Microclimates and microclimatic setups of the Alleghenies

  The Alleghenies, stretching from Pennsylvania, Western Maryland and West Virginia and some will argue portions of Virginia.

In general this is the vicinity in discussion.

And a closer in view

    This region is made up of high plateaus, bogs, deeper river valleys.  A wide range of plants, some of which are more common well north of the area.

Those Cold Pockets….

   While elevation is a key factor to the micro climate that allows for these plants to survive this far south, something much more important to that elevation is topography. The high elevation topography, which features these high plateaus of the Alleghenies is loaded with high valleys, bogs. High elevation low spots if that makes sense. 

  Some of the more common spots when one thinks of these coldest spots with plant life generally found much further north are Canaan Valley, bogs of Dolly Sods, Cranesville Swamp, The Glades, Finzel Swamp, Cranberry Glades to name a few,  amongst many others.

  These areas in general will contain your coldest average minimum temperatures year round in the Alleghenies, and in some cases will contain your overall coldest average temperatures of the year.

   For example in the year 2023, January 1 through December 31.  Lets look at 2 stations. The Canaan Valley Refuge station and the Cabin Mountain station.

The Canaan Valley Refuge station installed by Virginia Tech at 3150′ had a yearly average temperature of 45.8°. While on top at the Cabin Mt Station installed by Virginia Tech at 4035′ 2 miles away as the crow flies, the yearly average temperature was 46.3°. That is the yearly average temp(every recorded temp added and  divided by number of readings) The actual mean temp spread is even greater. (That’s max+min÷2) That displays reads Canaan Valley Refuge  45.3 and Cabin Mt at 47.3. So here,  a nearly 900′ vertical difference displays the lower area as colder overall. How does that occur?

  It occurs mainly from night time mins. The yearly average min at the Canaan Valley Refuge station was 31.7° for 2023. The yearly average min at the high Cabin Mt Station read 39.6°. A difference of 7.9°.  The daytime maxes nearly always will be lower at the higher location. There are exceptions and those will be talked about a bit later. The average daytime max for the Canaan Valley Refuge station for 2023 was  58.9°. While at the high Cabin Mt Station the yearly average max read 55.0°. A difference of 3.9°.

On a typical calm, clear morning with a dry or at least a semi dry air mass in place, and a little return SW flow often gives the greatest temperature spreads. As the high responsible for the calm clear skies moves off to the east, light SW winds keep the tops mild, while the valley “decouples” from that air mass and  may reveal a temperature profile that looks something like this;

Decoupling, a disconnect to the actual airmass in place. The colder, denser heavier air takes hold and the light warmer drier winds ride over top with little impact to the valley below. Should the winds become stronger, it simply mixes and forces that lower cold air out and the disconnect is over and the temperature profile rapidly shifts back to the standard lapse rate. Then you will see the Cabin Mt Station read lower than the valley station. As all things are equal. The frequency though of the colder valley at night,  far outweighs the frequency of the colder tops at night. Winds, dewpoints are the big player.

Over the years, it’s often seen posted on social media, etc. Whatever the Davis temperature is, subtract 10° for Dolly Sods.  Davis being down river along the Blackwater, not as nicely situated topographically as Canaan Valley, still has impacts of lower, cold air drainage and if the winds are light can often run 10° to even 20° than the 4000′ ridgetops. The notion higher is colder is so often misunderstood and always tool as a given.  Notice I mention the 4000′ ridgetops.

Those ridgetops are a far cry from meaning elevation only. On top the high Sods plateau are bogs, open land allowing maximum radiational heat loss, they are higher meaning they will hold less heat. The 3800’+ bogs,  these high bogs can see frost at any point of the year and at a higher summertine frequency than the 2500′ to 3500′ cold pockets.

In 2023 I recorded frost at Alder Bog, Bear Rocks Bog on August 1st. Neither 2meter(standard recording height) reached 32°, but as you went closer to the surface I recorded a 6″ temp of 29.7 at Alder. Some of those details are found in the link below.

DOLLY SODS EDITION AUGUST 1, 2023

As you’ll notice in the link, the Bear Rocks parking lot temp remained at 47° and light winds did occur through the night. These high shallow bogs decouple at a less frequency than the deeper cold pockets as simply they are not as wind protected being high,open, and shallow. When they do however, they can be the coldest locations around and in plenty of years these locations can see the first frost in the east, or last frost even as the actual growing season between frost can be very brief. 

Let’s go back to the previous August in 2022

August 13, 2022-Sods Edition

This particular night saw temps did below freezing at 2m at Alder Bog. In the summer time, these high shallow bogs perform better than the deeper cold pockets, such as the Canaan Valley Refuge site. The deeper bogs + summertime begin to fog in as the air masses often have more moisture in them, and once the temp reaches the dewpoints, fog begins to form and fill the valley bottom creating a cap on radiational heat loss. These high bogs, are too shallow for that to occur and continue to lose heat with often just a thin shallow fog layer that doesn’t inhibit heat loss like the deeper areas.

Example of the thin fog layer

And below is an example of Canaan Valley fogged in.

In those warmer season setups, the valley typically fogs in and the high bogs run colder. All these setups will have variations. Winds, moisture, cloud cover etc.  Sometimes the bogs a bit further north at The Glades in Garrett County can run a bit colder. They are all frequently the coldest and under equal conditions, the WV high bogs will run colder, but equal conditions are not always the case. Again, air mass/moisture, winds, cloud cover.

The Glades another example of a high shallow open bog. Allows for good radiational heat loss and is settled in enough it decouples well. It is shallow enough to avoid fogging in heavy enough to eliminate heat loss, but will fog in enough it does inhibit it at times. Air mass at hand plays that role. The Glades on a good calm clear night can run 25-30° colder at near 2700′ than The Garrett County Airport at 2930′.

Example of such a case below

These extreme cases occur with the most frequency in the transition seasons. The high pressure air masses are still quite dry(low dewpoints), night times are longer(more heat loss) and the overall air masses are a bit milder(what impacts the high ground). However, winter time too can offer these extremes just as great as well with a departing cold high + snow pack with a milder return flow kicking on across the ridgetops first. 

Let’s look at a extreme example from Jan 2022

On this same day, The Glades in Garrett County recorded -28.0 while the Garrett County Airport fell to +6.8. These cold pockets running 35-40° colder than the ridges a short distance away. They also rose to the low 30s creating a +60 rise on the day.

Across the plateau and Garrett County being a great example of the plateau with the exception of the deeper Savage River Valley and Yough River valley as you go towards Friendsville. The high bogs, high valleys will be the coldest average locations vs the ridgetops. There is not enough vertical to create enough of a daytime lapse variance vs the frequency of colder nights.

  These are just some examples of a few locations and times of one of the most fascinating microclimate of the high Alleghenies. Not the only micro climate occurrence, but the most common extreme one. 

   

Refrigeration….

On a less frequent scale, would be the microclimate of the high sheltered valleys. These are much different than the more open exposed valleys. These valleys chill off at night more than the ridgetops, not as extreme as the open exposed bogs, valleys. They don’t warm up as much as the higher exposed bogs being a bit more sheltered. In fact, one occurrence can keep them colder than the ridgetops during the daytime. That occurrence is a combo of snowcover and fairly calm conditions to light winds. Notice calm and light winds are a key factor with the microclimates.  Otherwise they do not occur.

This type of microclimate is a snow preserver and often holds snow much much longer than surrounding areas. I happen to record in one of these types of locations. Bittinger 2NW Valley, a narrow valley with Glade Run being the low point as it works it’s way to the North Branch of the Cassleman River.  These types of locations often contain hemlock, sporadic red Spruce, birch, beech,  maples, oaks etc. The hemlock, spruce play a big role in this microclimate along with topography.

The topography often acts as a natural wind barrier. It also along with evergreens makes shortened diurnal periods even shorter with natural shading.  Surrounding by snow covered hillsides, natural windbreak, limited sun and the snowpack preserves itself like a giant refrigerator. An example of how a afternoon may look under such conditions below;

Not the same process as the nighttime radiational cooling process, but a process of the colder, heavier, dense air holding its ground with no mechanism to kick it out. Colder, lack of wind, less sun equals a snow cover that long outlast most surrounding areas.  Snow will melt from the top down. Opposite of most folks thought process.

There are more examples of these areas. Even the higher north facing Wv Spruce covered ridges can act like this also. The Spruce limit the wind, sun and the snow cover aids it’s preservation by refrigerating the location it’s in.

So now that we covered a few valley microclimates that feature higher locations running milder, there is one more setup where this occurs and most frequently occurs on the east side of the Alleghenies. Cold air damming. 

Cold air damming/wedge setups…

  This setup is probably most noted in the wintry weather setups, but it does occur through the transition seasons and the least frequency ot occurs would be in the summer time.

  Not at all uncommon, a high pressure will pass north with a incoming low from the southwest. An example below

The high north feeds cold air down the east side of the mountains. Sometimes extended to Georgia. Remember high pressure clockwise, low pressure counter clockwise. This drives southeast or east winds back against the Allegheny Frost.  All the while, the low pressure to the southwest is driving milder south, or southwest winds up the west side of the mountains.

For a time in the initial process, the colder east flow will be modified by the time it reaches the west side of the Alleghenies, even the Smokies and downslope warming even further to the west. That area rapidly warms up. But what happens further east is an entirely different world.  

There are multiple variations of this setup, but the boundaries nearly always play out the same. 

Snowshoe is often the southern west extend if the cold air reach, then up along the Allegheny Front, Dan’s Mt, Savage Mt and north in to PA. That’s the prime areas that are in play when the high is driving dry east winds back and forces it up the eastern slopes. That boundary often comes back west in Garrett County to the 219 corridor near and north of Sand Flat road and points east. This is helped by the higher ridges south forcing the warmer south winds higher allowing the already colder, heavier air to undercut. These ice event are often longest lasting from Mt. Storm to Savage Mt.  

   The elevations in which are most wintry prone, and the duration of wintry precip, really depend on how strong and cold the high is north. The lower elevations east of the mountains will be colder than the west ridges and much colder than the valleys west side of the Alleghenies. If its a cold artic high departing north and east. Those low valleys may just hold straight below freezing weather for the duration, or if it’s only a weak cold high. Those valleys may just hold chilly but not cold enough for anything wintry, and these locations thats almost always the case in transition seasons. The high ground can see ice events Oct-Apr.

Let’s look at a few setups of temps

You can go back through this page and see so many repeats of this style of temp setup. Some colder, some milder, but often the same boundaries.

At a vertical profile view, the process would be seen like this. That wedge can advance west further or begin east further.  With this type of profile you may see snow falling in Petersburg at 960′ and 28°,  snow/sleet along the Allegheny Front and while that air is forced up, it may be 26°, low 30s at Cabin Mt with sleet, freezing rain, and likely plain rain and mid 30s+ Canaan Valley and west.

Another  example of the under cutting wedge

Further north

These over running/ wedge events that vary in temps/precip based on the high and the low tracks and strength often maintain similar boundaries based on the topography of the Alleghenies. 

As the high exits further east, the wedge shrinks, the east winds die, often a lull in wind occurs and the south, southwest winds kick in. In many cases during the winter season this is often rapidly followed by a cold front within 6-12 hours. 

The last example I’ll add is a cutoff low that meanders in the mid Atlantic.

The dreary dreaded cutoff low…a low with no where to go

While these can occur anytime, late spring often seems to be prime time for these to occur and again as we go in to the fall season. The low sits, spins meanders along the east side of the Appalachians spinning back moisture banking it against and over the Alleghenies. The Allegheny Front often becomes obscured in dense fog and depending on where that low is, that moisture and fog may extend west across the high ground.  Spruce Knob, Dolly Sods, Savage Mt are prime fog zones with these.

These events often have lots of blocking over the top and the low can’t escape. One example last May looked like

A weaker example last fall

September 27, 2023

If you progress through the following day you can see the slow progression east as the back edge of the fog holds to Cabin Mt.

While off to the east the high Sods remained in the fog

The  Upslope Flow…

   The prevalent upslope and most favorable direction is the NW flow and that provides the Alleghenies with the brunt of the snow accumulation each winter. More snowfall occurs from this than synoptic systems alone.

  As with coastal and overrunning systems east is beast, west is less in many cases , this is the opposite with west is best, east is least. This upslope setup even often follows those over running systems.

  The best areas of upslope are

Purple zones often the best, light blue a step below and within the dark blue still good, but another notch down. Once you break over the Allegheny Front and east, things dissipate to a stray snow shower, flurries and sometimes nothing at all.

In these upslope events, that are most always accompanied by winds of some westerly component, the high ground will always run colder. The western valleys in these cases will be colder than the Eastern valleys for 2 reasons. 1- cloud cover and 2 the winds are downsloping on the east side warming things up further.