Weather Headlines
Reference History Of Christmases Past
*A nasty pre-Christmas storm system will pass mainly south of the Cumberland Mountains
*Unseasonably mild weather conditions will dominate the Christmas Holiday period
*A transition back to a colder, wintry pattern will occur as the new year arrives by January 2020
NASTY Pre-Christmas Storm
Although this storm has been very well predicted in the modeling, the northern extent of the precipitation shield has been highly variable in the modeling.
With that noted, a review of more than 100 models of the most recent runs (includes ensemble members) places the northern extent somewhere between the Kentucky-Virginia and Tennessee-North Carolina statelines. Rain amounts within the southern Appalachians will be greatest in southwestern North Carolina.
While a few wet snowflakes can not be ruled out for crestlines of the Smokies, Great Balsams and Black mountains, it will mostly be a cold rain as the main pocket of cold air passes farther south. Wind driven rainfall and low clouds engulfing the high country of these southern mountain ranges will be the greatest impacts for those traveling, hiking, and hunting from Sunday through Monday (22-23 December 2019).
The greatest flood threat will be across the low country of the Deep South, especially the Georgia and Carolinas. Strong rises along streams draining the eastern slopes of the southern Appalachians will be possible.
National Weather Service Watches-Warnings-Advisories
The European Model, which has consistently been farther north, brings light rain as far north as Breaks Interstate Park on the Virginia-Kentucky stateline.
Clearly, heaviest rainfall (as always anticipated) will impact a large corridor from parts of Mississippi east across Alabama, Georgia, the Carolinas and Florida.
Unseasonably Mild Christmas Pattern
A broad region of upper-level ridging will generate unseasonably warm conditions during the period between Christmas and New Year’s.
Most of the continental USA will have unseasonably mild-warm conditions during this upcoming forecast period.
Pattern Change By Early January 2020
A change back into a colder, wintry pattern is currently expected as the new year begins. This also has support from past analog years featuring an analogous pattern to that of Christmas Day 2019.
Although the hemisphere pattern this year is very similar to the analogs, local conditions with respect to snow cover was certainly not identical with variations highlighted in the History Of Christmases Past section.
One would certainly be hard pressed to find a more similar upper air match across the Northern Hemisphere to that featured between analog years (above) and the forecast for this year (below) on Christmas Day.
Although every pattern and year is different, this does spark interest in seeing what came next in the analog years for their winter seasons.
Winter seasons with a Christmas Day pattern most similar to 2019 featured high-latitude blocking as a dominate circulation mode, associated with negative phases of the Arctic and North Atlantic Oscillations.
The repetitive appearance of high-latitude blocks near Greenland and in the Gulf of Alaska have already been observed this season, driven by warm sea surface temperature anomalies and other factors.
To have these blocks reappear in similar locations during coming weeks would not be surprising and has been an expected trend already highlighted in the major features to watch for Winter 2019-20.
Previous Discussion
The potential for a nasty storm system to impact the Mountain Empire is increasing for late this weekend into early next week, with more and more models beginning to follow the early trend of the GEM (Canadian) Model and JMA (Japanese) Model, and some ensembles, that have displayed a more northward track.
The Canadian and Japanese models, and many of their ensemble members, have been consistent in bringing the precipitation shield with this southern stream storm far enough north to impact much of the Mountain Empire.
While the GFS Model and its ensembles have consistently kept the precipitation shield well to the south, the NAM Model and European have recently come into better agreement with the Canadian and Japanese models.
While impacts are likely to be greatest toward the south, versus the north, with a sharp gradient and cut-off on precipitation amounts to the west and north, exact impacts remain to be worked out for given locations.
It is important to realize that 100-200 air miles is large from the perspective of a given point, but for global-scale models such as the European, GFS, Japanese (as examples) this is only a small difference.
Past climatology of similar systems suggests that the precipitation shield will likely reach northward to the mountains of southwestern Virginia and far southeast Kentucky where orographic gravity waves (mountain waves) become prolific and weather systems “feel” the underlying topography (as exemplified by the famous Mountain Empire of the southern Appalachians).
While a cold rain will be the primary mode for most, the potential for wet snow can not be ruled out for highest elevations in the southern Appalachians (e.g., Mount Mitchell and backbone of the Great Smokies).
Stay tuned for later updates.
Previous Statements
Caution For Low Temperatures And Wind Chills Into Thursday Morning
Temperatures in the 10s and low 20s will combine with breezy to gusty NW-N winds to generate bitter conditions into Thursday morning across exposed mountain ridges and plateaus. Cooling in valleys will accelerate toward morning as drainage of cold air increases.
Mountain valleys are “milder” than exposed ridges and plateaus this evening, with less wind to generate chill factors, but cold air drainage will increase overnight into morning as winds decrease and clouds diminish.
Wind chills in the 10s on exposed middle elevation ridges and plateaus, with single digits at upper elevations (sub-zero chills in gusts on highest peaks), will continue into the overnight before winds diminish in the predawn.