050416 Forecast

My Forecast For Today ( May 3-5 )

Very cold air aloft will support the development of numerous showers & thunderstorms, especially from late Tuesday into Thursday.  Many will be capable of producing hail.  As cold air deepens some snow and sleet will become possible by Wednesday Night into Thursday, especially across upper elevations.

*Some accumulation of snow-sleet will be possible above 3500 feet into Thursday Morning as unseasonably cold air, and wind chills, impacts the mountain region.  Some mix of wintry precipitation will be possible at lower-middle elevations.

Overnight Into Tuesday Morning

A chance of rain showers.  Areas of dense fog at highest elevations and in places that had heavy Monday rainfall.  Winds shifting WNW-NW at 5-15 mph along middle-upper elevation mountain ridges into morning.  Temperatures dropping into the 40s to middle-upper 50s ( coolest along highest mountain ridges ).

Tuesday Afternoon

Mostly cloudy.  Cooler.  A chance of showers.  Local thunder possible.  WNW-NW winds at 5-10 mph, with higher gusts.  Temps varying from upper 40s to lower 50s at highest elevations to the middle to upper 60s.

Tuesday Night Into Wednesday Morning

Mostly cloudy.  Chilly.  A chance of showers.  Local thunder & small hail possible.  W-NW winds generally less than 10 mph below 2700 feet.  W-NW winds 5-15 mph, with higher gusts, on mountain ridges above 2700 feet.  Temperatures varying from upper 30s to lower 40s at highest elevations to the mid 40s to around 50 degrees.

Wednesday Afternoon

Partly to mostly cloudy.  A chance of showers and hail producing thunderstorms.  SW-W winds at 5-15 mph, with higher gusts.  Temperatures varying from the 40s to lower 50s at highest elevations to the middle-upper 60s ( warmest at lower elevations and south toward the Tri-Cities ).

Wednesday Night Into Thursday Morning

A chance of showers and hail producing thunderstorms during the evening.  Rain showers becoming mixed with and changing to snow-sleet overnight into morning in the upper elevations ( mixed precipitation possible in middle to lower elevations ).  WNW-NW winds 5-15 mph, with higher gusts, on mid-upper elevation mountain ridges-plateaus.  Temperatures varying from upper 20s to lower 30s at the highest elevations in the High Knob Massif to the upper 30s to lower 40s.  Wind chills in the 20s and 30s along mountain ridges, with 10s in gusts on the highest peaks ( some rime formation possible along the highest ridges-peaks ).


Weather Discussion ( May 2-5 )

The stormy weather pattern that has opened May will be continuing through coming days, with forces driving this storminess shifting toward instability based cold air aloft.

High Knob Massif Webcam
High Knob Massif Webcam – University Of Virginia’s College At Wise

Increasingly cold air aloft will begin to deepen in the vertical during coming days, with a large lapse rate or difference between surface layers and altitudes above 15,000 feet within the atmosphere, where air temps will plunge to -10 to -20 degrees ( Fahrenheit ) below zero.

European Model 500 MB Height Anomaly Forecast
European Model 500 MB Height Anomaly Forecast – 8 AM Wednesday ( May 4 )

This pattern is simply straight out of winter, with only modification by the time of year ( e.g., high May sun angles, longer days, warmer ground temperatures and low-level moisture via evaporation and transpiration ) working to ease some of the pain of a late season cold blast.

European Model 500 MB Height Anomaly Forecast
European Model 500 MB Height Anomaly Forecast – 8 AM Thursday ( May 5 )

During May a setting analogous to the heart of winter creates other complicating factors, namely it enhances instability driven by increasingly cold air aloft and lapse rates ( differences in temperature between surface layers and altitudes above 10,000 to 20,000 feet in the free air ).

Numerous showers and thunderstorms will become possible as this instability increases, with many being capable of producing hail as the freezing level aloft drops.  Past settings like this have occasionally generated copious amounts of small to medium size hail.  A spring example being April 15 in 1998 when VDOT snow plows had to be called out for 3-4″ hail depths on routes such as U.S. 23 and State Route 80.
Pink Lady's-slipper Orchid ( Cypripedium acaule )
Pink Lady’s-slipper Orchid ( Cypripedium acaule ) – May 1, 2016

At a time of year when gorgeous spring ephemeral wildflowers are part of the main show, such a late season blast of cold air might seem unheard of; however, it is actually rather common ( and over the long-term locally gave rise to a May 10 spring planting rule…not to plant tender vegetation prior to this time due to cold air ).

At times the cold air blasts can be substantial enough to support snow, with May falls of snow recently being documented during May 1989 ( 2-4″+ ) and May 1992 ( 2″ up to 2-3 feet ).  May snow flakes being most recently observed during May 2005.

 It is too early to know what night or nights will be most favorable for valley frost, but an early estimate would be following the break-down of the northerly-northwest low-level flow into the upslope side of the mountains by late Thursday into Friday morning.  Stay tuned for updates.