012117 Forecast

Mountain Area Forecast ( Jan 21-24 )

An Alert For Development Of A Major Storm System, With Deep Low Pressure, Is Now In Effect From Late Sunday Into Tuesday Morning. 

A powerful low pressure will develop along the Appalachians by late Sunday into Monday.  Showers & thunderstorms Sunday will give way to the development of heavy rainfall Sunday Night into Monday, with significant rainfall amounts expected.
Strong rises on streams will be likely into Monday.  Folks living & driving through low-lying and flood prone locations will need to remain alert for changing conditions.
An increase in wind speeds and colder air will develop by late Monday into Tuesday, with a change from rain to accumulating snow at elevations above 3500 feet. 

Overnight Into Saturday Morning

Mostly cloudy ( high clouds ).  Areas of valley fog possible.  Winds SSW-SW at 5-15 mph, with higher gusts, on ridges & plateaus below 2700 feet.  SSW-SW winds 10-20 mph, with higher gusts, on upper elevation mountain ridges.  Temps varying from 30s to around 40 degrees in valleys to the upper 40s to low-mid 50s on gusty mountain ridges.

Saturday Afternoon

Increasing clouds with a chance for showers or steady rain by middle to late afternoon.  Unseasonably mild.  Light SE-S winds at mostly less than 10 mph.  Temperatures varying from upper 40s to lower 50s at highest elevations to the upper 50s to lower 60s.

Saturday Night Into Sunday Morning

Mostly cloudy.  Chance of showers.  SE-S winds increasing to 5-15 mph, with some higher gusts along mountain ridges by morning.  Temperatures in the 40s to lower 50s.  Areas of dense valley fog.

Sunday Afternoon

Mostly cloudy.  A chance of showers & thundershowers.  Mild.  ESE-SSE winds at 10-20 mph, with some higher gusts, especially across middle-upper elevation mountain ridges & plateaus.  Temperatures from the 40s to lower-middle 50s, coolest at the highest elevations.

Strong water level rises on streams will be possible into Monday.  Folks living and driving through low-lying, flood prone locations will need to remain alert to changing conditions.

Sunday Night Into Monday Morning

Rain developing.  Becoming windy.  Heavy rainfall totals.  Winds shifting from easterly to northerly into morning.  Wind speeds 10-20 mph, with higher gusts, on mid-upper elevation plateaus-ridges.  Dense fog ( orographic clouds ) at mid-upper elevations.  Temperatures dropping into the 40s.  Wind chills falling into the 30s in upper elevations.

Monday Afternoon

Rain & showers.  Low clouds with dense fog at elevations above 2500-3000 feet.  Patchy fog at lower elevations.  NW-N winds 10-25 mph with higher gusts.  Temperatures falling into the 30s to low-mid 40s ( coldest at highest elevations ). Wind chills dropping into the 20s & 30s.

A change from rain to snow is expected in upper elevations by late Monday into Tuesday AM.  Accumulations of 1-3″ are expected at elevations above 3500 feet, with 3-6″+ at elevations above 4000 to 5000+ feet in the southern Appalachians.  Slushy to hazardous driving conditions are expected at these high elevations.

Monday Night Into Tuesday Morning

Rain & showers.  Rain changing to snow at the highest elevations.  Winds NW-N at 10-25 mph, with higher gusts.  Dense fog at elevations above 2500-3000 feet, becoming freezing fog at elevations above 3400 feet.  Temperatures dropping into the upper 20s to mid-upper 30s ( coldest at highest elevations ).  Wind chills in the 10s and 20s ( again coldest at highest elevations ).

A storm system tracking across mid-continent will push milder air briefly back into the mountain region by the middle of next week,  before temperatures crash during late week to mark the start of a prolonged wave of cold conditions.


Weather Discussion ( Major Storm )

Saturday Night Update

The strongest low pressure of the winter season to date will be impacting the mountain region Sunday into Tuesday AM, with home barometers expected to plunge toward 29.00″ .

Any time pressure has reached that low in the past, strong winds develop as the center begins moving away with rapidly rising air pressures as air flow rushes across the mountains to fill the “void” of low pressure ( i.e., the atmosphere works to achieve balance ).
NAM 12 KM Model 500 MB Forecast To 7:00 PM Monday – January 23, 2017
Although varying at the surface with details, models remain fairly consistent at upper levels.  Past climatology says that significant weather impacts will occur with this type of storm development, which will present an array of conditions varying from showers and possible downpours in thunderstorms Sunday to heavy rain Sunday Night into Monday.  Winds increase and air turns colder later Monday as barometric pressure begins rising rapidly as the storm center shifts east.  Rain turns to accumulating snow at upper elevations ( cold air is limited, so this will be a strongly elevation biased event with respect to snow accumulations ).
NAM 12 KM Model 850 MB Forecast To 7:00 PM Monday – January 23, 2017


Previous Discussion

This is the “quiet” before major storm development occurs by late weekend into early next week over top of the great southern Appalachians.

Sunset From Powell Valley Overlook Along U.S. 23 On January 20, 2017

The energy responsible for this major storm system is just entering the western USA, and will track eastward across the southern states this weekend.

NAM 12 KM Model Initialized 500 MB Vector Wind-Vorticity At 7 PM Friday ( Jan 20 )
Models are in excellent agreement with the wave developing a negatively tilted ( from NW to SE ) upper trough, with strong low pressure development from the surface upward to 500 MB by late Sunday into Monday.
NAM 12 KM Model 500 MB Vector Wind-Vorticity Forecast At 7 AM Monday ( Jan 23 )

A couple of different low pressure centers are currently forecast to give way to rapid development of a deep, strong low center over the southern Appalachians early Monday.  It will be important to check back as new model runs come out through the next 24-hours.

NAM Model Surface Winds & Mean Sea Level Pressure Forecast: 7 AM Jan 23

The Friday evening ( 00z ) run of the European Model has a 984 MB ( 29.06″ ) low near the Tri-Cities at 1:00 AM Monday.

Either scenario, by the NAM or European, would create a high impact upon the mountain area with strong winds and heavy to potentially excessive rainfall ( enhanced by orographic forcing along the Cumberland Mountains and TN-NC border area ).

Since the wavelength between major waves is relatively short, only a brief window of opportunity will exist from Monday afternoon into Tuesday morning for a possible transition to snow at mid-upper elevations ( forced by a combination of dynamic + upslope + advective cooling of the vertical column…before warm air advection aloft develops in advance of the next mid-continent system ).

The Bottom Line:

Major Storm Development is expected by late Sunday into Monday with deep low pressure forming over top of the southern Appalachians.  Heavy rain and strong water level rises on streams will be possible.

Folks living & driving through flood prone, low-lying areas will need to remain alert to changing conditions.  Strong winds will also develop, especially at middle-upper elevations.  In addition, a change to wet snow is being monitored for upper elevations by later Monday into early Tuesday ( with snow levels and any possible amounts yet to be determined ).