# Presentation: 474084

## Unique Meteorological Characteristics of the Central Appalachians and their Implications on Aviation Forecasting

- Kevin Lipton and Aaron Tyburski
- Forecasters, State College PA

## Central Appalachians - Topography

- Source: U.S. Geological Survey

## Common Meteorological Phenomena...

- Cold Air Damming
- Low Level Wind Shear
- Persistent Low Ceilings/Visibilities
- Precip.-Type Problems
- Thunderstorm Forecasting
- Orographic Effects (mainly Plateau Regions)
- Upslope/Downslope effects on clouds
- Precip./Lake Effect Events

## Common Meteorological Phenomena...

- Topographical
- Fog formation/dissipation
- Convective Initiation
- Mesoscale Wind Circulations
- Other...
- Haze Layers

## Cold Air Damming

- Cold air remains trapped at low levels, while warmer air moves in aloft, above the cold low-level layer.
- Most prevalent _*between*_ the Coastal Plain and the Appalachian Plateau region.
- Has many effects on sensible weather, and thus on aviation!

## Factors that favor cold air damming (from HPC)...

- A cold surface high passes north of the Mid Atlantic States and New England.
- Cold air supplied by the surface high is channeled southward along the east slopes of the Appalachians.
- A ridge of high pressure develops between the mountains and the ocean. This helps to maintain a northerly component to the low level winds over the land.
- Easterly or northeasterly low level winds over the ocean and the more northerly winds over the land tighten the thermal gradient near the coast.
- When shallow arctic air is present, the computer models often have a hard time holding onto cold-enough low level temps when the surface high is still over New England or the Great Lakes regions.

## Synoptic Overview of CAD...

## Conceptual Dynamical Model of Cold Air Damming

- Diagram from UCAR/Comet Course “Cold-Air Damming”

## Effects of Cold Air Damming...and on Aviation...

- _Low Level Wind Shear_ – as NE or N winds remain within Boundary Layer, then rapid increase in wind speeds, along with strongly veering direction with height above inversion.
- Terminals within valley locations, immediately adjacent to a ridge are particularly at risk. (Example: Williamsport, PA – KIPT)

## Example of CAD Cross Section...

- Diagram from UCAR/Comet Course “Cold-Air Damming”

## Effects of Cold Air Damming...and on Aviation (cont’d)...

- _Low Clouds and Visibilities_ – extended IFR-VLIFR conditions.
- Moisture remains trapped underneath inversion – creates extended periods of low clouds, drizzle, and fog.
- _Precip.-Type Problems_ (Cold-Season):
- Freezing Rain/Sleet
- Often affects ridge-tops the worst, as slight moist adiabatic profile creates coldest air just above the surface, such as 2000-3000 Ft. AGL, with warmer above-freezing air higher up, above the inversion layer.

## Sounding Example within Cold-Air Damming...

- Diagram from UCAR/Comet Course “Cold-Air Damming”

## Effects of Cold Air Damming...and on Aviation (cont’d)...

- _Thunderstorm Forecasting_
- Cool/stable low levels often weaken thunderstorms approaching from the west.
- Weak surface lows occasionally develop on warm front near Mason-Dixon line as cool wedge begins to weaken/retreat.
- Sometimes cause thunderstorms to ‘flare’ up again.
- Increased low level shear near/just north of warm frontal boundary occasionally can aid in tornadogenesis, especially when coincident with low LCL’s.

## Example of Orographic Effects...

- From Comet/UCAR “Hydrometeorology Course”

## Orographic Effects

- _Upslope/Downslope Effects_ – can produce localized pockets of VFR conditions, embedded within broader area of MVFR/IFR conditions.
- _UPSLOPE Flow_:
- _Appalachian Plateau Region_ – worst conditions with westerly, upslope flow.
- Additional low level moisture added by Great Lakes.
- Lake Effect Snow Squalls – can produce highly variable conditions over very small distances.
- _Appalachian Ridge/Valley Locales eastward_ – worst conditions with Easterly low level flow.
- Added moisture from Atlantic Ocean.

## Orographic Effects (cont’d)

- _DOWNSLOPE flow_
- Appalachian Plateau Region – easterly wind component.
- Often KLBE (Latrobe, PA) will be VFR, while KJST (Johnstown, PA) will remain locked in LIFR conditions.
- Appalachian Piedmont – westerly wind component.

## Topographic Effects

- _Radiation Fog_
- Particularly Prevalent within Ridge/Valley section of Appalachian, although occurs elsewhere.
- Within deeper valley locations, especially in the fall, dense fog could prevail well into the midday hours, especially when adjacent to a river.
- Forecasting Challenge: When Fog will lift, and low clouds will dissipate.

## Topographic Effects (cont’d)

- From UCAR ATOC 5600, Physics and Chemistry of Clouds and Aerosols,
- “Interactions between Aerosols and Fogs”

## Example of Valley Fog...

## Topographic Effects (cont’d)

- _Convective Initiation_
- Generally favored across higher terrain (where parcels reach LFC faster).
- Often develops within Plateau region first, then shifts toward the Ridge/Valley areas, and finally Piedmont areas.
- Forecasting Challenges:
- When will convection develop?
- Will convection strengthen, weaken, or maintain its intensity as it tracks toward the Piedmont?

## Topographic Effects (cont’d)

- _Mesoscale wind circulations_
- Thermally-induced, with uneven heating between adjacent ridges, and valley floors.
- Have a diurnal nature.
- More prevalent within steeper terrain – especially Rockies, and southern Appalachians.
- Generally comprised of slope and valley winds.

## Examples of Valley Winds

- From COMET/UCAR Module “Thermally-Forced Circulations II – Mountain/ValleyBreezes”

## Diurnal Variation of Valley Winds

- From COMET/UCAR Module “Thermally-Forced Circulations II – Mountain/ValleyBreezes”

## HAZE – Appalachians and many other areas across the eastern U.S...

- Where does eastern haze come from?
- Anthropogenic SO2 comes mainly from combustion of sulfur-bearing fossil fuels in power plants, oil refineries and steel mills.
- Oxidation of SO2 creates *sulfates* (sulfate aerosols) – which are the main particles that comprise eastern haze.
- Higher relative humidities increases the oxidation rate of SO2 into sulfates.
- Sulfates are HYGROSCOPIC – in that they absorb water molecules from the environment, even when the relative humidity is below 100%.

## Haze – cont’d

- Where does eastern haze come from (cont’d)...
- Sulfate aerosols grow as water vapor condenses on them.
- Reach a size of .1 to 1, reaching a maximum in scattering efficiency per unit particle mass.
- IT IS THIS SCATTERING (OF SUNLIGHT) BY THE SULFATE AEROSOLS THAT IS RESPONSIBLE FOR MOST OF THE “EASTERN HAZE.”

## HAZE (Cont’d)...

- Most of the anthropogenic sources of sulfur dioxide are located in the Ohio Valley, and southern Great Lakes.
- Summer circulation frequently brings these sulfates around western periphery of ‘Bermuda High’, and thus across Appalachians!

## Distribution of SO2 Emissions during summer in Eastern U.S. in metric tons/day

## Long-Term Distribution of Haze...expressed as average visibility in km

## Summary

- There are numerous meteorological conditions to the Appalachian Region that produce unique challenges to Aviation Forecasters.
- Understanding the local climatology of specific regions within the Appalachians can significantly enhance forecasting skills, which can also help aviation interests within the region.

## References...

- Corfidi, S. F., 1996: **The Development and Movement of Warm Season Haze Over the Central and Eastern United States.** *Natl. Wea. Dig*., **20:3**, 2-14.
- Whiteman, C.D., 2000: *Mountain Meteorology: Fundamentals and Applications*. Oxford University Press, 355 pp.