**Weather**

**What you want:** Weather tool for all situations

**Type:** Tool

**IFPS/GFE build in which developed:** 16.2+

**Field(s) Edited:** Wx

**Data Requirements:** Always: PoP

Optional: T, WorkT, QPF

**Additional forecaster Input:**

**Always**: Probability term, character, intensity, thunder or not,
sprinkles/flurries or

> not, temperature scheme to use

**Optional:** -Probability Threshold to begin stratiform, probability
word to use for

> sprinkles or flurries, model, model level
>
> -surface temperature thresholds for snow/sleet vs. mix vs.
> rain/freezing rain, upper level thresholds for snow vs. snow/sleet,
> snow/sleet vs. sleet, sleet vs. sleet/rain and sleet/rain vs. rain.
>
> -If upper level threshholds are not used, but surface threshholds
>
> are, then these threshholds would simply be for snow vs. mixed snow
> and
>
> rain/freezing rain vs. rain/freezing rain.

**Limitations**: -Can only use 850 or 700mb levels for upper level
temperatures from

> GFS, MRF.
>
> -RUC is only available out to forecast projections of 12 hours.
>
> \-**The precipitation type scheme is simplified**. It does not yet
> take into account the absence of ice crystals in the cloud (assumes
> they're present), nor does it take into account a low level cold
> layer, other than just the surface temperature. Furthermore, the
> upper-level temperature thresholds for mixed precipitation depend on
> user input.

General Discussion:

The **Weather** tool assumes that at least the PoP has been completed,
and, depending upon the options chosen, that the T and/or the QPF grids
are also done. The idea here, particularly when it comes to winter
weather, is that, if one could come up with a good PoP, T and QPF
forecast, than a good Weather forecast could easily be obtained.

When the **Weather** tool is invoked, a rather large dialogue box covers
most of the screen:

![](media/image1.png){width="5.998611111111111in"
height="3.1694444444444443in"}

Note that this second piece is actually the right half of the first:

![](media/image2.png){width="5.997916666666667in"
height="3.4659722222222222in"}

The remainder form the bottom portion of the Weather tool dialogue box:

![](media/image3.png){width="5.334722222222222in"
height="4.973611111111111in"}

![](media/image4.png){width="5.996527777777778in" height="1.95625in"}

Many of the options can be left as is, depending upon how much or how
little information the forecaster wants utilized.

The **Weather** tool:

-   generates Wx grids that are consistent with the PoP grids

-   yields probability terms and character for Wx based on user
    preferences

-   can yield output with both stratiform and showery weather present,
    with the user able to choose whether the stratiform type kicks in at
    likely probability or not until definite probability

-   lets the user choose whether thunder is included or not, or is
    included for showery precipitation only in the case where combined
    stratiform and showery precipitation is chosen

-   whether sprinkles and/or flurries are to be output for PoPs of 5
    through less than 15, and, if so, what term is to be used for the
    sprinkles or flurries, and at what PoP (5 to 14) to start sprinkles
    or flurries

-   yields intensity based on user input including QPF, in which
    intensity is based on the NWS standard for hourly rainfall rate for
    liquid precipiation

Additionally, the user has a number of temperature schemes to choose
from for precipitation type. In addition to generic rain only, snow only
and rain and snow mixed only options, one can chose to use only surface
temperature, or use surface temperature together with a temperature from
a level aloft, either straight from the model or modified.

For surface temperatures, one chooses thresholds for snow vs. rain.
Because rain and snow are sometimes desired at the same grid point at
the same time, one has the option to enter a threshold for snow vs. snow
and rain mixed, and a higher threshold for snow and rain mixed vs. rain.
Making these values equal will yield just snow or rain at any given grid
point.

For temperatures aloft, one can chose to have the tool access a D2D
temperature grid directly, or use the WorkT field (as created at RLX).
The WorkT field facilitates making adjustments to the upper level
temperature field before running the **Weather** tool to determine
precipitation type. One simply loads the WorkT field from a model
temperature field aloft, or from a warm layer.

The **WorkTfmD2D** tool will allow one to populate the WorkT field with
temperatures from a model and pressure level of choice. As with any tool
that uses model data, one must be aware of what levels are available for
what models.

The **WarmLayerFmD2D** tool finds the pressure level that has the
highest temperature for each grid point. It may be different levels for
different grid points; the resulting grid is a mosaic of the highest
temperatures aloft over the area from the pressure levels chosen.

Either way, one has a grid of temperature values in degrees Celsius to
work with. One can then modify this in any way before running the
**Weather** tool, picking the WorkT option and choosing the temperature
thresholds for precipitation type.

The idea is to determine precipitation type based on temperatures. The
scheme used here is actually a small portion of the widely known top
down method.

**The presence of ice crystals is assumed.**

Next, the warm layer aloft is considered. If a model and single level is
simply chosen, this is still regarded as a warm layer aloft.
Precipitation falls into this layer as snow. It does not melt, partially
melts and refreezes below (sleet), or melts altogether, depending upon
the temperature. Thresholds are chosen to allow a mix: snow, snow and
sleet mixed, sleet, sleet and rain mixed, and rain are all allowed.
Thus, there are four temperature thresholds to delineate between these
five categories. To have just snow, sleet or rain, just make the top two
thresholds the same and the bottom two the same. [Careful consideration
should be used when selecting the upper-air temperature thresholds for
mixed precip types.]{.underline}

**This scheme does not yet look for a cold layer underneath the warm
layer, other than the surface temperature.**

Once the precipitation reaches the surface, its final form is
determined.

If the surface temperature is at or below 32F and below the surface
threshold for snow, any snow or sleet that falls remains unchanged,
while rain becomes freezing rain. If the temperature is above freezing
but still below the threshold for snow, all precipitation types remain
unchanged. If the snow to rain threshold is set below freezing, anything
that falls becomes freezing rain up to a temperature of 32F, with rain
above 32F. Of course, anything that falls becomes rain if the surface
temperature is above 32F, and above the surface threshold chosen for all
rain.

Two surface temperature thresholds are actually chosen, to allow for the
three possibilities: snow, snow and rain mixed, and rain. If the surface
temperature falls in this window, then rain aloft remains rain, except
for surface temperatures of 32F or lower, when it becomes freezing rain.
Otherwise, anything that falls becomes mixed with rain if the surface
temperature is above 32F, and mixed with freezing rain if the surface
temperature is 32F or lower.

Note that, for all temperature thresholds in this tool, the "colder"
precipitation type occurs at temperatures up to and including the
threshold, with the "warmer" precipitation type occuring at temperatures
above the threshold.

The output of the **Weather** tool, then, is no weather below 15% PoP
except flurries or sprinkles at or above 5%; and weather whose character
and probability term is chosen by the user, and determined by the PoP
according to the traditional NWS definitions. Precipitation type is
determined according to the temperature scheme used, and the appropriate
temperature grids. The presence of thunder as determined by the user.
Note that all combinations of precipitation type can be output by this
scheme.

In complicated situations, one can actually output weather ranging from
a stratiform snow, to a complicated winter mix, to scattered showers and
thunderstorms in the warm sector, all with one iteration of the tool.

Here's an example using these forecast surface temperatures and pops,
along with the following h850 temperature grid from the gfs. Though the
gfs grid is real, the gfe grids were made up for illustrative purposes.

Surface temperatures:

![](media/image5.png){width="5.99375in" height="5.739583333333333in"}

And the pops...

![](media/image6.png){width="5.997222222222222in"
height="5.763888888888889in"}

And h850 temperatures from the gfs for the same time:

![](media/image7.png){width="5.99375in" height="5.622916666666667in"}

And the resulting Wx grid using both for the shower/stratiform option,
with showers only for the thunderstorm option, SFC and model T aloft
option using the gfs at h850, surface threshold of 33 and 35F, and upper
level thresholds of -1, 0, and 0:

![](media/image8.png){width="5.999305555555556in" height="5.74375in"}