ALERT For Convective Snow Bursts With Accumulating Snow Through The Evening (January 4) Along With Snow Showers And Flurries
Local bursts of heavy snow, with snow squalls, will continue into the evening. Rapid drops in visibility will occur in wind driven snow to create hazardous conditions for travelers.
Slick conditions will develop on roadways in areas impacted through this evening into the overnight hours of Sunday (January 5). The greatest general coverage will be in W-NW-N flow upslope locations.
New Year And Decade_Weather Headlines
*The first significant storm of 2020 will begin impacting the mountain region Thursday into Friday (January 2-3).
A deep trough developing over the eastern USA will transport Gulf of Mexico moisture across the region with a heavy rain potential along and just west of the Appalachians.
*Heavy rainfall amounts with the potential for strong rises on streams is being monitored for late Friday into early Saturday (January 3-4).
The 51-member European Ensemble group is predicting amounts varying between 1.00″ and 3.00″ at the Wise gridpoint, such that the axis of heaviest rainfall is yet to be determined. It is most likely to be located over the Tennessee Valley and western slopes-foothills of the Appalachians given a westerly component to flow streamlines.
*Much colder air with rain showers changing to snow showers is expected Saturday afternoon into Saturday evening. Initial cold air transport on SW flow will shift NW with snow showers and possible squalls Saturday night into early Sunday (January 4-5).
Stay tuned for better resolved snowfall amounts and positioning as the event comes into view of high-resolution terrain models.
*A cross-barrier jet is expected to generate high winds along the Blue Ridge during Saturday night into Sunday (January 4-5).
*Milder conditions are expected January 6-7 as flow shifts southerly in advance of a second system.
*A second disturbance in NW flow aloft is being watched for more rain and snow during the January 7-8 period.
*A milder than average pattern is currently being indicated for the January 9-14 period.
Hemispheric Flow Pattern_No Winter Lock-down Seen Yet For Eastern USA
A review of the 2019 upper air flow regime finds persistent troughing across the western-central portion of the USA, keeping the Mountain Empire and Appalachians within a relatively mild, wet pattern (with above average total precipitation but below to much below average snowfall).
The year of 2019 will end having produced the least snowfall of the past 26 years at upper elevations in the High Knob Massif, with amounts only around 37% of the longer-term average.
A southerly to easterly anomaly to the flow pattern within the surface to 500 MB sector contributed to anomalous wetness during 2019 (and previously during 2018).
It is not surprising that this pattern wants to repeat, and that is what models are showing as the general flow pattern into mid-January.
Despite many factors favorable for persistent winter, that has not yet materialized as the polar vortex has become strong and hemispheric flow has featured limited blocking since the beginning of Meteorological Winter and a stronger zonal than meridional component to the flow field (below).
In other words, the polar jet stream has not been highly amplified and southward dips have been transitory, progressive and short-lived.
Although not at record strength, the zonal mean zonal wind flow has been stronger than average (in the 70-90% percentile) following a notable weakening in early December (above).
Temperatures have fallen to record cold levels within the stratospheric polar vortex (above), in the 50-90 degree North zone, indicating that the vortex has been stronger than average.
Although seemingly not intuitive, a strong polar vortex favors warmer than average conditions across middle-latitude continents as it promotes more zonal (west to east) than meridional (north to south) flow across the hemisphere.
A recent upward trend in temperatures above the North Pole, toward the long-term average (gray line above), appears to be associated with a reflective disturbance that will allow for some amplification of the polar jet stream into the eastern USA in the coming week, however, this will be progressive in nature and a setting which is favorable for cross-polar flow will not develop to help lock winter into the region.
The MJO is also working against USA winter, with a forecast movement into phases 4-6 which are mild phases for the USA during winter.
This certainly does not mean that no snow and cold air will occur, but for those looking for the development of a prolonged, persistent wintry pattern it is not favorable in the short-term.
These are reasons for a revision of the Climate Prediction Center forecast for January 2020, which predicts wetter & milder than average conditions across much of the southeast USA.
There are some signs of changes beyond mid-January, but those are merely trends to be followed (like analogs of past patterns).
Why may the use of analogs be less effective today? Could this be due to the forcing response of a atmosphere that is different from recent decades in terms of its chemistry and water vapor (sensible-latent heat) content?