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 2WFO State College, PA Northeast Regional Operational Workshop XIV – 11 December 2013

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.)

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

…how do we mitigate this? Long history of deadly accidents with snow squalls (~ 1”)… NBC33 - Indianapolis Some events from winter 2012-13. …how do we mitigate this? Burlington Free Press

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

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 (2001-02 through 2010-11). Logged surface data characteristics for each case. MSS BTV MPV New York Vermont 3 ASOS locations used

Surface and Synoptic Features

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

Modest Snow Amounts Small Liquid Equivalent Continental SLRs

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

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”.

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

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

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

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 C @ 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).

2005-06 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)

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.

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

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

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)

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

SNOW SQUALL OBSERVATIONS: CYYU 161900Z 33020KT 1/8SM +SN +BLSN VV001 M14/M35 A2944 RMK SN8 PRESRR SLP962 CYNM 162300Z AUTO 33028G37KT 1/4SM +SN BLSN VV005 M17/M19 A2936 RMK MAX WND 33037KT AT 2300Z PRESRR SLP968 CYXR 162354Z AUTO 32028G38KT 230V330 1/8SM +SN SCT014 BKN019 BKN027 OVC070 M08/M10 A2948 RMK MAX WND 27038KT AT 2345Z PRESRR SLP998 CYSB 170146Z 35021G28KT 1/2SM R22/2800VP6000FT/D SHSN DRSN VV003 RMK SN8 CYVO 170148Z 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

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

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

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)

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) http://www.erh.noaa.gov/btv/html/4kmwrf/index.php http://www.erh.noaa.gov/btv/html/12kmwrf/index.php SPC is working on adding the SNSQ Parameter to Mesoanalysis page (full CONUS availabiliity)

peter.banacos@noaa.gov, andrew.n.loconto@noaa.gov, QUESTIONS? E-MAIL peter.banacos@noaa.gov, andrew.n.loconto@noaa.gov, greg.devoir@noaa.gov