A large vertical temperature spread, which is typical during much of the year in the mountains, will continue to be a prevalent factor tonight and again tomorrow night ( perhaps to a lesser degree by Wednesday Night into Thursday if wind speeds can increase enough to generate better vertical mixing ).
Overnight Into This Morning
Variable clouds ( intervals of high altitude cloudiness ). Gusty across mid to upper elevation ridges-plateaus. S-SW winds 5-15 mph, with higher gusts, below 2700 feet. Winds SSW-SW 10-20 mph, with higher gusts, on mountain ridges above 2700 feet. Temperatures varying from the 30s within colder mountain valleys to the upper 40s to middle 50s on exposed mid-upper elevation ridges-plateaus.
This Afternoon
Hazy sunshine & high clouds. Unseasonably warm. Winds S-SW 5-15 mph, with higher gusts. Temperatures varying from 60-65 degrees at upper elevations to the upper 60s to middle 70s ( coolest at highest elevations ).
Tonight Into Thursday Morning
High altitude clouds. Gusty across mid to upper elevation mountain ridges & plateaus. SSW-SW winds increasing to 10-20 mph, with higher gusts, on exposed ridges-plateaus below 2700 feet. SW winds 15-25 mph, with higher gusts, across upper elevation ridges. Mild with widespread temps in the lower 50s to lower 60s on mid-upper elevation ridges and plateaus verses upper 30s to lower 40s in sheltered valleys protected from gusty winds.
Showers, with a chance for thunderstorms, will become likely Thursday Night into Friday. Local downpours may be possible.
Weather Discussion ( March 8-9 )
Although pollen like this is not yet being generated, neither are such flowers, there is and will be increasing amounts of pollen from maple, willow, and various other species in the air during this unseasonably warm spell of early spring.
Loading Up With Pollen – Cultivated Coreopsis – May 2015
Pollens which actually can not be seen with the naked eye are the ones which tend to cause most of the misery to those who suffer.
Strong temperature inversions, with cold air drainage into mountain valleys, tend to hold back the pollen season some in the mountains ( as it does along chillier upper elevation ridges ) if you live in these naturally cooler locations.
Thermal belt sites, with colder air beneath them ( especially at night ) as well as above them, are the most favored places for surges of early season pollination. In this area thermal belts are typically found upon middle elevation ridges and plateaus at elevations somewhere between 2000-3000 feet.
As I have written about many times in the past, pioneers were the first to discover thermal belts in the mountains and quickly began locating orchards and other frost sensitive crops in these sites. I will have much more on such microclimatology in weeks ahead.
Focus now shifts to a slowly increasing moisture regime with models showing a significant spike upwards in total water amid the atmosphere by Thursday Night into Friday.
NAM 12 KM Model Precipitable Water Analysis At 7 PM Tuesday ( March 8 )
The precipitable water value above the Wise gridpoint at 7:00 PM Tuesday was only 0.30″ on the NAM 12 KM Model analysis. That means if all the moisture between the surface and top of a vertical column were to be precipitated out it would generate a depth of 0.3″ on the surface.
We can judge the amount of water in the atmosphere by using a quantity known as precipitable water, which is a measure of all the moisture in a vertical column between the surface and upper troposphere ( if precipitated directly out the depth of this water on a surface would identically equal the PW value ( Precipitable Water value ) ).
NAM 12 KM Model Precipitable Water Forecast At 1:00 AM Friday ( March 11 )
Computer models can quickly calculate these values by using calculus & integrating the dewpoint ( or saturation mixing ratio ) between the surface and top of the troposphere ( where moisture typically becomes a minimum ). Integration, of course, yielding by definition a summation whose value = the PW value for any given vertical column of air above a given gridpoint or interpolated point amid the larger-scale forecast grid.
The NAM 12 KM Model ( above ) predicts a rather dramatic increase in PW values between now ( very dry air ) and late Thursday into Friday morning when values rise to around 1.50″ ( moderately high for any given time of year ).
While PW values are always important, they also need to always be looked at with respect to mean climatological values for the time of year.
For example, mean monthly PW values for March between the surface and 500 MB are roughly around 1.20″ ( closer to 1.00″ during early March ). So if a 1.50″ PW value develops into early Friday it will be approximately 20% to 30% above average for this time of year.
By comparison, the current PW values at 7:00 PM Tuesday was only 25% to 30% of average for this time of year ( approximately 4 times below average for early March ).
NAM 12 KM Model MSLP and 1000-500 MB Thickness Forecast – 1 AM Friday
Another forecast factor I like to look at is the thickness of the atmosphere between the surface and 500 MB. Heavy rain often has a preferred thickness range within which climatology shows that it tends to fall during any given time of year. Early March preferences for this mountain region being roughly around 556 dkm ( +/- ) 5 dkm.
In other words, the vertical depth of the atmosphere between the surface and 500 MB tends to be around 5,560 meters give or take some 500 meters ( 551 to 561 dkm ). The forecast above by 1 AM Friday is a little higher than the preferred thickness here, at around 565 dkm or 5,650 meters ( by contrast, the thickness predicted over Arkansas is within the preferred range ).
NAM 12 KM Model Total Rainfall Forecast – Up To 7 AM Saturday ( March 12 )
Looking strictly at the above, one might say that rainfall will become likely ( with local downpours possible ) but not yet within the preferred range for widespread heavy rain. Remembering, most importantly, the above is a forecast and this will need to be followed and adjusted accordingly as time for rain to occur gets closer.
The latest European 9 KM Model run is keeping the bulk of heavier rain well to the north and west of the Mountain Empire through Saturday Morning. A trend to follow.
There are many other factors that forecasters should look at to form the complete picture, with the above being only a glimpse into how moisture values should be considered.
***An important note is that when convection is able to form it can compensate for lower PW values, at any given point or time, via inflow which tends to concentrate PW values and increase them over localized areas ( thus greatly complicating what might seem to be a rather clear-cut way of forecasting rainfall intensity ). Superimposed upon a boundary or favorable topography adds more layers to this “rainfall equation.”
