Generally
looking at an inch or two over portions of central and northern
NM, with localized 4" totals in some of the higher elevations. A
bit more uncertainty from far eastern NM into the TX Panhandle.
Most guidance suggests we should have enough cold air aloft
underneath the upper low for some light snow accumulations across
this corridor late tonight into Saturday morning. A swath of 1-2"
seems plausible near the NM/TX border, with a lower chance of a
narrow axis of 2-3" if rates get high enough underneath the upper
Snow will continue tonight across the higher elevations of the WA
Cascades, with another round of snow possible on Monday. This
system on Monday is a progressive but pretty dynamic system...with
a strong shortwave bringing the potential for a quick shot of
moderate to heavy snow to the higher elevations (mainly above 4000
feet). Some model spread with this system is noted...with the
latest ECMWF shifting the system south and missing the WA
Cascades. This is more of an outlier at this time, with the latest
snowfall probabilities depicting a 40-60% chance of 6"+ of snow
An area of low pressure will track from the Gulf Coast northeast
into WV by 12z Monday. We should begin to see some phasing of the
southern and northern streams on Monday resulting in a deepening
of the low pressure system. This should support a deformation band
on the northwest side of the low, and an expanding area of lighter
snows/snow showers further west underneath the developing
mid/upper level low. Thus we should end up with some accumulating
snowfall across portions of the OH Valley by late Sunday night
into the day Monday.
Models are beginning to cluster on the low track and overall QPF
placement, however the snowfall accumulation remains uncertain.
Boundary layer temperatures will be marginal, and ground
temperatures warm. So will likely take decent snowfall intensity
to get accumulations. This will be possible within any deformation
band...and even underneath the upper low with temperatures cooling
enough to eventually get some light accumulations. Looking at the
last several runs of the ECMWF and GFS, along with the ECMWF
ensemble, suggests the highest probabilities for accumulating
snowfall through 00z Tuesday will be from far northern KY into
eastern IN and western OH...with moderate probabilities of 2"+
across this corridor. A broader area of coating to 1" is likely
within the more extensive light snow/snow showers underneath the
developing upper low...but any higher accumulations should end up
within any deformation axis on the west/northwest side of the low.
These dynamic systems can sometimes over perform with snowfall as
dynamic cooling takes over, so some risk of higher accumulations
certainly exists...but a general 1-4" seems most probable at this
time, through 00z Tuesday, over the aforementioned most favored
areas. Additional snow after this time will be possible over OH,
especially near Lake Erie.
For Days 1-3, the probability of significant icing (0.25 inch or
greater) is less than 10 percent.