Date:
Run:
Sector:
SPC Guidance
Probabilistic
4-hr HREF/SREF Calibrated:
[info]
At every grid point for the valid forecast hour, two probabilities are paired:
1. Probability of UH ≥ 75 m2/s2 over the previous 4 h (from the HREF)
2. Probability of environmental field(s) meeting a threshold over the previous 4 h (from the SREF; see table below)
The historical frequency of a hazard report occurring within 25 miles of that grid point and within the 4-h period for that forecast pair of probabilities is substituted as the 4-h calibrated hazard probability.

HazardSREF Conditions
HailMUCAPE ≥ 1000 J/kg, Eff. Shear ≥ 20 kt
WindMUCAPE ≥ 250 J/kg, Eff. Shear ≥ 20 kt
24-hr HREF/SREF Calibrated:
[info]
Uses the 4-h hazard probability forecasts that cover the 24-h convective day (i.e., 12z-12z). At every grid point, the cumulative sum of the 4-h probabilities and the maximum 4-h probability are paired. The historical frequency of a hazard report occurring within 25 miles of that grid point and within the 24-h period for those 4-h calibrated hazard probabilities is substituted as the 24-h calibrated hazard probability.
24-hr STP Calibrated:
[info]
A distribution of STP is formed for each gridpoint from points where UH exceeds the 99.99th percentile (within each member's climatology) within a 40 km radius. The 10th percentile of STP from that distribution is then assigned to each point at each hour, and then the maximum daily STP value for each point is used to assign a probability based on the climatological frequency of a tornado given a right-moving supercell and an STP value for each ensemble member. The mean at each point is taken across the members, and then a Gaussian smoother with σ = 50 km is applied.
HREF Calibrated Thunder:
[info]

The probability of at least one cloud-to-ground (CG) lightning strike within 12 miles (20 km) of a point over the forecast period is determined by:

[0.6 * P(4 km Refl > 40 dBZ) + 0.4 * P(QPF > 0.08 in.) + 0.1 * P(MU LI < -1)]

• P(4km Refl > 40 dBZ) is the fraction of HREF members with simulated 4 km radar reflectivity values greater than 40 dBZ
• P(QPF > 0.08 in.) is the fraction of HREF members with accumulated precipitation greater than 0.08 in over the forecast period.
• P(MU LI < -1) is the fraction of HREF members with a most-unstable lifted index less than -1

The forecast probabilities are calibrated to be statistically reliable based on CG lightning strikes recorded by the National Lightning Detection Network in 2017-2019.

Overlays of mean storm motion only use HRW members and do not include NAM or HRRR members.

Synoptic
Upper Wind/Height
500 mb Height/Wind:
[info]
Height and temperature fields are smoothed by interpolating to a 1° grid
700 mb Height/Wind:
[info]
Height and temperature fields are smoothed by interpolating to a 1° grid
850 mb Height/Wind:
[info]
Height and temperature fields are smoothed by interpolating to a 1° grid
Surface
2 m AGL Temperature/MSLP/Wind:
[info]
MSLP field is smoothed by interpolating to a 1° grid
2 m AGL Dew Point/MSLP/Wind:
[info]
MSLP field is smoothed by interpolating to a 1° grid
Clouds and Moisture
Cloud Cover:
[info]
Cloud layers are defined by pressure levels: high-level (350-150 mb), mid-level (642-350 mb), and low-level (below 642 mb).
Precipitable Water:
[info]
Overlays of mean storm motion only use HRW members and do not include NAM or HRRR members.
Severe
Instability
Surface Based CAPE:
Most Unstable CAPE:
[info]
The maximum CAPE value is chosen from among parcels originating from 6 mixed layers in the lowest 180 hPa AGL (0-30 mb AGL, 30-60 mb AGL, 60-90 mb AGL, 90-120 mb AGL, 120-150 mb AGL, and 150-180 mb AGL) and 2 m AGL. Thus, when the most unstable parcel in the layer lies above 2 m AGL, the plotted 30-mb mixed layer value may modestly underestimate the true MUCAPE.
Shear
0-3 km SRH:
0-1 km SRH:
Composite Indices
Fixed-layer STP:
[info]
Uses a version of the fixed-layer STP calculation described in Gallo et al. (2018), but with surface-500 mb bulk shear magnitude substituted for 0-6 km AGL bulk shear magnitude.
Winter
Precipitation Type
1-hr QPF and Precip Type:
[info]
Using a priority list of (1 - freezing rain, 2 - snow, 3 - rain), a type is chosen if more than half of precipitating ensemble members indicate the type is occurring at the top of the hour; if no category reaches that threshold, then the type is labeled mix
Snow
1-hr Snowfall:
[info]
For each ensemble member, the precipitation falling during each 1-hour period is assumed to be *all snow* if the member indicates that snow comprises at least some of the precipitation at the top of the hour (overestimation can thus occur during mixed precipitation). Uses WPC forecast snow-to-liquid ratio (SLR) when available, or 10:1 SLR otherwise.
12-hr Snowfall:
[info]
For each ensemble member, the precipitation falling during each 1-hour period is assumed to be *all snow* if the member indicates that snow comprises at least some of the precipitation at the top of the hour (overestimation can thus occur during mixed precipitation). Uses WPC forecast snow-to-liquid ratio (SLR) when available, or 10:1 SLR otherwise.
24-hr Snowfall:
[info]
For each ensemble member, the precipitation falling during each 1-hour period is assumed to be *all snow* if the member indicates that snow comprises at least some of the precipitation at the top of the hour (overestimation can thus occur during mixed precipitation). Uses WPC forecast snow-to-liquid ratio (SLR) when available, or 10:1 SLR otherwise.
Blizzard:
[info]
Combined probabilities of low visibility and high sustained wind speed are displayed only where at least one ensemble member indicates snowfall during the previous hour.
Freezing Rain
3-hr Freezing Rain:
[info]
For each ensemble member, the precipitation falling during each 1-hour period is assumed to be *all freezing rain* if the member indicates that freezing rain comprises at least some of the precipitation at the top of the hour (overestimation can thus occur during mixed precipitation). QPF plots are not necessarily reflective of accretion potential. FRAM plots attempt to estimate accretion (thickness on elevated horizontal surfaces) using a simplified version of the technique in Sanders and Barjenbruch (2016) where accretion efficiency is related to precipitation rate, and the ILR is capped at 1.4 for light rates.
24-hr Freezing Rain:
[info]
For each ensemble member, the precipitation falling during each 1-hour period is assumed to be *all freezing rain* if the member indicates that freezing rain comprises at least some of the precipitation at the top of the hour (overestimation can thus occur during mixed precipitation). QPF plots are not necessarily reflective of accretion potential. FRAM plots attempt to estimate accretion (thickness on elevated horizontal surfaces) using a simplified version of the technique in Sanders and Barjenbruch (2016) where accretion efficiency is related to precipitation rate, and the ILR is capped at 1.4 for light rates.
Fire
Indices
Fosberg Index:
Surface Fields
10 m AGL Wind (Hourly Max):
[info]
This is the time-max sustained wind speed during the hour; gusts are not considered.
10 m AGL Gust:
[info]
Gusts are calculated on the hour, and do not represent the peak gust over the previous hour.
Relative Humidity:
Combined Probabilities
Relative Humidity and Wind:
Moisture and Precipitation
QPF:
Precipitable Water:
[info]
Overlays of mean storm motion only use HRW members and do not include NAM or HRRR members.
Precipitation
QPF
1-hr QPF and Precip Type:
[info]
Using a priority list of (1 - freezing rain, 2 - snow, 3 - rain), a type is chosen if more than half of precipitating ensemble members indicate the type has occurred over the past hour; if no category reaches that threshold, then the type is labeled mix
1-hr QPF:
3-hr QPF:
6-hr QPF:
24-hr QPF:
[info]

Conceptually, PMM (probability-matched mean) is a variation of the ensemble mean with the original ensemble amplitude restored. At each grid point, the ensemble mean value is replaced with a value from the full distribution of individual member forecasts whose rank matches the point's rank within the ensemble mean distribution. See Ebert (2001, MWR) for more details.

LPMM (localized PMM) is a new technique from Clark (2017, WAF) wherein the PMM calculation is restricted to grid points inside some radius of influence, preventing precipitation in geographically distant areas from influencing the local value. On this site, we use r=110 km.

RF probs (random forest threshold exceedance probabilities) report the probability (PQPF) that the mean 24-h precipitation totals over the surrounding 20-km x 20-km area will exceed the given threshold (currently 0.1-, 0.5-, 1-, or 3-in.). The PQPFs are created using one random forest (RF) for each threshold. Inputs to the RFs include latitude and longitude as well as daily mean HREF ensemble mean forecast values of: temperature and dewpoint temperature at four atmospheric levels, 1-km above-ground-level reflectivity, surface-based CAPE and CIN, PWAT, hourly maximum wind components, 2-5 km updraft helicity, and forecast precipitation. These RF-based PQPFs perform best for the lower precipitation thresholds, which have the greatest climatological frequency.

48-hr QPF:
Storm Attributes
Reflectivity:
[info]
Instantaneous composite reflectivity
4-hr max Reflectivity:
[info]
Maximum reflectivity at 1 km AGL
24-hr max Reflectivity:
[info]
Maximum reflectivity at 1 km AGL
Updraft Helicity
4-hr min/max Updraft Helicity (2-5 km):
[info]

Percentile values (e.g., P99.85) are derived from a spring 2019 climatology of 3-km CAMs where various experimental runs were binned by dynamical core. The P99.85 (intended to replace old 75 m2 s-2 products) values are: 75 m2 s-2 for ARW; 100 m2 s-2 for NMMB; and 160 m2 s-2 for FV3. The P99.95 (intended to replace old 150 m2 s-2 products) values are: 150 m2 s-2 for ARW; 175 m2 s-2 for NMMB; and 250 m2 s-2 for FV3..

The substantially different UH magnitude distributions for the three dynamical cores represented in HREFv3 necessitate a transition from fixed-value to climatological percentile thresholds in order to treat members equitably.

For now, the old fixed-value UH thresholds remain available as paintballs and as optional neighborhood probability contour overlays. When using these plots, note that FV3 members may exceed the fixed thresholds in more marginal cases than ARW and NMMB members.

24-hr min/max Updraft Helicity (2-5 km):
[info]

Percentile values (e.g., P99.85) are derived from a spring 2019 climatology of 3-km CAMs where various experimental runs were binned by dynamical core. The P99.85 (intended to replace old 75 m2 s-2 products) values are: 75 m2 s-2 for ARW; 100 m2 s-2 for NMMB; and 160 m2 s-2 for FV3. The P99.95 (intended to replace old 150 m2 s-2 products) values are: 150 m2 s-2 for ARW; 175 m2 s-2 for NMMB; and 250 m2 s-2 for FV3..

The substantially different UH magnitude distributions for the three dynamical cores represented in HREFv3 necessitate a transition from fixed-value to climatological percentile thresholds in order to treat members equitably.

For now, the old fixed-value UH thresholds remain available as paintballs and as optional neighborhood probability contour overlays. When using these plots, note that FV3 members may exceed the fixed thresholds in more marginal cases than ARW and NMMB members.

Updraft
4-hr max Updraft:
[info]
Column-maximum updraft below 100 hPa for HRW members; below 400 hPa for NAM members
24-hr max Updraft:
[info]
Column-maximum updraft below 100 hPa for HRW members; below 400 hPa for NAM members
10 m Wind
4-hr max Wind Speed:
[info]
Maximum 10 m AGL wind speed coinciding with composite reflectivity >20 dBZ (wind in other areas is ignored)
24-hr max Wind Speed:
[info]
Maximum 10 m AGL wind speed coinciding with composite reflectivity >20 dBZ (wind in other areas is ignored)
Member Viewer
Reflectivity:
[info]
Composite reflectivity
Storm Attributes
Updraft Helicity:
[info]

NAM Nest members do not output hourly-minimum 2-5 km UH or hourly-maximum 0-3 km UH, so those fields are not plotted.

HRW NMMB members do not output hourly-minimum 2-5 km UH, so that field is not plotted.

HRW ARW and HRW NSSL members output all required fields, so plots for those members are complete.

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• HREFv3 is an operational version of SSEO
?
The High Resolution Ensemble Forecast version 3 (HREFv3) is an operational version of the SPC Storm Scale Ensemble of Opportunity (SSEO). It was developed at the NCEP/Environmental Modeling Center (EMC) and is run daily at 00, 06, 12, and 18 UTC by NCEP Central Operations (NCO). This page displays the 00 UTC and 12 UTC cycles only, and uses a 12-h time-lagged NAM Nest member rather than a 6-h time-lagged NAM Nest member. This prototype page replaces the previous SPC SSEO web page.
• Neighborhood probability details
?
Neighborhood (nh) probabilities (NP) are calculated on the 3-km HREF grid. The neighborhood is a box specified by its radius; if r=40 km, the neighborhood is an 80x80 km box centered on the grid point. Grid point probabilities for exceeding the threshold within the neighborhood are calculated at each point. After these grid point probabilities are calculated, a Gaussian smoother is applied with σ=40 km. The smoothed NP field is plotted on this site.
• HREF members
• HREF/SREF Calibrated Guidance FAQ
• Site update history
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