FORECASTING WINTER PRECIPITATION TYPE, Exercises of Kinematics

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FORECASTING WINTER

PRECIPITATION TYPE

A Review Prepared by Gary K. Schmocker

Winter WX Workshop– November 2004

Partial Thickness

•^

Used as a first guess to estimate if a layer is belowfreezing

-^

1000-700 mb: 2840 m (rain/snow line)

-^

Divide this layer to determine potential of freezingrain or sleet

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850-700 mb: 1540 m

-^

1000-850 mb: 1290-1310 m

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Look for pattern of thickness lines (overlapping)

Pcpn Change Forecast during

CAA using Partial Thicknessesred line 1000-850 mb & green

line 850-700 mb thickness

• Favors rain to snow

•^

Favors rain to freezing rain/sleet then snowrain/sleet then snow

Pcpn Type with Closed (Cutoff)

Mid-Upper Low

Favors rain followed by possible briefperiod of wet snow underneath or justnorth or west of low–sfc based warm

layer may partially or completely

melt snowflakesmelt snowflakes

Soundings

• Superior to using partial thickness
  • can resolve thin warm layers (>0°C)
    • For STL use ACARS soundings or model soundings

(initial or forecast from RUC, ETA, LAPS, or GFS)- use BUFKIT Program or display Bufr files on AWIPSAWIPS

• ACARS Website: http://acweb.fsl.noaa.gov/
  • displays weather data from automated sensors oncommercial aircraft
    • Interactive Soundings Website: http://www-

frd.fsl.noaa.gov/mab/soundings/java/- interactively displays soundings from RUC2,MAPS, RAOBs, Profilers, & ACARS

An Example of ACARS Data at STL

Is it Cold Enough for Ice Crystals to

Form in the Clouds?

•^

Heterogeneous nucleation: supercooled liquid condensingonto an ice nuclei (usually clay in the midwest) to form anice crystal

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Temperature has to be below –4° C for any ice nucleation tooccur in a cloud: –

°^

to

°C yields a high chance (

• 90%) of ice being in

°^

to

°C yields a high chance (

• 90%) of ice being in

the cloud- Shallow (low level) cloud decks

often not cold enough for icecrystal development

Ice Crystal Growth H2O

Vapor

H2O

H2O

H2O

H2O

H2O

• Deposition – ice crystals growing at the expense of

supercooled water droplets, maximized at -15°C(dendrites grow at -12°C to -18°C)- look for strong vertical motion around -15°C ontime/cross sections for potential heavy snowfall/highsnowfall rates (Auer & White, 1982)

Seeder-Feeder Mechanism

•^

A cloud with ice crystals can “seed” a lower warmercloud (feeder)

-^

About 5000 ft or > between cloud layers - ice willevaporate/sublimate before reaching the feeder cloud- seeder-feeder most effective with a separation of 3500 ft or lessft or less

-^

Use IR Satellite for higher cloud temperatures

-^

WSR-88D VWP to assess distance between clouds

1500m (5000 ft)

VWP shows Seeder-Feeder Mechanism FZDZ

| -SN

Example of BUFKIT Display

with P-Type Forecast

Using the Partial Thickness Nomogram in

BUFKIT

•^

Nomogram developed from adatabase of cases over the SEU.S. (Keeter et al.)

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X axis: 850 –700 mb thicknessY axis: 1000-850 mb thickness

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The largest circle represents the

-^

The largest circle represents the last p-type fcst

-^

“Looping” function allowsforecaster to see model p-typefcst trends- low to mid level thermaladvections and changes instability

1. Warm Layer Aloft

• P-Type (usually sleet or

freezing rain) based onamount of (+) energy(energy or size of an area is proportional toarea is proportional to the mean temperatureand depth of layer) inelevated warm layer(>0°C) versus (–)energy in cold surfacebased layer (<0°C)

2. SFC Based Warm Layer

• Uses both the height

of the freezing level and the amount ofand the amount of (+) energy in the sfcbased warm layer todetermine p-type(either rain or snow)