Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
408 AM EDT Mon Oct 15 2018

Valid 12Z Mon Oct 15 2018 - 12Z Thu Oct 18 2018


Day 1...

...Ranges of Arizona and New Mexico...
Vertically stacked upper low over Arizona will drop slowly
southward through tonight. E/SE flow ahead of this feature will 
advect moisture pooled across Texas into the southwest, with some
upslope enhancement likely. Total moisture is limited and the
overall airmass is warm, and WPC probabilities are below 30
percent for 4 inches of snow except in the highest terrain of the
Mogollon Rim.

...West Texas and New Mexico...
Freezing rain is likely this evening and into Day 2 as moist
southerly flow overruns E/NE surface winds behind a cold front.
Isentropic lift is modest at 305K, and forecast soundings depict
only a shallow layer of moisture between 850 and 750mb. However, a
long duration of light freezing rain is expected in the terrain of
West Texas into southern New Mexico. Freezing rain is likely to
accrete efficiently, both due to low hourly rates, as well as
continued low-level dry/cool advection which will offset warming
from the latent heat release of freezing. There has been a noted
upward trend in guidance in freezing rain accretions, and WPC
probabilities now show a moderate slight risk for 0.25 inches
across the Davis Mountains, with lighter, but still significant
amounts of over 0.1 inches across the Guadalupe and Sacramento
Mountains. Some of this accretion is expected during Tuesday
morning, Day 2.

Day 2...

...Mountains near the Four Corners...
Closed 500-700mb low will spin slowly across Arizona while lifting
gradually to the north. This will continue to advect moisture from
Texas into the Four Corners region on S/SE flow ahead of the low.
Total forcing is modest, but 700mb omega enhanced by upslope
low-level flow will produce snow showers across the higher
terrain. WPC probabilities show a slight risk for 4 inches of snow
across the San Juans, southern Sangre De Cristos, Wasatch, and
highest terrain of the Mogollon Rim.

...West Texas and New Mexico...
Freezing rain will continue from Day 1 into the morning of Day 2.
Up to an additional 0.1 inches is possible across the Sacramento
Mountains, but WPC probabilities are very low for significant
(0.25 inch or more) additional icing.

Day 3...

...Mountains near the Four Corners...
Closed upper low will lift northward into Utah, forcing the 700mb
flow to become more southerly. This will continue to drive
moisture into the mountains, with highest WPC probabilities for 4
inches across the San Juans where flow is more orthogonal to the
terrain forcing better upslope enhancement.

The probability of significant icing (0.25 inch or more) is less
than 10 percent.

Weiss