Weekend Weather Blog: A technical look into this week’s winter storm

Forecast snowfall through Wednesday morning
Forecast snowfall through Wednesday morning(Nicholas Snider)
Published: Dec. 11, 2021 at 10:52 PM MST
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TWIN FALLS, Idaho (KMVT/KSVT) — A Winter Storm Warning is in effect for central and northern Blaine county, and a Winter Weather Advisory is in effect for Lincoln, Minidoka, and southern Blaine counties. Both are in effect until 5 AM Tuesday.

In this week’s Weekend Weather Blog, you’ll see what exactly goes into forecasting these types of events - with all the technical terms there are. First, let’s define a few acronyms.

PVA stands for ‘positive vorticity advection’. This means that the wind is blowing vorticity, or atmospheric energy to some locations. This causes cooler temperatures to advance into an area where this occurs. PDVA stands for ‘positive differential vorticity advection’. This means that PVA is stronger higher up in the atmosphere than in lower levels.

NVA and NDVA stand for ‘negative (differential) vorticity advection. These mean the opposite of P(D)VA. PDVA can bring rising motion, allowing for cloud and precipitation formation.

LLWAA stands for ‘low level warm air advection’. This means the winds are blowing warmer temperatures to another location. LLCAA stands for ‘low level cold air advection’. This means the winds are blowing cooler temperatures to another location.

On top of this making a region warmer or cooler, LLWAA can enhance upward motion for precipitation and cloud formation.

PDTA stands for ‘positive differential temperature advection’. This means warmer temperatures are being blown more quickly to another location higher up in the atmosphere than lower in the atmosphere. NDTA means the opposite of PDTA. This has in impact on how warm or cold a location can get.

FROPA stands for ‘frontal passage’, meaning a front has passed, and cyclogenesis means an area of low pressure has formed. FGEN stands for ‘frontogeneis’, and means a front has formed. CORF stands for the Coriolis force, or the force that steers wind to the right of center.

Now that we’ve defined the technical terms, here’s the technical forecast through Wednesday morning.

Stronger PDVA will look to work its way into the forecast area as we go throughout the nighttime hours of Saturday night into Sunday. While initially, the first burs won’t likely be enough for the onset of precipitation, the second burst should be able to generate much stronger evaporative cooling for the onset of precipitation as we go throughout the nighttime hours up in the WRV.

RAP soundings suggest onset of precipitation between 7 and 9z Sunday (12 AM-2 AM local time). Strong omega through a deep low-mid level dendritic growth zone will be favorable for some modest snowfall rates throughout the nighttime hours of Saturday night into Sunday.

Primary forcing for this will be the strong LLWAA present throughout the duration of the event, as VORT advection doesn’t appear to be quite as strong. Weak instability, with lapse rates showing near the MALR, and plentiful moisture within the DGZ is concerning me for the release of some marginal CSI developing near the onset of precipitation.

One limiting factor on this, however, is the fact that mid-level winds, while unidirectional, are proving to be fairly weakly sheared, which (once again) may hinder geostrophic absolute momentum contours from becoming more horizontally oriented than the isentropes. Cross sectional analysis is also lacking on isentropic ascent, which will NOT allow for the release of CSI.

With that, upslope forcing, and WAA will be the primary forcers behind this event as we go throughout the initial event. With a WSW’ly component of flow, western and southern sides of the slopes will see the majority of accumulation. Outside of upslope areas, moisture, and negative omega will still be present in the mid and upper levels, but insufficient in the lower levels for the generation of precipitation there.

With that, will keep clouds in the forecast overnight Saturday night into Sunday. With the lack of strong PDVA, as well as CSI banding, will keep snowfall rates progged near .25-.5 in/hr in the WRV, with higher rates in the upper levels of the mtns, with the increased upslope forcing (.5-.75″/hr).

With the presence of cloud coverage throughout the nighttime hours OLR will be trapped in the atmosphere, allowing for some warmer temperatures for lows. With that, lows should reach into the middle and upper 30s for the SRP, upper 20s and lower 30s for the SH, and middle and upper 20s for the WRV.

Weak PDVA should continue throughout much of the morning on Sunday, providing for sufficiently negative omega for continued generation of precipitation throughout the morning hours. Eventually some NDVA will begin working into the area around the late morning and early afternoon hours of Sunday (I’d say between 18Z Sunday and 3Z Monday (between 11 AM and 8 PM Sunday).

This will allow for slight adiabatic compression in the mid and upper levels of the atmosphere. However, continued strong LLWAA will occur throughout the day on Sunday, allowing for negative omega to continue in the low and middle levels of the atmosphere. Soundings are indicative of this, as the DGZ is continuing to show negative omega throughout the day on Sunday (albeit on a much lower magnitude).

With this, will decrease rates down to around .25″/hr or less outside of the beaks, .25″ to-5″/hr in the peaks during the day on Sunday. Outside of the precipitation areas, LLWAA should continually provide sufficiently negative omega for the generation of mid-level cloudiness over the entire forecast area throughout the day on Sunday, keeping some shallow altostratus around.

With the RAP overmixed as always, and the NAM undermixed as always, and progged fairly steep lapse rates throughout the SRP, will mention the potential of some stratocumulus throughout the day on Sunday as well. These may produce some precipitation over the SH, allowing me to believe in some light accumulations there.

With a fairly similar pattern to Saturday developing throughout the day on Sunday (with the exception to the WRV), temperatures are expected to remain relatively the same throughout the day. Highs are expected to reach into the middle and upper 40s for the SRP, upper 30s and lower 40s for the SH, and middle and upper 20s for the WRV.

NDVA Sunday will be replaces by strong PDVA that will move into the forecast area throughout the night Sunday night into Monday, as an approaching potent trough and VORT max off the Pacific coast will begin to slowly propagate its way closer and closer to the forecast area.

This increased PDVA will really enhance negative omega throughout the entire forecast area, allowing for the development of very strongly negative omega throughout the entire forecast area Where precipitation is already falling, rates will be very much increased during this time, allowing for once again the .25-.5 in/hr range in the mountain towns (due to once again, the lack of CSI developing in), and /5-.75 in/hr developing into the peaks throughout the nighttime hours of Sunday night into Monday.

Despite the strongly negative omega throughout the entire forecast area at this point, forcing doesn’t look to be strong enough for the development of precipitation into the lower SRP elevations throughout the day on Monday as well, although evaporative cooling will be taking place. However, with continued influx of aloft moisture into the forecast area and the negative omega continuing, cloud coverage is expected to continue throughout the day on Monday across the SRP.

I do believe, despite guidance showing otherwise, that some SH locations will continue to see very light precipitation throughout the day on Monday, allowing for perhaps up to 3″ of snow in the higher elevations (and by that, I mean Pomerelle). With continued LLWAA occurring throughout the day on Monday, winds shouldn’t be an issue at this time.

Temperatures will continue to be relatively warm as well, due to LLWAA continuing throughout the day. Highs will likely reach into the middle and upper 40s for the SRP, middle and upper 30s for the SH, and middle and upper 20s for the WRV. FINAL TOTLAS FROM EVENT EXPECTED: Mtn towns: higher end 12-18″…Peaks: higher end 24-30″ Go based on elevation outside of that.

Off the Pacific coast, a fairly amplified trough and associated VORT max is expected to reach onshore by the early morning hours of Tuesday. Multiple perturbations in the temperature gradient associated with this are expected to further dig this feature down, and outward due to PDTA).

Watching closely the outward perturbation that is expected to occur just north of the Bay in California. PVA from this will jump the sierras and strengthen as it slopes down the Sierras into the Great Basin. With VORT maximized in this relative area, a jet streak will be generated.

This, combining with preexisting divergence due to subgeostrophic flow becoming supergeostrophic out of the trough will allow preexisting SFC VORT along the temperature gradient to become stretched vertically, increasing this in magnitude as well. This will generate a low-pressure perturbation in the region where net mass was lost, which in turn will generate isallobaric flow rushing into the center of this net mass loss as well.

This will allow for the development of a stronger CORF, which in turn will allow for the development of heightened temp advection throughout the forecast area, allowing for FGEN of a cold front and warm front in the normal locations (as well as obviously cyclogenesis over the great basin).

The temperature advection in the lower levels will generate a positive feedback, further amplifying aloft features, enhancing the deepness of the cyclone itself, which will enhance temp advection and so on, until the onset of occlusion.

With the process of cyclogenesis occurring by the morning hours of Tuesday the two terms contributing to the presence of negative omega will be enhanced simultaneously, allowing for very strongly negative omega to occur throughout the entire forecast area by Tuesday morning.

This will once again enhance snowfall rates over the WRV to .5-.75 in/hr in the mtn towns, and .75-1 in/hr in the peaks, and also allow for evaporative cooling to FINALLY allow steady moderate precipitation to manifest itself over the forecast area by the morning hours of Tuesday.

With the magnitude of omega through the DGZ, rates in the valleys should be nearly .5 in/hr to the north (near Shoshone and Richfield n such), and around .25 in/hr elsewhere. This should occur between 5 and 8 AM Tuesday over much of the SRP, with progged freezing levels (and wet bulb 0 levels) near the height of both BYI and TWF.

With this, precipitation type will be quite tricky to decipher throughout the day on Tuesday. Near the snake river, and the Canyon themselves, will keep precip a rain/snow mix throughout the day on Tuesday. Near the airport, and higher elevations near the Snake river, will allow for precip to be all snow throughout the day on Tuesday.

In areas with all snow, lower end 5-8 (north), lower end 3-5 (south). In areas that will eventually change over to snow later in the day on Tuesday, will go higher end 1-3″. Peaks of the Mtns should see 36-48″ by the time precipitation ends, with 18-24″ by the time it’s all set and done in the mtn towns.

South hills, with increased rates, should see totals approaching 8-12, with 12-18 in the higher peaks. FROPA and cyclone passage will look to occur by near sunset Tuesday, where CAA will eventually take over and generate some positive omega back into the forecast area by Tuesday night into Wednesday.

With much less diabatic heating occurring on Tuesday, as well as evaporative cooling generated by precipitation, temperatures will likely be cooler for Tuesday. Highs should likely only reach the lower and middle 30s for the SRP, upper 20s and lower 30s for the SH, and middle and upper 20s for the WRV.

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