Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
417 AM EDT Sat Apr 27 2019

Valid 12Z Sat Apr 27 2019 - 12Z Tue Apr 30 2019

...Northern Plains through the Great Lakes...
Day 1...

Compact but robust shortwave trough will race southeast from SD to
MI today and tonight before weakening and ejecting into New
England Sunday. This feature will race on the nose of a 90kt upper
jet, which will form a broad coupled structure with a secondary
jet max to induce lift and spawn cyclogenesis over the central
Plains. This low will intensify while moving eastward, spreading a
narrow but impressive plume of moisture across the area.

Initially temperatures are likely to be warm enough in the low
levels that most of the precipitation will fall as rain. However,
intense ascent through the aforementioned synoptic forcing, as
well as a maturing TROWAL and steeply sloped 850-600mb fgen
coincident with a saturated DGZ in an environment characterized by
weakly unstable MUCape and periodically negative theta-e lapse
rates suggests intense snowfall rates which will dynamically cool
the column to change precipitation over to snow. Additionally, dry
advection from the east will lower near-surface wet bulb
temperatures, so as precipitation begins to fall this will
additionally aid in cooling the column.

As this occurs, snow will likely come down heavily, and HREF
probabilities are high for 1-2"/hr snowfall rates. Despite a
relatively short temporal duration of intense ascent of 6-12
hours, and marginal low-level thermal profiles, these types of
snowfall rates will allow for rapid accumulation and a narrow
stripe of 6-10" of snow is likely. Guidance has trended even
further south overnight, leaving the NAM as a northern outlier
compared to much of the global consensus and other high-res
guidance. This leaves the highest WPC probabilities for 8 inches
aligned from far NE IA southeast towards the Chicago metro area,
and isolated 12" amounts cannot be ruled out, especially if
thundersnow occurs in the weakly unstable environment. However, a
sharp snowfall gradient is likely, so minor fluctuations in the
low track and placement of strongest frontogenesis could lead to
significant variations in snowfall amounts.

Surrounding the max swath of snow, WPC probabilities are moderate
for 4 inches from SW MN into the western half of the LP of
Michigan and even east into western New York, where the best
overlap of forcing is either of shorter duration, or less intense
such that some QPF is lost to rain before transitioning over to
accumulating snow.


...Northern Rockies into Northern Plains...
Days 1-2...

Upper trough diving through British Columbia, Canada Saturday will
dig into Montana Sunday while closing off. This will spawn surface
cyclogenesis in the lee of the Canadian Rockies, which will
intensify and drop into the CONUS Sunday. This system will be
accompanied by robust lift due to PVA around the stacked low,
upper diffluence as the jet max pivots around the trough, WAA and
focused frontogenesis. Although thermal profiles are marginal
noted by a shallow above freezing layer near the surface, snowfall
rates are forecast to be enough to dynamically cool the column,
and heavy snow accumulation is likely, and supported by nearly all
available guidance. This feature will be fast moving from west to
east, and despite a short window of heavy snow Sunday into Monday,
WPC probabilities are high for 4 inches from eastern MT into
northern ND, with a moderate risk for 8 inches where the best
deformation lingers north and west of the 700mb low.


...Central Rockies and Central High Plains...
Day 3...

Impressive closed low SW of California Sunday will lift northeast
and begin to open Monday as it approaches the Four Corners region.
As this occurs, a potent 100kt jet streak will round the base of
the trough, and as it opens into a positive tilt, strong moist
advection through the column will drive PWAT anomalies to +2 to +3
standard deviations above the mean across the Central and Southern
Rockies Monday. At the same time, a 140kt jet streak centered over
the Great Lakes will place Colorado beneath the favorable RRQ for
ascent, all while a cold front drops southward and banks against
the Front Range producing easterly low-level flow. All the
ingredients appear to be in place for a significant snow event
late Sunday and through Monday as this robust lift takes place in
an anomalously moist environment. Snow levels will feature a tight
gradient from NE to SW, and it is likely that heavy snow will pile
up to more than 12 inches in the terrain above 6000 ft in the San
Juans and Colorado Rockies, with upslope enhancement favoring the
Front Range just west of Denver for the highest totals. Further
east into the Plains, the jet streak will likely produce a swath
of snow across NE CO and into Nebraska where WPC probabilities for
4 inches range from high in the foothills of CO, to moderate in
the southern Panhandle of NE.


The probability for significant icing is less than 10 percent all
3 days.


Weiss