1. An Overview of Northwest Flow Snow in Western North Carolina
National Weather Service
Greenville-Spartanburg Airport
Greer, SC
Title page. A view of Cold Mountain in the Shining Rock Wilderness Area. Picture taken near Wagon Road Gap on U.S. 276 looking toward the northwest. Photo: NPS/DOI

2. What is Northwest Flow Snow in Western North Carolina?
Snowfall in WNC occurring during periods of low-level upslope (northwest) flow across the southern Appalachians TN NC
Definition of Northwest Flow Snow in western North Carolina. Three dimensional view of terrain in the southern Appalachians. SC
Johns Hopkins University Applied Physics Research Lab

3. Along the North Carolina / Tennessee border
Upslope Flow
Along the North Carolina / Tennessee border
Elevation increases of 4000 ft to 5000 ft occur over very short distances
Knoxville NC Mountains Greenville
Terrain cross section from near Knoxville, TN across the North Carolina mountains to the vicinity of Greenville, SC. The arrows in the top pane depict the upslope flow on the west side of the mountains and the downslope flow on the east side.

4. Northwest Flow Snow Minor Event Moderate Event
The image on the left depicts accumulations resulting from a typical minor northwest flow snow event. Only an inch or two of snow has accumulated in the TN border counties of the northern mountains. The image on the right shows accumulation produced by a scenario in which the moisture field impinging upon the mountains was more extensive both in terms of horizontal extent and depth. Events similar to the one depicted on the right are associated with an extensive cloud field west of the mountains. It is not uncommon for snow flurries or snow showers to be occurring in east TN and southeast KY when accumulating snows subsequently develop in western NC during periods of northwest flow.
Minor Event Moderate Event

5. Northwest Flow Snow Major Event
This image shows the snowfall distribution associated with the northwest flow snow event of December This particular event is a good example of the enhanced snowfall and significant accumulations that occur when dynamic support in the form of short waves aloft and associated surface troughs are embedded in the northwest flow. Even in northwest flow events with the added support of dynamic features, most of the snowfall is confined to the TN border counties. However, measurable snow can occur in mountain counties bordering SC and GA. Flurries and brief snow showers can also occur east of the mountains in association with well-defined upper level troughs.
Major Event

6. Primary Characteristics
Forced by orography
Often not associated with extratropical cyclone precipitation shield
Synoptic scale environment can be dominated by forcing promoting downward motion
Occasionally… troughs embedded in northwest flow enhance snowfall (e.g., December 2003)
Snowfall distribution quite irregular and accumulations highly variable
Sometimes only flurries
Sometimes warning criteria
The primary cause of the upward vertical motion necessary to produce northwest flow snow is the rapid increase in elevation that occurs along the TN/NC border.
The “classical” northwest flow snow is not associated with the “comma head” portion of an extratropical cyclone precipitation shield. Instead the snow is associated with the lifting of the broad area of stratus and stratocumulus clouds that accompany the cold air intrusion following the passage of a low pressure system that has moved along the eastern seaboard or through the interior U.S. toward New England. The synoptic scale forcing across the southern Appalachians in these cold advection situations is dominated by processes that favor downward motion.
One of the important characteristics of northwest flow snow events is one of the key forecast problems. Our current level of understanding of the these events and the current capability of numerical models allows the forecaster to anticipate the occurrence of a northwest flow event. However, the difficult part of the forecast is to know with confidence how much snow will fall and where the significant accumulations will occur. The terrain variability and the cellular or linear nature of the precipitation elements impinging upon the mountains creates a situation in which the mesoscale details cannot be known.

7. Categories Post Frontal “Classical” northwest flow snow in WNC
Strong winds
Blowing and Drifting
Comma Head
(“Wrap Around”)
Cut-Off Low
Late winter and early spring
An informal categorization of northwest flow snows in western North Carolina has been made after examining a number of events over a long period of time. The categories are certainly subject to modification as more becomes known about these events. There are three primary categories in the current system:
The “classical” northwest flow snow is termed “post frontal” because the snow occurs following the passage of a cold front. Typically, when a post-frontal event is underway, cold air advection is occurring across the southern Appalachians and deep low pressure is moving northward along the mid-Atlantic or New England coast. The Ohio and Tennessee valleys are generally overcast with stratus clouds that produce occasional flurries or snow showers. This large stratus cloud field is the moisture reservoir for northwest flow snow in western NC. Usually, the wind flow in post frontal events is blowing from the northwest through a deep layer of the atmosphere (i.e., surface through 500 or 300 mb).
The “comma head” or “wrap around” category is assigned to those cases in which the back edge of the extratropical cyclone cloud shield is still over western North Carolina. The flow aloft (e.g., 500 mb) can still be from the southwest. However, low level flow is northwesterly, thus the precipitation processes are augmented by orographic lift. (The event of 20 January 2001 falls into this category.) Comma head cases can evolve into post frontal events as the surface low pressure center moves to the northeast and the flow aloft becomes northwest.
Cut-off low cases occur primarily during the late winter and spring. (The heavy snowfall event of 8 May 1992 is a good example.) Much of the precipitation during these events is caused by synoptic scale processes (e.g., isentropic lift), but the circulation around these features when they are centered east of the Appalachians results in an upslope wind component that can enhance precipitation. The northwest flow can last for a day or two because of the slow movement of many cut-off lows.
Buckner Gap (Elev ft) - Interstate 26
Madison County (Photo: NCDOT)

8. Post Frontal HPC Surface Analysis - 1200 UTC 10 February 2005
This surface map shows a typical post frontal (i.e. classical) northwest flow snow scenario for western North Carolina.
HPC Surface Analysis UTC 10 February 2005

9. Post Frontal HPC 500 mb Analysis - 1200 UTC 10 February 2005
This 500 mb analysis accompanies the surface analysis in the previous slide.
HPC 500 mb Analysis UTC 10 February 2005

10. 5086 ft MSL View from Purchase Knob
Haywood County
Toward the Northeast
10 February 2005
2000 UTC
NPS/DOI
TERRA MODIS – 10 February – UTC
SSEC UW-Madison
The frame on the left is a TERRA MODIS image showing the cloud cover in the southern Appalachian region during the northwest flow snow event of 10 February The frame on the right is a picture (courtesy: National Park Service) of the snowfall on Purchase Knob during the same event.

11. Northwest Flow Snow Base Reflectivity
A WSR-88D Base Reflectivity image from WFO Morristown, TN. Note the cellular – and even linear – character of the precipitation elements moving into the southern Appalachians.
MRX (Morristown) UTC
10 February 2005

12. TERRA MODIS - 11 February 2005 - 1550 – 1601 UTC
SSEC UW-Madison
The frame in the upper, left portion of this slide is a TERRA MODIS image on 11 February It shows the snow on the higher terrain of WNC following the northwest flow snow event of the previous day. The frame on the right is a picture from the Purchase Knob National Park Service camera looking northeast from the southeastern boundary of the Great Smoky Mountains National Park. The picture was taken approximately four hours after the MODIS image on the left.
View from Purchase Knob
11 February 2005
2000 UTC
NPS/DOI

13. Fundamental Forecast Considerations
Horizontal and vertical extent of post frontal moisture
Flurries and snow showers west of mountains
Temperature
Stability
Wind direction and wind speed
Upwind short waves embedded in NW flow
Key items of concern to forecasters when assessing the general character and magnitude of an expected northwest flow snow event:
How much moisture is available? Is the upwind cloud field extensive both vertically and horizontally? Are snow showers and snow flurries occurring in eastern KY and east TN?
Are the in-cloud temperatures going to be in the range favorable for snow flake formation?
Will orographically induced upward vertical motion be enhanced by instability?
Is the wind direction going to be optimum (i.e., nearly orthogonal to mountains) for best upslope flow? Will the wind speed be sufficiently strong (at least 15 mph)?
Are upwind short waves embedded in the northwest flow? If so, these can enhance snowfall by augmenting the upward vertical motion.

14. Cloud and Precipitation Physics
Clouds should extend to –15oC ( 2 or 3 degrees) for most efficient generation of snowflakes (refer to Nakaya diagram)
Optimum production of dendritic snow crystals
Optimum diffusive growth rates of ice
Nakaya IR enhancement curve in GSP AWIPS
Snowflake formation is optimized when the in-cloud temperatures are approximately -15 degrees Celsius. Dendritic ice crystals are most common and the diffusive growth rate of ice is maximized near this temperature. A satellite enhancement curve (“Nakaya curve”) was developed at WFO GSP to identify cloud top temperatures favorable for dendritic ice crystal formation during northwest flow snow events.

15. IR Satellite Imagery (Nakaya Curve)
Light Blue…….. 0.0o C
Darker Blue… -10.0o C
Dark Blue…… -13.0o C
Purple……… o C
Light Purple… o C
Light Gray……-23.0o C
This infrared satellite image is an example of the Nakaya curve used to enhance cloud top temperatures during periods of northwest flow. The dark blue color focuses attention on temperatures that are within two degrees (plus or minus) of -15 degrees Celsius.
Dark Blue Highlights temperatures within 2oC of -15oC
1545 UTC April 2001

16. One of Several Moisture Sources?
Would this cloud field exist without the Great Lakes?
An issue that has not been totally resolved is identification of the moisture source (or sources) for northwest flow snow in the southern Appalachians. This example shows that lower tropospheric air parcels during the 10 February 2005 event had spent some time in the Great Lakes region. The backward trajectories originate near Asheville and can be traced 60 hours back in time to demonstrate they were in the vicinity of Lake Michigan. Other northwest flow snow events are characterized by trajectories that do not have residence time over the Great Lakes. It is not definitively known whether or not Great Lakes moisture plays a role in upslope snow events in the southern Appalachians.
1644 – 1657 UTC February SSEC

17. Model Guidance GFS – NAM – RUC
Reasonably good at depicting global and synoptic scale aspects of northwest flow snow events
Devil is in the details
The basic model guidance available to NWS forecasters does a reasonably good job of indicating whether or not northwest flow snow will occur. The primary forecast challenges are: 1) How much snow will accumulate, and 2) Where, precisely, will the accumulating snow occur. Improved model physics and improved model resolution will result in better mesoscale and local scale numerical guidance.

18. WRF DTC Winter Weather Forecast Experiment
ARW Precipitable Water
The DTC Winter Weather Forecast Experiment exposed forecasters to the nature and behavior of the Weather Research and Forecasting (WRF) model at very high resolution. Two configurations of the WRF model, both operating at 5 km resolution with explicit microphysics in the absence of any convective parameterization were available. The WRF Nonhydrostatic Mesoscale Model is designated the WRF-NMM. The Advanced Research WRF is the designated the WRF-NMM. A unique forecast field – previously not available to NWS forecasters – is the forecast radar reflectivity.
ARW Composite Reflectivity Forecast
NMM Precipitable Water

19. THE END