The first in a series of strong upper level troughs and associated surface low-pressure systems (one of which helped spawn several days of severe weather and heavy thunderstorms) is exiting the western Great Lakes region today (Fig. 1). In its wake, a second, stronger system will be moving into the inter-mountain west (Fig. 2). This system, with unseasonably cold air throughout the atmospheric column, should lead to some significant mountain snowfall across Utah and Colorado later in the weekend. Snowfall will likely be enhanced on east-facing slopes of the Rockies as a strong northeast low-level wind flow sets up by Sunday (Fig.3).
In fact, earlier today, the National Weather Service (NWS) office in Denver/Boulder, CO issued a winter storm watch for Mother’s Day Sunday which noted that snow was expected to form over the mountains Saturday, with a snow level of around 9,000 feet. Rain is expected to transition to snow at successively lower elevations Saturday night, such that snow will arrive across the eastern Plains during the day Sunday. According to the NWS, look for 1 to 2 feet of snow in mountain locations and 4 to 9 inch snowfalls across the foothills. Snowfall amounts in the Plains will depend upon when the transition to snow occurs on Sunday.
Accompanying the rain and snow, strong winds (gusting to 30 miles per hour or more), in conjunction with heavy snow, could lower visibilities to a quarter of a mile or less. Lowered visibilities and snow-covered roads could lead to some road closures.
The heavy wet snow should accumulate sufficiently on leafed out trees to break tree limbs. Downed limbs and trees will likely lead to power outages.
To the east of this storm system, an upper level ridge will allow many places to experience above average temperatures well into next week. This is on top of the myriad of warm temperature records (both maximum and minimum) set at several eastern U.S. cities on May 8, 2014.
Finally, warm, dry air from the southwest U.S. will still be affecting parts of southern and central Plains states.
In between these three widely different air masses will be a zone of conflict. Daily bouts of severe weather and heavy thunderstorms can be expected across the lower Mississippi River valley to south and east Texas today, retrograding westward to the southern Plains northward to Iowa and Illinois by Monday. Retrograding means moving against the normal wind pattern (which at these latitudes, is from west to east).
Computer models show widespread coverage of one to two inch rainfall amounts across this zone, as well as localized heavier amounts. Given the history of the tropical air mass and a predicted tendency for storms to train (move) over the same location, rainfall amounts could easily reach double or triple the average predicted amounts. Over parts of the Colorado Rockies, several inches of rainfall (or melted snow) are expected, as well (Fig. 4). Localized flooding should be anticipated wherever heavy precipitation occurs.
By midweek, the storm system should migrate eastward, crossing the Mississippi River Valley and begin affecting the Appalachians (Fig. 5). They system should also start to assume a “negative tilt,” meaning that it would become oriented northwest to southeast. This enhances upper level divergence and increases the risk for heavy rainfall and severe weather to the east of the system.
By next weekend, computer models show this system “cutting off” and becoming a closed low-pressure system over the Great Lakes (Fig. 6). At the same time, a second cut-off low moves into the Pacific Northwest. Both of these lows are linked to the larger polar vortex expected to be located over far northern Canada.
It isn’t until the beginning of the following week (Fig. 7) that the upper level wind pattern becomes “zonal,” blowing mainly west-to-east, across the U.S. It’s at this time that more seasonal and tranquil weather will again return U.S.-wide.
© 2014 H. Michael Mogil