1. Snow Squalls: Forecasting and Hazard Mitigation
11 December 2013 NROW XIV Virtual Presentation.
PETER BANACOS 1, ANDREW LOCONTO 1, and GREG DEVOIR 2
1WFO Burlington, VT WFO State College, PA
Northeast Regional Operational Workshop XIV – 11 December 2013

2. Outline Background information on squalls and societal impacts
Synoptic and mesoscale snow squall environments
Snow Squall Forecasting Parameter
R20: Research  Operations
Conveying the message (Special Wx Statements, Social Media, Interactive Highway signs, action plans, etc.)

3. A mesoscale convective system producing gusty winds & heavy snow.
What is a Snow Squall?
A mesoscale convective system producing gusty winds & heavy snow.
Tend to be short-lived
Don’t reach NWS snow advisory criteria
Falling temperatures can produce a “flash freeze” situation
Can have deadly road consequences (high-impact, sub-advisory, HISA)
Time-Matched Imagery 30 November 2007
TYX 0.5o reflectivity loop
Elm St., Potsdam, NY

4. …how do we mitigate this?
Long history of deadly accidents with snow squalls (~ 1”)…
NBC33 - Indianapolis
Some events from winter
…how do we mitigate this?
Burlington Free Press

5. PREVIOUS STUDIES OF SNOW SQUALLS (non-lake effect)
A few case studies and one forecast technique (WINDEX, Lundstedt 1993).
Cool-season MCS work may be relevant (e.g., Low-Dewpoint MCS/derechos, Corfidi et al. ‘06).
Some focus on external partnerships (Devoir 2004, NWS – PENNDOT)
WHAT WE WANT TO DO:
Create a snow squall database and improve our meteorological understanding through compositing
Improve forecaster situational awareness by developing a new snow squall parameter (these are low QPF events)
Validate snow squall parameter against individual cases
Continue to improve operational messaging, education, and state/local partnerships
END RESULT:
More certainty in forecast products, better lead times. Better communication.
Road crews pre-treat surfaces

6. Creating a Snow Squall Database
METHODS
Searched 10 years of ASOS data for moderate or heavy snow (VSBY ≤ ½ SM) with a west wind component.
Each S or S+ observation was compared with 2-km radar mosaics to subjectively determine if the event was associated with a cold front or mobile upper trough (and not a stratiform/WAA case).
Found 36 total snow squall events ( through ).
Logged surface data characteristics for each case.
MSS
BTV
MPV
New York
Vermont
3 ASOS locations used

7. Surface and Synoptic Features

8. Snow squalls are short-lived events…
Here’s a box-and-whisker chart showing the duration (in minutes) of moderate and/or heavy snow. This is based entirely on the observation (i.e. S or S+); but based on the ASOS reports, it was found that most cases (e.g. falling within the 25th to 75th percentiles) experienced a 14 to 24 minute period of heavy snow, and a 15 to 36 minute period of moderate snow. However, durations of heavy snow were as few as 4 minutes and as much as 47 minutes; while for moderate snow, durations ranged from 3 minutes to almost an hour and a half.
VSBY ≤ ¼ SM
VSBY ≤ ½ SM
ASOS

9. Modest Snow Amounts
Small Liquid Equivalent
Continental SLRs

10. Potential instability
Dendrite growth zone (-12 to -18C)
In the first BUFKIT image we have an overlay of RH (wrt ice) shaded, omega (negative = rising motion), VAD Wind profiler forecast winds, and precipitation shown in the hourly bars at the bottom. Note that around 20-22z there is an intense zone of omega accompanied by a quick burst of forecast snow.
Now if we overlay the dendritic snow growth zone (-12 to -18 degrees C) temperatures, it’s observed that the strongest omega is nearly co-located within the dendritic snow growth zone, but the amount of time that this occurs is brief. There still is some rising motion within the growth zone after 23z but it is not as intense. Note also that the dendritic growth zone is saturated, so that is also sufficient for significant crystal growth. The key is that the process for significant snow growth is there (much like in banded precip/synoptic-scale snow events), but only for short but intense periods.
The next overview shows theta-e isentropes. Note two things: the isentropes around 20-22z are nearly vertical (so approximately dry adiabatic theta-e lapse rates), and note the layer of instability (isentropes spaced far apart) in the 1-2km layer. In the theta overview you can also make out the sharp frontal boundary (sloped isentropes) in the lowest 1km. The wind speed also increases sharply in the lowest 1-2km to a maximum close to 40 kts.
When you overly the omega field on the potential temperature field, you’ll note the omega just precedes the frontal boundary, nearly co-located with the steep lapse rates themselves.
Wind speed max (38 kts)
𝜕𝜃𝑒 𝜕𝑧 <0𝜕
Potential instability

11. BUFKIT Summary Time-height cross-sections:
A brief, intense zone of UVV in the lowest 2km AGL.
Intersects Dendritic Growth Zone (-12 to -18C) in saturated (RH ≥ 90%) atmosphere
Along/just ahead of cold front.
Examine θe profile for vertically-oriented or folded isentropes (steep lapse rates, potential instability).
Well-defined wind shift with strongest wind speeds/ mixing just behind front.
Don’t get hung up on QPF. Likely only ~0.05”.

12. Parameters Examined using 3-hrly NARR data in GEMPAK (analysis times immediately before and after squall time)
SBCAPE
MUCAPE (0-180mb)
Sfc-2km Theta-e difference
Sfc-2km RH
Sfc-2km mean wind
Sfc-2km wind shear
925mb frontogenesis
Sfc isallobars (3hrly)
850mb frontogenesis
WBZ height
Precipitable Water
300mb Divergence
925mb theta-e Adv.
850mb theta-e Adv.
850mb temperature advection
0-2km lapse rate

13. NARR Variable Distribution for 36 Snow Squall Events
Normalized between 25th and 50th percentile values… 9ms-1, 75%, 0K/2km

14. Favored parameter space
NARR SCATTERPLOT of 0-2km mean RH vs. THETA-E Difference (36 SNSQ CASES)
Favored parameter space

15. Necessary Ingredients for Squalls
Moist convection, but cold enough for snow.
(1) moisture, (2) lift, (3) instability, (4) wind, and (5) vertical temperature structure (to support snow)
Snow Squall Parameter (SNSQ, non-dimensional)
plot only for values > 0 and where Tw ≤ 1 2m
Cold enough for snow
Moisture low-level Instability Wind
Calibration was done using NARR data, but can be tweaked for operational models.
SNSQ approaches zero as any of these variables approaches zero.
Lift (forcing) would be assessed independently (isallobaric rise/fall couplet, F-GEN).

16.
Winter
SNSQ Parameter One # to highlight where kinematic and thermodynamic conditions are favorable for snow squalls.
3-hrly NARR SNSQ time series at BTV (5 months)

17. Initialized with the GFS and no convective parameterization.
Case Examples
11-12 February 2003 (using the NARR)
17 January 2013 (using the NARR & BTV-12km WRF)
BTV-12km WRF:
Initialized with the GFS and no convective parameterization.

18. Testing the SNSQ Parameter Against Past Events: 11-12 Feb 2003
“SNOW DERECHO” CASE
EXAMPLE 1
CG Lightning (11/20z through 12/12z)

19. A simple way to assess convergence (lift) /propagation
RADAR
11-12 Feb 2003 WI
SNSQ Parameter
Shaded, left panels
3-hr Isallobars
A simple way to assess convergence (lift) /propagation
Pres. couplets often the difference between convective snow showers and organized squalls
SNSQ
21z
11/21 Z WI
12/00 Z
00z WI
12/03 Z
03z

20. SBCAPE (J kg-1), 925mb Frontogenesis (K 100km-1 3hr-1)
0-2km qe Diff.
0-2km Mean RH (%)
925mb
F-gen
Isallobaric
Convergence
SBCAPE (J kg-1), 925mb Frontogenesis (K 100km-1 3hr-1)
3-hr Isallobars (mb)

21. 16-17 January 2013 EXAMPLE 2 SNSQ (03z, 9 h forecast)
BTV-12km WRF 17 Jan 2013
Intense snow squalls along sharp cold front
Lake effect snow showers

22. SNOW SQUALL OBSERVATIONS:
CYYU Z 33020KT 1/8SM +SN +BLSN VV001 M14/M35 A2944 RMK SN8 PRESRR SLP962
CYNM Z AUTO 33028G37KT 1/4SM +SN BLSN VV005 M17/M19 A2936 RMK MAX WND 33037KT AT 2300Z PRESRR SLP968
CYXR Z AUTO 32028G38KT 230V330 1/8SM +SN SCT014 BKN019 BKN027 OVC070 M08/M10 A2948 RMK MAX WND 27038KT AT 2345Z PRESRR SLP998
CYSB Z 35021G28KT 1/2SM R22/2800VP6000FT/D SHSN DRSN VV003 RMK SN8
CYVO Z 32020G28KT 1/2SM -SN BLSN VV007 M14/M15 A2951 RESN RMK BLSN8 PRESRR SLP022
17/0040Z
Reflectivity – Landrienne, Quebec
16-17 January 2013 Case
EXAMPLE 2

23. NARR SNSQ parameter with PMSL and isallobars (03z 17 Jan 2013)
Convergence maximized on leading edge of isallobaric gradient (usually near zero change line).
isallobaric wind

24. 925mb
F-gen
Isallobaric
Convergence
Isallobaric convergence part of thermally direct frontogenetic circulation.

25. Mosaic Composite Reflectivity (17/1155z)
SNSQ Parameter (17/12z) (BTV-12km WRF 12-hr FCST)
Mosaic Composite Reflectivity (17/1155z)
View from Burlington, VT of snow squall crossing Lake Champlain (17/13z)

26. See the SNSQ Parameter in Real-Time
On the Web (BTV 4 and 12km WRF runs)
URLs:
AWIPS 4-panel procedures
(Volume browser changes sent to the SOO mail list)
SPC is working on adding the SNSQ Parameter to Mesoanalysis page (full CONUS availabiliity)

27. peter.banacos@noaa.gov, andrew.n.loconto@noaa.gov,
QUESTIONS?