Preliminary Freezing Rain/Drizzle Climatology for EAX Mike July Winter Weather/Cool Season Seminar November 3, 2006

Why Focus on Freezing Rain and Drizzle? □ To fill a time slot in the seminar? □ They produce hazardous weather conditions which can have significant impact on the power, insurance, and transportation industries and on public safety. □ Average annual loss nationally – $313 million Jan 29 – 31, $32 million (KC Metro – Boonville)….worst ice storm ever in KC….ice over 1” thick.... at its peak over 409,00 customers without power in CWA; some without power for 2 weeks. □ Accounts for 20% of all weather related injuries □ Challenge to forecast in time and space

Usually do not mix with other types of precipitation (east of Rockies) Usually short lived (Nationally……≤ 2 hours/~70%) Usually end by cessation Tied to the diurnal solar cycle. Max occurs just before sunrise and drops off sharply during the morning with a late afternoon minimum. Cloud-top temperatures almost always warmer than -10C….i.e. little if any ice nuclei available 850mb and 700mb winds show a strong bias from the southwest Characteristics (common to both)

Characteristics (Differences) Freezing Rain………… ….if a transition - usually rain during the day - evenly distributed among several pcpn types at night Normally associated with the classic “melting” process Most frequent north of surface warm front/occlusion Depth of moist/cloud layer deeper than FZDZ soundings Surface winds peak from the northeast to east East of Rockies approximately 80% of FZRA events occur with sfc temps 28-32F Freezing Drizzle………. ….if a transition - strongly dominated by snow Most cases form via collision-coalescence or supercooled “warm rain” process Most frequent with passage of Arctic fronts Relatively shallow cloud layer Surface winds most common from the north East of Rockies approximately 90% of FZDZ events occur with sfc temps 21-32F

Conditions Which Determine Ice Accumulation 1) Precipitation Rate Model showed increasing rain rate = greater accretion rate. 2) Precipitation Amount (duration) Bennett (1959) showed amount of accretion on wires was 40-60% of rainfall. If correct 2 inch rain = ~ 1 inch of ice. ice accumulation > 3/8” starts significant damage to trees/wires ice accumulation > 1” will cause most wires to break 3) Droplet Sizes and Temperatures 2m temperatures < 32F do not affect how much ice will form 4) Winds accretion rate increases with increasing wind speeds (Simple Model) winds at right angles to ice loaded wires are more damaging….leads to “wire dancing”. > 15 mph often causes wire breakage

30 yr average ( ) – 3.2 events/yr 30 yr average ( ) – 3.2 events/yr 53 yr average ( ) – 3.2 events/yr

FZRA events peak in December/January. Kansas City December, January and February have an equal distribution of FZRA events.

92% chance FZRA will occur in during the cold season (Nov-Mar). 83% chance FZRA will occur on 1 to 5 days per cold season. 58% chance FZRA will occur on 2 to 4 days (annual avg 3.2/cold season)

( ) – 3.2 days/yr ( ) – 3.2 days/yr ( not shown ) ( ) – 3.2 days/yr Kansas City Averages

Annual Mean Freezing Precipitation Days (FZRA & FZDZ)

Monthly Frequency of FZRA/FZDZ

Primary Weather Patterns Associated with Freezing Precipitation Studies by Bennett (1958), Changnon (2003) and Rauber, et al (2001) came to very similar conclusions……… 1)Arctic Fronts account for 42% 2)Warm Front / Occlusion – 19% 3)Cyclone/Anticyclone – 26% Pattern 3 most severe due to heavy icing plus high winds. Area of FZRA/FZDZ is typically narrow and just north of 0C surface isotherm.

Operational Application Forecast Challenges ◊ Freezing Drizzle vs Freezing Rain ► Depth of moist/cloud layer ► Any chance of ice falling into the cloud layer ► What type of weather pattern expected ► How much rain forecast…..FZRA advisory, ice storm warning ► Surface winds > 15 mph? ► How much/strong is the vertical motion in cloud layer to enhance the collision- coalescence process? ► Time of day when precipitation is expected ◊ Model Forecast/Observed Soundings ► Rauber, et al (1999)…25 year study; super-cooled warm rain process responsible for 75% of all freezing precipitation soundings east of Rockies……~72% of them produced only FZDZ. ► Top-Down Approach is best tool to use. Is the model sounding correct? Check the 12z/00z analysis!!!

Summary  FZDZ occurs much more frequently than FZRA.  Super-cooled warm rain process highly favors FZDZ formation.  Classic “melting” process highly favors FZRA occurring.  In KC freezing rain peaks in Dec/Jan.  Three primary weather patterns account for the vast majority of all freezing precipitation in Kansas City (possibly as high as 90%).  Between 1 and 5 freezing rain events per cold season with an annual average of 3.2/season.  Around 80% of all FZRA events occur with 2m temperatures within a tight range (28-32F) whereas the vast majority of FZDZ events occur within a much broader range (21-32F).  Top-Down Method best tool to investigate soundings to determine precipitation type.  The Freezing Precipitation Frequency graphics from NCDC look suspect….especially for December.