My Forecast For Today ( February 23 )

ALERT For A Major Winter Storm Impacting The Mountain Region Wednesday-Friday Morning With Strong Winds, Rain, Possible Thunderstorms, And Significant Snow-Rime At Mid-Upper Elevations

Overnight Into Tuesday Morning

Rain developing.  Becoming steady and soaking into the overnight, especially along and southeast of the VA-KY border.  Winds becoming ESE to SSE at 5-15 mph, with higher gusts, on mid-upper elevation mountain ridges.  Temps widespread in the 40s ( dense fog developing in upper elevations – patchy fog in other locations ).  Wind chills in the 30s along upper elevation mountain ridges.

This Morning Through This Afternoon

Mostly cloudy.  A continued chance for rain showers.  Winds SE-SSE at 5-15 mph, with higher gusts, below 2700 feet.  SE to S winds 10-20 mph, with higher gusts, on upper elevation mountain ridges.  Temperatures widespread in the 40s to lower 50s.  Areas of dense fog possible, especially along high mountain ridges.

Mountain waves will become possible Tuesday Night into Wednesday morning, with strong to locally severe wind gusts in favored breaking zones from Powell Valley in Wise County to the Clinch Valley of Russell-Tazewell counties ( and immediately next to the Kentucky side of Pine Mountain and Black Mountain ).

Tonight Into Wednesday Morning

Rain showers.  Becoming very windy along middle-upper elevation mountain ridges ( and in mountain wave zones ).  Downpours possible, with a chance of lightning-thunder overnight into morning.  SE-SSE winds increasing to 10-25 mph, with higher gusts, on mountain ridges-plateaus below 2700 feet.  SSE winds 20-35 mph, with gusts 40-50+ mph, on upper elevation mountain ridges.  Unseasonably mild with temperatures in the 40s to middle 50s.

A squall line of rain, with embedded thunderstorms, will be possible early Wednesday well in advance of an approaching cold front.  Instability during the afternoon may aid formation of low-topped convection, with northeast movement through the Great Valley toward the Cumberland Mountains along this cold frontal boundary.  The greatest threat will be potential for wind damage.

Given recent wetness, and Winter 2015-16 wetness in general, any moderate to heavy rainfall will be capable of causing strong rises along creeks as well as ponding of water in poor drainage and low lying locations during Wednesday.

A sharp temperature drop will follow with rain changing to snow during Wednesday evening in upper elevations of the High Knob Massif.  Snow levels will drop in elevation into Thursday AM.

Significant snow accumulations will be possible at mid-upper elevations during the Wednesday Night-Friday Morning period, with preliminary snow totals of 3″ to 5″ in the Norton-Wise area, 4″ to 8″ in the High Knob Massif ( above 2700 feet ), and 1″ to 3″ amid lower elevations around Pound, Clintwood & Clinchco.

Stay tuned for later updates. 

 

Weather Discussion ( February 22-23 )

Monday ended up being the best day of this work week to come, with lifting of cloud bases and gloomy but rain free skies as temperatures ranged in the 40s to middle 50s.

Rainfall totals of 0.70″ at the City of Norton Water Plant and 0.64″ in Clintwood were measured Sunday into early Monday morning.

A lead wave is spreading a steady, light to moderate rain across the mountain area tonight well in advance of the main system developing over Texas ( the big storm ).

Models had initially predicted rain tonight to remain mostly southeast of the area; however, a progressive northward shift will now allow for the most widespread, steady rain to fall along and southeast of Pine Mountain of the Cumberland Overthrust Block.

This rain will further saturate an already soggy ground to make water level rises with the main system on Wednesday potentially more problematic than if today had remained dry.

Focus now shifts to the main storm event.

Exceptional storm relative helicity, driven by strong wind shear, will be in place early Wednesday as showers & storms develop.  This will need to be closely watched despite a lack of any SBCAPE ( surface-based convective energy ).

SBCAPE = Surface Based Convective Available Potential Energy

An idea of the wind shear driving the strong SRH values can be obtained by looking at the cross-over flow in the vertical ( surface-850 MB-500 MB vectors overlaid ).

Both speed + directional shear in the vertical is acting to drive these extreme storm relative helicity values ( above ).  Superimposed upon this setting will be a mountain wave formation potential in breaking zones.

One of the interesting aspects of this storm event is that height falls will be so significant that the 850 MB level will actually drop down below the summit level of the High Knob Massif for a time.  The inversion level above is very low and it will be interesting to see how this may impact mountain wave formation.

Heights refer to the level of a particular isobaric surface, such as the 850 MB level.  It is typically around or just below 5,000 feet.  In this case, it will be dropping down to around the 4,200 foot level of the High Knob Massif.  That tends to indicate a very strong storm system developing, with falling heights indicative of large changes in mass within a vertical column of air.

Since warm air advection will be occurring against these height falls it means that the warm advection is being mostly applied to upward vertical motion, or large-scale lift.  Otherwise, the WAA would act to pump up heights and raise them to higher levels.

During Wednesday afternoon the SRH values begin to decrease amid the warm sector ( remaining very high within the most stable wedge zone with CAD to the east ).

SRH = Storm Relative Helicity

CAD = Cold Air Damming

There remains a notable gradient in the High Knob Massif, northeast Tennessee Valley zone, that needs to be watched.

The difference is models are showing some 500-1000 J/Kg of surface-based CAPE, which could generate low-topped convection with a wind damage potential ( as climatology during past such settings would suggest ).  This will need to be closely followed through the day on Wednesday.

CAPE = Convective Available Potential Energy

During this time period an inverted-V type of low-level sounding is suggestive of a marginal tornado risk, or brief spin-up, should this SBCAPE actually develop ( this will be more likely should breaks develop in the overcast to allow surface heating in between rain showers ).  Otherwise, beneath solid overcast this risk would remain minimal.

The current Storm Prediction Risk regions up to 7 AM Wednesday ( February 24, 2016 )

The main risk region for severe thunderstorms is forecast to shift east of the Appalachians through Wednesday into early Thursday.

The cold side of this storm begins Wednesday evening as rain changes to snow from top to bottom over time, with early evening featuring a change at the High Knob Massif summit level as cold air surges quickly in from the SW.

Moisture remains deep from the surface up through 700 MB to signal that significant snow will be likely, with a notable Lake Michigan connection currently being forecast.

I show the 700 MB RH and Flow Charts since 925-850 MB moisture remains copious = great moisture for snowfall production as was observed during much of the January 2016 storm event ( although snowfall amounts this time will be much less but still productive ).

In this case the 700 MB moisture field being especially important since it will maintain dendritic crystal growth as moisture reaches into -12C air or colder ( in this case that would not occur if the 700 MB moisture was not present since the summit level pilatus cloud temperatures reach only around -10 C at the end of this event ).

Reference The Physics Of Snow Crystals for more information.

Due to the thermal structure the best snow accumulations will occur across middle-upper elevations above 2000 feet, with heaviest snow likely above 3000 feet in the High Knob Massif where excellent orographic forcing and exposure will exist during this event.

Orographic Forcing = not only wind speed + direction but many other factors such as moisture and riming of snowflakes which increases snowfall volume, massif width which enhances snow amounts as well & tends to reduce carry-over to lee side valleys

This will again be a setting where more snow is likely to fall and accumulate on above ground objects than the ground, with depths being less than the total snowfall outside of drifting amid upper elevations, where wind blown snow and riming will be factors.

Lower elevations, below 2000 feet, and especially those downslope of the mountains will have a harder time accumulating snow in daylight hours ( best opportunities being after darkness falls ).

Make it a great Tuesday.