Adapting the P-Type Nomogramto the GFE

Patrick D. MooreNWS Greenville-Spartanburg

Motivation• Add some science to the “dark art” of

creating Wx grids for winter precipitation events…

• …by applying proven techniques to every grid box in the forecast area…

• …while keeping the forecaster engaged in the process…

• …and maintaining database integrity.

What we want to avoid…

Two techniques have been adapted to the GFE to accomplish

this task• Predominate Precipitation Type Trends

Technique– Kermit Keeter (WFO Raleigh)

• Top-Down Method of Precip. Type Forecasting– Dan Baumgardt (WFO LaCrosse)

Image courtesy of Dr. Baker Perry, Flat Springs, NC

The TREND Technique and thePartial Thickness Universal

Nomogram• Empirical technique

developed by correlating data from upper air soundings and surface observations taken at GSO (elevation 897 ft.)

• Specifies and verifies the occurrence of p-types continuously over six hour intervals

• Accounts for trends in the evolution of p-types

References: Keeter and Cline (1991)See also http://www4.ncsu.edu/~nwsfo/storage/trend/

Before the NDFD…A Problem…Where do we put transitions?

With the Digital Forecast Database…

Transitions are easier to define• We could draw

transitions between precipitation types that don’t conform to zone boundaries

• However, this is very tedious

• How to maintain consistency?

In the GFE…• Model data is ingested

and interpolated to grids which can be used to populate database

• Partial thickness values can be calculated at each grid box

The Nomogram Adapted to the GFE

• Using partial thickness values from grids, find position within Nomogram at every grid box

• Display on PTypeNomogram grid in the GFE

The Nomogram Adapted to the GFE

• Using partial thickness values from grids, find position within Nomogram at every grid box

• Display on PTypeNomogram grid in the GFE

• Indeterminate split between RA and SNra

Task of creating Wx grid is broken up into two main steps

• Creating PTypeNomogram grid using CreatePTypeNomogram procedure

− ThickH100toH85, ThickH85toH70

• Creating Wx from PTypeNomogram grid using WxFromNomogram procedure

− PtypeNomogram, PoP, T, QPF, FzLevel, Snowlevel

Changes to Nomogram P-Types • If elevation of grid box

is…– above FzLevel, change

all p-types to SN– below SnowLevel,

change all p-types to RA or ZR, depending on T

– within melting layer, allow RA and SN to coexist accordingly

• Surface temperature controls RA or ZR

Image adapted from SRH JetStream

The WxFromNomogram Procedure

• Conditional probability of primary precipitation type taken from PoP grid

• Conditional probability of a secondary precipitation type selected by user

• Intensity taken from QPF grid according to FMH-1

The WxFromNomogram Procedure

• Combine attribute and duration qualifier selected by user

• Initial distribution of p-types taken from PTypeNomogram grid

• Final distribution of p-types modified by surface temperature and grid box elevation relative to FzLevel and SnowLevel, if desired

Creating Wx from the Nomogram

Some things to consider…

• Incorporation of FzLevel and SnowLevel allows the technique to be extended to high elevations with reasonable results

• Wx grids of less than 6-hr length violate the experimental design of the Nomogram!!!

• Does not account for very shallow arctic air masses• Does not account for cloud microphysics issues• Will maintain internal consistency among elements!

The Problem With The Wx GridOr, why do my Wx grids look like hell?

• The Wx grid is a catch-all for…– Conditional

probability– Precipitation type

• Temp aloft• Temp at surface

– Intensity, Visibility– Attributes

Wx = probability : ptype : intensity : visibility: attribute+ prob2 : ptype2 : intensity2: visibility2 : attribute2

The World is a messy place…

Operational Considerations…• Beyond forecast hour 36, consider

choosing WxFromNomogram because of its built-in climatology

• Create a new weather element group for winter weather grids

• Configure colors and shades for different precipitation types

Other Important Considerations...

• These are “advanced” techniques!(Not simple Wx-from-PoP tools, but a methodology)

• Objective application of techniques used in a subjective process…(Grids will sometimes appear “messy”)

• The Procedures will only take you so far down the road…(May need to finish up with “Showerize” or “RemoveWx”)

• Impact on forecast process…(Should be done later instead of earlier)

• How will text formatters react?(Really intended for a point-and-click forecast)

Questions?