Winter Weather Event Nov 10th 2006

Chris Jakub - NWS Wichita - Dec 17th 2006

 



The following web review was created to help improve and refresh winter weather forecasting skills for surprising significant snow events from meso-scale bands. This event took place on November 10th 2006 across the northern plain states of Minnesota and Wisconsin.

This web review will diagnose the synoptic and meso-scale setup for a sneaky storm system that produced significant snowfall across the northern plains on November 10th, 2006. Several winter weather advisories were issued across the region by the local NWS offices, however the winter weather advisories were later upgraded to winter storm warnings by NWS offices as numerous reports of six to eight inch snow depths started coming into the NWS offices. A few locations witnessed up to sixteen inches of snow! See image 1 for snowfall totals from this event which affected mainly (Twin Cities, MN), (La Crosse, WI), and (Green Bay, WI) NWS offices.

 

Synoptic Set-up:

Note all systems with well developed TROWALS usually produce significant snowfall, but systems with weak or no TROWAL-like signatures can still produce surprising heavy snowfall. This particular heavy snow event actually parallels three other heavy snowband cases that I have studied and archived from the past with no TROWAL-like signature, and it still remains a challenge for meteorologists to predict today...See below.

Figure 1 (click on image for full view)

 

Figure 1 (left) is an 4-panel loop showing 650mb Theta E, 650mb heights and vorticity, 650mb Fn Divergence, and 700mb temperatures, winds, and RH. This 4-panel loop shows a very important characteristic that goes undetected by many meteorologists, but plays a key role in creating heavy meso-scale snow bands. Looking at the top right panel in the beginning of the loop one will see a short wave race eastward across the northern plains then across the Great Lakes. A second short wave then develops further west and moves across the plains about 36hrs later as an open wave system versus a wrapped-up system. The second short wave was responsible for the heavy snowfall across the northern plains, however it was the first short wave that set the stage for the second system to create a significant snow event.

Watch the loop again and notice the strong 650mb Theta E gradient(top left panel) move southward and stall out over southern Minnesota and central Wisconsin as the first short wave moves into the Great Lakes region. The 650mb Theta E gradient(baroclinic zone) then sits and lingers overhead after the passage of the first short wave, and becomes a crucial characteristic that many forecasters fail to notice. Probably one of the main reasons this Theta E gradient(baroclinic zone) goes undetected or becomes invisble to many forecasters is that precipitation rarely occurs with the first short wave. Once the second short wave moves into the picture the lingering Theta E gradient(baroclinic zone) from the first short wave becomes a prime focus area for intense mid-level frontogenesis, as strong isentropic lift/moisture transport interacts with the Theta E gradient(baroclinic zone).

This is where the term "Frontogenesis Stage-Setter" comes from.

 

 

Moisture Transport, Surface pressure falls, and the Warm Conveyor Belt(WCB):

Figure 2a (below left) shows the beginning stages of modest moisture transport northward across Missouri into Iowa which resulted in elevated thunderstorm development around 06Z across eastern Nebraska and western Iowa. Also notice the northward expansion of the shallow moist zone from Figure 2a to Figure 2b (green highlighted region in the bottom left pane), the transport direction/expansion of this shallow moist zone was towards the concern area versus away from the concern area as the short wave approached from the west. The shallow moist zone expansion in this manner suggests that moisture depth will become sufficient for heavier precip and convection-off of moisture transport will be limited. Figure 2c (below right) identifies the location of the narrow heavy snow band with (Heavy Snowband wording). A forecast discussion remark from a NWS office in the concern area during the early stages of the meso-scale snow band "A CONCERN FOR ADDL HVY SNOW IS THE DEVELOPMENT OF CONVECTION ACROSS IOWA AND ILLINIOS. SOME MSTR INTERCEPT IS PSBL WHICH WOULD THEN LIMIT ACCUMULATIONS." The convection continued to become more widespread from northern Missouri through eastern Iowa into southern Wisconsin by 18Z, but seemed to have little effect on moisture transport towards the heavy snow band.

If the convection areas were to develop in a solid west to east line across southern Wisconsin prior to the development of the heavy snow band, then its conceivable that some of the moisture could be cut-off. However that scenario seems unusual during the cold season so convection cut-off beliefs could be just a myth. Another signal to look for is the 3hr surface pressure falls (yellow highlighted region top left pane) and its movement/behavior over time. The 3hr surface pressure tendency gives us insight about the WCB(warm conveyor belt) and how it is evolving in the mid-levels of the atmosphere. Notice the 3hr surface pressure center (yellow highlighted region top left pane) deepening and advancing to the northeast. This tells us that the WCB is shifting east over time and intensifying, however system occlussion becomes less likely over the area due to the rapid eastward moving surface pressure fall center. The WCB and moisture transport are directly related to the pressure falls at the surface...this is due to hydrostatic effects in the atmosphere. Click on ---> Conveyor Belts to view a short audio/video clip from Dr. Moore explaining this process and what the surface pressure falls are telling us. Keep in mind that weather systems DO NOT need to be closed off or occluded to produce significant heavy snowfall.

Figure 2a

Figure 2b

Figure 2c

(Click on images above to see a full screen view)

 

Mid-Level Frontogenesis/Upper Jet Dynamic Coupling and CSI analysis:

The vertical motion enhancements from two seperate atmospheric circulations being coupled has been well documented and studied by many researchers. This particular event is another case that fits the pattern...See Figure 3a & 3b (below).

Conditional Symmetric Instability is not easy to visualize physically. Basically, the idea is that a parcel may be stable with respect to vertical displacement (convective stability) and the parcel may be stable with respect to horizontal displacement (inertial stability), but the parcel can be unstable when it travels upward along a slantwise path. This is why CSI is sometimes called slantwise convection. Generally, the conditions for CSI are met in areas to the north of a shallow front. This can be either a warm-front or shallow cold-front. The region north of a shallow front is usually characterized by a convectively stable environment, and conditions that are near saturation. Another typical feature is the presence of a jet (and usually it is jet entrance region) just north of the area. This provides the anticyclonic horizontal shear that is typical of inertially unstable regions. The jet also provides the strong vertical shear found in CSI events. Finally, CSI is nearly always found near regions of frontogenesis. The frontogenesis produces a "secondary" circulation that often results in a larger-scale band of precipitation.

Figure 3a (click on image for full view)

Figure 3a (left) contains two images superimposed, NAM 300mb isotachs(upper jet) and NAM 650mb Fn Divergence (mid-level frontogenesis), NAM 650mb Theta E(red contours), NAM 700mb winds(black wind plots), and 700mb profiler winds(green wind plots). The key thing to notice is how the right rear quad of the 300mb upper jet axis is orientated parallel to the 650mb Theta E gradient and 650mb Fn Divergence couplet. Ageo-strophic circulation effects from the upper jet work in tandem with the mid-level frontogenetic circulation. The response is a deep coupled circulation in the atmosphere as illustrated by the two dimensional view in Figure 3a (left).

 

Figure 3b (click on image for full view)

Figure 3b (left) shows a cross section line (line C) cutting through the NAM 300mb upper jet axis and NAM 650mb Theta E gradient(red contours). It is important to have the cross sectional cut-line nearly perpendicular to the Mid-Level thermal wind (700-300mb thickness line) to diagnose for CSI(Conditional Symmetric Instability) and analyze vertical circulations along the baroclinic zone. Another thing to note is that we don't see a TROWAL-like signature in the NAM 650mb Theta E contours.

 

Cross Section Analysis:

Cross Section analysis is a great way visualize vertical circulations in the atmosphere. Click on ( image 2 ) to see a vertical cross section for line C (above in Figure 3b). Image 2 shows Theta E (black contours), Fn Divergence (green/yellow couplet image) favored location for mid-level frontogenesis, and Upper Jet Core image.

Click on ( image 3 ) to see the same cross section as illustrated in image 2 but with ageo-strophic circulations overlaid. Notice how efficient the coupling of the two separate circulations became during this surprise heavy snow event. This cause the upward vertical motion to elevate to significant proportions!

Figure 4a (left) shows a four panel cross section which comes from one of my AWIPS procedures.

The low levels were fairly dry early on during the event but deep moisture transport (bottom left panel) moving towards a very strong and compact mid-level frontogenesis circulation (bottom right panel) quickly defeated the dry low levels.

This AWIPS procedure can be found under "Winter WX" then look up CSI X-Section/Line C.

Figure 4b (left) shows how upward vertical motions became enhanced thermodynamically with values of EPV < 0.25 above the frontogenetic circulation. This will cause the vertical ascent portion of the frontogenetic circulation to slope more upright and contract. This is a common CSI signal found in the real atmosphere.
Figure 4c (left) is a NAM model time height series output for a particular point using the "Cross-Hair Technique." Notice the high values of Omega co-located with the dendritic layer -12 to -18C and region of low EPV values inside and just above the favored snow growth area. Residence time predicted by the NAM model can also be identified in the time height series, and in this event the residence time was generally around 8 hours or less.

 

Figure 5a

Figure 5a (above) is a NAM sounding for a location inside the meso-scale snow band. The sounding indicates a snow producing thermal profile with moist adiabatic lapse rates above the mid-level inversion. Another key signal is the nearly uni-directional wind profile and increasing speed shear in the layer (highlighted by orange box above). A uni-directional wind shear profile favors stationary meso-scale band movement compared to strong directional wind shear profiles. Directional shear decreases the moisture convergence into the mid-level baroclinic zone and causes the snow to spread over a larger area, versus concentrated moisture transport from a uni-directional shear profile. Monitoring vertical wind changes from NOAA Profiler Network(NPN) or VAD winds from local WSR-88Ds will provide us with insight on how the meso-scale band will behave.

 

Radar Analysis:

Figure 6a (below left) shows a radar image of the intense meso-scale snow band. The narrow snow band remained stationary with a life cycle of almost five hours. Embedded thundersnow with visibility less than a quarter of a mile was observed at KAEL during the early stages of band development (see figure 6b below right)which indicates that high snowfall rates were occurring.

Figure 6a (click on image above to see a radar loop for this event)

Figure 6b (click on image above to see ASOS observation)

 

Summary:

Figure 7a (below) is our local office significant snow forecasting table (this table can be found on the SOO page under Winter). Notice that most ingredients were in the strong/very favorable categories, however no TROWAL signature was linked to this event and residence time for the precip was generally less than 8hrs. Two factors combined together to play a major role in this significant snow event. The very favorable ingredients of instability and coupled forcing created unusually high snowfall rates (~ 3"/hr) at times, and the other factor was the "Frontogenesis Stage-Setter" characteristics from the first short wave. Forecaster situational awareness of the strong mid-level Theta E gradient(baroclinic zone) left in place from the first short wave is usually low since little or no precipitation occurs with the first short wave.

Utilizing the significant winter storm ingredients table and understanding the "Frontogenesis Stage-Setter" characteristics should improve forecaster situational awareness for meso-scale snow bands that produce significant snowfall totals.

Significant Winter Storm Ingredients Table: November 10th, 2006

COUPLED FORCING
STRONG
MODERATE
WEAK
NONE
FRONTOGENESIS
STRONG
MODERATE
WEAK
NONE
INSTABILITY
STRONG
MODERATE
WEAK
NONE
SNOW GROWTH
VERY FAVORABLE
SOMEWHAT
SLIGHTLY
NONE
RESIDENCE TIME of PRECIP
LONG ( > 8hrs )
MODERATE ( 4 - 8 hrs )
SHORT ( < 4hrs )
NONE
TROWAL
YES
NO

Figure 7a (Ingredient parameters for this particular event are highlighted in Red above)

 


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